Pump swashplate control assist
BACKGROUND OF THE INVENTION 
     The present invention relates to hydraulic systems including axial piston pumps and, in particular, relates to closed center hydrostatic drive systems with variable displacement axial piston pumps. 
     Closed center hydrostatic drives with variable displacement axial piston pumps require a source of pressure to provide a control pressure for the servo pistons which control the pump swashplate angle. In many such systems, the charging pump pressure is utilized in conjunction with large servo pistons for controlling swashplate angle. Other such systems utilize the output or system pressure as this control pressure with reduced size servos. This system pressure is normally adequate, except when the swashplate is in a neutral or low pump output position or when the pump is supplying fluid to an overrunning load. In these latter situations, the system pressure may be less than what is required for adequate control of the servo pistons or, in otherwords, less than what is required to overcome the friction which tends to resist changes in the position of the swashplate and piston inertia which tends to increase the swashplate angle. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an adequate swashplate servo piston control pressure in a hydrostatic drive having a variable displacement axial piston pump when the inlet and outlet ports are at the same pressure. 
     This and other objects are achieved by the present invention which includes a variable displacement axial piston pump with a servo piston controlled swashplate and intake and output ports. A charge pump provides a pressurized charge fluid to the pump intake port via a shuttle valve which is responsive to fluid pressure in the intake and output ports and which prevents fluid communication between the output port and the charge pump. The shuttle valve provides a flow restriction when the piston pump inlet and outlet ports are at the same pressure. This restriction disappears as the shuttle valve moves to the side in response to pressure from either piston pump port. A displacement control valve controls fluid communication to the servo pistons from a check valve unit. The check valve unit check valve unit includes three check valves, connected to the intake port, the outlet port and the charge pump, respectively, so that the higher of the intake ports, the outlet port and the charge pump pressures is communicated to an inlet of the displacement control valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic of a hydrostatic drive system which includes the present invention. 
     FIG. 2 is a more detailed schematic of a hydrostatic drive system, including the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a conventional hydrostatic drive system 10 includes a variable displacement axial piston pump 12 which rotates in response to a torque applied to an input drive shaft 14 and pumps fluid to and from a hydraulic motor 16. A charge pump 18 supplies pressurized charge fluid to the system 10 via charge valve 20 to make up for fluid losses due to leakage. 
     Referring now to FIG. 2, the axial piston pump 12 includes first and second ports 22 and 24 which alternately function as an inlet and an outlet, depending upon the direction of rotation and position of swashplate 26. Ports 22 and 24 communicate high pressure and return pressure fluid to and from the motor 16 via lines 28 and 30. The position of swashplate 26 is controlled by servo pistons 32 and 34 which receive fluid from control valve 36 which is connected to a manual input receiving linkage 38. 
     The charge pump 18 supplies pressurized charge fluid to inlet port 40 of charge valve 20. This charge fluid is limited to a predetermined pressure level, such as 18 bar, by pressure relief valve 42. The charge valve 20 includes a valve bore 44 which slidably receives a pair of valve members 46 and 48. When the swashplate 26 is in a neutral position and the pressure in lines 28 and 30 is equalized, springs 50 and 52 urge valve members 46 and 48 together to the neutral position, shown in FIG. 2. This causes a restriction to charge valve outlets 54 and 56. The result is that pressure in line 53 is higher than the pressure in ports 22 or 24. Valve member 46 is movable in response to fluid pressure in chamber 58 which is communicated with line 28 via pump port 22. Valve member 48 is movable in response to fluid pressure in chamber 60 which is communicated with line 30 via pump port 24. 
     When valve member 46 is moved in response to pressure in chamber 58, it pushes valve 48 back against its spring 52. This reduces the restriction at outlet 56. When valve 48 is bottomed out in this manner, the restriction at outlet 56 essentially disappears and the pressure at the outlet of charge pump 18 is essentially the same as charge pressure in port 24. Due to symmetry, the same is true when port 24 is at high pressure and port 22 is at charge pressure. 
     A three check valve unit 62 includes a first check valve 64 which permits one-way fluid flow from line 28 and port 22 to control valve 36, a second check valve 66 which permits one-way fluid flow from line 30 and port 24 to control valve 36 and a third check valve 68 which permits one-way fluid flow from charge pump 18 to control valve 36. Thus, check valve unit 62 will communicate the highest of the fluid pressures in lines 28, 30 or charge pump pressure to the control valve 36 for operation of the servo pistons 32 and 34. 
     This system may also be provided with a pressure-regulating means 70 for regulating the fluid pressure in whichever of lines 28 and 30 is the return line to a maximum pressure, which is lower than the pressure provided by charge pump 18, for example, 10 bar. The pressure regulating means 70 includes a valve bore 72 which slidably receives a shuttle valve member 74. Inlet ports 76 and 78 communicate the valve bore 72 with pump ports 22 and 24, respectively. Sensing ports 80 and 82 communicate the ends of valve bore 72 with pump ports 22 and 24, respectively. Outlet 84 communicates valve bore 72 with a reservoir via regulating valve 86. When pump 12 is pumping fluid to and from motor 16 via lines 28 and 30, the high system pressure in one of lines 28 and 30 moves shuttle valve member 74 to open to outlet 84 the one of inlets 76 and 78 which is communicated with the return pressure in the other of lines 28 and 30, thereby limiting the return pressure in that other of lines 28 and 30. At the same time, shuttle valve member 74 closes communication between outlet 84 and the other of inlets 76 and 78. 
     Normally, this system pressure in this one of lines 28 and 30 will be higher than the charge fluid pressure from charge pump 18 and higher than the return pressure in the other of lines 28 and 30. In this case, one of check valves 64 and 66 will communicate this higher system pressure to control valve 36, thus providing a control pressure which is sufficient to operate the servo pistons 32 and 34. 
     However, if the pump 12 is providing fluid to a motor 16 which is under the influence of an overrunning load, or if the swashplate 26 is in a neutral position so that no or very little fluid is flowing between pump 12 and motor 16, then the pressure in both lines 28 and 30 may be lower than the charge fluid pressure and thereby insufficient for operation of the servo pistons 32 and 34. In this case, check valve 68 will communicate the higher charge pump pressure from charge pump 18 to control valve 36 so that there will be sufficient control pressure available to control the position of swashplate 26 under this neutral or overrunning load condition. 
     While the invention has been described in conjunction with a specific embodiment, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications, and variations which fall within the spirit and scope of the appended claims.