Abstract:
A hydraulic device includes a rotatable drum, at least one piston, a transition arm, and at least one compensating piston. The rotatable drum defines a cylinder that houses the piston. The transition arm is coupled to the piston to translate between linear motion of the piston and rotation of the rotatable drum. The compensating piston is in fluidic communication with the cylinder and is configured to counteract a force caused by an inlet or an output pressure during operation.

Description:
[0001]     This application claims priority under 35 USC § 119(e) to U.S. patent application Ser. No. 60/602,865, filed on Aug. 20, 2004, the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     This description relates to pumps and motors, for example, hydraulic pumps and motors.  
         [0003]     In some hydraulic pumps, rotary motion is translating into reciprocating motion of a piston, which pumps the hydraulic fluid. Conversely, in some hydraulic motors, a pressurized fluid causes reciprocating motion of the piston, which is translated into rotary motion.  
       SUMMARY  
       [0004]     In one aspect, a hydraulic device includes a rotatable drum, at least one piston, a transition arm, and at least one compensating piston. The rotatable drum defines a cylinder that houses the piston. The transition arm is coupled to the piston to translate between linear motion of the piston and rotation of the rotatable drum. The compensating piston is in fluidic communication with the cylinder and is configured to counteract a force caused by an inlet or an output pressure during operation.  
         [0005]     Implementations may include one or more of the following features. For example, the hydraulic device may include a shaft and a member, such as a plate, mounted on the shaft. The shaft may be connected to the rotatable drum such that the rotatable drum does not move axially along the shaft. The compensating piston then may be configured to counteract the force caused by at least one of the inlet pressure and the output pressure during operation by applying a force to the member. A flanged nut and a spring also may be mounted to the shaft, with the spring between the member and the flanged nut such that the spring applies a force to the flanged nut in the same direction as the force applied to the member by the compensating piston. The member may be mounted on the shaft such that the member moves axially along the shaft. The compensating piston may be configured to counteract the force caused by at least one of the inlet pressure and the output pressure during operation by applying a force to the member such that the member moves axially along the shaft to compress the spring, resulting in an increase in the force applied to the flanged nut by the spring.  
         [0006]     Alternatively, the shaft maybe connected to the rotatable drum such that the rotatable drum moves axially along the shaft. The member, such as a support, may be mounted to the shaft such that the member does not move axially along the shaft. The compensating piston then may be configured to counteract the force caused by at least one of the inlet pressure and the output pressure during operation by applying a force to the member. The compensating piston applying a force to the member may result in a force being applied to the rotatable drum in a direction opposite to the force caused by the inlet or outlet pressure. The hydraulic device may include a U-joint coupled to the member and the transition arm. A spring may be mounted between the member and the rotatable drum such that the spring applies a force to the rotatable drum in the opposite direction of the force caused by the inlet or output pressure during operation.  
         [0007]     The hydraulic device may be a hydraulic motor or a hydraulic pump. The hydraulic device may include a bulkhead that defines an inlet or outlet manifold, a cylinder housing the compensating piston, and a port between the inlet or outlet manifold and the cylinder housing the compensating piston. The port may provide the fluidic communication between the compensating piston and the cylinder defined by the rotatable drum. Alternatively, or additionally, the bulkhead may define a port between the cylinder housing the compensating piston and the cylinder defined by the rotatable drum to provide the fluidic communication.  
         [0008]     The hydraulic device may include a face valve between the bulkhead and the rotatable drum. The compensating piston may be configured to counteract the force caused by at least one of the inlet pressure and the output pressure during operation to maintain a seal between the face valve and the bulkhead or the face valve and the rotatable drum. A spring may be coupled to the compensating piston such that the spring applies a force to the rotatable drum in the opposite direction of the force caused by the inlet or output pressure during operation. The compensating piston may be configured to counteract the force caused by at least one of the inlet pressure and the output pressure during operation by creating a force that acts on the rotatable drum in a direction opposite to the force caused by the inlet or output pressure.  
         [0009]     The hydraulic device may include a non-rotating member coupled to the transition arm. A radial position of the non-rotating member relative to an axis of rotation of the rotatable drum may be adjustable to change an angle between the transition arm and the axis of rotation of the rotatable drum. The transition arm may be coupled to the piston and the non-rotating member such that a change in the angle between the transition arm and the axis of rotation of the rotatable drum changes a stroke of the piston. The non-rotating member may include teeth that mesh with a gear such that rotation of the gear adjusts the radial position of the non-rotating member relative to the axis of rotation of the rotatable drum.  
         [0010]     The hydraulic device may include a piston joint assembly coupling the transition arm to the piston. The piston joint assembly may be configured to reduce centrifugal forces acting on the piston as a result of rotation of the rotatable drum and piston. The piston joint assembly may include a casing that defines a hole through which a portion of the piston extends. The hole may have a diameter larger than the portion of the piston extending through the hole. The portion extending through the hole of the casing may terminate in a head piece. The head piece may have a diameter larger than the hole defined by the casing and the casing may be dimensioned to provide a spacing between an inner surface of the casing and the head piece.  
         [0011]     In another aspect, a method for counteracting a force in a hydraulic device caused by at least one of an inlet pressure and an outlet pressure during operation may include adjusting a counteracting force as at least one of the inlet pressure or the outlet pressure changes during operation.  
         [0012]     In another aspect, a method for counteracting a force in a hydraulic device caused by at least one of an inlet pressure and an outlet pressure may include rotating a drum that defines a cylinder; reciprocating a piston in the cylinder; and communicating a pressure in the cylinder to a compensating piston such that the compensating piston creates a force that counteracts the force caused by at least one of an inlet pressure and an outlet pressure during operation.  
         [0013]     Implementations may include one or more advantages. For example, the compensating piston may be able to produce a variable force to counteract the force cause by the inlet or output pressure. The varying force may increase as the inlet or output pressure increases, and decrease as the inlet or output pressure decreases. This may allow the friction between the between the drum and the face valve and between the face valve and bulkhead to be low at low output or inlet pressures, while increasing to maintain a seal between the drum and the face valve and between the face valve and the bulkhead at higher pressures. This may improve efficiency, particularly when the pump or motor is operating at low inlet or output pressures.  
         [0014]     In addition, the use of a spring may provide an initial force that is appropriate to counteract the force from the pressure during start-up conditions. The initial force from the spring may be designed so as to maintain the seal between the drum and the face valve and between the face valve and bulkhead during low start-up pressure, without being greater than necessary to maintain the seal by completely or partially counteracting the force generated by the low start-up pressure. In this way, friction during start-up may be decreased, which may improve pump efficiency and may reduce wear.  
         [0015]     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1A  illustrates a combination hydraulic motor/pump.  
         [0017]      FIG. 1B  is an end view of the combination hydraulic motor/pump taken along line  1 B of  FIG. 1A .  
         [0018]      FIG. 2A and 2B  show a face valve of the combination hydraulic motor/pump of FIG  1 A and  1 B.  
         [0019]      FIG. 3  is an end view of the valve cylinder of the combination hydraulic motor/pump of  FIGS. 1A and 1B .  
         [0020]      FIG. 4  is a side view of the valve cylinder of the combination hydraulic motor/pump of  FIGS. 1A and 1B .  
         [0021]      FIG. 5A  illustrates a hydraulic assembly with reduced centrifugal force effects.  
         [0022]      FIG. 5B  is a cross-sectional view taken along line  5 B of  FIG. 5A .  
         [0023]      FIG. 5C  is a cross-section taken along line  5 C in  FIG. 5A .  
         [0024]      FIG. 5D  shows a piston assembly of the hydraulic assembly of  FIG. 5A .  
         [0025]      FIG. 5E  is a cross-sectional view of a piston assembly taken along line  5 E of  FIG. 5C .  
         [0026]      FIG. 5F  is a view of the piston assembly and support members taken along line  5 F in  FIG. 5C .  
         [0027]      FIG. 6A  illustrates a hydraulic assembly.  
         [0028]      FIG. 6B  is a cross-sectional view of the hydraulic assembly of  FIG. 6A  taken along line  6 B of  FIG. 6A .  
         [0029]      FIG. 7A  illustrates an alternate implementation of the hydraulic assembly of  FIGS. 6A and 6B .  
         [0030]      FIG. 7B  is a view of a variable pressure mechanism of the hydraulic assembly of  FIG. 7A .  
         [0031]      FIG. 7C  is a view of an alternate implementation of the variable pressure mechanism of the hydraulic assembly of  FIG. 7A . 
     
    
     DETAILED DESCRIPTION  
       [0032]     Referring to  FIGS. 1A and 1B , a combination hydraulic motor/pump  30  particularly useful, e.g., in deep well applications, includes a first side  31  that acts as a hydraulic motor and a second side  32  that acts as a hydraulic pump. The motor side  31  is designed to operate with a high pressure/low volume fluid input and directly drives the pump side, which is designed for higher volume/lower pressure fluid output. Motor/pump  30  is placed within a well or borehole and the space between motor/pump  30  and casing  47  of the well is sealed with a seal  1 . Accordingly, when placed below the liquid level of the well or borehole, a low volume/high pressure fluid is pumped into the motor side  31 , which then directly drives the pump side  32  to pump the fluid in the well or borehole to the surface at a lower pressure, but higher volume than the fluid pumped into the motor side  31 .  
         [0033]     Specifically, combination hydraulic motor/pump  30  includes stationary housing parts  2 ,  9 ,  13 , and  14  that define an open region  35  within motor/pump  30 . Located within open region  35  is a transition arm  22 . Transition arm  22  is mounted on, e.g., a universal joint (U-joint)  23 . For example, U-joint  23  is a cardon type U-joint, however, other types of U-joints can be used. Transition arm  22  includes a nose pin that is coupled to a rotating member  21  via a self-aligning nose pin bearing such that transition arm  21  is at an angle β with respect to assembly axis A. The nose pin is axially fixed within rotating member  21 .  
         [0034]     Piston assemblies  5  (e.g., three piston assemblies  5 ) are mounted circumferentially about transition arm  22  via drive pins coupled to piston joint assemblies  6 , such as, e.g., one of the piston joint assemblies described in FIGS. 14-16, 23-23A or 56-56F in PCT Application WO 03/100231, filed May 27, 2003, incorporated herein by reference in its entirety, or, e.g., the piston joint assembly described below with respect to  FIGS. 5C-5E . Piston assemblies  5  include double-ended pistons having a first piston  7  (the drive piston) on one end and a second piston  3  (the pump piston) on the other end. Pistons  7  are received in cylinders  36   a  formed in stationary housing part  9  on motor side  31 , while pistons  3  are received in cylinders  36   b  formed in stationary housing part  2  on pump side  32 . Pistons  7  are centered in cylinder  36   a  by seals  8  such that they do not touch the sides of cylinders  36   a.  Similarly, pistons  3  are centered in cylinders  36   b  by seals  4  such that they do not touch the sides of cylinders  36   b.  In an exemplary implementation, the distance between the end face  43  of pistons  3  and end wall  42  of cylinders  36   a  when pistons  3  are at the end of their intake stroke (i.e., when pistons  3  are moving in the direction of arrow D) is the same as the distance between the end face  44  of pistons  7  and the end wall  45  of cylinders  36   a  when pistons  7  are at the end of their power stroke (i.e., moving in the direction of arrow C). The area of the end face  43  of pistons  3 , however, is larger than the area of the end face  44  of pistons  7  (for reasons described further below).  
         [0035]     Transition arm  22  is supported on U-joint  23 , which is connected to stationary housing part  2  such that U-joint  23  does not rotate. U-joint  23  acts as a pivot point for transition arm  22  such that linear movement of pistons  3  and  7  results in the nose pin of transition arm  22  moving in a generally circular fashion about assembly axis A. Because the nose pin is connected to rotating member  21 , the circular movement of the nose pin causes the rotating member  21  to rotate about assembly axis A. Rotating member  21  is connected to a shaft  33 , which rotates with rotating member  21 .  
         [0036]     Referring also to  FIGS. 2A, 2B ,  3 , and  4 , mounted on shaft  33  is a face valve  10  and a valve cylinder  11  that control the flow of fluid in motor half  31 . Face valve  10  and valve cylinder  11  are mounted on shaft  33  such that face valve  10  and valve cylinder  11  rotate with shaft  33 . For instance, shaft  33  has a splined end that mates with splined holes  37  and  40  defined in face valve  10  and valve cylinder  11 , respectively.  
         [0037]     Shaft  33  is attached to valve cylinder  11  via a bolt  18 . Between the bolt head and valve cylinder  11  is a spring washer  19 , such as a Belleville washer. Spring washer  19  provides a force against valve cylinder  11  so that valve cylinder  11  and face plate  10  remain in contact during operation, thereby maintaining a seal between the two to limit the possibility of leakage of the drive fluid.  
         [0038]     Face valve  10  includes an inlet section  38  that is connected to an inlet tube  16  through a port  12  and space  41  in valve cylinder  11 . Inlet tube  16  extends from space  41  in valve cylinder  11  through stationary housing part  14 . Valve cylinder  11  rotates about inlet tube  16  during operation and o-rings  17  seal the area between valve cylinder  11  and inlet tube  16 . Face valve  10  also includes an outlet section  39  connected to a port  20  of valve cylinder  11 . Port  20  is connected to holes  34  located about stationary housing part  13  such that the drive fluid is exhausted from motor side  31  into the well.  
         [0039]     During operation, fluid is delivered from the surface via a low volume/high pressure line to inlet tube  16  and delivered to inlet section  38  via port  12  in valve cylinder  11 . Inlet section  38  is in fluidic contact with some of cylinders  36   a  (e.g., two of the cylinders  36   a ) at a time as valve cylinder  11  and face valve  10  rotate. Thus, the fluid from the surface enters the aligned cylinders  36   a  and causes the corresponding pistons  7  to move along piston axis P in the direction of arrow C ( FIG. 1A ). The movement of the aligned pistons  7  results in movement of the nose pin of transition arm  22  in a generally circular fashion about assembly axis A, which cause rotating member  21  and shaft  33  to rotate. As a result, face valve  10  and valve cylinder  11  also rotate about assembly axis A, which brings other cylinders  36   a  and corresponding pistons  7  into alignment with inlet section  38 , thereby allowing the delivered fluid to cause those pistons  136   a  to move along piston axis P in the direction of arrow C.  
         [0040]     For simplicity of illustration, port  12  is shown as aligned with cylinder  36   a  in  FIG. 1B  while drive piston  7  is at the end of its power stroke and getting ready to enter an exhaust stroke. During operation, however, valve cylinder  11  is rotated 90° from the position shown when the stroke of drive piston  7  is at this point. That is, cylinder  36   a  is aligned with one of the ends  46   a  or  46   b  (depending on the direction of rotation) of inlet section  38 .  
         [0041]     At the same time that some of the pistons  7  are moving along piston axis P in the direction of arrow C, the piston(s)  7  not aligned with inlet section  38  are moving along piston axis P in the opposite direction, i.e., in the direction of arrow D. The cylinders  36   a  corresponding to the piston(s)  7  moving in the direction of arrow D are aligned with outlet section  39 . Thus, as the pistons  7  aligned with outlet section  39  move in the direction of arrow D, fluid contained in their corresponding cylinders  36   a  (which entered those cylinders through inlet section  38 ) is expelled through outlet section  39  to port  20  and out of motor/pump  30  through holes  34 .  
         [0042]     In this manner, motor half  31  acts as a hydraulic motor. Delivery of the drive fluid to inlet tube  16  causes the pistons  7  to reciprocate along piston axis P. The reciprocation of pistons  7  results in pistons  3  also reciprocating along piston axis P in cylinders  36   b.    
         [0043]     Cylinders  36   b  are in fluidic communication with inlet ports  28  located on pump half  32 . Inlet ports  28  open to one side of seal  1  such that they are in fluidic communication with the fluid to be pumped. Inlet ports  28  include a check valve  27  (e.g. a poppet valve) that only allows fluid to flow through inlet ports  28  in the direction of arrow D. Consequently, as pistons  3  move in the direction of arrow D, fluid is pulled into inlet ports  28  and cylinders  36   b;  however, as pistons  3  move in the direction of arrow C, fluid does not flow out of pump half  32  through inlet ports  28 .  
         [0044]     Instead, the fluid in cylinders  36   b  are output via output ports  24 , which are in fluidic communication with inlet ports  28  via a port  26 . Outlet ports  24  are open to the opposite side of seal  1  than inlet ports  28  such that the fluid is pumped towards the top of the well or borehole. The fluid is guided towards the top of the well or borehole by the well casing  47 . Outlet ports  24  also include a check valve  25  that only allows fluid to flow in the direction of arrow D. Consequently, as pistons  3  move in the direction of arrow C, fluid is pumped out of cylinders  36   b,  through ports  26 , and out of outlet ports  24 ; however, as pistons  3  move in the direction of arrow D, fluid is not pulled into pump half  32  through outlet ports  24 .  
         [0045]     Accordingly, fluid from the surface entering cylinders  36   a  that are aligned with inlet section  38  of face valve  10  causes the corresponding pistons  7  and  3  to move in the direction of arrow C. This results in fluid being pumped out of the corresponding cylinders  36   b  through ports  26  and out outlet ports  24 . At the same time, the pistons  7  on the motor half  31  not aligned with inlet section  38  and the corresponding pistons  3  on the pump side  32  are moving in the direction of arrow D. This results in the fluid in the corresponding cylinders  36   a  being pumped into outlet section  39  of face valve  10 , through outlet port  20  of valve cylinder  11 , and out of holes  34 , along with fluid in the well or borehole being pulled into the corresponding cylinders  36   b  on the pump side  32  via inlet ports  28 .  
         [0046]     Thus, during operation, a high pressure/low volume down line is connected to the inlet port  16  on motor side  31 , and combination hydraulic motor/pump  30  is placed below the liquid level in the well or borehole such that inlet ports  28  on pump side  32  are in fluidic communication with the fluid in the well or borehole to be pumped (the pump fluid). A high pressure/low volume drive fluid is pumped to the inlet tube  16  on motor side  31  via the down line. The drive fluid enters some of cylinders  36   a  by port  12  of valve cylinder  11  and inlet section  38  of face valve  10  and cause pistons  7  to move linearly within cylinders  36   a,  which causes rotating member  21  and, consequently, face valve  10  and valve cylinder  11  to rotate. As face valve  10  and valve cylinder  11  rotate, the drive fluid causes other of pistons  7  to move linearly within cylinders  36   a,  and pistons  136   a  begin reciprocating in cylinders  36   a.  This expels the drive fluid in cylinders  36   a  out through outlet section  39  in face valve  10 , port  20  in valve cylinder  11 , and holes  34 .  
         [0047]     The movements of pistons  7  also directly drive pistons  3 , thereby causing pistons  3  to reciprocate. The reciprocating motion of pistons  3  pumps the fluid in the well or borehole into cylinders  36   b  through inlet ports  28 , and out ports  26  to outlet ports  24 . Because the area of the end faces  43  of pistons  3  is greater than the area of end faces  44  of pistons  7 , pumped fluid at a lower pressure than the drive fluid, but a higher volume than the drive fluid, is pumped back up. The ratio of the area of the end faces  43  of pistons  3  to the area of end faces  44  of pistons  7  determines the ratio of the volume pumped to the volume delivered and can be, e.g., in the range of 3:1 to 10:1.  
         [0048]     Ideally, the ratio of volume pump to volume delivered would be exactly equal to the ratio of the area of the end faces  43  of pistons  3  to the area of end faces  44  of pistons  7 . Absent friction and with an incompressible fluid, the product of the pressure times fluid capacity of the drive fluid would be the same as the pumped fluid. The combined area of end faces  43 , in  FIGS. 1A and 1B , is three times that of the combined area of end faces  44 . Thus, the volume of drive fluid supplied to motor part  31  through inlet port  16  ideally produces three times the volume of pump fluid out of ports  24  at one-third the pressure. For example, if the drive fluid is supplied at a rate and pressure of 10 gallons per minute and 9,000 PSI respectively, the combined output of output ports  24  is ideally 30 gallons per minute at 3,000 PSI.  
         [0049]     But, since the friction and the volumetric efficiency is unlikely to be zero, this ideal is unlikely to be achieved. However, the efficiency of hydraulic motor/pump  30  can be in the mid-ninety percent range. This is because the force provided by pistons  7  is transferred directly to pistons  3 , without passing through the rotating parts of hydraulic motor/pump  30 , which instead serve simply to control the timing of pistons  7  and  3  so that they are evenly spaced in time for proper motoring and pumping purposes. In addition, pistons  7  and  3  are moving in straight lines and generate little or no side load on cylinders  36   a  and  36   b.  Consequently, the frictional losses are low.  
         [0050]     Accordingly, for example, if the ratio of the area of end faces  43  to the area of end faces  44  is 3:1 and efficiency is ninety percent, then for every gallon of drive fluid delivered to motor half  110   a,  approximately 2.7 gallons of pump fluid would be returned (in addition to the one gallon of drive fluid).  
         [0051]     Combination hydraulic motor/pump  30  advantageously allows the drive fluid to be the same or compatible with the pumped fluid, such that a return hydraulic line is not needed. Rather, both fluids are returned up the well or borehole. For instance, if potable water is being pumped, then high pressure water can be used as the drive fluid. Similarly, if the fluid being pumped is crude oil, then a fluid such as cleaned up crude oil, semi-refined oil, or even water can be used to drive the motor so that the drive fluid and pumped fluid are able to be returned along the same up line. In the event an application requires that the drive fluid and pumped fluid not mix, the combination hydraulic motor/pump  30  can be implemented with a hydraulic return line for the drive fluid.  
         [0052]     The fluid pressure needed to run combination hydraulic motor/pump  30  is generated at the top of the well, for example, by an electric motor or internal combustion engine used to run a hydraulic pump that provides a high pressure fluid flow. The ratio of fluid pressure into the well to the fluid pressure out of the well is roughly the inverse of the respective flow rates into and out of the well, with allowances for losses in the pipes owing to viscosity and other mechanical forces. Because the fluid/power source is located at the top of the well or borehole, it is easily serviced, resulting in low maintenance costs. The power source may be a combination internal combustion engine/hydraulic pump similar to combination hydraulic motor/pump  30  (with motor side  31  implemented as an internal combustion engine, such as is described with respect to FIGS. 32 and 32a in PCT Application WO 03/100231, filed May 27), which provides high energy efficiency and low power costs. Similarly, a combination electric motor/hydraulic pump similar to that described with respect to FIGS. 65 and 67 in PCT Application WO 03/100231, filed May 27, 2003, can be employed.  
         [0053]     Changing the pressure or rate of flow of the drive fluid in the high pressure drive line changes the rate of flow of the pumped fluid in the low pressure return. When a combination engine and hydraulic pump are used as the power source, changing the engine speed varies the rate of flow of the drive fluid. Alternatively, the hydraulic pump portion of the power source can be a variable stroke pump (such as the ones described with respect to FIG. 50 and 54 in PCT Application WO 03/100231, or described below with respect to  FIGS. 5A and 5B ), which allows the flow rate to be changed without varying the engine speed. This provides a simple mechanism for controlling the rate of pumping.  
         [0054]     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, the flow of fluid can be controlled on pump side  32  through a second face valve whose rotation is synchronized with face valve  10 , instead of through the check valves  28  and  25 . In addition, a variable stroke mechanism can be included in combined motor/pump  30 , such as the ones described with respect to FIG. 50 and 54 in PCT Application WO 03/100231, or described below with respect to  FIGS. 5A and 5B . Furthermore, while three double ended pistons and corresponding cylinders have been shown, more or less pistons and cylinders can be used.  
         [0055]     Referring to  FIGS. 5A-5C , a hydraulic assembly  500  (e.g., hydraulic pump or motor) designed to reduce misalignments caused by centrifugal forces includes a stationary housing  502  defining a chamber  504 , and a rotating drum  506  located within chamber  504 . Drum  506  is mounted on a shaft  508 . Application of torque to shaft  508  rotates drum  506  about assembly axis A in housing  502 , and rotation of drum  506  causes rotation of shaft  508 . Drum  506  includes a first section  506   a,  a second section  506   b,  and support members (e.g., cylindrical tie rods)  506   c  that connect first section  506   a  and second section  506   b  so that the first and second sections rotate together (and for other reasons further described below).  
         [0056]     First section  506   a  and second section  506   b  define an open region  512  between them. Located within open region  512  is a transition arm  514 . Transition arm  514  is mounted to, e.g., a universal joint (U-joint)  516 . For example, U-joint  516  is a cardon type U-joint, however, other types of U-joints can be used. Transition arm  514  includes a nose pin  518  that is coupled to a bearing block  520  via a self-aligning nose pin bearing  558  such that transition arm  514  is at an angle β with respect to assembly axis A. Nose pin  518  is axially fixed within bearing block  520 .  
         [0057]     Bearing block  520  is received in an arced channel  560  defined in housing  502  such that bearing block does not rotate, and includes a gear-toothed surface  562  that mates with a pinion gear  564  within housing  502 . A mechanism (not shown), such as an external knob with a shaft engaged with pinion gear  564 , is used to turn pinion gear  564 . Turning pinion gear  564  causes bearing block  520  to slide in arced channel  560  to and away from assembly axis A, thereby changing the angle β. Changing the angle β changes the piston stroke and, consequently, the motor/pump capacity. This allows an operator to adjust the piston stroke of assembly  500 . In other implementations, other mechanisms of controlling the angle β are employed, such as the mechanisms described with respect to FIG. 50 and 54 in PCT Application WO 03/100231, filed May 27, 2003.  
         [0058]     Transition arm  514  also includes drive pins  522  coupled to piston assemblies  524  (e.g., seven piston assemblies  524 ) via piston joint assemblies  526 , such as, e.g., piston joint assemblies described below with respect to  FIGS. 5C-5E  or, e.g., the piston joint assemblies described in FIGS. 56-56F in PCT Application WO 03/100231, filed May 27, 2003. Piston assemblies  524  include single ended pistons having a piston  556  on one end. Pistons  556  are received in cylinders  530  formed in first section  506   a.    
         [0059]     During operation as a hydraulic pump, shaft  508  is rotated, causing drum  506  to rotate, which results in pistons  556  and transition arm  514  rotating with drum  506 . Because nose pin  518  is fixed and transition arm  514  is at an angle β with respect to assembly axis A, rotation of drum  506  causes pistons  556  to reciprocate in cylinders  530  along piston axis P. As pistons  556  reciprocate, fluid is pumped from an inlet  582  to an outlet  538 . Inlet  582  connects to an inlet manifold  584 , while outlet  538  connects to an outlet manifold  570 .  
         [0060]     Pump/motor  500  includes a face valve  536  to control the fluid pumping during operation. Face valve  536  allows fluid to be pulled from the inlet  582  through inlet manifold  584  into a cylinder  530  when the cylinder&#39;s corresponding piston  556  is on an intake stroke (moving in the direction of arrow F), while directing fluid from a cylinder  530  through outlet manifold  570  to outlet  538  when the cylinder&#39;s corresponding piston  556  is on a pump stroke (moving in the direction of arrow G).  
         [0061]     During operation as a hydraulic motor, the operation is reversed. A pressurized stream of fluid is provided to inlet  582 . This fluid causes some of the pistons  556  to move in the direction of arrow F. This movement causes drum  506  and transition arm to  514  to rotate, which causes the other pistons  556  to move in the direction of arrow G. The movement of these pistons expels any drive fluid in their corresponding cylinders  530  out of outlet  538 . As this process continues, pistons  556  reciprocate in cylinders  530  as drum  506  rotates. Rotation of drum  506  causes shaft  508  to rotate.  
         [0062]     Referring to  FIGS. 5D-5F , piston joint assemblies  526  include a spherical or cylindrical bearing  540  that is coupled to drive pin  522 . Bearing  540  is seated between bearing pads  542   a  and  542   b  located in a casing  544 . Casing  544  includes a piston end  544   a  and an end  544   b  opposite the piston end. A hole  546  is formed in piston end  544   a.  In addition, piston end  544   a  and bearing pad  542   a  define a space  534  therebetween.  
         [0063]     Piston  556  has a circular head  548  that is received in space  534  and abuts against a face of bearing pad  542   a,  while piston  556  projects through hole  546 . Circular head  548  has a diameter larger than hole  546  such that circular head  548  can not pass through hole  546 . Hole  546  has a diameter larger than the outer diameter of piston  556  passing through hole  546 . In addition, there is spacing  554  between the inner surface of the sides of casing  544  and circular head  548 . The larger diameter of hole  546  and the spacing  554  between the inner surface of casing  544  and circular head  548  allows casing  544 , bearing  540 , and bearing pads  542   a  and  542   b  to slide relative to circular head  548  and piston  556  in the direction of arrow E for reasons described further below. The faces of circular head  548  are polished to minimize friction.  
         [0064]     Referring to  FIGS. 5B, 5E , and  5 F, each side  544   a,    544   b  of casing  544  includes two extensions  550 , each having, e.g. half circle, cutouts  552  that mate with support members  506   c  on either side of the piston joint assemblies  526 , as shown in  FIG. 5B , so that piston joint assemblies  526  ride along support members  506   c.  Support members  506   c  are polished and extensions  550  have a low friction surface to reduce friction as piston joint assemblies  526  ride along support members  506   c.    
         [0065]     Support members  506   c,  extensions  550  with cutouts  552 , and the sliding of piston  556  relative to casing  544  act to reduce the effects of centrifugal force. During operation as a pump, the rotation of drum  506  and piston assemblies  524  results in transition arm  514  (through drive pins  522 ) driving piston joint assemblies  526  back and forth along piston axis P (i.e., causing piston joint assemblies  526  to reciprocate along piston axis P). Piston joint assembly  526  applies this drive force to piston  556  through circular head  548 . On the pump stroke, this force is applied to circular head  548  by bearing pad  542   a.  On the intake stroke, this force is applied to circular head  548  by the inner surface of piston end  544   a  of case  544 . Similar forces are applied during operation as a motor.  
         [0066]     The rotation of drum  506  and piston assemblies  524  produces centrifugal force on piston joint assemblies  526 , causing them to move outward (i.e., radially away from assembly axis A). This movement of piston joint assemblies  524  can cause misalignment of pistons  556  in cylinders  530  if the movement also acts on pistons  556 . The attachment of piston joint assemblies  524  to support members  506   c  via cutouts  552  reduces the magnitude of this movement by transferring the centrifugal force of the piston joint assemblies  526  to support members  506   c.  In addition, the ability of piston  556  to slide relative to piston joint assembly  526  isolates piston  556  from any outward movement of piston joint assembly  526  that does occur. Thus, any movement of piston joint assembly  526  that does occur is prevented from being applied to pistons  556 , particularly if the diameter of hole  546  and spacing  554  are designed such that circular head  548  and piston  556  can slide without contacting casing  544  for the expected amount of movement of piston joint assembly  526 .  
         [0067]     As a result, the centrifugal force acting on pistons  556  is reduced to the centrifugal force generated by the mass of pistons  556 , rather than the centrifugal force generated by the mass of pistons  556  and piston joint assemblies  526 . The mass of pistons  556  can be kept relatively small, in the range of one-sixth to one-eighth of the mass of piston joint assemblies  526 . Consequently, pistons  556  are able to have a closer fit to their respective cylinders  530  because the frictional heating and resulting size change is lower than when the centrifugal force acting on pistons  556  is the result of the mass of pistons  556  and piston joint assemblies  526 . When the centrifugal force acting on pistons  556  is the result of the mass of pistons  556  and piston joint assemblies  526 , pistons  556  experience greater misalignment and, therefore, rubbing against the cylinder sidewalls. This results in frictional heating that increases the size of pistons  556 , which causes a greater surface area of pistons  556  to touch the cylinder sidewalls if not enough spacing is providing between pistons and the cylinder sidewalls.  
         [0068]     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, other mechanisms to control fluid flow can be used; for example, poppet valves such as those shown in  FIG. 1  can be used. Support members  506   c,  extensions  550  with cutouts  552 , and the sliding of piston  556  relative to casing  544  can be implemented in other types of assemblies that use rotating pistons and cylinders. In addition, while seven single ended pistons  556  and corresponding cylinders  530  have been shown, more or less pistons and cylinders can be used and the pistons can be double-ended, rather than single ended, as shown, for example, in the assembly described with respect to  FIG. 1 .  
         [0069]     Referring to  FIGS. 6A and 6B , a hydraulic assembly  600  (e.g., hydraulic pump or motor) designed to counteract forces that tend to move the face valve  636  away from the rotating drum  606  or bulkhead  684 , similar to pump/motor  500 , includes a stationary housing  602  defining a chamber  604 , and a rotating drum  606  located within chamber  604 . Drum  606  is mounted on a shaft  608  such that drum  606  does not slide along shaft  608 . For example, shaft  608  fits through a hole in drum  608  and a pin is used to prevent sliding and causes drum  606  to rotate with shaft  608 . Alternatively, a hole in drum  606  can be press fit to shaft  608 . Drum  606  includes a first section  606   a,  a second section  606   b,  and connecting members  606   c  that connect first section  606   a  and second section  606   b  so that the first and second section rotate together.  
         [0070]     First section  606   a  and second section  606   b  define an open region  612  between them. Located within open region  612  is a transition arm  614 . Transition arm  614  is mounted to, e.g., a universal joint (U-joint)  616 . For example, U-joint  616  is a cardon type U-joint, however, other types of U-joints can be used. Transition arm  614  includes a nose pin  618  that is coupled to a bearing block  620  via a self-aligning nose pin bearing  658  such that transition arm  614  is at an angle β with respect to assembly axis A. Nose pin  618  is axially fixed within bearing block  620 . Bearing block  620  is mounted in an arced channel  660  such that bearing block  620  does not rotate and allows the angle β to be adjusted as described above with respect to  FIG. 5A .  
         [0071]     Transition arm  614  also includes drive pins  622  coupled to piston assemblies  624  (e.g., seven piston assemblies  624 ) via piston joint assemblies  626 , such as, e.g., piston joint assemblies described above with respect to  FIGS. 5D-5F  (and assembly  600  may include support members  506   c ) or, e.g., the piston joint assemblies described in FIGS. 56-56F in PCT Application WO 03/100231, filed May 27, 2003. Piston assemblies  624  include single ended pistons having a piston  656  on one end and a guide rod  628  on the other end. Pistons  656  are received in cylinders  630  formed in first section  606   a.  Guide rods  632  are received in a sleeve-bearing  688  held by second section  606   b.    
         [0072]     During operation as a pump, as shaft  608  is rotated, drum  606  rotates, which results in pistons  656  and transition arm  614  rotating with drum  606 . Because nose pin  618  is fixed and transition arm  614  is at an angle β with respect to assembly axis A, rotation of drum  606  causes pistons  656  to reciprocate in cylinders  630  along piston axis P. As pistons  656  reciprocate, fluid is pumped from an inlet (not shown, but similar to inlet  582 ) to an outlet  638 . Pump/motor  600  includes a face valve  636  (like face valve  10 ) to control the fluid pumping during operation. Face valve  636  allows fluid to be pulled from the inlet through an inlet manifold (not shown, but similar to inlet manifold  584 ) when the cylinder&#39;s corresponding piston  656  is on an intake stroke (moving in the direction of arrow F), while directing fluid from a cylinder  630  to through outlet manifold  670  to outlet  638  when the cylinder&#39;s corresponding piston  656  is on a pump stroke (moving in the direction of arrow G).  
         [0073]     During operation as a hydraulic motor, the operation is reversed. A pressurized stream of fluid is provided to the inlet. This fluid causes some of the pistons  656  to move in the direction of arrow F. This movement causes drum  606  and transition arm to  614  to rotate, which causes the other pistons  656  to move in the direction of arrow G. The movement of these pistons expels any drive fluid in their corresponding cylinders  630  out of outlet  638 . As this process continues, pistons  656  reciprocate in cylinders  630  as drum  606  rotates. Rotation of drum  606  causes shaft  608  to rotate.  
         [0074]     The pressure of the hydraulic fluid through face valve  636 , whether from the inlet when operated as a motor or from outlet  638  and pistons  656  when operated as a pump, causes face valve  636  to separate from drum  606  or bulkhead  684 , which can result in leakage and valve failure. To counteract this force, a variable force mechanism  682  is included. The variable force mechanism  682  includes compensating pistons  662  (e.g., two compensating pistons  662 ) located in cylinders  664  defined by bulkhead  684 , a plate  666 , a washer spring  668 , and a nut  678  with flange  680 . Compensating pistons  662  and cylinders  664  are situated parallel to and on opposite sides of assembly axis A. Cylinders  664  are connected via ports  672  to the inlet or output manifold  670  that leads from face valve  636  to outlet  638 . When assembly  600  is operated as a pump, cylinders  664  are connected to outlet manifold  670 . Conversely, when assembly  600  is operated as a motor, cylinders  664  are connected to the inlet manifold.  
         [0075]     Heads  674  of compensating pistons  662  contact plate  666 . Plate  666  is mounted around shaft  608  such that plate  666  does not rotate when shaft  608  rotates, but plate  666  can slide linearly along assembly axis A. For example, the outer diameter of plate  666  may be sized such that it has a close fit with the inner diameter of casing  602  in the space  686  so that plate  666  is supported and can slide linearly along assembly axis A. Plate  666  then has a hole (not shown) through which shaft  608  passes. The hole&#39;s diameter is a sufficient amount so that shaft  608  may pass through a hole in plate  666  without touching plate  666 . A tang (not shown) extends from plate  666  and fits into a groove (not shown) in the inner wall of casing  602  in space  686  to prevent plate  666  from spinning.  
         [0076]     Also mounted on shaft  608  next to plate  666  is a thrust bearing  676 , spring washer  668 , and nut  678  with flange  680 . Nut  678  and spring washer  668  are situated on shaft  608  such that they rotate with shaft  608 , but do not slide along shaft  608 . For example, a portion of shaft  608  is threaded. After spring washer  668  is placed onto shaft  608 , nut  678  is screwed onto the threaded portion. Nut  678  is screwed onto shaft  608  to a position that partially compresses spring  668 , thereby exerting a force in the direction of arrow G on nut  678  through flange  680 .  
         [0077]     In addition, as compensating pistons  662  exert a force on plate  666  (as described below), plate  666  moves in the direction of arrow G. Movement of plate  666  applies the force exerted on plate  666  to spring washer  668  through thrust bearing  676 , which results in spring washer  668  being further compressed, thereby increasing the force exerted in the direction of arrow G on nut  676  through flange  680 . Because drum  606  is mounted on shaft  608  such that drum  606  does not slide along shaft  608 , and nut  678  does not slide along shaft  608 , the force from spring  668  on nut  678  is exerted on drum  606  through shaft  608 , thereby urging drum  606  towards face valve  636 . This force acts to counteract the force in the direction of arrow F caused by the hydraulic pressure.  
         [0078]     When there is no inlet or output pressure (e.g., before pump/motor  600  begins operation), spring  668  provides an initial force on drum  606  in the direction of arrow G because spring  668  is partially compressed as described above. This initial force maintains a seal between drum  606  and face valve  636 , and between face valve  636  and bulkhead  684 , thereby preventing the leakage of fluid when pump/motor  600  begins operating. The initial force exerted by spring  668  depends on the amount spring  668  is initially compressed, which depends on the position nut  678  is mounted on shaft  608 . The amount of the initial force is sufficient to maintain a seal between drum  606  and face valve  636  and between face valve  636  and bulkhead  684  during start-up conditions.  
         [0079]     During operation, a force proportional to the input pressure, when operated as a motor, or output pressure, when operated as a pump, is exerted on drum  606  in the direction of arrow F. This force tends to urge face valve  636  away from bulkhead  684  in the case of a motor, and drum  606  away from face valve  636  in the case of a pump. To compensate for this force, the pressure is communicated from inlet (for a motor) or output manifold  670  (for a pump) to compensating pistons  662  by ports  672 . The pressure causes compensating pistons  662  to move in the direction of arrow G, thereby exerting a force on plate  666 , which is transferred to drum  606  through bearing  676 , spring  668 , nut  678 , and shaft  608 . This compensating force counteracts the force acting on drum  606  and/or face valve  636  in the direction of arrow F so as to maintain the seal between drum  606  and face valve  636 , and between face valve  636  and bulkhead  684 .  
         [0080]     The amount of compensating force exerted by one of the compensating pistons  662  on plate  666  (and hence drum  606 ) is proportional to the input or output pressure times the area of the end face of compensating piston  662 . Thus, as the input or output pressure increases, the force exerted on drum  606  in the direction of arrow F increases, and the compensating force exerted by compensating pistons  662  in the direction of arrow G also increases. The total compensating force exerted on drum  606  depends on the total compensating force exerted by compensating pistons  662  and the spring force of spring  668 . Accordingly, spring  668 , the number of compensating pistons  662 , and the area of the compensating pistons  662  are designed to exert an additional force for a given pressure that, when added to the initial force provided by spring  668 , is sufficient to counteract the force in direction F that results during operation.  
         [0081]     The use of spring  668  to provide an initial force and compensating pistons  662  to provide additional compensating force proportional to the input or output pressure allows the friction between drum  606  and face valve  636  and between face valve  636  and bulkhead  684  to be low at low inlet or output pressures, while increasing to maintain a seal between drum  606  and face valve  636  and between face valve  636  and bulkhead  684  at higher pressures. This improves efficiency when the pump or motor is operating at low input or output pressures and also reduces the friction and wear during start-up conditions.  
         [0082]     Referring to  FIGS. 7A and 7B , an alternate implementation of a hydraulic assembly  700  (e.g., hydraulic pump or motor) designed to counteract forces that tend to move the face valve  736  away from the rotating drum  706  or bulkhead  784 , which is designed and operated similar to pump/motor  600 , includes a variable force mechanism  782  having compensating pistons  762  located in cylinders  764  defined in drum  706 . Cylinders  764  are positioned parallel to cylinders  730  and circumferentially spaced about assembly axis A. Cylinders  764  are fluidically connected to cylinders  730  (and, hence, the input or output pressure) by ports  772 . There are, for example, the same number of compensating pistons  762  as there are pistons  756 . However, the same number of compensating pistons  762  as pistons  756  does not need to be used. For instance, for an even number of pistons  756 , half the number of compensating pistons  762  can be used (e.g., 4 compensating pistons  762  for 8 pistons  756 ).  
         [0083]     A first end portion  774  of pistons  762  contact a support portion  780  of U-joint  716 , which is mounted to shaft  708  such that it rotates with, but does not slide along shaft  708 . Furthermore, because U-joint  716  is connected to transition arm  714 , which is connected to bearing block  720  located in an arced channel (not shown in  FIG. 7 ), when pistons  762  exert a force on support portion  780 , U-joint  716  does not move linearly along assembly axis A.  
         [0084]     In addition, drum  706  is mounted to shaft  708  such that drum  706  can slide along shaft  708  and rotate with shaft  708 . For example, shaft  708  has a splined end that mates with a splined hole in drum  706 . A spring washer  768  (e.g., a Belleville washer) is mounted on shaft  708  between drum  706  and support portion  780 . Spring washer  768  is partially compressed.  
         [0085]     As with pump/motor  600 , when there is no input or output pressure (e.g., before pump/motor  700  begins operation), spring  768  provides an initial force on drum  706  in the direction of arrow G because spring  768  is partially compressed as described above. This initial force maintains a seal between drum  706  and face valve  736  and between face valve  736  and bulkhead  784 , thereby preventing the leakage of fluid when pump/motor  700  begins operating. The initial force exerted by spring  768  depends on the amount spring  768  is initially compressed.  
         [0086]     During operation, as the pistons are reciprocating, a force proportional to the inlet or output pressure is exerted on drum  706  in the direction of arrow F, which is compensated for by compensating pistons  762 . The inlet or output pressure is communicated from cylinders  730  to compensating pistons  762  by ports  772 . The pressure in cylinders  730  causes a force to be exerted at a second end  774   b  of compensating piston  762 , which results in compensating piston  762  applying the force to support portion  780 , causing an equal but opposite force to be exerted on an end wall  764   a  of cylinder  764  in the direction of arrow G. Because support portion  780  does not move linearly along assembly axis A, but drum  706  does slide along shaft  708 , the forces cause drum  706  to be urged in the direction of arrow G. The force acting on drum  706  from the pressure counteracts the force acting on drum  706  in the direction of arrow F so as to maintain the seal between drum  706  and face valve  736 .  
         [0087]     The amount of compensating force exerted on drum  706  is proportional to the inlet or output pressure times the cross-sectional area of the compensating piston  762 . Thus, as the inlet or output pressure increases and, consequently, the force exerted on drum  706  in the direction of arrow F increases, the compensating force exerted in the direction of arrow G increases. The total compensating force exerted on drum  706  thus depends on the cross-sectional area of the pistons  762  and the number of pistons  762 . Accordingly, the number of compensating pistons  762  and the cross-sectional area of the compensating pistons  762  are designed to exert an additional force for a given pressure that, when added to the initial force provided by spring  768 , is sufficient to counteract the force in direction F that results during operation.  
         [0088]     As with pump/motor  600 , the use of spring  768  to provide an initial force and compensating pistons  762  to provide additional compensating force proportional to the input or output pressure allows the friction between drum  706  and face valve  736  to be low at low input or output pressures, while increasing to maintain a seal between drum  706  and face valve  736  at higher pressures. This improves efficiency when the pump is operating at low pressures and also reduces the friction and wear during start-up conditions. In addition, the design of pump/motor  700  allows pump/motor  700  to be more compact in design than pump/motor  600 .  
         [0089]     Referring to  FIG. 7C , in an alternate implementation of the assembly of  FIGS. 7A and 7B , rather than spring washers  768 , an assembly  700  includes springs  784 , such as coil springs, coupled to the compensating pistons  762 , to provide an initial compensating force. Compensating piston  762  includes a pin  762   a  at end portion  774   b  near port  772 . Pin  762   a  has a smaller diameter than the rest of piston  762 . Coil spring  784  is seated in cylinder  764  between an end face  764   a  of cylinder  764  and an end face portion  774 d of piston  762 , with pin  762   a  of received within coil spring  784  to center coil spring  784  in cylinder  764 .  
         [0090]     Each of the coil springs  784  are partially compressed, which causes springs  784  to exert a force on end face portion  774 d of compensating pistons  762  in the direction of arrow F and an equal but opposite force on an end wall  764   a  of cylinder  764  in the direction of arrow G. Because support portion  780  does not move linearly along assembly axis A, but drum  706  does slide along shaft  708 , the forces cause drum  706  to be urged in the direction of arrow G. The compensating force exerted on drum  706  in direction G by springs  784  is sufficient to maintain a seal between drum  706  and face valve  736  and between face valve  736  and bulkhead  784  during start-up conditions, thereby preventing the leakage of fluid when pump/motor  700  begins operating. Then, as described above with respect to  FIGS. 7A and 7B , during operation, the compensating pistons  762  result in an additional force that depends on the input or output pressure, which maintains the seal between drum  706  and face valve  736  and between face valve  736  and bulkhead  784 .  
         [0091]     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, other mechanisms to control fluid flow can be used, such as poppet valves as described with respect to  FIG. 1 . In addition, other numbers of pistons and cylinders can be used, and the pistons can be double ended, rather than single ended, as shown in  FIG. 1 .  
         [0092]     Furthermore, elements of one or more implementations described above may be combined, deleted, supplemented, or modified to form further implementations. For example, the variable force mechanisms of  FIGS. 6A-6B  or  7 A- 7 B may be incorporated into assembly  500 , and supports  506   c  and piston joint assemblies  526  may be used in assemblies  600  or  700 . Accordingly, other implementations are within the scope of the following claims.