Nuts, bolts, screws, and washers
This chapter starts with tips on drawing hexagon nuts and hex bolts and comprehensively covers, using illustrations, tables of size and explanations on usage, the majority of metric fixings and fasteners used in engineering today i.e. screws of the Hexagon Socket type such as Cap Head Screws, Shoulder Screws, Button Head Screws, Countersunk Head Screws and Set Screws. Machine Screws such as Phillips and Slotted Pan Head, Countersunk and Raised Countersunk Head, Slotted Cheese Head are also included as are Machine Screw Nuts, Wing Nuts and Locking and Retaining Devices such as Slotted Nuts and Castle Nuts Simmonds Locknut, Spring Washers, Shakeproof Washers, Wire Locking, Tab Washers, Locking Plates, Taper and Parallel Pins, Split Cotter Pins, locking by Adhesives and Peening. Finally thread cutting screws are covered with recommendations on installation.
A bolt, as you may recall, is a parallel-sided shaft with an inclined plane or helical groove wrapped around it. A screw bolt is similar except that its sides are tapered, not parallel. Alternatively, one could say that a screw is cone shaped while a bolt is cylindrical. This fine distinction between a bolt and a screw is not appreciated by most people, who might believe that screws are little fasteners tightened with a screwdriver while bolts are larger fasteners tightened with a wrench. No matter how you view them, bolts and screws have much in common. Both stretch a bit while being tightened, both spread the load over several threads, and both will break if over tightened. Screws, however, unlike bolts, cut their own mating thread as they are tightened. This is a key difference from a bolt, which must have a machine-threaded mating hole. Furthermore, repeated removal and reinsertion will cause the screw hole to become just a bit larger in diameter. After too many cycles, the hole no longer fits the screw (sometimes termed hole “wearout”) and we must employ some remediation technique—see “Remediating Hole ‘Wearout’.”
Screws are often categorized in terms of application (wood, sheet metal, drywall, concrete, etc.); head configuration; and sometimes (when it's uncommon) driving method. Button-head sheet metal, roundhead wood, flathead drywall, and TORX-head cabinet screws are but a few common examples. Head descriptions such as pan, button, truss, and oval confuse most people, and for good reason. Each description evokes different mind pictures for different people—my pan probably isn't shaped like your pan, and would that be a saucepan or a sauté pan? What is a “fillister” and what does it look like, and just what exactly is a cabinet screw anyway?
You likely know the two main screw driving types—slotted and Phillips—but there are many others out there. Besides a number of Phillips-lookalikes, screw manufacturers have devised other slot designs that facilitate assembly line operations or prevent tampering by keeping unauthorized individuals from gaining access to the interior of equipment. While the Phillips-design screw and driver combination purposely allows the driver to slip out under high torque conditions to prevent over tightening, other similar styles such as the Pozidriv and the Reed & Prince (also known as the “Frearson”) screw drive have a slightly different shape, designed not to slip out under high torque conditions. Both are more likely to shear the screw head off than allow the driver to slip out of the screw head. The same holds true for the Japanese Industrial Standard (JIS) screw that is commonly found in Japanese-manufactured equipment.
Other drive styles include the TORX, Hex (or “Allen”), Robertson, Square, Tri-Wing, Torq-Set, Spanner, and Clutch Types “A” and “G.” Many of us who work on our own automobiles or computers are familiar with the TORX drive's six-rounded-point star pattern. Both the Robertson (used primarily in Canada) and Square (the American clone) drive screws are similar in appearance, but the Robertson head has a slight wedge shape, allowing the driver to hold the screw horizontally or even downward without it falling off the driver. The Square-drive head is not tapered, and is therefore slightly larger than the driver, thus making it more likely to strip or round-out than the Canadian original. Tri-Wing screws, with their triangular slotted configuration like a three-lobe Phillips design, are used by some video game manufacturers to hide their inner workings from curious eyes, but are rarely found on medical equipment. Spanner heads are frequently seen in elevators securing the control panel in the elevator's cab. Both Tri-Wing and Spanner designs are meant to be tamper-resistant due to their unique head design and rarity of drivers. Clutch Type “A” screws resemble a bow tie and were commonly used to secure body panels on General Motors vehicles during the 1940s and 1950s. The Clutch Type “G,” commonly used in the manufacture of mobile homes and recreational vehicles, looks like a butterfly.
For the do-it-yourselfer at home (which many biomeds are, whether it is building cabinetry or working on cars), there are several techniques to remediate hole wearout. I don't, however, recommend employing any of these on the job in critical or load-bearing applications for obvious reasons!
The most common technique when faced with hole wearout is to simply use a larger screw. This is not always advisable—some would object to the appearance of a single larger screw in a row of screws, thus requiring the replacement of all screws and the need to enlarge all the other holes as well. Other times, the mating material is too thin to use a larger diameter screw with its wider thread. Fortunately, if one is working with wood, shimming the hole with wood (flat toothpicks work well for this) and glue works in most cases. Metal is another story, however. Sometimes you can shim the hole with a dab of epoxy, using the screw to cut threads in the glue until it is in its plastic state, then removing the screw while the epoxy completes hardening. If one is very careful, a nut can be glued (cyanoacrylic adhesives are good for this) to the backside of the oversize mating hole and a so-called “machine screw” be used in place of the original screw. If one has access to the blind side, a nut and “machine screw” might be used in place of the original screw. In desperation, resort to any of a number of specialty devices intended to mount sheet metal and provide a captured machine screw joint.
“Tamper-resistant” or “security-head” screws are usually variants of the common designs. A supposed tamper-resistant version of the TORX screw includes a small pin in the center recess to prevent using a slotted or Phillips screwdriver, or even a common TORX drive, which can be purchased at a hardware store. The downside of this tamper-resistant design is the ease with which the pin can be removed with a pair of needle-nose pliers or a hand-held grinder. Variants feature sloping edges so that the screw can be driven in, but the bit slips out when trying to remove the screw. A third type of security or tamper-resistant design features unusual proprietary designs mating with drivers only available from the screw manufacturer and only sold to registered owners. These types of screws are not popular with medical equipment manufacturers, and biomeds seldom run across them. When we do, we have several courses of action to follow:
Purchase the tool from the medical equipment manufacturer.
Attempt to buy the appropriate tool from the screw manufacturer (generally the organization's purchase order or a letter request on letterhead is sufficient to prove that the purchase is not for a nefarious purpose).
Have the appropriate tool fabricated by a local machine shop.
Grind or chisel the head off, use a screw extractor to remove the remains, and then replace it with a more common screw. (If some measure of security is desired, use a tamper-resistant TORX screw in its place.)
Screws, washers, and other fastening hardware are made from a wide range of materials. Steel is the most common, but special applications call for other metals more suited to the environment. Copper, brass, and bronze are most commonly used in damp or submerged applications where rusting cannot be tolerated. Where higher physical strength and rust resistance is required, a nickel-base alloy, corrosion resisting (a.k.a. “stainless”) steel, or titanium is used. Plastics such as nylon or Teflon are used when moderate strength is needed and absolutely no rust or fluid interaction can be tolerated. Where electrolytic action (from the mating of different metals) is a concern, fasteners are either made of the same material as the metals being joined—aluminum instead of steel, for example—or of plastic. Where electrical insulation is required, plastic fasteners are most commonly used.
Washers were originally used for three purposes—to spread the compressive load or anchoring pressure over a larger load-bearing area, to relieve friction, or to prevent leakage. Common flat washers are, as the name implies, a flat disk, usually round and with a hole in the middle, made of metal, plastic, rubber, or leather. Their thickness allows the relatively small diameter head of a fastener, such as a screw or small bolt, to spread its compressive force over a larger diameter (approximately that of the washer's outside diameter) thus reducing stress at the edges of the mounting hole. For example, a printed circuit board could be fastened to a standoff with a screw and a flat washer. The washer spreads the pressure of the screw over a larger area than just the screw head, thus preventing the board from cracking. “Thrust washers” absorb friction between shaft-mounted components by acting as an intermediate or buffer piece between two rotating parts. Thrust washers are often used inside motors and in linear mechanical assemblies as spacers. Washers used as seals around immersion heaters and in water lines are familiar to just about every biomed in the field.
Over time, washers have evolved from the three basic types to a plethora of designs for a number of common and unique purposes. The most commonly encountered include four types of lockwashers (split, internal tooth, external tooth, and spring); fender washers; trim washers; and square washers. The split lockwasher, as the name implies, looks like a regular flat washer made of spring metal (usually a steel) with a cut from the center to the perimeter. The ends of the cut appear to have been bent apart in an up-and-down fashion. (Note: If the ends appear to be opposite each other, without a distinct bend in the washer, discard the washer and use a new one.) As the washer is compressed, the ends are slightly wedged into the fastener (usually a screw, bolt, or nut) and the item being fastened (like a cover or a clamp) to prevent the fastener from unscrewing under vibration. Internal and external tooth lockwashers are similar in that they have teeth pointing either toward or away from (respectively) the center hole and spread their compressive force in the same direction that the teeth point. When compressed, the teeth grab both parts being compressed to prevent the fastener from unscrewing. The last type of lockwasher is the spring washer. Spring washers are formed in an irregular shape, usually wavy, so that it acts like a spring when compressed. The resultant pressure prevents (within the limits of its design) the fastener from unscrewing. Biomeds often will encounter thrust and spring washers on the same rotating shaft, with the spring washer providing a fairly constant tension to eliminate rattle, take up slack, reduce play to a tolerable level, and/or to provide a controlled reaction to intermittent shock. Some commonly found biomed applications include clutch assemblies, air compressors, and some portable x-ray unit drive trains.
A fender washer is an oversized flat washer, distinguished by its relatively large diameter compared to the hole at the center. These washers spread their compressive force over a larger area than a common washer does and are especially useful in preventing cracking when securing plastic parts with smallish screws. Trim washers appear in a variety of forms, all of which provide a “finished” look in final assembly. By design, trim (or finishing) washers blend well with both their fastener and surroundings. They provide a smooth and eye-appealing transition between a screw and a panel. Often a trim washer, a cabinet screw (a screw with a hole in its top designed to hold a plastic cap piece), and its cap form an “inconspicuous” fastening system in consumer-assembled furniture such as bookcases, entertainment centers, and chests of drawers. Square washers can be used in special applications where round hardware would inappropriate or unusable, such as a corner application.
Armed with this basic knowledge of fastener design, terminology, and application, the biomed is now able to better select the correct fastener for the application. By the way, about that “panhead” screwhead—I think it looks more like a rounded sauté pan than a saucepan.
Robert Dondelinger, CBET-E, MS, is the medical equipment manager at the U.S. Military Entrance Processing Command in North Chicago, IL. An internationally certified biomedical electronics technician, he entered the U.S. Army in 1970 and retired from active duty in 2002.