Therefore, when the conditions of Eq. (14) or Eq. (18) are satisfied, the worm gear achieves backdrivability. Now, the condition that is satisfied faster is considered. The specifications of the worm gear are shown in Table 1. Using the lead angle γ = 7.18◦ and the friction coefficient µ = 0.175 in Eqs. (14) and (18), �>7.94 and �>0.05 can be obtained respectively. Therefore, when the vibration is added to the axis of the worm wheel Wz , the backdrive is created by the slippage. Finally, we compare Eqs. (7), (10), (14) and (18). In these equations, is the ratio of the exciting force Fz to the pushing force F between the gear teeth generated by the torque of the worm wheel. Therefore, when the exciting force is added to the axis of the worm wheel, it can be backdriven by a smaller exciting force.