The first is friction and anti loosening. This is the most widely used anti loosening method, which produces a positive pressure between the thread pairs that does not change with the external force to produce a friction force that can prevent the relative rotation of the thread pairs. The positive pressure can be achieved by pressing the thread pair axially or simultaneously. For example, elastic washer, steel connecting sleeve, self-locking nut and nylon insert lock nut are used. This anti loosening method is convenient for nut disassembly, but in the case of impact, vibration and variable load, the pre tightening force of the bolt will decrease at the beginning due to relaxation. With the increase of vibration times, the lost pre tightening force will slowly increase, which will eventually lead to nut loosening and thread coupling failure.
The second is mechanical locking. It is to directly limit the relative rotation of the thread pair with the stopper. Such as the use of cotter pin, steel wire and stop washer. Since the stop has no pre tightening force, the stop can only work when the nut is loosened and retreated to the stop position. Therefore, this method does not prevent loosening but falling off.
The third way is riveting and punching. After tightening, punching, welding, bonding and other methods are used to make the thread pair lose the characteristics of the moving pair and become a non detachable connection. The disadvantage of this method is that the bolt rod can only be used once, and it is very difficult to disassemble, so the bolt pair must be destroyed before disassembly.
The fourth way is to prevent the structure from loosing. It uses the thread pair's own structure, i.e. down thread locking mode.