Effects of Bearing Preload, Oil Volume and Operating Temperature on Axle Power Losses

In order to boost the fuel economy of their vehicles, automotive OEMs and suppliers have been investigating a range of options from alternate vehicle propulsion systems down to optimized component level technologies. A rear axle differential with a hypoid gear set is one of the least efficient drive train components, and as such, provides unique opportunities for improvements. It has therefore attracted significant attention from researchers to reduce the power losses. Both loaded and unload power losses have been studied before and found to vary significantly with load and speed conditions. This paper will focus on the effects of the axle pinion bearing preload, axle gear oil levels and operating temperatures on axle power losses during the fuel economy driving cycles where both axle load and speed vary significantly. In this paper, power loss measurements from experiments conducted on an automotive rear drive axle on a dedicated dynamometer will be presented. Tests were conducted under a range of speed and load conditions that were developed from EPA fuel economy driving cycles. Both urban and highway cycles were included in the tests. Separate tests were conducted for unloaded spin losses and loaded power losses. The tests were conducted at a few different controlled levels of gear oil operating temperatures, gear oil volumes and pinion bearing preloads, and their influence on power losses were quantified. The measured power losses, at a matrix of load and speed conditions provide a series of power loss maps as a function of gear oil operating temperature, oil volume, and bearing preload. Using these power loss maps, the overall axle efficiency or power loss during any driving cycle can be quantified by integrating the instantaneous power losses as the axle goes through the driving cycles. Similar maps can be created for other influences and the proposed procedure can be utilized to quantify their influences on a given driving cycle. Results from this study indicate that with the combination of appropriate preloads, gear oil volume and temperature control, axle efficiency can potentially be improved by roughly 3% in the tested axle.