Abstract:
A bent axis pump/motor includes a back plate positioned within a casing, and a check valve positioned in the back-plate, the check valve configured to control passage of fluid from within the casing to an interior of the back plate. A yoke, coupled to the back plate, includes trunnions, positioned within respective apertures in the casing, upon which the yoke rotates. Bearings, occupying less than the complete circumference of the respective trunnion, are positioned between each of the trunnions and respective inner walls of the apertures. Trunnion apertures, for passage of fluid, are positioned in a portion of the circumference not occupied by the respective bearing. A valve positioned within the casing selectively couples high- and low-pressure fluid to the trunnions. Fluid supply channels, formed integrally with the casing, transmit fluid from the valve to the trunnions via fluid apertures provided within the apertures in the casing.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This disclosure relates generally to improvements of various components and systems commonly found in bent-axis pump/motors. 
   2. Description of the Related Art 
   Bent-axis pump/motors provide a high degree of efficiency in converting energy supplied as a pressurized fluid, from a hydraulic accumulator, or some other pressurized fluid source, into kinetic energy. Additionally, bent-axis pump/motors provide a further advantage over many other hydraulic technologies, inasmuch as bent-axis pump/motors operate equally well as pumps or motors, providing the potential, in automotive applications, of reclaiming and storing kinetic energy during braking, for use during a subsequent acceleration. 
     FIG. 1  shows a simplified diagrammatical representation of a hydraulic pump/motor system  100 . The system  100  comprises a bent-axis pump/motor  102 , which includes a casing  125 , a yoke  118  and a cylinder barrel  104 . 
   The cylinder barrel  104  has piston cylinders  106  radially spaced around a common center. The barrel  104  is configured to rotate around an axis A. Each of the cylinders  106  includes a piston  108  having a first end  110  positioned within the cylinder  106 , and configured such that there is a pressure tight seal between the first end  110  of the piston  108  and the wall of the respective cylinder  106 . A second end  112  of each of the pistons  106  engages a drive plate  114 , which is coupled to an input/output shaft  116  of the pump/motor  102 . 
   The angle of the barrel  104  relative to the drive plate  114  dictates the displacement volume of the pump/motor  102  and hence the amount of energy converted by the pump/motor  102 . 
   The angle of the barrel  104  is controlled by the yoke  118 , which includes a back plate  119  to which the barrel  104  is rotatably coupled. The yoke  118  further includes a pair of trunnions  120 ,  121  upon which the yoke  118  rotates, around an axis B. The trunnions  120 ,  121  are received by apertures  122 ,  123  in the pump/motor casing  125 , and their rotation is accommodated by bearings  126 ,  127  that are positioned within the apertures  122 ,  123  of the casing  125 , and which encircle the trunnions  120 ,  122 , respectively. As the yoke  118  rotates around axis B, so also does the barrel  104 , thereby changing the barrel angle relative to the drive plate  114 . 
   Fluid channels  128 ,  129  are coupled from the yoke  118 , via a valve plate surface  130  of the back plate  119 , to each of the cylinders  106  of the barrel  104 , as the barrel  104  rotates over the valve plate  130 . The fluid channels  128 ,  129  run down respective arms  132 ,  133  of the yoke  118  to the trunnions  120 ,  121 . The channels  128 ,  129  within the yoke  118  terminate at the trunnions  120 ,  121  at respective ports  134 ,  135  that are positioned to couple with corresponding fluid ports  136 ,  137  within the pump/motor casing  125 . 
   The fluid ports  136 ,  137  of the pump/motor casing  125  are each coupled to low- and high-pressure fluid sources  138 ,  140 , via respective switching valves  142 ,  143  configured to selectively couple the low-pressure source  138  to one side of the pump/motor  102  via the arm  132  of the yoke  118  and the high-pressure source  140  to the other side of the pump/motor  102  via the other arm  133 , or alternatively, to reverse this arrangement. In this way, the device can be selectively configured to apply rotational force to the output shaft  116  in a clockwise or counter-clockwise direction. The coupling between the valves  142 ,  143  and the fluid ports  136 ,  137  of the pump/motor casing  125  is generally accomplished using respective pressure hoses  144 ,  145 . 
   The casing  125  encloses the moving parts of the pump/motor  102 . In some systems, the space  117  within the casing  125  is filled with hydraulic fluid and may be in fluid communication with the low-pressure fluid source  138  via a high volume, low loss fluid connection such as a large-bore pressure hose (not shown). This connection maintains the fluid in the casing  125  at a pressure substantially equal to the pressure at the low-pressure fluid source  138 . Accordingly, the pump/motor casing  125  may be manufactured to withstand the pressure of the low-pressure fluid source  138 . This pressure may be on the order of 100 to 300 psi. 
   In operation, for example, in an application in which the pump/motor system  100  is coupled to the drive train of a vehicle, fluid from the high-pressure source  140  is coupled to fluid port  137  of the pump/motor  102  by valve  143 . The other fluid port  136  is simultaneously coupled to the low-pressure fluid source  138  by the other valve  142 . High-pressure fluid enters the pump/motor  124  via the fluid port  137 , passes from trunnion  121 , through the channel  129 , to the valve plate  130  and into the cylinders  106 , as the barrel  104  rotates over the valve plate  130 . The pistons  108  are sequentially driven against the drive plate  114 , causing the drive plate  114  to rotate around a “bent” axis A to achieve displacement. As the barrel  104  also rotates around axis A, the fluid in the cylinders  106  is sequentially released through the valve plate  130  and into the channel  128 , to be vented back through the valve  142  to the low-pressure fluid source  138 . In this manner, energy from the high-pressure source  140  is converted to kinetic energy by the pump/motor  102  to be transmitted via the rotating shaft  116  to the drive train of the vehicle or other mechanical system. 
   To slow the vehicle or other mechanical system, the high- and low-pressure connections are reversed, such that the low-pressure source  138  is coupled by the valve  143  to the port  137 , while the high-pressure source  140  is coupled by the valve  142  to the port  136 . Such a configuration, with the pump/motor  102  at rest, would cause the shaft  116  to rotate in the opposite direction. However, inasmuch as the shaft  116  is coupled to the drive train of the vehicle, the shaft  116  is driven, by the forward momentum of the vehicle, to rotate in the forward direction. Because the pressure connections have been reversed on the pump/motor  102 , the pump/motor is now resisting the rotation of the shaft  116 . As a result, the vehicle is slowed and, at the same time, fluid is drawn from the low-pressure side of the circuit and forced into the high-pressure fluid source  138 , the pump/motor  102  functioning as a pump to store energy to be used subsequently. This is commonly referred to as regenerative braking. 
   If the vehicle is traveling in reverse mode, the sequence of operation will be opposite that previously described. However, the results will remain the same, namely, high-pressure fluid at the port  136  will drive the vehicle in reverse, while reversing the connection and placing high pressure at port  137  will slow the vehicle as it travels in reverse. 
   A pump/motor and its operation are described in much greater detail in U.S. patent application Ser. No. 10/379,992, entitled HIGH-EFFICIENCY, LARGE ANGLE, VARIABLE DISPLACEMENT HYDRAULIC PUMP/MOTOR, which is incorporated herein by reference, in its entirety. This application will provide additional background on the features and operation of a bent-axis pump/motor. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an embodiment of the invention, a bent axis pump/motor is provided, including a casing configured to be substantially filled with fluid, a back plate positioned within the casing and configured to receive or include a valve plate, and a check valve positioned in the back-plate and configured to permit passage of fluid from within the casing and outside of the back plate through the check valve to an interior of the back plate. The check valve is further configured to restrict flow of fluid from the interior of the back plate through the check valve. 
   According to another embodiment, the casing of the pump/motor comprises first and second apertures positioned coaxially on opposite sides of the casing and traversing from the interior of the casing to the exterior thereof. The pump motor further comprises a yoke coupled to the back plate. The yoke includes first and second trunnions positioned within the first and second apertures, respectively, and the yoke is configured to rotate on the trunnions around an axis. First and second bearings are positioned between the first and second trunnions and an inner wall of each of the first and second apertures, respectively, the position of each of the first and second bearings further defined by respective inner and outer planes, parallel to each other and transverse to the axis, with the respective bearing positioned therebetween. Each of the first and second bearings occupies less than the complete circumference of the respective trunnion. Each of the trunnions includes a respective aperture for passage of fluid therethrough, positioned between the inner and outer planes in a portion of the circumference not occupied by the bearing. 
   According to an additional embodiment, the pump/motor includes first and second fluid supply channels formed integrally with the casing. The supply channels are configured to transmit fluid from valves or other fluid switching means to the first and second trunnions via apertures provided within the first and second apertures and positioned and configured to couple with the apertures provided in the trunnions. 
   A further embodiment of the invention provides a valve positioned within the casing and configured to selectively couple high- and low-pressure fluid supplies to the first and second trunnions, via the first and second fluid supply channels. 
   According to an embodiment of the invention, a yoke configured to carry a rotatable barrel is provided, a trunnion coupled to the yoke and configured to be received by an aperture of a pump casing, and further configured to receive a bearing between the trunnion and a wall of the aperture in a position defined by two parallel planes transverse to an axis of the trunnion, and a fluid channel passing within the yoke to the trunnion and exiting the trunnion via an aperture positioned between the two planes. 
   According to an additional embodiment, a pump/motor is provided, having a casing configured to receive components of the pump/motor, a valve configured to selectively control fluid flow, the valve including a valve body, integral to the casing; and a first fluid channel, integral to the frame, having a first terminus at the valve and a second terminus at a first fluid port configured to transmit fluid to a first trunnion of the pump/motor. The pump/motor may also include a second fluid channel, integral to the frame, having a first terminus at the valve and a second terminus at a second fluid port configured to transmit fluid to a second trunnion of the pump/motor. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a diagrammatical representation of a pump/motor according to known art. 
       FIG. 2  is an elevation of a yoke of a pump motor according to an embodiment of the invention. 
       FIG. 3A  is a cross section of the yoke of  FIG. 2 , taken along line  3 - 3 . 
       FIG. 3B  is a detail of a check valve of the type illustrated in the sectional view of  FIG. 3A . 
       FIG. 4  is a side elevation of a pump/motor according to an embodiment of the invention. 
       FIG. 5A  is a cross section of the pump/motor of  FIG. 4 , taken along line  5 - 5 . 
       FIGS. 5B-5D  are details of the pump/motor of  FIG. 5A , according to various embodiments of the invention. 
       FIG. 6  is a cross section of the pump/motor of  FIG. 4 , taken along line  6 - 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The improvements described below with reference to various embodiments of the invention deal generally with minimizing losses occurring in the various channels, couplings, valves, and components of a hydraulic pump/motor system. For example, any time a hydraulic fluid is obliged to change directions within a conduit, energy is lost. When the directional changes are very sharp, or occur in restricted passages, the energy loss is exacerbated. In pump/motors according to current technology, such losses occur in locations such as hose couplings, valve passages, and the passages through the yoke trunnions. 
   These energy losses are expressed as a difference in pressure between the high-pressure fluid source, or accumulator, and the high-pressure present at the valve plate of the pump/motor, and between the low-pressure fluid source, or accumulator, and the low-pressure present at the valve plate of the pump/motor. The actual power available to the motor is directly proportionate to the difference between the high-pressure and low-pressure found at the valve plate. When pressure losses are reduced between the motor and the accumulators, the pressure difference at the valve plate is increased, and thus the available power to the motor is increased. 
   In the various embodiments of the invention illustrated in  FIGS. 2-6 , sources of high- and low-pressure fluid are not shown. Such fluid sources are well known in the art. A common type of pressurized fluid storage is an accumulator, which is referred to occasionally in the present descriptions, and is well understood in the art. Other types of fluid supply and storage may be employed and are considered to fall within the scope of the invention. 
   As previously explained, during a regenerative braking operation a pump/motor is configured to operate as a pump, forcing fluid at high pressure into the high-pressure source, and drawing fluid from the low-pressure source. For example, given the pump/motor and conditions previously described with reference to  FIG. 1 , with the vehicle traveling in a forward direction, the pump/motor  102  draws low-pressure fluid from port  137  during braking and pumps high-pressure fluid to port  136 . There is an energy loss associated with the passage of the low-pressure fluid through the pressure lines, channels, trunnion, and valves between the valve plate  130  and the low-pressure fluid source  138 , or accumulator  140 . 
     FIG. 2  shows a yoke  150  of a pump/motor  190  (the pump/motor  190  is shown in  FIGS. 4-6 ). As shown in  FIGS. 2 and 5A  the yoke  150  of pump/motor  190  includes a back plate  152 , arms  154 ,  155 , and trunnions  156 ,  157 . The yoke  150  also includes check valves  160  in the back plate  152 , which will be described in detail hereafter. 
   As seen in  FIG. 3A , a cross-section of the back plate  152  is shown, including details of the check valve  160 . An enlarged view of a check valve  160  is shown in  FIG. 3B . More particularly, the check valve  160  of this embodiment includes a threaded insert  162  configured to engage a threaded aperture  164  in the back plate  152 . Seal  166  provides a fluid seal between the insert  162  and the back plate  152 . Poppet valve  168  is biased in a closed position by spring  170 . 
   The yoke  150  further includes fluid channels  172 ,  173  located within the arms  154 ,  155 . It may be seen, in  FIG. 3A , that there are two fluid channels  172  within the arm  154 , and two fluid channels  173  within the arm  155 . The provision of two fluid channels  172 ,  173  in each of the arms  154 ,  155  enhances the stiffness of the arms  154 ,  155  as compared with arms having single, larger fluid channels in each of the arms. 
   In operation, when pump/motor  190  is coupled to the drive train of a vehicle, high-pressure fluid is introduced to the yoke via port  175  (see  FIG. 2 ) and travels up the arm  155  to the back plate  152  via channels  173 . The high-pressure fluid is supplied to the valve plate  178  and to the barrel  158  via fluid cavities  177 . The yoke  150  is sealed within a casing  192  (see  FIG. 5A ). Space within the casing  192  around the yoke  150  may be filled with hydraulic fluid, and coupled to a low-pressure fluid source, such as an accumulator, via a high volume, low loss fluid connection such as a large-bore pressure hose (not shown). 
   While fluid pressure within the cavities  177  is greater than, or equal to fluid pressure outside of the yoke  150 , the poppet  168  of the check valve  160  remains in a closed position. Accordingly, operation in a forward mode is unaffected by the check valve  160 . High-pressure fluid enters the cylinders  180  of the barrel  158  from the fluid cavities  177 , driving pistons (not shown) downward, and causing the drive plate (not shown) to rotate, as described with reference to the pump/motor  102  of  FIG. 1 . The drive plate is connected to the barrel  158  via a flexible shaft means (not shown) and rotates the barrel  158  in unison. As the barrel  158  continues to rotate, fluid from the cylinders  180  is released into fluid cavities  176  at low pressure, whence it is returned to the low-pressure accumulator, via the channels  172  and the trunnion port  174 . 
   To slow the vehicle, the fluid pressure connections at trunnion ports  175 ,  174  are reversed, as described in more detail hereafter, such that the high-pressure fluid source, a high-pressure accumulator, for example, is coupled to trunnion port  174 , while the low-pressure fluid source is coupled to trunnion port  175 . In this configuration, low-pressure fluid is drawn into the cylinders  180  of the barrel  158  via the fluid cavities  177 , and pumped at high pressure from the cylinders  180  into the fluid cavities  176 , and thence to the high-pressure accumulator via the trunnion port  174 . 
   When the pump/motor is operating in pump mode, as occurs during a braking operation, fluid pressure within the fluid cavities  177  drops below the fluid pressure at the low-pressure accumulator. In known systems, such as that described with reference to  FIG. 1 , the pump/motor must develop enough suction to draw fluid through the valves and channels of the pump/motor, as previously described, which consumes energy. However, in the embodiment illustrated in  FIG. 3 , as soon as the pressure within the fluid cavities  177  drops below the pressure of the fluid within the casing  192  around the yoke  150 , the poppet valve  168  opens, permitting fluid to pass directly from the space around the yoke  150  into the fluid cavities  177 . In this way, low-pressure fluid is permitted to enter the pump/motor directly at the back plate  152 , without the need to pass through the valves and passages of the pump/motor. Accordingly, the pressure losses previously encountered are substantially eliminated. As previously explained, the casing is provided with a high-volume, low-loss coupling to the low-pressure accumulator, which minimizes pressure losses.  FIG. 3B  shows a detail of a check valve  160  similar to that shown in  FIG. 3A . The check valve  160  of  FIG. 3B  is shown in an open position, as described above. It may be seen, with reference to  FIG. 3B , that when the poppet  168  is in the open position, fluid may pass freely around the poppet and into the fluid cavities  177 . 
   While not shown, it will be understood that if the back plate  152  is provided with check valves on the opposite side, that is, between the fluid cavities  176  and the exterior of the yoke  150 , regenerative braking may be carried out while the vehicle is traveling in reverse. 
   According to an alternate embodiment, the check valves may be configured to remain open under reverse pressures greater than the pressure found in the low-pressure side of the circuit, but to close under pressures much lower than the pressure present in the high-pressure side (spring biased open). In this way, low-pressure fluid may flow in either direction through the check valves, thus further reducing losses by generally bypassing most of the restrictive passages between the back plate of the pump/motor and the low-pressure fluid source, for example on the motor discharge side. On the other hand the valves will close instantly when high pressure is present in the corresponding fluid cavity. High pressure fluid must enter or exit the yoke. 
   Referring again to  FIG. 1 , it may be seen that in the prior art, fluid traversing the trunnions  120 ,  121  must execute several sharp turns in entering or leaving the pump/motor  102 . For example, fluid entering via trunnion port  135  makes a sharp turn to pass axially through the trunnion  121  and through the bearing  127 , and then another sharp turn to rise into the channel  129  of the arm  133 . The fluid returning from the pump/motor must pass through a similar series of turns as it exits the trunnion  120 . These sharp turns are due in large measure to the need for the trunnions  120 ,  121  to be of a length sufficient to pass through the bearings  126 ,  127 , and to mate with fluid ports  136 ,  137  on the outside of the pump/motor casing  125 . 
     FIG. 4  shows the pump/motor  190  according to an embodiment of the invention.  FIG. 5A  shows a cross-section of the pump/motor  190  of  FIG. 4 , taken along line  5 - 5 . 
   Referring now to  FIG. 5A , it may be seen that, according to an embodiment of the invention, in place of full bearings, such as the bearings  126 ,  127  of  FIG. 1 , partial bearings  196 ,  197  are shown, which occupy only an upper portion of a region of the respective trunnion  156 ,  157 . While not limiting the invention in anyway, applicant believes that in operation, only an upper portion of a trunnion bearing is subjected to force of any significance, inasmuch as the net effect of all the forces exerted by the pump/motor is to push the yoke and trunnion away from the motor casing in an upward direction, as viewed in  FIGS. 1  or  2 . Consequently, the lower part of the trunnion bearing receives virtually no force or pressure. 
   Trunnion ports  174 ,  175  are located in positions occupied, in pump/motors of known art, by the lower portion of the trunnion bearings. For example, trunnion port  174  is shown exiting trunnion  156  between vertical planes that also define the horizontal limits of trunnion bearing  196 . Fluid seals  200 ,  201  are positioned on either side of the trunnion bearings  196 ,  197  and trunnion ports  174 ,  175  of trunnions  156 ,  157 , respectively. 
   The cross-section of  FIG. 5A  shows the yoke  150  and cylinder barrel  158  of  FIG. 2 , and provides a cross-sectional view of the trunnions  156 ,  157 . Trunnion ports  174 ,  175  are shown coupled to fluid ports  182 ,  183  of the pump/motor casing  192 . It may be seen that the fluid channels  172 ,  173  are much straighter as compared to those of a conventional pump/motor such as pump/motor  102  of  FIG. 1 , inasmuch as the trunnion ports  174 ,  175  can now be positioned in a location that, in the pump/motor of  FIG. 1 , is occupied by the lower half of bearings  126 ,  127 . By straightening out the fluid channels  172 ,  173 , and eliminating the sharp right-angle turns found in the passages  128 ,  129  of the pump/motor  102  of  FIG. 1 , fluid resistance is greatly reduced. This reduction in resistance in fluid passages  128 ,  129  results in a reduced pressure drop through these channels, which in turn results in a greater pressure differential available at the valve plate of the barrel  158 , producing a greater availability of power, and improved efficiency of the pump/motor  190 . 
   Additionally, because the trunnion ports  174 ,  175  are positioned closer to the center of the pump/motor, the trunnions  156 ,  157  may be made shorter than previously known trunnions, such as trunnions  120 ,  121  of  FIG. 1 , reducing the size and mass of the pump/motor  190  as compared to previously known pump/motors. 
   Because of the tremendous forces exerted on the trunnions  156 ,  157  when the pump/motor  190  is in operation, the arms  154 ,  155  and the trunnions  156 ,  157  undergo a distortion, with each of the arms  154 ,  155  tending to pivot upward and outward on the fulcrums formed by the bearings  196 ,  197 . As a result, not only are the forces concentrated on the upper portions of the bearings  196 ,  197 , but the forces are concentrated in a small area of the top of each bearing along an inner rim closest to the respective arm  154 ,  155 . According to various embodiments of the invention, several bearing configurations are provided to improve efficiency and reduce wear on the trunnions  156 ,  157  and bearings  196 ,  197 . 
     FIGS. 5B-5D  illustrate three of the bearing configurations provided in accordance with various embodiments of the invention. In each of the  FIGS. 5B-5D , a sectional detail of the trunnion  157  is shown, together with a portion of the pump/motor casing  192  and trunnion end cap  205 . It will be understood that, while trunnion bearings configured to operate with trunnion  157  are shown, corresponding bearings are also provided to operate with trunnion  156 , which are substantially identical, and so need not be illustrated separately. 
     FIG. 5B  shows trunnion bearing  197 . Bearing  197  is a roller bearing comprising a cage frame  203  and a plurality of needle rollers  215 . 
     FIG. 5C  shows a conical bushing  207 . Bushing  207  is in the form of a section of a hollow cone. The bushing  207  tapers in thickness from an outboard edge  211  to an inboard edge  213 , as may be seen by phantom lines T, which indicate the tapering thickness of the bushing  207 . In operation, the bushing  207  is positioned on the trunnion  157  such that the inboard edge  213  is closest to the arm  155 . Because of the taper of the bushing  207 , when the pump  190  is idle, the upper surface closest to the inboard edge  213  does not contact the corresponding inner surface of the pump casing  192 . However, when the pump  190  is in operation, the forces within the pump cause the arm  155  to deform slightly, flexing outward. As a result, the trunnion  157  is biased in a clockwise direction, as viewed in  FIG. 5C , bringing the entire surface of the bushing  207  into contact with the inner surface of the pump casing  192 , effectively distributing the load across the surface of the bushing  207 , thereby reducing localized wear. The bushing  207  may be formed of bronze or some other suitable material, and may be impregnated with a lubricant. 
     FIG. 5D  illustrates a cylindrical bushing  209 . In addition to having a cylindrical cross-section in a first axis C, in order to accommodate the cylindrical shape of the trunnion  157 , bushing  209  also has a cylindrical cross-section in a second axis D, as may be clearly seen in the sectional view of  FIG. 5D . This shape permits the bushing  209  to adjust slightly within the space provided for it in the trunnion  157  of  FIG. 5D  as the varying forces placed on the trunnion  157  cause it to rotate slightly on the second axis D within the pump/motor casing  192 . In this way, the stresses can be evenly distributed across the upper and lower surfaces of the bushing  209 , preventing localized wear and stress. As with the bushing  207  of  FIG. 5C , the bushing  209  may be formed of bronze or some other suitable material, and may be impregnated with an appropriate lubricant. 
   Currently known pump/motors employ couplings and hoses to carry high- and low-pressure fluid between the pump/motor and control valves located externally to the pump/motor. As has been previously explained, each time the fluid in a hydraulic circuit passes through a restriction in the passage or is required to make a sharp turn, there is an associated energy cost. Additionally, there is a pressure drop associated with any fluid channel. This “line loss” varies in direct proportion to the length of the channel. 
     FIG. 6  is a cross-sectional view of the pump/motor  190  taken along line  6 - 6  of  FIG. 4 . Referring to  FIG. 4 , a fluid supply channel  198  may be seen as it curves up toward the trunnion cover plate  204 . The fluid supply channels  198 ,  199  are integrated into the structure of the pump/motor frame, eliminating the need for an external hose in this location. Referring to  FIG. 6 , the fluid supply channels  198 ,  199  may be clearly seen, positioned to carry fluid to and from the yoke  150  via spool valve  210 . It may be seen, with reference to  FIGS. 4 and 6 , that the fluid supply channels  198 ,  199  are configured to provide passage for hydraulic fluid, while avoiding sharp turns and tight restrictions, wherever possible. Additionally, a spool valve  210  is integrated into the pump/motor frame. Because high- and low-pressure switching is accomplished by the spool valve  210 , couplings and transmission lines between exterior switching valves and the pump/motor  190  are eliminated. Furthermore, by combining the function of the two valves  142 ,  143  of  FIG. 1  into a single valve  210  of  FIG. 6 , complexity is reduced, and durability and safety are improved. 
   The structure and operation of a spool valve similar to that illustrated with reference to  FIG. 6  is described in more detail in U.S. patent application Ser. No. 10/731,985, which is incorporated herein by reference, in its entirety. 
   Other valves may also be incorporated into the structure of the pump/motor  190 , such as pilot valves, check valves, and actuator valves. For example, generally referring to  FIGS. 6 and 5A , an actuator  218  controls the rotation of the yoke  150  on trunnions  156 ,  157 . The actuator  218  is controlled by actuator control valve  216 , which may be incorporated into the structure of the pump/motor  190 . A detailed description of the operation of an actuator and actuator control valve of the type referenced in  FIG. 6  may be found in U.S. patent application Ser. No. 10/767,547, which is incorporated herein by reference, in its entirety. 
   The pump/motor  190  of  FIG. 6  also includes pressure input ports  212 ,  214 , configured to receive a high-pressure fluid supply and a low-pressure fluid supply, respectively. 
   By incorporating the housings for the associated valves in the body or casing of the pump/motor, fluid channels formed within the casing can be routed directly to the valves with a minimum of obstruction and without passage through couplings or hoses. Additionally, because the channels are machined, or otherwise formed in the steel casing of the pump/motor, they do not have even the minimal resiliency associated with flexible pressure lines, thereby eliminating another source of energy loss. 
   Channels formed within the pump/motor casing are almost always shorter than equivalent channels formed using hoses, since a hose channel is required to follow a longer path around the pump/motor. The pressure loss is reduced over known systems and, additionally, the number of components of the pump/motor is reduced. It is known that, in hydraulic systems in general, hoses and hose connections are among the most frequent sources of failure and down time. Thus, by eliminating such from the system, the overall durability and dependability of the system is improved. 
   In known systems, such as that previously described with reference to  FIG. 1 , a first valve  142  is used to couple the fluid supply line  144  alternately to the high- or low-pressure fluid source, while a second valve  143  is used to perform the same function for the fluid supply line  145 . Such an arrangement required that the valves  142 ,  143  be carefully coordinated in their operation. Otherwise, while reversing the sources of each of the valves  143 ,  145 , there is a potential for a period during which both fluid supply lines  144 ,  145  may be connected to the high-pressure source  140  or to the low-pressure fluid source  138 , simultaneously. While such a configuration does not damage the pump/motor  102 , there is no energy transfer during this period. Thus, if a rapid switch is required, undesirable delays may occur. Additionally, high-pressure fluid on both sides of the pump/motor  102  results in unnecessary drag and wear on the motor. 
   By incorporating the valves into a single valve with multiple ports configured to control a coupling of both fluid supply lines with both the high- and low-pressure fluid sources, such as through spool valve  210  of  FIG. 6 , the coordination of the switching is improved, while the circuitry required to control the switching is simplified. If pressure losses in the high- or low-pressure sides of the hydraulic circuit of the pump/motor are reduced, the pressure differential at the valve plate of the pump/motor will be closer to that between the high- and low-pressure fluid sources. This will result in an increase in available power as well as improved fuel economy for an associated vehicle. 
   Additionally, if losses on the low-pressure side of the pump/motor circuit are reduced through the employment of one or more of the improvements described herein, the maximum pressure required in the low-pressure side of the circuit to overcome those losses may also be reduced. This makes possible the reduction of the overall pressure in the low-pressure accumulator, resulting in a further increase in the pressure differential at the motor, with a concomitant increase in available power to the motor. 
   Finally, if the maximum pressure in the low-pressure side of the circuit is reduced, the pressure within the pump/motor casing will also be reduced. With lower pressure in the pump/motor casing, the casing may be manufactured to lower pressure tolerances. Additionally, the low-pressure accumulator may also be manufactured to lower pressure tolerances. This allows a reduction in mass and weight of the casing and accumulator, which further increases the operational economy of the pump/motor while reducing its overall size, without reducing its power output. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.