Effect of Lubrication on Friction and Torque-Tension Relationship in Threaded Fasteners

In this research, three types of lubricants are investigated for their effects on the friction and torque-tension relationships in threaded fastener applications. Investigated lubricants included oils, greases, and solid film lubricants that were applied to the threads of the fastener and the nut, and the turning bolt underhead surface as well as the joint surface in contact with the bolt head. The oils tested were mineral oils with different viscosities, i.e. 21cst, 115cst, 1000cst respectively at 40°C. Three types of grease were tested; namely, regular grade, pure nickel special and heavy metal free grease. For the solid lubricants, an olefinic type, an olefinic base with a polytetrafluoroethylene additive, and an olefinic base with an inorganic additive (Molybdenum disulfide), were applied. Class 8.8 M12 fasteners with both fine and coarse threads were used. The experiments were carried out using a Laboratory Fastener Testing System which measures the input torque, the fastener tension, the thread friction torque, and the angular rotation of the bolt head. The system computes the thread and bearing friction coefficients from the measured data. In each test, the fastener was repeatedly tightened and loosened five times. The tests were also conducted under different tightening speeds, varying from 1 rpm to 100 rpm. The nut factor, the coefficients of thread and underhead friction were obtained from the experiments. The effects of the number of tightening and loosening cycles, the tightening speed, and the type of lubricant on friction and nut factor were investigated. It was found that lubrication had a significant effect on the friction and the torque-tension relationship in threaded fasteners.