Abstract:
A hydraulic valve assembly, for use in a hydrostatic transmission, for controlling fluid transfer between a first, a second and a third line, wherein two of the lines define first and second pressure lines, within a closed-loop circuit. The valve assembly comprises: a valve body having ports in communication with the three lines; a spool bore; a valve spool, adapted for sealing reciprocation within the spool bore, having a first and second end portion, a connecting portion, and a first and second bypass orifice within the valve spool; and dampers for centering the valve spool. This spool is movable from a neutral position occurring when the fluid pressure forces in the first and second pressure lines are substantially similar, to a first or a second position occurring when the fluid pressure force in the first pressure line is greater or less than that in the second pressure line, respectively. The bypass orifices are enabled in the neutral position, but are substantially disabled in the first and second positions. A hydraulic system utilizing the valve assembly and a method for increasing the width of the dead band of the hydrostatic transmission in a neutral mode of operation are also set forth.

Description:
CROSS-REFERENCE TO RELATED CASES  
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Application Serial No. 60/395,865, filed Jul. 12, 2002, the disclosure of which is expressly incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a valve assembly and method for increasing the width of the dead band of a hydrostatic transmission in a neutral mode of operation without impairing the performance of the hydrostatic transmission in operating modes. The present invention further relates to a hydraulic system including the above mentioned valve assembly.  
         BACKGROUND OF THE INVENTION  
         [0003]    Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines, and offer a stepless control of the machine&#39;s speed. A typical hydrostatic transmission system includes a variable displacement main pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is connected directly or through suitable gearing to the vehicle&#39;s wheels or tracks. Acceleration and deceleration of the transmission are controlled by varying the displacement of the main pump from its neutral position. The present invention relates generally to the hydrostatic transmission and, more specifically, to the hydraulic pump/motor having integrated valves for providing a smoother operation during the acceleration phase of the transmission operation near its neutral position.  
           [0004]    The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with the pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. A charge pump is added to the hydraulic circuit in order to charge the closed-circuit with hydraulic fluid through check valves, thus making up for possible lost fluid due to internal leakage. Other valves can be added to the closed-circuit. For example, high pressure relief valves can be used to protect the hydrostatic transmission from overloading during its operation, bypass valves can be used to allow oil to be routed from one side of the transmission to the other side without significant resistance, and hot-oil shuttle valves can be used to reduce the loop temperature by connecting the low pressure side of the closed loop to a drain, thus allowing replenishment with fresh, cooled replacement hydraulic fluid.  
           [0005]    In hydrostatic applications, an over center variable displacement axial piston pump is used. The displacement of the pump is determined by the size and number of pistons, as well as the stroke length. A control handle enables the operator to control the direction and amount of flow from the pump. When an operator pushes the handle in one direction, the pump delivers flow for one direction of motor operation. When an operator pulls the handle in the opposite direction, the pump delivers flow for the opposite direction. To avoid a rough, jerky start of the motor, the prior art has utilized an orifice with a fixed diameter that is added to the closed-loop circuit to widen the width of the dead band of the hydrostatic transmission. The dead band of a hydrostatic transmission is the non-response range of the transmission near its neutral position where the motor will not turn over due to internal cross-port leakage across the bypass orifice. The orifice creates a bypass flow passage for the closed-loop, increases the dead band of the transmission, and allows the motor to start moving smoothly when the transmission is originally at neutral position. The size of the orifice is very important and the optimum diameter can be determined by carefully checking the change of stoking effects on the machine due to the change of orifice diameter. The orifice can also be integrated onto other hydraulic components, for example the aforementioned valves, within the closed-loop circuit.  
           [0006]    Although the additional bypass orifice helps a machine obtain smooth operation near the neutral position of the hydrostatic transmission, there are disadvantages if the bypass orifice is fixed. A fixed bypass orifice allows a certain amount of flow routed from the high pressure side to the low pressure side of the closed-loop during all phases of the transmission&#39;s operations. This unwanted cross-port leakage not only reduces the overall efficiency of the hydrostatic transmission, but also generates substantial heat that increases the operating temperature of the closed loop. This can cause a safety issue for the machine and reduces its service life. An additional cooling device can be added, but this increases the cost and presents possible encumbrances when space is limited. It is desired that an orifice only performs its cross-port bypassing near the neutral position of the hydrostatic transmission, and then is disabled during the continuous operation of the motor.  
           [0007]    Prior art, such as U.S. Pat. No. 3,740,950 to Polaski sets forth an example of a valve block design for use in a hydrostatic transmission application that consists of a cross-port bypass passage and two check valves interconnected by a spring. Flow though the valve is shut off when the spring between the two check valves is compressed. When one of the check valves is seated, flow through the bypass passages, as well as all flow through the valve block, is obstructed. This valve block design does not allow continued charging fluid to reach the low-pressure side without use of separate make-up check valves. Another prior art reference, U.S. Pat. No. 6,295,811 to Mangamo et al., also sets forth a design, which utilizes a bypass orifice in a valve for use in hydrostatic transmission applications. This design differs from the present invention in that the orifice can be disabled, but separate check valves are needed.  
         SUMMARY OF THE PRESENT INVENTION  
         [0008]    The present invention provides a hydraulic valve assembly, comprised of a shuttle valve with integrated bypass orifices and an optional check valve connected at each end, for use in a hydrostatic transmission in order to provide improved efficiency, cooler operation, a longer life expectancy, as well as a smoother start-up for the transmission. This invention overcomes the obstacle of controlling the fluid flow through the bypass orifices during the operation and neutral cycles of the hydrostatic transmission.  
           [0009]    A feature of the present invention is to provide a hydraulic valve assembly for use in a hydrostatic transmission for controlling fluid transfer between a first, second and third line within a closed-loop circuit, wherein two of the lines define a first and second pressure line and are located at similar longitudinal distances from the remaining line which is rotationally displaced relative to the first and second pressure lines. The valve assembly is comprised of a valve body having a first port for connection to the remaining line, a second port for connection to one of the first and second pressure lines, and a third port for connection to the other of the first and second pressure line, the valve body also includes a spool bore in fluid communication with the first, second and third lines. The valve assembly further includes a valve spool, adapted for sealing reciprocation within the spool bore, having a first end portion, a second end portion, a connecting portion with a cross-sectional area smaller than the cross-section of the first and second end portions, a first bypass orifice within the valve spool extending between the first end portion and the connecting portion, and a second bypass orifice within the valve spool extending between the second end portion and the connecting portion.  
           [0010]    The valve spool is movable from a neutral position in which the valve spool is longitudinally centered within the spool bore and where the pressure forces in the first and second pressure lines are substantially similar, to a first position occurring when the pressure forces in the first pressure line is greater than the pressure forces in the second pressure line, or to a second position occurring when the pressure forces in the first pressure line are less than the pressure forces in the second pressure line. During each of these positions, the connecting portion is in fluid communication with at least a portion of the first port. While in the neutral valve spool position, the first bypass orifice is aligned with the first pressure line for fluid communication with the remaining line and the second bypass orifice is aligned with the second pressure line for fluid communication with the remaining line. While in the first valve spool position, the first and second bypass orifices are at least substantially disabled and the connecting portion is in fluid communication with one of the first and second pressure lines. While in the second valve spool position, the first and the second bypass orifices are at least substantially disabled and the connecting portion is in fluid communication with the other of the first and second pressure lines. Dampers are located at both ends of the valve spool for centering the valve spool relative the to the remaining line while in the neutral valve spool position.  
           [0011]    In the noted valve assembly the substantially disabling of the first and second bypass orifices occurs as a result of the orifice ends in the valve spool end portions being in a juxtaposed relationship with the valve bore during the first and second position of the valve spool. In one embodiment of the noted valve assembly each of the first and second bypass orifices have a cross-sectional area as large as that of the inlet line. One of the noted valve assemblies utilizes springs for use as the dampers. Also, the volume of fluid transfer, while the valve spool is in the neutral position, is less than the volume of fluid transfer while the valve spool is in one of the first or second positions. During movement of the noted valve assembly, the first and second bypass orifices are disabled simultaneously when the valve spool reaches one of the first and second positions, and the first and second bypass orifices are enabled simultaneously when the valve spool reaches the neutral position.  
           [0012]    In one of the noted valve assemblies the first line is an inlet line for a charge pump outlet fluid and the second and third lines are outlet lines. Furthermore, the first port is longitudinally centered relative to the second and third ports. This valve assembly includes a length of the first bypass orifice, located in the valve spool first end portion, having a cross-section smaller than a length of the first bypass orifice located in the valve spool connecting portion, and includes a length of the second bypass orifice, located in the valve spool second end portion, having a cross-section smaller than a length of the second bypass orifice located in the connecting portion. In another version of this noted valve assembly the distance from the connecting portion to the first bypass orifice on the first end of the valve spool is equal to the diameter of the second port, and the distance from the connecting portion to the second bypass orifice on the second end of the valve spool is equal to the diameter of the third port.  
           [0013]    In another version of the noted valve assembly the first and second lines are inlet lines and the third line is an exhaust line. In this version, the third port is longitudinally centered between the first and second ports. Also the distance from the connecting portion to the first bypass orifice on the first end of the valve spool is equal to the diameter of the first port, and the distance from the connecting portion to the second bypass orifice on the second end of the valve spool is equal to the diameter of the second port.  
           [0014]    Another feature of the present invention includes having a hydraulic valve assembly similar to the previously noted assembly wherein fluid transfer is controlled from a single inlet line to a first and second outlet line within a closed-loop assembly. The valve assembly is comprised of a valve body having a first port connected to the inlet line, a second port connected to the first outlet line, a third port connected to the second outlet line, and a spool bore in fluid communication with the inlet line, first outline line and the second outlet line. This assembly is further comprised of a unitary valve spool, adapted for sealing movement within the spool bore, having a first end portion, a second end portion and a connecting portion having a cross-sectional area smaller than that of the first and second end portions, the valve spool having at least one orifice in each of the first and second end portions in communication with the connecting portion which is always in fluid communication with at least a portion of the first port. The valve spool is longitudinally movable, via fluid pressure, within the spool bore from a neutral position where the fluid pressure forces acting on the first and second end portions are approximately equal to a first position where the fluid pressure forces acting on the first end portion is greater than the fluid pressure forces acting on the second end portion, or to a second position where the fluid pressure forces acting on the first end portion are less than the fluid pressure forces acting on the second end portion.  
           [0015]    The hydraulic valve assembly further includes a first check valve, in physical contact with the first end portion of the valve spool, having a fully open position permitting fluid transfer from the inlet line to the second outlet line when the valve spool is in the second position and having a closed position when the valve spool is in the first position. This assembly also includes a second check valve, in physical contact with the second end portion of the valve spool, having a fully open position permitting fluid transfer from the inlet line to the first outlet line with the valve spool is in the first position and having a closed position when the valve spool is in the second position. Fluid transfer occurs from the inlet line through the orifices in each of the first and second end portions of the valve spool to the first and second outlet lines when the valve spool is in the neutral position, and transfer is substantially stopped through the orifices of both end portions when the valve spool is in either the first or second positions.  
           [0016]    This noted valve assembly includes a spool bore comprised of a central first cross-sectional portion interposed between two second larger cross-sectional end portions wherein each intersection between the first and second cross-sectional portions defines a valve seat. In one version of this assembly the check valves are comprised of a check ball and a spring adapted to bias said ball into sealing engagement with an associated valve seat. Movement of the valve spool from the first position to the neutral position or to the second position dislodges the second checkball from its associated valve seat. Movement of the valve spool from the second position to the neutral or first position dislodges the first check ball from its associated valve seat.  
           [0017]    Another feature of the present invention includes having a hydraulic system for use with a hydrostatic transmission comprising, in combination, a variable displacement pump, a hydraulic motor, a hydraulic circuit operatively interconnecting the main pump and the motor, a charge pump, within the circuit, having an outlet line, and a valve block, within the circuit, having an inlet line in fluid communication with the charge pump outlet line, and having a first and second outlet line in fluid communication with the hydraulic circuit. The valve block is comprised of a valve body having a first port connected with the inlet line, a second port connected with the first outlet line, a third port connected with the second outlet line, and a spool bore in fluid communication with the inlet, first outlet, and second outlet lines.  
           [0018]    This valve block further includes a valve spool, adapted for sealing movement within the spool bore, having a first end portion, a second end portion, and a connecting portion having a cross sectional area smaller than that of the first and second end portions. The valve spool has at least one orifice in each of the first and second end portions in communication with the connecting portion and the connecting portion is in fluid communication with at least a portion of the first port at all times. The valve spool is longitudinally movable, via fluid pressure, within the spool bore from a neutral position where the fluid pressure forces acting on the first and second end portions are approximately equal to a first position where the fluid pressure forces acting on the first end portion are greater than the fluid pressure forces acting on the second end portion, or to a second position where the fluid pressure forces acting on the first end portion are less than the fluid pressure forces acting on the second end portion. The at least one orifice in each of the first and second end portions has fluid flow therethrough when the valve spool is in the neutral position and has substantially no fluid flow therethrough when the valve spool is in either the first or second positions.  
           [0019]    The noted valve block also includes a first check valve in physical contact with the valve spool first end portion, having a fully open position when the valve spool connecting portion is in fluid communication with both the inlet line and the first outlet line thus permitting fluid transfer from the inlet line to the first outlet line when the valve spool is in the second position, and having a closed position when the valve spool is in the first position. The valve block further includes a second check valve in physical contact with the valve spool second end portion, having a fully open position when the valve spool connecting portion is in fluid communication with both the inlet line and the second outlet line thus permitting fluid transfer from the inlet line to the second outlet line when the valve spool is in the first position, and having a closed position when the valve spool is in the second position.  
           [0020]    Another version of the noted hydraulic system includes valve spool orifices that are at least substantially disabled simultaneously when the valve spool reaches one of the first and second positions, and are enabled simultaneously when the valve spool reaches the neutral position. In the noted system the first port is longitudinally centered relative to the second and third ports.  
           [0021]    Another feature of the present invention includes a method for increasing the width of the dead band of the hydrostatic transmission in a neutral mode of operation without impairing the performance of the hydrostatic transmission in non-neutral modes of operation, where the hydrostatic transmission includes a variable displacement main pump, a hydraulic motor, a hydraulic circuit operatively interconnecting the main pump and the motor, a charge pump interconnected within the circuit having an outlet line, and a valve block operatively interconnected with the circuit. The valve block, which has an inlet line in communication with the charge pump outlet line and a first and a second outlet line in communication with the hydraulic circuit, further includes a valve body having a first port connected with the inlet line, a second port connected with the first outlet line, and a third port connected with the second outlet line. The valve block also includes a valve spool adapted for sealing longitudinal movement within the spool bore, having a first end portion, a second end portion and a connecting portion having a cross-sectional profile smaller than that of the first and second portions. Dampers center the valve spool in a neutral mode of operation.  
           [0022]    The method comprises: including a first bypass orifice within the valve spool extending between the first end portion and the connecting portion; including a second bypass orifice within the valve spool extending between the second end portion and the connecting portion; keeping the connecting portion in fluid communication with the first port at all times; permitting substantially equal fluid flow from the first port, via the first and second bypass orifices, to the first and second outlet ports, respectively, in the neutral mode of operation when the fluid forces acting on the first and second end portions are about equal; and shifting the valve spool from the neutral mode of operation to the non-neutral mode of operation during which the fluid forces acting on the first and second end portions are unequal, to thereby at least substantially disable the fluid flows via the first and second bypass orifices while simultaneously permitting fluid flows from the inlet line to one of the first and second outlet ports.  
           [0023]    The noted method also includes locating the valve spool in a first position where the pressure in the first outlet line is greater than the pressure in the second outlet line and in which fluid flows from the inlet line to the second outlet line, or locating the valve spool in a second position where the pressure in the first outlet line is less than the pressure in the second outlet line and in which fluid flows from the inlet line to the first outlet line. The noted method further includes preventing cavitation within the hydraulic circuit when the fluid flows from the inlet line to one of the first and second outlet ports. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a hydraulic schematic of a typical prior art hydrostatic transmission closed-loop circuit.  
         [0025]    [0025]FIG. 2 is a hydraulic schematic of a typical prior art hydrostatic transmission closed-loop circuit having a fixed bypass orifice interposed between both sides of the closed-loop, as well as, in the alternative, having fixed orifices integrated into other hydraulic components of the hydrostatic transmission.  
         [0026]    [0026]FIG. 3 is a hydraulic schematic of a special valve block of the present invention having a directional valve and a bypass orifice.  
         [0027]    [0027]FIG. 4 is a hydraulic schematic diagram of a first embodiment of the present invention showing a hydrostatic transmission closed-loop circuit with a special valve block, consisting of two check valves, two bypass orifices, and a directional valve.  
         [0028]    [0028]FIG. 5 is a cross-sectional view of the actual design of the special valve block schematically shown in FIG. 4.  
         [0029]    [0029]FIG. 5 a  is an enlarged version of the elliptical area circumscribed in FIG. 5, showing the two check valves, two bypass orifices, and directional valve in greater detail.  
         [0030]    [0030]FIG. 5 b  is an elevational view of the valve spool of one embodiment of the present invention.  
         [0031]    [0031]FIG. 6 is a cross-sectional view, substantially similar to that of FIG. 5, showing the special valve block in its neutral position, where the fluid pressure in lines  23  and  24  are approximately equal.  
         [0032]    [0032]FIG. 6 a  is a cross-sectional view of the special valve block of FIG. 6 shown in the position where the fluid pressure in line  23  exceeds the fluid pressure in line  24 .  
         [0033]    [0033]FIG. 6 b  is a cross-sectional view of the special valve block of FIG. 6 shown in the position where the pressure in line  24  exceeds the pressure in line  23 .  
         [0034]    [0034]FIG. 7 is a hydraulic schematic of another embodiment of the present invention showing a hydrostatic transmission closed-loop circuit having a spool type shuttle valve with integrated bypass orifices.  
         [0035]    [0035]FIG. 8 is a cross-sectional view of the actual design of the spool type shuttle valve schematically shown in FIG. 7.  
         [0036]    [0036]FIG. 8 a  is an enlarged version of the elliptical area in FIG. 8, showing the neutral position of the spool shuttle valve with integrated orifices and springs on both ends of the spool.  
         [0037]    [0037]FIG. 8 b  is a view, similar to that of FIG. 8 a , but showing the position of the spool type shuttle valve where the fluid pressure in line  23  exceeds the fluid pressure in line  24 .  
         [0038]    [0038]FIG. 8 c  is a view, similar to that of FIG. 8 a , but showing the position of the shuttle valve where the fluid pressure in line  24  exceeds the fluid pressure in line  23 .  
         [0039]    [0039]FIG. 9 is a hydraulic schematic of a further embodiment of the present invention showing a hydrostatic transmission closed-loop circuit having a hot oil shuttle valve with integrated bypass orifices.  
         [0040]    [0040]FIG. 10 is an elliptical cross-sectional view of the actual design of the hot oil shuttle valve schematically illustrated in FIG. 9 showing the hot oil shuttle valve with integrated orifices and springs on both ends of the valve in a neutral position.  
         [0041]    [0041]FIG. 10 a  is a view, similar to that of FIG. 10, but showing the position of the shuttle valve when the fluid pressure in line  23  is greater than the fluid pressure in line  24 .  
         [0042]    [0042]FIG. 10 b  is a view, similar to that of FIG. 10, but showing the position of the shuttle valve when the fluid pressure in line  24  is greater than the fluid pressure in line  23 .  
         [0043]    [0043]FIG. 11 is a graph showing the efficiencies of a 10 cc pump, as part of the closed-loop circuit, utilizing a conventional bypass orifice.  
         [0044]    [0044]FIG. 12 is a graph showing the efficiencies of a 10 cc pump, as part of the closed-loop circuit, utilizing the special valve block of this invention as illustrated in FIGS. 4, 5 and  6 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0045]    [0045]FIG. 1 shows a schematic of a typical prior art hydrostatic transmission closed-loop circuit  10  consisting of a variable displacement main pump  12  and a hydraulic motor, such as a fixed displacement motor  14 , connected to each other by lines  23  and  24 . Pump  12  can be an over center axial piston or bent-axis piston pump. With an over center variable displacement axial piston pump, the displacement of the pump is determined by the size and number of pistons, as well as the stroke length. An input shaft  11  for pump  12  is driven by a prime mover (not shown), such as an internal combustion engine or an electrical motor, at a predetermined speed in a predetermined direction. Although the size and number of pistons are fixed, changing the piston stroke length can change the displacement of the pump. The stroke length is determined by the angle of pump&#39;s  12  swashplate, which can be tilted by any corresponding stroke controlling device, for example a trunnion shaft (not shown). The trunnion shaft is connected to a control handle through a linkage installed in the machine. When an operator pushes the handle forward, pump  12  delivers flow for one direction of motor  14  operation. Changing the displacement of pump  12  will change its output flow rate, which controls the speed of motor  14 . Moving the swashplate or yoke (not shown) of pump  12  overcenter will automatically reverse the flow out of pump  12 , thus reversing the direction of motor  14 . Depending on the direction of the overcenter movement of the pump swashplate (or yoke) line  23  (or line  24 ) can be a high pressure supply line or a low pressure return line.  
         [0046]    A charge pump  16 , also driven via input shaft  11 , supplies additional hydraulic fluid to closed-loop circuit  10  at the rate of approximately 10-30% of the flow rate that main pump  12  can deliver. Charge pump  16  draws fluid from a reservoir  13  which can be passed through a filter  15  and supplies this fluid into closed-loop circuit  10  through a conduit line  17  by way of one-way check valves  18  and  19  to compensate for any possible flow loss due to internal leakage. Charge pump  16  also continuously provides fluid flow for cooling main pump  12  through a conduit line including a cooling orifice  21  during the operation of main pump  12 . A charge pump relief valve  22  is used to provide a relief path to reservoir  13  when more than required flow from charge pump  16  cannot enter closed loop circuit  10 , and also regulates the pressure of the low pressure side of circuit  10 . Relief valves  26  and  27  are positioned between lines  23  and  24  and protect each line from pressure overload during the operation. Valve  26  provides relief for line  23  and valve  27  provides relief for line  24 .  
         [0047]    In certain applications, closed-loop circuit  10  will also have a bypass valve  29  positioned between lines  23  and  24  in order to transfer oil from one line to the other. The use of bypass valve  29  will enable motor  14  to turn over with little resistance when it is desirable, for example, to move a machine for a short distance without operating the transmission. Again, in certain applications, a hot-oil shuttle valve  31  is provided to reduce the loop temperature by connecting the low pressure side of closed-loop circuit  10  to a drain line. This valve allows a certain percentage of the hot oil discharging from motor  14  to flow back to reservoir  13  for cooling and filtering, and replaces the discharged hot oil with cooled, filtered oil from charge pump  16 . Line  32  connects a forward/reverse charge pressure relief valve  33  with hot oil shuttle valve  31  to provide a lower resistance on the low pressure side of closed-loop circuit  10 . Relief valve  33  maintains a certain amount of fluid pressure on the low pressure side of circuit  10 . Since charge pump relief valve  22  is in parallel with relief valve  33 , charge pump relief valve  22  should be set at a pressure higher than that of relief valve  33 . When the transmission is in neutral and hot oil shuttle valve  31  is centered, charge pump flow is relieved over relief valve  22 .  
         [0048]    In order to avoid a rough, jerky start of the machine (in the forward or reverse direction), a fixed orifice  35   a , shown in FIG. 2 and interposed between the high and low pressure sides of circuit  10 , can be used to widen the width of the dead band of the hydrostatic transmission. The dead band of a hydrostatic transmission can be defined as the non-response range of the transmission near its neutral position where motor  14  will not be turned due to internal cross-port leakage of the transmission at very low fluid flow, near the neutral swashplate position. Adding an orifice, such as orifice  35   a , creates a bypass flow passage in the closed-loop. Increasing the dead band of the transmission allows the machine to start moving smoothly when the transmission is originally at neutral position. The size of orifice  35   a  is important and the optimum diameter is generally determined by carefully checking the change of stoking effects on the machine due to the change of orifice diameter. Normally the orifice diameter is in the range of 0.5 to 1.0 mm. Two fixed orifices  35   b  can also be integrated into other hydraulic components of the hydrostatic transmission, as also shown in FIG. 2. For example, in lieu of using previously described interposed fixed orifice  35   a , fixed orifices  35   b  are integrated into system check valves  18  and  19 . If desired, fixed orifices  35   c  are integrated into high pressure relief valves  26  and  27 . Furthermore fixed orifice  35   d  can be integrated into bypass valve  29 . Finally fixed orifice  35   e  can be integrated into hot oil shuttle valve  31 .  
         [0049]    Although hydrostatic transmissions with the noted fixed orifices  35   a  to  35   e , as shown in FIG. 2, help a machine obtain smooth operation near its neutral position, there are several drawbacks. The use of one or more of fixed orifices  35   a  to  35   e  provide a flow path from the high pressure side to the low pressure side of closed-loop circuit  10  during all phases of the transmission&#39;s operation. While a fixed orifice  35   a  to  35   e  enhances smooth operation near the neutral position of the hydrostatic transmission, it also hinders the operation, when not in the neutral position, by continuing to allow a certain amount of fluid flow once the machine is operating. This then unwanted cross-port leakage reduces the overall efficiency of the hydrostatic transmission since the effective capacity of flow delivery of pump  12  is decreased. Cross-port leakage also generates substantial heat, which has the negative effect of increasing the operating temperature of closed-loop circuit  10 . This excessive operating temperature is not only a safety issue for machine operators, but also reduces the service life of the machine. Adding an additional oil cooling device not only increases the cost of the machine, but also adds complexity and may encumber possible space and location limitations.  
         [0050]    It is thus desirable that an orifice performs its “cross-port bypassing” function only near the neutral or dead-band position of the hydrostatic transmission and that thereafter the orifice be disabled during continuous operation of the machine away from the neutral or dead-band position. FIG. 3 shows a schematic of a special valve block  37  having a directional valve  38  with an orifice  35 . Movement of directional valve  38  away from its neutral, or centered, position disables orifice  35 . This orifice disablement occurs during normal operation of the hydrostatic transmission and significantly increases the efficiency of the transmission and substantially reduces the working or operating temperature of closed-loop circuit  10 .  
         [0051]    [0051]FIG. 4 shows a schematic diagram of a hydrostatic transmission closed-loop circuit  10   a  with an integrated special valve block  40 . The componentry of circuit  10   a  is similar to the aforementioned closed-loop circuit  10  in FIG. 1 with the addition of special valve block  40 , in the former, in place of check valves  18  and  19  in the latter. Therefore, the numbering of the remaining componentry in FIG. 4 will be the same as that in FIG. 1. Valve block  40  is comprised of two check valves  41  and  42 , two orifices  43  and  44 , and a directional valve  45 . During operation, charge pump  16  fills both sides of the loop with hydraulic fluid through orifices  43  and  44  when the system operates in its neutral position. A slight amount of swashplate movement caused by operation of the control by an operator will cause main pump  12  to pump fluid into the corresponding side of the loop. Motor  14  will not yet rotate because this flow is so small that it will bypass motor  14  through orifices  43  and  44  and other internal leakage paths in the system without significant pressure build-up. As the operator continues to increase the swashplate angle, the increased fluid pressure will start to turn motor  14 . At that point directional valve  45  shifts so that orifices  43  and  44  are disabled and the appropriate low pressure check valve, either  41  or  42 , is opened. Charge pump  16  then continuously replenishes the closed-loop on the low pressure side through the open check valve,  41  or  42 , with fluid, thus making up for internal leakage throughout the closed-loop. A supply of fluid to the low pressure side also prevents cavitation, which may occur at the pump inlet from a lack of fluid pressure.  
         [0052]    [0052]FIGS. 5 and 5 a  show the actual design of a valve block  40 , schematically shown in FIG. 4, as having orifices  43  and  44  which can be disabled after the start-up of the motor. Valve block  40  is comprised of a valve spool  47 , shown in detail in FIG. 5 b , having two opposed generally cylindrical end portions  48  and  49  with at least one, but preferably multiple equally spaced orifices  43  and  44  in end portions  48  and  49  respectively. Orifices  43  and  44 , which may be of any desired shape, are illustrated in FIG. 5 b  as being generally triangular in cross section, and are located in a peripheral band portion on the outermost edge of end portions  48  and  49 . Similarly orifices  43 ,  44  can be placed on other locations on the end portions  48 ,  49  as long as the disabling function occurs. Valve spool  47 , whose end portions  48 ,  49  also include relieved portions  48   a ,  49   a  respectively are interconnected by a smaller central cross-sectional area columnar portion  46 , with valve spool  47  being interposed between opposed spring-loaded check valves  41  and  42  as best seen in FIG. 5 a . If so desired, orifices  43  and  44  may extend along the full longitudinal extent of valve spool end portions  48  and  49 . The design of check valves  41  and  42  is simple and inexpensive and allows low cost spheres or balls  41   a  and  42   a  to be used as check valve poppets. The use of steel balls  41   a  and  42   a  improves the reliability of sealing of check balls and reduces the cost of valve seat manufacturing compared with other types of valve poppets.  
         [0053]    [0053]FIGS. 6, 6 a , and  6   b  show all three working positions of valve block  40  when the hydrostatic transmission is operated. Position  50  in FIG. 6 shows valve block  40  in a neutral position, i.e., when charge pump  16  (not shown) is supplying a low pressure fluid through line  17  into an inlet port (not shown) of valve block  40 . By virtue of being centered in its associated bore  47   a , valve spool  47  positions both check valve balls  41   a  and  42   a  off their respective seats  54  and  55  and enables similar fluid flow through both orifices  43  and  44  into lines  23  and  24 , respectively, which are connected to outlet ports, not shown, in valve block  40 . In this position the fluid pressure in lines  23  and  24  is approximately equal.  
         [0054]    Position  51  in FIG. 6 a  shows valve block  40  in a non-neutral position where the fluid pressure in line  23  is greater than the fluid pressure in line  24 . Due to this pressure differential, valve spool  47  pushes check valve ball  42   a  completely off its associated seat  55 , while the spring in check valve  41  pushes ball  41   a  into full sealing engagement with its associated valve seat  54 . Since the movement of valve spool  47  opens check valve  42  there is no energy loss in the system due to the pressure that is typically needed to crack open check valve  42 . Since check valve  42  is opened fully, there is substantially less power loss in the system as well. Fluid flow from line  17  will pass through relieved portion  49   a  of spool  47  and flows past spool cylindrical portion  49  and past valve seat  55  of open check valve  42  into line  24 . This fluid flow is necessary since it replenishes any fluid lost due to internal leakage. Continued fluid flow through low pressure line  24  ensures that cavitation does not occur at the pump inlet. Any purported fluid flow towards higher pressure line  23  is stopped by closed check valve  41 , and specifically by ball  41   a , which is sealingly engaged with valve seat  54 . With spool  47  in position  51 , charge pump  16  can continuously charge closed-loop circuit  10  on the low pressure side (line  24 ). Fluid flow from high pressure line  23  cannot pass check valve  41 , thus disabling orifices  43  in this direction as well.  
         [0055]    Position  52  in FIG. 6 b  shows valve block  40  in a non-neutral position where the fluid pressure in line  24  is greater than the fluid pressure in line  23 . Due to this pressure differential, valve spool  47  pushes check valve ball  41   a  fully away from seat  54 , while the spring in check valve  42  pushes ball  42   a  against valve seat  55 . As a result, charging fluid from line  17  flows through widely opened relieved portion  48   a  and continues into line  23 . Any flow towards line  24  from line  17  will be stopped by check valve  42  which is sealingly engaged with valve seat  55 . Likewise any flow from high pressure line  24  cannot pass check valve  42 , thus disabling orifices  44 . With spool  47  in position  52 , charge pump  16  will continuously charge the low-pressure side (line  23 ) of the closedloop circuit.  
         [0056]    [0056]FIGS. 7, 8,  8   a - c  show another embodiment of the present invention having a valve  60  that performs a function similar to that of the previously described embodiment. Unlike the earlier embodiment where two check valves  41  and  42  are incorporated into valve block  40 , valve  60  of this embodiment takes the form of a spool type shuttle valve having a spool  61  sealingly reciprocatable within a bore  61   a  and having integrated orifices  64  and  65 , each having a receiving end always in communication with a central smaller cross-sectional spool mid-portion  66 . Similar to the previously described embodiment, valve  60  utilizes springs  62  and  63 , which can be compression springs, on opposite ends thereof. Again, like previously described valve block  40 , valve  60  communicates the high and low pressure sides of the closed-loop circuit with charge pump  16 . At very low fluid flow, near the neutral position of the hydrostatic transmission, as depicted by position  67  in FIG. 8 a , leakage across orifices  64  and  65 , the discharge ends of which are, in this position, in communication with lines  23  and  24 , respectively, ensures that both lines  23 ,  24  are equally charged. Position  67  shows valve  60  in a neutral position when charge pump  16  (not shown) is supplying low pressure fluid through line  17 , which is connected to an inlet port  70  in valve block  40 . This low pressure fluid is supplied to both sides of the closed loop through lines  23  and  24 , which are connected to outlet ports  71  and  72 , respectively, in valve block  40 , for loop charging. Centered spool  61  positions both orifices  64  and  65  so that the discharge ends thereof are aligned with lines  23  and  24  respectively. In this position the fluid pressures in lines  23  and  24  are approximately equal.  
         [0057]    When the operator further pushes/pulls the control handle (not shown) from neutral position  67  shown in FIG. 8 a , the increased fluid flow from main pump  12  will increase the pressure differential across valve  60 , causing it to shift and thereby disable both orifices  64  and  65  by moving their discharge ends from communication with lines  23 ,  24  respectively. In non-neutral position  68  in FIG. 8 b , where the fluid pressure in line  23  is greater than the fluid pressure in line  24 , spool  61  is shown biased towards low pressure line  24 . As illustrated, both orifices  64  and  65  are blocked in a juxtaposed position against the wall of valve bore  61   a , spring  63  is compressed, and due to the design of spool  61  by virtue of the use of smaller cross-sectional area spool mid-portion  66 , line  17  is still able to be in fluid communication with line  24 . Fluid flow from line  17  will pass through spool mid-portion  66  and flow into low pressure line  24 . With spool  61  in position  68 , charge pump  16  can continuously charge the closed-loop circuit on the low pressure side, thus replenishing the circuit with fluid that may have been lost due to internal leakage. As previously noted spool mid-portion  66  provides a wide opening for fluid flow to low pressure line  24 , thereby allowing ample fluid into line  24  in order to minimize any power loss. The distance from the discharge end of orifice  65  to spool mid-portion  66  is substantially the same as the diameter of port  72 . Therefore there is no interruption of fluid flow to line  24  when valve  60  shifts in this direction. Fluid will flow from orifice  65 , then from spool mid-portion  66  during this transition.  
         [0058]    When the operator pushes/pulls the control handle (not shown) from the neutral position  67  shown in FIG. 8 a  in the direction in opposition to that of position  68  shown in FIG. 8 b , the pressure differential in lines  23 - 24  will cause valve  60  to shift towards line  23 , which now becomes the low pressure line. Position  69  in FIG. 8 c  shows valve  60  in a position where the fluid pressure in line  24  is greater than the fluid pressure in line  23 . Both integrated orifices  64  and  65  are again blocked in a juxtaposed position against the wall of valve bore  61   a , spring  62  is compressed, and line  17  is able to be in fluid communication with line  23  through spool midportion  66 . Fluid from line  17  can thus only flow through spool mid-portion  66  into low pressure line  23 . As previously noted, due to the design of spool  61  all fluid flow to line  24  is now blocked. With spool  61  in position  69 , charge pump  16  will continuously charge the low-pressure side (line  23 ) of the closed-loop circuit to minimize power loss. The distance from the discharge end of orifice  64  to spool mid-portion  66  is substantially the same as the diameter of port  71  in valve block  40 . Therefore there is no interruption of fluid flow to line  23  when the valve shifts in this direction.  
         [0059]    A further embodiment of this invention is schematically shown in FIG. 9 where a hot oil shuttle valve  73 , similar to the previously noted hot oil shuttle valve  31  in FIG. 1, herein utilizes integrated orifices  75  and  76 . The construction and function of valve  73  is substantially similar to that of previously described valve  60  except that the connecting lines are reversed. While valve  60 , in FIG. 8, utilizes one inlet line  17  and two outlet lines  23 ,  24 , valve  73 , in FIG. 10, utilizes both lines  23  and  24  for inlet flows while line  32  comprises the single outlet conduit, or exhaust line, connected with relief valve  33 . Lines  23  and  24  are connected to inlet ports  87  and  88 , respectively, in the valve body, while line  32  is connected to an outlet port  89  in the valve body. Position  80  shows the actual construction and orientation of valve  73  during low fluid flow from charge pump  16  when the fluid pressures in lines  23  and  24  are approximately equal. Valve spool  61  is centered so that the receiving or inlet ends of orifices  75  and  76  are aligned with lines  23  and  24 , respectively.  
         [0060]    Referring to FIG. 10 a , when the operator activates the stroke controlling device in one direction in order to initiate turning of the motor  14 , main pump  12  will pump fluid into the corresponding side of the loop, either line  23  or  24 . When the increased fluid pressure reaches a predetermined or set value sufficient to turn motor  14 , valve  73  will shift, as shown in non-neutral position  81 , so that orifices  75  and  76  are disabled, or shutoff in a juxtaposed position against the wall of valve bore  61   a , and fluid can flow through low pressure line  24 . Charge pump  16  then continuously charges the closed-loop on the low pressure side through line  24 . Fluid flowing through low pressure line  24  ensures that cavitation does not occur in the hydrostatic transmission system. The distance from the inlet end of orifice  76  to a mid-portion  77  in valve  73  is substantially the same as the diameter of port  88 . Therefore there is no interruption of fluid flow from line  24  when valve  73  shifts in this direction. Fluid will flow from line  24  to orifice  76 , then to mid-portion  77  during this transition.  
         [0061]    Referring to FIG. 10 b , when the operator changes the direction of movement of the control handle, main pump  12  will alter the direction of the fluid flow. When the pressure differential between lines  23  and  24  reaches a predetermined value, valve  73  will move to position  82 . In position  82 , the fluid pressure in line  24  is greater than the fluid pressure in line  23 , thus biasing spool  61  towards low pressure line  23 . As in position  81  (FIG. 10 a ), both orifices,  75  and  76 , are disabled in a juxtaposed position against valve bore  61   a  and pressurized fluid can only reach line  32  through low pressure line  23 . The distance from the inlet end of orifice  75  to mid-portion  77  in valve  73  is substantially the same as the diameter of port  87 . Therefore there is no interruption of fluid flow from line  23  when valve  73  shifts in this direction. Fluid will flow from line  23  to orifice  75 , then to mid-portion  77  during this transition.  
         [0062]    In all hydrostatic transmissions, two check valves are used in order to enable the charge pump to replenish the closed-loop system with fluid during operation. The high pressure side check valve closes while the low pressure side check valve opens allowing for the replenishing fluid to flow into the closed loop. It is undesirable for the replenishing fluid from the charge pump to encounter the resistance of a spring, causing cracking pressure, at the backside of the check valve. Overcoming this cracking pressure of the check valve requires pressure from the charge pump supply. All the embodiments of this invention provide the unique valve design that allows for simultaneous opening of the low pressure side check valve and closing of the high pressure side check valve, while disabling the orifices which perform the desired fluid bypass function when the main pump is at its neutral position. This combination significantly improves the performance efficiency of the hydrostatic transmission not only by eliminating the unwanted cross-port fluid bypass or leakage at normal operation, but also by eliminating the cracking pressure of the make-up check valves. This fluid bypass feature provides a smooth transition of the motor while moving from neutral into forward or reverse motion. The instantaneous opening of the low pressure side check valve also prevents unwanted noise, which may result from pump cavitation due to fluid starvation.  
         [0063]    [0063]FIGS. 11 and 12 illustrate the change of performance efficiencies of two  10  cc pumps with respect to the differential pressure between inlet and outlet ports. FIG. 11 shows the test results of a commercially available pump having a fixed 0.031″ orifice on a make-up check valve. FIG. 12 shows the test results of a pump having a check valve, as detailed in an embodiment of the present invention (shown in FIGS. 4, 5, and  6 ), comprised of orifice spool  47  (having integral bypass orifices  43  and  44 ) interposed between two make-up check valves,  41  and  42  in valve block  40 . Comparing FIG. 12 to FIG. 11, the use of special valve block  40  significantly increases the performance of a hydrostatic pump at normal operational conditions, while still maintaining a bypass function near the neutral position for smooth transition of motor  14 , from neutral to operating, near zero speed. Specifically, for the conventional bypass orifice-equipped pump the efficiencies, illustrated in FIG. 11, show a decrease in pump volumetric efficiency from about 98% at 500 psid to about 79% at 2500 psid, which translates into a drop of about 19%. In contrast thereto, a pump equipped with the disabled bypass orifice  40  (FIGS. 4, 5, and  6 ) of this invention, in FIG. 12, shows a decrease in pump volumetric efficiency from about 99% at 500 psid to about 95% at 2500 psid, a drop of only about 4.0%, while having its overall efficiency increased from about 78% at 500 psid to about 82% at 2500 psid, an increase of about 5.6%. A comparison of FIGS. 11 and 12 also shows that the noted pump of this invention had an initial overall efficiency of about 78% at 500 psid which is equal to the prior art pump overall efficiency of about 78% at 2500 psid vs. 82% at 2500 psid for the noted pump of this invention. A further comparison of the overall efficiencies of the two designs also shows that the maximum efficiency, 86%, of the pump of this invention occurs at a much greater pressure, 2000 psid, compared with the prior art&#39;s pressure of 1500 psid when its peak overall efficiency is but 79%.  
         [0064]    It should be noted that the present invention is not limited to the specified preferred embodiments and principles. Those skilled in the art to which this invention pertains may formulate modifications and alterations to the present invention. These changes, which rely upon the teachings by which this disclosure has advanced, are properly considered within the scope of this invention as defined by the appended claims.