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The threaded connection may be loosened under the action of variable load, vibration and impact, and when the working temperature varies greatly. The loosening causes the reduction or even loss of the pre-tightening force of the connection, which can not guarantee the tightness of the connection, and even causes the loosening of the connection or the premature fatigue failure of each part of the connection, which eventually leads to serious accidents of the machine or equipment.

(1) Loosening Principle
Generally, bolts and nuts are made of ordinary threads, because under static load, the thread lift angle is less than the equivalent friction angle of the threaded pair. Therefore, the screw pair of bolt and nut satisfies the self-locking condition, coupled with the friction between the screwed nut (or bolt) and the support surface of the connector or washer, so if the support surface is not too deep, the connection will not be loosened under static load.
When nuts begin to loosen, they must overcome the obstacles of thread moment and nuts support surface moment. Under the action of bolt pre-tightening force Qp, the total moment T'which prevents nut from loosening makes the moment of nut loosening equal, and T' is approximately equal to 80% of the tightening moment T. It can be considered that the bolt connection tightened by moment T will not loosen as long as the loosening moment acting on the nut or bolt head is not more than 0.8T. However, under the action of variable load, vibration and impact, the friction coefficient of the screw pair decreases sharply, and the friction resistance of the screw pair and the nut support surface will disappear instantaneously. The screw pair can not satisfy the self-locking condition and slip slightly, which causes the nut to rotate, so repeated will lead to loosening of the bolt connection.
In the tension bolt connection of nuts under compression, when the tension acts, the nuts expand radially because of the radial component of the lateral positive pressure of the threaded teeth, while the bolts shrink radially, so the soy sauce slips slightly between the threaded pairs. Experiments show that the tangential slip of nuts relative to the rotation of bolts is easy to occur when the radial relative slip occurs. Repeated occurrence of this phenomenon will lead to loosening of the connection.
Above are two analyses of the principle that the bolt slides from trace relative slip to rotation under tension load and finally leads to the loosening of the connection.
In addition, even if the nut does not rotate, the support surface of the nut and bolt head and the connector or gasket will collapse due to excessive pressure, and this plastic deformation may continue to occur in use, resulting in the reduction of the preload of the connection and the loosening of the connection.
There are also contact surfaces (such as the side of threaded teeth, supporting surfaces and the contact surfaces of jointed parts) in bolted connections. Because of roughness, corrugation and shape errors, local plastic deformation occurs during tightening, and under the cumulative effect of external forces in use, some deformation will continue to develop, which will also lead to loosening of connections. For a specific bolt connection, its loosening may be caused by a variety of factors, and thus involves several loosening principles, including primary and secondary. Therefore, measures should be taken to prevent the loosening of the bolt connection, and the root is to prevent the relative movement of the thread pair towards the loosening direction. Generally speaking, increasing pretension force is beneficial to loosening prevention. Another common way to overcome the above two loosening due to plastic deformation is to use large gaskets to reduce pressure and screw nuts after a period of connection work to eliminate loosening.

(2) Anti-loosening methods
According to the principle of anti-loosening, it can be divided into three kinds: depending on friction, locking directly and destroying the relationship between thread pairs. 1. Anti-loosening by friction: A positive pressure between the threaded pairs does not vary with the external force to produce a friction that can prevent the relative rotation of the threaded pairs. This positive pressure can be achieved by compressing the threaded pairs in both directions, either axially or horizontally or simultaneously. 2. Direct locking: use stop to restrict the relative rotation of thread pair directly. 3. Destroy the kinematic relationship of threaded pairs: after tightening, the threaded pairs lose the characteristics of the kinematic pairs and become a non-removable connection by means of punching point, welding and bonding.
It is a good design idea to adopt fine thread, use its small lift angle to achieve friction and anti-loosening or adopt two anti-loosening methods at the same time, complement each other, etc. This is one of the reasons why many mechanical bolt nuts used in aerospace machinery, whose thread diameter is greater than 10 mm, and many mechanical bolt nuts withstanding variable loads, adopt Rovena Fine Teeth.
The commonly used anti-loosening method of bolt connection is also applicable to bolt connection and double-head bolt connection.
Benign Development of Fastener Enterprises and Improvement of Competitiveness
Fastener enterprises in China should cultivate their own talents and have their own technology, so as to win better development.
In order to improve the technical level, fasteners enterprises must start with equipment improvement and technology improvement, and produce high-end fasteners instead of imported products. I think that after the technical performance reaches the level of imported products, domestic enterprises will not give up the near and far to import, which is a favorable trend for fastener enterprises in the current situation.
The healthy development of a fastener enterprise must establish the core competitiveness of the enterprise and cultivate the internal power of the industry. But at present, most of the fastener enterprises in our country are short of innovative ability, lack of advanced technology of intellectual property rights, lack of technical personnel, unstable production team, lack of adequate training and mobility of front-line employees, and their daily production is too poor to cope with. Complex low-end or low-end products, passive production mode and unstable market environment. It is undeniable that the existence of endogenous defects, such as lack of human resources, lack of core technology and lack of market expansion, will inevitably have a negative impact on the development of the industry.