Verification of micro pitting occurring process considering frictional force at the gear tooth mesh surface of power transmission gears

Abstract

To reduce weight and improve efficiency of the power transmission, the gears must be miniaturized and lubricant viscosity must be lower. However, these changes will bring about augment of contact pressure and severe lubricating condition. These conditions might occur micro pitting. Accordingly, designing method for micro pitting is required to reduce weight and improve efficiency of the transmission. For designing micro pitting, the calculated flash temperature is used. On the other hand according to ISO/TR, micro pitting can be designed using ratio of sum of arithmetic average roughness and elastic hydraulic lubrication (EHL) minimum oil film thickness. However, in the designing method using flash temperature, it cannot take into account surface roughness and EHL oil film thickness. And if using designing method shown ISO/TR, the limit of each gear has to be measured. So, new theory about gear surface failure was focused. This theory says that false mode of the gear tooth surface is influenced by the frictional force. Based on this theory coefficient of friction was calculated using Matsumoto's formulae and frictional force was compared with micro pitting pattern on gear tooth surface. Results obtained are shown as follows. (1) The pattern of the area micro pitting was occurred is corresponded with the pattern of the area calculated frictional force is higher. (2) The result shown (1) gives that the occurrence of micro pitting has correlation with the frictional force on the tooth surface. (3) The reason why micro pitting is not necessarily grown into pitting is expected that caused load distribution change due to wear of tooth surface.