ABSTRACTThe prediction of churning power loss has been a difficult problem in the analysis of spur gears. Thus, an analytical prediction model based on the redefinition of churning power loss and energy transformation is proposed to estimate the churning power of spur gears. Churning power loss is defined as the combination of the power loss due to the drag on the end face, the power loss due to the tangential flow, the power loss due to the acceleration of lubricants in the tooth space, and the power loss due to the centrifugal force. Several comparisons of prediction and experimental results are made and good agreement of those is obtained. Finally, the components of churning power loss under different gears and work conditions are analyzed, and the influences of each part on churning power loss are obtained.