GB2493706A

GB2493706A - Combined closed-loop hydraulic circuit and hydraulic energy storage system

Info

Publication number: GB2493706A
Application number: GB1113884.9A
Authority: GB
Inventors: Andrew John Smith; Robert Nicholas Harper
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2011-08-11
Filing date: 2011-08-11
Publication date: 2013-02-20
Anticipated expiration: 2031-08-11
Also published as: GB201113884D0; EP2557239A1; RU2605514C2; RU2012134393A; GB2493706B; CN102927069A

Abstract

Disclosed is a hydraulic control system 10, 47, 54 comprising a combined closed loop hydraulic circuit 14 for controlling two or more hydraulic actuators 21, 22 and a hydraulic energy storage system comprising one or more accumulators 19, 20. The hydraulic control system 10, 47, 54 further comprises a power unit 11. The closed loop hydraulic circuit 14 comprises a hydraulic machine 16 operably connected to the power unit 11, first and second accumulators 19, 20 for storing hydraulic fluid, first and second hydraulic actuators 21, 22, a first valve 17 for selectively directing hydraulic fluid between the hydraulic machine 16 and either the first accumulator 19 or the first hydraulic actuator 21and a second valve 18 for selectively directing hydraulic fluid between the hydraulic machine 16 and either the second accumulator 20 or the second hydraulic actuator 22. The first hydraulic actuator 21 is fluidly connected 20 to the second hydraulic actuator 22 by one or more conduits 29.

Description

COMBINED CLOSED LOOP HYDRAULIC CIRCUIT AND HYDRAULIC ENERGY

STORAGE SYSTEM

Technical Field

This disclosure is directed towards a hydraulic control system comprising a combined closed loop hydraulic circuit for controfling two or more hydraulic actuators and a hydraulic energy storage system comprising one or more accumulators.

Background

Work machines, including backhoe loaders, excavators, loaders and the like, commonly comprise a hydraulic control system for controlling one or more work tools, such as buckets, booms, backhoes, grapples and the like. The hydraulic control system may comprise an open loop hydraulic circuit for controlling one or more actuators connected to each work tool. The open loop hydraulic circuit may utilise one or more pumps to direct hydraulic fluid to the actuators via one or more valves and conduits. However, the valve arrangements in such circuits may be complex and, if two or more actuators perform the same function on a work tool, ensuring that consistently similar loads are applied by each of the actuators may require complex control software and circuitry. The hydraulic control system may, therefore, comprise a closed loop hydraulic circuit rather than an open loop hydraulic circuit to overcome these issues.

An example of a closed loop hydraulic system for controlling a work tool is disclosed in the document US-B- 6,520,731. The hydraulic control system disclosed is used to control the swing of a backhoe on a backhoe loader. A closed loop circuit comprises a variable displacement pump which directs hydraulic fluid to a pair of swing cylinders via conduits. The swing cylinders are attached to a boom on the backhoe via a frame and control the side to side movement, or swing, of the backhoe. A swashplate inside the variable displacement pump is adjusted to direct hydraulic fluid in either direction around the closed loop circuit, thereby allowing hydraulic fluid to be directed into either of the swing cylinders, The closed loop circuit is arranged such that, when the piston of one of the swing cylinders extends, the piston of the other swing cylinder retracts. The closed loop circuit also comprises a fixed displacement charge pump to supply supplemental hydraulic fluid in the case of any leakages existing in the closed loop circuit.

However, US-B-6,520,731 does not disclose a means by which the closed loop hydraulic circuit may be integrated with further hydraulic circuits for controlling other actuators connected to work tools. Furthermore, the use of a fixed displacement charge pump may add complexity and cost to the hydraulic control system. In addition, tJS-E-6,520,731 does not disclose a means of supplying power to the variable displacement pump.

Typically, pumps in hydraulic control systems are supplied with power from the work machine's main power unit, such as an internal combustion engine, a micro turbine, an electrical engine or the like. Power units with a capacity for high power outputs are commonly utilised since the hydraulic control system and work machine drivetrain may have high power requirements. However, power units with high power output capacities may require relatively high amounts of fuel, may be relatively heavy and may emit relatively excessive amounts of waste products. It has, therefore, become commonplace to incorporate hydraulic energy storage systems in work machines comprising power units with a relatively low power output capacity and to release the stored energy when extra power is required. US-B-6,520,731 does not disclose the use of a hydraulic energy storage system with the closed loop hydraulic circuit.

Summary

According to one aspect of the present disclosure there is provided a hydraulic control system comprising a power unit and a closed loop hydraulic circuit, said closed loop hydraulic circuit comprising:-a hydraulic machine operably connected to the power unit; first and second accumulators for storing hydraulic fluid; first and second hydraulic actuators; a first valve for selectively directing hydraulic fluid between the hydraulic machine and either the first accumulator or the first hydraulic actuator; and a second valve for selectively directing hydraulic fluid between the hydraulic machine and either the second accumulator or the second hydraulic actuator; wherein the first hydraulic actuator is fluidly connected to the second hydraulic actuator by one or more conduits.

The disclosure further provides a method of controlling a hydraulic control system comprising a power unit and a closed loop hydraulic circuit, said closed ioop hydraulic circuit comprising:-a hydraulic machine operably connected to the power unit; first and second accumulators for storing hydraulic fluid; first and second hydraulic actuators; first and second valves; wherein the valves are operable to effect either; a charging mode in which hydraulic fluid is directed S from the second accumulator to the first accumulator; or a discharging mode in which hydraulic fluid is directed from the first accumulator to the second accumulator such that the hydraulic machine outputs power; or a steering node in which hydraulic fluid is directed from the first to the second hydraulic actuator or vice-versa.

By way of example only, embodiments of a combined swing and accumulator energy storage system are now described with reference to, and as shown in, the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic representation of one embodiment of the hydraulic control system of this

disclosure;

Figure 2 is a schematic representation of a further embodiment of the hydraulic control system of this

disclosure;

Figure 3 is a schematic representation of yet another embodiment of the hydraulic control system of this

disclosure; and

Figure 4 is a side elevation of one type of work machine which may be controlled by the hydraulic control

system of this disclosure.

Detailed Description

The present disclosure is generally directed towards a hydraulic control system for a work machine which combines a closed loop hydraulic circuit with a hydraulic energy recovery system. A closed loop hydraulic circuit may be used to store hydraulic fluid for later release when extra power is required by other parts of the hydraulic control system.

The closed loop hydraulic circuit may also be used to control two or more hydraulic actuators. The hydraulic control system may further comprise a secondary hydraulic circuit for controlling further hydraulic actuators.

The present disclosure is directed towards a work

machine of any type, examples of which include backhoe loaders, excavators, forest machines, harvesters, track loaders, front shovels, bulldozers and wheel loaders. Such work machines typically comprise a power unit which directs power, via a transmission, to a hydraulic circuit and/or a driveline. The hydraulic circuit may control the position of a work tool, which typically comprises one or more booms, arms or sticks, to which at least one work implement is attached. The work implement may be of any type, examples of which include buckets, blades, augurs, cold planers, felling heads, forks, grapples, hammers, shears, thumbs and material handling arms. The driveline may transfer power from the transmission to the wheels and/or tracks such that the work machine may move.

Figure 1 illustrates an embodiment of a hydraulic control system 10 for controlling a work machine comprising a power unit 11, a transmission 12, a driveline 13, a closed loop hydraulic circuit 14 and a secondary hydraulic circuit 15. The power unit 11 may be of any type, examples of which include internal combustion engines, micro turbines and electric motors. The transmission 12 may be of any type, for example manual, automatic or semi-automatic. A clutch or torque converter (not shown) may be positioned in between S the power unit 11 and the transmission 12 to enable the transmission 12 to be engaged and disengaged from the power unit 11. A clutch or torque converter may be positioned in between the transmission 12 and the driveline 13 to enable the driveline 13 to be engaged and disengaged from the transmission 12.

The closed loop hydraulic circuit 14 comprises a hydraulic machine 16, a first valve 17, a second valve 18, a first accumulator 19, a second accumulator 20, a first hydraulic actuator 21, a second hydraulic actuator 22 and a number of interconnecting conduits 23, 24, 25, 26, 27, 28, 29, the arrangement of which is described below. The hydraulic machine 16 may be reversible, such that it may behave as a hydraulic fluid pump or as a motor driven by the movement of hydraulic fluid. The hydraulic machine 16 may be a variable displacement pump, such as an axial piston pump, a radial piston pump or a bent axis pump. The hydraulic machine 16 may comprise a driveshaft (not shown) attached to an adiustable device (not shown), such as a swash plate, which controls the flow rate of hydraulic fluid through the hydraulic machine 16.

The hydraulic machine 16 is operably connected to the power unit 11. An operable connection enables the transfer of power between components. The power output from the power unit 11 can, therefore, be transferred to the hydraulic machine 16. This may be achieved by directly connecting an output shaft from the power unit 11 to the driveshaft of the hydraulic machine 16. The driveshaft of the hydraulic machine 16 may run through the transmission 12, but not supply power to the transmission 12, and/or be engaged by S splines to the output shaft of the power unit 11. The output shaft of the power unit 11 and the driveshaft of the hydraulic machine 16 may be comprised of a single shaft.

Alternatively, the driveshaft of the hydraulic machine 16 may be operably connected to an output shaft of the transmission 12. The driveshaft of the hydraulic machine 16 and the output shaft of the transmission 12 may, for example, be operably connected to one another using engaged splines on the shafts or via a clutch or torque converter.

The hydraulic machine 16 is fluidly connected to the first valve 17 by the conduit 23 and to the second valve 18 by the conduit 24. These fluid connections enable hydraulic fluid to flow in either direction between components. The first valve 17 is fluidly connected to the first accumulator 19 by the conduit 25 and the first hydraulic actuator 21 by the conduit 27. The second valve 18 is fluidly connected to the second accumulator 20 by the conduit 26 and the second hydraulic actuator 22 by the conduit 28. The first valve 17 and the second valve 18 may be valves of any suitable type, examples of which include selector valves, spool valves, cartridge valves or poppet valves. The first valve 17 and the second valve 18 may be operated in any suitable mariner, for example manually, by using pilot hydraulics, or by using electronic controls. The first valve 17 is operable to enable hydraulic fluid to be directed to or from any of the first accumulator 19, the hydraulic machine 16 and the first hydraulic actuator 21. The second valve 18 is operable to enable hydraulic fluid to be directed to or from any of the second accumulator 20, the hydraulic machine 16 and the second hydraulic actuator 22.

The pressure of the hydraulic fluid in one or both of the first accumulator 19 and second accumulator 20 may be relatively high and may be higher than the ambient pressure of the hydraulic control system 10. The hydraulic fluid may be stored in the first accumulator 19 at a pressure higher than, lower than or approximately equal to the pressure of the hydraulic fluid stored in the second accumulator 20. The accumulators 19, 20 may be directly connected to, or integrated with, the first and second valves 17, 18 such that the conduits 25, 26 are not required.

The first and second hydraulic actuators 21, 22 may be of any type of hydraulic actuator, such as tie-rod, welded, single-acting, double-acting, telescoping, ran or rodless.

In the embodiment illustrated in Figure 1, the first hydraulic actuator 21 and the second hydraulic actuator 22 are double-acting hydraulic actuators. Each of the first and second hydraulic actuators 21, 22 comprises a actuator body 30, 31, a piston 32, 33, a first hydraulic chamber 34, 35 and a second hydraulic chamber 36, 37.

The second chambers 36, 37 may be fluidly connected to each other by means of the conduit 29. Alternatively, the second hydraulic chamber 36 of the first hydraulic actuator 21 may be fluidly connected by a conduit (not shown) to the first hydraulic chamber 35 of the second hydraulic actuator 22, and the second hydraulic chamber 37 of the second hydraulic actuator 22 may be fluidly connected by a conduit (not shown) to the first hydraulic chamber 34 of the first hydraulic actuator 21.

The pistons 32, 33 may each comprise a piston rod 38, 39 and a piston head 40, 41. The first hydraulic chambers 34, 35 and the second hydraulic chambers 36, 37 may be positioned inside the cylinder bodies 30, 31 on either side of the piston heads 40, 41. Hydraulic fluid may not be transferred between the first hydraulic chambers 34, 35 and the second hydraulic chambers 36, 37 due to the piston heads 40, 41 forming a seal with the inner side of the cylinder bodies 30, 31.

There may be more than one cylinder body 30, 31 and piston 32, 33 in each of the first and second hydraulic actuators 21, 22. For example, the first hydraulic actuator 21 may comprise two cylinder bodies and two pistons, wherein a first hydraulic chamber in each of the two cylinder bodies is fluidly connected to the first valve 17 by the conduit 27 and a second hydraulic chamber in each of the cylinder bodies is fluidly connected to the second hydraulic chamber 37 of the second hydraulic actuator 22 by the conduit 29.

The piston rods 38, 39 are illustrated in Figure 1 as extending through the cylinder bodies 30, 31 from the second hydraulic chambers 36, 37. However, the piston rods 38, 39 may extend through the cylinder bodies 30, 31 in the opposite direction from the first hydraulic chambers 34, 35.

The pistons 32, 33 may be of the double rod type, wherein the piston rods 38, 39 extend through both the first hydraulic chambers 34, 35 and the second hydraulic chambers 36, 37. The first and second hydraulic actuators 21, 22 may -10 -be single-acting hydraulic actuators wherein a spring is positioned inside the second hydraulic chamber 36, 37.

The piston rods 38, 39 of the first and second S hydraulic actuators 21, 22 may be attached to at least one part of a work tool, such as a boom, such that the position of the work tool may be manipulated. The pistons 32, 33 may also operate a work implement, such as a set of shears or felling heads, attached to the work tool. The first and second hydraulic actuators 21, 22 and any conduits or components in between may be arranged such that when one of the pistons 32, 33 extends the other piston 32, 33 retracts.

The secondary hydraulic circuit 15 may comprise a hydraulic pump 42, a valve block 43, a main work tool hydraulic circuit 44 and conduits 45, 46. The hydraulic pump 42 may comprise an input shaft (not shown) operably connected to the drive shaft of the hydraulic machine 16, such that the hydraulic pump 42 is operably connected to, and may receive power from, the power unit 11, The operable connection may comprise engaged splines on the drive shaft of the hydraulic machine 16 and the input shaft of the hydraulic pump 42. Alternatively, the input shaft of the hydraulic pump 42, the drive shaft of the hydraulic machine 16 and the output shaft of the power unit 11 may be comprised of a single shaft. The hydraulic pump 42 may be a variable displacement pump. The hydraulic pump 42 may comprise an adjustable device, such as swash plates, connected to the input shaft which controls the flow rate of hydraulic fluid through the hydraulic pump 42.

-11 -In Figure 1, the hydraulic pump 42 is illustrated on the opposite side of the hydraulic machine 36 to the transmission 12. However, the hydraulic pump 42 may alternatively be placed between the transmission 12 and the hydraulic machine 16.

The hydraulic pump 42 may be arranged so as to direct hydraulic fluid from a hydraulic reservoir (not shown) to the valve block 43 via the conduit 45. The flow rate and pressure of the hydraulic fluid in the secondary hydraulic circuit 15 may be controlled by manipulating the position of the adjustable device in the hydraulic pump 42, The valve block 43 may direct hydraulic fluid around the main work tool hydraulic circuit 44 via the conduit 46. The main work tool hydraulic circuit 44 may comprise hydraulic actuators attached to one or more work tools and/or work implements.

The main work tool hydraulic circuit 44 may, therefore, control the position of one or more work tools and/or work implements. The main work tool hydraulic circuit 44 may also operate the work implement. The hydraulic actuators of the work tool hydraulic circuit 44 and the first hydraulic actuator 21 and second hydraulic actuator 22 of the closed loop hydraulic circuit 14 may be attached to the same, or different, parts of the work tool and/or work implement.

The hydraulic fluid in the closed loop hydraulic circuit 14 remains in the closed loop hydraulic circuit 14 when displaced by the hydraulic machine 16, the first hydraulic actuator 21 or the second hydraulic actuator 22.

However, closed 1oop circuits comprising components such as valves, pumps and actuators will also typically comprise means by which the hydraulic fluid may leak. As a result, a -12 -hydraulic charge circuit may be incorporated in the hydraulic control system 10 to supply supplemental hydraulic fluid to the closed ioop hydraulic circuit 14 to compensate for any hydraulic fluid leakages or losses. Thereby an approximately constant volume of hydraulic fluid in the closed loop hydraulic circuit 14 may be maintained, and the volume maintained may be held within a predetermined range.

The hydraulic charge circuit may draw hydraulic fluid from a hydraulic reservoir or the secondary hydraulic circuit 15 and may supply supplemental hydraulic fluid to any component, including the conduits 24, 27, 29, 28, of the closed ioop hydraulic circuit 14.

Figure 2 illustrates an embodiment of a hydraulic control system 47 which includes a hydraulic charge circuit 48. In this example the hydraulic charge circuit 48 comprises a pressure reducing valve 49, check valves 50, 51 and conduits 52, 53. The conduit 52 is illustrated as connected to the conduit 45. However, the conduit 52 may be connected to any part of the secondary hydraulic circuit 15.

The pressure reducing valve 49 directs hydraulic fluid from the conduit 52 to the check valves 50, 51 via the conduit 53. The check valves 50, 51 may be fluidly connected to, and therefore direct hydraulic fluid into, the conduits 23, 24.

The pressure reducing valve 49 reduces the pressure of the hydraulic fluid supplied to the conduit 53 from the secondary hydraulic circuit 15. The magnitude of the change in hydraulic fluid pressure implemented by the pressure reducing valve 49 may be fixed or adjustable and may be related to the pressure in the closed loop hydraulic circuit 14. The pressure reducing valve 49 may be operated manually, -13 -by using pilot hydraulics, or by using electronic controls.

The check valves 50, 51 may have a cracking pressure, being the pressure at which the check valves 50, 51 open, suitable such that hydraulic fluid only enters the closed loop hydraulic circuit 14 from the conduit 53 once the hydraulic fluid in the closed ioop hydraulic circuit 14 falls below a predetermined pressure. The cracking pressure may be the pressure of the hydraulic fluid in the conduit 53 and therefore the check valves 50, 51 may release hydraulic fluid into the closed loop hydraulic circuit 14 when the pressure in the closed loop hydraulic circuit 14 falls below the pressure in the conduit 53.

In a further embodiment, as illustrated in Figure 3, the hydraulic control system 54 comprises a hydraulic charge circuit 55, wherein a hydraulic charge pump 56 is utilised to supply hydraulic fluid to the closed loop hydraulic circuit 14. The hydraulic charge pump 56 directs hydraulic fluid from a hydraulic reservoir 57 via a conduit 58 to check valves 59, 60 and a pressure reducing valve 61 via a conduit 62. The check valves 59, 60 may be fluidly connected to the conduits 23, 24 or any other component of the closed loop hydraulic circuit 14 and the pressure reducing valve 61 may direct hydraulic fluid to a hydraulic reservoir 63 via a conduit 64. The hydraulic reservoirs 57, 63 may be the same hydraulic reservoir and may be the same as the hydraulic reservoir which supplies hydraulic fluid to the hydraulic pump 42.

The hydraulic charge pump 56 may be a fixed displacement hydraulic pump and may comprise internal or external gears. The hydraulic charge pump 56 may be operably -14 -connected to the drive shaft of the hydraulic machine 16, an output shaft from the power unit 11, the output shaft of the transmission 12 or the input shaft of the hydraulic pump 42.

The hydraulic charge pump 56 is, therefore, operably S connected to, and driven by, the power unit 11. The hydraulic pump 42 may be operably connected to the transmission 12 via the hydraulic machine 16 and/or via the hydraulic charge pump 56.

The pressure reducing valve 61 may be fixed or adjustable and may control the pressure of the hydraulic fluid in the conduit 62. The pressure maintained in the conduit 62 may be related to the pressure in the closed ioop hydraulic circuit 14. The pressure reducing valve 61 may be operated manually, by using pilot hydraulics, or by using electronic controls. The check valves 59, 60 may have a cracking pressure suitable such that hydraulic fluid only enters the closed 1oop hydraulic circuit 14 once the pressure of the hydraulic fluid in the closed loop hydraulic circuit 14 falls below a predetermined pressure. Any type of directional control valve may be used in place of the check valves 59, 60.

The pressure reducing valve 61 ensures that the pressure of the hydraulic fluid in the conduit 62 is maintained at a constant level regardless of the output of the hydraulic charge pump 56. Hydraulic fluid may, therefore, be released into the closed loop hydraulic circuit 15 by the check valves 59, 60 if the pressure of the hydraulic fluid in the closed 1oop hydraulic circuit 15 falls below the pressure of the hydraulic fluid in the conduit 62. Since leaks in the closed loop hydraulic circuit -15 - 14 will cause a reduction in hydraulic fluid pressure, the volume of hydraulic fluid in the closed loop hydraulic circuit 14 Will be maintained at a relatively constant level -During operation of the hydraulic control system 10, 47, 52 the power unit 11 may supply power to the transmission 12, the hydraulic machine 16, the hydraulic pump 42 and/or to the driveline 13. The closed loop hydraulic circuit 14 and the secondary hydraulic circuit 15 may be operational simultaneously, operational individually or not operational.

The input shaft of the hydraulic machine 16 and the drive shaft of the hydraulic pump 42 may rotate when they receive power via the operable connection to the power unit 11. The adjustable devices in the hydraulic machine 16 and/or the hydraulic pump 42 may be orientated such that hydraulic fluid is not displaced in the closed loop hydraulic circuit 14 and/or the secondary hydraulic circuit when the input shaft of the hydraulic pump 42 and the drive shaft of the hydraulic machine 16 are rotating. Where one of the adjustable devices is a swashplate, the angle between the normal to the shaft and the face of the swashplate may be set to an angle of zero degrees to prevent hydraulic fluid from being displaced.

In a further arrangement, one or more clutches or torque converters (not shown) may be positioned in between the drive shaft of the hydraulic machine 16 and the output shaft of the transmission 12 or the power unit 11 to enable the hydraulic machine 16 to be engaged and disengaged from -16 -the transmission 12 or the power unit 11. A clutch or torque converter (not shown) may be disposed in between the drive shaft of the hydraulic machine 16 and the input shaft of the hydraulic pump 42 to enable the engagement and disengagement of the hydraulic pump 42 from the hydraulic machine 16. The clutches or torque converters may be engaged to provide power to either/both of the hydraulic machine 16 or/and the hydraulic pump 42. The hydraulic pump 42 may be a fixed or variable displacement pump. If fixed, and thereby only capable of supplying a fixed flow rate of hydraulic fluid for a certain level of power, the hydraulic pump 42 may be operated by engaging the appropriate clutches or torque converters. If variable, the hydraulic pump 42 may be operated by engaging the appropriate clutches or torque converters and by adjusting the orientation of the adjustable devices. The hydraulic machine 16 may be operated by engaging a clutch or torque converter and by adjusting the orientation of the adjustable device in the hydraulic machine 16.

When the closed loop hydraulic circuit 14 is operational, hydraulic fluid is directed around the closed loop hydraulic circuit 14. In one mode of operation, which may be referred to as a "charging mode", the adjustable device in the hydraulic machine 16 is orientated such that the hydraulic machine 16 behaves as a pump by utilising the power supplied by the power unit 11. The valves 17, 18 are actuated and hydraulic fluid is directed from the second accumulator 20 to the first accumulator 19. The hydraulic fluid is stored at a higher pressure in the first accumulator than in the second accumulator.

-17 -In a further mode of operation of the hydraulic control system 10, which may be referred to as a "discharging mode", the adjustable device in the hydraulic device 16 may be orientated such that the hydraulic machine 16 behaves as a motor. The valves 17, 18 are actuated and the first accumulator 19 releases hydraulic fluid, which is directed through the hydraulic machine 16 and into the second accumulator 20. The hydraulic machine 16 thereby supplements the power from the power unit 11 and may supply power to the transmission 12 and/or the hydraulic pump 42 via the operable connections. The hydraulic fluid is transferred from the first accumulator 19 to the second accumulator 20 as a result of the hydraulic fluid being stored at a higher pressure in the first accumulator 19 than in the second accumulator 20.

A person skilled in the art would recognise that during the discharging and charging modes described above, the first and second accumulator may be interchanged. Therefore, the hydraulic fluid may be directed from the second accumulator 20 to the first accumulator 19 in the discharging mode and from the first accumulator 19 to the second accumulator 20 in the charging mode.

In a further mode of operation of the hydraulic control system 10, which may be referred to as a "steering mode", the adjustable device of the hydraulic machine 16 is orientated such that the hydraulic machine 16 behaves as a pump by utilising the power supplied by the power unit 11.

The hydraulic fluid is pumped into the first hydraulic chamber 34 of the first hydraulic actuator 21 from the first hydraulic chamber 35 of the second hydraulic actuator 22 via -18 -the first valve 17, the second valve 18 and the conduits 23, 24, 27, 28. The pressure of the hydraulic fluid in the first hydraulic chamber 34 therefore rises and the piston 32 moves, extending the piston rod 38 further out of the cylinder body 30. Hydraulic fluid in the first or second hydraulic chamber 34, 36 of the first hydraulic actuator 21 is transferred to the first or second hydraulic chamber 35, 37 of the second hydraulic actuator 22. The piston 33 therefore moves and the piston rod 39 retracts further inside the cylinder body 31.

Alternatively, during the steering mode the hydraulic fluid may be pumped into the first hydraulic chamber 35 of the second hydraulic actuator 22 from the first hydraulic chamber 34 of the first hydraulic actuator 21. The piston 33 may therefore extend and the piston 32 may therefore retract.

The steering, charging and discharging modes may be effected at any time either manually or automatically by utilising control circuitry. The steering mode may be engaged when the work tool or work implement attached to the pistons 32, 33 is operated.

The charging mode may be effected when the power required from the power unit 11 by the secondary hydraulic circuit 15 and the driveline 13 is low or the power output of the power unit 11 is below a predefined value. The predefined value may be the maximum power output, or power output capacity, of the power unit 11. The power output of the power unit 11 may be increased to supply sufficient -19 -power to the hydraulic machine 16 to allow the charging mode to be effected.

The discharging mode may be effected when the power required from the power unit 11 by the secondary hydraulic circuit 15 and the driveline 13 is high or the power output of the power unit 11 is above a predefined value. The predefined value may be the maximum power output, or power output capacity, of the power unit 11. When the discharging mode is effected, power may be supplied to the secondary hydraulic circuit 15 via the hydraulic pump 42 and/or to the driveline 13 via the transmission 12 or the power unit 11.

The power supplied may be independent of, or supplemental to, the power supplied by the power unit 11 to the closed loop hydraulic circuit 14, the secondary hydraulic circuit 15, the transmission 12 and the driveline 13.

Figure 4 illustrates one type of work machine 65, in the form of a backhoe loader, which may utilise the hydraulic control system 10, 47, 54. The work tools 66, 67 of the backhoe loader are a loader 66 and a backhoe 67, each comprising a series of booms 68, 69, 70, 71. work implements 72, 73, illustrated in Figure 4 as buckets, are attached to the booms 68, 71. The hydraulic control system 10, 47, 54, as previously described herein, may be used to control the position and orientation of the backhoe loader itself, the backhoe 67 and the loader 71.

In one arrangement, the pistons 32, 33 may be attached to the backhoe 67 and control the side to side movement, or swing, of the backhoe 67. The actuators of the secondary hydraulic circuit 15 may be attached to, and thereby control -20 -the position of, the booms 68, 69, 70, 71 and buckets 70, 71 of the backhoe 67 and the loader 66.

Industrial Applicability

The disclosed hydraulic control system 10, 47, 54, in which the closed loop hydraulic circuit 14 combines a closed loop circuit and a hydraulic energy storage system, may be incorporated into a wide variety of work machines.

By utilising a single hydraulic machine 16 to control both of the closed loop hydraulic circuit and the hydraulic energy storage system, excessive complexity and costs are avoided.

The hydraulic control system 10, 47, 54 may comprise a power unit 11 with a power output capacity which is less than that required by the driveline 13 and/or the secondary hydraulic circuit 15, The discharging mode may be engaged when extra power is required by the driveline 13 and/or the secondary hydraulic circuit 15.

The hydraulic control system 10, 47, 54 allows for relatively smooth, simple and consistent control of the first and second hydraulic actuators 21, 22 due to the closed loop closed loop hydraulic circuit 14.