Abstract:
A mechanical front wheel drive roller wedging control system includes a 4WD switch in a vehicle operator station, a roller cage drag mechanism electrically activated by the 4WD switch and providing a drag on a roller cage if the 4WD switch is in an on position, and a throttle pedal switch actuated by the throttle pedal and that deactivates the roller cage drag mechanism when the throttle pedal is released.

Description:
FIELD OF THE INVENTION 
     This invention relates to utility vehicles and other off road machines having a mechanical front wheel drive (MFWD). More specifically, the invention relates to an MFWD control system to prevent wedging of rollers in roller clutches. 
     BACKGROUND OF THE INVENTION 
     Utility vehicles and other off road machines may have an MFWD to provide power to the front wheels if the rear wheels slip and lose traction. With the operator controlled 4WD switch in the on position, roller clutches in the front axle gear case will engage and provide drive to the front wheels if rear wheel speed is faster than front wheel speed. With no speed differential between the front and rear wheels, the front drive does not engage and the vehicle is driven by the rear wheels only. 
     With the 4WD switch in the on position and the transmission in either forward or reverse, voltage may be supplied to a roller cage drag mechanism such as a solenoid. The voltage is stopped when the machine is shifted into neutral. For example, if a solenoid is energized, a plunger may be extended into the path of the rotating tabs of an actuating washer, stopping the outer washer. Wave washers between the inner and outer washers place a drag between the outer (stopped) washer and the inner washer. The inner washer is keyed to a roller brake assembly which is connected to a roller cage by tabs. 
     The roller cage lies inside a ring gear hub. Each side of the roller cage contains cylindrical rollers that roll around the output hub to each axle. The rollers may be generally cylindrical or may have other geometries. 
     The drag imposed on the roller cage through the wave washers and tabs forces the rollers slightly off center in the openings of the roller cage. When the rear tires slip and spin more than about 15% faster than the front tires, the rollers move against ramps in the rotating ring gear hub and are forced inward. The inward movement presses the rollers against the left and right output hubs. At this point, all components begin rotating together. 
     Alternatively, the roller cage drag mechanism may be an electromagnet instead of a solenoid. The 4WD switch may energize an electromagnet that imposes drag on an armature plate, and through direct contact with the plate, to the roller cage. 
     When the vehicle stops and the operator shifts the transmission from forward to reverse, the voltage to the roller cage drag mechanism is momentarily stopped (neutral switch opened). This allows the springs on each roller to rotate the roller cage and rollers back into a neutral position. 
     However, certain driving maneuvers may allow the rollers to become wedged tightly between the ramps in the ring gear and output hub, and not move back to the neutral position. For example, in a reverse panic stop, an operator may drive the vehicle in reverse and then apply the brake suddenly to lock the front wheels and tires. High forces tend to push the rollers further up the ramps where they may become wedged against the output hub. Rollers also may become wedged by driving the vehicle up a sloped surface in forward or reverse, then allowing it to roll back down without applying the brakes or shifting the transmission out of forward or reverse. 
     If a vehicle is driven with wedged rollers, the vehicle drive train may be damaged by high sustained torque transferred through the driveshaft. For example, driveshaft components including the CV joint may be damaged. A mechanical front wheel drive roller wedging control system is needed to prevent wedging of rollers between the ramps in the ring gear and the output hub. 
     SUMMARY OF INVENTION 
     A mechanical front wheel drive roller wedging control system includes a roller cage drag mechanism activated by a 4WD switch to drive at least one front wheel if rear wheel slip occurs, and a throttle pedal switch to deactivate the roller cage drag mechanism when the throttle pedal is released. The system prevents wedging of rollers between the ramps in the ring gear and the output hub. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a mechanical front wheel drive roller wedging control system, partially in cross section, according to a first embodiment of the invention. 
         FIG. 2  is an exploded perspective view of a MFWD system with the mechanical front wheel drive roller wedging control system according to a first embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a first embodiment shown in  FIGS. 1-2 , MFWD roller wedging control system  100  includes roller cage drag mechanism  101 , throttle pedal switch  104 , and  4 WD switch  106 . The roller cage drag mechanism may be attached or mounted to the MFWD system  107 . Throttle pedal switch  104  may be provided on or adjacent throttle pedal  112 .  4 WD switch  106  may be located on the dash or operator station of the utility vehicle. Electrical wiring harness  110  may connect the roller cage drag mechanism, throttle pedal switch and  4 WD switch in a circuit connected with a power supply on the vehicle. When the  4 WD switch is in the on position and the throttle pedal switch is actuated by depressing the throttle pedal, electrical power may be provided through the circuit to activate the roller cage drag mechanism. 
     In one embodiment, the MFWD system  107  may include a roller cage  116  with a plurality of cylindrical rollers  118 , or rollers having other geometries. Each side of the roller cage contains rollers  118  that roll around an output hub  122  to each axle. The roller cage lies inside a ring gear hub  120 , which may be mounted inside a housing  108  and enclosed by a cover plate  113 . Extension  109  may be connected to the front driveshaft. 
     In one embodiment, the operator may use 4WD switch  106  to electrically activate roller cage drag mechanism  101 . The drag mechanism imposes drag on the roller cage  116  to force the rollers  118  slightly off center in the openings of the roller cage. When the rear tires slip and spin more than about 15% faster than the front tires, the rollers move against ramps  123  in the rotating ring gear hub  120  and are forced inward. The inward movement presses the rollers against the left and right output hubs  122 . 
     In one embodiment, the roller cage drag mechanism may be a solenoid  102 . The solenoid may include plunger  114  that may be extended to engage rotating tabs of the outer or actuating washer  119  when the solenoid is powered. When the extended plunger engages the tabs of outer or actuating washer  119 , wave washers  121  place a drag between the outer (stopped) washer  119  and the inner washer  124 . The inner washer may be keyed to a roller brake assembly  125  which is connected to roller cage  116  by tabs. 
     In one embodiment, if the operator releases the throttle pedal  112 , the throttle pedal switch  104  cuts power to the roller cage drag mechanism. If the roller cage drag mechanism is a solenoid, plunger  114  may be retracted out of engagement with the outer or actuating washer  119 . As a result, wave washers  121  stop imposing a drag on the roller cage  116 , and the rollers  118  move back to the neutral position before they become wedged between the ramps  123  in ring gear  120  and the output hubs  122 . The plunger  114  may remain in the retracted position and the rollers cannot reengage the output hub  122  until the operator depresses the throttle pedal again to actuate the throttle pedal switch while the 4WD switch  106  remains on. 
     In a second or alternative embodiment, the roller cage drag mechanism may include an electromagnet. When the 4WD switch is in the on position and the throttle pedal switch is actuated by depressing the throttle pedal  112 , electrical power may be provided through a circuit to the electromagnet. The electromagnet may impose drag on an armature plate, and through direct contact with the plate, to the roller cage. If the operator releases the throttle pedal  112 , the throttle pedal switch  104  cuts power to the electromagnet. The electromagnet releases the armature plate, which stops imposing a drag on the roller cage  116 , and the rollers  118  move back to the neutral position before they become wedged between ramps  123  in the ring gear  120  and the output hub  122 . The armature plate stays released and the rollers cannot move back up the ramps  123  to engage the output hubs  122  until the operator depresses the throttle pedal again to actuate the throttle pedal switch again while the 4WD switch  106  is on. 
     The MFWD roller wedging control system may reduce or eliminate the roller wedging problem by employing a throttle pedal switch to cut power to a roller cage drag mechanism such as a solenoid or electromagnet before the rollers become wedged between the ring gear and output hubs. The throttle pedal switch cuts power immediately, and before an operator can apply vehicle brakes and before high forces urge the rollers into a wedged position. 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.