WO2018178921A1

WO2018178921A1 - An open-loop hydrostatic drive

Info

Publication number: WO2018178921A1
Application number: PCT/IB2018/052182
Authority: WO
Inventors: Norman Grant
Original assignee: Ducere Holdings (Pty) Limited
Priority date: 2017-03-31
Filing date: 2018-03-29
Publication date: 2018-10-04

Abstract

This invention relates to an open-loop hydrostatic drive with engine optimization, regenerative braking and engine braking abilities. The hydrostatic drive comprises a first hydraulic mechanism which has a first and second hydraulic port. The first hydraulic mechanism is linked to a first drive means. The system furthermore comprises a second hydraulic mechanism which is selectively configurable as one of a pump and a motor, which has a third and fourth hydraulic port. The second hydraulic mechanism is linked to an output drive. A first hydraulic fluid line is connected in fluid flow communication between the first and third hydraulic ports.

Description

AN OPEN-LOOP HYDROSTATIC DRIVE

INTRODUCTION AND BACKGROUND

This invention relates to a mechanical drive system. More particularly, the invention relates to an open-loop hydrostatic drive with engine optimization, regenerative braking and engine braking abilities.

Generally, hydrostatic drives comprise a hydraulic pump driven by a first source, typically an internal combustion motor or an electric motor, and a hydraulic motor driven by the hydraulic pump via a common hydraulic pressure line. The hydraulic motor in turn drives a mechanical output, such as an output drive of a vehicle, a hoist of a crane etc.

Two main configurations of hydrostatic drives, namely open-loop and closed-loop hydrostatic drives, exist. In both configurations the high- pressure lines of the hydraulic pump and motor are connected so that the motor is directly driven by the pump via a common high-pressure line.

In broad terms, a closed-loop hydrostatic drive is characterised in that the ports of the hydraulic pump and motor are interconnected. This allows a closed-loop hydrostatic drive to operate in both forward and reverse direction, by directing the high-pressure fluid to the various ports as needed. Typically, to reverse the direction of rotation, the low-pressure and high-pressure ports swop around, typically by altering an angle of a swashplate arrangement of the hydraulic mechanism. Consequently, all of the ports have to be designed to be able to withstand high fluid pressures. This increases the complexity and cost of the hydraulic pump and motor. Furthermore, a charge pump and associated check valve shuttle system is required to direct flow to the control circuit and to the low-pressure line to accommodate volumetric losses and hydraulic leakages, and to facilitate cooling. The closed-loop configuration also requires an expensive high- pressure filter, which are known to easily get obstructed by debris. The use of high pressure hydraulic filters, together with the fact that substantially all the high pressure hydraulic fluid continuously circulates within the closed hydraulic circuit, (circulating from the low-pressure line, through the pump to the motor and back to the pump again in a continuous circuit), can cause closed-loop hydrostatic drives to overheat.

An open-loop hydrostatic drive on the other hand is characterised in that the hydraulic pump and motor do not share a common low-pressure line. The low-pressure lines drain into a common reservoir. This leads to lower pressure handling requirements on the part of the low-pressure ports of the pump and the motor. However, the unit is not capable of reverse operation or to use engine braking without significant and costly modifications, including the use of full directional control valves. This however, furthermore results in pressure spikes or a momentary loss of power (during the transition phase) and inadvertent power losses and reduced efficiency.

Known hydrostatic drives furthermore includes a charge pump which is typically around 15% of the size of the hydraulic pump and is used to generate control pressure and make up fluid losses caused by leakage and volumetric losses. This consumes a significant portion of available power and requires a large number of different components such as shuttle valves and pressure relief valves to direct the fluid to the control circuit and the low-pressure lines. Stocking these pumps and valves increases maintenance and inventory costs.

Current open-loop hydrostatic drives furthermore do not have an inherent regenerative braking capability, while neither open-loop nor closed-loop hydrostatic drives have an inherent engine optimisation capability.

A need exists for a hydrostatic drive that incorporates the functionality of a closed-loop hydrostatic drive with the simplicity of an open-loop hydrostatic drive, while having engine optimization and regenerative braking capabilities. OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide an open-loop hydrostatic drive with which the applicant believes the aforementioned disadvantages of known open-loop and closed-loop hydrostatic drives may at least be alleviated or which may provide a useful alternative for the known open-loop and closed-loop hydrostatic drives.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided an open-loop hydrostatic drive, comprising:

a first hydraulic mechanism having a first and second hydraulic port, the first hydraulic mechanism being linked to a first drive means; a second hydraulic mechanism which is selectively configurable as one of a pump and a motor, and having a third and fourth hydraulic port, the second hydraulic mechanism being linked to an output drive; and

a first hydraulic fluid line connected in fluid flow communication between the first and third hydraulic ports. The second and fourth ports may be provided in fluid flow communication with a reservoir. The second hydraulic mechanism may be selectively configurable as one of a pump and a motor, in any one of a forward and reverse rotational direction.

The second hydraulic mechanism may comprise an open-loop, over- center, variable displacement hydraulic mechanism in the form of a plurality of axially reciprocating pistons and an associated manipulatable and variable swash-plate arrangement which is controllable to move over- center. Alternatively, the second hydraulic mechanism may be a bent axis hydraulic device configurable to an over-centre configuration.

The hydrostatic drive may further comprise an accumulator arranged in fluid flow communication with the first hydraulic fluid line.

A control valve arrangement may be used to regulate the flow of hydraulic fluid into and out of the accumulator. The control valve arrangement may be provided between the accumulator and the first hydraulic line.

The control valve arrangement may comprise a control valve which is arranged in series with a pilot operated check valve. The control valve may be operable between an open configuration to allow hydraulic fluid to flow therethrough to and from the accumulator, and a closed configuration to isolate the accumulator from the first hydraulic fluid line. The pilot operated check valve may be operable between an open configuration where hydraulic fluid may be allowed to flow therethrough to and from the accumulator, and a closed configuration where hydraulic fluid may be allowed to flow therethrough to the accumulator but may be prevented from flowing there through from the accumulator

The first and second hydraulic mechanisms may be substantially similar. Therefore, the first hydraulic mechanism may be selectively configurable as one of a pump and a motor in any one of a forward and reverse rotational direction.

The first drive means may comprise an electrical motor, an internal combustion motor, a wind turbine, a water turbine or a flywheel. The output drive may typically form part of a drivetrain of a vehicle, a turbine, a machine, a crane hoist, a slewing drive, a generator or an alternator.

The hydrostatic drive may further comprise a central control system which may be utilized to control the interaction, configuration and settings of each component of the hydrostatic drive. According to a second aspect of the invention there is provided a method of operating an open-loop hydrostatic drive, the method comprising selectively configuring a second hydraulic mechanism, which is connected in fluid flow communication with a first hydraulic mechanism via a first hydraulic fluid line, as one of a i) a motor, receiving hydraulic fluid via the first hydraulic line, the motor driving an output drive in one of a forward and reverse direction; and ii) a pump being driven in one of a forward and a reverse direction by the output drive, the pump supplying hydraulic fluid to the first hydraulic line.

When the second hydraulic mechanism is configured as a motor, it may be driven by hydraulic fluid supplied to it via the first hydraulic fluid line. The first hydraulic fluid line may be supplied with hydraulic fluid by either or both of the first hydraulic mechanism configured as a pump, and a hydraulic accumulator, which may be arranged in fluid flow communication with the first hydraulic fluid line.

The second hydraulic mechanism, when configured as a pump, may provide pressurised hydraulic fluid via the first hydraulic fluid line, to either or both of the first hydraulic mechanism configured as a motor, and the accumulator. When the first hydraulic mechanism is configured as a pump, it may provide pressurised hydraulic fluid via the first hydraulic fluid line, to either or both of the second hydraulic mechanism configured as a motor and the accumulator.

When the first hydraulic mechanism is configured as a motor, it may be driven by hydraulic fluid supplied to it via the first hydraulic fluid line from either or both of the second hydraulic mechanism configured as a pump, and the hydraulic accumulator.

The configuration of either or both of the first and second hydraulic mechanisms may be varied in real time, thereby configuring the system as a variable speed/power transmission system. The first and second hydraulic mechanisms may therefore rotate at similar or opposite rotational directions. Furthermore, the first and second hydraulic mechanisms may rotate at similar or different rotational speeds. The rotational speeds of the first and second hydraulic mechanisms may be varied individually.

In use, the first and second hydraulic mechanisms may be configured as a pump and a motor respectively.

The flowrate of hydraulic fluid at the first port may be substantially identical to the flowrate of hydraulic fluid at the third port. Alternatively, the flowrate of hydraulic fluid at the first hydraulic port may differ from the flowrate of hydraulic fluid at the third hydraulic port. The difference in the flowrates at the first and third ports may be facilitated by the accumulator.

The output drive may drive the second hydraulic mechanism configured as a pump. The hydrostatic drive may thus be configured in a regenerative braking configuration.

By varying the configuration of the second hydraulic mechanism, the output drive may be driven in one of a forward or a reverse direction.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein: figure 1 is a diagrammatic representation of an open-loop hydrostatic drive for driving an output drive; figure 2 is an exploded view of an example open-loop, over-centre variable displacement hydraulic mechanism selectively operable as one of a pump and a motor, comprising a plurality of reciprocating pistons with an associated manipulatable swashplate arrangement; figure 3 is a side section view of the open-loop, over-centre variable displacement hydraulic mechanism of figure 2; and figure 4 is a side section view of an alternative example hydraulic mechanism in the form of a bent axis open-loop, over-centre variable displacement hydraulic mechanism selectively operable as one of a pump and a motor. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An open-loop hydrostatic drive is generally indicated by reference numeral 10 in figure 1 . The hydrostatic drive 10 is linked an output drive 12.

The hydrostatic drive 10 comprises a first hydraulic mechanism 14 which is linked via a first linking arrangement (preferably in the form of a first coupling or shaft 16) to a first drive means 18. The first hydraulic mechanism 14 furthermore comprises a first hydraulic port 20 and a second hydraulic port 22. The hydrostatic drive 10 further comprises a second hydraulic mechanism

24 which is selectively configurable as either a pump or a motor. The second hydraulic mechanism 24 is linked via a second linking arrangement (preferably in the form of a second coupling or shaft 26) with the output drive 12. The second hydraulic mechanism 24 furthermore comprises a third hydraulic port 28 and a fourth hydraulic port 30.

The third port 28 is connected in direct and uninhibited fluid flow communication with the first port 20 via a first hydraulic line 32. The second and fourth ports (22, 30) are provided in fluid flow communication with a reservoir 34. Hydraulic fluid ejected from the second and fourth ports (22, 30) is allowed to drain into the reservoir 34, which is maintained at a pressure which is substantially below the working pressure of the accumulator. The pressure of the reservoir may be around, or substantially near an atmospheric pressure.

In a further, and preferable embodiment, an accumulator 36 is provided in fluid flow communication with the first hydraulic fluid line 32. The accumulator 36 allows the flow rates at the first and second ports (20, 28) to differ. For instance, in use, in cases where the hydraulic fluid flow rate at the first port 20 exceeds the hydraulic fluid flow rate at the third port 28, the accumulator takes up excessive fluid provided by the first hydraulic mechanism 14. Similarly, if the hydraulic fluid flow rate into the third port 28 exceeds the hydraulic fluid flow rate from the first port, the accumulator supplements the hydraulic fluid flow rate from the first port 20. The second hydraulic mechanism 24 is configurable as either a pump or a motor, in any rotational direction (forward and reverse) of the second shaft 26. Therefore, when configured as a motor, the second hydraulic mechanism 24 receives pressurised hydraulic fluid via the first hydraulic fluid line 32, which is supplied by the first hydraulic mechanism configured as a pump, the accumulator, or both. The second hydraulic mechanism then drives the output drive in either a forward or reverse direction of rotation. On the other hand, when configured as a pump, the second hydraulic mechanism 24, may provide pressurised hydraulic fluid to the first hydraulic fluid line 32, by being driven by the output drive 12 (such as during a regenerative braking cycle as will be described in more detail below). In such a case, the output drive may drive the second shaft 26 in any one of a forward or reverse rotational direction. The first drive means 18 may typically be one of an electrical motor and internal combustion motor. Alternatively, the first drive means may be one of a wind turbine, a water turbine and a flywheel.

The output drive 12 may form part of a drivetrain of a vehicle, a turbine, a machine, a crane hoist, a slewing drive, a generator and an alternator. A central control system (not shown) may be utilized to control the interaction, configuration and settings of each component of the hydrostatic drive 10. A pressure relief valve 40 may be provided between the first hydraulic line

32 and the reservoir 34.

In cases where the first drive means 18 is in the form of an electrical machine, or a flywheel, the first hydraulic mechanism 14 may be selectively configurable as both a pump and a motor, in both a forward and reverse rotational direction of the first shaft 16. In such a case, the first drive means may act as an energy storage device, to absorb surplus energy supplied to the system 10 (as will be described in more detail below). The first and second hydraulic mechanisms (14, 24) may therefore be substantially similar.

As described in more detail below, each of the first and second hydraulic mechanisms (14, 24) is an open-loop, over-centre, variable displacement hydraulic mechanism 100. The hydraulic mechanism 100 comprises a plurality of axially reciprocating pistons 1 18 and an associated manipulatable and variable swash-plate arrangement 102 which is controllable to move over-center. Alternatively, each of the first and second hydraulic mechanisms (14, 24) may be a bent axis hydraulic mechanism (as shown in figure 4) configurable to move over-center.

Each of the first and second hydraulic mechanisms (14, 24) is operable as either a pump or a motor, in any rotational direction of the first and second shafts (16, 26) respectively. For example, when the second hydraulic mechanism 24 is acts as a motor, the second shaft 26 is an output shaft, whereas, when the second hydraulic mechanism 24 acts as a pump, the second shaft acts as an input shaft. The swash plate 102 (or the axis of the bent axis hydraulic mechanism) is moved to an over-centre position to, in concert with the rotational speed of the shaft and the hydraulic pressure, facilitate the second shaft 26 moving in a reverse direction (irrespective of the rotational direction of the first shaft 16, or whether the second shaft 26 is an input or an output shaft). The first hydraulic mechanism 14 may be substantially similar or identical to the second hydraulic mechanism 24.

As stated above, the displacement of the first and second hydraulic mechanisms (14, 24) is variable. By varying the displacement of one or both the first and second hydraulic mechanisms (14, 24), the hydrostatic drive acts as a variable speed/power transmission system. Thus, by variation of the displacement as mentioned above, the first and second shafts (16, 26) may rotate at different rotational speeds, whereas, by varying input parameters (mainly the angle of the swash plate 102 or the axis) the first and second shafts (16, 26) may rotate in different rotational directions. This may be varied in real time.

The first hydraulic mechanism may be operated as a motor. This is typically used to slow down a vehicle while it is travelling down-hill or slowing down, or to controllably lower a load using a crane.

In cases where the accumulator is not used to receive or supply hydraulic fluid to or from the first hydraulic fluid line, the flowrate of hydraulic fluid at the first port 20 may be substantially identical to the flowrate of hydraulic fluid at the third port 28.

A control valve arrangement 38 is provided to regulate the flow of hydraulic fluid into and out of the accumulator 36. The control valve arrangement 38 is positioned between the accumulator 36 and the first hydraulic line 32.

The control valve arrangement 38 is required to regulate flow into and out of the accumulator 36, especially during changes in the configuration of the components of the system 10. The first and second hydraulic mechanisms (14, 24) are configurable to inhibit flow of hydraulic fluid (for instance, in the case of a swashplate type hydraulic mechanism 100, as described in more detail below, by pivoting the swash-plate 102 to substantially zero degrees). However, configuration changes of the hydraulic mechanisms from a pump to a motor configuration (or vice versa) is not instantaneous, which might lead to unwanted torque supplied to one of the first or second shafts (16, 26). For this reason, at least hydraulic fluid flow from the accumulator 36 needs to be controlled by the control valve arrangement 38. In some instances, such as when pressure within the accumulator 36 is very low, the accumulator 36 needs to be temporarily isolated from the first and second hydraulic mechanisms (14, 24).

The control valve arrangement 38 comprises a control valve 38.1 (which may be a directional control valve) arranged in series with a pilot operated check valve 38.2.

The directional control valve 38.1 is operable between an open configuration in which hydraulic fluid is allowed to flow therethrough to and from the accumulator 36, and a closed configuration in which the accumulator 36 is isolated from the first hydraulic fluid line 32.

The pilot operated check valve 38.2 is operable between an open configuration where hydraulic fluid is allowed to flow therethrough to and from the accumulator 36, and a closed configuration where hydraulic fluid is allowed to flow therethrough to the accumulator 36 but prevented from flowing therethrough from the accumulator 36. The pilot-operated check valve 38.2 is therefore a one-way type valve that can be prompted to an open position. The configurations of the directional control valve 38.1 and the pilot operated check valve 38.2 is controlled by command signals.

The directional control valve 38.1 and pilot operated check valve 38.2 can therefore be configured to either allow hydraulic fluid flow into or out of the accumulator 36 uninhibited (when the directional control valve 38.1 is open and the pilot operated check valve 38.2 is prompted to the open position), to inhibit flow into or out of the accumulator 36 therefore isolating the accumulator 36 from the first hydraulic fluid line 32 (when the directional control valve 38.1 is closed) or to allow fluid flow into the accumulator 36, but not out of the accumulator (when the directional control valve 38.1 is open, and the pilot-operated check valve 38.2 is in the closed configuration).

Normally, the flow rates of hydraulic fluid in the first fluid line 32 is regulated by internal settings and configurations of the first and second hydraulic mechanisms (14, 24). Therefore, under normal circumstances the control valve arrangement 38 is not utilized to throttle the flow rate of fluid into or from the accumulator 36. However, utilizing the control valve to throttle flow into or from the accumulator 36 is feasible.

It will be appreciated that, by controlling the directional control valve 38.1 , the pilot operated check valve 38.2 and the internal configurations of the first and second hydraulic units (14, 24), the hydraulic fluid flow can be effectively controlled. For instance, the system can be configured so that hydraulic fluid flows into the accumulator from one or both of the hydraulic mechanisms, (both configured as pumps, or one configured in neutral (i.e. no fluid flows into or from the specific hydraulic mechanism)). Secondly, a portion of hydraulic fluid may flow from one of the hydraulic mechanisms configured as a pump to the other configured as a motor, while a further portion may be either supplied by, or to, the accumulator. Furthermore, both hydraulic mechanisms may simultaneously or individually receive fluid from the accumulator.

The accumulator 36 acts as an energy storage device. As is described in more detail below, energy may be stored in the accumulator 36 by either one or both of the first and second hydraulic mechanisms (14, 24) acting as pumps.

In use, the system 10 may be configured in a number of different system 10 configurations, which are described below. It should again be noted that the rotational speeds of the first and second shafts (16, 26) need not be equal in any of the examples, owing to the system's 10 capability to act as a variable speed/power transmission (as described above). Similarly, the direction of rotation of the first and second shafts (16, 26) need not be the same in any of the examples.

In a first example configuration, the first drive means 18 drives the first hydraulic mechanism 14 configured as a pump. The flowrate of hydraulic fluid from the pump 14 is substantially equal to the flowrate of fluid into the second hydraulic unit 24, configured as a motor and used to drive the output drive 12.

In a second example configuration, the first drive means 18 drives the first hydraulic mechanism 14 configured as a pump. The flow of hydraulic fluid from pump 14 is divided between the accumulator 36, to store excess energy in the accumulator 36, and the second hydraulic mechanism 24, configured as a motor, to drive the output drive 12. This configuration is useful when the power required from the output drive 12 is below the power delivered by the first drive means 18 when operating on its most efficient line. Thus, instead of operating the first drive means 18 inefficiently, surplus power is channeled to, and stored in the accumulator 36 for later use. In a third example configuration, the accumulator 36 and the first hydraulic mechanism 14 configured as a pump both deliver fluid to the second hydraulic mechanism 24 configured as a motor. The flow of hydraulic fluid supplied from the pump 14 is therefore supplemented by the accumulator 36. This configuration is useful when the power required by the output drive 12 exceeds the power available from the first drive means 18. Thus, the power delivered to the output drive 12 is boosted by the accumulator 36 to a level above what can be delivered by the first drive means 18. Alternatively, this configuration is used when the power required by the output drive 12 exceeds the power delivered by the first drive means 18 operating on its most efficient line. Therefore, instead of operating the first drive means inefficiently, surplus power previously stored in the accumulator 36 is channeled to the output drive 12. In a fourth example configuration, the first hydraulic mechanism 14 may be idling or stationary or at substantially zero displacement while the accumulator 36 provides the required flow of hydraulic fluid to the second hydraulic mechanism 24 configured as a motor, thereby driving the output drive 12.

In a fifth example configuration, the first drive means 18 drives the first hydraulic mechanism 14 configured as a pump, supplying hydraulic fluid to the accumulator 36 and thereby storing energy in the accumulator 36 while the second hydraulic mechanism 24 is stationary or at substantially zero displacement. The stored energy may then be channeled to the second hydraulic mechanism 24 when occasion calls for same. In a sixth example configuration, the first drive means 18 may be idling or even held stationary, or the first hydraulic mechanism 14 may be substantially at zero displacement. The second hydraulic mechanism 24 is configured as a pump, supplying hydraulic fluid to the accumulator 36. This configuration is generally termed "regenerative braking" and generally refers to a configuration where energy is absorbed by the system 10 from the output drive 12. The second shaft 26 therefore acts as an input shaft to the system 10. By varying the settings and configuration of the second hydraulic mechanism 24, the rate at which the output drive 12 is braked, or at which energy is absorbed and thus the rate at which kinetic energy is converted into, and stored as, potential energy, is varied.

In a seventh example configuration, the system is configured in the regenerative braking configuration of example configuration six above, with the exception that the first drive means 18 supplies power to the first hydraulic mechanism 14 configured as a pump, thereby supplementing the potential energy in the form of pressurized hydraulic fluid stored in the accumulator 36. In an eighth example configuration, the first hydraulic mechanism 14 is configured as a motor, and the first drive means 18 is a flywheel or an electrical machine configured as a generator or alternator. The electrical machine is connected to a charge storage or receiving means, such as a battery, capacitor or power grid. The motor 14 is supplied with hydraulic fluid from one or both of the accumulator 36 and the second hydraulic mechanism 28, configured as a pump. The motor 14 is therefore used to absorb kinetic energy and convert same into potential energy stored in the flywheel or charge storage/receiving means for later use.

In a first example embodiment as is shown in figures 2 and 3, each of the first and second hydraulic mechanisms (22, 46) comprises an open loop over-center variable displacement hydraulic device 100 that can be operated as a pump or as a motor, such as an axial piston-type hydraulic device utilizing a variable swash plate 102 wherein the angle of the swash plate 102 may be varied to so that the swash plate may move over-center.

The hydraulic mechanism 100 comprises a swash plate 102 having a face 104. The swash plate 102 is arranged to pivot about pivot point 106, so that the angle 108 between the face 104 and a reference line 1 10 can be adjusted. In the current example, the reference line 1 10 lies in a vertical plane. The hydraulic mechanism 100 furthermore comprises a rotatable, cylindrical barrel 1 12, having a first face 1 12.1 and a second face 1 12.2. The second face 1 12.2 of the barrel 1 12 is connected to a shaft 1 14, which can serve either to drive the barrel 1 12, in the case of the hydraulic mechanism 100 operating as a pump, or alternatively to be driven by the rotatable barrel 1 12 in the case of the hydraulic mechanism 100 operating as a motor. The arrangement is such that the angle 108 is zero when the face 104 of the swash plate 102 and the first face 1 12.1 of the barrel 1 12 are parallel. The angle 108 may vary through a range of both positive and negative angles.

The barrel 1 12 comprises a plurality of radially and equidistantly spaced cylinder bores 1 16 extending from the first face 1 12.1 towards the second face 1 12.2 and thus through the barrel 1 12. The bores 1 16 extend parallel to a centerline 128 of the barrel 1 12. Each bore 1 16 is associated with a respective piston 1 18 which is allowed to reciprocate within the bore 1 16. The pistons 1 18 extend from the bores 1 16 beyond the first face 1 12.1 , towards the swash plate 102 and terminate against a bearing surface 120 on the face 104. Each piston is connected to the bearing surface 120 by a slipper (not shown).

In use the barrel 1 12 rotates relative to the swash plate 102. Because of the angle 108, the swash plate 102 causes the pistons 1 18 to reciprocate within the cylinder bores 1 16 between a top dead center (TDC) and a bottom dead center (BDC). The TDC represents a situation where the piston 1 18 is closest to the second face 1 12.2, while the BDC represents a situation where the piston 1 18 is furthest away from the second face 1 12.2. The order in which adjacent pistons 1 18 reach either the TDC or

BDC is determined by the direction in which the barrel 1 12 is rotating.

The hydraulic mechanism 100 further comprises a porting plate 130 having a first face 132 and second face 134. The porting plate 130 has a diameter similar to the barrel 1 12. The first face 132 of the porting plate

130 abuts against the second face 1 12.2 of the barrel 1 12, so that a substantially fluid tight seal forms between the porting plate 130 and the second face 1 12.2 of the barrel 1 12 (a degree of leakage is present, which results in minor losses). The porting plate has an aperture 136 through which the shaft 1 14 protrudes. The porting plate 130 is fixed in position so that, in use, the barrel 1 12 and shaft 1 14 rotates relative to the porting plate.

The porting plate comprises a first fluid channel 138 and a second fluid channel 140 that extends from the first face 132 to the second face 134.

The first fluid channel 138 is associated with a high-pressure fluid line 54 whereas the second fluid channel 140 is associated with a low-pressure fluid line 144. Hydraulic fluid is therefore transferred between the bores 1 16 and the high-pressure fluid line 54, through the first fluid channel 138, while hydraulic fluid is transferred between the bores 1 16 and the low- pressure fluid line 144, through the second fluid channel 140. The operation of the hydraulic mechanism 100 will now be described from the viewpoint of one of the cylinder bores 1 16 and its concomitant piston 1 18. It will be understood that all of the cylinder bores 1 16 and pistons 1 18 progress in similar fashion, albeit out of phase with the one described, resulting in a smooth motion of the barrel 1 12.

When the hydraulic mechanism 100 is operating as a pump, high pressure hydraulic fluid flows along the high-pressure line 54 towards the mechanism 100 and through the first fluid channel 138. The swash plate 102 is angled such that, taking into account the desired direction of rotation of the barrel 1 12, the pistons 1 18 in fluid flow communication with the first channel 138 has at least reached or passed the TDC. High pressure hydraulic fluid thus exerts a force on the pistons 1 18 in fluid flow communication with the first fluid channel 138, forcing the pistons 1 18 in a direction towards the first face 1 12.1 of the barrel 1 12. The piston 1 18 therefore transfers an axial force on the face 104 of the swash plate 102.

The angle of the swash plate 102, which is not at zero, results in a transverse component of the axial force, which translates into a torque causing the barrel 1 12 to rotate. The rotation of the barrel 1 12 in turn rotates the shaft 1 14. At the time a piston 1 18 reaches the BDC, a maximum volume of high pressure hydraulic fluid is thus contained within the bore 1 16. The barrel 1 12 has now rotated to a point where it is no longer in fluid flow communication with the first fluid channel 138. When the barrel 1 12 rotates further, the bore 1 16 comes into fluid flow communication with the second fluid channel 140 which is left at a relatively low pressure (atmospheric pressure or slightly above this). Further rotation of the barrel 1 12, together with the interaction of the piston 1 18 with the swash plate 102 causes the piston 1 18 to start moving back towards the TDC, which causes the volume of hydraulic fluid contained within the bore 1 16 to be deposited through the second fluid channel 140 into the low pressure hydraulic fluid line 144. A single revolution of the barrel 1 12 has thus been completed. One revolution of the barrel 1 12 thus results in the bore 1 16 being in alternating fluid communication with the first and second fluid channels (138, 140) respectively.

When the hydraulic mechanism operates as a pump, the piston 1 18 has at least reached TDC by the time it comes into fluid flow communication with the second fluid channel 140. Unlike when the mechanism 100 is configured as a motor, the barrel 1 12 is now driven by the shaft 1 14. The rotation of the barrel 1 12, and the interaction of the piston 1 18 with the swash plate 102 causes the piston 1 18 to start moving towards the BDC, which causes the piston to create low pressure (or suction) within the bore 1 16. Hydraulic fluid from the low-pressure line 144 thus enters the bore 1 16 through the second fluid channel 140. By the time the piston reaches the BDC, a maximum volume of hydraulic fluid has thus entered the bore 1 16. Further rotation of the barrel 1 12 caused by the shaft 1 14, and the interaction of the barrel 1 12 with the porting plate 130 terminates the fluid flow communication between the bore 1 16 and the second fluid channel 140. Further rotation of the barrel 1 12 causes bore 1 16 to come into fluid flow communication with the first fluid channel 138, while interaction of the piston 1 18 with the swash plate causes the piston 1 18 to start moving towards the TDC. This causes the piston 1 18 to exert a force on the hydraulic fluid contained in the bore 1 16, which is deposited under pressure, through the first fluid channel 138 into the high-pressure fluid line 54. When the piston 1 18 reaches the TDC, a full revolution of the barrel 1 12 has been completed.

Thus, when configured as a motor, high-pressure hydraulic fluid kept in a high-pressure source is used to cause the shaft 1 14 to rotate, while, when configured as a pump, rotation of the shaft 1 14 is used to provide high pressure hydraulic fluid to an actuator or sink.

By having a variable swash plate 102, the load created the hydraulic mechanism 100 when operating as a pump may be finely controlled from zero load (when the angle 108 is zero) to a maximum load through an infinite number of steps. Similarly, the power delivered through the shaft 1 14 (and thus the output torque and speed of the shaft 1 14) may be controlled from zero to a maximum available power through an infinite number of steps.

When the angle 108 is substantially zero, no torque will be transferred to or from the shaft 1 14, and effectively, no flow of hydraulic fluid into or from the bores 1 16 will occur. In a second example embodiment as is shown in figure 4, each of the first and second hydraulic mechanisms (22, 46) comprises an open loop over- center variable displacement hydraulic device that can be operated as a pump or as a motor, in any one of a forward and reverse direction, which hydraulic mechanism 200 is in the form of a variable displacement, variable axis, bent axis hydraulic mechanism 200.

The mechanism 200 operates on substantially the same principle as the mechanism 100 described above, in that a rotatable cylindrical barrel 202 houses a plurality of pistons 204, within a concomitant number of cylindrical bores 206, formed within the cylindrical barrel 202.

The pistons are pivotably fixed to a holder 208 that is fixed to a shaft 210. When the mechanism 200 is configured as a pump, the shaft 210 is an input shaft, while, when the mechanism 200 is configured as a motor, the shaft 210 is an output shaft. The shaft 210 has a central axis 212, while the cylindrical barrel also has a central axis 214. The barrel 202 is pivotable such that the central axis 214 of the barrel 202 is pivotable relative to the central axis 212 of the shaft 210. By pivoting the central axis 214 of the barrel through an angle 216 relative to the central axis 212 of the shaft (to constitute a "bent axis"), the pistons 204 reciprocate within the cylindrical bores 206 when the shaft 210 is rotated. Therefore, when the central axis 214 of the barrel and the central axis 212 of the shaft are substantially in line, the pistons 204 will not reciprocate within the barrel 202, and the mechanism 200 will act as neither a pump nor a motor. A porting plate (not shown) performing a similar function as the porting plate 130 described above, is provided in fluid flow communication with high and low-pressure lines respectively. The pistons reciprocating within the barrel causes hydraulic fluid to be expelled and received in the cylindrical bores in similar fashion as described above in relation to the hydraulic mechanism 100.

An actuator (not shown) is used to pivot the barrel 202. By pivoting the barrel 202 over-centre (in other words, pivoting the central axis 214 of the barrel up to, and beyond the central axis 212 of the shaft) the configuration of the mechanism 200 is changed from a pump to a motor, or vice versa, for a specific rotational direction of the shaft 210. However, this will not cause the high-pressure and low-pressure lines to change around.

The bent-axis mechanism 200 has a number of advantages over the swashplate mechanism 100.

Firstly, the bent axis mechanism 200 is inherently more powerful than a swashplate mechanism 100 of a comparable size, as the angle 216 of the bent-axis mechanism 200 may inherently be greater than the swashplate angle 108. Consequently, the stroke of the pistons of the bent axis mechanism 200 is larger than that of a swashplate mechanism 100 of comparable size, resulting in a larger volume of hydraulic fluid that can be displaced by the pistons per revolution of the shaft 210. The angle 216 may typically reach a maximum of around 40 to 45 degrees, while the angle 108 of the swashplate is typically restricted to 22 to 23 degrees.

Secondly, the increased piston stroke, and the concomitant increased length of the pistons 204, results in an increased leakage path length. This in turn results in a lower rate of leakage, and increased efficiency, even though drag and viscous losses may be slightly higher. Furthermore, the holder 208 is more robust than the slippers of the swashplate mechanism 100. This means that the rotational speed at which the pistons 204 start to pull out of, or get dislodged from the holder 208, is a lot higher than the rotational speed at which the pistons 1 18 pull out of, or get dislodged from, the slippers of the mechanism 100. Consequently, a bent axis mechanism 200 is capable of producing more power than a swashplate hydraulic mechanism 100 of a comparable size.

The reservoir 34 may comprise a low-pressure filter (not shown) to remove debris from the hydraulic fluid and a fluid cooling device (not shown).

It will be appreciated that the use of the first and/or second hydraulic mechanisms (14, 24) enables the power supplied to the system 10 by the first drive means 18 to be relatively constant and to be provided at a high level of efficiency. In cases where surplus power is supplied to the system

10, the surplus energy is channeled to the accumulator 36. In cases where a shortage exists, power can be supplemented from the accumulator 36.

It will further be appreciated that the second and fourth ports (22, 30) will never be in fluid flow communication with the first hydraulic line 32, or the first or third ports (20, 28). Thus, the second and fourth ports will never be exposed to high fluid pressures, thereby reducing the stress experienced in these areas and improving service life of the components. Design and manufacturing costs of the hydraulic units are therefore reduced.

Furthermore, since the first and second hydraulic units (14, 24) may be identical, maintenance and running costs of the system is drastically reduced in that a single set of spare parts may be provided to cater for both hydraulic units.

It will also be appreciated that, in addition to the aforementioned advantages, that the current invention retains all of the advantages of an open-loop hydrostatic drive, while providing the functionality of a closed- loop hydrostatic drive. Many disadvantages of closed-loop configurations, for example fluid friction losses in a second, high pressure hydraulic fluid line, are also avoided. The current invention, having engine braking capabilities, enables it, despite being an open-loop system, to be utilized in systems that would conventionally require closed-loop systems (such as crane hoist systems and tracked vehicle drives typically used in heavy commercial vehicles such as earth moving equipment and bulldozers).

It will also be appreciated that the first and second couplings (16, 26) may be in the form of any suitable coupling or linking arrangement, including a chain and sprocket arrangement, a belt and pulley arrangement, direct coupling between spur, bevel or other types of gears, etc. The accumulator is of a known kind.

It will be appreciated that a "forward and reverse" direction, when used in relation to the hydraulic mechanism, refers to a clockwise and anticlockwise rotational direction of a shaft of the mechanism (the shaft is an input shaft when the mechanism is configured as a pump, and an output shaft when the mechanism is configured as a motor). It will be appreciated by those skilled in the art that the invention is not limited to the precise details as described herein and that many variations are possible without departing from the spirit and scope of the invention as claimed.

Claims

1 . An open-loop hydrostatic drive, comprising:

a first hydraulic mechanism having a first and second hydraulic port, the first hydraulic mechanism being linked to a first drive means;

a second hydraulic mechanism which is selectively configurable as one of a pump and a motor, and having a third and fourth hydraulic port, the second hydraulic mechanism being linked to an output drive; and

a first hydraulic fluid line connected in fluid flow communication between the first and third hydraulic ports.

2. An open-loop hydrostatic drive according to claim 1 wherein the second and fourth ports are provided in fluid flow communication with a reservoir.

3. An open-loop hydrostatic drive according to any one of claims 1 and 2, wherein the second hydraulic mechanism is selectively configurable as one of a pump and a motor, in any one of a forward and reverse rotational direction.

An open-loop hydrostatic drive according to claim 3, wherein the second hydraulic mechanism comprises an open-loop, over-center, variable displacement hydraulic mechanism in the form of one of i) a plurality of axially reciprocating pistons and an associated manipulatable and variable swash-plate arrangement which is controllable to move over-center and ii) a bent axis hydraulic device configurable to an over-centre configuration.

An open-loop hydrostatic drive according to claim any one of the preceding claims, further comprising an accumulator arranged in fluid flow communication with the first hydraulic fluid line.

An open-loop hydrostatic drive according to any one of the preceding claims, wherein a control valve arrangement used to regulate the flow of hydraulic fluid into and out of the accumulator is provided between the accumulator and the first hydraulic line.

An open-loop hydrostatic drive according to claim 6, wherein the control valve arrangement comprises a control valve arranged in series with a pilot operated check valve, the configuration being such that the control valve is operable between an open configuration to allow hydraulic fluid to flow therethrough to and from the accumulator and a closed configuration to isolate the accumulator from the first hydraulic fluid line, and such that the pilot operated check valve is operable between an open configuration where hydraulic fluid is allowed to flow therethrough to and from the accumulator, and a closed configuration where hydraulic fluid is allowed to flow therethrough to the accumulator but prevented from flowing there through from the accumulator

An open-loop hydrostatic drive according to any one of the preceding claims, wherein the first and second hydraulic mechanisms are substantially similar and wherein the first hydraulic mechanism is selectively configurable as one of a pump and a motor in any one of a forward and reverse rotational direction.

An open-loop hydrostatic drive according to any one of the preceding claims, wherein the first drive means comprises one of: i) an electrical motor; ii) an internal combustion motor; iii) a wind turbine; iv) a water turbine; and v) a flywheel.

An open-loop hydrostatic drive according to any one of the preceding claims, wherein the output drive forms part of a drivetrain of one of: i) a vehicle; ii) a turbine; iii) a machine; iv) a crane hoist; v) a slewing drive; vi) a generator; and vii) an alternator.

1 1 . An open-loop hydrostatic drive according to any one of the preceding claims, further comprising a central control system, for controlling the interaction, configuration and settings of each component of the hydrostatic drive.

12. A method of operating an open-loop hydrostatic drive, the method comprising selectively configuring a second hydraulic mechanism, which is connected in fluid flow communication with a first hydraulic mechanism via a first hydraulic fluid line, as one of: i) a motor, receiving hydraulic fluid via the first hydraulic line, the motor driving an output drive in one of a forward and reverse direction; and ii) a pump being driven in one of a forward and a reverse direction by the output drive, the pump supplying hydraulic fluid to the first hydraulic line.

13. A method of operating an open-loop hydrostatic drive according to claim 12, wherein an accumulator is provided in fluid flow communication with the first hydraulic fluid line, and wherein the second hydraulic mechanism configured as a motor, is driven by hydraulic fluid supplied via the first hydraulic fluid line from at least one of the first hydraulic mechanism configured as a pump, and the hydraulic accumulator. A method of operating an open-loop hydrostatic drive according to claim 13, wherein the second hydraulic mechanism, when configured as a pump, provides pressurised hydraulic fluid via the first hydraulic fluid line, to at least one of the first hydraulic mechanism configured as a motor, and the accumulator.

A method of operating an open-loop hydrostatic drive according to any one of claims 13 and 14, wherein the first hydraulic mechanism, when configured as a pump, provides pressurised hydraulic fluid via the first hydraulic fluid line, to at least one of the second hydraulic mechanism and the accumulator.

A method of operating an open-loop hydrostatic drive according to any one of claims 13 to 15, wherein the first hydraulic mechanism, when configured as a motor, is driven by hydraulic fluid supplied via the first hydraulic fluid line from at least one of the second hydraulic mechanism configured as a pump, and the hydraulic accumulator.

A method of operating an open-loop hydrostatic drive according to any one of claims 13 to 16, comprising varying the configuration of at least one of the first and second hydraulic mechanisms in real time, to configure the system as a variable speed/power transmission system, so that the first and second hydraulic mechanisms rotate at one of similar and opposite rotational directions, and at one of similar and different rotational speeds.