Abstract:
A compact, integrated hydrostatic transmission without reduction gearing wherein the pump and motor are contained within a single housing, and an output shaft extends from the housing inline with a shaft of the motor. The output shaft is directly connectable to a wheel of a vehicle. The pump and motor share a common sump within the housing in which pump and/or motor leakage is collected.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/588,125 filed Jul. 15, 2004, which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to hydrostatic transmissions. More particularly, the invention relates to hydrostatic transmissions for use in vehicles, such as mowing machines.  
       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 hydraulic pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. 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 a pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. 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 typically generally connected through suitable gearing to the vehicle&#39;s wheels or tracks.  
         [0004]     Fluid connections between the pump, motor and conduits should generally be leak free. However, as is often the case, the hoses or other conduits connecting the pump and motor can leak causing a loss of hydraulic fluid and a decrease in transmission performance. In response to the leakage problem, prior art hydrostatic transmissions were created which generally prevent leakage of fluid from the pump and motor by containing the leakage and returning it to the closed hydraulic circuit as needed. However, these prior art transmissions are typically large, expensive and complex, and often include reduction gearing which further increases the size, cost and complexity of the transmission.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a compact, integrated hydrostatic transmission without reduction gearing wherein the pump and motor are contained within a single housing, and an output shaft extends from the housing inline with the rotational axis of the motor. The output shaft may have a portion thereof forming an axle to which a wheel of a vehicle may be mounted. The pump and motor preferably share a common sump within the housing that collects pump and/or motor leakage. The invention also provides a unique way of sealing passages between housing parts that eliminates the need for high pressure gaskets at the mating faces of the housing parts.  
         [0006]     Accordingly, the invention provides a hydrostatic transmission comprising a hydraulic pump, an input shaft for rotatably driving the hydraulic pump, an output shaft, a hydraulic motor for driving the output shaft, and a housing enclosing at least a portion of the hydraulic pump and the hydraulic motor. The hydraulic pump and hydraulic motor share a common sump within the housing, and the output shaft extends through a wall of the housing in axial alignment with the rotational axis of the pump.  
         [0007]     In a preferred embodiment, the hydraulic pump and the hydraulic gear motor to form part of a closed hydraulic loop that further includes at least one passageway formed in the housing. The hydraulic pump may be a piston pump and the hydraulic motor may be a gear motor, such as a gerotor. The hydraulic motor may be mounted to and carried by a mounting plate that is removably attachable to the housing. The mounting plate permits installation of the motor within the housing as a unit.  
         [0008]     The hydrostatic transmission can be mounted to the frame of a vehicle, and a wheel can be mounted to an outer axle portion of the output shaft of the motor for supporting the vehicle for movement over the ground.  
         [0009]     In accordance with another aspect of the invention, a hydrostatic transmission is provided including a pump, a motor, and a housing containing at least one of the pump and the motor. The housing has first and second mating parts each including a fluid passageway for fluidly connecting the pump and the motor. The fluid passageways of the first and second mating parts open to mating faces thereof for receiving a seal insert. The seal insert has an insert passageway that extends from an opening at one end of the insert to an opening at the other end of the insert for providing a flow passage therethrough. The insert passageway is in communication with the respective passageways of the first and second mating parts, and a sealing element, such as an O-ring, is disposed at each end of the seal insert for sealing an outer diameter of the seal insert to an inner diameter of the passageways of the first and second mating parts.  
         [0010]     The fluid passageways in the first and second mating parts may each have counterbores for receiving and axially positioning the seal insert. Circumferential recesses may be provided on the outer diameter of the seal insert for retaining the sealing elements.  
         [0011]     In accordance with another aspect of the invention, a hydrostatic transmission is provided comprising a pump, a motor, and a housing containing the pump and at least a part of the motor. The pump includes an input shaft and at least one control member for controlling pump flow output. The housing includes opposed side wall portions aligned in a direction parallel to the rotation axis of the input shaft, either one of the side wall portions being selectable to provide thereon a through hole for passage of a rotatable actuating member from the exterior of the housing to the interior of the housing for connecting to the control member. The actuating member is mounted for rotation and coupled to the control member for control of the pump flow output. Each of the opposed sidewall portions can include a preformed recess in which the through hole can be located, the preformed recess corresponding to a location at which the actuating member can be coupled to the control member for control of the pump.  
         [0012]     In a preferred embodiment, the opposed sidewall portions are formed by a common housing part and the control member is a swash block. The actuating member is a trunnion shaft.  
         [0013]     In accordance with yet another aspect of the invention, a hydrostatic transmission is provided comprising a pump, a motor, and a housing containing the pump and at least a part of the motor. The pump includes an input shaft extending from a bottom side of the housing, the input shaft having a drive wheel fixed for rotation therewith and connectable to a prime mover, such as an internal combustion engine. The prime mover, when coupled to the drive wheel, may have its center of gravity disposed lower than if the drive member was located on an output shaft extending from a top side of the housing. The hydrostatic transmission may be mounted in a vehicle along with the prime mover.  
         [0014]     In accordance with still another aspesct of the invention, a hydraulic pump assembly is provided comprising a first pump, a second pump, a housing enclosing at least a portion of the first and second pumps, wherein the first and second pumps share a common sump within the housing.  
         [0015]     In a preferred embodiment, the first and second pumps are connected to first and second motors thereby forming a first and second closed loop. At least one positive displacement charge pump is configured to supply fluid drawn from the common sump to the first and second closed loops. The common sump serves as a fluid reservoir and the first input shaft and second input shaft are oriented parallel to each other.  
         [0016]     According to a further aspect of the invention, a vehicle comprises a frame, a hydraulic pump including an input shaft and having a housing mounted to the frame with the input shaft extending from the bottom of the housing, and an engine mounted to the frame and coupled to a drive wheel on the input shaft of the motor.  
         [0017]     Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a perspective view of a hydrostatic transmission in accordance with the present invention, wherein an input shaft and an output shaft are oriented at 90 degrees relative to each other.  
         [0019]      FIG. 2  is a side view of the hydrostatic transmission of  FIG. 1 .  
         [0020]      FIG. 3  is a top view of the hydrostatic transmission of  FIG. 1 .  
         [0021]      FIG. 4  is a front view of a the hydrostatic transmission of  FIG. 1 .  
         [0022]      FIG. 5  is a cross-sectional view taken along the line A-A in  FIG. 2 .  
         [0023]      FIG. 6  is a cross-sectional view taken along the line B-B in  FIG. 2 .  
         [0024]      FIG. 7  is a cross-sectional view taken along the line C-C in  FIG. 3 .  
         [0025]      FIG. 8  is a cross-sectional view taken along the line D-D in  FIG. 7 .  
         [0026]      FIG. 9  is a cross-sectional view taken along the line E-E in  FIG. 4 .  
         [0027]      FIG. 10  is a cross-sectional view taken along the line F-F in  FIG. 8 .  
         [0028]      FIG. 11  is a side view of a hydrostatic transmission in accordance with the present invention, wherein a drive pulley is on an input shaft extending from a bottom side of the housing.  
         [0029]      FIG. 12 a  schematic diagram illustrating a vehicle including the hydrostatic transmission of  FIG. 11 .  
         [0030]      FIG. 13  is a is a cross-sectional view of another hydrostatic transmission in accordance with the present invention, wherein an input shaft and an output shaft are oriented parallel relative to each other.  
         [0031]      FIG. 14  is a schematic diagram illustrating a hydraulic circuit of a hydraulic pump assembly in accordance with the present invention.  
         [0032]      FIG. 15  is a cross-sectional view of a hydraulic pump assembly in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0033]     Referring now to the drawings in detail, and initially to  FIGS. 1-4 , an exemplary integrated hydrostatic transmission (herein referred to as IHT)  10  will be described. The IHT  10  comprises a housing  14  containing a reversible pump  18  fluidly connected to a motor  22  in a closed loop hydraulic circuit. An input shaft  26  for driving the pump  18  extends vertically from a top  30  of the housing  14  and includes a fan  34  and pulley  38 . The pulley  38  can be connected by a belt to a prime mover, such as an internal combustion engine of a vehicle (not shown), for turning the input shaft  26 . An output shaft  42  driven by the motor  22  extends horizontally from a side of the housing  14 . As will be appreciated, the pump  18  supplies pressurized hydraulic fluid to an inlet of the motor  22  for driving the output shaft  42 .  
         [0034]     IHT  10  can be mounted to a frame  43  of a vehicle, generally indicated by reference number  44 , using bolt holes  45  through which bolts can extend to secure the IHT  10  to the vehicle frame  43 . A wheel  46  can be mounted to an outer axle portion of the output shaft  42  of the IHT  10  for supporting the vehicle  44  for movement over the ground. In the illustrated embodiment, the output shaft  42  has a tapered portion provided with a key  54  that engages in a key slot in the hub of a wheel  46  so that the wheel  46  will rotate with the output shaft  42 . The wheel  46  can be secured in place by means of a nut  58  which is threaded onto the end of the axle output shaft  42 .  
         [0035]     The housing  14  of the IHT  10  includes an upper housing part  62 . and a lower housing part  66  secured together with bolts  70 , or by other suitable means. A sealing gasket  72  is typically provided at the interface of the upper housing portion  62  and the lower housing portion  66 . The upper housing portion  62  and the lower housing portion  66  together form a generally circular opening into which the motor  22  is received. The opening is closed by a mounting plate  74  that carries the motor  22 . The mounting plate  74  is secured to the upper housing portion  62  and the lower housing portion  66  by bolts  78 , or by other suitable means. A sealing gasket  75  ( FIG.7 ) is provided at the interface of the upper housing portion  62 , the lower housing portion  66 , and the mounting plate  74 . The mounting plate  74  may be provided with a bypass valve  76  for creating a short fluid circuit in the motor  22  to allow free rotation of the output shaft  42  when desired.  
         [0036]     The upper housing portion  62 , the lower housing portion  66 , and the mounting plate  74  together form a sealed interior space of the housing  14  that contains the motor  18  and pump  22 . The interior of the housing  14  serves as a shared sump for the motor  18  and the pump  22 , and further may function as a reservoir.  
         [0037]     The housing  14  further includes cooling fins  82  at various locations, generally on the top surface  30  near the fan  34 , for assisting in dissipating heat generated during operation of the pump  18  and motor  22  of the IHT  10 . As will be described below, the housing  14  can further include an externally mounted oil filter  86  for filtering the hydraulic fluid of the IST  10 . The externally mounted oil filter  86  can be easily accessed for replacement when the filter becomes dirty. A breather  90  is provided for allowing air to exit and enter the housing  14  as necessary to compensate for thermal expansion and contraction of the hydraulic fluid. The breather  90  can also function as a fill cap for adding hydraulic fluid to the shared sump. Although not shown, a drain plug can be included at the bottom of the housing  14  for draining the hydraulic fluid so that fresh hydraulic fluid can be added to the housing  14 .  
         [0038]     Turning now to  FIGS. 5-10 , internal details of the IHT  10  will be described.  FIG. 5  is a cross-sectional view of the pump portion of the IHT  10  showing the pump  18  and input shaft  26 . In the illustrated embodiment, the pump  18  is a variable flow reversible piston pump. The input shaft  26  is supported in the upper housing portion  62  by a bearing  92 , and a seal  94  is provided to seal the shaft  26  to the housing  14  to prevent fluid from escaping from the interior of the housing  14 . A cylinder barrel  95  having a plurality of pistons  96  mounted for reciprocal movement therein is coupled to the input shaft  26  for rotation therewith. The input shaft  26  extends through a swash-block  97  which serves as a control member for controlling pump flow output. A thrust bearing  98  supports the input shaft  26  below the swash block  97 .  
         [0039]     The swash block  97  is arranged such that the pistons  96  of the cylinder barrel  95  abut its upper surface. The swash block  97  is connected by a linkage to an actuating member which in the illustrated embodiment is a trunnion shaft  99  ( FIG. 1 ). For further description of the linkage between the trunnion shaft  99  and the swash block  97 , reference may be had to U.S. Pat. No. 6,766,715 issued Jul. 27, 2004, which is hereby incorporated herein by reference.  
         [0040]     Trunnion shaft  99  protrudes from the housing  14  through a hole  100  in the lower housing part  66  (linkage and trunnion shaft  99  not shown in  FIG. 5 ). The hole  100  can be preformed in the lower housing part  66 , or may be bored as desired. As will be appreciated, the swash block  97  can be inclined in both directions from its neutral point, or zero-inclination (the horizontal plane in  FIG. 5 ). Rotation of the trunnion shaft  99  in one direction inclines the swash block  97  such that the pump  18  pumps fluid in a first direction, while rotation of the trunnion shaft  99  in the opposite direction inclines the swash block  97  such that the pump  18  pumps fluid in the opposite direction. As will be further appreciated, suitable control means can be connected to the trunnion shaft  99  for allowing an operator of a vehicle to rotate the trunnion shaft  99  as desired.  
         [0041]     The lower housing part  66  of the IHT  10  includes opposing side wall portions  101  and  102 . The trunnion shaft  99  can extend from the side wall  102  of the housing  14  as shown, or alternatively may extend from the opposing side  101  of the housing  14 . By way of further example, two trunnion shafts (and corresponding linkages) can be provided, each trunnion shaft extending from an opposite side wall  101  and  102  of the housing  14 . In this regard, a preformed recess, such as recess  103  in the illustrated embodiment, can be provided in which a through hole can be located if desired. The location of the preformed recesses can correspond to a location at which the actuating member, once installed in the through-hole, can be coupled to the control member for control of the pump.  
         [0042]     Turning to  FIGS. 5 and 8 - 10 , it will be appreciated that pressurized fluid is supplied to the motor  22  from the pump  18  via passageways formed in the upper housing member  62  and the mounting plate  74 . In particular, a first passageway between the pump  18  and the motor  22  begins at pump pressure port  104 . Pump pressure port  104  connects to a passageway  106  in the upper housing member  62  as seen in  FIG. 5 . Passageway  106  connects to passageway  108  which in turn connects to passageway  110 . Passageway  110  connects to passageway  112  in the mounting plate  74 , as best seen in  FIGS. 8 and 9 . As shown in  FIG. 10 , passageway  114  connects to a pressure port of the motor  22  (not shown).  
         [0043]     Returning to  FIG. 5 , a second passageway between the pump  18  and motor  22  begins at pump pressure port  105 . Pump pressure port  105  connects to passageway  116  in the upper housing member  62 . Passageway  116  connects to passageway  118 , which in turn connects to passageway  120  in the mounting plate  74 , as seen in  FIGS. 8 and 9 . Passageway  120  connects to passageway  122  which in turn connects to passageway  124  and thereby to the other pressure port of the motor  22 , as seen in  FIG. 10 . In this manner, a closed loop hydraulic circuit is formed between the pump  18  and the motor  22  by the first and second passageways.  
         [0044]     As shown in  FIG. 8 , seal inserts  130  and  134  are provided for sealing the connections between fluid passageways  110  and  112  in the lower housing part  66 , and fluid passageways  118  and  120  in the upper housing part  62 , respectively. The seal inserts  130  and  134  include passageways  138  and  142  that extend from an opening at one end of the seal inserts  130  and  134  to an opening at the other end of the seal inserts  130  and  134  for providing a flow passage therethrough. Counterbores  146  are provided in the faces of the mating surfaces of the upper housing portion  62  and the mounting plate  74  for receiving and axially positioning the seal inserts  130  and  134 . Sealing elements, such as O-rings  150 , are disposed at each end of the seal inserts  130  and  134  for sealing an outer diameter of the seal inserts  130  and  134  to an inner diameter of the respective passageways formed in the upper housing portion  62  and the mounting plate  74 . In illustrated embodiment, the O-rings  150  are retained in circumferential notches at the terminal ends of the seal inserts  130  and  134 . By way of further example, the O-rings could be retained in circumferential recesses in the outer diameter of the seal inserts  130  and  134 . The seal inserts  130  and  134  described above can be utilized in other applications for sealing a fluid connection between mating housing parts.  
         [0045]     During operation of the IHT  10 , the prime mover (not shown) drives the input shaft  26  via pulley  38  which in turn rotatably drives the pump  18 . As the trunnion shaft  99  is rotated, the hydraulic pump supplies hydraulic fluid via either the first or second passageways to the motor  22 , depending on the direction of rotation of the trunnion  99 . The motor  22  in turn drives the output shaft  42 .  
         [0046]     As will be appreciated, during normal operation of the IHT  10  some fluid leakage from the pump  18 , the motor  22  and/or the first and second passageways is common, or even desired. Returning to  FIG. 7 , it will be recalled that the pump  18  and motor  22  of the present invention share a common sump  154  within the housing  14 . The common sump  154  in the illustrated embodiment is formed by interior surfaces of the upper housing member  62 , the lower housing member  66 , and the mounting plate  74 . Fluid leakage from the hydraulic circuit drains to the common sump  154  where it can be returned to the closed loop as make-up flow to replenish the fluid in the closed circuit. To this end, a charge pump  158  may be provided for supplying fluid drawn from the sump  154  to the closed loop.  
         [0047]     In the illustrated embodiment, the charge pump  158  is a positive displacement pump also driven by the input shaft  26 . A charge pump inlet line  162  formed in the lower housing member  66  is connected via filter  86  to the common sump  154 . A charge pump outlet passage  170  also formed in the lower housing member  66  is connected to pump pressure ports  104  and  105  via check valves  174  and  178 , as best seen in  FIG. 9 . The charge pump  158  supplies a steady flow of hydraulic fluid to the pump return line.  
         [0048]     Fluid only flows into the closed loop when the pressure in the closed loop drops below a predetermined level. However, the charge pump  158  runs continuously whenever the input shaft  26  is being driven by the prime mover. As such, the charge pump  158  includes a pressure release bypass valve  182  and bypass passageway  186  for bypassing flow from the outlet passageway  170  to the inlet passageway  162  when the pressure in the outlet passageway  170  exceeds a predetermined level. For further details of the hydraulic circuit of the IHT  10 , reference may be made to U.S. Patent Application No.______, filed Jul. 15, 2005 entitled “HYDROSTATIC TRANSMISSION WITH BYPASS VALVE” (Attorney Docket No. PARKP0185USA) which is hereby incorporated herein by reference in its entirety.  
         [0049]     As will be appreciated, by drawing relatively cool fluid from the sump  154  and adding it to the closed loop, the charge pump  158  functions not only to maintain a desired operating volume of hydraulic fluid in the closed loop, but also to circulate the fluid between the sump  154  and the closed loop to facilitate cooling of the hydraulic fluid.  
         [0050]     It will be appreciated that the charge pump  158  also distributes hydraulic fluid to the bearing  92  for cooling and lubrication. The fluid supplied to the bearing  92  also drains to the common sump  154 .  
         [0051]     Turning now to  FIGS. 11 , an IHT  200  is shown comprising a housing  204  containing a reversible pump  208  fluidly connected to a motor  212  in a closed loop hydraulic circuit. An input shaft  214  for driving the pump  218  extends vertically through a bottom side  222  and a top side  224  of the housing  204 . The input shaft  214  includes a fan  226  on the portion thereof extending from the top side  224  of the housing  204 . The input shaft  214  also includes drive wheel, such as a pulley  230 , fixed for rotation therewith on the portion thereof extending from the bottom side  222  of the housing  204 . As in the above embodiment, the pulley  230  can be connected by a belt to a prime mover, such as an internal combustion engine of a vehicle (not shown), for turning the input shaft  214  to drive the pump  208 .  
         [0052]     As will be appreciated, the external and internal details of IHT  200  are identical to the IHT  10  described above in  FIGS. 1-10 , with the exception of input shaft  214  extending through a bottom surface  222  of the housing  204 . When installed in a vehicle and coupled to a prime mover, such as an internal combustion engine, the IHT  200  facilitates mounting of the prime mover such that its center of gravity may be disposed lower than if the pulley  230  was located on the portion of the input shaft  214  extending from the top side  224  of the housing  204 .  
         [0053]     Turning to  FIG. 12 , a schematic diagram of a vehicle  250  including the IHT  200  is shown. The vehicle  250 , which may be a lawn mower, includes an engine  254  having a vertically aligned output shaft  256  extending downwardly from the bottom of the engine  254  and coupled to the pulley  230  mounted on the input shaft  216  of the IHT  200  by a belt  258 . It will be appreciated that other methods of coupling the IHT  200  to the motor  254  are possible. By way of example, a sprocket can be substituted for the pulley  230  and a chain can be provided to couple the IHT  200  and the engine  254 . As will be appreciated, because the pulley  230  and output shaft  256  generally should be aligned in a horizontal plane, by providing the pulley  230  below the IHT  200 , the engine  254  can be located at a lower position than if the pulley  230  was located on an input shaft  216  extending from a top side  224  of the housing  204 . Thus, the IHT  200  enables the overall center of gravity of the vehicle  250  to be lower which thereby can make the vehicle  250  more stable.  
         [0054]     Turning to  FIG. 13 , a hydrostatic transmission according to another embodiment of the present invention will be described. The IHT  300  is similar in all general aspects to the IHT  10  described above, with the exception of the input shaft  310  and output shaft  320  being oriented parallel to each other. In the illustrated embodiment, the input shaft  310  and output shaft  320  extend from opposite sides of the housing  330 . It will be appreciated that other configurations are possible, such as the input shaft  310  and output shaft  320  being parallel and extending from a common side of the housing  330 .  
         [0055]     Turning to  FIG. 14 , a schematic diagram is shown illustrating a simplified fluid circuit  350  of a hydraulic pump assembly  355  in accordance with the present invention. The fluid circuit  350  includes a first pump  358  and a second pump  362 . The first and second pumps  358  and  362  are connected in closed hydraulic loops  363  and  364 , respectively, via pressure lines to first and second motors  366  and  370  for supply and return of fluid. The pump assembly  355  also includes a common sump  374  for collecting any leakage from the first and second pumps  358  and  362 .  
         [0056]     A charge pump  378  is provided for supplying fluid drawn from the common sump  374  to the closed loops  363  and  364 . A filter  382  can be provided as shown to filter the fluid drawn from the common sump  374 . The charge pump  378  supplies flow to closed loop  363  via check valves  384  and  385 , and to closed loop  364  via check valves  386  and  387 . It will be appreciated that the arrangement of check valves permits flow of fluid from the charge pump  378  to the closed loops  363  and  364  only when the pressure in one of the lines of the closed loops drops below a predetermined pressure.  
         [0057]     Turning now to  FIG. 15 , the first and second pumps  358  and  362  of the hydraulic pump assembly  355  are contained within a housing  400 . The pumps  358  and  362  in the illustrated embodiment are identical to the pump  18  as described above. However, it will be appreciated that a variety of different types of pumps may be used as desired. The pumps  358  and  362  can be connected via conventional porting and pressure lines to externally located hydraulic motors for the supply and return of pressurized fluid. It will be appreciated that the pumps  358  and  362 , when connected to motors, are connected in first and second closed hydraulic loops, each hydraulic loop including a pump, a motor, and supply and return lines as described in relation to  FIG. 14 .  
         [0058]     The interior of the housing  400  serves as the shared sump  374  for both pumps  358  and  362 , and further may function as a reservoir. As such, any leakage from the pumps  358  and  362  is collected in the shared sump  374  where it can be returned to the first and second closed loops as make-up flow.  
         [0059]     Accordingly, each pump  358  and  362  includes a charge pump  408  and  412  as for supplying fluid drawn from the shared sump  374  to the first and second closed loops in the manner described above in connection with the IHT  10  of  FIGS. 1-10 . Thus, fluid is drawn from the shared sump  374  through the filter  382  into a charge pump inlet line  432 . The fluid is then pumped by the charge pumps  408  and  412  into a charge pump outlet line  436  and supplied to the pressure return line of the closed loops for providing makeup flow thereto. In some applications only one charge pump may be provided for supplying fluid drawn from the sump to the first and second closed loops.  
         [0060]     The two pumps  358  and  362  can be driven separately by connecting each pump separately to one or more prime movers, such as internal combustion engines. The pump assembly  355  can also be provided with a single input, double output mechanism such as bevel gears or spur gears to thereby drive both input shafts together.  
         [0061]     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.