Abstract:
A hydraulic control system for steering rear caster wheels of a work machine. The control system has a steering mode that proportionally controls the steering of rear caster wheels while compensating the circuit to keep steering performance independent from the load generated by the rear caster wheels, a no steering mode that maintains the position of the rear caster wheels in the absence of a steering command, and a freewheeling mode that permits the rear caster wheels to rotate freely 360°.

Description:
BACKGROUND OF THE INVENTION 
     This invention is directed to a hydraulic control steering system and more particularly to a hydraulic control steering system for a rear caster wheel. 
     Work machines sometimes include one or more caster wheels which are carried by a machine frame and are free to rotate about a generally vertical axis 360°. As an example, self-propelled Windrowers are typically driven through a dual-path hydrostatic system. Speed changes are made by adjusting the speed of both drive (front) wheels simultaneously. Direction changes are made by adjusting the relative speed of the drive wheels. The rear wheels are castered to allow the machine to pivot during direction changes. 
     When direction changes are made, hydraulic pressure builds in one drive wheel circuit to increase speed and is reduced in the other drive wheel to lower the speed. The pressure difference continues until the inertia of the machine and the turn resistance of the casters are overcome. When turn resistance is high enough to create a delay in reaction to the steering wheel input, control of the machine can be difficult. 
     Due to the machine&#39;s inherent instability, the steering system is not ideal for high speed transport. Better machine control is needed at higher speeds without sacrificing the spin steer agility of the system at lower speeds. Also, improved machine handling is needed when towing an attachment. Thus, a control system is desired that addresses these deficiencies. 
     An objective of the present invention is to provide a control system that provides proportional steering control to rear caster wheels while also allowing for the wheels to rotate freely upon command. 
     Another objective of the present invention is to provide a control system that provides better machine control when towing an attachment. 
     A still further objective of the present invention is to provide better machine control and higher transport speed when driving on a road. 
     These and other objectives will be apparent to those skilled in the art based upon the following written description, drawings and claims. 
     SUMMARY OF THE INVENTION 
     A control system for steering one or more rear caster wheels on a work machine that has at least one hydraulic circuit. The hydraulic circuit has three modes of operation. The first is a normal steering mode where the rear caster wheel is turned in response to a steering command independent of the load generated by the rear caster wheel. The second is a no steering mode where the position of the rear caster wheel is maintained. The third is a freewheeling steering command that permits the rear caster wheel to spin freely 360°. 
     Preferably, in the normal steering mode, the hydraulic circuit has fluid supplied from a pump port that flows through a proportional solenoid valve to a rear caster wheel steering cylinder port. A logic element valve is added to the circuit so that the circuit is compensated and the steering performance will be independent of both the load generated by the rear caster wheel and the input pressure. For the freewheeling steering mode, a solenoid valve and a plurality of pilot operated check valves are included in the circuit such that when the solenoid valve is energized, a signal is sent to open the pilot operated check valves providing a free path for fluid to flow from the rear caster wheel cylinders to a discharge tank and also from the tank to the rear caster wheels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a hydraulic circuit of a control system for steering a rear caster wheel to be supplied with hydraulic fluid using, for example, a fixed displacement pump; and 
         FIG. 2  is a schematic view of a hydraulic circuit of a control system for steering a rear caster wheel to be supplied with hydraulic fluid using, for example, a variable displacement pump. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a hydraulic control system  10  is shown to control the steering of the rear caster wheel(s) of a work machine. While not shown, the rear caster wheel typically is coupled to a chassis and has a sensor and one or more steering cylinders configured to control a steering position of the rear caster wheel. 
     The control system  10  has a fluid supply line  12  connected to a gear pump (not shown) through port  14 . Fluid line  12  is connected to and in fluid communication with logic element valves  16  and  18 . Logic element valve  16  is connected to a proportional solenoid valve  20  via line  22  and logic element valve  18  is connected to proportional solenoid valve  24  via line  25 . 
     Proportional solenoid valve  20  is connected to counterbalance valve  26  via line  28  and counterbalance valve  30  via line  32 . A shuttle valve  34  is connected to lines  28  and  32  and is also connected to logic element valve  16  via line  50 . 
     Similarly, proportional solenoid valve  24  is connected to counterbalance valve  38  via line  40  and counterbalance valve  42  via line  44 . A shuttle valve  46  is connected to lines  40  and  44  and is also connected to logic element valve  18  via line  51 . Shuttle valves  34  and  46  are also connected to a third shuttle valve  52 , which is also connected between logic element valves  16  and  18 . 
     Counterbalance valve  26  is connected to a first port  54  connected to a cylinder (not shown) for steering a left rear caster wheel via line  55  and counterbalance valve  30  is connected to a second port  56  connected to the same cylinder via line  58 . Counterbalance valve  38  is connected to a first port  60  connected to a cylinder (not shown) for steering a right rear caster wheel via line  62  and counterbalance valve  42  is connected to a second port  64  connected to the same cylinder via line  66 . 
     Line  55  is also connected to pilot operated check valve  70  and line  58  is connected to pilot operated check valve  74 . Line  62  is also connected to pilot operated check valve  76  and line  66  is also connected to pilot operated check valve  72 . Check valves  70  and  74  are connected to line  78  which is connected to a left pilot pressure gauge port  80  at one end and to a solenoid valve  82  at an opposite end. Check valves  72  and  76  are connected to line  84  which is connected to a right pilot pressure gauge port  86  at one end and to a solenoid check valve  88  at an opposite end. 
     Solenoid valves  82  and  88  are connected to a pilot pump port  90  and pilot pressure gauge port  92  via line  94 . 
     Line  68 , which serves as a discharge tank line, is connected to a tank port  98 . Proportional valves  20  and  24 , solenoid valves  82  and  88 , and check pilot operated check valves  70 ,  72 ,  74 , and  76  are also connected to line  68 . Shuttle valve  52  is connected to discharge line  68  via line  100  having a relief valve  102  and to logic element valve  104 . Logic element valve  104  is connected to line  106 , which is connected to supply line  12  and discharge line  68 . 
     Alternatively in  FIG. 2 , shuttle valve  52  is connected to a load sensing port  108  via line  100  while relief valve  102  is connected to supply line  12  and discharge line  68  via line  106 . 
     In operation, when a normal steering mode is commanded, fluid flows from pump port  14  through line  12  to logic element valves  16  and  18 . From logic element valves  16  and  18 , fluid flows to proportional solenoid valves  20  and  24  via lines  22  and  25 , respectively. From valve  20 , fluid flows to first port  54  through counterbalance valve  26  to provide pressure to act upon the cylinder of the left rear caster wheel. During this operation, a pressure signal is also sent from line  28  through shuttle valve  34 . From valve  34 , the signal is sent to logic element valve  16  and to shuttle valve  52  through line  50 . Fluid also flows from valve  24  to port  60  through counterbalance valve  38  to provide pressure to act upon the cylinder of the right rear caster wheel. During this operation, a pressure signal is also sent from line  40  through shuttle valve  46 . From valve  46 , the signal is sent to logic element valve  18  and to shuttle valve  52  through line  51 . 
     Fluid also flows from port  56  through counterbalance valve  30 , through valve  20  and back to discharge tank port  98 . Likewise, fluid flows from port  64  through counterbalance valve  42  through valve  24  and back to discharge tank port  98 . To reverse the direction of the steer, the opposite coil of proportional valves  20  and  24  will be energized, providing pressure to ports  56  and  64  and a path to tank port  98  for ports  54  and  60  respectively in the same manner as previously disclosed. 
     In this manner, proportional solenoid valves  20  and  24  are compensated by logic element valves  16  and  18  so that steering performance is independent from the load generated by the rear caster wheels. The degree of the steering command will be controlled proportionally by the input current that is applied to valves  20  and  24 . A zero current command will result in no steering, while a full current command will result in a full turn command to the caster wheel. When there is no steering commanded, counterbalance valves  26 ,  30 ,  38  and  42  and pilot operated check valves  70 ,  72 ,  74 , and  76  will maintain the position of the rear caster wheels. 
     When a command is given to allow the rear caster wheels to freewheel, solenoid valves  82  and  88  are energized which sends a pilot signal to open pilot operated check valves  70 ,  72 ,  74  and  76 . When these valves are opened, fluid from ports  54 ,  56 ,  60  and  64  has a free path to and from tank port  98  which allows the rear caster wheels to spin freely. 
     Additionally in  FIG. 1 , logic element valve  104  has multiple functions in the system. When no steering is commanded, pressure in line  100  will be minimal. This will allow logic element valve  104  to open a path from line  12  (inlet pressure) to line  68  (tank) and bypass the inlet flow at low pressure. As steering is commanded through energizing valve  20  and/or  24 , a pressure signal will be sent from line  28 / 32  through valve  34  to valve  52  and from line  40 / 44  through valve  46  to valve  52 . Valve  52  will select the higher of the two pressure signals and send this result to logic element valve  104  through line  100 . Logic element valve  104  will modulate open and closed based on the highest pressure signal demanded by the steering operation. It will only provide the flow needed by the operation and allow the remainder to bypass from line  12  to  68  at lower pressure with improved efficiency. Logic element valve  104  in conjunction with relief valve  102  will also act to provide system relief protection. 
     The system disclosed in  FIG. 2  will operate in the same way as the system in  FIG. 1  with the exception of items  102  and  104 . The output of valve  52  will provide a pressure signal to a load-sensing pump via line  100  and port  108 . Relief valve  102  is now connected to input line  12  and will provide protection for the system against pressure spikes. 
     Thus, a control system for steering a rear caster wheel has been disclosed that at the very least meets all the stated objectives.