Hex bolts Full-time Job

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?

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.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

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.