Abstract:
A four wheel drive system for bilaterally symmetric vehicles is characterized by separate drive systems for the front and rear wheels. Each drive system is operable to independently drive each wheel. The rear wheels of the vehicle are steering wheels which are connected with the vehicle frame for independent rotation about a vertical axis. Steering and driving of the wheels is controlled by a controller. The combination of independent steering for the rear wheels and independent powering of all four wheels provides the vehicle with a zero turning radius for improved mobility as well as improved traction on unstable surfaces.

Description:
[0001]     This application claims the benefit of U.S. provisional application No. 60/714,588 filed Sep. 7, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     There has always been a trade off between traction and maneuverability in vehicles, particularly heavy equipment vehicles, tractors, and the like. Tractors were generally not considered for four wheel drive systems because it was believed that deliberate weight distribution and bilaterally independent brakes on the drive wheels would be sufficient for maximum tractive force. Moreover, there are difficulties in providing a driving force to the front steering wheels of such vehicles. If the steering wheels were given drive capability, it would be advantageous to make the steering wheels larger for greater traction. However, larger front steering wheels would interfere with the framework of the vehicle, thereby limiting their usefulness. In addition, conventional differentials required to drive the steering wheels are complex and expensive. With the advent of zero turning radius vehicles capable of turning in a reverse direction within the length of the vehicle, there is a greater need for improved traction. Some such vehicles have steering wheels which pivot about vertical axes in place of less directionally stable caster wheels. The present invention relates to zero turning radius vehicles in which the steering wheels are both driven and rotated about their vertical axes to control the steering and propulsion forces thereto.  
       BRIEF DESCRIPTION OF THE PRIOR ART  
       [0003]     Various devices have been used to decrease the turning radius of four wheel drive tractors. Kubota uses a Bi-Speed turning mechanism wherein variable speeds are used for the front axle relative to the back. Kubota also manufactures a system where clutches are used in place of a differential between the rear wheels so that in tight turns, the system is practically a four wheel vehicle with the three outside wheels being driven. Others have used steerable axles where the entire axle pivots about its vertical axis. Other methods of decreasing turning radii while maintaining four wheel drive include four wheel steering, skid steering such as provided by Ingersoll Rand, and articulated steering. However, other than skid steering, none of these devices allow a zero turn radius. Moreover, the zero turning radius vehicles known in the art are not provided with four wheel drive because of the complexities of driving the steering wheels as set forth above. The present invention was developed in order to overcome these and other drawbacks of the prior devices by providing a four wheel drive vehicle which also has zero turning radius capabilities.  
       SUMMARY OF THE INVENTION  
       [0004]     The four wheel drive system of the invention is suitable for use with bilaterally symmetrical vehicles having a pair of front wheels and a pair of rear wheels. Each of the rear wheels is connected with the vehicle for rotation about a vertical axis to provide steering for the vehicle. A first drive system independently rotates the front wheels about horizontal axes and a second drive system independently rotates the rear wheels about horizontal axes to propel the vehicle in forward and reverse directions. The second drive system further independently rotates the rear wheels about their vertical axes to steer the vehicle. The rear wheels can be turned through 360 degrees to provide the vehicle with a zero turning radius. A controller is connected with the first and second drive systems to control the operation thereof to propel and steer the vehicle in a desired direction. The controller is also preferably connected with each wheel to receive signals from the wheels corresponding to the horizontal rotation thereof and to the turning position of the rear wheels.  
         [0005]     The drive systems comprise hydraulic pumps or motors. The second drive system includes two pumps, one for delivering a propulsion force to the rear wheels and another for delivering a steering force to the rear wheels.  
         [0006]     In an alternate configuration, the first drive system for the front wheels includes a power source, a power splitter connected with the power source, and a pair of opposed drive mechanisms connected with the power splitter and the front wheels. Each drive mechanism includes a first differential clutch connected with the power splitter and a second differential clutch connected with the first differential clutch. First and second braking devices are connected between the first and second differential clutches to control the delivery of power from the power splitter to each wheel. Operation of the first and second braking devices controls the degree and direction of driving power being delivered from the power source to the wheels. In a preferred embodiment, hydraulic pumps act as the braking devices and appropriate valves and controls avoid the need for separate pumps. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]     Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in the light of the accompanying drawing, in which:  
         [0008]      FIG. 1  is a schematic view of the four wheel drive system according to the invention;  
         [0009]      FIGS. 2 and 3  are front and perspective views, respectively, of one of the rear steering wheels of the vehicle;  
         [0010]      FIG. 4  is a schematic view of a preferred drive system for the front wheels of the vehicle; and  
         [0011]      FIG. 5  is a schematic view of a preferred drive system for the rear wheels of the vehicle. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Referring to  FIG. 1 , the four wheel drive system according to the invention will be described. The system is used to drive and steer a bilaterally symmetrical vehicle. Such vehicles include farm equipment, tractors, lawn mowers, and other types of heavy machinery which include front wheels  2 ,  4  and rear wheels  6 ,  8 . A first drive system  10  is connected with the front wheels to independently rotate each wheel in forward or reverse directions about horizontal axes to propel the vehicle. The wheels have a fixed vertical axis relative to the vehicle and thus are not steerable except that limited steering is achieved by rotating the wheels  2  and  4  at different speeds or in different directions. A controller  12  is connected with the first drive system to control the delivery of drive power or force to the front wheels via the first drive system.  
         [0013]     A second drive system  14  is connected with the controller and with the rear wheels  6  and  8  in order to independently rotate each wheel in forward or reverse directions about horizontal axes to propel the vehicle. Thus, the vehicle has four wheel drive capability under control of the controller  12  which assists the vehicle in traversing unstable ground.  
         [0014]     Unlike the front wheels, the rear wheels  6 ,  8  are steering wheels. In order to accommodate steering thereof, the rear wheels are connected with the vehicle for rotation about a vertical axis. In  FIGS. 2 and 3  is shown one of the rear wheels  6  which has a horizontal axis H which passes through the center of the axle  16  on which the wheel is mounted and a vertical axis V which passes through the center of a vertical shaft  18 . The vertical shaft is connected at its lower end with a frame  20  which passes above and down the side of the wheel. The horizontal axle  16  is connected with the lower end of the frame. As shown more particularly in  FIG. 3 , the wheel vertical shaft  18  passes through the frame  22  of a vehicle and is rotatable with respect to the frame. This allows the wheel  6  to be rotated about its vertical axis V in order to steer the vehicle. Preferably, the wheel is rotatable through 360 degrees. The other rear wheel  8  is connected in the same manner as the wheel  6  to be steerable as well.  
         [0015]     A steering system is connected between the controller  12  and the rear wheels  6  and  8  as shown in  FIG. 1 . The steering system is operable to independently rotate the wheels about their vertical axes. Because the rear wheels are rotatable through 360 degrees, the vehicle has a zero turning radius which allows it to turn around within its length. A motor  26  is operated by the controller  12  to rotate the vertical shaft  18  to turn the associated steering wheel  6 . The motor can be an electric motor, a hydraulic motor, or an air motor as will be appreciated by those of ordinary skill in the art. According to a preferred embodiment of the invention as shown in  FIG. 3 , the motor  26  comprises a hydraulic pump. The controller controls valves (not shown) between the pump  26  and the vertical shaft  18  to control the delivery of hydraulic fluid to the shaft to rotate the shaft in opposite directions.  
         [0016]     Each wheel is also connected with the controller  12  to provide feedback signals which indicate the direction and speed of horizontal rotation of each wheel as well as the rotational position of the rear wheels with respect to their vertical axes. The feedback signals can be used to override certain inputs to the controller by the operator of the vehicle to prevent the vehicle from being driven in a dangerous manner. For example, as the forward speed of the vehicle increases, the turning radius of the vehicle is increased to prevent the vehicle from tipping over by turning too sharply at high speed.  
         [0017]     A preferred drive system for the front wheels of the vehicle is shown in  FIG. 4 . This drive system is disclosed in U.S. Pat. No. 6,957,731. The driving system of  FIG. 4  includes a power source  102  connected with a power splitter  104  via a drive shaft  106 . The power splitter includes output shafts  108  and  110 , which are connected to the first differential clutches  112  and  114 , respectively. First differentials  112  and  114  have a single input which receives power from the output shafts  108 ,  110  from the power splitter, and two outputs, which are connected to second differentials  130 ,  132  via their respective output shafts. More particularly, output shafts  134  and  136  connect first differential  112  to the second differential  130 , while output shafts  138  and  140  connect first differential  114  to second differential  132 . Output shafts  134 ,  138  rotate in a first direction, while output shafts  136 ,  140  rotate in the opposite direction. Second differentials  116 ,  118  have two inputs and a single output. Because the splitter delivers the same output to the differential clutches, the drive system is bilaterally symmetrical.  
         [0018]     Each second differential clutch has an output drive shaft  116 ,  118  connected with a wheel  2 ,  4 . The operation of each differential clutch is individually controlled by braking devices  124 ,  126 ,  142 , and  144 . The braking devices may be of any conventional type including pumps or generators. Braking action from one of the braking devices slows or stops the rotation of the corresponding shaft spinning in a first direction and engages the differential clutch of the first differential shaft with which it is connected, thereby allowing power to be transmitted to the output shaft rotating in the opposite direction. The rotating shaft transmits power turning the other output shaft which is also connected to the second differential. Because the braking devices are controlled independently via the controller  12  by the operator, the amount of driving force applied to each wheel from the power source can be controlled to propel the wheel in a forward or reverse direction as well as to provide coordinated steering of the vehicle.  
         [0019]     For example, if braking elements  124 ,  126  are engaged, power from the power source is transmitted to second differential clutches  130 ,  132  by output shafts  136  and  140 , respectively, thereby facilitating reverse motion. If braking elements  142 ,  144  are engaged, power from the power source is transmitted to second differential clutches  130 ,  132  by output shafts  134  and  138 , respectively, thereby providing forward motion.  
         [0020]     To execute a right turn, braking elements  142 ,  126  are engaged, thereby causing power from the power source to transmit to second differential clutches  130 ,  132  by output shafts  134  and  140 , respectively, thereby facilitating a zero-radius turn to the right. A zero-radius left turn is accomplished by engaging braking elements  124  and  144 .  
         [0021]     The preferred drive and steering system for the rear steering wheels  6  and  8  will be described with reference to  FIG. 5 . A first variable displacement pump  150  is connected with the controller and feeds hydraulic fluid to a first hydraulic motor  152  which in turn is connected with the horizontal axle  16  of one of the rear wheels. The connection would be via the wheel frame  20 . A second variable displacement pump  154  feeds hydraulic fluid to a second hydraulic motor  156  which is connected with the vertical axle  18  of the wheel under control of the controller. The first hydraulic motor  152  actuates the horizontal axle to drive the wheel forward or backward. The second hydraulic motor  156  actuates the vertical axle to turn the wheel left or right. Feedback signals are sent from the horizontal  16  and vertical  18  axles to the controller  12 .  
         [0022]     Vertical axis rotation is achieved by indexing the second hydraulic motor  156 . An encoder sends a signal to the controller  12  so that the controller knows the angle at which each of the rear wheels is pointing. With such an arrangement, the rear wheels can rotate about their vertical axes to a much greater angle than traditional traction or steering wheels.  
         [0023]     Hydraulic motors can also be used to independently drive the front wheels  2  and  4  in place of the drive system shown in  FIG. 4 . The motors would be supplied by a variable displacement pump under control of the controller in a manner similar to the horizontal axle drive system of  FIG. 5  for the rear wheels. In addition, each of the drive and steering motors may comprise electric or pneumatic motors as is known in the art.  
         [0024]     In operation, an operator can use a joystick, not shown, to provide input to the controller which is used to propel and steer the vehicle. If the operator pushes the joystick all the way to the left, the vehicle would sit still but the rear wheels will rotate about their vertical axes and would be pointed at 90 degrees to the direction of the front wheels and also therefore 90 degrees to where the machine is facing. If the operator then moved the joystick forward, still holding it all the way to the left, the vehicle would begin making a very sharp left hand turn. The right front wheel would roll forward, and the left front wheel would roll backward, both under power. The rear wheels would drive forward, that is, they would push the rear of the vehicle to the right at a right angle to where the vehicle is facing. If the operator then pulls the joystick backwards, still with the joystick all the way to the left, the left front wheel  2  would be driven forward and the right front wheel  4  would be driven in reverse.  
         [0025]     The rear wheels would not change relative to their vertical axes, but their horizontal rotation would be reversed. If the drive system for the front wheels is of the type shown in  FIG. 4 , the pumps which are used in place of brakes can be used to provide the hydraulic oil to drive the rear wheels. A valve is integrated into the system so that the oil goes in the proper direction  
         [0000]     past the wheel motors on the rear wheels.  
         [0026]     While the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the inventive concepts set forth above.