Surface Integrity Characterization of Gears with respect to Running-in

Licentiate thesis, 2016

A gearbox with gears of different sizes is part of a vehicle transmission system that plays an important part in transmitting the engine power to the wheels. The efficient energy transmission highly relies on the performance of gears. Together, the mesh efficiency and durability determines the gear performance. It has been reported that the final smoothening of surface by running-in process has increased the mesh efficiency of ground gears. This is not the case for superfinished gears. However, in comparison to ground, superfinished gears produced a higher overall efficiency but only at higher speeds. Micropitting is a surface contact fatigue failure which occurs in all type of gears and for all heat treatments. Depending on the initial surface micro-geometry and contact conditions used, the micropitting can initiate after a relatively short running time. Further progression of micropitting due to continuous operation leads to degradation of profile and thereby gears fail in the form of pitting, spalling or tooth breakage. The hard finishing of gear surface by different manufacturing methods, for example grinding, honing and superfinishing etc., produces a unique characteristic surfaces in terms of roughness, surface lay and residual stresses. These topographical characteristics of tooth flank affects the efficiency and durability of gears. In addition, contribution of microstructure and surface chemistry should not be neglected. Running-in process is known to alter the aforementioned surface characteristics via smoothening the surface asperities. This creates an interest to understand the initial running period with the purpose to improve efficiency and longevity of gears. The aim of this study is to investigate the effect of running-in load on the surface characteristics of gears set by generating grinding. Secondly, to follow how these characteristics influence and further develop during the initial usage. To characterize the surface layers of gears a methodology is developed by combining analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that running-in process smoothened the surface asperities through plastic deformation and the severity of deformation increased with the magnitude of the load. On the other hand, higher load produced more micropits and this trend continued for in-line efficiency testing as well. It has been found that on one side of the as-ground teeth the stresses were rather uniform while there were stress gradients from tip to dedendum and in axial direction on the other side. The deformation created by running-in increased the compressive residual stresses but only at the surface confined to 5 µm. It has also been found that surface conditioned by higher running-in load gave subsurface cracks during efficiency testing. High atomic concentration of phosphorous-containing extreme pressure additive (EP) was also observed after efficiency test that ran at high load. Keywords: Gears, running-in, micropitting, surface roughness, residual stresses, surface chemistry, surface asperities