This paper discusses background studies needed for the prediction of sliding resistance on gear teeth. Various
elastohydrodynamic and mixed lubrication theories for coefficient of traction are examined. These theories are
evaluated with respect to experimental results from two-disk tests for several parameters that simulate the continuously
varying properties during gear engagement. For one mesh cycle, coefficient of friction for each tooth in contact is
predicted as a function of roll angle. Normal load is estimated by the Load Distribution Program and is combined with
friction coefficient to compute the total friction force. Dynamics tests are carried out on a pair of gears to measure shaft
displacement in the off-line of action direction. Using the bearing stiffnessand shaft motions, measured friction force is
calculated and compared with analytical results predicted from lubrication principles. Frequency response of the test rig
is used to eliminate the influence of cross-coupling terms with line-of-action forces and filters are applied to the
measured data to curtail the dynamic effects of the test rig. Based on the comparison of theoretical predictions and test
results on a spur gear pair, the lubrication models are evaluated, with special emphasis being placed on the dynamic
modeling of friction.