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Stover Lock Nuts
Stover lock nuts are one piece, all-metal 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 be capable of withstanding severe vibration and shock loads. Since they are top locking and only the bottom surface is flat, Stover lock nuts are considered one way lock nuts because they are installed one way—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 the automotive, agricultural equipment and metalworking industries, their cone shape allows for proper orientation during automated assembly making them popular for high volume applications. Stover lock nuts are also called:
automation lock nuts, Autolock nuts, Crownlock nuts, Unitorque 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 disassembly is unlikely even if preload diminishes completely because of the remaining rotational resistance. Despite the fact they are called "lock nuts," Stover 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).
Stover lock nut size refers to its nominal thread diameter. Typically, sizes range from about 1/4" to 1". Size is specified in inches, usually fractional rather than decimal.
Grade T9, Cadmium Dichromate Plated, is a common grade and finish. Never replace a graded Stover lock nut with a lower grade or lower strength nut.
Nut width and thickness vary with the size of the Stover 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.
Stover 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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