Lift off 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 / motor assembly further includes lift-off means in the form of an hydraulic lift-off piston (40) and cylinder (41), selectively operable to displace the cylinder block marginally away from the valve plate, thereby to minimise rotational resistance between the respective mating valve faces (21, 22) under predetermined operational conditions. 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.

Description

[0001] The present invention relates generally to hydraulic motors and pumps, and more particularly to positive displacement axial piston motors and pumps.[0002] The invention has been developed primarily for use with a pump / motor assembly which forms part of a regenerative drive system ("RDS"), and will be described predominantly hereinafter in that context. It should be appreciated, however, that the invention is not limited to this particular field of use, being readily adaptable to any axial piston hydraulic motor or pump for use in virtually any application.BACKGROUND TO THE INVENTION[0003] In the present context, the invention has been developed more specifically as an improvement to the RDS described by the present applicant in an earlier patent application filed via the Patent Cooperation Treaty (PCT) as international application No. PCT / AU99 / 00740, the full contents of which are hereby incorporated by reference.[0004] As previously described in that earlier patent applicati...

AI Technical Summary

Benefits of technology

[0021] the pump / motor assembly 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.

[0028] The lift-off means preferably 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. Preferably, the exposed 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. The lift-off cylinder would normally only be activated with the swash plate in the neutral position, when there is minimal internal pressure within the pump / motor assembly. Advantageously, this lift-off mechanism enables 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.

[0033] The valve actuator is preferably adapted to supply pressurised oil into the hydraulic hold-down cylinder, and release oil from that cylinder, according to the pressure and flow characteristics of the pump / motor assembly. In general, the system is ideally 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. Conversely, under conditions of relatively high motor or pump load, the supply pressure to the hold-down cylinder is substantially increased, so as to minimise leakage from the ports at the interface between the cylinder block and the valve plate.

[0040] The inner surface of the cylinder block is preferably tapered to match the outer surface profile of the locking collet. The outer surface of the locking collet is preferably 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. In this way, the locking collet positively transmits rotary drive from the shaft to the cylinder block, while still allowing a limited degree of axial displacement of the rotational group along the shaft. This limited degree of axial displacement, with the splines engaged, preferably permits 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. Upon full axial disengagement, however, the shaft is able to spin independently of the surrounding rotational group. Advantageously, this mechanism selectively allows frictional and hydrodynamic drag to be further reduced in situations where the pump / motor assembly is not under load.

Problems solved by technology

The system as described, however, is subject to several significant limitations, many of which are common to previously known axial piston hydraulic motors and pumps.

In this regard, one significant limitation in the pump / motor assembly as previously described, relates to the inherent drag associated with the seals, bearings, valve faces and other elements that are in direct sliding contact with each other, as the cylinder block and other components in the rotational group rotate with respect to the valve plate, housing and other stationary components of the system.

Nevertheless, a residual drag factor remains, which consumes power, generates heat, and compromises the potential efficiency of the system.

While effective in preventing excessive leakage, the relatively high axial force between the cylinder block and the valve plate causes significant frictional drag between these components as they slide rotationally relative to one another, separated only by a thin film of pressurised oil.

The associated inefficiency is particularly significant in situations where the pump / motor assembly is operating in a neutral or "free-wheel" mode, in which the unit is neither pumping nor driving but is nevertheless rotating, often at high speed, as a result of direct connection with a rotary source of power or load.

While applying to some extent in almost any application of axial piston hydraulic motors and pumps, this limitation is particularly significant in the context of an PDS unit fitted to a vehicle, in the manner previously described.

In such situations, the RDS may do no effective work for prolonged periods, and yet introduce an inherent drag factor into the drive line, which ultimately compromises the efficiency of the power train of the vehicle.

Moreover, the basic problem is compounded in such conditions because the drag factor itself is exacerbated when the pump / motor unit is operating in the neutral or free-wheeling mode.

This is because the unit is not producing hydraulic pumping pressure sufficient to sustain the film of hydrostatic pressurised oil required to minimise the effect of frictional drag between the components in direct rotational contact with one another.

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