Under the load change, vibration and impact during the working process of bolted parts, the compression force and pre tightening force between the connectors will gradually decrease or even disappear. Because under the action of axial load, the bolt will elongate axially and shrink elastically in radial direction, while the connecting nut will expand in opposite direction. There will be a small amount of relative radial sliding between the two contact surfaces, so under the repeated action of long-term and multiple bad loads, the nut will eventually turn loose. Similarly, when the connecting bolt is subjected to repeated radial load, the bolt will have the tendency of elastic torsional deformation. With the deformation accumulating, the displacement of torsion deformation will eventually be caused, and the component force that makes the nut whirl loose appears in the screw direction.
When using bolts and nuts for fastening connection, the first condition is to ensure that the expected clamping force between the fasteners must reach the corresponding size (refer to the loosening torque of ordinary bolts, which is generally 80% of the tightening torque and 85% of the fine teeth). The bolt and nut mainly through the friction torque to ensure the locking effect, at the same time, there are many factors affecting the friction torque, which mainly include the size of bolt pre tightening force, the size of friction coefficient, the outer diameter and inner diameter of bolt contact with the surface of connected objects. In general, higher strength grade or fine tooth bolt and nut can be used to achieve better locking effect [2]. However, in the actual working conditions, the expected anti loosing effect or aging requirements can not be achieved only by the above selection. It can be considered to increase the clamping force of bolts (generally the maximum pre tightening force shall not exceed 80% of the yield strength of the material) or increase new torque to ensure the purpose of preventing rotation loosening. Therefore, various forms of anti loose structures can be added Anti loosing purpose.