Effect of fastener friction coefficient on assembly-torque method

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Used to verify reference equipment: fastener friction coefficient testing machine, thread friction coefficient testing machine, friction coefficient testing machine, nut friction coefficient testing machine, multi-function threaded fastener system, bolt friction coefficient testing machine, standard part friction coefficient Testing machine, thread fastener test analysis system.

Effect of friction coefficient on different torque methods

At present, the "torque method" and "torque-angle control method" are the most used, and both of these tightening methods will be affected by the friction coefficient of the product, thereby affecting the pre-tightening force when the bolt is tightened. The "yield point control method" and the "bolt length method" avoid the influence of the friction coefficient on the assembly.

Torque method

When the torque assembly method is used, it stops when the specified torque is reached, only a certain tightening torque is controlled, the operation is simple, the torque is easy to measure and control, and the torque is easily rechecked afterwards. However, after a large amount of trial and practical experience accumulation, the friction coefficient is affected by the actual tightening process, only 5%~10% of the torque is converted into the required pre-tightening force, and 90% of the torque is tightened during the tightening process. Use up. When the friction coefficient of the bearing surface is reduced by 20%, the frictional torque of the bearing surface is reduced by 40%, and the axial clamping force of the bolt is doubled (20% of the tightening torque is converted into the clamping force). It can be seen that with the torque assembly method, the friction coefficient has a great influence on the bolt clamping force.

Torque-angle control

When tightening the bolt, first tighten the bolt with the set torque, then rotate the bolt to a specified angle, and use the elastic deformation of the bolt to ensure that the coupling force meets the specified requirements. This is the corner torque method. This method can be divided into elastic region tightening method and plastic region method.

This method can accurately control the pre-tightening force, and the bolt material is completely utilized, which is only related to the thread pair friction and the yield strength of the bolt. The set torque value is usually about 25% of the required tightening torque value. Although the coefficient of friction of the thread has a certain influence on the "stage pre-tightening force" generated by the smaller torque, the influence is small, and the friction coefficient of the thread has no influence on the pre-tightening force generated by the corner tightening, because in the elastic zone or In the plastic zone, if the modulus of elasticity is constant, the preload is only related to the elongation of the bolt, and the elongation is proportional to the angle of rotation of the bolt.