Analysis on the causes of loose connection of threaded fasteners

The threaded connection is subject to a tightening force in advance before bearing the working load, which is called pre tightening force. Looseness of threaded fasteners refers to the complete or partial loss of pre tightening force of threaded connections. The reasons for looseness of threaded fastener connections mainly include the following aspects.

(l) Initial loosening. After the threaded fastener is tightened, in the working process, such as under the vibration environment, the roughness of its various contact surfaces, including the thread profile, the bearing surface and the contact surface with the connected parts will be reduced due to continuous running in, which will lead to changes in the fastening state, loss of preload, and then loosening. In this case, it is necessary to check the fastening state of threaded fasteners after a period of work, and tighten them again to make them have pre tightening force again.

(2) The bearing surface is compressed and loosened. When the bearing surface of the bolt or nut is subjected to too much pressure, the contact surface between the bearing surface of the bolt or nut and the connected parts will be subject to indentation deformation, which is a plastic deformation. Even if the pressure is removed, it can no longer be restored to the original state, resulting in reduced or even lost pre tightening force of threaded fasteners and loose connection.

(3) Self loosening of threaded connections. Self loosening of threaded connections is the most common cause of failure. We know that when an object is placed on an inclined plane, it is subjected to downward gravity, the support force perpendicular to the inclined plane and the friction force parallel to the inclined plane. When the lateral component is greater than the friction force, the object will slide along the inclined plane. The same is true for bolts. The thread is an equidistant helix, and any section of thread is a diagonal line. The pressure on the bolt is decomposed into axial force and transverse component. When the bolt is subject to vibration, the instantaneous transverse component is greater than the friction force, and the bolt will rotate along the slope. If this happens for a long time, it will become loose.

Common locking measures for threaded fastener connection

(1) Increase friction between threaded fasteners. The looseness of threaded fasteners can be prevented by increasing the friction between threaded fasteners or nut contact. It is not limited by space and can be disassembled and used circularly. It is the most widely used measure so far.

(2) Use welding, gluing or punching riveting and other means to destroy the relationship between the thread movement pairs of threaded fasteners. This method will destroy the original structure of threaded fasteners, making threaded fasteners can not be reused. This method is mainly used for one-time threaded connections.

(3) Add mechanical fasteners. The locking washer, cotter pin and other mechanical fasteners can be used to fix and lock the threaded fasteners. This locking method is widely used in important connection parts, which can effectively prevent the loosening of threaded fasteners. However, the production and installation process of this method is complicated, and the weight of threaded fasteners will also increase.

(4) Double nuts are used for locking. Replace the original single nut with two nuts of the same specification. This double nut assembly can first use 80% of the assembly torque to tighten the inner nut of the double nut, and then use 100% of the torque to lock the outer nut, which can effectively increase the fit of the threads on the two nuts and bolts, and can significantly improve the friction resistance of thread fasteners against loosening. However, this method has the same disadvantage as the third method, that is, it will increase the weight of threaded fasteners. In addition, this method also has certain requirements on the installation space of nuts.

(5) Use elastic washers. Elastic washers mainly include spring elastic washers, wave shaped elastic washers, saddle shaped elastic washers and tooth shaped elastic washers. The principle of using elastic washers is to use the external elastic force of the spring or the friction force of the tooth warping carried by the washers to prevent the connection of threaded fasteners from loosening and provide a certain locking effect for threaded connections. The elastic washer is widely used as one of the anti loose methods because of its simple connection, low cost, convenient installation and use. However, it is worth noting that the elastic gasket material is easy to hydrogen embrittlement, and the anti loosening effect is general. It is not suitable to use elastic gaskets for anti loosening in the parts with severe impact or vibration, and it is not allowed to use elastic gaskets alone for anti loosening in important threaded connections.

(6) Use self-locking nuts. The self-locking nuts are classified according to their functions, including nylon ring embedded nuts, necked nuts and metal locking devices. They are all effective torque type locknuts and are widely used in various important connection parts.