Plastic deformation of balls in safety coupling by collision with V-Hole was investigated in the current study. Generally, when the applied torque is greater than the maximum allowable torque, balls in V-Hole get out from the holes and the coupling loses the torque transfer capability. After balls are out from the V-Holes, the balls and V-Hole rotate at a different velocity. When balls meet the next V-Hole, they collide into the wall of the V-Hole. Due to this collision, plastic deformation and wear take place. The plastic deformation and wear may reduce the torque transfer capability of the safety coupling. The reduction in torque transfer capability was observed in the experiment. In this study, plastic deformation of balls and flange was investigated through dynamic analysis of the safety coupling. Also, the effect of relative rotational velocity on the plastic deformation was investigated.