Research on Bolt Friction and Anti-loosening

1.Reasons for loose thread
When the static load and working temperature change little, the tightening thread connection will not automatically loosen, and the connection is very reliable. If the threaded connection is working in an environment with impact, vibration, variable load, or high temperature and large temperature changes, the friction and pre-tightening force of the connection will gradually decrease or even disappear, and the threaded connection will become loose after repeated many times. , Finally invalidated.

1.1 Affected by axial load
When the thread pair is subjected to an axial load, if the pre-tightening force is greater than the axial load received, the bolt will not loosen. When the pre-tightening force is less than the axial load, it is greater than the bolt yielding and tightening axial force, causing plastic tensile deformation of the bolt, and relative sliding of the threaded contact surface, causing the nut to loosen and the connection to fail.
In order to avoid loosening of the thread pair, in the design process, the appropriate pre-tightening force should be selected, that is, the appropriate bolt should be selected.

2.2 Affected by radial load
When the thread pair is subjected to a transverse load perpendicular to the axis, under the repeated action of the transverse force (alternating stress), the thread will undergo elastic torsional deformation or there will be relative slippage between the contact surfaces of the parts in the direction perpendicular to the thread axis. . The gradually accumulated torsional displacement forces the screw pair to slide down in the spiral direction, thereby gradually reducing or even disappearing the pre-tightening force, thereby causing the threaded connection to loosen.

In the case of a certain clamping force, in order to improve the self-locking ability, it is necessary to increase the friction capacity of the bolt. Under some limited conditions (such as the nominal diameter of the bolt and the surface friction coefficient of the connected part have been determined), the friction of the thread pair can only be increased. The area and the friction coefficient of the surface, such as replacing the hexagonal head bolts with hexagonal flanged bolts, increase the friction of the thread pair bearing surface.

2.3 Deformation of connected parts
In the threaded connection, the connected part is subjected to the pre-tightening force of the thread pair, and the pre-tightening force exceeds the yield strength of the connected part material, and plastic deformation occurs during the tightening process, and this plastic deformation continues to exist in the threaded connection. It will expand under certain conditions. It is the existence of this initial deformation that causes the initial loosening of the threaded connection. In order to avoid this phenomenon, it is necessary to calculate the connected parts from the design
Select the appropriate material of the connected parts.