Bi-Directional Overrunning Clutch: Operation: Notes

In order to achieve all-wheel drive operation in reverse, the overrunning clutch locking functional direction must be reversible. The bi-directional overrunning clutch (BOC) changes the operational mode direction depending on the propeller shaft direction. The propeller shaft rotates in the clockwise (when viewed from the front) direction when the vehicle is moving forward, which indexes the BOC to the forward overrunning position. When the vehicle is in reverse, the propeller shaft will rotate counter-clockwise and index the BOC to the reverse overrunning position.

The BOC acts as a mechanical stator. It is active (transmitting torque), or it is not active and in overrunning mode (not transmitting torque). This "all or nothing" approach to torque transfer would cause a sudden application of all available power to the rear wheels, which is not desirable. Therefore it is run in series with a viscous coupler to smooth, dampen, and limit the transmission of torque to the rear axle and to prevent a step style torque input to the rear axle.