Why are assembly bolts loose?

The connection method of bolted connection is widely used in mechanical equipment, automobiles, trains, aerospace and other fields. It is the most basic structural form of fastener connection. For assembly factories, especially construction machinery assembly factories, bolted connection has the advantages of convenient Standardized mass production, simple structure, low cost, convenient installation, easy disassembly and other advantages have been the most widely used. When the equipment is in use due to vibration, high and low load changes, impact or long-term working conditions, the bolts are prone to loosening, which directly affects the operation performance of the equipment and reduces its safety and reliability. , the severe cases will affect personal safety.

1 Design flaws
(1) The strength of the bolts selected is insufficient
The bolt connection generally adopts the yield point tightening method, that is, the pre-tightening force of the bolt should reach close to the yield strength of the bolt material. Considering the safety factor, it should generally not exceed 80% of the yield limit of the material. The yield strength of bolts of different materials is different. , which also means that the maximum preload force that bolts of different materials can withstand are different.

(2) Lack of anti-loose connection
When the bolted parts are subjected to changes in load, vibration, shock, etc., the compression force and pre-tightening force of the connection will gradually decrease or even disappear. Looseness, the form of bolt and nut mating connection is particularly serious.

2 Insufficient preload during assembly
The pre-tightening force that the bolt can achieve during the tightening process directly determines the clamping force between the two connecting parts. Insufficient pre-tightening force will inevitably lead to the loosening of the connecting bolt and eventually the loosening of the connecting parts.

3 Improper assembly method
There are single bolts and multiple bolts tightened in the assembly process. For the tightened state of multiple bolts in groups, especially those with regular distribution, the method and method of bolt tightening are extremely important, which directly affects the actual acquisition of each bolt. The size of the preload.

4 Machining quality of the mounting hole of the connector
When two connectors are connected, the size of the threaded hole or the mounting bolt hole is particularly important. The thread size of the threaded hole directly affects the preload obtained by the bolt. If the threaded hole is too large or too small, the preload obtained by the bolt cannot meet the requirements. If the size of the mounting bolt hole is too large, the surface of the connected part will produce a plastic annular indentation at the contact with the bearing surface of the bolt or nut, and severe indentation will reduce the bolt's pre-tightening force or lose the pre-tightening force. , resulting in loosening of nuts or bolts.

Commonly used bolt anti-loosening methods
Anti-loosening of bolts is to eliminate (or limit) the relative movement between thread pairs, or increase the difficulty of relative movement, prevent the connection from loosening, and ensure the safety and reliability of the connection. There are various measures to prevent loosening. In addition to taking corresponding effective anti-loosening devices according to the relevant standards and actual working conditions of the equipment during design, it is also necessary to control the tightening torque of the mounting bolts during the assembly process to avoid loosening of the bolts before leaving the factory. Therefore, the bolt anti-loosening measures can be roughly divided into the following two aspects: physical device anti-loosening and process method anti-loosening.