Abstract:
A hydraulic pump for a closed-loop hydrostatic transmission circuit having an integrated shuttle valve for diverting hot fluid from the hydrostatic transmission circuit to a reservoir. The circuit operatively interconnects the hydraulic pump and a motor and includes a first line connecting a first port within the hydraulic pump to a first port within the motor and a second line connecting a second port within the hydraulic pump to a second port within the motor.

Description:
CROSS-REFERENCE TO RELATED CASES  
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Application Serial No. 60/458,109; filed Mar. 26, 2003, the disclosure of which is expressly incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a hydrostatic transmission circuit having a hydraulic pump with an integrated shuttle valve.  
         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. 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.  
           [0004]    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 which has an integrated valve for cooling the hydraulic fluid within closed loop hydraulic circuit.  
           [0005]    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. The drain leads to a heat exchanger that cools the hot oil before depositing the oil in a reservoir tank. The charge pump provides the oil replenishment when it draws the cooled oil through a filter from the reservoir tank.  
           [0006]    It is necessary for the drain line to be connected with a component within the closed loop circuit. Prior art designs have added drain lines to manifolds which are attached onto the pump or the motor. These drain lines add further componentry to the circuit which provides leak points, size and expense to the system. An example of such a design is shown in prior art U.S. Pat. No. 6,062,405 to Pech et al. Certain motors and pumps don&#39;t have manifolds so adding a manifold further complicates the circuit. It is advantageous to avoid adding componentry to the system.  
           [0007]    Other prior art designs add a case drain line to the motor. Motors and pumps have housings, or cases, that capture leaking fluid and return such fluid to the reservoir through the case drain line. Typically these motors are three pressure stage motors. If the motor used is not a three-pressure stage motor and, for example, is a two-pressure stage motor than a case drain line orifice is not provided. In order to supply a case drain line, a separate manifold (discussed above) is then needed. This can be disadvantageous since the added manifold provides more leak paths, size and expense to the system. As can be seen, it is advantageous to use an existing case drain port for connection with the case drain line. Since the purpose of the hot-oil shuttle valve is to return hot oil to the reservoir tank through a drain line, it simplifies the system to use the component that is connected with the case drain line.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides improvements with hydrostatic transmission circuits having a hydraulic pump fluidly connected with a motor. According to one feature of the present invention, the hydrostatic transmission comprises a hydraulic circuit operatively interconnecting a variable displacement main pump with a hydraulic motor. The hydraulic circuit includes a first line connecting a first port within the main pump to a first port within the hydraulic motor and a second line connecting a second port within the main pump to a second port within the hydraulic motor. The hydrostatic transmission also includes a charge pump operatively connected to the circuit and a reservoir. The variable displacement pump has a first input passage fluidly connected to the hydraulic circuit first line, a second input passage fluidly connected to the hydraulic circuit second line, an output passage fluidly connected to a pump case drain line that leads to the reservoir, and a valve bore integrated within the pump in fluid communication with the first input passage, the second input passage and the output passage, for receiving a hot oil shuttle valve. The hot oil shuttle valve 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 and in fluid communication with at least a portion of the output passage at all times. The valve spool is longitudinally movable, via fluid pressure, within the spool bore from a neutral position to one of a first and second position. The fluid pressure forces, acting on the first and second end portions, are approximately equal in the valve spool neutral position. The fluid pressure forces acting on the first end portion are greater than the fluid pressure forces acting on the second end portion in the first position. The fluid pressure forces acting on the first end portion are less than the fluid pressure forces acting on the second end portion in the second position. The second input passage communicates hot oil fluid to the output passage while the valve spool is in the first position and the first input passage communicates hot oil fluid to the output passage while the valve spool is in the second position.  
           [0009]    Another feature of the noted circuit has the pump having a case with at least one orifice for connection with a case drain line. A further feature of the noted circuit has the motor being a two-stage motor. Still another feature of the noted circuit has the hot oil shuttle valve taking the form of a spool valve.  
           [0010]    According attribute of the present invention includes providing a hydraulic pump for a closed-loop hydrostatic transmission circuit having an integrated shuttle valve for diverting hot fluid from the hydrostatic transmission circuit to a reservoir, where the circuit operatively interconnecting the hydraulic pump with a motor.  
           [0011]    Another feature of the noted hydraulic pump has the circuit including a first line connecting a first port within the hydraulic pump to a first port within the motor and a second line connecting a second port within the hydraulic pump to a second port within the motor. Still a further feature of the noted hydraulic pump has the shuttle valve being housed within a bore in the pump, the bore being fluidly connected to a first passage, a second passage and a third passage. The first passage is fluidly connected to the first line in the closed-loop hydrostatic transmission circuit, the second passage is fluidly connected to the second line in the closed-loop hydrostatic transmission circuit, and the third passage is fluidly connected to a case drain line connecting the pump to the reservoir. The first passage has fluid flow therethrough when the pressure in the first line is less than the pressure in the second line. The second passage has fluid flow therethrough when the pressure in the second line is less than the pressure in the first line. The third line has fluid flow therethrough when either the pressures in the first and second lines are not equal.  
           [0012]    Still, yet another feature of the noted hydraulic pump has the shuttle valve housed within a bore in the pump and being able to reciprocatingly move from a centered position, in which the pressure in the first line is equal to the pressure in the second line, to a first position, in which the pressure in the first line is greater than the pressure in the second line, and to a second position, in which the pressure in the first line is less than the pressure in the second line. Still a further feature of the noted hydraulic pump has the hot fluid passing from the second line to the reservoir for cooling when the shuttle valve is in the first position and the hot fluid passes from the first line to the reservoir for cooling when the shuttle valve is in the second position. Another attribute of the noted hydraulic pump has the pump being of the variable displacement variety. Further features and advantages of the present invention will become apparent to those skilled in the art upon review of the following specification in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a hydraulic schematic of the present invention showing a hydrostatic transmission closed-loop circuit with a pump having an integrated shuttle valve.  
         [0014]    [0014]FIG. 2 is a hydraulic schematic of a typical prior art hydrostatic transmission closed-loop circuit.  
         [0015]    [0015]FIG. 3 is a hydraulic schematic of the present invention similar to FIG. 1 but with the pump in the neutral position.  
         [0016]    [0016]FIG. 4 is an elliptical cross-sectional view of the actual design of the hot oil shuttle valve schematically illustrated in FIG. 1 showing the hot oil shuttle valve with integrated orifices and springs on both ends of the valve in a neutral position.  
         [0017]    [0017]FIG. 5 is a view, similar to that of FIG. 4, but showing the position of the shuttle valve when the fluid pressure in line  43  is greater than the fluid pressure in line  45 .  
         [0018]    [0018]FIG. 6 is a view, similar to that of FIG. 4, but showing the position of the shuttle valve when the fluid pressure in line  45  is greater than the fluid pressure in line  43 .  
         [0019]    [0019]FIG. 7 is a frontal view of the pump according to the present invention.  
         [0020]    [0020]FIG. 8 is a sectional view of the area of the pump which houses the hot oil shuttle valve taken along the line  8 - 8  in FIG. 7.  
         [0021]    [0021]FIG. 9 is a sectional view of the pump taken along the line  9 - 9  in FIG. 7.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The present invention relates to a hydraulic pump, and in particular to a light duty pump used, for example, in a closed-loop hydrostatic transmission circuit. The pump is of the axial piston design and combines with a motor and other accessories to comprise the hydrostatic transmission. The pump is a variable displacement pump and is typically used in turf equipment propulsion systems. As is well known in the art, a variable displacement pump enables the equipment to smoothly transition from neutral to forward or reverse.  
         [0023]    [0023]FIG. 2 shows a schematic of a typical prior art closed-loop hydrostatic transmission circuit  80  consisting of a variable displacement pump  81  and a fixed displacement motor  85  connected to each other by lines  83  and  84 . An input shaft (not shown) for pump  81  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. Changing the displacement of pump  81  changes its output flow rate, which controls the speed of motor  85 . Moving the swashplate or yoke (not shown) of pump  81  overcenter will automatically reverse the flow out of pump  81 , thus reversing the direction of motor  85 . Depending on the direction of the overcenter movement of the pump swashplate (or yoke), line  83  (or line  84 ) can be a high pressure supply line or a low pressure return line.  
         [0024]    A charge pump  86 , also driven via an input shaft, supplies additional hydraulic fluid to closed-loop circuit  80  at the rate of approximately 10-30% of the flow rate that main pump  81  can deliver. Charge pump  86  draws fluid from a reservoir  92  which passes through a filter  94  and replenishes closed loop  80  with fluid to compensate for any possible flow loss due to internal leakage. A charge pump relief valve  97  is used to provide a relief path to reservoir  92  when more than required flow from charge pump  86  cannot enter closed loop  80 .  
         [0025]    [0025]FIG. 2 shows main pump  81  in the forward position such that hydraulic fluid flows through line  83  to motor  85 , causing same to rotate. A hot-oil shuttle valve  90  is shown integrated within motor  85 . The integration of the shuttle valve within the motor of a closed-loop circuit is common, specifically with motors having attached case drain lines  88 . Such motors, for example three pressure zone motors, are well known in the art. Hot-oil shuttle valve  90  is provided to reduce the loop temperature by connecting the low pressure side of closed-loop circuit  80  to case drain line  88 . Any resistance created by the fluid flowing through motor  85  creates pressure which causes shuttle valve  90  to shift, opening the low pressure side (at low pressure line  84 ) of the closed loop to shuttle valve  90 . Shuttle valve  90  allows a maximum possible percentage of the hot oil discharging from motor  85  to flow back to reservoir  92  for cooling and filtering, and replaces the discharged hot oil with cooled, filtered oil from charge pump  86 . Specifically, about 10% of the oil exhausted from motor  85  flows through low pressure line  84  and enters shuttle valve  90 . From shuttle valve  90  the fluid enters a forward/reverse charge pressure relief valve  99 . Relief valve  99  maintains a certain amount of fluid pressure on the low pressure side of circuit  80 . Since forward/reverse charge pressure relief valve  99  (which functions for forward and reverse) is in parallel with charge pump relief valve  97  (which functions for neutral), valve  97  is set at a pressure higher than that of relief valve  99  thus allowing a maximum possible flow to pass through shuttle valve  90 . This fluid joins other fluid, such as any created from internal leakage of motor  85  (caused by high pressure and lubrication that is in the motor case), in case drain line  88  downstream of motor  85 . Together this fluid flows through pump  81  housing and is cooled by a heat exchanger  96  before entering reservoir  92 .  
         [0026]    Referring to FIGS. 1 and 7- 9 , the present invention relates to a closed-loop hydrostatic transmission circuit  10  similar to that described above and also provides a hot oil shuttle valve  30  within circuit  10  for the purpose of removing heat from the closed-loop. However, the placement of shuttle valve  30  in the present invention differs from that shown in FIG. 2. In the present invention, a pump unit  15 , comprising the componentry within dotted line  71 , houses hot oil shuttle valve  30  rather than having same integrated within motor  50 , as the prior art has done. The actual placement and location of hot oil shuttle valve  30  within pump unit  15  is best shown in FIGS. 8 and 9. As discussed above, a closed-loop hydrostatic transmission circuit, including circuit  10 , consists of a variable displacement pump  20  and a fixed displacement motor  50  connected to each other by lines  43  and  45 . Also discussed above and well known in the art, hot oil shuttle valves divert hot oil back to the reservoir (for cooling) and need a low pressure case drain line for such return. Closed-loop circuit  10  utilizes an existing case drain line  48  fluidly connected with pump  20  and positions hot oil shuttle valve  30  within pump unit  15 .  
         [0027]    Operation of the present invention will now be discussed. Referring to FIG. 3, upon start-up and while the machine/vehicle utilizing the hydrostatic transmission is in neutral, fluid flows from a reservoir  36 , through a suction filter  39 , through a line  33  connecting reservoir  36  to a charge pump  40 , and into charge pump  40 . Flow from charge pump  40  enters closed-loop circuit  10  via a line  35 , passes through dual charge check valves  27 ,  28  and charges (fills) circuit  10  with fluid. Specifically the fluid fills line  43  from pump  20  to motor  50  and line  46  from motor  50  to pump  20 . Shuttle valve  30  remains centered since there is equal pressure in lines  43  and  45 . This fluid flow is represented by arrows  57 . When circuit  10  is entirely full of fluid, flow from charge pump  40  diverts through a neutral charge pressure relief valve  52  and returns to charge pump inlet.  
         [0028]    Referring to FIG. 1, in order to move the machine/vehicle in the forward or reverse direction, the swashplate (not shown) of pump unit  15  is moved either side of center. As soon as the swashplate moves off the center position, fluid flows out of pump  20  in the direction indicated by arrow  61 , through line  43  (which becomes the high pressure side of the closed-loop) to motor  50  causing it to rotate. The resistance created by the fluid flow through motor  50  begins to build pressure within line  43  and causes shuttle valve  30  to shift towards line  45  thus opening shuttle valve  30  to line  45 . A portion (indicated by arrow  63 ) of the flow exiting motor  50  through line  45  enters shifted shuttle valve  30 . The remaining flow  61  continues past shuttle valve  30  and returns to pump  20 .  
         [0029]    Since hydrostatic transmission circuit  10  is closed, it is necessary to remove a portion (e.g. 10-15%) of the fluid for cooling and filtering on a continuous cycle. Shuttle valve  30  creates a secondary flow path (as indicated by arrows  63 ) so the maximum possible fluid can be cooled and filtered. Flow  63  enters the housing of pump unit  15 , combines with any internal leakage of pump  20  and other componentry, and exits pump unit  15  as flow  67 . A dotted line  71  is shown surrounding the majority of components integrated into the hydrostatic pump unit. All leakage within line  71  will travel into the case of pump unit  15  and returns to reservoir  36  through case drain line  48 . Therefore all leakage from charge pump  40 , all relief valves, etc. inside dotted line  71  enters the case of pump unit  15  for cooling and filtering before returning to circuit  10 . Pump unit  15  has a housing or case with a port for connection with case drain line  48 . Flow  67  travels through case drain line  48  and enters (and is cooled by) a heat exchanger  72  before returning to reservoir  36 . The equivalent amount of fluid leaving loop  10 , indicated by flow arrow  67 , is replenished into loop  10  by charge pump  40 . This replenishment fluid is indicated by flow arrow  65 . Replenishment fluid  65  combines with fluid  61  leaving motor  50  (and not flowing through shuttle valve  30 ) and re-enters pump  20  for another loop. The hot fluid flow  67  leaving pump unit  15  through case drain  48  is cooled by heat exchanger  72  and enters reservoir  36 . This same amount of fluid is re-introduced into loop  10  by charge pump  40  (through line  33 ) after it is cleaned by filter  39 . The cool fluid reintroduced by charge pump  40  also cools off the case of pump unit  15  when it travels through the pump inlet. Replenishment fluid  65  to the low pressure side also prevents cavitation, which may occur at pump  20  inlet from a lack of fluid pressure.  
         [0030]    Since hot-oil shuttle valve  30  is utilized for cooling and cleaning fluid in a closed-loop, it needs to be connected to case drain line  67  which returns fluid to reservoir  36  for cooling and cleaning. As is well known in the art, most motors and pumps have cases, or housings. Case drain lines connected to motors, such as three zone motors, are well known in the art. The present invention is primarily intended for closed-loop circuits having a two-zone motor which do not have case drains. If the shuttle valve was separate from both the motor and pump, a separate manifold would be needed. This would add componentry and conduit that is undesirable since this adds possible leak paths and expense to the closed-loop. By integrating shuttle valve  30  into pump unit  15 , closed loop circuit  10  is more compact, has less leak points, less componentry and less plumbing.  
         [0031]    Referring to FIGS. 4-6 and  8 , shuttle valve  30  is shown as a spool type shuttle valve. Shuttle valve  30  has a spool  31  sealingly reciprocatable within a bore  42 , having a mid-portion  32  with a smaller cross-section. The opposite end of mid-portion  32  provides a path for fluid communication between lines  43  and  45  with line  48 . Shuttle valve  30  utilizes springs  77  and  78 , which can be compression springs, on opposite ends thereof. As described above, shuttle valve  30  communicates the low pressure side (line  45  in FIG. 1) with case drain line  48  when the swashplate of pump  20  is in the forward position. FIG. 4 details the centered position of shuttle valve  30  when closed-loop circuit  10  is in neutral. The pressure within lines  43  and  45  are equal and the force exerted by springs  77  and  78  is equal, so valve  30  remains centered. Referring to FIG. 8, the center position is also shown. As can be seen, flow from lines  43  and line  45  can not reach line  48  since spool midportion  32  is centered.  
         [0032]    When the swashplate of pump  20  is moved into the forward position, the pressure in line  43  exceeds the pressure in line  45 . Referring to FIG. 5, spool  31  shifts downwardly towards low pressure line  45 , compressing spring  78  and opening spool midportion  32  to both lines  45  and  48 . This allows fluid communication, as indicated by arrow  38 , from low pressure line  45  into case drain line  48 . With spool  31  in the FIG. 5 position, low-pressure line  45  can continuously supply hot oil to heat exchanger  72 .  
         [0033]    Referring to FIG. 6, when the swashplate of pump  20  is moved into the reverse position, the pressure in line  45  exceeds the pressure in line  43 . Spool  31  shifts upwardly towards line  43 , compressing spring  77  and opening spool midportion  32  to both line  43  and pump case drain line  48 . This allows fluid communication from line  43  into case drain line  48 , indicated by flow arrow  37 , so the hot fluid can reach heat exchanger  72  and reservoir  36 .  
         [0034]    Referring again to FIGS. 1 and 9, in certain applications, closed-loop circuit  10  will also have a bypass valve  75  positioned between lines  43  and  45  in order to transfer oil from one line to the other. The use of bypass valve  75  enables motor  50  to turn over with little resistance when it is desirable, for example, to move the machine/vehicle for a short distance without operating the transmission.  
         [0035]    Closed-loop circuit  10  can also utilize a fixed restriction orifice  55  within shuttle valve  30  in order to ensure that the low pressure side of the loop maintains a desirable pressure so that fluid flows into the reservoir which is at ambient pressure. It should be noted that a pressure relief valve, such as that shown in FIG. 2 (as relief valve  99 ), could be used to replace orifice  55 .  
         [0036]    When the swashplate (not shown) of pump  20  is returned to its neutral position, flow through closed-loop circuit  10  ceases immediately. Shuttle-valve  30 , being spring-centered to the neutral position, immediately returns to the position shown in FIGS. 3 and 4. Flow from charge pump  40  travels through neutral charge pressure relief valve  52  and maintains charge pressure within circuit  10 . While in neutral, the transmission isn&#39;t generating significant heat so heat dissipation, via shuttle valve  30 , isn&#39;t needed. Moving the swashplate of pump  20  over-center (opposite of that described above) will cause fluid to flow out of the opposite side of pump  20  (and opposite of fluid arrows  61 ). Reversing the fluid flow in closed-loop circuit  10  causes motor  50  to reverse. The function of the system in reverse is a mirror image of the function of the system in forward.  
         [0037]    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.