Differential mechanisms are conventionally used to transmit driving forces to a pair of drive wheels of a vehicle and to permit said wheels to change their speed of rotation and travel when the vehicle is turning. Under certain road conditions such as mud, ice, snow, or rough irregular terrain, the traction of one drive wheel with the ground surface may differ from the traction of the other drive wheel. Such difference in traction with the differential mechanism in operation may result in a loss of driving force imparted to one of the drive wheels. In some instances, one drive wheel may slip to such an extent that it may spin freely with a complete loss of driving force at that wheel. Substantial driving power is thereby lost. Under such conditions, locking of the differential to prevent differential operation permits transmission of driving forces through the locked differential mechanism directly and substantially equally to the wheels.
Locking devices for differential mechanisms have been employed heretofore to alleviate such conditions. A simple prior proposed locking device has included a lock pin moved into engagement with a hole in one of the parts of the differential mechanism. The shear stress imparted to such a pin was very great and undesirable because such stress often resulted in shearing of the lock pin and a resumption of differential operation.
Another type of prior proposed locking device for a differential mechanism included the movement of a locking collar splined to a half axle and movable axially to engage a toothed locking member fixed for rotation with a rotatable differential cage, U.S. Pat. No. 3,215,000. Another prior proposed locking device for a tractor transmission included the axial movement of a sliding second pinion gear along a drive shaft having a fixed pinion gear for engagement with juxtaposed bevel gears on spaced brake drums, U.S. Pat. No. 2,836,084.