Effect of elastohydrodynamic film thickness on a wear model for worm gears

Worm gears used for power transmission commonly consist of a steel worm running against a bronze or phosphor bronze worm wheel. The wheel teeth are usually manufactured using an 'oversize' bob so that the initial elastic contact between the teeth is in the form of an ellipse with lubricant entrainment predominantly in the major axis direction. As a result of this unfavourable contact geometry, the lubricant film generated between the worm and wheel teeth is relatively thin. the kinematic configuration is close to that of simple sliding as the velocity of the wheel tooth surface relative to the nominal contact point is very low in comparison with that of the worm. The combination of poor film forming, high sliding and a soft gear material (to avoid scuffing) leads to the continuous wear seen in these gears. The article presents results obtained using a model for the prediction of wear for this configuration based on the well-known Archard wear law extended to take the variation of pressure and film thickness over the contact area into account in determining the wear rate. The pressure and film thickness distributions are obtained from elastohydrodynamic lubrication (EHL) modelling of the contact over the meshing cycle. Wear patterns are presented corresponding to different film thickness sensitivities in the wear formula, and these predictions, together with the calculated increase in backlash due to wear, give a basis for calibrating the model against experimental wear tests.