Study of Fluid Penetration and Lubrication in a Threaded Contact

Penetrating oil producers consider the loosening rate of seized bolted joints as the key performance indicator of their products. Penetrating oils are low viscosity lubricants, which are used to loosen rusted and seized mechanical parts, i.e., bolted joints. The oil penetrates by moving through narrow spaces between the bolt and nut threads and reduces the coefficient of friction. To improve the effectiveness of a penetrating oil product, their penetration and lubrication performance need to be understood. Both of these issues are investigated in this thesis. In the first phase of the research, a non-intrusive ultrasonic method was developed to measure the penetration time of various fluid samples in fasteners. This technique, utilizing piezo-electric crystals and ultrasonic physics, detects the presence of penetrant in a threaded contact based on the portion of the ultrasound waves reflected, which is different at a dry and lubricated contact. An experimental rig was built and a signal processing procedure was used to obtain the ultrasonic data. The findings show that for lower viscosity fluid samples, penetration time decreases. The second phase of the project involved modelling the penetration process based on the principles of laminar fluid flow in capillary tubes. Equation for fluid flow under capillary forces in micro channels of the threaded contact were derived, which is essentially the Washburn equation but with a slight difference due to the geometry of the channel. Viscosity and surface tension of the fluid and the geometrical characteristic of the threaded contact were the data input into the model. Two assumed fluid flow paths were modelled based on the interpretation of the ultrasonic data. The theoretical model was then compared with the experimental data. The last phase of work focused on studying the lubrication behaviour of the penetrants. An experimental set up was prepared, whereby the tightening torque was first measured and then the loosening torque was measured at different stages of fluid penetration. It was found that different penetrants show different rate of lubrication as they progressed along further threads. However, there was not much difference in the loosening torque after complete fluid penetration. Furthermore, the tension force was measured using a load cell which allowed the calculation of coefficient of friction at the threads. The coefficient of friction before and after fluid penetration was found to be within 0.24 to 0.26 and 0.17 to 0.18 respectively. The lubrication behaviour was also modelled analytically and a good agreement was found between the coefficient of friction obtained experimentally and analytically. The findings of the work can be used to formulate penetrants with enhanced penetration and lubrication performance.