SELF-LOCKING NUTS. Self-locking nuts provide tight connections that will not loosen under vibrations. Self-locking nuts approved for use on aircraft meet critical strength, corrosion-resistance, and temperature specifications. The two major types of self-locking nuts are prevailing torque and free spinning. The two general types of prevailing torque nuts are the all-metal nuts and the nonmetallic insert nuts. New self-locking nuts must be used each time components are installed in critical areas throughout the entire aircraft, including all flight, engine, and fuel control linkage and attachments. The flexloc nut is an example of the all-metal type. The elastic stop nut is an example of the nonmetallic insert type. All-metal self-locking nuts are constructed with the threads in the load-carrying portion of the nut out of phase with the threads in the locking portion, or with a saw cut top portion with a pinched-in thread. The locking action of these types depends upon the resiliency of the metal when the locking section and load-carrying section are forced into alignment when engaged by the bolt or screw threads. 

PLAIN HEX NUTS. These nuts are available in self-locking or nonself-lotting styles. When the nonself-locking nuts are used, they should be locked with an auxiliary locking device such as a check nut or lock washer. See figure 2-26.

CASTLE NUTS. These nuts are used with drilled shank bolts, hex-head bolts, clevis bolts, eyebolts, and drilled-head studs. These nuts are designed to be secured with cotter pins or safety wire.

CASTELLATED SHEAR NUTS. Like the castle nuts, these nuts are castellated for safetying. They are not as strong or cut as deep as the castle nuts.

CHECK NUTS. These nuts are used in locking devices for nonself-locking plain hex nuts, setscrews, and threaded rod ends.

PLATE NUTS. These nuts are used for blind mounting in inaccessible locations and for easier maintenance. They are available in a wide range of sizes and shapes. One-lug, two-lug, and right-angle shapes are available to accommodate the specific physical requirements of nut locations. Floating nuts provide a controlled amount of nut movement to compensate for subassembly misalignment. They can be either self-locking or nonself-locking. See figure 2-27.

CHANNEL NUTS. These nuts are used in applications requiring anchored nuts equally spaced around openings such as access and inspection doors and removable leading edges. Straight or curved channel nut strips offer a wide range of nut spacings and provide a multinut unit that has all the advantages of floating nuts. They are usually self-locking.

BARREL NUTS. These nuts are installed in drilled holes. The round portion of the nut fits in the drilled hole and provides a self-wrenching effect. They are usually self-locking.

INTERNAL-WRENCHING NUTS. These nuts are generally used where a nut with a high tensile strength is required or where space is limited and the use of external-wrenching nuts would not permit the use of conventional wrenches for installation and removal. This is usually where the bearing surface is counterbored. These nuts have a nonmetallic insert that provides the locking action.

Figure 2-26.Nuts.

Figure 2-27.Self-locking nuts.