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Flexloc nuts are one piece, all-metal prevailing torque lock nuts that resist impact, shock and vibration. They are hex nuts with six opposing narrow slots (60° apart) aligned with the nut corners in a cylindrical top a little smaller in diameter than the width of the nut; slot depth and cylindrical top height are the same (they look like castle nuts expect the slots are much narrower). All Flexloc nut threads, even those in the locking section, are said to carry their full share of the load. 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 aerospace and agricultural industries, and in industrial and automotive engine and suspension applications, Flexloc nuts are also known as:
flex lock nuts, flex top lock nuts, one way lock nuts, split beam 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 disassembly is unlikely even if preload diminishes completely because of the remaining rotational resistance. Despite the fact they are called "lock nuts," Flexloc 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).
Flexloc nut size refers to its nominal thread diameter. Typically, sizes range from about #2 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.
Nut width and height vary with the size and style of the Flexloc nut. They are available in heavy duty hex and light hex, and in full height and thin height types.
- Heavy duty hex full height provides maximum strength, both in terms of locking power and tensile strength. Said to be impervious to shock or moisture and will resist loosening under extremes of vibration or dynamic loading, their large size improves wrenchability.
- Light hex full height offers high performance and additional wrenching clearance due to reduced width. They are lower in weight and cost yet withstand extremes of moisture, shock and vibration.
- Heavy duty hex thin height provides exceptional strength in a low profile nut while light hex thin height conserves the most space and weight. Compared to their full height counterparts, thin height Flexloc nuts weigh about 20% less and are 30% lower in height. Although primarily intended for shear applications, both can be used in many tension applications due to the inherent strength of their design. Also, their lower profile allows for a more compact, lighter weight assembly because shorter bolts/screws can be used.
Flexloc nuts spin freely until the mating threads enter the top locking area. As the nut is wrenched into position, the slotted section expands and locks the nut. Full thread engagement occurs when 1 1/2 threads of a standard bolt extend beyond the top of the nut.
Steel is a common material and plain finish and cadmium plated are typical finishes. (Plain finish is unplated so there is no surface treatment to inhibit rust.) They are available from domestic (USA) manufacturers.
Flexloc nuts are reusable for a limited number of times except unplated stainless steel nuts, which are not recommended for reuse. 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.
The manufacturer says "Vibration will not loosen an SPS® Locknut. There is never any galling of threads."
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.