Hold-down cylinder for axial piston hydraulic pump

Abstract

A positive displacement hydraulic pump/motor assembly ( 1 ) includes a rotary cylinder block ( 3 ) having a central axis ( 5 ) and a generally circular array of cylinders ( 6 ) disposed in parallel relationship around the axis. A corresponding plurality of axial pistons ( 10 ) is reciprocably disposed within the respective cylinders. A drive shaft ( 12 ) effects rotation of the cylinder block about the central axis and a drive plate ( 15 ) is disposed at one end of the cylinder block ( 3 ) to effect sequentially staggered reciprocation of the pistons ( 10 ) in response to rotation of the cylinder block. A stationary valve plate ( 20 ) is disposed at an opposite end of the cylinder block. The valve plate ( 20 ) includes a valve face ( 21 ) adapted for sliding rotational engagement with a complementary mating face ( 22 ) formed on the cylinder block. The valve plate further includes at least one inlet port ( 24 ) adapted for fluid communication with a source of hydraulic fluid and at least one outlet port ( 26 ) adapted for fluid communication with an hydraulic load. The ports ( 24, 26 ) are disposed such that in use, hydraulic fluid is progressively drawn into the cylinders in sequence as the respective pistons are displaced away from the valve plate and subsequently expelled from the cylinders as the pistons are progressively displaced toward the valve plate. The pump further includes selectively variable bias means in the form of a hold-down piston ( 30 ) and cylinder ( 32 ) disposed to apply a variable bias force urging the respective mating faces ( 21, 22 ) on the cylinder block and the valve plate into sealing engagement. The pump/motor assembly is adapted for incorporation into an energy management system operable in a driving mode, a braking mode and a neutral motor to provide supplementary drive from regenerative braking in a vehicle.

1 . A positive displacement hydraulic pump/motor assembly including:—
 a rotary cylinder block having a central axis and incorporating a generally circular array of cylinders disposed in parallel relationship around the axis;  
 a corresponding plurality of axial pistons reciprocably disposed within the respective cylinders;  
 drive means to effect rotation of the cylinder block about the central axis;  
 a drive plate disposed at one end of the cylinder block to effect sequentially staggered reciprocation of the pistons in response to rotation of the cylinder block;  
 a stationary valve plate disposed at an opposite end of the cylinder block, the valve plate having a valve face adapted for sliding rotational engagement with a complementary mating face formed on the cylinder block;  
 the valve plate further including at least one inlet port adapted for fluid communication with a source of hydraulic fluid and at least one outlet port adapted for fluid communication with an hydraulic load;  
 the ports being disposed such that in use, hydraulic fluid is progressively drawn into the cylinders in sequence as the respective pistons are displaced away from the valve plate and subsequently expelled from the cylinders as the pistons are progressively displaced toward the valve plate;  
 the pump further including selectively variable bias means disposed to apply a variable bias force urging the respective mating faces on the cylinder block and the valve plate into sealing engagement.  
 
     
     
         2 . A pump/motor assembly according to  claim 1 , wherein the drive means include a drive shaft, disposed in coaxial relationship with the cylinder block.  
     
     
         3 . A pump/motor assembly according to  claim 1 , wherein the drive shaft extends through a complementary bore formed in the cylinder block.  
     
     
         4 . A pump/motor assembly according to  claim 3 , further including coupling means disposed drivingly to connect the shaft to the cylinder block.  
     
     
         5 . A pump/motor assembly according to  claim 4 , wherein the coupling means include a splined or keyed connection between the shaft and the bore of the cylinder block.  
     
     
         6 . A pump/motor assembly according to  claim 4 , wherein the coupling means are fixed.  
     
     
         7 . A pump/motor assembly according to  claim 4 , wherein the coupling means are selectively releasable.  
     
     
         8 . A pump/motor assembly according to  claim 1 , wherein the drive plate takes the form of a stationary swash plate, which is inclined or inclinable with respect to the central rotational axis of the cylinder block.  
     
     
         9 . A pump/motor assembly according to  claim 8 , wherein the ends of the pistons remote from the valve plate include followers adapted to slide over the swash plate as the cylinder block rotates.  
     
     
         10 . A pump/motor assembly according to  claim 9 , wherein a hold-down plate is disposed to locate the floating ends of the pistons and retain the followers in sliding contact with the swash plate.  
     
     
         11 . A pump/motor assembly according to  claim 9 , further including springs to facilitate retention of the followers in contact with the swash plate.  
     
     
         12 . A pump/motor assembly according to  claim 8 , wherein the angle of inclination of the swash plate is selectively adjustable, to provide selectively variable flow rate characteristics.  
     
     
         13 . A pump/motor assembly according to  claim 12 , wherein the swash plate is adapted to be selectively inclined in a positive or a negative sense, thereby enabling the assembly alternately to operate as a motor or a pump.  
     
     
         14 . A pump/motor assembly according to  claim 13 , wherein the swash plate can also be oriented in an intermediate or neutral position effectively normal to the central axis, such that rotation of the cylinder block causes no axial movement of the pistons, hence induces no net flow into or out of the cylinders through the ports, and therefore induces no substantial load or drive on the shaft.  
     
     
         15 . A pump/motor assembly according to  claim 1 , wherein the pump/motor unit is a bent axis type hydraulic pump, further including piston connecting rods pivotably attached to a thrust plate adapted to rotate with the cylinder block.  
     
     
         16 . A pump/motor assembly according to  claim 1 , further including lift-off means being selectively operable so as axially to displace the cylinder block marginally away from the valve plate, thereby to minimise rotational resistance under predetermined operational conditions.  
     
     
         17 . A pump/motor assembly according to  claim 16 , wherein the lift-off means include a selectively operable hydraulic lift-off cylinder effectively interposed between the cylinder block and the valve plate, such that upon actuation, the cylinder block is axially displaced marginally away from the valve plate.  
     
     
         18 . A pump/motor assembly according to  claim 17 , wherein one end of the lift-off cylinder is in direct contact with a thrust bearing, to facilitate continued rotation of the cylinder block with minimal frictional resistance with the lift-off cylinder activated.  
     
     
         19 . A pump/motor assembly according to  claim 17 , wherein the lift-off cylinder is adapted to be activated with the swash plate in the neutral position, when there is minimal hydraulic pressure within the pump/motor assembly, thereby enabling selective minimisation of frictional drag between the cylinder block and the valve plate in situations where sealing of the valve ports in order to prevent leakage flow is non-critical to performance, because the pump/motor unit is doing no work.  
     
     
         20 . A pump/motor assembly according to  claim 1 , wherein said bias means include an hydraulic hold-down piston disposed within a complementary hold-down cylinder.  
     
     
         21 . A pump/motor assembly according to  claim 20 , further including a control actuator disposed to regulate pressure in the hold-down cylinder.  
     
     
         22 . A pump/motor assembly according to  claim 20 , wherein the hold-down cylinder is supplied by pilot pressure from the pump/motor unit.  
     
     
         23 . A pump/motor assembly according to  claim 20 , wherein the hold-down cylinder is supplied from a supplementary external source of hydraulic fluid pressure.  
     
     
         24 . A pump/motor assembly according to  claim 20 , wherein the hold-down piston takes the form of an annular sleeve disposed coaxially around the drive shaft, at an end of the cylinder block remote from the valve plate.  
     
     
         25 . A pump/motor assembly according to any one of  claim 21 , wherein the control actuator is adapted to regulate the hold-down cylinder according to predetermined pressure and flow characteristics of the pump/motor assembly.  
     
     
         26 . A pump/motor assembly according to  claim 25 , wherein the hold-down cylinder is regulated such that under conditions of minimal motor or pump load, the pressure to the hold-down cylinder is released or substantially reduced so as to minimise frictional drag between the rotary cylinder block and the stationary valve plate, and such that under conditions of relatively high motor or pump load, the pressure to the hold-down cylinder is substantially increased, so as to minimise leakage between the cylinder block and the valve plate.  
     
     
         27 . A pump/motor assembly according to  claim 26 , wherein the hold-down cylinder is controlled in a linear, step-wise or other manner in response to pressure changes at selected points in the system.  
     
     
         28 . A pump/motor assembly according to  claim 27 , wherein the hold-down cylinder is further controlled wholly or in part according to other system parameters including one or more of: leakage flow; frictional drag; temperature changes; or combinations of such parameters.  
     
     
         29 . A pump/motor assembly according to  claim 1 , wherein the bias means include an actuating element selected from the group comprising:—an electromagnetic solenoid; a mechanical screw; an electrical actuator; a magnetic actuator; a mechanical linkage; or an hydraulic, pneumatic, electrical or mechanical hybrid actuating arrangement.  
     
     
         30 . A pump/motor assembly according to  claim 1 , further including a retention spring disposed to provide a minimum threshold level of substantially constant initial bias force sufficient to hold the cylinder block against the valve plate at relatively low pump/motor pressure levels, independently of the selectively variable bias means.  
     
     
         31 . A pump/motor assembly according to  claim 30 , wherein the retention spring takes the form of a preloaded annular bevel or frusto-conical washer disposed within an annular recess formed between the hold-down piston and the cylinder block.  
     
     
         32 . A pump/motor assembly according to  claim 31 , when dependent upon any one of  claims 20  to  30 , wherein the threshold force provided by the retention spring is sufficient to allow the pump to supply a predetermined level of pilot pressure to the hydraulic hold-down cylinder with minimal leakage, at an initial non-zero swash plate angle.  
     
     
         33 . A pump/motor assembly according to  claim 32 , wherein the initial swash plate angle for the predetermined level of pilot pressure is between 0.1 degrees and around 5.0 degrees.  
     
     
         34 . A pump/motor assembly according to  claim 33 , wherein the initial swash plate angle for the predetermined level of pilot pressure is between 0.3 degrees and around 1.0 degree.  
     
     
         35 . A pump/motor assembly according to  claim 34 , wherein the initial swash plate angle for the predetermined level of pilot pressure is around 0.5 degrees off the neutral or zero position.  
     
     
         36 . A pump/motor assembly according to  claim 1 , wherein the coupling means include a releasable locking mechanism, adapted to enable selective disengagement of the cylinder block from the drive shaft.  
     
     
         37 . A pump/motor assembly according to  claim 36 , wherein the releasable locking mechanism includes a tapered locking collet disposed coaxially around the drive shaft.  
     
     
         38 . A pump/motor assembly according to  claim 37 , wherein the locking collet is splined internally for engagement with a correspondingly splined section of the shaft, so as to transmit rotary drive, while accommodating a limited degree of relative axial displacement between the collet and the shaft.  
     
     
         39 . A pump/motor assembly according to  claim 38 , when dependent upon  claim 20 , wherein the hydraulic hold-down cylinder, in addition to its function of applying a variable hold-down force, is configured positively to disengage the locking collet mechanism when the hydraulic lift-off cylinder is simultaneously de-pressurised.  
     
     
         40 . A pump/motor assembly according to  claim 39 , wherein an inner surface of the cylinder block is tapered to match a corresponding outer surface profile of the locking collet.  
     
     
         41 . A pump/motor assembly according to  claim 40 , wherein the outer surface of the locking collet is also splined, for engagement with complementary splines formed on the inner surface of the cylinder block such that upon axial engagement, the two components become mechanically interlocked for conjoined rotation.  
     
     
         42 . A pump/motor assembly according to  claim 41 , wherein the locking collet positively transmits rotary drive from the shaft to the cylinder block while allowing a limited degree of axial displacement of the cylinder block along the shaft, said limited degree of axial displacement, with the splines engaged, permitting the cylinder block to be alternately held positively against the valve plate by the hold-down cylinder and displaced marginally away from the valve plate by the lift-off cylinder, whereas upon full axial disengagement, the shaft is able to spin independently of the cylinder block.  
     
     
         43 . An energy management system operable in a driving mode, a braking mode and a neutral mode, said energy management system including:—
 energy accumulation means operable selectively to store and release energy through controlled receipt and release of hydraulic fluid;  
 a positive displacement hydraulic pump/motor assembly, as defined in any one of the preceding claims, in fluid communication with the energy accumulation means;  
 an hydraulic reservoir in fluid communication with the pump/motor assembly;  
 and coupling means for coupling the pump/motor assembly to a drive shaft;  
 the system being arranged such that in the braking mode the pump/motor assembly retards the drive shaft by pumping hydraulic fluid into the accumulation means, in the driving mode the pump/motor assembly supplies supplementary power to the drive shaft using pressurised hydraulic fluid from the accumulation means, and in the neutral mode the pump/motor assembly is effectively inoperative and exerts no substantial driving or retarding influence on the drive shaft.  
 
     
     
         44 . An energy management system according to  claim 43 , wherein the drive shaft forms part of the drive train of a vehicle.  
     
     
         45 . An energy management system according to  claim 43 , wherein the drive shaft extends through a complementary bore formed in the cylinder block, such that the drive shaft and the cylinder block are coaxial, with the pistons of the pump/motor assembly being uniformly disposed in parallel relationship around the drive shaft.  
     
     
         46 . An energy management system according to  claim 43 , wherein the coupling means include a connection adapted to transmit drive directly between the vehicle drive shaft and the cylinder block.  
     
     
         47 . An energy management system according to  claim 46 , wherein said connection includes spline formations formed respectively in the drive shaft and the cylinder block.  
     
     
         48 . An energy management system according to  claim 43 , wherein said connection includes a transmission interposed to transmit rotary drive between the vehicle drive train and the pump/motor unit.  
     
     
         49 . An energy management system according to  claim 48 , wherein said transmission includes mechanical, hydraulic, pneumatic or electromagnetic transmission elements.  
     
     
         50 . An energy management system according to  claim 48 , wherein said transmission is adapted to the permanently engaged.  
     
     
         51 . An energy management system according to  claim 48 , wherein said transmission is selectively decouplable.  
     
     
         52 . An energy management system according to  claim 43 , wherein the pump/motor assembly includes at least three external ports to permit ingress and egress of hydraulic fluid, with a first port communicating with an inlet of the hydraulic reservoir, a second port communicating with an outlet of the hydraulic reservoir, and a third port communicating with the accumulation means.  
     
     
         53 . An energy management system according to  claim 43 , wherein a heat exchanger is disposed between the first port and the hydraulic fluid reservoir.  
     
     
         54 . An energy management system according to  claim 43 , wherein a plurality of said pump/motor assemblies is arranged axially along the drive shaft, and connected hydraulically to operate in series, parallel, or a combination of both.  
     
     
         55 . An energy management system according to  claim 43 , further including a flow control circuit through which hydraulic fluid may be selectively directed, the control circuit being adapted to provide a controllable resistance enabling the pump/motor unit selectively to exert a retarding force on the drive shaft when required, irrespective of the charge state of the accumulation means.  
     
     
         56 . An energy management system according to  claim 43 , wherein the accumulation means include a gas/liquid accumulator comprising a double-ended cylinder and a piston adapted to float sealingly within the cylinder.  
     
     
         57 . An energy management system according to  claim 56 , wherein one side of the cylinder contains a compressible inert gas, while the other side of the cylinder is connected hydraulically to the pump/motor assembly, whereby the accumulator is adapted to store energy by pumping hydraulic fluid into one side of the cylinder, so as to compress the gas on the other side by displacement of the floating piston, and subsequently to release that energy by expulsion of hydraulic fluid as the compressed gas expands.  
     
     
         58 . An energy management system according to  claim 43 , wherein the accumulation means include a bladder or diaphragm type accumulator.  
     
     
         59 . An energy management system according to  claim 43 , wherein the accumulation means include a plurality of accumulators connected in series.  
     
     
         60 . An energy management system according to  claim 43 , wherein the accumulation means include a plurality of accumulators connected in parallel.