Friction and thermal behaviour in elastohydrodynamic lubrication power transmission contacts

• Main Author: Al-Hamood, Amjad
• Published: Cardiff University 2015
• Subjects: 621.8; TJ Mechanical engineering and machinery
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.655946

This thesis reports experimental and theoretical analyses to study the heat generation and partition between contacting bodies under rolling/sliding elastohydrodynamic (EHL) point contact. A twin disk test rig was re-commissioned and used for the experimental work. 76 mm diameter crowned, super-finished test disks, fixed on parallel shafts, were used in the experimental EHL tests. In each disk, 6 thermocouples were installed to measure the temperature during sliding/rolling contact under EHL conditions. In addition, ceramic washers were fixed on the plane sides of each disk to minimize heat transfer over those surfaces. A Labview data acquisition system was built for acquiring data from the sensors installed on the rig. The EHL experimental results show the fast disk has significantly higher bulk temperature than the slow disk. A transient two dimensional numerical model was constructed to calculate the average circumferential temperature distribution within the disks using three thermocouples as a boundary condition. The heat partition factor,  , and convection heat transfer coefficient, h, were varied systematically in the model. A linear relationship between  and hf for the fast disk and 1- and hs for the slow disk were obtained. These two linear relations were related together by the variation of h with the rotational speed. Over the approximately steady state condition, the calculated  using this approach was in the range of 0.65 to 0.77 for 32 EHL tests at different loads and sliding speeds. This means that the majority of heat is conducted to the fast disk, which is compatible with the previous findings. In order to carry out transient modelling, the unloading mechanism was developed to ensure rapid disk separation. This improved the temperature trend in the cooling phase. In addition, a microswitch was used to give accurate monitoring of the disks contacting and separation. The thermocouple arrangement within the disks and their installation technique were assessed and developed. An immersion test in a hot oil bath was used to test the thermocouples dynamic response. This test was modelled, and the results showed significant lag in the measurements. It was concluded that a system identification method is required to identify the dynamic characteristics of the temperature measurement system in order to compensate for time lagging Extensive studies were carried out to determine the cause behind some of the experimental uncertainties. It was found that the most likely reason behind these uncertainties is the contact pressure variation along the disk-shaft interference fit, which would result in significant thermal conductance variation. The traction force measurement within the EHL contacts has shown broadly linear increase with increased load at constant sliding speed. This indicates a constant coefficient of friction at constant sliding speed. The results also showed that the coefficient of friction decreases as the sliding speed increases.