Gear Tribology - Friction and Surface Topography

Doctoral thesis, 2005

The quest for energy efficiency leads to the development of highly efficient power transmissions for vehicles. Gears are some of the most rugged and durable torque transmitters among machine elements. Friction between sliding surfaces at gear teeth is one major source of power loss. To increase efficiency and reduce damage accumulation in gears has always been a background motivation to this thesis. It focuses on the frictional performance of manufactured gear surfaces with emphasis on the effect of surface topography and the orientation of its lay. To achieve the goal of this work, a test rig was designed and constructed. The rig makes it possible to examine gear tooth friction on double crowned rollers. Five alternative gear-finishing processes are implemented in roller surface preparation. They comprise grinding, shotpeening, phosphating, chemical deburring and DLC coating. Theoretically, a friction model is suggested which helps to explain some of the testing results. Advanced surface metrology techniques and statistical tools are implemented in identifying surface features that influence friction. The result demonstrates that surface topography is a significant factor affecting the lubricated frictional behaviour. For ground-finishing, the smoother the surfaces are finished, the lower friction coefficient they show. Chemical deburring and DLC coating as well as certain degrees of phosphating provide minimum friction coefficient and improved load carrying capacity, which are competitive to fine-ground finishing. Properly designed lay combinations of meshing gear surfaces can minimise friction and improve gear transmission efficiency. Theoretically, the friction coefficient can be explained by means of a combination of the slide-to-roll ratio and the real mean contact pressure within the range of loads considered in this thesis. Finally, features that correlate surface topography to friction are identified.