Common loosening problems of threaded fasteners, anti-loose measures, anti-loose structure

The embankment of a thousand miles was destroyed by an ant hole. A small ant hole can break the embankment of a thousand meters long. Screws are known as the rice of industry. Although they are tiny, they are by no means insignificant. However, in history, incidents that have caused catastrophes due to the neglect of screws abound.

In response to the problem of loosening of threaded fasteners, technicians have taken various active and effective measures to inject new vitality into the development of threaded fasteners. There are many anti-loose technologies and anti-loose structures for threaded fasteners. The solution is as follows.

Common loosening problems of threaded fasteners, anti-loose measures, anti-loose structure

Control preload

Controlling the installation pre-tightening force is one of the economical and effective measures to prevent the loosening of threaded fasteners. This method uses the self-locking condition of the thread and does not need to make any changes to the bolt and nut structure, by ensuring the appropriate pre-tightening force Come to prevent loosening. For use occasions with particularly high installation control requirements, the direct control method is adopted to measure the pre-tightening force during the installation process and control it. Under normal circumstances, direct control of the installation pre-tightening force requires the use of special devices or mastering special technology. Difficult to promote. In order to obtain satisfactory pre-tightening force in an economical way, more methods of indirect measurement and control of pre-tightening force, namely torque control method, are adopted.

Common loosening problems of threaded fasteners, anti-loose measures, anti-loose structure

The torque control method uses the torque coefficient to convert the pre-tightening force into the assembly torque. Use a fixed torque or torque measurement assembly machine or wrench to control the assembly torque, or use the fastener's own structure to ensure the tightening torque (such as torsion shear type bolt connection pair ), indirectly achieve the purpose of controlling the pre-tightening force. In order to achieve the desired purpose, it is required that the torque coefficient of the connecting pair can be accurately measured in advance, and to ensure that the torque coefficient of the same batch of parts has little dispersion. For example, GB/T1231-1991 clearly stipulates that the average value of the torque coefficient of the same batch of connection pairs is 0.110-0.150, and the standard deviation of the torque coefficient should be less than or better than 0.001%. In engineering practice, there are also control methods such as turning angle method and yield point tightening method. (Guide: Standard keys and pins commonly used in mechanical equipment)

Effective torque type fasteners

Effective torque type fasteners increase the effective torque part on the basis of ordinary fastener structure, and its function is to add a resistance torque that does not change with external force in the connection pair.

Common loosening problems of threaded fasteners, anti-loose measures, anti-loose structure

The effective torque part is mainly added to the nut, and the product with the effective torque part added to the external thread is relatively rare. All-metal effective torque type lock nuts, one type uses the deformation of the nut body after the thread processing on the nut body is completed, so that the thread is deformed in the axial or radial direction, causing local interference between the internal and external threads during assembly to produce an effective torque. And the influence of the deformation resistance and geometric accuracy of the blank before deformation, it requires high processing technology, and the effective torque control is difficult; the other type is to thin the effective torque part, and close it after closing or grooving. At present, it is mainly used in the military industry in China. Many; the third type is to embed a metal elastic element in the nut body. The external thread forces the elastic element to deform during assembly and produce an effective torque. This type of nut has higher requirements for the elastic element elasticity and the position of the insert, and sometimes scratches the external thread. surface.

Use washers

The washers currently in use mainly include flat potential rings, spring washers, and elastic washers. Common loosening problems of threaded fasteners, anti-loose measures, anti-loose structure

The flat washer is mainly used to improve the contact state of the supporting surface, to ensure that the friction coefficient of the supporting surface is stable, and has a certain effect on the anti-loosening; the spring washer uses its elasticity to generate axial force, improve the elasticity of the connection, and the results of the transverse vibration test It shows that its anti-loosening effect is poor under this test condition; the twisted teeth of the elastic washer are flattened by the tightened nut, so that the threaded auxiliary shaft body is compressed, and at the same time, it is partially embedded in the supporting surface, with uniform elasticity and good anti-loosening effect. It will scratch the surface of the part. In some specific occasions, scratching the surface of parts is what people want. For example, the terminals on the parts used for surface painting can scratch the paint and ensure the conductivity.

Lock directly

After tightening the nut, use a locking (stop) element to lock the nut and bolt to prevent them from rotating relative to each other. The most commonly used is the use of split pins, tandem steel wires, and stop washers. The cotter pin is used in conjunction with the bolt with a hole at the end and a slotted nut to prevent loosening and reliable.Generally, the slotted angle of the nut is 60°, and the slot must be aligned during installation, which is inconvenient for assembly; use low-carbon steel wire to penetrate the bolt head or In the wire hole of the nut, several bolts or nuts are connected in series to restrict each other, which is reliable and reliable; the stop washer is blocked by the plastic deformation of the washer. When disassembling, the washer must be flattened and restored before loosening the nut. Heavy, dynamic load connections that are frequently disassembled, such as flywheel nuts.

Destruction of sports associate relations

Use the punch to locally deform the threads of the bolts and nuts, deviate from the original tooth profile, so that the part cannot be engaged with the normal thread direction, destroy the movement relationship of the original motion pair, and form a non-reusable connection. If you want to disassemble, It is necessary to use a larger torque to unscrew the nut or destroy it. This method is rarely used at present.

Bonding

Bonding is to bond the bolts and nuts or the connected parts together to achieve the purpose of preventing loosening. Bonded bolts used in mass production are generally coated with anaerobic glue on the parts in the fastener manufacturer and dried to form microcapsules. The surface of this microcapsule is dry and does not have a sticky feeling. The capsule is squeezed and ruptured, the glue overflows, and the bolt and the nut are firmly bonded. As long as sufficient torque is applied during disassembly, under normal circumstances, within a certain period of time, it can be reused a limited number of times.

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