Abstract:
Improvement in a hydrostatic valve assembly for use in a hydrostatic transmission, for controlling fluid transfer between a first, second and 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 and valve spool reciprocating therewithin, having first and second end portions joined by a connecting portion, and first and second bypass orifices within the valve spool; and dampers for centering the valve spool in a neutral position. The bypass orifices utilize increased cross-sectional areas that permit the passage of substantially the full flow of the charge pump, without using a charge pump relief valve, at a low pressure drop. A hydraulic system utilizing this valve assembly and a method for increasing the transmission efficiency, in the neutral mode, are also set forth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a non-provisional patent application claiming the benefit of the filing date of U.S. Provisional Application No. 60/613,889, filed Sep. 28, 2004, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention pertains to a valve assembly and method for increasing efficiency thereof, in the neutral mode of operation, without impairing the performance in non-neutral operating modes. The present invention further relates to a hydraulic system that includes the noted valve assembly and an improved method of operation. Specifically, a separate charge pump relief valve is eliminated and the valve bypass orifices utilize increased cross-sectional areas to permit the passage of substantially the full flow of the charge pump at a low restriction to flow through these orifices. 
   2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
   Hydrostatic transmissions have many uses, including the propelling of vehicles, such as grass mowing machines, and offer a stepless control of the machine&#39;s speed. A typical hydrostatic transmission system includes a variable displacement main pump coupled in a closed hydraulic circuit with a fixed displacement hydraulic motor. For most applications, the main pump is driven by a prime mover, at a predetermined speed, in a given direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the coupled motor. Main pump outflow can be reversed, thus reversing the directional rotation of the motor. In a vehicle, the motor is connected directly, or via suitable gearing, to the vehicle&#39;s wheels or tracks. Both acceleration and deceleration of the transmission are controlled by varying the displacement of the main pump from its neutral position. 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. Additional valves, such as high pressure relief valves, bypass valves and hot oil shuttle valves, for example, are also often utilized, in a manner well known in the art. The present invention relates specifically to the hydraulic main pump and motor combination having improved integrated valves for providing smoother operation, particularly during the acceleration phase of the transmission, near its neutral position. 
   In hydrostatic transmission applications, an over-center variable displacement main pump is normally utilized, with a control handle enabling the operator to control the direction and amount of flow from the main pump. By pushing the handle in one direction, the main pump delivers flow in one direction of motor operation. By pulling the handle in the opposite direction, the main pump delivers flow for the opposite direction. In order 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 increase the width of the dead band of the hydrostatic transmission. The dead band of a hydrostatic transmission refers to 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. 
   Prior art U.S. Pat. No. 6,837,047 B2, also assigned to the assignee of the present invention, and which will be more fully discussed in the “Detailed Description of the Invention”, sets forth a hydraulic valve assembly, as well as a hydraulic system that utilizes this valve assembly, together with a method for increasing the width of the transmission dead band, wherein the bypass orifices are enabled in the neutral position, but are substantially disabled in non-neutral positions. While this has improved transmission performance, the present invention represents an improvement over these prior art constructions by eliminating the previously-required prior art charge pump relief valve and modifying the hot oil shuttle valve by increasing the sizes of the bypass orifices so as to allow the passage of substantially the full flow of the charge pump, at a low pressure drop, i.e., at a low restriction to flow through these bypass orifices, thus resulting in a less costly and more efficient hydrostatic transmission that also operates at a lower working temperature. 
   The patent literature sets forth a large number of hydrostatic transmission pump/motor systems, including, for example: U.S. Pat. No. 2,961,829 to Weisenbach; U.S. Pat. No. U.S. Pat. No. 3,326,049 to Reinke; U.S. Pat. No. 3,734,225 to Kobald et al.; U.S. Pat. No. 5,211,015 to Schroeder, and U.S. Pat. No. 6,263,670 B1 to Gluck et al. However, none of these prior art structures pertains to the specific structure, system, and method of operation of the present invention. 
   BRIEF SUMMARY OF THE INVENTION 
   Accordingly, in order to overcome the deficiencies of the prior art devices and methods, the present invention provides an improved hydraulic valve assembly that eliminates the previously-required charge pump relief valve and utilizes a modified hot oil shuttle valve, having increased cross-sectional area bypass orifices that allow the passage of substantially the full flow of the charge pump, at low pressure drop, i.e., at a low restriction to flow through these orifices. This results in a less costly and more efficient hydrostatic transmission that also operates at a lower working temperature. 
   Specifically, in terms of structure, a feature of the present invention is to provide a hydraulic system for use with a hydrostatic transmission, comprising in combination: a variable displacement main pump; a hydraulic motor; a closed loop hydraulic circuit, including low and high pressure leg portions, operatively interconnecting the main pump and motor; a charge pump, within the circuit, having an outlet line only to the circuit; a valve block within the circuit, for controlling fluid transfer between a first, second and third line, within the hydraulic circuit, wherein two of the first, second and third lines define first and second pressure lines and are located at substantially similar longitudinal distances from the remaining one of the first, second and third lines, the remaining line being rotationally displaced relative to the first and second pressure lines, the valve block comprising:
         i. a valve body defining 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 lines, the valve body further including a spool bore in fluid communication with the first, second and third lines;   ii. a valve spool adapted for sealing reciprocation within the spool bore, having a first end portion, a second end portion, a connecting portion having a cross-sectional area smaller than the cross-sectional areas 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, the valve spool being 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 are greater than the pressure forces in the second pressure line, or to a second position, occurring when the pressure forces within the first pressure line are less than the pressure forces in the second pressure line, with the connecting portion being in fluid communication with at least a portion of the first port at each of the positions of the valve spool, wherein: 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 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;   iii. dampers located at both ends of the valve spool, for centering the valve spool, relative to the remaining line, in the neutral valve position; and iv. wherein the bypass orifice cross-sectional areas are of a size to allow the passage of substantially the full flow of the charge pump at a low restriction to flow through the bypass orifices; and one of a relief orifice and a low pressure forward/reverse charge pressure relief valve interconnected with the valve block and the hydraulic circuit low pressure leg portion, the bypass orifices exposing both of the hydraulic circuit low and high pressure leg portions to the one of a relief orifice and relief valve when the main pump is substantially centered to stop rotation of the hydraulic motor.       

   In one version thereof, the one of the relief orifice and relief valve is a relief orifice, while in another version thereof, the one of the relief orifice and the relief valve is a relief valve. 
   In a further version, the only fluid that needs to be passed through the bypass orifices is the fluid that is produced as a result of any undesired slight inclination of the angle of an internal swashplate of the main pump, when the main pump is placed in the neutral position by an operator. 
   In a differing version, the first and second bypass orifices have a cross-sectional area sufficient to permit equalization of the fluid pressure between the hydraulic circuit low and high pressure leg portions. In one application, the valve block functions as a hot oil shuttle valve. 
   A further embodiment of this invention pertains to a hydraulic valve assembly for use in a hydrostatic transmission, the transmission including a variable displacement main pump, an interconnected charge pump, an interconnected hydraulic motor, and an interconnecting closed loop hydraulic circuit having low and high pressure leg portions, the hydraulic valve assembly controlling fluid transfer between a first, a second and a third line, within the hydraulic circuit, wherein two of the first, second and third lines define first and second pressure lines and are located at substantially similar longitudinal distances from the remaining one of the first, second and third lines, the remaining line being rotationally displaced relative to the first and second pressure lines, the valve assembly comprising in combination: a valve body defining 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 lines, the valve body further including a spool bore in fluid communication with the first, second and third lines; a valve spool adapted for sealing reciprocation within the spool bore, having a first end portion, a second end portion, a connecting portion having a cross-sectional area smaller than the cross-sectional area 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, the valve spool being 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 are 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, with the connecting portion being in fluid communication with at least a portion of the first port at each of the positions of the valve spool, wherein: 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 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 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, located at both ends of the valve spool, for centering the valve spool, relative to the remaining line, in the neutral valve position; and wherein the first and second bypass orifices have a cross-sectional area sufficient to permit the equalization of the fluid pressure between the hydraulic circuit low and high pressure leg portions. 
   In a variation thereof, the only fluid that needs to be passed through the bypass orifices is the fluid that is produced as a result of any undesired slight inclination of the angle of an internal swashplate of the main pump, when the main pump is placed in the neutral position by an operator. 
   In a differing variation, the bypass orifice cross-sectional areas are of a size to allow the passage of substantially the full flow of the charge pump at a low restriction to flow through the bypass orifices. In one version, the only fluid that needs to be passed through the bypass orifices is the fluid that is produced as a result of any undesired slight inclination of the angle of an internal swashplate of the main pump, when the main pump is placed in the neutral position by an operator. The valve assembly can function as a hot oil shuttle valve. 
   Another feature of the present invention includes a method for increasing the efficiency of a hydrostatic transmission, in a neutral mode of operation, without impairing the performance in non-neutral modes of operation, wherein the hydrostatic transmission includes: a variable displacement main pump; a hydraulic motor; a closed loop hydraulic circuit operatively interconnecting the main pump and motor; a charge pump having an outlet line operatively interconnected only to the circuit; a valve block within the circuit, for controlling fluid transfer between a first, second and third line, within the hydraulic circuit, wherein two of the first, second and third lines define first and second pressure lines, the remaining line defining an outlet line; the valve block comprising a valve body defining 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 line for connection to the other of the first and second pressure lines, the valve body further including a spool bore in communication with the first, second and third lines; a valve spool adapted for sealing reciprocation within the spool bore, having a first end portion, a second end portion and a connecting portion having a cross-sectional area smaller than the cross-sectional areas of the first and second end portions; and dampers for centering the valve spool in a neutral mode of operation, the method comprising: a. including a first bypass orifice, within the valve spool, extending between the first end portion and the connecting portion; b. also including a second bypass orifice, within the valve spool, extending between the second end portion and the connecting portion; c. sizing the cross-sectional areas of the first and second bypass orifices to allow the passage of substantially the full flow of the charge pump, at a low restriction to flow, through the bypass orifices; d. keeping the connecting portion in fluid communication with the first port at all times; e. permitting substantially equal fluid flows from the second and third ports, via the first and second bypass orifices, respectively, to the first port, in the neutral mode of operation when fluid forces acting on the first and second end portions are about equal; and f. shifting the valve spool from the neutral mode of operation to non-neutral modes 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 one of the pressure lines to the outlet port. 
   The noted method also includes that the only fluid passing through the bypass orifices is the fluid that is produced as a result of any undesired slight inclination of the angle of an internal swashplate of the main pump, when the main pump is placed in the neutral position by an operator. 
   The noted method further includes that the recited sizing step alternatively includes keeping the cross-sectional areas of the first and second bypass orifices of a sufficient size to permit equalization of the fluid pressure between the hydraulic circuit low and high pressure leg portions. 
   Finally, the noted method includes that the recited sizing step alternatively includes keeping the cross-sectional areas of the first and second bypass orifices of a sufficient size to allow the passage of substantially the full flow of the charge pump at a low restriction through the orifices. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a hydraulic schematic of a typical prior art hydrostatic transmission closed loop circuit, similar to that of FIG. 9 of U.S. Pat. No. 6,837,047 B2; 
       FIG. 2  is an elliptical cross-sectional view of the actual design of the hot oil shuttle valve schematically illustrated in prior art  FIG. 1 , showing the hot oil shuttle valve with integrated orifices and springs on both ends of the valve in a neutral position; 
       FIG. 2   a  is a view, similar to that of  FIG. 2 , but showing the position of the prior art shuttle valve when the fluid pressure in line  23  is greater than the fluid pressure in line  24 ; 
       FIG. 2   b  is a view, similar to that of  FIG. 2 , but showing the position of the prior art shuttle valve when the fluid pressure in line  24  is greater than the fluid pressure in line  23 ; 
       FIG. 3  is a hydraulic schematic of the present invention showing a hydrostatic transmission closed loop circuit, without the charge pump relief valve of  FIG. 1 , together with a modified hot oil shuttle valve.; 
       FIG. 4  is a schematic of the hot oil shuttle valve in the circuit of  FIG. 2 ; 
       FIG. 5  is a view, similar to that of  FIG. 2 , showing the hot oil shuttle valve of this invention with modified integrated orifices in a neutral position; 
       FIG. 5   a  is a view, similar to that of  FIG. 5 , but showing the shuttle valve when the fluid pressure in line  23   a  is greater than the fluid pressure in line  24   a ; and 
       FIG. 5   b  is a view, similar to that of  FIG. 5 , but showing the position of the shuttle valve when the fluid pressure in line  24   a  is greater than the fluid pressure in line  23   a.    
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to the several prior art drawings,  FIG. 1  shows a schematic diagram of a typical prior art hydrostatic transmission closed-loop circuit or loop  10 , similar to that of FIG. 9 of U.S. Pat. No. 6,837,047 B2, consisting of a variable displacement main radial piston pump  12  and a hydraulic motor  14 , such as a fixed displacement motor, connected to each other by lines  23  and  24  of circuit  10 . Pump  12  can be an over-center axial piston pump or a 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 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 the swashplate of pump  12 , 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 upon the direction of the overcenter movement of pump swashplate or yoke, line  23  (or line  24 ) of circuit or loop  10  can be a high pressure supply line or a low pressure return line. 
   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. A charge pump relief valve  22  is used to provide a relief path to reservoir  13  when more than the 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 operation. Valve  26  provides relief for line  23  and valve  27  provides relief for line  24 . 
   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  73  is provided to reduce 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 being discharged 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  73  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 closed-loop 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  73  is centered, charge pump flow is relieved over relief valve  22 . 
   As best seen in prior art  FIG. 2 , prior art hot oil shuttle valve  73  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 substantially aligned with lines  23  and  24 , respectively. 
   Referring now to prior art  FIG. 2   a , when the operator activates the stroke controlling device in one direction in order to initiate turning of 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 shut-off in a juxtaposed position against the wall of valve bore  61 ′, 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. 
   Turning now to prior art  FIG. 2   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. 2   a ), both orifices  75  and  76  are disabled in juxtaposed position against valve bore  61 ′ 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. 
   In recapitulation, when the transmission is in operation, hot oil shuttle valve  73  senses which leg  23  or  24  of circuit or loop  10  is at high pressure and shifts to expose relief valve  33  to the low pressure side of loop  10 . Charge pump relief valve  22  is now in parallel with relief valve  33  which is set to relieve hydraulic pressure at a lower setting than charge pump relief valve  22  so that valve  22  does not open. Hot, contaminated hydraulic working fluid exits from the outlet of hydraulic motor  14  via hot oil shuttle valve  73  and across relief valve  33  with the hot, contaminated hydraulic fluid going back to reservoir  13  through the case of main pump  12 , either through the case of hydraulic motor  14 , as shown, or directly into the case of main pump  12 , bypassing hydraulic motor  14 . Relief valves  33  and  22  can take the form of either relief valves or orifices (not shown per se). The full flow of cool, filtered hydraulic working fluid provided by charge pump  16  enters loop  10  since it cannot exit across relief valve  22 . A volume of hot, hydraulic working fluid, equal to the flow of charge pump  16 , exits loop  10  across hot oil shuttle valve  73  and relief valve  33 . When main pump  13  is centered, so as to provide no flow of hydraulic working fluid, and stops the rotation of hydraulic motor  14 , there is no pressure differential across hot oil shuttle valve  73  and it centers. Relief valve  33  is now out of circuit  10  and the flow from charge pump  16  exits across charge pump relief valve  22  at a higher pressure than the pressure that was experienced when relief valve  33  controlled the pressure from charge pump  16 . This noted higher pressure builds heat and inefficiency within circuit  10 . 
   As described in previously noted prior art U.S. Pat. No. 6,837,047 B2, when it is desired to stop rotation of hydraulic motor  14 , the swashplate of main pump  12  is centered by the operator. If it does not center exactly and is at a slight unintended angle, pressure will build up in one of the legs  23  or  24  of circuit  10 , thus causing hydraulic motor  14  to slowly rotate and the machine to creep. Orifices  75  or  76  are designed to allow the transfer of a small amount of hydraulic working fluid from leg  23  or  24  to the opposite leg, thus equalizing the pressure across hydraulic motor  14  and eliminating its tendency to slowly rotate. 
   Proceeding now to  FIG. 3 , illustrated therein is a hydraulic schematic diagram of the present invention, showing a hydrostatic transmission closed loop circuit  10   a , without the charge pump relief valve  22  of previously-described prior art closed loop circuit  10  of  FIG. 1 . In addition, hot oil shuttle valve  73   a  is modified in a manner to be described hereinafter. The schematic diagram of  FIG. 3  is quite similar to that of prior art  FIG. 1  and like parts are identified with like numerals with the addition of the suffix “a”. Specifically, as noted, prior art charge pump relief valve  22  is eliminated and, very importantly, hot oil shuttle valve  73   a  is modified by increasing the sizes of orifices  75   a ,  76   a , to allow the passage of substantially the full flow of charge pump  16   a , at a low pressure drop, i.e., at a low restriction to flow through orifices  75   a ,  76   a . These orifices need only to be large enough to permit equalization of the fluid pressures between circuit legs  23  and  24 . The only fluid that needs to be passed is the fluid that is produced by any undesired slight inclination angle of the swashplate when the pump is placed in its neutral position by the operator. The noted orifice modification exposes both legs  23   a ,  24   a  of loop  10   a  to low pressure relief valve  33   a  when main pump  12   a  is substantially centered to stop the rotation of hydraulic motor  14   a . The benefits of new circuit  10   a  include the elimination of the cost of the previously-required prior art charge pump relief valve  22  and allows lower hydraulic working fluid pressure, via charge pump  16   a , when main pump  12   a  is centered and hydraulic motor  14   a  is at rest. The result is a less costly and more efficient transmission that also operates at a lower working temperature. 
   In terms of operation, hydrostatic transmission circuit  10   a , except as noted directly above, operates very similar to that of previously-described prior art circuit  10 . 
     FIG. 4  is basically a schematic diagram of hot oil shuttle valve  73   a,  showing both lines  23   a  and  24   a  for inlet flows while line  32   a  comprises the single outlet or exhaust line. Similarly,  FIG. 5  is similar to that of prior art  FIG. 2  in showing hot oil shuttle valve  73   a  in physical position  80   a  during low working fluid flow from charge pump  16   a  (not shown here) when the fluid pressures in lines  23   a  and  24   a  are approximately equal. Valve spool  61   a  is substantially centered so that the receiving inlet ends of orifices  75   a  and  76   a  are substantially aligned with inlet lines  23   a  and  24   a , respectively. 
   Turning now to  FIG. 5   a , which is similar to that of prior art  FIG. 2   a , shows hot oil shuttle valve  73   a  in non-neutral physical position  81   a , wherein both orifices  75   a  and  76   a  are disabled or shut-off in a juxtaposed position against the wall of valve bore  61 ′ a  and fluid in low pressure line  24   a  can only reach line  32   a  through valve spool  61   a.    
   Finally, turning to  FIG. 5   b , which is similar to that of prior art  FIG. 2   b , shows hot oil shuttle valve  73   a  in position  82   a , wherein the working fluid pressure in line  24   a  is greater than the working fluid pressure in line  23   a , thus biasing spool  61   a  towards low pressure line  23   a . As is the case in position  81   a  ( FIG. 5   a ), both orifices  75   a  and  76   a  are disabled in juxtaposed position against valve bore  61 ′ a  and fluid in low pressure line  23   a  can only reach line  32   a  through valve spool  61   a.    
   At this point it should be well understood that in circuit  10   a , prior art charge pump relief valve  22  is eliminated and hot oil shuttle valve  73   a  is modified, by increasing the sizes of orifices  75   a ,  76   a  to allow the passage of substantially the full flow of charge pump  16   a , at a low pressure drop. Thus, both legs  23   a  and  24   a  of loop  10   a  are exposed to low pressure relief valve  33   a  when main pump  12   a  is centered to stop the rotation of hydraulic motor  14 . The result is a less expensive but more efficient transmission. 
   It is deemed that one of ordinary skill in the art will readily recognize that the present invention fills remaining needs in this art and will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as described herein. Thus, it is intended that the protection granted hereon be limited only by the scope of the appended claims and their equivalents.