6 CHA PTER OI':E OPENINGS FOR TOOLS Sioned Ph illips All en I nternal torx External torx cA~~~ ~ MACHINE SCREWS Nut s Self - Locking f asten er s Nuts are manufactured in a variety of types and sizes. Most are hexagonal (6-sided) and fit on bolts, screws and studs with the same diameter and pitch. Several type s of bolts, screws and nuts incorporate a system that develops an interference between the bolt, screw, nut or tapped hole threads. Inte rf erence is achieved in various ways: by distorting threads, coating threads with dry adhesive or nylon, distorting the top of an all-metal nut, or using a nylon insert in the cen ter or at the top of a nut.
Self-locking fasteners offer greater holding strength and better vibration resistance. Some self-locking fasten- ers can be reused if in good condition. Others, like the ny- lon insert nut, fo nn an initial locking condition when the nut is first insta lled - the nylon forms closely to the bolt thread pattern, thus reducing any tendency for the nut to loosen. When the nut is removed, the locking efficiency is greatly reduced. It is recommended thai new self-locking fasteners be installed after they are removed. f igure 11 shows severaltypes of nuts. The common nul is generally used with a lockwasher.
Self-locking nuts have a nylon insert thai prevents the nut from loosening; no lockwas her is required. Wing nuts are designed for fast removal by hand. Wing nuts are used for convenience in non-crit icalloca tions. To indicate the size of a metric nut, manufacturers spec- ify the diameter of the opening and the thread pitch. This is similar to bolt specifications, but without the length di- mension. The measurem ent across two flats on the nut in- dicates the wrench size 10 be used.