Push-pull tubes are used as linkage in various types of mechanically-operated systems. This type linkage eliminates the problem of varying tension and permits the transfer of either compression or tension stress through a single tube.

A push-pull tube assembly consists of a hollow aluminum alloy or steel tube with an adjustable end fitting and a check nut at either end. [Figure 7-71] The check nuts secure the end fittings after the tube assembly has been adjusted to its correct length. Push-pull tubes are generally made in short lengths to prevent vibration and bending under compression loads.

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To ensure fasteners do not separate from their nuts or holding ends, various safetying methods are used in aircraft from heavy aircraft to gliders to recreational aircraft. Safetying is the process of securing all aircraft, bolts, nuts, screws, pins, and other fasteners so that they do not work loose due to of safetying equipment on an aircraft is necessary to perform maintenance and inspection.

There are various methods of safetying aircraft parts. The most widely used methods are safety wire, cotter pins, lock washers, snap rings, and special nuts, such as self-locking nuts, pal nuts, and jam nuts. Some of these nuts and washers have been previously described in this chapter.

The three main types of pins used in aircraft structures are the taper pin, flathead pin, and cotter pin. Pins are used in shear applications and for safetying. Roll pins are finding increasing uses in aircraft construction.

Plain and threaded taper pins (AN385 and AN386) are used in joints that carry shear loads and where absence of play is essential. The plain taper pin is drilled and usually safetied with wire. The threaded taper pin is used with a taper pin washer (AN975) and shear nut (safetied with a cotter pin or safety clip) or self-locking nut.

Commonly called a Clevis pin, the flathead pin (MS20392) is used with tie rod terminals and in secondary controls, which are not subject to continuous operation. The pin is customarily installed with the head up so that if the cotter pin fails or works out, the pin remains in place.

The AN380 cadmium-plated, low-carbon steel cotter pin is used for safetying bolts, screws, nuts, other pins, and in various applications where such safetying is necessary. The AN381 corrosion-resistant steel cotter pin is used in locations where nonmagnetic material is required or in locations where resistance to corrosion is desired.