FASTENERS (THREADED)

is essential. See figure 2-17. The threaded taper pin is used with a taper pin washer and a shear nut if the taper pin is drilled, or with a self-locking nut if undrilled. When a shear nut is used with the threaded taper pin and washer, the nut is secured with a cotter pin. FLAT-HEAD PINS.—The flat-head pin is used with tie rod terminals or secondary controls, which do not operate continuously. The flat-head pin should be secured with a cotter pin. The pin is normally installed with the head up. See figure 2-17. This precaution is taken to maintain the flat-head pin in the installed position in case of cotter pin failure. SNAP RINGS.—A snap ring is a ring of metal, either round or flat in cross section, that is tempered to have springlike action. This springlike action will hold the snap ring firmly seated in a groove. The external types are designed to fit in a groove around the outside of a shaft or cylinder. The internal types fit in a groove inside a cylinder. Special pliers are designed to install each type of snap ring. Snap rings can be reused as long as they retain their shape and springlike action. External snap rings may be safety wired, but internal types are never safetied. STUDS.—There are four types of studs used in aircraft structural applications. They are the coarse thread, fine thread, stepped and lockring studs. Studs may be drilled or undrilled on the nut end. Coarse (NAS183) and fine (NAS184) thread studs are manufactured from alloy steel and are heat treated. They have identical threads on both ends. The stepped stud has a different thread on each end of the stud. The lockring stud may be substituted for undersize or oversize studs. The lockring on this stud prevents it from backing out due to vibration, stress, or temperature variations. Refer to the Structural Hardware Manual, NAVAIR 01-1A-8 for more detailed information on studs. HELI-COIL INSERTS.—Heli-coil thread inserts are primarily designed to be used in materials that arc not suitable for threading because of their softness. The inserts are made of a diamond cross-sectioned stainless steel wire that is helically coiled and, in its finished form, is similar to a small, fully compressed spring. There are two types of heli-coil inserts. See figure 2-18. One is the plain insert, made with a tang that forms a portion of the bottom coil offset, and is used to drive the insert. This tang is left on the insert after installation, except when its removal is necessary to provide clearance for the end of the bolt. The tang is notched to break off from the body of the insert, thereby providing full penetration for the fastener. The second type of insert used is the self-locking, mid-grip insert, which has a specially formed grip coil midway on the insert. This produces a gripping effect on the engaging screw, For quick identification, the self-locking, mid-grip inserts are dyed red. FASTENERS (THREADED) Although thousands of rivets are used in aircraft construction, many parts require frequent dismantling or replacement. For these parts it is more practical to use some form of threaded fastener. Furthermore, some joints require greater strength and rigidity than can be provided by riveting. Manufacturers solve this Figure 2-17.—Types of aircraft pins. 2-14