In recent years, with the introduction and digestion of China's automotive products and related process technology, people's understanding of threaded fasteners has gradually deepened, especially for the quality of threaded fasteners and bolt tightening speed control. During the assembly process, the bolt friction speed is controlled to effectively control the bolt friction coefficient, thereby reducing the dispersion of the torque coefficient and improving the bolt performance.
In this paper, the torque control method is used to indirectly realize the axial force control.
2.3 Test data processing and result analysis The multi-functional bolt fastening analysis system developed by the company fixed the bolts and carried out five sets of experiments on the nuts at the same tightening speed of 15 r/min; the tightening device applied a linearly increased torque to the nuts. (until stop around 200Nm), the instrument automatically records the curve of the axial preload of the bolt as the torque increases. Then change a pair of bolts and nuts, assemble with the No. 2 hole on the orifice plate, and repeat the above experiment process to 5 times. Similarly, the tightening speed is set to 30, 45, 50, 55, 60! 7/1! 111, and five sets of tests are performed at various tightening speeds, and the instruments record their data separately.
Table 2 shows the mean data of the fasteners at different tightening speeds.
Table 2 Average value of fasteners at different speeds Bolts Maximum tightening speed Torque / (Nm) Maximum axial preload / (KN) Total friction coefficient Bearing surface friction coefficient Thread friction coefficient Torque tightening speed The histogram of the coefficient of friction and the torque coefficient is as shown. Through analysis, it can be concluded that: 1) As the tightening speed of the fastener increases, the total friction coefficient decreases slowly to 0.12, and the friction coefficient of the thread gradually decreases. The tightening speed is increased from 30r/min to 45r/min, and the total friction coefficient and the friction coefficient of the thread are greatly reduced. It is known from the data that the tightening speed of the fastener has a great influence on the friction coefficient, and the friction coefficient is higher under the condition of higher tightening speed. The reduction is smaller than the low speed condition. The test results show that when the tightening torque tends to yield torque, the friction coefficient decreases with the increase of the tightening speed, and gradually becomes stable. And when the speed is 55,60r/min, the friction coefficient reaches 0.12, which is in line with the control range of fastener friction coefficient in European automotive assembly process, the range is 0.070.12. It can be seen that the fastener tightening speed is low. The torque factor under conditions is significantly higher than the torque factor at higher speeds. When the tightening speed of the fastener is controlled to 15r/min, the torque coefficient of the fastener is about 0.25, the axial preload is 72.05kN; when the tightening speed of the fastener is increased to 60r/min, tight The torque coefficient of the firmware is about 0.15, and the axial pre-tightening force is 85.11kN; the axial pre-tightening force increases greatly. According to the torque calculation formula, the torque coefficient determines the proportion of the axial preload in the conversion of the tightening torque. When the tightening torque tends to the yield torque, the smaller the torque coefficient K, the corresponding conversion is The value of the axial preload is also large, and the reliability of the axial connection is high, which ensures the reliable service of the bolt.
The torque coefficient of the bolt at different tightening speeds 2) From the experimental data analysis, the friction coefficient is generally reduced as the tightening speed increases. Explain the reasons from two aspects: (1) It can be seen from the experimental data that when the tightening speed is low, the tightening torque is converted into an axial pre-tightening force, and the friction coefficient is relatively large, which is due to the adhesion of the plating layer and the coupling member. The joint force is small and it is easy to fall off. At the same time, the actual contact area of â€‹â€‹the fastener is relatively small, and the peeling coating will increase the roughness of the contact surface and increase the friction coefficient. When the tightening speed is large, the tightening torque is converted into an axial pre-tightening force, and the appropriate axial pre-tightening force contributes to the lubrication between the fastener and the coupled member to suppress the roughening of the contact surface, resulting in The friction coefficient is inevitably reduced; 2) On the other hand, because during the tightening process of the fastener, the tightening speed causes the contact surface to heat up and the temperature changes to change the properties and contact conditions of the friction pair surface material, thereby affecting the friction coefficient. When the tightening speed is low, the frictional heat of the contact surface is less affected. At this time, the friction surface is less likely to form an oxide film, and the actual contact surface is prone to sticking, resulting in a large friction coefficient; when the tightening speed is increased, the contact surface is rubbed. When the temperature is raised, the molecular thermal motion reduces the shear strength of the adhesion point and reduces the friction coefficient.
3The effect of tightening speed on the allowable strength of the bolt is the common bolt connection. When the nut is tightened, the bolt is not only subjected to the tensile stress generated by the pre-tightening force F, but also the shear stress generated by the torque. The bolt is stretched and twisted. For the common threaded bolt, the fourth strength theory is applied to calculate the equivalent stress of the bolt dangerous section. It can be known from equation (6) that the ordinary bolt subjected to the preloading force is subjected to the composite stress after preloading, and the strength can be calculated when the strength is calculated. The tensile stress is increased by 30%, and the strength of the bolt is calculated only by the axial tensile force. Therefore, the strength condition of the tight bolt connection only by the pre-tightening force is: from the above, in the tightening process, the tightening torque must be controlled to convert the axial pre-tightening force to ensure the tensile strength of the bolt section is allowed to be pulled. Within the stress range. It can be seen from Table 2 that when the tightening speed is 15r/min and 60r/min, the required tightening torque is reduced from 200.02Nm to 149.14Nm, which is reduced by 25%; the axial preload is increased from 72.05kN to 85.11kN, an increase of 18%, indicating that the tightening speed is controlled during the fastener assembly process, and only a small tightening torque is applied to the fastener to increase the axial preload force to the desired purpose. Therefore, in the elastic region, the tightening speed is appropriately increased. Before the tightening torque reaches the yielding state, the smaller the torque coefficient, the greater the axial preload force generated, and the tightening bolt connection is more effective.
4 Conclusion Through theoretical and experimental analysis, it is known that the friction coefficient is the main factor affecting the torque coefficient. The torque coefficient is an increasing function of the friction coefficient. Within a certain range, the torque coefficient and the friction coefficient increase and decrease.
The tightening speed has a great influence on the friction coefficient, and the friction coefficient tends to be stable as the tightening speed increases. Therefore, the tightening speed should be strictly controlled to better control the dynamic tightening torque.
The torque factor is not constant throughout the tightening process, and the torque factor plays an important role in the overall tightening before yielding.
During the tightening process of the fastener, the friction will generate high temperature, and the temperature change is an important factor that changes the friction coefficient.
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