SHEAR NUTS. These nuts are designed for use with devices such as drilled clevis bolts and threaded taper pins that are normally subjected to shearing stress only. They are usually self-locking.

SHEET SPRING NUTS. These nuts are used with standard and sheet metal self-tapping screws to support line clamps, conduit clamps, electrical equipment, and access doors. The most common types are the float, the two-lug anchor, and the one-lug anchor. The nuts have an arched spring leek that prevents the screw from working loose. They should be used only where originally used in the fabrication of the aircraft. See figure 2-28.

WING NUTS. These nuts are used where the desired tightness is obtained by the use of your fingers and where the assembly is frequently removed.

KLINCHER LOCKNUTS. Klincher locknuts are used to ensure a permanent and vibrationproof, bolted connection that holds solidly and resists thread wear. It will withstand extremely high or low temperatures and exposure to lubricants, weather, and compounds without impairing the effectiveness of the locking element. The nut is installed with the end that looks like a double washer toward the metal being fastened. Notice in figure 2-29 that the end that looks like a double hexagon is away from the metal being fastened.

The most common threaded fastener used in aircraft construction is the screw. The three most used types are the structural screw, machine screw, and the self-tapping screw.

STRUCTURAL SCREWS. Structura.l screws are used for assembling structural parts. They are made of alloy steel and are heat treated. Structural screws have a definite grip length and the same shear and tensile strengths as the equivalent size bolt. They differ from structural bolts only in the type of head. These screws are available in round-head, countersunk-head, and brazier-head types, either

Figure 2-29.Typical installations of the Wincher locknut.

Figure 2-30.-Structural screws. 

slotted or recessed for the various types of screwdrivers. See figure 2-30.