An important part of fastener technology has included the development of several methods for locking mated threads of fasteners. Many of the latest methods include the locking device or method as an integral part of the fastener assembly and are referred to as self-locking nuts or bolts. Self-locking fasteners are more expensive than some older methods but compare favorably in cost with pin or wiring methods.

Length of Protrusion

Male threads on threaded fasteners, when installed and tightened, will protrude the distance of at least one thread length beyond the top of the nut or plastic locking ring. Excessive protrusion is a hazard, particularly where necessary clearances, accessibility, and safety are important. Where practicable, the number of threads protruding should not exceed five. In no case should thread protrusion exceed 10 threads unless specifically approved by the work supervisor. (This is the 1-to-10 rule.)

Where screw threads are used for setting or adjusting (such as valve stem packing glands and travel stops) or where installed threaded fasteners do not strictly follow the 1-to-10 rule but have given satisfactory service, the rule does not apply. An example of an acceptable existing installation would be where a male thread is flush with the top of a nut or where more than 10 threads protruding is of no foreseeable consequence.

Repair of Damaged Threads

You can remedy damaged external threads by replacing the fastener. In large equipment castings you must repair damaged internal threads to save the part. You can repair internal threads by redrilling the damaged thread; clean and either install a solid wall insert or tap for a helical coil insert. These inserts, in effect, return the tapped hole to its original size so it takes the original mating fastener.

LOCKNUTS

Locknuts are used in special applications where you want to ensure that the components joined by the fasteners will not loosen. Two types of locknuts are in common use. The first type applies pressure to the bolt thread and can be used where frequent removal may be required. The second type deforms the bolt thread and is used only where frequent removal is unnecessary. The first type includes plastic ring nuts, nylon insert nuts, jam nuts, spring nuts, and spring beam nuts. The second type includes distorted collar nuts and distorted thread nuts; they are not commonly found in gas turbine equipment and will not be covered in this section.

Figure 9-65.-Plastic ring nut.

Figure 9-66.-Nylon insert nut.

Plastic Ring Nuts

Plastic ring nuts (fig 9-65) deform the plastic insert when they are installed. The resilient plastic material is forced to assume the shape of the mating threads, creating large frictional forces.

Nylon Insert Nuts

Nylon insert nuts (fig 9-66), have plastic inserts (plugs) that do not extend completely around the threads. They force the nut to the side, cocking it slightly. This produces frictional forces on one side of the bolt thread. Although the plastic insert locks without seating, proper torque applied to the nut stretches the bolt, creating clamping forces that add to the locking abilities of the nut. Before reusing nylon insert nuts, check the inserts. If worn or torn, discard the nut. Install the nut (on clean lightly lubricated threads) finger tight. If you can install the nut to the point where the bolt threads pass the insert without a wrench, discard the nut and use a new one.

Jam Nuts

You should install jam nuts fig 9-67 with the thinner nut to the working surface and the thicker nut to the outside. The thin nut is deformed by the wider nut and pressed against the working surface and threads.

Figure 9-67.-Jam nuts.

Figure 9-68.-Spring nuts.

Figure 9-69.-Spring beam nuts.

Spring Nuts

Spring nuts (fig 9-68) lock by the side grip on the bolt. When tightened, the spring nut flattens, or straightens, a spring section. Many types of spring nuts use curved metal springs, bellows, and coil springs. All spin on and off without locking until the pressure against the working surface straightens the spring.

You should always consult equipment manuals for the proper torque value. Be sure threads are always clean and lightly lubricated with the proper lubrication. Discard any with damaged threads.

Spring Beam Nuts

Spring beam nuts (fig 9-69) are formed with a light taper in the threads toward the upper portion of the nut. Slots are cut in the outer portion, forming segments that can be forced outward when the nut is installed. Elastic reaction causes the segments to push inward, gripping the bolt. Like the nylon insert nut, this nut does not deform the bolt threads and can be used on frequently removed items. If you can thread the nut past the deflection segments without a wrench, discard the nut and replace it with a new one.