The contact stiffness of a mechanical bonding surface is an important parameter in determining the normal and radial contact force.To improve the calculation accuracy of the contact force model,the surface roughness of the bonding surface and the energy loss that necessarily occurs during the impact process should be considered comprehensively.To study the normal contact force of a revolute joint with clearance more accurately in the case of dry friction,a nonlinear stiffness coefficient model considering the surface roughness was established based on fractal theory,which considers the elastic,elastic-plastic,and plastic deformations of the asperities of the contact surface during the contact process.On this basis,a modified nonlinear spring damping model was established based on the Lankarani-Nikravesh contact force model.The laws influencing the surface roughness,recovery coefficient,initial velocity,and clearance size on the impact force were revealed,and were compared with the Lankarani-Nikravesh model and a hybrid model using MATLAB.The maximum impact force was obtained using a modified contact force model under different initial velocities,different clearances,and different degrees of surface roughness,and the calculated results were then compared with the experiment results.This study indicates that the modified model can be used more widely than other models,and is suitable for both large and small clearances.In particular,the modified model is more accurate when calculating the contact force of a revolute joint with a small clearance.