A vacuum pump includes a rotor assembly mounted on a driven shaft, and a motor for rotating a drive shaft in forward and reverse directions. A drive member on the drive shaft engages a driven member on the driven shaft. Each member has first and second impact surfaces. The members are configured to permit at least one quarter of a revolution of the drive member relative to the driven member in either the forward or the reverse direction before one of the impact surfaces of the drive member impacts upon a corresponding impact surface of the driven member. This enables the drive member to acquire sufficient angular momentum before it impacts the driven member such that the amount of energy transferred to the driven shaft upon impact can be sufficient to free a pump that has become locked by process deposits.