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Center Lock Nuts
Center lock nuts are low cost, one piece, all-metal prevailing torque hex lock nuts with chamfered (beveled) corners. The locking action is created by two or three equally spaced rectangular or round indentations centered in the flats (sides) of the nuts that slightly distort the threads. Center lock nuts are considered "two way" because they can be installed either way, which reduces automated assembly costs (their tops and bottoms are the same like regular hex nuts). 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 commercial and industrial appliances, the automotive and truck industries, and in garden and medical equipment, center lock nuts are also known as:
bi-way lock nuts, centerlock nuts, reversible lock nuts, two way 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," center 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).
Center lock nut size refers to its nominal thread diameter. Typically, sizes range from about #8 to 1". 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 thickness vary with the size of the center lock nut.
Grade A, steel, zinc plated, is a common grade, material and finish.
The nuts spin freely until their center distorted threads engage the threads of the mating part. For the locking action to be effective, the locking element must contact fully formed threads. As a standard practice—to ensure full thread engagement with the nut—bolts/screws should be long enough to allow at least two full threads to extend beyond the nut face after tightening.
Center lock nuts are reusable for 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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