Abstract
Slow-speed strain records, which were obtained by means of strain gages positioned at gear tooth fillets, show sudden changes in the strain values as the contact passes through the pitch point. However, the calculated values of tooth fillet strains, which are due to the application of a pure constant normal load at different positions along the tooth profile and obtained by means of the Finite Element Method, do not show such changes. It is, therefore, being assumed that the sudden changes indicated by the strain records are caused by the reversal of the frictional force which occurs at the pitch point. Based on the above observation, a technique for the determination of the instantaneous coefficient of friction at gear tooth contact is suggested. In this technique, tooth surface loads (i.e. frictional and normal loads), and hence coefficient of friction, are related to the strains at tooth fillets by means of influence coefficients. These influence coefficients, which represent tooth fillet strains when pure constant normal and pure constant tangential loads are applied at different positions along tooth profile, were determined by the Finite Element Method. The technique yields realistic values for the coefficient of friction at gear tooth contact, and its accuracy depends on accurate measurements of tooth fillet strains. The results obtained for the instantaneous coefficient of friction show that, as the contact moves away from the pitch point along the tooth face, the coefficient of friction increases sharply to reach a maximum value at a position close to the pitch point and then falls gradually as the contact proceeds towards the tooth tip. Similar variation is obtained along the tooth flank. Also, the results obtained show that, at any position of contact along the tooth profile, the coefficient of friction increases as the tooth load increases and decreases as the test rig speed increases. These observations agree with the observations concluded from friction measurements made on disc machines. The maximum values obtained for the coefficient of friction at different loads and speeds were compared with disc machine results and it is found that the results obtained are slightly greater. This increase in the calculated values of gear tooth friction may be attributed to the unsteadiness effect involved in gear teeth contact.--AA.