Fatigue wear may take place even where gear oil provides a satisfactory lubricating film and there seems to be some question as to what quality of a gear lubricant is responsible for decreasing fatigue wear of gear sets. Hutt^30 presented data from an IAE machine which showed that a 190 per cent increase in fatigue life resulted from an increase in viscosity from 90 to 140 cs at 158 degree (F) . On the other hand, Hundere^29 is of the opinion that it is not viscosity as measured by standard viscometers that controls the effect on surface fatigue. Davidson and KU^20 likewise state: It was found that lubricant viscosity, at least as measured by the conventional viscometric method, did not have as predominant an effect on gear tooth surface fatigue as some other undetermined lubricant characteristics.
In tests conducted by these investigators the lubricant having the highest rating on a Ryder Gear Test ran three to four times as long before pitting occurred as did the other oils examined.
