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Prevailing Torque Lock Nuts
Prevailing torque lock nuts are one piece, prevailing torque hex nuts with a conical top and a flat bottom bearing surface with chamfered corners. The locking action, created by distortion of their top threads, is said to resist loosening caused by shock, vibration and other dynamic forces. Since they are top locking and only the bottom surface is flat, prevailing torque lock nuts are considered one way lock nuts because they are installed one way—conical top up. Being all-metal, they are not subject to the temperature and chemical limitations of non-metallic (such as nylon) insert type lock nuts. Used in agricultural equipment and the automotive and metalworking industries, prevailing torque lock nuts are also known as:
all metal lock nuts, all steel lock nuts, automation style lock nuts, top lock nuts.
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 self disassembly is unlikely even if preload diminishes completely because of the remaining rotational resistance. Despite the fact they are called "lock nuts," prevailing torque lock nuts are not permanently locked in place so they can be adjusted or removed after installation. And because they stay put without being seated, they are 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).
Prevailing torque lock nut size refers to its nominal thread diameter. Typically, sizes range from about 1/4" to about 2". Size is specified in inches, usually fractional rather than decimal. Not all types are available in all sizes.
Grades B, C and 9 are common grades and zinc plated and cadmium & wax are prevalent finishes. Never replace a graded prevailing torque lock nut with a lower grade or lower strength nut. Surface markings identify the grade. Some nuts are available from domestic (USA) manufacturers.
Nut width and height vary with the size of the prevailing torque lock nut.
The nuts spin freely until their distorted top threads engage the threads of the mating part. For the locking action to be effective, the locking element must contact fully formed threads so several threads of a standard bolt/screw should extend beyond the top of the nut.
Prevailing torque lock nuts are reusable a limited number of times because the prevailing torque declines each time the nut is used. 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 or the mating threads. Therefore, the length of the bolt/screw should not be much longer than that needed to ensure full thread engagement with the nut.
Refer to ASME B18.16.6 and IFI 101 for specifications relating to prevailing torque lock nuts. However, in response to customer demand, at least one manufacturer continues to supply lock nuts that meet the requirements of IFI 100/107 even though the standard was replaced by ASME B18.16.6.
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