Influence Of Tooth Face Roughness On Gearing Contact Fatigue Stress

The contact fatigue strength design of an involute cylinder gearing drive is based on Hertz theory, However, this theory is only suitable for surfaces that are ideally smooth with no lubrication. In the actual production, absolute smooth tooth face is not exist, and tooth face roughness and oil film thickness is usually in the same order of magnitude, tooth face roughness peak and valley will cause the time-varying of oil film thickness, which will no doubt cause gear tooth face pressure distribution and gear subsurface contact stress distribution change dynamically. Therefore, we must consider the effect of tooth face roughness for the perfection of the gear contact fatigue strength design.Taking into account the effect of thermal and characters of non-Newtonian lubricating oil, gear tooth face roughness is assumed to take a cosine distribution, using multiple grid integration method to solve oil film thickness distribution, using multiple grid method to solve EHL pressure distribution, using the sequential column sweeping scheme to solve the temperature distribution, using elastic contact theory to solve subsurface contact stress distribution, lots of mixed EHL numerical calculations have been completed for involute cylinder spur gearing. The influence of tooth face roughness on tooth surface pressure, frictional coefficient, average film thickness, temperature and subsurface contact fatigue stress is theoretically investigated, so it provides certain theoretical basis to further perfect gear contact fatigue strength design. Analyzing and concluding calculation results, the main conclusions are as follows:(1) the presence of the moving roughness features has led to very significant subsurface stress concentrations near tooth surfaces within contact zone. The smaller the roughness wavelength and the larger tooth surface pressure peak and valley is, the more obvious the stress concentration is and the closer it is to the tooth surface.(2) Tooth face roughness can make average film thickness hav and maximum principal shear stress rmax along the meshing line occur sawtooth wave shape change, haV and rmax is overall greater than the corresponding values for smooth tooth faces.(3) When the roughness wavelength is less than a certain value, if roughness wavelength is fixed, the average value of subsurface maximum principal shear stress rmax,r increases as the roughness amplitude rises, there is a approximate exponential relation between the average value of subsurface maximum principal shear stress and the roughness amplitude. If roughness amplitude is fixed, rmax,rdecreases as the roughness wavelength rises, there is a approximate inverted exponential relation between the average value of subsurface maximum principal shear stress and the roughness wavelength;When roughness wavelength is more than a certain value, if roughness wavelength is fixed, rmax,r increases after decreasing as the roughness amplitude rises, there is a approximate parabolic relation between the average value of subsurface maximum principal shear stress and the roughness amplitude. If roughness amplitude is fixed, rmax,r increases slowly and tends to achieve the respective value corresponding to that under full lubrication conditions in smooth tooth surfaces contacts as the roughness amplitude rises.(4) When roughness wavelength is fixed, the average value of the average film thickness hiV,r increases as the roughness amplitude rises, there is a approximate exponential relation between the average value of the average film thickness and the roughness amplitude;When roughness amplitude is fixed, if roughness wavelength is less than a certain value, hzv,r would fluctuate as the roughness amplitude rises. If roughness wavelength is more than a certain value, hav,r decreases and tends to achieve the respective value corresponding to that under full lubrication conditions in smooth tooth surfaces contacts as the roughness amplitude risesIn addition, the fitted equation between the average value of subsurface maximum principal shear stress rmax,r,the average value of the average film thickness hav,rand the roughness amplitude,the roughness wavelength have been given through curve fitting analysis of the result.Gear tooth face roughness is assumed to take a cosine distribution in this paper, however, the real gear tooth face roughness changes irregularly. EHL numerical calculations discussing based on real tooth face roughness will be one of the research objectives in the future.