Top lock nuts are also called top locking nuts, stiff nuts, or self-locking nuts. Top lock nuts are able to resist loosening under vibrations and torque with the crimped top. In most cases, the top lock nut cannot be used without a mating part, for example, a screw or a threaded shaft. The top lock nuts as fastening accessories on machinery and mechanical equipment have to withstand a certain level of loads and forces and high operating temperatures; therefore, these fasteners are mostly made from stainless steel or other steel-based alloys because stainless steel has excellent tensile strength and temperature resistance.
There are a wide variety of lock nuts that are used to prevent loosening under vibrations and torque yet applying different designs and mechanisms. The friction lock nuts and positive locking nuts are of the most common types. These types of top lock nuts are used frequently in the automotive industry where wheels of the machinery are held in place with these nuts. Once the top lock nuts are fastened, they are not expected to be loosened; the parts the nuts hold in place are required to be fixed permanently.
As the name suggests, friction lock nuts apply friction to resist loosening. Greater torque is required to tighten and loosen a friction lock nut. The friction the lock nuts have makes it less easy to loosen and tighten the nuts, which also indicates that it is less easy to spin the nuts along the length of a thread. In other words, it takes more time and effort to assemble a friction lock nut with its mating part. Based on the top design, the friction lock nuts can be further divided into several subtypes such as jam nuts, distorted thread lock nuts and nylon lock nuts.
Jam nuts are also known as half nuts. It has a thinner body than the regular friction lock nuts. This type of lock nut is designed to be jammed (tightened) against a primary nut. The use of jam nuts goes like this: a jam nut is fastened in place first and then the primary nut is tightened over the top of the jam nut. The extra friction brought about by the two nuts prevents the loosening of the part.
This type of nuts has a section of shaped thread so that it deforms plastically to mate with the threaded shaft. The deformation of the shaped thread creates a normal force which increases the friction and hence prevents the nut from loosening. The normal force refers to the force that surfaces apply to prevent objects from passing through each other. Top lock nuts are considered a kind of distorted thread lock nut because they have a distorted thread at one end.
The nylon lock nuts have a polymer collar that inserts into the recess at the top of the nut. The polymer collar is made slightly smaller than the outer thread diameter so that it can be placed into the nut. The collar deforms plastically when the nut is tightened and friction is hence created. Since the friction is created by the polymer collar, this type of nut is not suitable for applications where elevated temperatures are involved.
The lock nuts that apply a positive locking device can be rotated freely to either tighten or loosen; it is easy to assemble the positive locking nuts with their mating parts. With the positive locking device, this type of nuts is locked only when a positive action is performed at their current position. For example, a pin is usually used as the positive locking device to lock the nut. With the use of such devices, the assembly of the nuts can be easier and faster and the fastening can be more secured.
The other examples of a positive locking device are the castle nuts and the slotted nuts. The castle nuts have a notch-shape cylindrical extension at one end which makes the nuts look like a castle. A pin can be inserted into the notches so that the rotation of the nut can be prevented. The slotted nuts are akin to the castle nuts but the notches are created into the body of the nut.
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