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Reasons for the overrun of fasteners and improvement measures

Update:21-05-2021
Summary:

Fasteners are widely used in all walks of life. Various […]

Fasteners are widely used in all walks of life. Various types of fasteners can be seen in various machinery, equipment, vehicles, railways, etc., and they are one of the most widely used mechanical basic parts. It is characterized by a wide variety of specifications, different performance and uses, and a very high degree of standardization, serialization, and generalization. Once the fastener fails, it will cause serious effects. Therefore, it is necessary to strengthen the analysis of the reasons for the failure of fasteners and find the corresponding improvement measures.

Torque alarms often occur in the bolt assembly process where the torque is controlled by the angle method. The failure modes and reasons that cause the fastener torque to exceed the limit are:
(1) After the assembly is completed, the torque of the part is higher than the upper control limit or lower than the lower control limit. The reason is that the assembly torque control range of the parts is unreasonable. The performance is that the set control range is too small and the control range shifts upward or downward.
(2) The torque reaches the upper limit without preloading to the preset angle. The reason is that the friction coefficient of the part itself exceeds the upper limit, and the friction coefficient of the part matching exceeds the upper limit, and the interference between the parts causes the assembly torque to rise sharply.
(3) Normal installation, lower torque limit alarm. The reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the part matching exceeds the lower limit. The fitting torque when the part is screwed is greater than the initial torque (that is, the screwing torque is too large), which is common in the tightening of the lock nut.

Improvement measures for distortion exceeding limit
The torque control method is to first screw the bolt to a small torque, generally 40% to 60% of the tightening torque (defined by the process verification), and then start from this point, screw a control method of a specified rotation angle. This method is based on a certain rotation angle, the bolt produces a certain axial elongation and the connector is compressed. The purpose of this is to screw the bolt to the close contact surface, and overcome some uneven unevenness of the surface, and the axial clamping force required later is generated by the angle of rotation. After calculating the rotation angle, the influence of frictional resistance on the axial clamping force no longer exists, so its accuracy is higher than that of the pure torque control method. The main point of the torque control method is to measure the starting point of the rotation angle. Once the rotation angle is determined, it can be obtained. High tightening accuracy.
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