This invention relates to an axle assembly having multiple wheels at each end of an axle, and more specifically, the invention relates to a mechanism permitting relative rotation between the wheels during a turn. 
Axle assemblies having dual wheels on either end of the axle have been used to increase the load bearing capability of heavy duty vehicles.  Typically the pair of wheels on each end of the axle assembly is secured together so that they rotate together about an axis.  Some heavy duty vehicles, such as lift trucks, undergo numerous turning maneuvers which wear the tires significantly.  The tire wear is caused when the tires scrub, or drag, since the wheels that are secured together must travel different distances at the inside and outside of the turning radius.  Tire wear and maintenance on heavy duty lift trucks due to scrub cost thousands of dollars annually per vehicle. 
Dual wheel assembly designs have been proposed that permit the wheels to rotate relative to one another to reduce scrubbing during vehicle turns while driving at least one of the wheels to propel the vehicle.  Certain driving conditions require that both wheels be driven to provide enough traction to propel the vehicle and its load.  Some designs have been developed which utilize a differential gear set between the wheels so that both wheels may be driven.  These designs either permit the wheels to move freely relative to one another, or include complicated devices for locking the wheels together.  Moreover, only one of the dual wheels may be driven.  Heavy duty vehicles such as lift trucks typically do not have suspensions so that if the non-driven wheel is raised off the ground by a road hazard the driven wheel cannot transmit torque to the road to propel the vehicle.  However, prior art designs do not provide a mechanism that will lock wheels together that are rotatable relative to one another for driving and/or braking conditions.  Therefore, what is needed is an inexpensive mechanism that permits the wheels to be selectively locked together to provide increased traction and permit the wheels to rotate relative to one another during a turn. 
The present invention provides a differential mechanism for a dual wheel assembly including a spindle having a drive axle defining a first rotational axis.  First and second wheels are supported on the spindle adjacent to one another.  The wheels are driven by the drive axle and are rotatable relative to one another about the first rotational axis.  A gear assembly having a second rotational axis transverse to the first rotational axis mechanically couples the wheels.  A biasing assembly engages the gear assembly thereby preventing relative rotation between the wheels when a torque is applied to one of the wheels below a desired amount from a road input.  That is, the biasing assembly and gear assembly cooperate with one another to lock the first and second wheels together so that they are driven together about the first rotational axis by the drive axle.  This is desirable when the vehicle is traveling a straight path to provide maximum traction.  However, when a torque is applied to one of the wheels above the desired amount, such as during a vehicle turn in which scrub commonly occurs, the biasing assembly and gear assembly permit the first and second wheels to rotate relative from one another. 
Accordingly, the above invention provides an inexpensive mechanism that permits the wheels to be selectively locked together to provide increased traction while permitting the wheels to rotate relative to one another during a vehicle turn.