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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?

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.