Surface roughness effect on the torque-tension relationship in threaded fasteners

Abstract

An experimental study is presented in order to investigate the effect of surface roughness on the torque-tension relationship in threaded fasteners. Three levels of surface roughness are considered for the fastener underhead and the joint surface; namely, low, medium, and high levels of surface roughness. The study is conducted for two joint materials, two fastener classes, and for coarse and fine threads. In this study, the torque-tension data are expressed in terms of the value of the nut factor as well as its scatter. The effect of the number of tightening on surface roughness and on the torque-tension relationship is investigated as well. The surface roughness is measured before tightening, and after each loosening using a WYKO optical profiling system. An M12 fastener is used in this study. Both fine and coarse threads and fastener material Classes 8.8 and 10.9 for M12 fasteners are used in this study. The torque-tension data is analysed for both steel and aluminium joints. The safety and reliability of bolted assemblies are mainly determined by the level and the stability of the clamp load provided by the initial tightening of the threaded fastener. The value of initial clamp load, which is achieved by a specific level of tightening torque, is highly sensitive to the friction torque components. This study provides an insight into the reliability of the existing engineering practices for estimating the clamp load level from the tightening torque. Hence, the findings of the study would help enhance the reliability and the safety of bolted assemblies, especially in critical applications.

Tightening and loosening of bolted joint

“…However this relationship is highly affected by friction of a bolted joint. This is confirmed by Nassar, S. A. et al in their experiments [18][19][20][21][22], which identified several factors affecting the relationship between input torque and preload generate at the bolt. They are the surface finish, tightening speed, coefficient friction of the bolt [21], and the lubricant used [22].…”

Calculation of the effective bearing contact radius for precision tightening of bolted joints

“…Otherwise, only equation (7) should be used in precision assembly applications. Considering that the allowed error of the torque-tension relationship might be as small as several percents for precision assembly applications, equations (6) and (8) are not suitable for these applications. The results and conclusions of this study can help to increase the accuracy of the torque-tension relationship for tightening bolted joints, thus ensuring mechanical accuracy and quality for precision assemblies.…”

“…Since the geometrical factors are fixed for a specific type of bolted joint, the accuracy of the torque-tension relationship depends mostly on the accuracy of the friction coefficients and the effective contact radii. In practice, friction coefficients are affected by various factors, such as material class, 5 surface roughness, 6 tightening speed, 7 and the number of tightening cycles. 8 Thus, friction coefficients may scatter considerably for the same type of bolted joint.…”

Friction-Induced Vibrations during Tightening of Bolted Joints—Analytical and Experimental Results

“…where G denotes the shear modulus of the material and J p the polar moment of inertia of the extension. With the motor transferring the torque to the head into the bearing of the screw, the latter will be rotated and increase the pretension force onto the clamped parts [24][25][26][27]. The frictional contact area underneath the bearing is approximated by [28]…”

“…It is notable that Equations (25) and (26) are absent of a dependency ofφ m . This is counterintuitive because with the increase of the tightening velocity, the frequencies should also increase.…”