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Nylon Insert Lock Nuts
Nylon insert lock nuts are hex nuts that have a captive nylon washer mounted in their top to create a prevailing torque type lock nut. The undersized washer forms a compression grip that binds on the threads of the mating fastener to resist loosening. Due to limitations of the non-metallic insert, the nuts cannot be used at elevated temperatures or with certain chemicals but the nylon washer does provide a gas and moisture seal. Various colors of nylon are used; some manufacturers use a specific color. Regular, thin pattern and machine screw nylon insert lock nuts are used in aerospace, agricultural equipment, appliances, avionics, computers, furniture, home and garden, lighting, medical equipment and vehicles (auto, truck and trailer). Heavy pattern types are used on bridges and buildings, in heavy industry, and on oil field machinery and railroad equipment. Nylon insert lock nuts are also known as:
aircraft lock nuts, elastic lock nuts, elastic stop nuts, nyloc nuts, nylock nuts, nylon insert stop nuts, stop nuts.
Because the washer largely returns to its original form, nylon insert lock nuts may be reused a limited number of times. Lock washers are not used with prevailing torque lock nuts.
By definition, "Prevailing-torque locking fasteners have a self-contained feature which creates frictional interference between the threads of the mating components." Consequently, unlike free spinning lock nuts, there is a resistance to rotation during both assembly and disassembly requiring them to be wrenched; that resistance is called prevailing torque. The advantage is that disassembly is unlikely even if preload diminishes completely because of the remaining rotational resistance. Despite the fact they are called "lock nuts," nylon insert lock nuts are not permanently locked in place so they can be adjusted or removed after installation. However, their "locking" feature does resist loosening without the nuts being seated, which allows them to be used as stop nuts or spacers for rotating or other components.
Threads are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse) or Unified inch fine (UNF, Unified National Fine).
Nylon insert lock nut size refers to its nominal thread diameter. Typically, sizes range from about 2-56 to 2". Sizes less than 1/4" are listed as a number size (the larger the number the larger the size) while sizes that are 1/4" and larger are specified in inches, usually fractional rather than decimal. Not all types are available in all sizes.
Nut width and thickness vary with the size and type of the nylon insert lock nut: thin, regular (full height), heavy and machine screw. See Table 1 for a description of each type.
The nuts are considered one-way lock nuts because they can only be installed one-way—top up. Top lock, or top locking, is another term used to describe this type of lock nut. Nylon insert lock nuts spin freely until the nylon washer engages the threads of the mating part. For the locking action to be effective, the washer must contact fully formed threads so several threads of a standard bolt/screw should extend beyond the top of the nut.
Grades 2 and 8 are available. Steel is a common material along with stainless steel. Zinc plated and yellow zinc plated are typical finishes for steel nylon insert lock nuts.
Always follow the manufacturer's recommendations regarding nut reuse, thread lubricants and torque values. Do not apply standard torque values to prevailing torque lock nuts because the joint may not be tightened to the correct clamping force due to the additional friction of the locking element.
Generally, prevailing torque lock nuts are not considered suitable for long threaded assemblies because extended nut travel along the threads could damage the nut's locking feature. Therefore, the length of the bolt/screw should not be much longer than that needed to ensure full thread engagement with the nut.
The different types of nylon insert lock nuts are summarized in Table 1 below…
Nylon Insert Lock Nuts |
|
|---|---|
| Nylon Insert Lock Nut Thin Height | |
| Thin height is primarily used for shear load applications. |  |
| Nylon Insert Lock Nut "Regular" (Full Height) | |
| Regular, full height is the most prevalent type used for the majority of commercial applications. |  |
| Nylon Insert Lock Nut Heavy | |
| Heavy pattern has a larger surface area, which improves wrenching, and is used in heavy industrial or structural applications. |  |
| Nylon Insert Lock Nut Machine Screw | |
| Machine screw style is available in smaller sizes and is normally used with machine screws in light-duty applications. |  |
Refer to ASME B18.16.6 for specifications relating to prevailing torque lock nuts.
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