Comprehensive Influence Of Lubricant Viscosity And Gear Speed On Gearing Contact Stress

The current design of contact fatigue strength of gear transmission is based on Hertz theory which is only suitable for the design of static dry contact between a pair of smooth gear surfaces. The Hertz theory shows big differences with the actual performance of gear transmission which is realized with the gear lubricated. In order to fill this gap, lubrication coefficient and velocity coefficient are recommended in international standard (ISO/6336:1996) and corresponding China national standard (GB3840-1997) of involute cylinder gear load capacity respectively. Some scholars have also probed into the effect of lubricant viscosity and gear rotating velocity on the contact stress generated during the process of gear transmission. It is easy to understand that lubricant’s viscosity and gear rotating velocity are closely related. Therefore, it is necessary to discuss the integral effect of these two factors has made on contact fatigue stress of gear transmission.This paper would firstly model lubrication for the transmission of involute cylinder gear and secondly solve the pressure distribution of tooth surface, thickness distribution of oil film, and temperature distribution with multi-grid method, multi-grid integration method, and column scanning method respectively in according to the thermal-elastohydrodynamic lubrication theory on which basis elastic contact theory is applied to solve the stress distribution of subsurface in the gear contact area. At last, the paper would make588numerical calculations regarding to medium load and heavy load, especially heavy load gear transmission in terms of four conditions:the product of lubricant viscosity ηo and integral velocity of tooth surface U,ηoU, keeps constant, the viscosity keeps constant and the integral velocity changes, the viscosity changes and the integral velocity keeps constant, and both the viscosity and the integral velocity increase monotonely. The following conclusions can be made according to the above calculations:1. If the product of the viscosity and the integral velocity, namely ηoU keeps constant, the change of either one would make no effect on the contact fatigue lifetime of gear transmission.2. When Hertz contact stress pH≥1.0GPa, the maximum value τmax of main shear stress of subsurface in the gear contact area is always lower than the corresponding values of Hertz contact stress no matter how lubricant viscosity changes. Especially when pH≥1.3GPa, rmax decreases monotonely with the increase of ηoU. This has indicated theoretically that lubrication is conducive to the decreasing of gear contact stress. It is obligatory to state here that the relationship between τmaK and lubricant viscosity ηo is consistent with parabolic when gear rotating velocity keeps constant under the load capacity condition that Hertz contact stress pH is no more than1.3. It means that gear contact stress decreases with viscosity increases once it exceeds a finite amount. This theory result has been verified with the results of previous researches. However, the lubrication coefficient recommended in the mentioned international standard (ISO/6336:1996) and corresponding China national standard (GB3840-1997) always increases monotonely with the increasing of lubricant’s viscosity. Therefore, it is reasonably considered that the mentioned coefficient is usually only suitable for the transmission of heavy load gear.3. The X coordinate value of the maximum value of subsurface shear stress τmax moves towards the exit part with the increasing of ηoU. While Z coordinate value of the maximum value of subsurface shear stress τmax keeps constant in0.8with the increasing of ηoU.4. The value of second stress Pmax waves heavily with the increasing of ηoU the the increasing amplitude decreases with the load increasing. The position of Pmax moves towards the entry part and its X coordinate value declines with the increasing of ηoU.5. The multiple regression theory is applied to deduce regression equation between the maximum value of subsurface main shear stress in gear contact area τmax and the product ηoU under heavy load condition. Based on this equation, the integral effect of lubricant viscosity and gear rotating velocity shall be qualitatively taken into consideration when the transmission of heavy load gear is designed to make the design of contact fatigue strength of gear transmission perfect and increase the reliability of design result positively.The shortcoming of this paper lies on that all the calculations are made under full film lubrication condition and take no consideration of the time-dependent nature caused by tooth surface roughness effect.