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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?










Washers

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.



Author notes

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.