**0 Introduction**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. In the experiment process, the relationship between the friction coefficient of the threaded fastener and the torque-preload force is analyzed by different tightening speeds, and the friction coefficient is obtained. And the torque coefficient has a great influence, so in the assembly process of the vehicle, controlling the tightening speed is of great significance for improving the bolting efficiency.

**1 Theoretical analysis of bolted joints**When tightening the bolt connection, the tightening torque of the threaded fastener is mostly used to overcome the torque generated by the screw friction and the friction of the support surface, and only a small part of the torque is converted into the axial preload force required for the bolt tightening. The main factors affecting the preload are the friction coefficient of the fastener in addition to the tools and tightening methods used. The factors affecting the friction coefficient mainly include tightening speed, surface treatment properties, surface roughness, processing accuracy, temperature, etc. This paper mainly studies the effect of tightening speed on the friction coefficient. Figure 1 is a schematic diagram of the tightening torque distribution when the bolt tightening speed is controlled.

**1.1 Theoretical analysis of the effect of tightening speed on friction coefficient**During the tightening process of the bolt, the contact surface will produce relative sliding, which will inevitably cause friction on the contact surface, causing the contact surface to wear and change the temperature of the friction surface. When the tightening speed is low, the wear of the fastener is mainly caused by abrasive wear and slight plastic deformation and furrow. At this time, the temperature of the contact surface is low, and it is difficult to form an oxide film. When the screw is tightened, the contact surface is prone to sticking and rubbing. The coefficient is large; as the tightening speed increases and the wear time increases, the wear is intense. The frictional contact surface transfers the surface elements due to a large amount of frictional heat. The formation of different formed films leads to changes in the surface properties of the material and the contact conditions of the contact surfaces. The wear resistance of the material is thus changed, thereby affecting the coefficient of friction. The higher the tightening speed, the more obvious the temperature of the contact surface is increased by the relative sliding of the contact surface, and the fastener is more susceptible to deformation, achieving a stable and relatively low coefficient of friction. By empirical formula:

Where: Î¼

_{0}is the coefficient of static friction; v is the speed; c is a constant.According to the formula, as the tightening speed increases, the friction coefficient decreases. At the same time, during the tightening process, the tightening torque is converted into the axial pre-tightening force. The influence on the friction is mainly on the friction film formed. At the low speed, many friction cracks are generated on the friction film. When the tightening speed is increased, the friction film is relatively complete. At this time, the friction coefficient is small, so the friction coefficient decreases as the preload force increases [1].

At present, the testing equipment for the coefficient of friction of the thread pair is mainly the German multi-functional bolt fastening analysis system. The following formula is obtained according to the national standard GB/T 16823.3-2010 [2], the formula of the total friction coefficient:

Where: P is the pitch; d

_{2}is the thread diameter mm; D_{b}is the effective diameter of the bearing surface friction, mm; D_{0}is the outer diameter of the bearing surface, mm; d_{h}is the diameter of the thread contact, mm.Thread friction coefficient:

Friction coefficient of bearing surface:

**1.2 Theoretical analysis of the influence of friction coefficient on torque coefficient**The torque factor of the bolt directly reflects the coefficient between the torque and the axial force during the tightening of the bolt. It depends not only on the friction coefficient but also on the geometry of the threaded connection pair. The national standard GB/T 16823.2-1997 "General Rules for Fastening of Threaded Fasteners" [3] clearly states that the torque coefficient K can be expressed by the following formula:

Where: d is the nominal diameter of the thread mm; Î± is the flank angle of the thread.

It can be seen from equation (4) that the torque factor determines the proportion of the axial force in the transformation of the tightening torque. It can be seen from equation (5) that the torque coefficient is determined by the thread constant and the friction coefficient. Therefore, the torque factor is very important for the study of bolt fastening.

**2 Experimental research****2.1 Test materials**The test bolts are unified into hexagon bolts and the nuts are hexagon flange nuts. The surface treatment methods are the same. After processing, the bolts and nuts are matched with 6H/6g. The material to be joined is 45#, the thickness of the plate is 3 mm, the surface roughness after machining is 3.2, and the acute angle is dull. Other parameters are shown in Table 1.

**2.2 Test methods**In order to improve the quality of bolted joints, computer-controlled integrated tightening technology is widely used in the assembly process of automobiles. This technology can automatically control the tightening process and monitor the tightening results. As far as the current technology is concerned, the torque control method has the advantages of simple control system, easy use of a torque sensor or a high-precision torque wrench to check the quality of the tightening and the low cost.

This test uses the German multi-functional bolt fastening analysis system to test the fasteners. The test rig has three sensors to measure the three parameters of rotation angle, torque and axial pre-tightening force. The following parameters can be obtained by the calculation software of the above formula: torque coefficient, friction surface coefficient (total friction coefficient, thread friction coefficient, friction coefficient of support surface), yield point axial force, yield point torque, rotation angle, maximum torque and maximum Torque. The torque control method is a method of tightening control in the elastic zone by using the linear relationship between the torque and the pre-tightening force. When tightening, only one determined tightening torque is controlled, and the torque and the corner are tightened. The relationship is shown in Figure 2.

**2.3 Test data processing and result analysis**The test uses the multi-functional bolt fastening analysis system developed by Schatz, Germany, to fix the bolts, and carries out five sets of experiments on the nut at the same tightening speed of 15r/min; the tightening device applies a linearly increased torque to the nut (up to 200N). Â·m stops around), 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, repeat the above experiment process to 5 times. Similarly, the tightening speed is set to 30, 45, 50, 55, 60r/min, 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.

According to the data in Table 2, the histograms of the fastener friction coefficient and torque coefficient of the fastener at different tightening speeds are respectively shown in Fig. 3 and Fig. 4. Through analysis, we can conclude:

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, and its range is 0.07~0.12. It can be seen from Fig. 4 that the torque coefficient under the condition that the fastener tightening speed is low is significantly higher than the torque coefficient in the higher speed state. When the tightening speed of the fastener is controlled to 15r/min, the torque coefficient of the fastener is about 0.25, the axial pre-tightening force 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 pre-tightening force is also large, and the reliability of the axial connection is high, which ensures the reliable service of the bolt.

2) From the experimental data analysis, the friction coefficient decreases 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 [4]; (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 Coefficient of friction. 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.

**3 The effect of tightening speed on the allowable strength of the bolt**Ordinary bolt connection. When tightening the nut, the bolt is not only subjected to the tensile stress Ïƒ generated by the pre-tightening force F, but also subjected to the shear stress Ï„ generated by the torque T. The bolt is in a combined action of stretching and torsion, and is applied to the ordinary threaded bolt. The fourth strength theory [5], the equivalent stress of the bolt dangerous section is calculated as:

It can be known from the formula (6) that the ordinary bolt subjected to the pre-tightening force is subjected to the composite stress after the pre-tightening, and the tensile stress can be increased by 30% when the strength is calculated, and the strength of the bolt is calculated only by the axial tensile force. Therefore, the strength conditions of the tight bolt joints that are only subjected to the preload force are:

Where: [Ïƒ] = Ïƒ

_{s}/ S; d_{1}is the bolt diameter, mm.It can be seen from the above that during the tightening process, the tightening torque must be controlled to convert the axial preload force to ensure that the tensile strength of the bolt section is within the allowable tensile 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 200.02NÂ·m to 149.14NÂ·m, which is reduced by 25%; the axial preload force is from 72.05kN increased 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 [ 6-7]. 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, as shown in Fig. 5.

**4 Conclusion**1) 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.

2) 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.

3) The torque factor is not constant throughout the tightening process. Before the yielding, the torque factor plays an important role in the entire tightening.

4) 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.

**references**[1] Wang Zhenting, Meng Junyi. Friction and wear and wear resistant materials [M]. Harbin: Harbin Institute of Technology Press, 2013.

[2] National Technical Supervision Bureau. GB/T 16823.3-2001 fastener torque-clamping force test [S]. Beijing: China Standard Press, 2011.

[3] National Technical Supervision Bureau. GB/T 16823.2-1997 thread fastener fastening rules [S]. Beijing: China Standard Press, 1997.

[4] Jiang Haoming, Yu Ningfeng. Effect of Positive Pressure and Sliding Speed â€‹â€‹on Friction Coefficient of Galvanized Steel Sheet[C]âˆ¥The 9th National Plastic Engineering Academic Annual Meeting, the 2nd Global Chinese Advanced Plastic Processing Technology Symposium Proceedings (1), Forging Technology, 2005(S ): 131-132.

[5] Peng Wensheng, Li Zhiming, Huang Hualiang. Mechanical design [M]. Beijing: Higher Education Press, 2008.

[6] Qiu Yibing, experimental design and data processing [M]. Hefei: China University of Science and Technology Press, 2008.

[7] CROCCOLO D; M. De AGOSTINIS; VINCENZI N Failure analysis effect of friction coefficients in torque-preloading relationship 2011 [J]. Engineering Failure Analysis, 2011, 18: 364-373.

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