Knowledge of fastener bolt torque method

The torque applied to the nut when tightening the bolt is proportional to the axial force introduced into the bolt. Therefore, when using bolts, nuts, and washers with a certain shape, the effective diameter of the threaded part (outer diameter + inner diameter), thread The half-angle of the thread, the rise angle of the thread, and the average radius of the contact surface between the nut and the washer are all fixed numbers. The specific ratio will also be determined. (Guide: Four reasons why screws and bolts often break)

The torque coefficient of the bolt is determined by the size of the thread, the coefficient of friction between threads, the size of the nut, and the coefficient of friction between the nut and the washer. However, for the value of the torque coefficient of the bolt, the variation of the friction coefficient has a greater impact than the difference in the value of the thread or nut size. However, the error of the thread pitch and the thread angle will increase the value of the friction coefficient of the thread on the surface, which will also affect the torque coefficient of the bolt. Compare the 4 threads of thread rolling after heat treatment (A), thread rolling heat treatment (B), wet galvanizing (C) and electric galvanizing (D). In addition, the results of the first torque test and the second test after loosening it are also shown. The test results show that the dispersion of galvanized bolts is large at the first time, and the friction coefficient becomes larger at the second time, and the synthetic stress generated by the thread part becomes larger to a considerable increase in axial stress by more than 50%.

Under normal circumstances, regardless of whether the thread is rolled after heat treatment, or the thread is rolled after the thread is well heat treated, due to the shear force generated by the torque, the effect on the composite stress is increased by 20 % Is enough to consider. In recent years, lubrication treatment has been widely used on nuts to reduce the torsional shear force. Most of the resultant stress is considered to increase the axial stress by about 10%.

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