Abstract:
A hydraulic fluid cooling system for a pair of integrated hydrostatic transmissions includes a common reservoir holding hydraulic fluid and having hydraulic lines connecting the common reservoir to each integrated hydrostatic transmission, a hydraulic fluid reservoir inside each integrated hydrostatic transmission, and a pump for pumping fluid from each transmission&#39;s hydraulic fluid reservoir to the other transmission. Hydraulic fluid lines link each transmission&#39;s hydraulic fluid reservoir to the other transmission. The system pumps hydraulic fluid from the sump or reservoir of each integrated hydrostatic transmission to the other transmission, providing the cooling capacity of a non-loaded transmission to cool hydraulic fluid for a fully loaded transmission.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to hydraulic fluid cooling systems for hydrostatic transmissions, and specifically to hydraulic fluid cooling systems for a pair of individual integrated hydrostatic transmissions on zero turning radius (“ZTR”) mowers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Grass mowing machines known as zero turning radius (“ZTR”) mowers have independently powered drive wheels on each side of a frame. One drive wheel may be operated in a forward direction while the other drive wheel may be stopped or operated in reverse. Many ZTR mowers have a twin stick control system. A pair of control levers or sticks may be provided side-by-side, with each lever or stick controlling one of the drive wheels. When both levers or sticks are advanced together forwardly out of their neutral position, both drive wheels rotate forwardly to cause the mower to move forward. A ZTR mower may be steered by advancing one lever or stick more than the other. 
         [0003]    Some ZTR mowers include a pair of individual integrated hydrostatic transmissions, each integrated hydrostatic transmission having a variable displacement pump and an independent wheel motor. Advantages of a pair of individual integrated hydrostatic transmissions include flexibility in the width of the mowing vehicle, and freeing up center space between the pair of transmissions. 
         [0004]    In the past, each integrated hydrostatic transmission has included its own hydraulic fluid reservoir system. However, when a ZTR mower operates on a side slope, the fluid used in the integrated hydrostatic transmission for the downhill working drive wheel will increase in temperature significantly. Under those circumstances, the hydraulic fluid temperature for the downhill transmission may climb beyond the allowable fluid temperature. 
         [0005]    One approach to reduce the hydraulic fluid temperature is to provide a hydraulic fluid cooler to provide extra cooling for each integrated hydrostatic transmission. However, the cooler and additional hydraulic lines can increase the cost of each integrated hydrostatic transmission significantly, and also may become plugged by debris in the mowing environment. 
         [0006]    Alternatively, the surface area of an integrated hydrostatic transmission housing can be increased to help dissipate the heat and reduce the fluid temperature. An air flow device such as a fan also can increase the cooling capacity. However, this alternative is limited by cost and space constraints on a ZTR mower, and does not increase cooling capacity sufficiently under severe conditions. 
       SUMMARY OF THE INVENTION 
       [0007]    A hydraulic fluid cooling system for a pair of integrated hydrostatic transmissions includes fluid lines connecting between the pair of integrated hydrostatic transmissions. A pump may urge hydraulic fluid through the fluid lines from each integrated hydrostatic transmission to the other integrated hydrostatic transmission. The pump may be a remote pump, such as an electric pump, or a charge pump in each integrated hydrostatic transmission. The system also may include a common reservoir holding hydraulic fluid for both integrated hydrostatic transmissions. 
         [0008]    The hydraulic fluid cooling system can send hydraulic fluid from the sump or reservoir of each integrated hydrostatic transmission to the other transmission, providing the cooling capacity of a non-loaded transmission to cool hydraulic fluid for a fully loaded transmission. The system does not require use of a hydraulic fluid cooler, is not prone to plugging, and does not require additional air flow systems to provide separate cooling air. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a pair of integrated hydrostatic transmissions with a hydraulic fluid cooling system according to a first embodiment of the invention. 
           [0010]      FIG. 2  is a schematic diagram of a hydraulic fluid cooling system for a pair of integrated hydrostatic transmissions according to the first embodiment. 
           [0011]      FIG. 3  is a schematic diagram of a hydraulic fluid cooling system for a pair of integrated hydrostatic transmissions according to a second embodiment. 
           [0012]      FIG. 4  is a schematic diagram of a hydraulic fluid cooling system for a pair of integrated hydrostatic transmissions according to a third embodiment. 
           [0013]      FIG. 5  is a schematic diagram of a hydraulic fluid cooling system for a pair of integrated hydrostatic transmissions according to a fourth embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    The hydraulic fluid cooling system of the present invention may be used with a pair of integrated hydrostatic transmissions on a zero turning radius (“ZTR”) mower. A ZTR mower has a drive wheel on each side that is independently powered to rotate independently of the other drive wheel. Each of the left and right drive wheels have an integrated hydrostatic transmission  101 ,  102  to transmit power from an internal combustion engine or other power source to an independent wheel motor. The left and right integrated hydrostatic transmissions  101 ,  102  may be housed in upper casings  106 ,  107  attached to lower casings  108 ,  109 . The upper and/or lower casings may have cooling fins to help dissipate heat generated by the transmission during operation. 
         [0015]    Each integrated hydrostatic transmission may be a closed loop system with a variable displacement reversible pump  132 ,  133  driven by pump input shaft powered by an internal combustion engine, which provides pressurized flow of hydraulic fluid through fluid lines to a fixed displacement motor  146 ,  147  that rotates an output shaft or axle  110 ,  111  for a traction drive wheel. Each pump provides fluid to a motor through either a forward line or reverse line, while the other line serves as a return line. For example, the pump displacement may be variable between 0 and 13.3 cc/revolution and may be driven by an internal combustion engine or other power source. The fixed displacement motor may have a speed proportional to the 0-13.3 cc/revolution flow from the pump. 
         [0016]      FIG. 2  is a schematic of a first embodiment of the hydraulic fluid cooling system. The bold arrows show the closed loop flow of hydraulic fluid between each variable displacement pump  132 ,  133  and hydraulic motor  146 ,  147 , while each steering control and trunion arm  124 ,  125  operates an integrated hydrostatic transmission in the forward direction. If the steering controls and trunion arms of the transmissions were reversed to operate the transmissions in the reverse direction, the flow of hydraulic fluid would be reversed and the bold arrows point in the opposite direction. 
         [0017]    A common hydraulic fluid reservoir or tank  103  provides hydraulic fluid through the upper casings  106 ,  107  into both integrated hydrostatic transmissions  101 ,  102 . Hydraulic lines  104 ,  105  including fittings or connectors connect the common reservoir  103  to the upper casings of each transmission. 
         [0018]    An internal combustion engine or other power source may drive each pump through a belt driven pulley  120 ,  121  mounted on a shaft extending through each upper casing. A cooling fan  112 ,  113  also may be mounted on each shaft above the pulley. Each integrated hydrostatic transmission may have a swash plate to define a pump stroke between a neutral position, a full forward position, and a full reverse position. An operator may use steering controls connected to trunion arms  124 ,  125  to pivot the swash plates for driving and steering the vehicle. 
         [0019]    Additionally, each integrated hydrostatic transmission  101 ,  102  may have a brake trunion arm  126 ,  127 . The brake trunion arm may provide a parking brake from the motor to the axle wheel hub, and may engage the motor shaft, axle shaft or reduction shaft. 
         [0020]    Each integrated hydrostatic transmission may include a positive displacement charge pump  134 ,  135  to make up fluid losses from the closed loop through internal leakage. Each charge pump may have a smaller capacity than pumps  132 ,  133 , to provide makeup flow of hydraulic fluid from each sump. Each charge pump may be driven by the internal combustion engine or other power source, and may provide pressurized flow of hydraulic fluid from the sump or reservoir  144 ,  145  in each casing, and may have a displacement of 4.125 cc/revolution, for example. 
         [0021]    In the first embodiment, the flow of charge fluid from each charge pump  134 ,  135  is indicated by the light arrows in  FIG. 2 . In this embodiment, the charge fluid is from an outlet of each charge pump  134 ,  135  through a hydraulic line or hose  114 ,  115  to an inlet to the other integrated hydrostatic transmission. The pair of hydraulic lines or hoses connecting between the pair of integrated hydrostatic transmissions may direct hydraulic fluid from one transmission to be used for charge fluid in the other transmission. Optionally, a screen or filter  136 ,  137  may be provided in each hydraulic line  114 ,  115  between the pair of integrated hydrostatic transmissions. Optionally, each hydraulic line  114 ,  115  may have a flex point to allow the integrated hydrostatic transmissions to move relative to each other. 
         [0022]    In the first embodiment, once charge fluid from charge pumps  134 ,  135  is directed through hydraulic lines  114 ,  115  to the other hydrostatic transmission, the charge fluid then may be directed through check valves and orifices to reach the low pressure side of a hydraulic motor  146 ,  147 . If each integrated hydrostatic transmission is in forward, as shown in  FIG. 2 , the charge fluid flow is primarily through check valves  154 ,  155  and orifices  150 ,  151 . If the transmission is in reverse, the charge fluid flow is primarily through check valves  152 ,  153  and orifices  148 ,  149 . 
         [0023]    Each integrated hydrostatic transmission may include a charge pump relief valve  140 ,  141  so that if hydraulic fluid pressure from the charge pump exceeds a predetermined pressure such as 0.25 MPa, the charge pump relief valve opens to send charge fluid to sump or reservoir  144 ,  145  in each transmission. 
         [0024]    Additionally, each integrated hydrostatic transmission may include a bypass valve  130 ,  131  which may be controlled by trunions  128 ,  129  to direct charge fluid flow around the hydraulic motor. 
         [0025]    Each integrated hydrostatic transmission optionally may include an implement relief valve  138 ,  139  so that if the line from the charge pump to the other transmission is blocked and the charge fluid pressure exceeds a predetermined pressure such as 4.5 MPa, the implement relief valve opens to send charge fluid from the charge pump to join the charge flow in the same transmission, or directly to the sump if charge flow needs of the loops are met. 
         [0026]    Check valves  142 ,  143  may be provided in hydraulic lines connected to the sump or reservoir  144 ,  145  of each integrated hydrostatic transmission. Check valves  142 ,  143  allow hydraulic fluid to flow around the charge pump when the engine is off, so that the loops remain charged with hydraulic fluid and can resist rolling downhill on a slope. 
         [0027]    In a second embodiment of the invention shown in  FIG. 3 , remote pump  161  is provided in hydraulic line  162  connecting between the pair of integrated hydrostatic transmissions. For example, the remote pump may be a positive displacement electric pump to direct flow of hydraulic fluid from the sump or reservoir of each integrated hydrostatic transmission through the hydraulic line to the sump of the other transmission. In this embodiment, return flow may be through lines  104 ,  105  via common reservoir  103 . Alternatively, according to a third embodiment shown in  FIG. 4 , return flow may be through hydraulic line  163  connecting between the sumps or reservoirs  145 ,  146  of the pair of integrated hydrostatic transmissions. 
         [0028]    In a fourth embodiment shown in  FIG. 5 , hydraulic line  164  is provided to link between the sumps or reservoirs  144 ,  145  of the pair of integrated hydrostatic transmissions, without a separate pump. Instead, the fourth embodiment functions as a natural heat pump to circulate hydraulic fluid between the pair of integrated hydrostatic transmissions when there is a temperature difference in the hydraulic fluid. Additionally, hydraulic fluid may be returned through lines  104 ,  105  connecting common reservoir  103  to the pair of integrated hydrostatic transmissions. 
         [0029]    By pumping hydraulic fluid from the sump or reservoir of each integrated hydrostatic transmission to the other transmission, the hydraulic fluid cooling system of this invention provides the cooling capacity of a non-loaded transmission to cool hydraulic fluid for a fully loaded transmission. The system is low cost because it does not require use of a hydraulic fluid cooler. Additionally, the system is not prone to plugging as are many coolers, and does not require additional air flow systems to provide separate cooling air. 
         [0030]    Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.