“Rotating Joints”

For leak-proof fluid transmission into piping or a drum which is rotating, these little dandies could save you a ton of time in having to design your own.  Making these on your own would be a very big design challenge in itself.  Some of these are rated for extremely high pressures, or for high rotation speeds.

“Inline Fuses and Fuse Holder”

Cool way to get a fuse into a circuit which wasn’t originally designed with one–adding an inline fuse (catalog page 908) into the wiring.

“Mechanically Operated Air Control Valves”

We use pneumaticss pretty frequently in my new job, but they’re typically monitored and controlled electronically.  We often buy them with hall sensors in them that detect the cylinder movement based on a magnet on the cylinder.  For position detection of other machine elements we often use prox sensors, but I could see there being situations where position detection/pneumatic reaction could be useful, which is what these valves enable.

“Linear/Rotary Ball Bearings and Spline Shafts”

Where torque needs to be transmitted to a component that also needs to be free to move linearly, these ball splines will get the job done.  McMaster also sells simpler combination linear/rotary ball bearings.  These bearings are going to to (hopefully) allow me to implement an innovative new design at my job:

“Ultra-Miniature Versa-Mount Guide Blocks & Rails”

The sizes on these guide blocks and rails are ridiculous; “we believe these are the smallest blocks and rails in the world.”  In the image above, that’s a fingertip holding one of them up.  McMaster also says they’re “ideal for electronic, medical, and other precision applications,” but I wouldn’t want to be the guy who has to assemble anything this small.  What I’m currently working on at my job will employ the “miniature versa-mount blocks”–and these are still really small, being tapped with paltry M2 threads, to give you a sense of size.  But these ultra-miniatures are tapped with M1 x 0.9′s.  What the heck is that.  You’d have to employ ants to tighten the screws.  Dimension “A” in the image above, for the smallest of these assemblies, is 4 mm!

“Miniature Adjustable-Speed Controllers”

A simple mechanical solution for achieving a constant feed-rate for short strokes–”as the piston rod is compressed, hydraulic fluid is forced through an adjustable internal opening, creating consistent velocity throughout the stroke.”  Could be useful for preventing the breaking of drill bits in repetitive drilling operations, once you dial in the right rate (p. 1181).

“Air-Glide Multidirectional Dolly Kits”

By hooking up a hose with 90 psi shop air, these dollys have a remarkable lifting capacity–a plate-shaped dolly 21″ x 21″ x 2″ can lift 7,000 lbs. (!)

“Pocket-Size Vibration Meter”

I thought this was a neat little device.  ”Flexible steel wire reed detects vibration while the scale on the side of the meter indicates the machine vibration in cycles per minute.”

“High Spot Blue Marking Paste”

“Also known as Prussian blue, this past marks high surfaces and interference spots on precision fittings such as bearings, gears, and valves.  Coat the parts and assemble them into working position.  Any improper fit will show up as a blue streak.”  Clever.

“Pipe Contour Gauge”

I thought this tool was awesome, and I had not seen it ever before.  ’Twill be useful whenever I get a place with a real garage and get into welding.  ”Transferring the exact outline of a pipe joint couldn’t be easier.  Simply slide the gauge over the pipe to be cut, hold pipe in joint position, and push the gauge probes down until they take the shape of the pipe joint.  Slide the gague up and you’re ready to mark the pipe.”  A similar tool is available on the same catalog page (2247) called a contour gauge; it operates on the same principle, just for replicating complex surfaces in a single plane:

“[...] Gauge” (Radius, Thread Pitch, Nut & Bolt, Wire & Sheet Metal, Hole Diameter (Plug))

I want one of each of these in my toolbox.  Well, some are probably too expensive to be practical, but they’re still useful.

“Electronic Water Level”

How damn clever is this?! Using water finding its own level in a tube, you find the level between two points!  ”Determine level around corners and over long distances.  This level lets water seeking its own level do the work.  Just fill the attached hose with water and raise the unattached end of the hose in a different area.  When the electronic sensor buzzes, the water level in the hose marks the matching level point.”  Of course, you don’t really need a fancy buzzer, just a clear tube.  But applying Bernoulli’s principle / hydrostatics to level-finding…  smart!

“Shaft Alignment Kits”

Pretty critical for high-speed rotation or static joining of rotating shafts.  Two versions, inexpensive stylus, or expensive dial indicator.

“Threaded Hole Transfer Punches”

Where an array of drilled and tapped holes needs to be transferred to another part manually, these punches can be threaded in and used to drive pilot countersinks into the new component.

“Adjustable Large Diameter Hole Cutter”

Clever device for turning a drill into a device for cutting large circular holes in sheet metal.

“Tap Extractor”

I’ve been pretty careful whenever I’m tapping something (3/4 turn forward, 1/2 turn back…) because I didn’t want to find out what happens when you snap a tap off in your workpiece.  But this is what you get to use, if you have to get one of them back out of a hole.

“Hinged Pipe-Wrenching Collar”

These keep you from dinging up the pipe, by wrenching off of the collar, instead of directly on the pipe.

“Torque Multipliers”

Rather than using a breaker bar and damaging your tool, you can use a torque multiplier which has “gears in the housing which multiply the amount of torque that can be applied to the fastener.”  I saw versions of these being used on impact tools at GE’s wind assembly group, where I worked as an intern.

“Screw Extractors”

I haven’t had occasion to use these before, but you use them by drilling a hole in a bolt or screw, drive the extractor in CW, then back the bolt out on its own threads (CCW) by backing the screw extractor out.

“Sealing Flat Head Phillips”

Neat addition to a common product; rubber O-ring under the countersink face to seal pressurized fluids behind the fastener.

“Tension Indicating Steel Bolts”

The head of these bolts have a color dot which changes from red to black, once it’s properly tightened.

“Wire Lockable Bolts”

These bolt heads have holes through them, through which wires can be run.  The wires will prevent the bolt from backing out of the material; good for high vibration applications.

“Clip-On Nuts”

For holes on edges of sheet metal, these fasteners could save you time.  Clever design, many shapes available.

“Rivet Nuts”

I can’t imagine that these are rated for very high loads, but for light load joining, these could be handy.

“Precision Mini Torch”?!

“Needle point flame gives you maximum control for precision welding, brazing, soldering and heating…  Torch produces enough heat to weld steel up to 3/32″ thick.  Flame reaches 6000 degrees F using oxygen and acetylene.”  (WANT)

I have a mind to acquire a rifle for hobby target shooting and I’ve been reading up on the subject.  During my move from Southern California to Washington to start my new job, I stopped in at Powell’s books in Portland, OR and was looking for a good “beginner’s guide to rifle purchasing.”  I was quite happy to elicit curled lips from the unkempt hipster bookstore employees by asking for directions to their store’s firearms section…  This is something which one simply does not do in the most enlightened looney-lefty anti-gun-nut city in our country.

Unfortunately I wasn’t able to find a “Dummies Guide to Rifle Buyin’,” but I did find an interesting book called “American Rifle, A Biography,” by Alexander Rose.  It’s an interesting history of the development of the technology and the major innovators who drove it forward.

One thing that I thought was particularly interesting was the technological transition from muskets to rifles.  Rifles superseded muskets because they were more accurate and breech loading makes for a far more efficient gun (that is, loading bullets into the barrel from the rear–the “breech”–as opposed to driving them down the muzzle–”muzzle loaders.”).  Rifles, as their name implies, have helically grooved rifling in their barrels (as illustrated above), which imparts spin to the bullet.  This gives the projectile a more stable flight and makes rifles more accurate than smoothbore muskets.

However, I had assumed that the transition from muskets to rifles would have been an abrupt one due to the details of the technology…  A gun that relies on rifling to impart spin to the bullet is essentially swaging (deforming) the bullet–it forces the bullet through a barrel which is slightly undersized to the outer diameter of the bullet.  The outer surface of the bullet takes on the helical shape of the rifled barrel, which cause it to spin down the barrel as it is driven forward and out of it (a great slow-motion video showing this grooving on bullets is available in an earlier blog post).

So, to my mind, this process of bullet-deformation-as-it-travles-down-the-barrel could not possibly allow a muzzle-loading musket to have a rifled barrel, right?  The deformation of the bullet through a rifled barrel would surely prevent someone from loading it from the muzzle; it would require the user to awkwardly and forcibly ram it down the barrel just to load the gun, right?

Wrong!  There was actually a significant span in rifle history (which included the Civil War) in which “rifle-muskets” were quite common.  But how did gun designers get around the mental trap which I got caught in?  How do you get a bullet to “go in small,” (permitting ease of muzzle loading), but then increase its size after firing so that the rifling could impart its effect to the bullet?

They relied on clever bullet design and material selection.  Two successive designs occupied the rifle-musket barrels of this era, but the “Minié” bullet eventually dominated as the rifle-musket round of choice.  This was the round used in the “Model 1855,” which was the primary rifle of the Civil War.  A personal anecdote: when I was probably 10 years old I was on a family vacation with my folks and my parents bought a bullet for me at a Gettysburg souvenir shop–and it does indeed look exactly like that Minié bullet pictured to the right below!  Little did I know about this bullet’s background at that point, or why it had this unique shape!  The alternative bullet–designed before Minié but eventually superseded by it–was designed by one Captain Delvigne of the French army and consisted of a lead bullet which was tamped down at the breech of the rifle barrel with a ramrod, flaring it outward.

Regarding the Delvigne bullet, Alexander Rose had this to say:

“Captain Henri-Gustave Delvigne of the French Royal Guard grasped that reducing windage–the gap between the sides of the barrel and the ball–was key to raising the musket’s accuracy.  The problem here was that muskets loaded faster than rifles because their bullets slid easily down their smooth, wide barrels.  Reducing the windage would make loading only more difficult, thereby lowering the musket’s rate of fire to that of the rifle.  Delvigne’s novel solution was conceptually similar to a ship-in-a-bottle, in which the folded, flattened vessel is slipped through the narrow neck and unfurled inside.  He placed a “rebated,” or slightly smaller, chamber at the bottom of a broad but rifled barrel.  The soldier poured the powder down so that it settled into this cramped space and, after rolling down a spherical ball, used a heavy ramrod to stamp on the soft lead bullet so that it flattened and expanded its diameter.  Upon firing, the bloated ball gripped the grooves, spun, and turned the musket into a rifle.”

Though this improved accuracy, the bullet flight was still a bit erratic due to the inherently erratic method of loading: a person using a ramrod cannot flatten a Delvigne bullet with the consistency required to make this round repeatably accurate.  But then came the Minié bullet:

“Around this time several British and French inventors, working independently, had grasped that since it was the human act of flattening the ball that made it irregularly shaped, what if the projectile self-expanded to fit the rifling?…  Relying on recent scientific investigations into the nature of projectile flight, gunsmiths, sporting shooters, and ballisticians were beginning to understand that an elongated bullet was subjected to weaker air resistance than a spherical one…  The advent of these more modern-looking, if stubby, projectiles meant the end of the spherical bullet was nigh–though soldiers, tipping their hats to the past, continue to fire “rounds” today.  The first popular “cylindro-conoidal” bullets had a broadly pointed nose that rather resembled a Romanesque arch atop a short, almost hollow cylinder.  Fighting off the competition, it was Captain Claude-Étienne Minié of the French Army whose design became the standard.  Minié placed an iron plug at the hollowed bottom of the cylindrical bullet so that the combusting powder’s gas forcefully thrust it forward.  As the plug expanded, the bullet’s sides pushed outward; it gripped the rifling and began spinning.  The beauty of Minié’s design was in its ease of loading: a bullet went in one size and emerged miraculously larger.”

I think that this was a pretty clever observation and then employment of the deformation of metals to improve older, less effective technologies!

One interesting final note: private gun manufacturers during the Civil War were well into developing breech-loading guns capable of firing bullets far more rapidly than the muzzle loaders which Army troops at the time were being issued.  Being able to quickly load a bullet into the breech of a rifle is a critical design feature if you are a soldier who does not wish to die from the slow reload rate of a muzzle-loading gun (innovative Minié bullet be damned, this is life and death here!).  One might find some sympathy for the armorers of the Union and Confederacy for their being skeptical about making a risky technology leap in the middle of a war by switching their soldier’s arms over to these new and “untested” breech loading guns…  However, there were thousands of soldiers in the field who did this of their own volition, as well as generals who wrote to these new manufacturers, pleading to spend their own money to equip their units with these new guns and the benefits they imparted.  Additionally, nearly all settlers moving West were defending their families and fortunes with these new arms.  One wonders if the War could have ended sooner and with potentially less blood if the government armorers of the Union–who were being actively courted by these gun manufacturers to supply the Army with their wares–had just adopted them earlier.  Alexander Rose pretty consistently paints a picture of government armorers resisting innovation and the adoption of new gun technology.  This was sometimes for reasons as trivial as “military tradition,” or for academically concocted statistical reasons which had no real meaning on a battlefield, even while all this frivolity was endangering the lives of the soldiers.  ”Good enough for government work,” eh?

In just a couple weeks I’ll be starting my new mechanical engineering job with Electroimpact, a major aerospace tooling and automated machine production supplier (The video above is one of their automated fiber placement machines; it lays down carbon fiber tows on the mandrel barrel sections of the Boeing 787 fuselage).  Since I have limited professional experience with CNC/mechatronics system design (beyond teaching simple design-build-test, kit-based projects to undergrads), I thought it would be appropriate for me to review the major subsystems which comprise the design space of a mechatronics-based engineering project.

Rather than focusing on the technical details of how to specify, design, and select and/or create individual components for each of these systems (which would be a diabolically large and unfeasible blog post), this write-up will just review the major subsystems–the “design building blocks”–of the technology which underpins CNC machinery & robotics–”mechatronics,” in the more general sense.  This write-up will describe and then illustrate these subsystems with some specific examples.  As an entry-level mechanical engineer, the field of mechatronics necessarily incorporates expertise which is beyond the realm of the knowledge which I possess at this point in my career.  And, due to the inherent breadth of disciplines required in mechatronics system design, it will be quite a while–if ever–before I could comfortably write up a thorough explanation on specifying and designing components for all of these subsystems…  Some of these subsystems are much better handled by Electrical Engineers, or engineers with a depth of experience in controls engineering.

Nevertheless, this write-up on mechatronics systems which has been written in broad brush strokes is outlined as follows:

• Mechatronics Defined
• Prime Movers
  • Electrically Powered Actuation
    • Linear Movement
      • Electric Actuators
      • Solenoids
      • Shape-Memory Alloys
    • Rotation
      • Servos
      • Stepper motors
  • Mechanically Powered Actuation
    • Linear Movement
      • Pneumatic cylinders
      • Hydraulic cylinders
    • Rotation
      • Air motors
      • Hydraulic motors
      • Combustion Engine
      • Steam Engine
• Motion Hardware
  • Rolling-Element Bearings
    • Ball Bearings
    • Roller Bearings
    • Needle Bearings
    • Tapered Roller Bearings
    • Spherical Roller Bearings
    • Thrust Bearings
    • Linear Bearings
    • Linear Guide Blocks & Rails
    • Ball Screws & Ball Nuts
  • Plain Bearings
  • Ball Joints
  • Leadscrews
  • Ballscrews
  • Cams
  • Gears
    • Spur Gears
    • Helical Gears
    • Double Helical Gears
    • Bevel Gears
    • Hypoid Gears
    • Worm Gears
    • Rack & Pinion Gears
    • Epicyclic / Sun & Planet Gears
  • Belts & Pulleys; Chain & Sprockets
  • Springs
• Linkages & Structure
  • Foundation
  • Frames & Bedplates
  • Four-Bar Linkages
• The “End Effector” / Tool
• Control Hardware
  • Switches
  • Relays
  • PLCs
  • Power Management Hardware
• Control Software
• Sensors
  • Limit Switches
  • Proximity Sensors
  • Photoelectric Sensors
  • Rotary Encoders
  • Machine Vision
  • Temperature Sensors
  • Pressure Sensors
• References

Mechatronics Defined

Mechatronics is defined as a “combination of the engineering disciplines of mechanical, electronic, computer, control, and systems design.” [1]  With the advent of electrically powered prime movers (motors) and then the development of computers, machines could begin to be controlled much more efficiently with electricity and then accurately and flexibly with computers.  Strictly mechanical systems (pre-electric, industrial revolution era machinery for example) had severe limitations relative to these new types of machines.  Computer-controlled machinery can generally operate much faster, much more efficiently, can be designed to operate usefully in a wider range of situations, and can operate much more independent of human input.  The main value driving this interdisciplinary field of engineering are these advantages which mechatronics systems have over their strictly mechanical forebears.

Prime Movers

Prime movers are those elements of a machine which provide the motive force driving the motion of the device.

Electrically Powered Actuation

Linear Movement

Electric Actuators

Devices called “electric actuators” are technically a combination of a motor and gearbox (which provides torque/rotational movement) coupled to an acme thread and ball-screw assembly to transform the motor’s rotation into extension of the rod (acme thread/ball screws will be described later in the “Motion Hardware” section).  However, they can be bought as electrically powered plug-and-play systems which do provide linear motion.

Solenoids

Solenoids are an induction-based technology which ejects a ferrous pin from the inductor barrel/coil when current is applied to the coil.  I couldn’t find a video of a solenoid in action which wasn’t part of a valve demonstration, but this video does show the guts of the valve (the actual solenoid), as well as a particularly dorky looking fellow in the role of narrator:

Controlling flow in valves is a very common application of this technology.  I employed these valves in my senior capstone design project to create a pneumatic t-shirt launcher mounted on a R.C. mobile platform for Michigan State sporting events.

Shape-Memory Alloys

Shape-memory alloys are applied in technology relatively rarely, but they commonly function as electrically powered actuators which provide linear motion.  Shape-memory alloys are special metals (or other materials) which, when heated (typically through resistance to electric current running through them), return to a shape which they had prior to being deformed by an external force.  Demonstrating that heat (a warm cup of water) can return the shape memory alloy to its original shape:

In an actual mechatronics application, a spring force opposing the memory allocy can be used to stretch the shape memory alloy, which achieves machine motion in one direction.  A current applied to the alloy causes the alloy to contract, which returns the device to its original shape or position.  A common shape memory material is Nitinol.

Rotation

Electric motors are one of the major technologies which makes the field of mechatronics a viable field worth pursuing.  There are many different types and classifications of electric motors, and a thorough listing of all types of motors would be the subject of another blog post.  However, the two most relevant types of motors with respect to precision mechatronics are listed below.

Servomotors

Servomotors are one dominant type of motor used in mechatronics.  Their electrical design (synchronous, asynchronous, AC or DC) is not limited to a specific design, but all servomotors share the feature of feedback control.  This relies on a controller, which can be a component separate from the motor itself known as the “servo drive,” which monitors the state of the motor or mechanical system with a sensor.  The controller then compares this sensor reading to a command input.  The servo drive then amplifies the differential electrical reading to power the motor towards the command input value.  A commonly used type of sensor is a rotary encoder (explained in the “Sensors” section), which measures rotational position of the motor shaft.

Stepper Motors

A stepper motor on the other hand–instead of relying entirely on external control hardware and a sensor–also relies on a specific geometric shape of the rotor and stator.  This type of rotor and stator design allows it to be rotated by a specific angle.  The stator consists of several toothed electromagnets placed around the periphery of a similarly toothed wheel on the rotor.  The electromagnets are energized sequentially such that torque is optimally generated based on the current position of the electromagnet teeth, relative to the position of the teeth on the rotor (a very good animation of stepper motor design is available at the wikipedia article on the subject).  Stepper motors have high starting torque and, apparently, can be used for more than just accurate positioning of CNC machines:

Mechanically Powered Actuation

Linear Movement

Pneumatic Cylinders

Pneumatic (compressed air) power is used in mechatronics applications requiring the application of a liner force.  Compressed air is already such a common thing in factories where impact tools are used, that it can be a natural extension to use equipment that runs on compressed air.

You can see in this video that the cylinder is double acting: some cylinders rely on a return spring to draw the cylinder back into place after extending it (the latter are called “single acting”).

Hydraulic Cylinders

Where high powered linear motion is required, hydraulic cylinders (compressed fluid) do the job.  Hydraulic cylinders require heavy duty pumps for anything require high throughput, but their power density allows them to perform even the largest of lifting operations: watch the “A heavy weight on stilts” video at Mammoet’s website to get a sense of just how large the loads can be, which can be lifted by hydraulics.

The tensile testing frames I used during my master’s thesis to test carbon fiber composite coupons were hydraulic cylinders, controlled by feedback control.  They could apply peak loads of approximately 1,000 lbs at 100 Hz (1oo cycles per second!).

Rotation

Air Motors

Less common than using compressed air to power pneumatic cylinders is using compressed air to power an air motor, which rotates the shaft of a ‘reversed blower’ type device.

Wind turbines too can be considered a mechatronics system relying on air to drive the system.  Wind turbines themselves are certainly mechatronics machines (being automatically operated, controlled, and monitored by a PLC system).  I spent about a year and a half working on these devices at General Electric.

Hydraulic Motors

Where a supply of compressed fluid exists, or electricity cannot be used, hydraulic motors may make sense, where a torque is required.  Hydraulic motors drive a shaft off of a set of vanes or gears which are propelled by hydraulic fluid rushing through the motor housing.  They supply much more torque than air motors.

And similar to wind turbines, river dams use much larger turbines relying on fluid to drive the mechatronics system / generate electricity.

Combustion Engine

Internal combustion engines which convert expanding combustion gases directly to mechanical work include variations of piston-based or turbine engines.  Obviously, a wide range of power can be derived from combustion engines–from small handheld portable generators to 600 MW gas turbines at power plants.  Modern controls applied to combustion engines has rendered them mechatronics systems in themselves–even before considering the possibility of using them to directly power another mechatronics system.

1,000 Watts of piston power by Honda

520,000,000 Watts of turbine power by General Electric

Steam Engine

The steam engine–either piston-powered or turbine-based–is another mechanical source of torque.  Steam piston engines were common during the industrial revolution but are rare nowadays (However, the floating relic known as the SS Badger–a coal-fired carferry I lived and worked on one summer on Lake Michigan–still relies on them.  We had a breakdown one night and I observed the ship engineers swapping a new two-foot diameter piston into the suffering steam-powered cylinder).  Modern steam-powered systems burn fossil fuels (coal or gas typically) to generate steam in boilers, which is then run through a steam turbine.  These are large complex systems which are usually attached to generators whose electricity is subsequently used to power smaller machines.  As with many combustion engines, steam engines are typically complex mechatronics systems in their own right.

Motion Hardware

“Motion hardware” are the machine elements which promote system motion, and enable it to move in a reliable manner.  They often also serve to prevent wear of other components (the linkages and structure).

Rolling-Element Bearings

Rolling-element bearings come in many forms, but greatly reduce frictional wear at a pivot point by replacing material-on-material sliding with elements rolling across each other.  There are many types of rolling element bearings.  The images below are taken either from the wikipedia article on rolling-element bearings, or from the McMaster-Carr catalog offerings on bearings.

Ball Bearings

Ball bearings are good for high speed applications and can be made fairly economically relative to other bearing types.  However, the high point-load contact with the race can limit the load they are able to bear.  The inner and outer races can tolerate some misalignment and ball bearings can tolerate axial and radial loads.

Roller Bearings

Roller bearings rely on cylindrical rolling elements.  Due to the line-contact of the cylindrical rolling element, they can bear a higher load than an equivalently sized ball bearing of similar material.  However, they do not tolerate race misalignment or axial loading very well.

Needle Bearings

Whereas roller bearings use rolling elements with a length only slightly greater than their diamter, needle bearings rely on elements whose length greatly exceeds their diameter.  This also incrases the number of elements while permitting the bearing to fit much more closely around the shaft.

Tapered Roller Bearing

Tapered roller bearings rely on conical races and conically shaped rolling elements.  Their large contact surface and ability to bear high radial and axial loads are beneficial, but their unique, conically based geometry makes them more difficult to manufacture than simple ball bearings.  The outer ring, or “cup,” can be separated from the inner roller cage and race.

Spherical Roller Bearing

Spherical roller bearings are designed with rolling elements whose outer surface is some portion of a sphere, with a race to run to match.  The primary benefit of these bearings is that they can run with misalignment of the inner and out race, but they are difficult to manufacture and run hot because different points along the contact line run at different speeds as the elements rotate, causing wear.

Thrust Bearings

These bearimgs can have cylindrical, conical, or spherical rolling elements, and can also be bought as “turntables.”  Obviously, they are designed for where the primary load is axial thrust.  Fluid and magnetic thrust bearings are also available, which do away with rolling elements altogether.

Linear Bearings

Linear bearings are used to promote sliding along the axis of a rod–as opposed to promoting rotation of the rod within the bearing (though linear bearings are available which permit both, if that type of unique application is what you are dealing with).

Linear Guide Blocks & Rails

Linear guide blocks and rails are co-engineered, precision-made units.  The rolling elements in the guide block are designed to fit into features on the rails with minimal clearance.  Due to the fact that the rolling elements ride in grooves on the rails, linear guide blocks typically can bear more load than simple linear bearings, and can handle offset loads better than simple linear bearings.

Ball Screws & Ball Nuts

Another type of rolling-element bearing which promotes linear translation is the “ball screw and ball nut.”  This is a bearing with a large number of spherical ball bearings that are captive, internal to the ball nut.  The ball nut itself is threaded on a ball screw threaded rod, and when the ball screw is rotated, the ball nut translates up or down the ball screw rod.  The ball bearings roll along inside the grooves in the threaded rod, and are recirculated to begin the process again, through a ball passageway that is external to the race.  Ball screws have a much higher efficiency than leadscrews (described later) because they rely on rolling elements, instead of frictional sliding.  They are fairly expensive to make, but are great where precise linear positioning is required.

Plain Bearings

Also known as a “sleeve bearings” or “journal bearings”…  where loads are not too high, it can be practical and much less expensive to install a sleeve bearing to promote sliding (along a shaft) or rotation (within the bearing).  Sleeve bearings come in many types of durable plastic, but also in porous metal which has been impregnated with oil to “self-lubricate” itself; some even have graphite cores embedded in them to promote lubrication.

Plain linear bearings are also available for promoting linear sliding along a shaft:

Plain bearing versions of the “guide block” format of linear bearing are also available:

Ball Joints

Ball joints are a type of bearing which “captures” a spherical metal ball in a housing, and allows rotation within a limited range of motion.  They are not technically rolling-element bearings because the sphere slides within the housing–it does not roll.  There are many different geometries of ball joints available.

Leadscrews

Leadscrews (also known as “power screws“) are used to translate rotational motion into linear translation.  Acme leadscrew threaded rod can be purchased and matched to internally threaded components which can be mounted to the device which requires linear translation.

Acme threaded rod:

Acme threaded nuts and flanges:

Leadscrews can also be bought as a prepackaged devices with a “slide” table mounted onto a component with internal threading matched to the leadscrew.

Leadscrews are commonly used in the  x and y axis positioning of milling machines, and anywhere else precise positioning is needed and a feasible length of threaded rod can be installed.

Cams

Cams are mechanical timing elements which are carefully shaped to cause a predicted motion output for a cam-follower that rides on the cam.  The most familiar application of cams is in internal combustion piston-based engines–they are used to lift the intake and exhaust valves at known positions of the engine crankshaft, preventing the piston from colliding with the valves.  Much more complicated motion can be accomplished than just a simple linear valve lift:

Gears

Gears transfer rotational motion accurately between two shafts and can increase/decrease torque while decreasing/increasing shaft rotational speed, respectively.  Gearboxes are combinations of multiple sets of gears which can achieve higher gear ratios in more reasonably sized and feasibly produced sizes, than if just one gear stage were employed.  There are many different types of gears which allow the engineer to achieve different ends.  Again, many of the images below are from a wikipedia article on gearing.

Spur Gears

This is the typical hob-cut, simple gear external gear.  Axes must be parallel for these types of gears since the teeth are parallel to the shaft axis; and they exhibit noise if running in high speed applications because the teeth do not engage gradually.

Helical Gears

Helical gears, while being more complex to make than spur gears, are much better suited for high-speed applications since the teeth engage more gradually than spur gears.  Spur gears engage instantaneously along a line across the whole tooth, but helical gears engage at a point, develop into a line of contact, then separate at a single point of contact.  This causes less noise / mechanical shock in the tooth.  The tooth shape of a helical gear is a segment of a helix, and their shaft axes can be oriented at any angle, though the contact area tends to be optimal when the shafts are parallel [2].  Thrust is generated along the shaft axis in helical gears, which has to be counteracted with thrust, taper, or spherical bearings.

Double Helical Gears

Double helicals (also known as “herringbone” gears) prevent the problem of axial thrust which is present in helical gears.  However, they are even more complex to manufacture.

Bevel Gears

Bevel gears consist of two conically profiled gears (in cross-section) whose hypothetical vertices intersect.  Teeth can be straight-cut or spiral-cut (which are analagous in functionality to straight hob-cut spur gears and helical gears).

Hypoid Gears

Similar to bevel gears except the shaft axes do not lie in the same plane.

Worm Gears

Worm gears function similar to leadscrews.  The worm can always drive the gear, but they can be designed such that the gear cannot backdrive the worm.

Rack and Pinion

When the teeth on the periphery of a helical or straight-cut gear are instead placed along a single line instead of the circumference of a circle, a “rack and pinion” is formed.  In this way, finite linear motion can be achieved directly from a rotating gear.

Epicylic / Sun & Planet Gears

Gears can be paired to create “epicyclic” gear sets, in which one or more of the gear axes rotates during gearing operation.  By fixing one or more sets of these axes, different output gear ratios can be obtained–and this is the principle and type of gearing in the modern automotive gearbox transmission (a great interactive illustration, in which various axes can be held fixed by the user, is available here).

Belts & Pulleys; Chain & Sprockets

Belts & pulleys (and cables) / chain & sprockets transfer force and motion from one shaft to another, and similar to gears, they can increase/decrease torque while decreasing/increasing rotational speed respectively.  Pulleys combined with a non-toothed belt or cable, such as a V-belt, provide a useful method of transferring power where an approximate (but not absolute) ratio of input speed to output speed is necessary.

Timing belts are pulleys with teeth features on the belt and pulley, which provide a much more exacting ratio of input-to-output rotational speed.

Of course, compound pulleys (a “block and tackle” arrangement) can be used to magnify mechanical force–trading off the distance a load is moved for the amount of force applied to it.

Timing chains and sprockets are more expensive, (can be) noisier than belting, and require lubrication, but they can transfer more power within a given space and have a direct and exact ratio of input-to-output shaft speed.  They also require exacting location of the shafts.

Springs

Springs store mechanical energy in the form of strain energy of the spring material.  They can supply a linear reactive force or a torque when they are deformed from their resting state (either compressed or extended).  A typical helical spring for linear tension force:

“Clock springs” can supply a reactive torque when deformed from their initial state:

Leaf springs and recurve bows function on the same basic geometry–applying a bending moment to a strip of material, applying tension between the two points of the spring element’s attachment.  They can also apply a force transverse to a line through the mounting points, if deformed from their initial static state.  This is what provides suspension force in the case of leaf springs, and the force to launch an arrow in the case of a recurve bow.

Linkages & Structure

The “Linkages & Structure” serve as the frame (dynamic or static) or platform/base upon which all the other elements are installed or work from.  These are often custom designed for unique process machinery.  Basic elements of mechtronics linkage and structure include the following categories.

Foundation

Some mechatronics systems are meant to be stationary, but involve very large forces–just from the sheer mass of the machine or what it is manipulating, or from dynamic loading from the machine’s motion.  This is especially true for process and assembly machinery and the design of the foundation for the machine can be critical to ensuring the machine will operate effectively–appropriately supporting and damping out any vibrations.  Companies such as Unisorb specialize in installation and foundation design specifically.

Frames & Bedplates

Generally, major mechatronics systems that require a serious foundation will also have machine components which could be described as the “machine frame” in order to do useful work.  Frames & bedplates serve as structures which all the other system components mount to, and serve as the “rigid base” which counteract any forces generated by the prime movers to move the other kinematic elements of the system.  A diagram from a GE turbine:

In that illustration, the “Main Frame” (labeled as number 5), as well as the “bedplate” (directly under the Gearbox-13), and the tower serve as the “frame” to which the dynamic elements are mounted–the rotor assembly, rotor shaft, (internal gears of the) gearbox, and the generator’s rotor (Though it should be noted that the machine head itself can rotate on top of the tower).

Four-Bar Linkages

A unique machine element which can effect complex machine motion, while still serving as part of the machine structure is the “four-bar linkage.”  Four-bars are exploited in robotics, and anywhere a unique but consistently repeatable path is required, or a mechanical advantage needs to be derived from a simple assembly of linkages in series:

Applications are numerous in mechatronics.  A parallel manipulator is shown in the video below–very similar to the ABB Flexpicker featured in an earlier writeup–and each of the three arms is a parallelogram of linkages with ball joints at their vertices:

Each of the kinematic parallelograms is a four-bar linkage.  A four-bar is also what enables the vise-grip to do such great work for us in the shop:

Four-bar linkages can be thought of as a means to do one of three things.  The first is to achieve a unique but repeatable tool path (the position of elements in the four-bar system are an exercise in advanced geometry).  The second is to increase force input to a system: by rotating one of the larger linkages in the system, input force can be multiplied in the motion of one of the smaller links (this is what vise-grip pliers do).  Four-bar linkages can also help you achieve the opposite process: by manipulating one of the shorter linkages in the system, the output speeds and distances translated at a larger linkage in the system can be greatly increased relative to the input.  Mounting a motor at one of the joints–driving one of the linkages to rotate–is one way in which a four-bar could be incorporated into a mechatronics system.

The “End Effector” / Tool

The end effector is the device which carries out the task which the machine is designed to perform.  This is typically a custom-designed device, and is specific to whatever the mechatronics machine has been designed to do.  In the first video at the very top of this write-up, the end effector is the head which contains multiple spools of carbon fiber tows, the motors and hardware for paying out the tows and guiding them onto the mandrel, and the shears used to trim the tows.  In this robot however, the end effector is a painting head:

In this robot, it’s a spot welder:

The end effector is specific to whatever the task of interest is.

Control Hardware

There is electronics or controls-related hardware associated with any mechatronics system.

Switches

A switch is typically defined as a component which can interrupt an electric circuit.  It usually also refers to an electromechanical device that is manually thrown by an operator.  Wide-scale use of switches to operate mechatronics systems is not common today–good system design and the use of PLCs and sensors can remove the need for an operator to “approve” every step of a machine’s operation by throwing a swtich.

Switches can be designed to be “thrown” by almost any sort of stimuli the engineer can think of, which then makes the “switch” then defined as a “sensor” (a binary, on-or-off, mechanical state detector): vibration, turning a key, the orientation of the switch, presence of a magnetic field, rotation, presence of a mechanical stop–the possibilities are numerous.  For machinery incorporating pneumatic and hydraulic power, switches (valves) are available for these systems as well.

Relays

Relays are a step up in terms of automation, relative to switches.  Instead of manually throwing a switch, relays close a circuit when current is provided to the relay from another circuit.  In this way, a sensor can close a relay when it detects a certain machine state, and the closed relay then powers some other device, reacting to the machine state which triggered the sensor.  For example, an optical switch monitoring a conveyor belt can detect the presence of a package and then send a low voltage signal to the relay wired to the conveyor belt’s motor.  When the optical switch detects a package on the belt, it closes the relay with a low voltage sensor reading; the closed relay supplies high-power electricity to the motor, which then drives the conveyor forward.

Electromechanical relays are based on induction: copper windings energized by the low power circuit drive an iron core out of the coil, which closes or opens the high-power relay contacts.

Relays do not have to be mechanical–physically closing a contact; they are available as solid-state devices.  Based entirely on circuitry and with no moving parts, these can be particularly useful where vibration can risk closing an electromechanical relay when the circuit is not actually energized.  For high-current applications or where frequent switching occurs, heatsinks are required to dissipate the power which is consumed during their operation.

Programmable Logic Controller (PLC)

Many, if not most, modern mechatronics devices are controlled by programmable logic controllers.  PLCs are simple computers which can continuously monitor multiple inputs, compare these inputs to user defined settings or operator commands, and then command govern the behavior of multiple components in the system as outputs–turning them on or off as appropriate.  In this way, PLCs are the “brain” of many mechatronics systems you see today.

PLCs were designed to replace the increasingly complex  control systems comprised entirely of huge systems of logic-based relay circuitry.  Modern PLCs commonly have the code which runs them written on and debugged on a separate personal computer.  This code is then downloaded to the PLC via ethernet or RS-232 cabling and stored on RAM or flash memory.  To control high-powered devices, PLCs often energize relays to close high-power circuits.  PLCs need not interact and control their mechatronic systems according to digital on/off inputs and outputs; analog inputs and outputs can be incorporated because PLCs can monitor the intensity of a voltage or current being sent to it (this enables quantifiable monitoring of things like pressure, temperature, or weight).  Additionally, PID control can be implemented to prevent overshooting system targets.

Power Management Hardware

With so many devices requiring different power and voltage levels (sensors, prime movers, end-effector machinery), it’s common for mechatronics systems to need power management hardware.  To be honest, I have very little experience with this hardware at this point in my career, but it’s worth mentioning that it can be necessary.

Control Software

The software controlling the PLC depends on the PLC which is selected for the job.  The logic controller’s manufacturer will have their own code language for their hardware.  For example, Fanuc has its own software for the robots it sells, I’ve worked with and taught the use of Parallax pbasic to students, and Arduino microcontrollers have a language that is similar to C++.

The details of coding are a topic way outside the purpose of this blog post, but logic commands like “IF,” “THEN,” “ELSE,” “FOR,” “WHILE,” and “COUNT,” were some of the common commands from pbasic code.  There are analogous commands between different manufacturer’s coding languages, and computer programming in general.  For example, lot of my training as an undergrad in MatLab code writing carried over pretty directly into coding in pbasic.

Sensors

Sensors monitor features of the system which the engineer deems important to the behavior of the system–either with respect to function, safety, or preventing system damage.  Some common sensors are presented below.

Limit Switch

These are mechanical switches with a toggle or button which, when depressed by an object moving past them, either activate or deactivate a circuit.  These provide true/false information about the machine status–has the machine reached this position–yes or no?  They don’t provide information about the absolute current position of the machine beyond whether it has reached the limit defined by the switch position itself.  They are useful in shutting off or slowing down machinery which are reaching a position which could be dangerous to an operator, or to damaging the system itself.  Limit switches come in many forms.

Proximity Sensor

Proximity sensors come in various forms.  They can detect the presence of a metal object (through the hall effect), they can detect the presence of liquid–some liquid level sensors close their circuit through the liquid itself.  Noncontact liquid level sensors can measure the presence of liquid by reflecting ultrasonic waves off the surface of the liquid.  A metal-sensing proximity switch looks like this:

While working as a mechanical engineering co-op for GE Wind Energy, I saw the switches in this image used as redundant measures of wind turbine tower rotation…  In addition to encoder measurement of yaw motor rotation, these prox switches measured the passage of gear teeth on the yaw bearing, relate to the bedplate’s motion.

Photolelectric Sensors

Photoelectric sensors are another sensor which provides information in a “yes or no” format–they provide a signal to the controller when an object is present or absent, when it interrupts a light beam.

Rotary Encoders

Most machine motion is facilitated by torque-providing prime movers–electric motors, or perhaps engines rotating a shaft.  If this is isn’t the case (for example, if hydraulic cylinders are being used), most machine motion can still be measured fairly easily as a function of rotation, and thus rotary encoders are the devices which are commonly used to measure the absolute position of a machine.  Rotary encoders measure the position of a rotating shaft.  Some encoders also have the ability to count the number of complete revolutions the shaft has turned,  which gives the engineer the ability to know how far the machine has moved from a specified datum.

If an encoder is being integrated with a PLC system, an analog to digital converter (ADC) will be required to convert the analog voltage or current signal to a digital signal that the PLC can read–if it’s not already integrated into the encoder itself.  Encoders optically measure a pattern etched into or through a surface with a photo detector, the photo detector yields a current/voltage reading, and this reading is converted to a digital signal.  The encoder etching pattern determines the shaft position which corresponds to the analog/digital.

It can be observed in the image above that each of the eight, 45-degree arc sections has a unique pattern in it, which then uniquely (in 45-degree increments) determines the position of the shaft.  However, encoders are available with more than 10,000 counts per revolution!  Obviously, very precise positioning can be achieved with encoders.  Precision can (typically) be further enhanced by measuring motor rotation instead of machine component rotation–this is true because motor rotation speeds are (typically) kicked down by a gearbox in order to attain speeds which are useful for most machinery.

In the days of yore–when the computer mouse relied on a trackball instead of optical surface sensing–encoders were what measured the movement of the trackball.

Machine Vision

Machine vision is a large topic in itself, but it is like a photoelectric switch on steroids…  Instead of measuring the presence or absence of light on a single sensor, an entire array of pixels on a CCD can be monitored for the presence or absence of specified light intensity levels.  In this way, objects of interest can be differentiated from their background, and these objects can then be reacted to by the controller system that’s monitoring them.  For example, if you fast forward to 1:50 in the video below, the ABB Flexpicker control system can be seen locating the presence (AND type!) of muffins on the conveyor belt:

Machine vision is a new but rapidly expanding field.

Temperature Sensors

Temperature can be measured as an analog signal by a temperature sensor, converted to a digital signal, and read and reacted to by a microcontroller.  Thermocouples can serve this purpose; infrared thermometers are also available.

Pressure Sensors

Similar to temperature sensors, pressure transducers can measure pressure–measured as an analog signal, converted to digital by an ADC, which is then read and reacted to by a PLC.

There are few limits on how many types of machine conditions could potentially be read by a sensor of some type, as either a digital or analog signal.

References

[1] http://en.wikipedia.org/wiki/Mechatronics

[2] http://www.roymech.co.uk/Useful_Tables/Drive/Hellical_Gears.html

Fortunately for sedan owners, automotive and optics engineers are smart enough to supply a simple solution to this problem with most cars–the rear-view mirror dimmer.  In many cars, this is simply  a mechanical toggle switch at the base of the mirror.  But I was curious, how does this switch make the image behind you dim?

This rear-view mirror dimmer relies on two things:

1. The optical properties of “mirror glass” and “mirror silver”–namely: the silver has a much higher reflectivity value than the mirror glass, and,
2. The mirror silver surface and the mirror glass surface are not parallel.

In the “flipped-down position” of the toggle switch, the rear-view mirror functions as a typical mirror–objects behind your car are reflected to your eye off of the mirror silver backing:

But while in the “flipped-up position,” the mirror is rotated up and the plane of the silver mirror backing is oriented so that it no longer reflects the image into your eyes (instead, this image is reflected up to the ceiling of your car, or down to the floor, depending on the design). In the “dimmed” case, the image behind your car is reflected off of the mirror glass surface:

If you have seen the outline of your reflection in a clean glass surface, you know that the intensity of the image is very faint and this is exactly what optics engineers count on when designing the “dimming” feature of a rear-view mirror.  By reflecting the image to your eye off of the glass surface–which has much lower reflectivity–the jackass behind you is far less able to blind you.

Given the number of times I use this feature in my car, the “rear-view mirror dimmer” is a valuable mechanism without qualification.

• Overview
  • Definition & Description
  • Installation Methods
    • Hot-Formed
    • Cold-Formed Deformation
    • Mechanical Forming
  • Benefits
  • Drawbacks
  • Applications
• Rivet Designs
  • Solid Rivets
  • Semi-Tubular Rivets
  • Blind Rivets
• Joint Design; Strength & Fatigue
  • Forms of Rivet Joints
  • Specification of Rivets; Design Parameters
  • Strength Calculation
  • Fatigue
• References

Overview

Definition & Description

A rivet can be defined as a permanent mechanical fastener.  It is a fastener–unlike say, welding, it does not chemically merge the materials of the subcomponents.  It is also permanent–unlike a threaded bolt, rivets are deformed into place in holes within the subcomponents to achieve a permanent connection (though some of them can be removed with relative difficulty).

Rivets come in many forms, and the different types will be described below in the “Rivet Designs” section.

Installation Methods

There are three general types of rivet installation methods.  Which installation method is used on what rivet depends on the design of the rivet itself.

Hot-Formed

In hot-forming of “solid rivets,” the rivet is heated to a red-hot, malleable state, slid into the workpiece, and the end of the rivet composed of a cylindrical shaft is then “upset” into a headed shape.

(above) Solid rivet before hot-forming

Solid rivet after forming, joining two steel plates

Hot-forming of rivets is old-school, and rarely used now.  The process was labor-intensive, requiring several workers:

• The two components to be joined are fixtured in position.  A hole is drilled (or punched and/or reamed) if one hasn’t been already.
• One worker slides a red-hot rivet in place from the rivet furnace (occasionally, the “rivet catcher” is a completely different job from the worker tending to the furnace).
• One worker hammers on the rivet to create the “factory head” while another worker holds the factory head in place with a “bucking bar.”

(And back in the day, a guy wouldn’t have had the benefit of a pneumatic hammer, either. He’d have to bash on it with a mallet.)

Old bridges, boilers, and ships were made in this way.  I recently visited the Queen Mary cruise ship (operated from 1936-1967, now docked in Long Beach California as a tourist attraction).  I was kind of in awe of how much tedious riveting was required to assemble this immense ship (which is even larger than even some modern cruise liners; image source):

Pneumatic and hydraulic machines have been created to make hot-riveting (and cold-riveting, too) a much less labor-intensive process, but the nature of the hot-riveting process–having to run a rivet immediately from a hot furnace into the workpiece–does not make it easy to automate.

Things that I would be particularly concerned about if I had to design with this fastening method include:

• A solid understanding of the heating/cooling cycle during the installation of the rivet would be necessary.  This heating could alter the rivet metal microstructure and thus, the mechanical properties of the rivet.
• Largely a manual process…  The process variables of heating time, differences in strength or ability across workers could affect the final upset shape of the rivets and the joint performance.  Bolted connections (which do not require expensive skilled labor) have almost completely replaced riveting in structural steel construction for this reason…  The hot-riveting process has become irrelevant enough that steel construction codes do not even contain design practice recommendation for riveting anymore [2].
• Slow.  It’s a slooooowww process.

Cold-Formed Deformation

The process required to buck a rivet into shape is significantly easier if the rivet does not need to be heated.  Modern rivets exploit optimized geometry and careful material selection to avoid the necessity of heating the rivet to allow it to be formed into shape.  Rivet materials are selected which are ductile and tough, and the rivet dimensions are engineered to allow the “factory head” of the rivet to deform into shape without rupturing during this cold working process.

A great example of this is the electromagnetic cold forming process which the company Electroimpact was founded on (good paper on this technology is located here, which is where the description and images below were taken from):

LVER (low voltage electromagnetic riveting) is based on eddy current repulsion between a copper pancake coil and a copper driver plate. Two actuators are typically employed on opposing sides of the rivet. Each actuator is connected to its own capacitor bank by a coaxial pulse cable. After the capacitors have been charged to a predetermined voltage, this stored energy is discharged through the coil. An intense magnetic pressure quickly develops between the coil and the driver plate, accelerating the driver into the rivet. Peak forces of over 30,000 lbs can be reliably and accurately generated using this technology.

Wild!

Mechanical Forming

Many types of cold-formed rivets rely on design features of the rivet–such as breakaway mandrels inside tubular rivets (so-called “pop rivets”)–to deform the rivet into shape.  These rivets, rather than relying on the standard externally applied compressive force to create the “factory head,” employ extra hardware which is part of the rivet and assists in bucking the rivet.  Rivets with these unique features will be discussed in the “Rivet Designs” section below.

Benefits

• Appropriate where vibration can threaten loosening of bolted assemblies
• Relatively strong and lightweight
• Lends itself to automation (cold rivets on reels or fed via pneumatic feeders, combined with automated drills)
• Joining dissimilar materials (tough to do with welding)

Drawbacks

• Disassembly is complicated
• Joint is not quite as neat as a welded structure
• Understanding of joint stress behavior can be complicated

Applications

Some applications of riveting include

• Aerospace assembly–riveting is lightweight and high strength [2].
• Fastening components made of metals which are difficult to weld.
• Fastening components of dissimilar metals.
• Pocket knives–the axis which the blades pivot on.

Rivet Designs

Solid Rivets

Solid rivets were illustrated earlier:

The cylindrical end of the shank must be deformed by an external mechanical force to form the “factory head.”  Solid rivets are used where reliability and safety are required [2], and thus they are heavily used in the aircraft industry.  Rivets do not have to have the rounded head (“universal” rivets) as illustrated above–another popular version has a 100 degree countersink.  Popular materials for aircraft rivets include aluminum alloys, titanium, and nickel-based alloys (e.g., Monel); some types of aluminum need to be annealed before they can be bucked, to prevent splitting [2].  A disadvantage of these rivets is that both side of the rivet have to be accessible for assembly.

Semi-Tubular Rivets

Semi-Tubular rivets are much easier to install than solid rivets because the shank which protrudes beyond the back of the second workpiece is hollowed out, which allows it to be bucked much more easily:

Besides being much easier to buck, the semi-tubular rivet prevents the solid part of the shank from expanding in the hole–solid rivets expand in the hole during the bucking process, typically forming an interference fit (hot formed rivets can contract in the hole, leaving a gap).  Thus, semi-tubular rivets can be used as pivot points.  The bucked end of a semi-tubular rivet will have a rolled-over shallow dimple in it.

Another unique type of semi-tubular rivet is the “self-piercing rivet,” which is sold by (among others) Emhart teknologies–check their website for a good video (.wmv) of the process.  The self-piercing rivet is semi-tubular and punches through the upper metal, but an anvil below compressed below the rivet and workpieces then forces the lower sheet metal to deform around the legs (which flare outwards from the shape of the anvil) to form a water and gas tight seal.

Blind Rivets

One major drawback about most rivet designs is that they require access to both sides of the workpiece–the factory head has to be held in place while the opposite end of the rivet is bucked into the “shop head” shape.  This is not true with “blind rivets.”  Blind rivets can be inserted and bucked with access to only one side of the panels being joined.  Their design typically incorporates a “mandrel” running through the center of a tubular rivet which draws and compresses the rivet into shape when the mandrel is pulled or threaded out of the rivet.  Manufacturers have different methods of achieving this form of rivet.

Several types of blind rivets are also made by Emhart teknologies, the most common being the “POP” rivet.  In the POP rivet, the mandrel is pulled through the center of the rivet.  The mandrel has a notch which causes most of the mandrel to snap off at a controlled load, leaving a small bead of metal in the shop head of the rivet. (I used this type of rivet in the construction of my senior capstone design project–the “Sparty Tank” remote control t-shirt launcher–described in the undergrad design portfolio here):

These rivets are not used frequently in aircraft [2] because of the possibility of the bead of metal falling out of the shop head, causing the joint to loosen.  Also, tubular rivets in general are weaker than solid rivets.  However, some tubular rivets leave a significant portion of the mandrel inside the rivet, and these have a better shear strength than those which remove the mandrel completely [3].  The “well-nut” by Emhart relies on elastomeric material that deforms outward when a threaded bolt is run into it (thus, perhaps technically disqualifying it from the status of “rivet”).

A third type of rivet by Emhart–the popnut–may not be able to be defined as a rivet either, since it involves a threaded connection.  However, it consists of a threaded insert installed into (typically) a single base piece.  A drill-like tool with a threaded, bolt-like bit installed pulls the popnut in and causes the grooved barrel section of the fastener to collapse, forming a ring of material securing the popnut into the workpiece:

Another type of blind rivet design [a "trifold"] available from Emhart and is, again, really intended for a threaded bolt connection only (though it is available from other suppliers without the necessity of a threaded inner ring, i.e, you can buy a traditional rivet in the form of a trifold).  Called a “jack nut,” by Emhart, it relies on three flared legs expanding outwards, with a threaded inner ring for threading a bolt into, to attach the other workpiece.  A tri-fold rivet is good where a distributed load is required–spreading it across the wide area formed by the compressed legs.

Another form of blind rivet is the “drive rivet” which has a short stub protruding above the factory head.  Tapping on the stub with the hammer forces the mandrel shaft down through the middle of the rivet, expanding the legs or other features on the back of the rivet to fasten the workpieces.  Drive rivets have relatively weak clamping forces compared to other rivet types [2].

Joint Design; Strength & Fatigue

Forms of Rivet Joints

There are two basic types of rivet joints.  One is the lap joint, where the two workpieces overlap each other and the rivet goes through both of them.  The second type of rivet joint is the butt joint, which requires a “cover plate” or “butt strap” to attach the rivets to, and which spans the butt joint edge.  A butt joint has the advantage of a smooth outer plane and a less complex loading state on the joint (lap joints introduce twisting into the joint due to the workpieces terminating in different planes).

Above: single riveted lap joint

Above: single riveted butt joint with butt strap

The images above display “single riveted” joints, but it is quite common to have multiple rows of rivets in lap or butt joints.

The spacing between rivets in a single row is termed “pitch,” and the spacing between rows of rivets is termed “back pitch” or “transverse pitch.”  The spacing between rivets in adjacent rows is called “diagonal pitch.”  The distance from the edge of the plate to the nearest row of rivets is called “margin.” [1]

Specification of Rivets; Design Parameters

• Type of joint (lap or butt)
• Spacing of rivets (some general guidelines provided below)
• Type and size of rivet (solid rivet? blind rivet? general guidelines on sizing of rivets provided below)
• Type and size of hole (countersink or button head rivet being used?  holes typically drilled 1/16″ larger diameter for hot driven rivets, but reamed for minimum clearance for cold-driven rivets)
• Rivet material (weight considerations, safe-failure considerations).

Strength Calculation

Some of the assumptions of bolted joint strength calculation are transferable to rivet joint calculation.  However, a major difference between the two is that tensile loads transmitted through rivets at say, a lap joint, are computed as transferred through the rivets in shear.  While this is not entirely true for hot rivets (which, on cooling, contract and apply a compressive force at the interface) or in situations where a solid understanding of the clamp force during riveting is known, this is definitely different than the behavior of a bolted joint.  Bolted joints, when properly designed and installed, rely on interfacial static friction shear to bear a tensile load transverse to the axis of the bolt.  This interfacial shear is due to the clamping force applied by the bolt when it is torqued down.  Only a bolted joint that is not installed properly, with the nut not torqued down at all, would cause the load to be imposed on the bolt in shear.

Rivets should NOT be combined with bolts in creating a fastening assembly.  Particularly with rivets that expand to fill the hole with an interference fit, they provide different joint mechanics than bolted connections.  Due to the interference fit, they will bear the loading on the joint earlier and thus fail earlier than bolts…  Therefore, assuming the load is shared evenly between a set of rivets and bolts is invalid.  The rivets will fail first and the joint will then “unzip”–the bolts will immediately fail due to the immediate transfer of high load.

As a general design guideline, the rivet diameter commonly falls between:

1.2√(t) < d < 1.4√(t)

Where “t” is the thickness of the plate [1].

To design a rivet joint for strength, several assumptions are made, which are analagous to a few of the assumptions made in bolted joint design [1].

1. Load is carried equally by the rivets.
2. No combined stresses act on a rivet to cause failure (engineering judgement is required to determine if this is a safe assumption).
3. The shearing stress in a rivet is uniform across the cross-section under question (this is an unconservative assumption if a thorough analysis is to be performed and strength knockdown parameters should be employed which account for stress concentrations at the top of the rivet and hole which would cause the shear-stress field to be anything but uniform).
4. Load causing failure in single shear would need to be doubled to cause failure in double shear.
5. The bearing stress of rivet and plate is distributed equally over the projected area of the rivet (another unconservative assumption which should be accounted for by determining stress concentrations on the rivet and plate-hole joint assembly, which cannot possibly be line-to-line fit-up).
6. The tensile stress is uniform in the section of metal between rivets (again a generous assumption; K [stress concentration factor] is known to be 3x the the far-field stress at the edge of a hole–compute accordingly).

Allowable stresses for rivet design calculations relating to steel construction and boiler manufacture are specified in their respective codes.  In the case of aerospace, qualification for a specific rivet design is an experiment-intensive process (while I was working on my master’s degree at GA Tech, a couple grad students in my lab were doing these sorts of tensile testing experiments).

When the center of the rivet is placed a minimum of 1.5x its own diameter away from the edge, the “shearing of plate,” and the “tearing of plate to margin” failure modes (in the picture above) are prevented.  If that design practice is followed, a simple analysis of rivet strength is then performed by analyzing three different failure modes:

Rivet Shear

This computation is performed to predict failure for modes of “single shear of rivet” and “double shear of rivet” illustrated above.  The shear strength of the rivet material must be known (or yield strength, depending on what you are computing for).  This calculation is based on assumptions (2) and (3) above.

Bearing Stress

To predict failure for the case illustrated as “crushing of plate or rivet” in the “failure modes” picture above.  The compressive failure strength (or, a more sensible calculation for materials in compression: yield strength) for the plate and rivet materials must be known.  This computation is based on assumption (5) above.  For the case illustrated below (single riveted, double butt-joint), whichever of the two cross-sectional areas is smaller will be the one which is pertinent to the calculation of bearing stress on the rivet.

Plate Stress

This computation is performed to predict failure of the plate area between the rivet holes (or the area of the butt straps if it is a butt joint)–the failure modes described as “tearing between rivets” in the “failure modes” picture above.  This computation is based on assumption (6) above.  If the workpiece plate and butt strap are the same material, then computation only needs to be performed for the smaller of the two cross-sectional areas.

Below, an image is taken out of reference [1].  Equations have been derived and compiled in books which save the engineer the necessity of computing the stress distribution for common rivet patterns.

Fatigue

Given the aforementioned caveats to the assumptions in just a simple monotonic strength calculation, an in-depth understanding of the specific details of any rivet design will be required for any rivet joint subject to fatigue.  Where appropriate, sub-units of repeating rivet patterns can be modeled in fatigue, just as they can be for static strength analysis.  Traditional fatigue analysis methods can be used to predict fatigue of riveted joints, but a strong understanding of the specific rivet and specific joint behaves will be required–are the rivets subjected to shear, or is the clamping force making it perform more like a bolt?  Is the rivet material more prone to fatigue damage, or the base workpiece metal–and which would make more sense to have fail, in order to allow for a “fail safe” design, or one that can allow early damage detection?

References

1.  ”Machinery’s Handbook, 28th Ed.,” Erik Oberg, Franklin Jones, Holbrook Horton, Henry Ryffel, c. 2008 Industrial Press New York, 2692 pages

2.  http://en.wikipedia.org/wiki/Rivet (nothing used from Wikipedia was used without corroboration from the other, “more serious” engineering sources)

3.  ”Marks’ Standard Handbook for Mechanical Engineers, 11th Ed.,” E. A. Avallone, T. Baumeister III, Ali M. Sadegh, c.  2007 McGraw Hill

One final treat…  A youtube video of an esoterically oddball rivet that (according to the video description) was once used to repair WWII airplanes in the field–it deforms from explosive chemicals ignited in its core!

Position repeatability is 0.1 mm!

The four-bar mechanism comprising the  arms of the robot is innovative.  Through a set of three motors and spherical joints at the shoulders and wrists, the ‘bot can move the end effector to anywhere within a sizable cylindrically-shaped working envelope.  In my machine design class as an undergrad, we learned the theory which predicts the location of elements within a four-bar linkage system.  I explicitly recall my prof saying, “Four-bar linkages are quite good for motion in-plane, but it’s quite difficult to get them to operate out-of plane.”  The FlexPicker effectively gets three four-bar assemblies in different planes to function together through the use of spherical joints at the elbows and wrists and–from the looks of it–it’s capable of speeds that traditional X-Y translation or robot-arm setups just couldn’t do for pick-and-place operations.  While single-plane four-bar linkage calculation is fairly straightforward–essentially geometry on steroids–I would be interested to see the control scheme used by ABB for the machine.  A whole new set of challenges is introduced by those spherical joints.

The unique three-arm, four-bar design also obviates the need to place motors at the elbow or wrist joint, allowing the “humerus” and “forearm” linkages to be quite light.  A traditional robot taking the form of a human arm requires heavy, torque-producing motors at those locations.  Granted, a traditional robot arm can do much more complex work than the FlexPicker can.

Technical datasheet for the ABB IRB 360.

It’s particularly interesting how the rifling deformation on some of the bullets causes them to split preferentially along the direction of the rifling.

True Life: I did this arm wrestling with an old housemate… ”Arm wrestling? Sure, I haven’t done that in forever, what’s the worse that could happen?”

While the bone was healing, my arm was in a sling for five weeks.  I learned to be pretty good at one-handed typing with my left hand, since I was submitting my master’s thesis in four weeks.  After getting my arm out of the sling, I had lost quite a bit of my range of motion.  I could not extend my arm because the bicep and connective tissue had become so taught from being held up in the sling for so long:

Shortly after getting out of the sling. (Coke museum in Atlanta with my sister).

I was distraught to see how messed up (and weak) my arm had become.  Fortunately for me, help was on the way IN THE FORM OF TECHNOLOGY.  My physical therapist hooked me up with a Dynasplint rental.  A Dynasplint is basically a medieval torture device in which you crank down on some springs with a key which then forces your arm (or leg or jaw(?) etc) to stop misbehaving, and extend like normal.

The Dynasplint representative said I should have worked up until I was able to wear it all night while sleeping but this thing HURTS LIKE HELL.  The rental was pricey but definitely worth it–I’m pretty sure that the Dynasplint did more to help me regain something close to my original range of motion than sessions with the physical therapist did.  I’ll mention again that it HURTS LIKE HELL.  I don’t think I ever made it much beyond an hour and fifteen minutes wearing the device.  The pain would just increase as time wore on while wearing it too–it was nuts.  I definitely never fell asleep wearing the Dynasplint, but the pain was worth getting my range of motion back.  And, after about five months in the gym, I was back to my pre-break weight lifting strength too.

Getting medieval on my misbehaving arm

Due to the construction of the thing, I couldn’t really tell how the springs in the design worked on your arm.  There might be powerful clock springs in the elbow joints on either side of your arm, and cranking on the key was what tightened them.

The key for tightening the springs is inserted at the “hand end” of the forearm rod

Alternatively, linear springs in the forearm rods may have connected to a feature in the elbow joint which was offset from the center of the elbow pivot point to provide the necessary torque.  I’m just not sure–too much of the hardware was hidden under the rods and joints.  The designers were smart to provide a graduated tension scale in the forearm rods on both sides of the forearm rods in order to allow the user to make sure that the torque was equal on both sides of the arm:

I highly recommend the Dynasplint for anyone fresh off of breaking a bone and is in physical therapy, trying to regain range of motion.  With pain comes gain, and I attribute getting most of my elbow extension back to the Dynasplint.

I was watching the movie “Road Warrior” yesterday on Netflix (which probably qualifies as required viewing for anyone who digs the science fiction genre).  I was intrigued by a helicopter-looking machine in the movie which looked a lot like that shown in the video above.

What really puzzled me about this helicopter-like machine was the spindly, relatively unsupported structure supporting the main rotor.  Helicopters with powered main rotors usually have beefy structures supporting the main rotor for transmitting engine torque and adjusting the pitch of the blades to maneuver the aircraft.  But this machine didn’t even look like power was being transmitted from the engine to the main rotor; this made me wonder, “How the heck is this thing being held aloft without a powered main rotor; and how is the main rotor even spinning then?”

As it turns out, there is actually a class of machines that do have unpowered main rotors which are still providing lift for the vehicle and they are called Autogyros.  They rely on a principle called autorotation, which is a term for the aerodynamic effect of air rising up through the main rotor, as opposed to being drawn down through the rotor, as helicopters do when moving forward in flight.

Air rising up through a rotor can force a properly designed airfoil to spin and provide lift.  While the principle of autorotation is the method by which helicopters can land safely if engine power is lost, it also can be exploited to provide lift for a machine whose main rotor is not powered by an engine at all, and this is what autogyros do.

By angling the main rotor backwards, so that air is forced up through the rotor while in forward flight, the main rotor is both forced to rotate by the motion of forward flight through the air, while simultaneously providing lift.

Image above: main rotor angled backwards clearly displayed here. Rear rotor provides forward thrust required to force air up through main rotor.

As the wikipedia article on the principle of autorotation explains, different regions along the span of the main rotor blades serve different purposes:

• Driven Region: aerodynamic drag on this portion of the blade provides lift, but counteracts torque on the rotor, slowing it down.
• Driving Region: net aerodynamic thrust on this portion of the blade provides the force which forces the rotor to rotate.
• Stall Region: blade in this region is stalling–i.e., operating above max. angle of attack, causing drag, and slowing the rotor down.

To simplify a very complex aerodynamics problem: by adjusting the pitch on the rotor blades, the ratios of the three regimes are adjusted and this alters the net forces acting on the vehicle which enables maneuvering.  I thought this was a pretty unique application of aerodynamic principles in an uncommon method of flight, similar to the Fanwing I discussed earlier.

Several of the companies I’m currently pursuing for interviews rely on welding for some portion of their operations, so I wanted to refresh that technology in my mind.  Welding is defined as “a materials joining process in which two or more parts are coalesced at their contacting surfaces by a suitable application of heat and/or pressure [1].”  Fusion welding–in which heat is used to melt and join the base materials with a filler metal sometimes added–is described here.  Solid-state welding is less common, but includes diffusion welding (two surfaces held together under pressure at high temperature, allowing atomic diffusion/fusion to occur), friction welding (friction between the two surfaces generates the heat for coalescence), and ultrasonic welding (moderate pressure is applied between the parts while an ultrasonic oscillation parallel to the contacting surfaces achieves atomic bonding) [1].  The following post will be divided into the following sections:

• Surface Preparation
• Weld Joint Geometry
• Safety
• The Various Welding Technologies
  • Oxyfuel
    • Oxyacetylene
  • Arc Welding
    • Consumable Electrode
    • Shielded Metal Arc (stick welding)
    • Gas Metal Arc
    • Submerged Arc
    • Nonconsumable Electrode
    • Gas Tungsten Arc (TIG)
  • Resistance Welding
    • Resistance Spot Welding
    • Resistance Seam Welding
    • Resistance Projection Welding
  • Heat Sealing
• Design Factors; Strength & Fatigue

Surface Preparation

Typically, the surface which is to be welded has to prepared–cleaned of surface contaminants and oxides to expose fresh metal.  Some types of welding are less susceptible to poorly prepared surfaces than others, and some types of consumable electrodes used in shielded metal arc welding can alleviate the need to have a well prepared surface by combining with surface oxides and removing them [8].

Weld Joint Geometry

There are five main types of weld joints: Butt, Corner, Lap, Tee, and Edge [1, 8].  The joint selection is dictated by the part design requirements, what shape the stock material is available in or can be shaped into, and the required strength of the weld joint.  Images below obtained from [2].

Butt Joint:

Formed when the two parts lie in the same plane and are joined at the edge.

Corner Joint:

The two parts form a right angle and are joined at the corner of the angle.

Lap Joint:

Consists of two overlapping parts.

Tee Joint:

One part is perpendicular to the other, in the shape of the letter “T.”

Edge Joint:

Parts are parallel with at least one of their edges in common and the joint is made at this common edge.

There are four main types of welds: groove, fillet, slot, and plug.  By themselves, fillet welds do not fully fuse the cross-sectional areas of the parts they join, though with a sufficiently sized weld it is still possible to achieve a full-strength connection.  Slot and plug are only used for connections that transfer small loads [8].

Welds can be partial joint penetration (PJP) or complete joint penetration (CJP) [8]:

Specification of weld geometry requires a joint specification (butt, corner, tee, lap, edge), type of weld and weld throat size (fillet, slot, plug, groove with groove geometry–U-groove, J-groove, etc.) and penetration depth (full or partial).

Safety

Precautions must be taken when welding.  Protective clothing needs to be worn to protect the welder from the heat of the operation and any spatter which may occur–gloves, apron, good shoes.  In the case of arc welding, special goggles or face masks must also be worn to protect the welder from UV damage to eyesight.  Some fluxes and molten metals release dangerous fumes which must be either ventilated by a good air system in the vicinity of the work station, or be accounted for by ventilation suits or hoods if the welder has to work in an enclosed area.  A series of safety recommendations is available from the American Welding Society here, and the entire pdf of their safety recommendations, ANSI Z49.1:2005, is available for download there as well.

The Various Welding Technologies

Oxyfuel Welding

In this form of welding, oxygen is combined with various types of gases to produce a flame used in the welding process.  ”Cutting torches” instead use the gas to cut metal instead of joining components.  By far, the most important oxyfuel welding process is oxyacetylene.  Filler metals are sometimes used and are typically in the form of rod (1/16″ – 3/8″ diameter), and must be similar to the base metals.  Filler rod is often coated with a “flux” that cleans the surface and helps to prevent oxidation [1].  Different torches are used depending on whether the operator is welding or cutting.

Oxyacetylene

General Info

Acetylene is not stable above 1 atmosphere.  The pressurized tanks storing the gas are packed with porous material saturated in acetone which can dissolve 25 times its own volume of acetylene [1].  During the welding operation, the outer envelope of the flame spreads out and covers the the work surfaces being joined, shielding the work from the atmosphere [1].

Applications

• Suitable for low-quantity production and repair jobs (good portability of equipment) [1].

Benefits

• Equipment is relatively inexpensive and portable [1].
• Oxfuel cutting is often regarded as the most economical way to cut plate greater than 1/2″ thick [8].
• Aluminum cannot be cut with oxyfuel, and highly alloyed steels (stainless) require flux or iron-rich powder to be injected into the cutting stream to promote material decomposition [8].

Disadvantages

• Rarely used to weld plate thicker than 1/4″–arc welding has advantages in this application [1].
• Explosive hazard of normally odorless gas (commonly processed to possess a garlic odor to alert operator of leaks) [1].
• Can be automated, but usually performed manually and thus highly dependent on operator skill [1].

Equipment

• Goggles, gloves, protective clothing.
• Tank of oxygen and a tank of acetylene–different threads on each to prevent accidental connection to the wrong gas [1, 3].
• Regulators on each tank.  Regulator displays tank pressure and fuel outlet pressure (pressure to hose). Dual stage regulators have a fixed intermediate pressure while allowing adjustment of pressure in hose.  Needle valves at the torch adjust flow rate / flame size.
• Hoses to run to the torch.  Green hose is oxygen, red is fuel.
• Welding rod compatible with the base materials, if necessary.
• Torch.  Cutting torches (as opposed to a welding torch, note difference in the third image in the series above) have an “oxygen blast” trigger which ignites a burning reaction by changing heated steel to iron oxide which (with a lower melt temp) burns, adding heat and causes the cut to progress [3].

Arc Welding

In arc welding, the material is fused through the heat of an electric arc at the work surface, stemming from an electrode held by the operator.  The arc is initiated by bringing the electrode into contact with the work and then drawing it away from the surface; temperature generated can be 5500 degrees C (10,000 degrees F), sufficiently hot to melt any metal [1].  The workpiece, cables, and welding machine form a circuit which is closed by the arc spanning the distance between the workpiece and the electrode.  The general configuration of an arc welding equipment setup is shown below.

Most arc welding operations employ a filler metal which is added to the weld as it is built up.  The electrodes themselves are classified as consumable or nonconsumable.  For nonconsumable electrodes, filler metal has to be added separately, as the electrode is not contributing material to the weld’s volume.

Shielding is also very critical for arc welding operations; the extremely high temperatures make the metal very reactive to the atmosphere, and an inert gas (argon and helium are common), a specialized flux (which prevents oxide formations from developing), or a combination of both must be employed.  During welding, flux melts to become a liquid slag which covers the cooling weld, and it can be chipped off later.  Flux can be applied in a granular format poured over the weld, as a stick electrode coating, or inside a tubular electrode as the electrode’s core.

The power source itself can be AC or DC current.  AC machines are less expensive to purchase and operate, but are generally restricted to welding ferrous metals.  DC equipment is generally noted for better arc control and and can be used on all metals with good results [1].

Shielded Metal Arc (“Stick Welding,” Consumable Electrode)

General Info

Shielded metal arc welding employs a consumable electrode–a filler metal rod coated with chemical flux.  The welding stick is typically 9″ – 18″ long and 3/32″ – 3/8″ in diameter.  The filler rod must be compatible with the base work metal.  The flux is powdered cellulose (cotton & wood powders) mixed with oxides, carbonates and other ingredients, held together with a silicate binder [1].  This flux melts to become a slag which covers the weld as it cools.

Currents range from 30 to 300A and voltages from 15 to 45V–the power setting depends on:

• Metal being welded
• Electrode type and length
• Depth of weld penetration

Applications

• Construction
• Pipelines
• Machinery Structures
• Shipbuilding
• Fabrication job shops
• Repair work
• Can be used on steels, stainless, cast iron, certain nonferrous alloys. Seldom or not used at all on aluminum, its alloys, Cu alloys, and Ti.

Benefits

• Relatively inexpensive equipment: can be bought for a few thousand dollars.
• Equipment is portable.
• Probably the most widely used of the arc welding processes [1].
• Has earned a reputation for providing high-quality welds, dependably [8].

Disadvantages

• Usually performed manually–dependant on operator skill.
• Use of consumable electrode stick requires frequent change-out, lowering arc time; generally slower than other welding methods [1, 8].  Lowered arc-time coupled with a manual operator actually makes the total cost of stick welding generally more expensive than other welding methods [8].
• Limited current level–as electrode length decreases, resistance changes which can overheat and melt the flux prematurely.
• Necessity of chipping away flux after the weld is completed.

Flux cored arc welding is similar to stick welding, except the electrode is tubular and continuously fed–the flux is contained in the center of the electrode tube.  Along with gas metal arc welding, this is one of the two most cost-effective manual arc welding processes [9].  Like stick welding, the flux core forms a protective slag over the weld.  Unlike stick welding, the electrode is continuously fed through a handheld “gun,” which improves arc-time and reduces the chance of weld discontinuities.  Also unlike stick welding, a shielding gas can also be fed to the weld area through the gun (carbon dioxide for mild steels; mixtures of argon and CO2 for stainless steels–remaining aware that drafty areas can negate the affect of inert gas shielding).  It’s primarily used for welding steels and stainless over wide stock thicknesses and is noted for very high-quality weld joints that are smooth and uniform [1]. This method has become quite useful in shops doing semiautomatic welding, where production welds that are short, change direction, or are out of position (vertical or overhead) are done [8].

Gas Metal Arc (Consumable Electrode)

General Info

In this arc welding process, the consumable electrode is a bare metal wire which is continuously fed from a spool.  Wire diameters are between 1/32″ and 1/4″, and selection of wire diameter depends on the thickness of the parts being joined and desired deposition rate [1].  The wire is fed through a gun held by the operator, which also floods the welding area with a gas for shielding the weld from the atmosphere.  Selection of gases–and mixes of gases–depends on the metal being welded.  Inert gases (argon and helium) are used for welding aluminum alloys and stainless.  CO2 is commonly used for welding low and medium carbon steels [1].  CO2, though it is an active gas which can degrade weld mechanical properties, is a much more economical shielding gas, and can be used in pure form as the shielding gas (on “globular transfer” gas metal arc welding, [8]) or mixed with argon in ratios of CO2/Argon ratios from 25%/75% to 10%/90% (“spray arc transfer” gas metal arc welding [8]) [4], while still allowing an acceptable weld to be created.  (FYI, the deposition of metal as it shorts to the base piece creates the characteristic buzzing heard when this process is performed on thin metals using “short arc transfer” Gas Metal Arc Welding, [9])

Applications

• Widely used in fabrication operations in factories for ferrous and nonferrous metals [1].
• Used extensively in sheet metal operations and thus heavily in the automobile industry [4].

Benefits

• Due to the shielding gas, there is no need to chip away flux after the weld is completed (thus, ideal for welds requiring multiple passes).
• Significant advantage in arc-time over stick welding because wire is fed continuously.
• Continuous feed wire lends itself to automation of this welding method.
• Higher deposition rates than stick welding [1].
• With Flux-cored arc welding, one of the two most cost-effective manual arc welding operations [9].

Disadvantages

• The use of a shielding gas generally makes it unsuitable for welding in drafty environments [4].

Submerged Arc (Consumable Electrode)

General Info

Submerged arc welding uses a continuous consumable electrode fed to the weld location which is submerged under a blanket of granular flux.  The flux is fed from a hopper ahead of the position being welded.  As illustrated in the first photo above, a vacuum device is also often used to pick up and recycle the flux which is not converted to slag by the heat of the arc welding process.

Applications

• Horizontal welding applications which can be easily automated.
• Frequently used in steel fabrication for structural shapes, an example being welded I-beams and heavy machinery [1, 5].
• Longitudinal and circumferential seams for large diameter pipes, tanks, and pressure vessels.
• Low-carbon, low-alloy, and stainless steels are readily welded by this process–high-carbon steels, tool-steels and nonferrous metals cannot be [1].  It can be used with Nickel based alloys [5].

Benefits

• Increases safety; blanket of flux reduces spatter [1].
• Relatively slow cooling due to slag and unused blanket of flux–this results in a high-quality weld joint noted for toughness and ductility [1].
• Very high deposition rates can be achieved–100 lb/h for this process vs. 10 lb/h for stick welding.  This is aided by the fact that multiple wire electrodes can be used [5].
• Fast deposition rates also mean less energy is transferred into the base parts, which helps to prevent part distortion [8].
• Thin sheet steels can be welded at very high rates (16 ft/min) [5].
• Requires little edge preparation [5].
• Deep weld penetration is achieved, which allows small welding grooves [5, 8].

Disadvantages

• Slag must be chipped off the finished weld.
• Parts must be oriented horizontally and a backing plate is often required.

Gas Tungsten Arc (TIG, Nonconsumable Electrode)

General Info

Gas tungsten arc welding uses a nonconsumable tungsten electrode to generate the arc and an inert gas for arc shielding.  Typical shielding gases include argon, helium, or a mixture of these two gases.  A filler metal, if it is used, is added from a separate rod or wire. Unlike other welding operations, “scratch starting” of the arc is not recommended unless the equipment possesses the electronic equipment to enable this; touching the tungsten to the work surface can contaminate the tungsten electrode and the weld pool.  Other equipment possesses the high-frequency, high-voltage circuitry required to initiate the spark at the arc gap [6].  The welding torch is equipped with a cooling system via air or water.

Applications

• Applicable to nearly all metals in a wide range of thicknesses.
• Can be used to join dissimilar metals.
• Most common application is aluminum and stainless steel.  Cast irons, wrought irons, lead, tungsten, and zinc are difficult to weld [1,6].
• Aerospace relies on this process heavily, as do many industries which weld thin wall tubing, such as the bicycle industry [6].

Benefits

• Produces high-quality welds [1].  Critical repair welds (such as molding dies [1]) and root passes on pressure piping are common applications [8].
• No weld spatter because metal is not transferred across the arc gap.
• Little or no post-weld cleaning because flux is not used.
• Can be performed manually and can be automated.
• Because metal is not transferred across the arc (and many metals are not suitable as electrodes due to spattering), a wide variety of filler metals are available [6].
• TIG welds most closely match the base metal, and they are highly resistant to corrosion, cracking, and hydrogen embrittlement [6].

Disadvantages

• Cast irons, wrought irons, lead, tungsten, and zinc are difficult to weld [1, 6].
• For steel welding applications, TIG is generally slower and more costly than consumable electrode arc welding processes, except where thin sections are involved and very high-quality welds are required [1].
• Generally considered the most difficult welding process to perform manually–a precise arc gap must be maintained by the operator, and skilled welders are required for the work [6].
• Care must be taken in preparing the surfaces to be welded.
• Drafty environments make the process impractical.

Stud Welding is another arc welding process which generates the arc across a gap between the piece to be attached to the base work part, and then plunges the attachment part into the weld pool formed at the interface.  This process is shown below.  In high production operations, stud welding can have advantages over rivets, manual arc welding of attachments, and drilled and tapped holes [1].

Resistance Welding

[1]

Resistance welding is a fusion-welding process which relies on resistance at the juncture of the pieces to be joined to create the weld pool and the two parts being pressed together merges the parts.  Resistance welding does not use shielding gases, flux, or filler metal.  The electrodes are nonconsumable.  The operation results in a fused zone between the two parts called a ‘weld nugget’ in spot welding.  Surface resistance generates high heat and fusion at the part interface initially, and heat is then generated and localized within the part material due to the resistance in the workpiece metal–the electrodes have high conductivity and are typically water-cooled and thus do no exhibit high internal heat generation because of this [7].  Variables in the resistance welding operation include current level, force applied at the interface, weld time, part thickness, and part material.  Care must be taken in adjusting these variables to ensure that the molten pool does not extend to the surface of the parts, or obliterate the metal with too much energy (expulsion) [7].  General advantages of resistance welding [1]:

• No filler metal is required.
• High production rates.
• Lends itself to automation.
• Skilled operators not required.
• Good repeatability.
• Can produce sparks, but doesn’t generally create harmful fumes.
• Multiple layers of sheet metal can be joined–up to four [7].
• Efficient use of input energy [7]

Drawbacks include [1]:

• Equipment cost is high–typically much higher than most arc-welding operations [1, 7].
• Mostly limited to lap-joints.

Other forms of resistance-based welding besides the major types described below include:

• Flash Welding: normally used for butt joints.  The surfaces to be joined are brought close together, current is applied to heat the surfaces and the parts are then forced together.  Flash may need to be trimmed afterwards.  The ends to be joined must have the same cross section and the equipment is expensive.
• High Frequency Resistance Welding: high frequency AC is used to heat the parts, followed by an upsetting force, which merges the parts.  Frequencies are in the range of 10-500 kHz and the electrodes are in contact with the work near where the weld is performed.
• High Frequency Induction Welding: similar to high frequency resistance welding, except an induction coil generates the heat in the part; the coil does not make contact with the work. Principal applications of both of these methods are butt welding the longitudinal seams of pipes [1].

Resistance Spot Welding

General Info

[1]

Resistance welding machines apply a compressive force across a lap joint, then apply a current to fuse the parts; the process is demonstrated above.  Resistance spot welding can joint sheet-metal parts with a thickness of 3 mm (0.125″) or less [1].  The size and shape of the weld spot is determined by shape of the electrode tip, and the weld nugget is typically 5-10 mm (0.2-04″) in diameter, with a heat affected zone extending slightly beyond this [1].  Strength is comparable to the surrounding metal.  Spot welding does not create watertight seals.  Electrodes have water passages in them for cooling when possible, to decrease temperature and wear of the copper alloy or copper/tungsten alloy electrode tips [1].  Spot spacing must be made wide enough to prevent current from shunting through the previous made spot weld.  Surfaces must be kept very clean [8].

Applications

• Resistance spot welding is frequently used in the creation of auto body components, with a robot arm moving the spot welding device about the stationary auto frame.

• Spot welding can be performed manually.  Stationary “rocker-arm” spot welding machines are available for this purpose, and portable “spot-welding guns” are also available.

[1]

• Other applications include appliances and metal furniture–virtually anything involving the joining of sheet metal.

Resistance Seam Welding

General Info

[1]

This form of resistance welding is similar to resistance spot welding, except that the electrode tips are replaced with wheels which are capable of rolling the workpiece between them.  The level of electric current or the wheel speed can be adjusted to make intermittent spot welds.  Resistance seam welding can be inappropriate where sharp corners in the seam are required.  To prevent warping of the parts as the seam is created, good workpiece fixturing is necessary with this method [1].  Surfaces must be kept very clean [8].

Applications

• Useful where airtight seals are needed.
• Gasoline tanks, automobile mufflers and other sheet metal containers.

Resistance Projection Welding

General Info

[1]

Resistance welding relies on small projections on the surface geometry of the parts to create the discrete location where the resistance weld is created.  Small projections, embossments in sheet metal, or localized intersections of parts can form these small locations where the weld will form.  Current is applied through the parts themselves to create the weld.  Examples of this form of resistance weld are shown above.

Applications

• Cross-wire welding, illustrated in (b) of the image above, is commonly used to make wire fencing, shopping carts, and stove grills.
• Fasteners can be welded to surfaces when the fastener has machined or formed projections on its head, as in (a) of the image above.
• While it is analogous to spot welding, multiple spot welds can be performed simultaneously, which also achieves better uniformity of the welds (current would shunt through closely spaced, individually formed spot welds) [8].

Design Factors; Strength & Fatigue

Design Factors

Beyond the practical constraint of some welding processes not being appropriate for certain types of metal, economic factors generally dictate what type of welding process will be used.  Factors in selecting a process include [8]:

• Base metal type
• Joint geometry (illustrated earlier)
• Section thickness
• Production quantity
• Joint access
• Position in which weld has to be formed
• Equipment availability
• Skilled personnel availability
• Location of work: in the shop or in the field

Specifying the correct size of weld is the first step in ensuring weld costs do not get out of hand [8].  The variables involved in specifying a weld’s geometry were discussed earlier, but sizing a weld properly is the responsibility of the weld engineer who has to perform weld strength calculations.

Strength Calculation

When sizing a weld for sufficient strength, the analysis begins with whether the weld is “primary” (critical weld that directly transfers full structural load–complete joint penetration groove welds make sense here) or “secondary” (merely hold parts together to form a built-up member–low loads and fillet welds are usually sufficient) [8].  With arc-welding, when the pieces are properly designed and fabricated,  the weld strength properties are basically those of the individual pieces before welding [8].  Weld strength calculation then becomes  an analysis of the weldment itself, as though it were constructed of a homogeneous metal.  However, weld metal can be “matching,” “undermatching,” or “overmatching” which means the filler metal is equal/less/more strong than the base metal.  When stronger, the area of concern for failure will likely be the fusion zone if the weld is stressed to its maximum value.  When undermatching, the filler metal area must be sized up to compensate for the lower strength, relative to the work.

For many welding situations, designing to the “allowable weld strengths” specified by the American Welding Society (AWS) or American Institute of Steel Construction (AISC) will ensure the weld will deliver the mechanical properties of the base parts being joined.  These organizations designate weld strengths for both steady and fatigue loads.  For static loads [8]:

(Note the handy table at the base, which matches weld metal with similar steels) [8]

CJP (complete joint penetration) groove welds are considered full-strength welds and are equivalent to strength capacity of the parts they join (when filler metal strength is equivalent to the base parts).  In the case of PJP (partial joint penetration) welds, the variables are more significant and the use of PJP welds frequently arises: for thick materials, a CJP weld may be too impractical and significant cost savings may be found in using a properly designed PJP weld.

When comparing the stress in a weld of interest to the allowable stresses established by the AWS or AISC, the stress is computed on the weld’s throat.  For a fillet weld, this “theoretical throat” is indicated in this diagram:

For a PJP groove weld, determining the “effective throat” is more complicated.  The depth of groove penetration (designated by “S” in weld drawings) is not necessarily equivalent to the effective throat (designated by “E” in the drawings).  Drawings can specify both of these dimensions, but it is mandatory for the designer to specify E, the effective throat, so that the fabricator can prepare an appropriate groove depth, S, and then perform a selected welding process correctly.  Some rules for calculation of the effective throat of PJP groove welds [8]:

• For V, J, and U grooves, it is assumed the welder can easily reach the bottom of the joint, and thus the effective throat is equal to the depth of the groove.
• When stick welding a bevel groove with an angle of less than 45 degrees, 1/8″ is deducted from the groove depth when calculating the effective throat (difficulty in accessing the bottom of the narrow groove with this form of welding).
• For gas metal arc welding and flux-cored arc welding, the 1/8″ reduction only applies for bevel grooves less than 45 degree when welding in the vertical or overhead position.

And, as mentioned earler, full penetration welds are assumed to have a “throat” which is equal to the thickness of the parts they join–i.e., the parts are a homogeneous metal.

The AISC and AWS has conducted tests which show that the allowable shear value for weld metal in a fillet or PJP bevel groove weld is:

τ = 0.30 x Specified Tensile Strength of Electrode

There is a minimum size for fillet and PJP groove welds, which, from AISC commentary on Table J2.4:

“Table J2.4 provides the minimum size of a fillet weld for a given thickness of the thinner part joined. The requirements are not based on strength considerations, but on the quench effect of thick material on small welds. Very rapid cooling of weld metal may result in a loss of ductility. Further-more, the restraint to weld metal shrinkage provided by thick material may result in weld cracking.”

For simple tensile, compressive, or shear loading, strength calculations are performed by dividing the load by the cross sectional area of the weld–calculated with the throat length and the lineal length of the weld line.  For parallel fillet welds:

…  the throat is stressed only in shear, and the max stress occurs on the 45 degree throat.  For transverse fillet welds:

…  the maximum shear stress occurs on the 67.5 degree throat plane, while the maximum normal stress occurs on the 22.5 degree throat [8].  Both should be computed to determine which will fail first.  This is partly a consequence of the fact that max normal stress and shear occur on planes which are oriented 45 degrees relative to one another–familiar to those who have used Mohr’s circle diagram.

Beams subjected to bending exhibit internal shear; if these beams are constructed by welding plates together, then these welds can be subjected to this “horizontal shear” in the beam.  To compute the horizontal shearing force in the welds exhibited in, for example, the image below [8]:

f=(V*a*y) / (I*n)  lbs / linear inch

Where

• f = force on weld [lb/linear inch]
• V = total shear on section at a given position along beam (refer to shear diagram!) [lb]
• a = area of flange held by weld (refer to shaded area ‘a’ in the beams shown in the image below) [in^2]
• y = distance between center of gravity of flange area and neutral axis of whole section [in]
• I = moment of inertia of whole section [in^4]
• n = number of welds joining flange to web

Obviously, if it is possible, it behooves designers to orient welded beams such that the welds are located where there is minimal horizontal shear and intermittent welds can be employed where there is low shear.

For welds subjected to bending and twisting, one way to address strength calculations is by treating the weld as a line with a definite length and cross-sectional shape.  By treating the weld as a line and inserting equivalent terms for moment of inertia and section modulus based on the line properties, standard design formula can be used to calculate f, the force in lbs/lineal inch of weld.  This load per unit length can then be used to compute an optimal throat length, either per AISC or AWS allowable loads, or from weld material strength data obtained elsewhere [8].

This table shows how regarding the weld as a line can then be translated into calculating section modulus and moment of inertia for some common weld outlines [8]:

This table shows how the formulas from mechanics of materials are used with the previously calculated section modulus and moment of inertias are used to compute “f,” the load in lb/inch of weld [8].

Definition of terms from the previous two tables are shown here [8]:

Of course the previous discussion is based on the assumption that quality welds are created according to the designer’s specifications.  Poor welds of course produce problems with weld strength.  If proper shielding is not employed, atmospheric oxygen can dissolve into the weld metal, which results in porosity as the weld cools to room temperature.  Nitrogen in the atmosphere can also dissolve into the metal itself, resulting in a decrease in notch toughness of the weld metal.  Post-weld inspection is common in the industry for critical welds–dye penetrants or radiography.

Fatigue Life Calculation

As mentioned earlier, allowable weld strengths in fatigue for many common welds are specified by the AWS and the AISC (for steel).  AISC research was directed towards bridge construction and focused on weld shapes common to that industry with steel being the material of interest.  The information presented below is a small part of an extensive AISC tabulation in “Manual of Steel Construction, 9th Ed”:

By selecting the appropriate weld geometry (the upper part of table, with the part diagrams), and then an appropriate range of cycles or the appropriate stress range (depending on what the engineer is attempting to solve for, or knows about the service life) in the lower table, then Goodman diagrams for specific: A) steels and filler material B) weld geometries and C) number of cycles are then used to ensure failure will not occur:

(Note the specified geometry: butt weld, the cycle specification of 500k-2M cycles, and the upper limits for specific, but common base metal and filler metal combinations)

This is a fairly straightforward way to predict weldment life when designing with typical materials and weld geometries, so it suffices for much of the welding which is performed in industry.

However, when atypical weld materials or weld designs are being used, the engineer will need to locate fatigue performance data for the metal of interest, and possibly do a finite element analysis of the weldment to find the weld areas which exhibit the highest stress.  With a knowledge of the cyclic loading that the structure will be subjected to, the same procedure for calculating weld stress described in the static strength calculation section of this essay can be used to calculate the pertinent peak and trough stresses of the fatigue cycle.

Also, though sound weld metal has about the same fatigue strength as unwelded metal [8], the change in section at the weld joint can lower the fatigue strength of the welded area; fillet welds in lap or tee joints are an abrupt change in cross section which acts as a stress raiser.  These stress raisers need to be accounted for in stress calculations for fatigue and static loading with appropriate stress-intensity factors.

References

[1]  ”Fundamentals of Modern Manufacturing, Materials, Processes and Systems, 3rd ed.,” Mikell P. Groover, John Wiley & Sons, Inc., c. 2007, 1022 pages.

[2] http://deltaschooloftrades.com/basic_joints.htm

[3] http://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting

[4] http://en.wikipedia.org/wiki/Gas_metal_arc_welding

[5] http://en.wikipedia.org/wiki/Submerged_arc_welding

[6] http://en.wikipedia.org/wiki/Gas_tungsten_arc_welding

[7] http://en.wikipedia.org/wiki/Electric_resistance_welding

[8] “Marks’ Standard Handbook for Mechanical Engineers, 11th Ed.,” Eugene Avallone, Theodore Baumeister III, Ali Sadegh, McGraw Hill Company, Inc., c. 2007.

[9] “Machinery’s Handbook, 28th Ed.,” Erik Oberg, Franklin Jones, Holbrook Horton, Henry Ryffel, Industrial Press New York, c. 2008, 2692 pages.

Note: Machinery’s Handbook in particular is a great resource for the gritty details of performing welding at the level of the welding operator, including such things as recommended current settings for arc welding, and a fairly complete listing of weld symbols.