WO2005054688A1

WO2005054688A1 - A valve assembly for a hydraulic cylinder assembly

Info

Publication number: WO2005054688A1
Application number: PCT/AU2004/001654
Authority: WO
Inventors: Norman Ian Mathers
Original assignee: Norman Ian Mathers
Priority date: 2003-12-01
Filing date: 2004-11-25
Publication date: 2005-06-16
Also published as: AU2003100983A4

Abstract

A valve assembly (1) for preventing a telescopic hydraulic cylinder assembly from retracting in an uncontrolled manner should there be an unexpected drop in hydraulic fluid pressure. The valve assembly has a body (20) having first and second inlets (21, 22) for fluid, a passage (24), and a piston chamber (25). A piston (26) is movable between resting and working positions within the chamber (25). A first check valve (40) controls the flow of fluid to and from the fluid chamber, and has a valve seat (41) and a valve member (43) biased into engagement with the valve seat (41). The valve member (43) has a passage (60) extending therethrough for communicating fluid between the fluid chamber and passage (24). A second check valve (70) located within the passage (60) controls the flow of fluid from the fluid chamber to the passage (24). The second check valve (70) has a valve member (71) biased into engagement with a valve seat (72). A push rod (80) extends between the piston (26) and the first and second check valves (40, 70) and moves the valve members (43, 71) out of engagement with the respective valve seat (41, 72). The push rod (80) is biased so as to not engage the valve members (43, 71) and is movable into engagement therewith by the piston (26) when in the working position. To fill the fluid chamber, fluid is applied to the second inlet (22) and the fluid moves the valve member (43) out of engagement with the valve seat (41). To empty the fluid chamber, fluid pressure is applied to the first inlet (21) and the push rod (80) initially moves valve member (71) out of engagement with the valve seat (72).

Description

A VALVE ASSEMBLY FOR A HYDRAULIC CYLINDER ASSEMBLY

This invention relates to a valve assembly for controlling the flow of fluid into and out of a fluid chamber of a telescopic hydraulic cylinder assembly. In particular, the invention concerns a valve assembly for preventing a telescopic hydraulic cylinder assembly from retracting in an uncontrolled manner.

Background of the Invention Telescopic hydraulic cylinder assemblies are used in many different types of machines, including tip/dump trucks. A telescopic hydraulic cylinder assembly includes a cylinder forming a fluid chamber and at least one piston extendable and retractable relative to the cylinder. To extend the piston, the fluid chamber is filled with fluid, and to retract the piston, fluid is drained from the fluid chamber. Tipping trays of tip/dump trucks are usually raised and lowered using multi- stage telescopic hydraulic cylinder assemblies. A directional control valve of the truck, employing a spool valve, may be used to direct hydraulic fluid to and from the multi-stage telescopic hydraulic cylinder assembly. The directional control valve may be connected to the air brake system of the truck and pressurised air may be used to switch the directional control valve between extension and retraction modes of operation. The directional control valve may alternatively be connected to any other pneumatic or electric signal system on the truck. Should there be an unexpected drop in hydraulic fluid pressure, due to failure of a hydraulic component of the tip/dump truck (e.g. a burst or damaged hose or fitting), then an extended telescopic hydraulic cylinder assembly under load may retract in an uncontrolled manner (i.e. "free fall") and may cause damage to the hydraulic cylinder assembly, tipping tray or chassis of the truck. It is also potentially dangerous to personnel working on or around the truck and represents a significant occupational health and safety issue. Summary of the Invention The present inventor has now developed a valve assembly which overcomes the problem referred to above. In a first aspect, the present invention provides a valve assembly for controlling flow of fluid to and from a fluid chamber of a telescopic hydraulic cylinder assembly, the valve assembly having: a valve body; a first port in the valve body; a second port in the valve body, the second port being in fluid communication with the fluid chamber; a first check valve comprising a first valve seat located in a flow passage between the first port and the second port and a first valve member for seating on the first valve seat; a second check valve comprising a second valve seat and a second valve member for seating on the second valve seat; actuation means for unseating the first valve member and for unseating the second valve member; said valve assembly being operable to allow fluid to drain from the fluid chamber when fluid pressure at the second port is greater than fluid pressure at the first port, wherein said actuation means operates to unseat the first valve member to drain fluid from the fluid chamber if fluid pressure at the second port is less than a predetermined maximum pressure, and wherein said actuation means operates to unseat only the second valve member to drain fluid from the fluid chamber if the fluid pressure at the second port exceeds the predetermined maximum pressure. Preferably, the actuation member operates to unseat both the first valve member and the second valve member if the fluid pressure at the second port is less than the predetermined maximum pressure. Suitably, the actuation means operates to unseat the second valve member prior to unseating the first valve member. In one embodiment of the invention, the second check valve operates to open and close a second flow passage, wherein at least a part of the second flow passage extends through the first valve member. The first valve member is suitably biased to a closed position in which the first valve member is sealed against the first valve seat. Similarly, the second valve member is suitably biased to a closed position in which the second valve member is seated against the second valve seat. In this fashion, both the first valve member and the second valve member have to move against the action of biasing means to unseat the valve members. It is preferred that the second check valve allows a lesser flow of fluid therethrough relative to the first check valve. The actuation means may comprise a plunger adapted for reciprocal movement, the plunger having a shoulder that abuts on the first valve member to unseat the first valve member and a projection extending beyond the shoulder, the projection extending through a passage in the first valve member and contacting the second valve member, the actuation means further comprising force applying means to apply a force to the plunger. The force applying means may comprise a piston moveable by pneumatic pressure such that applying pneumatic pressure applies a force to the piston which causes the piston to move and apply a force to the plunger. Other force applying means may also be used. For example, a solenoid may be used to apply a force to the plunger. The solenoid may move an arm or a camming member to apply a force to the plunger. A spring is suitably interposed between the force- applying arm or camming member and the plunger. The biasing means for biasing the first valve member and the biasing means for biasing the second valve member suitably comprises a biasing spring that acts on both the first valve member and the second valve member. Suitably, the second valve member is positioned between the biasing spring and the first valve member. In this fashion, a single biasing spring can bias both valve members to the closed position. Alternatively, each of the first valve member and the second valve member may be provided with separate biasing springs. In some embodiments, the second flow passage includes an orifice for causing a pressure drop as fluid flows therethrough, said orifice being located between the second valve member and the second port. In some embodiments, the apparatus further includes a spring located between the actuating means and the first valve member. The valve body may be of any suitable shape and size. Preferably, the valve body is elongate. The first port and second port may correspond to respective openings in the body, located in a side wall of the body adjacent. The body preferably comprises two or more attachable pieces so that internal components of the valve assembly may be accessed. The body may be connected to the telescopic hydraulic cylinder assembly in any suitable way. The body may be located within a cavity or port of a wall of a cylinder of the telescopic hydraulic cylinder assembly. Alternatively, a manifold housing the valve assembly may be connected to a wall of the cylinder. Manifolds connectable to hydraulic cylinder assemblies are well known in the art. Preferably, the valve assembly is in the form of a cartridge that may be screwed into a port or cavity of the hydraulic cylinder assembly or manifold. The valve assembly may have sealing members, such as o-rings, to provide a fluid-tight fit within the port or cavity. Preferably, fluid lines connect the first port to a directional control valve for extending and retracting the telescopic hydraulic cylinder assembly. Directional control valves employing spool valves are well known in the art. Any pneumatic or electric signal may be used to control the directional control valve and to apply air to an air inlet which forms part of the actuation means. A hydraulic pump of the vehicle may be connected to the directional control valve and may provide the first port with hydraulic fluid. The valve member and valve seat of the first check valve may be of any suitable shape and size. The first valve member may have a sealing portion located outside the body for engaging the first valve seat and a stem portion located in a passage of the body. More preferably, the first valve member has a sealing portion located within the valve body for engaging the first valve seat. The first valve member may be biased into engagement with the valve seat in any suitable way. Preferably, the first valve member is biased by a spring, such as a coil spring. The spring may extend about the stem portion within the passage of the body. The valve member and valve seat of the second check valve may be of any suitable shape and size. The valve seat may extend laterally across the passage of the valve member and the valve member may be a ball. The valve member of the second check valve may be biased into engagement with the valve seat in any suitable way. The valve assembly may have a spring, such as a coil spring, extending within the passage of the valve member. Preferably, the valve assembly is associated with a safety control means that prevents lowering of the tipper body if it detects that the pressure in the extended hydraulic cylinder is less than a predetermined valve. This can occur if the truck is being used on a poor work site, such as one having pot holes or is otherwise uneven. Such poor work sites can cause the truck body to twist, which can cause the hydraulic cylinder to bind when lowering the tipper body. If this occurs and the value assembly permits hydraulic fluid to continue draining, the tipper body can remain raised (due to binding of the cylinder) even though there may be little or no hydraulic fluid remaining in the cylinder. When the truck is subsequently moved to level ground, the cylinder can unbind and the tipper body can come crashing down. This can cause damage to the hydraulic cylinder, the tipper body and the truck. It is also potentially dangerous to personnel working on or around the truck. To prevent this, a safety control means may be used with the valve assembly of the present invention. The safety control means may comprise a pressure sensing means for sensing pressure at the second port and control means for stopping flow of fluid from the second port to the first port of the sensing means senses that the pressure at the second port has dropped below a predetermined level. Any suitable sensing and control means may be used. In a preferred embodiment, the safety control means comprises a safety control valve for use with a valve assembly that is actuated by pilot pneumatic pressure. The safety control valve may include a valve body having a spool positioned in a passage in the valve body, with the spool being reciprocally movable in the passage. The spool may have a first end sensing pressure at the second port, suitably by being in fluid communication with the second port. The spool may have the other end sensing pressure of the pilot pneumatic fluid. If the pressure at the second port drops below the predetermined pressure (which is determined by the minimum pressure required to allow safe lowering of the tipper body), the pressure of the pneumatic air acting on the spool moves the spool and shuts off the supply of pilot air to the valve assembly. This results in the flow passages in the valve assembly closing and thereby preventing further drainage of hydraulic fluid from the cylinder. In a second aspect, the present invention provides a valve assembly for controlling flow of fluid to and from a fluid chamber of a telescopic hydraulic cylinder assembly, the valve assembly having: a valve body; - a first port in the valve body; a second port in the valve body, the second port being in fluid communication with the fluid chamber; a first check valve comprising a first valve seat located in a flow passage between the first port and the second port and a first valve member for seating on the first valve seat; a second check valve comprising a second valve seat and a second valve member for seating on the second valve seat; actuation means for unseating the first valve member and for unseating the second valve member; - said valve assembly being operable to allow fluid to drain from the fluid chamber when fluid pressure at the second port is greater than fluid pressure at the first port, wherein said actuation means operating to unseat the second valve member prior to unseating the first valve member. The valve assembly of the present invention also allows novel and inventive hydraulic circuits to be implemented in order to control the extension and retraction of an extendable hydraulic cylinder. In a third aspect, the present invention provides an hydraulic circuit for moving an hydraulic cylinder between extended and non-extended positions comprising a source of hydraulic fluid, a hydraulic pump receiving hydraulic fluid from the source of hydraulic fluid, a valve assembly in accordance with the first or second aspect of the present invention, a first hydraulic line connecting the hydraulic pump to the first port of the valve assembly, an extendable hydraulic cylinder in fluid communication with a second port of the valve assembly, a bypass in the first hydraulic line having a bypass valve for selectively passing pressurised hydraulic fluid from the pump to the first port of the valve assembly or to pass pressurised hydraulic fluid from the hydraulic pump to a bypass line, and control means for controlling fluid flow through the hydraulic circuit, said control means being operable in a first mode to extend the hydraulic cylinder and operable in a second mode to allow the hydraulic cylinder to move from the extended position to the retracted position, wherein in the first mode the control means actuates the bypass valve to prevent flow of pressurised fluid from the pump through the bypass line such that pressurised fluid from the pump passes to the first port of the valve assembly and thereafter into the extendable cylinder to thereby extend the cylinder, and wherein in the second mode the control means actuates the bypass valve to allow pressurised fluid from the pump to pass through the bypass line and actuates the valve assembly such that pressurised hydraulic fluid in the extendable cylinder drains from the extendable cylinder. Preferably, in the second mode, the hydraulic fluid draining from the extendable cylinder passes through the valve assembly and thereafter passes through the bypass line. Suitably, the bypass line returns hydraulic fluid to the source of hydraulic fluid. The source of hydraulic fluid is suitably a hydraulic fluid reservoir or a hydraulic fluid tank. The control means may comprise a conventional controller for raising or lowering a tipper body connected to the hydraulic cylinder. The valve assembly of the present invention also allows a unique and novel hydraulic circuit to be assembled using a hydraulic pump as described in my co-pending international patent application no. PCT/AU2004/000951, the entire contents of which are herein incorporated by cross-reference. This international patent application describes an hydraulic pump that has vanes that can be retracted into the rotor of the pump. The pump also includes retaining means for selectively retaining the hydraulic vanes in a retracted position. When the hydraulic vanes are retained in the retracted position, the rotor can rotate without pumping the fluid (as the vanes, which normally provide the pumping action, are retracted in the rotor). Alternatively, the vanes may be selectively released so that pumping of the fluid can again take place. In a fourth aspect, the present invention provides a hydraulic circuit including a source of hydraulic fluid, a hydraulic pump having vanes that can be selectively retained within a pump rotor so that the pump does not pump hydraulic fluid and selectively released so that the pump pumps hydraulic fluid, the hydraulic pump being in fluid communication with the source of hydraulic fluid, a valve assembly in accordance with the first or second aspect of the present invention, the first port of the valve assembly receiving pressurised hydraulic fluid from the hydraulic pump, an extendable hydraulic cylinder in fluid communication with the second port of the valve assembly, and control means for controlling fluid flow through the hydraulic circuit, wherein the control means operates in a first mode in which the vanes of the hydraulic pump are not retained in the retracted position such that the hydraulic pump delivers pressurised hydraulic fluid to the valve assembly and thereafter to the extendable hydraulic cylinder to thereby extend the hydraulic cylinder, and a second mode in which the vanes are retained in the retracted position such that the hydraulic pump does not supply pressurised hydraulic fluid to the valve assembly, whereby in the second mode the control means also actuates the valve assembly such that pressurised hydraulic fluid in the extendable hydraulic cylinder can drain from the hydraulic cylinder and through the valve assembly such that the hydraulic cylinder moves from the extended position to the retracted position. Preferably, in the second mode, the hydraulic fluid that is draining from the extendable cylinder drains through the valve assembly and thereafter passes through the pump to return to the source of hydraulic fluid. The hydraulic circuits of the third and fourth aspects of the present invention may suitably omit the directional control valve conventionally used in hydraulic circuits on trucks used to raise and lower tipper bodies. The hydraulic circuit of the fourth aspect of the present invention may also allow a power take off, which typically drives the hydraulic pump in conventional hydraulic circuits used to raise and lower tipper bodies on trucks, to be omitted.

Brief Description of the Drawings Figure 1 depicts a valve assembly connected to a multi-stage telescopic hydraulic cylinder assembly of a tip/dump truck, according to an embodiment of the invention; Figure 2 is a longitudinal cross-sectional view of the valve assembly of Figure 1; Figure 3 is a perspective view, partly in cross-section, of a valve assembly in accordance with another embodiment of the present invention; Figure 4 is an exploded perspective view of the valve assembly of Figure 3 showing the components of the valve assembly in-line for assembly; Figure 5 is a side view in cross-section of the valve assembly of Figure 3; Figure 6 is a graph of oil flow through the second check valve vs pressure for a test conducted using a valve assembly as shown in Figure 3. The graph of Figure 6 represents an example of oil flow vs pressure; Figure 7 is a cross-sectional side view of another embodiment of a valve assembly in accordance with the present invention. In Figure 7, only a part of the valve assembly is shown, with the parts of the valve assembly not shown being identical to Figure

3; Figure 8 is a cross-sectional perspective view of a valve assembly in accordance with a further embodiment of the present invention; Figure 9 is an enlargement of detail A of Figure 8; Figure 10 is an enlargement of the flow control attachment of the valve assembly shown in Figure 8; Figure 11 is a cross-sectional perspective view of a valve assembly in accordance with another embodiment of the present invention; Figure 12 is an enlargement of detail A of Figure 11; Figure 13 is a perspective view of plug 300 of the valve assembly shown in

Figure 11; Figure 14 is a cross-sectional view of the plug shown in Figure 13; Figure 15 is a cross-sectional view of a safety valve or a jam lock valve for use with valve assemblies as shown in Figures 1-7 and 10-14; Figure 16 is a schematic diagram of a pneumatic and hydraulic circuit incorporating the safety valve of Figure 15 and a valve assembly as shown in Figures 1-7 or 10-14; Figure 17 is a flow diagram showing a conventional hydraulic circuit used to raise and lower a tipper body on a truck; Figure 18 is a flow diagram showing an hydraulic circuit in accordance with an embodiment of the third aspect of the present invention; and Figure 19 is a flow diagram showing an hydraulic circuit in accordance with an embodiment of the fourth aspect of the present invention. Figure 1 depicts a valve assembly 1 connected to a multi-stage telescopic hydraulic cylinder assembly 2 of a tip/dump truck. Assembly 2 includes a cylinder 5 forming a fluid chamber 8 and a multi-stage piston (8a) extendable and retractable relative to cylinder 5. To extend the multi-stage piston, fluid chamber 8 is filled with fluid under pressure and the pressurised fluid causes piston 8a to extend. To retract the piston, fluid is drained from fluid chamber 8. A tipping tray of the truck is raised when assembly 2 extends and is lowered when assembly 2 retracts. A manifold 4 is connected to cylinder 5. The manifold 4 has two laterally extending passages 7, 18 and a longitudinally extending passage 6. Valve assembly 1 is in the form of a screw-in cartridge valve and is located within passage 6. Various o-rings (not labelled) provide a fluid tight seal. The assembly 1 controls the flow of fluid into and out of the fluid chamber 8 via passage 7. The tip/dump truck has a directional control valve 3, employing a spool valve, for extending and retracting the assembly 2. The directional control valve 3 is connected to an air brake system of the truck or other source of pilot pressure as well as to an oil pump 14 and reservoir 15 of the truck. Pressurised air travelling through hoses 12 and 13 controls the directional control valve 3. An air hose 9 connects the valve assembly 1 to directional control valve 3 and hose 12 connects the valve assembly 1 and the directional control valve 3 to the pilot source. A hydraulic hose 11 is connected to passage 18 and extends to directional control valve 3. When pressurised air is directed through hose 13, pressurised hydraulic fluid flows from the pump supply 14 to the valve assembly 1 via hose 11, and piston 8a of multi-stage telescopic hydraulic cylinder assembly 2 extends. When pressurised air is directed through hoses 12 and 9, oil flows from fluid chamber 8 to tank via hoses 11 and 15, and assembly 2 retracts by virtue of the load imposed on assembly 2 by the truck/tipper body and any load in the truck/tipper body. Referring now to Figure 2, the valve assembly 1 has a cylindrical body 20 having a threaded port 21 connected to air hose 9, port 22 for communicating oil to passage 18, a passage 24 extending along the longitudinal axis of body 20, and a piston chamber 25. Piston chamber 25 has a shoulder 30. The body 20 comprises a first cylindrical piece 50, a cap 31 screwed to the first cylindrical piece 50, a second cylindrical piece 51 screwed to the first cylindrical piece 50, and a sleeve 53 extending within pieces 50 and 51. An annular flange 49 of sleeve 53 abuts a shoulder 54 of piece 51. The screw cap 31 seals one end of the piston chamber 25 and the first port 21 extends through the cap 31. A piston 26 is movable between resting and working positions within chamber 25. A coil spring 28 located within chamber 25 returns and biases piston 26 within the resting position against cap 31. A nut 29 is threaded onto an external thread of the piston 26. The stroke of the piston 26 is defined by cap 31 and the point at which nut 29 collides with shoulder 30. The stroke of the piston 26 is adjustable by changing the position of nut 29 relative to piston 26. A breather port 33 allows for volume changes within chamber 25 to maintain essentially atmospheric pressure in chamber 25. A first check valve 40 has a first valve seat 41 and a first valve member 42. First valve member 42 consists of a sealing portion 43 and a stem portion 44. The sealing portion 43 is located outside body 20 and stem portion 44 extends within passage 24 and sleeve 53. A coil spring 45 extends around stem portion 44 and sleeve 53 and biases sealing portion 43 into engagement with valve seat 41. Lock nuts 47, 48 extend from stem portion 44. Spring 45 extends between lock nuts 47, 48 and annular flange 49 of sleeve 53. An end of spring 45 is connected to lock nut 48. First valve member 42 has a passage 60 extending longitudinally through sealing portion 43 and stem portion 44, and laterally through stem portion 44. Passage 60 communicates fluid between fluid chamber 8 and passage 24. A second check valve 70 regulates the flow of fluid from fluid chamber 8 to passage 24. Check valve 70 has a ball valve member 71 and a valve seat 72. A coil spring 73 is anchored within the passage 60 and biases the ball valve member 71 into engagement with the valve seat 72. A push rod 80 extends between piston 26 and the first and second check valves 40, 70. Push rod 80 extends through an o-ring 84 and a narrow portion of push rod 80 extends within stem portion 44. An end 81 of push rod 80 is connected to or in abutment with piston 26 and the other end 82 of the rod 80 is situated adjacent to ball valve member 71. When piston 26 is moved to the working position, push rod 80 first moves ball valve member 71 out of engagement with valve seat 72, following which a shoulder 85 of push rod 80 engages stem portion 44 and moves sealing portion 43 out of engagement with valve seat 41. In order to extend hydraulic cylinder assembly 2, the engine of the truck drives the hydraulic oil pump and hydraulic oil is directed through hose 11 to sealing portion 43 via ports 22 and passage 24. The oil pressure causes sealing portion 43 to move out of engagement with valve seat 41, at which point oil flows through passage 7 to fluid chamber 8, and consequently the piston extends. In order to retract the assembly 2, the directional control valve is operated to stop the flow of pressurised oil from the hydraulic at pump (normally by controlling the power take-off that controls the pump). Pressurised air is then directed to piston chamber 25 via hoses 9 and 12 and port 21. As the part of the piston chamber 25 above piston 26 fills with air, piston 26 moves push rod 80 into engagement with ball valve member 71 and moves it from valve seat 72. With passage 60 open, fluid bleeds from fluid chamber 8 to hose 11 via passages 24 and 60 and ports 22. Shoulder 85 of push rod 80 also engages stem portion 44 to move sealing portion 43 out of engagement with valve seat 41, at which time fluid may also flow through valve seat 41 to ports 22. The rate at which oil flows from fluid chamber 8 may be controlled by adjusting the stroke of piston 26. To move sealing portion 43 out of engagement with valve seat 41, the piston force must be greater than the holding force acting on valve member 43. The piston force is equal to the air pressure within chamber 25 multiplied by the surface area of piston 26 labelled as Al. The holding force is equal to the oil pressure within passage 7 multiplied by the area of sealing portion 43 labelled as A2, combined with the force imparted by spring 45. In tip trucks, the fluid pressure acting on the valve member 43 at this stage is largely a function of the weight of the tipping tray that is connected to the cylinder assembly. To ensure that sealing portion 43 can be moved out of engagement with valve seat 41, area Al is typically four to five times larger than area A2. Typically the air system of a truck will operate at 80-100 PSI. An air pilot supply of 80 PSI will move sealing portion 43 out of engagement with valve seat 41 as long as the pressure within the fluid chamber 8 is not greater than about 320 PSI. Typically, the tipping tray would be lifted by a pressure of 200 PSI within fluid chamber 8. Should the holding force exceed the piston force such that check valve 40 cannot be opened, check valve 70 will nevertheless be opened by piston 26 at a pilot pressure of about 80 PSI. This is because ball valve member 71 has an effective seat area about 40 times less than area Al of piston 26, and the holding force (i.e. the oil pressure within passage 60 multiplied by the effective seat area of the ball valve member 71, combined with the force imparted by spring 73) will be far less than the piston force. An air pilot supply of 80 PSI could lift a loaded tipping tray of over 3000 PSI. The valve assembly of the present invention can thus prevent a telescopic hydraulic cylinder assembly from retracting uncontrollably should a hydraulic hose or fitting fail. This is achieved by the valve assembly preventing fluid from leaving the fluid chamber. Furthermore, the valve assembly allows retraction to take place in a regulated manner by allowing fluid to bleed from the fluid chamber at a controlled rate. Figures 3 to 5 show various views of a valve assembly in accordance with another embodiment of the present invention. The valve assembly 100 includes a valve body 102. A piston cap 104 is connected in a sealing manner to valve body 102. Similarly, a plug 106 is also sealingly connected to the valve body 102. The valve body 102 includes a first port 108 and a second port 110. First port 108 is normally connected via an appropriate hydraulic hose to a directional control valve which is used to selectively apply pressurised hydraulic fluid to first port 108 (when it is desired to raise the hydraulic cylinder) and to interrupt the supply of pressurised hydraulic fluid to port 108. Second port 110 is connected via an appropriate fluid connection to the fluid chamber of the hydraulic cylinder assembly. Although not shown in Figure 3 to 5, ports 108 and 110 may be threaded or formed with other appropriate connections to enable hydraulic connections to be made. A first flow passage 112 is formed in the valve body 102. First flow passage 112 allows for fluid communication between the first port 108 and second port 110. A first check valve is provided to open and close first via passage 112. First check valve includes a first valve seat 114 that mates with the sealing surface of first valve member 116. First valve member 116 is in the form of a spool valve member. As most clearly shown in Figure 5, first valve member 116 includes a passage 118 that extends from an upper surface 120 of first valve member through to a hollow cavity formed in first valve member. The hollow cavity is partly defined by a depending cylindrical skirt 122 of the first valve member 116. The valve assembly 100 shown in Figures 3 to 5 also includes a second check valve. Second check valve includes a ball valve member 124 which seats upon a second valve seat 126. Second valve seat 126 is formed at the lower end of passage 118 which extends through first valve member 116. The first valve member 116 and second valve member 124 are biased to a normally closed position. In order to achieve biasing of the first and second valve members to a normally closed position, a coil spring 128 is mounted in passage 130, which is formed in the valve body 102. Coil spring 128 presses against a spring cap 132. Spring cap 132, in turn, presses against second valve member 124 to bias second valve member 124 into a normally closed position. It will also be appreciated that coil spring 128 also biases first valve member 116 in an upwardly direction in order to bias first valve member 116 into a normally closed position. The valve assembly 100 is also provided with a plunger 134. Plunger 134 extends through a passage formed in the upper part of the valve body 102. The upper part of plunger 134 is connected to or in abutment with a piston 136. Piston 136 is mounted in piston cap 104. Appropriate pneumatic seals, as shown by reference numeral 138, form a seal between piston 136 and piston cap 104. The piston cap 104 also includes a pneumatic port 140 which, in use, is connected to a source of pressurised air, such as an air pilot line. Pressurised air may be selectively supplied to port 140. The lower end of the chamber in the piston cap is provided with a breather 142 to allow air to move in and move out of the piston chamber as the piston 136 moves upwardly and downwardly, respectively. As best shown in Figure 5, the plunger 134 is provided with a shoulder 144. A lower end of the plunger 134 is formed as a projection 146 extending beyond the shoulder 144. Projection 146 is suitably slightly longer than the length of passage 118 through the first valve member 116. The end of projection 146 contacts ball valve member 124. It will be appreciated that piston 136 is mounted for reciprocating movement in the piston chamber formed in piston cap 104. In order to control the extent of reciprocal movement of the plunger 134, the plunger is fitted with a control washer having shims 148. The number of shims 148 can be adjusted to adjust the stroke of the piston and plunger. As can also be seen from figures 3 and 5, the valve body 102 is provided with a gallery 152 that is in fluid communication with second port 110. Gallery 152 opens up into passage 130 that is formed in the lower part of the valve body 102. The upper end

154 of plug 106 defines a lower part of the passage 130. As can be seen from Figures 3 and 5, the plug 106 also includes a recess 156 for receiving and locating the lower end of coil spring 128. O-ring 158 ensures a fluid tight seal is formed between the cap 106 and valve body 102. From reviewing Figure 3 to 5, it can be seen that two possible flow passages exist for oil flow from first port 108 to second port 110, these being: a) oil flows from port 108 through passage 112 and into port 110 (and vice versa). This oil flow can occur when first valve member 116 is in an open or unseated position; and b) oil flows from first port 108 via passage 118 in first valve member, through passage 130, into oil gallery 152 and into second port 110 (and vice versa). Oil can flow through this flow passage when second valve member 124 is in the unseated or open position. In order to raise the hydraulic cylinder that is in fluid communication with second port 110, pressurised hydraulic fluid is provided to port 108. Pressurised hydraulic fluid is conventionally provided by an hydraulic fluid pump. Control of pressurised hydraulic fluid to port 108 is conventionally achieved by an appropriate direction control valve. The pressurised hydraulic fluid impinges upon the upper surface 120 of first valve member 116. This causes the first valve member to open. Pressurised hydraulic fluid then flows from first port 108, through passage 112, into second port 110 and subsequently out of second port 110 and into the hydraulic cylinder. This causes the hydraulic cylinder to extend, thereby raising the tipping tray or tipping body of a tip truck. Once the tipping operation has been completed and it is desired to lower the tipping tray, the supply of pressurised hydraulic oil to first port 108 is stopped. Due to the weight of the tipping tray (and any contents remaining in the tipping tray), the hydraulic fluid at port 110 is under pressure. Once the supply of pressurised hydraulic fluid to port 108- is stopped, the hydraulic fluid pressure at second port 110 is higher than the hydraulic fluid pressure at first port 108. Removing the supply of pressurised hydraulic fluid from first port 108 causes the first valve member 116 to close such that fluid flow through passage 112 is stopped. The biasing spring 128 and the pressure of the hydraulic fluid at port 110 each assist in closing the valve member 116. Indeed, it is a feature of some embodiments of the present invention that the first valve member is biased to a closed position and is arranged such that the supply of pressurised hydraulic fluid to the first port 108 opens the first valve member whilst the ceasing of supply of pressurised hydraulic fluid to the first port 108 results in the first valve member closing. In order to lower the tipping tray, the air pilot line connected to port 140 receives pressurised air from the directional control valve. This pressurised air can be selectively delivered by the operator of the truck. The pressurised air delivered through port 140 causes piston 136 to move downwardly. If the pressure of the hydraulic fluid in port 110 is below a predetermined maximum (which predetermined maximum is suitably calculated to correlate to the hydraulic pressure arising due to the weight of the empty tipping assembly and load), the pneumatic air supplied via port 140 causes the piston 136 to move downwardly. This, in turn, pushes plunger 134 downwardly. The end of projection 146 of plunger 134 pushes ball valve member 124 off its seat, which allows hydraulic fluid to flow through passage 118 and first valve member 116. Continued downward movement of piston 136 then results in shoulder 144 contacting the upper surface 120 of first valve member 116. This moves valve member 116 away from a closed or seated position, thereby allowing hydraulic fluid to flow past the now-open first valve member 116. As the flow passage that is opened by unseating of first valve member 116 is relatively large, this allows for rapid but controlled lowering of the tipper tray. The valve assembly shown in Figures 3 to 5 also allows for a safe lowering of a tipper tray that has not been fully emptied and hence, still carries some load. It will be appreciated that loaded tipper bodies apply a much greater force onto both the first valve member 116 and the second valve member 124. This much greater force arises by virtue of the extra weight of the loaded tipper body increasing the pressure of hydraulic fluid at second port 110. The extra force applied to the first valve member 116 by the loaded tipper body may exceed the force applied via the pilot air supply that acts on piston 136. If this is the case, the first valve member 116 remains closed. However, due to the small area of passage 118, the force applied by the higher pressure hydraulic fluid at second inlet 110 may not be sufficiently large to prevent second valve member 124 from being opened by the pilot air acting on piston 136. (It being appreciated that force = pressure x area). Thus, when the tipper body remains loaded, the second valve member 124 can still open, with the result that oil can be drained from the fluid chamber of the hydraulic cylinder via oil gallery 152, passage 130, passing through orifice 160 in spring cap 132 and through passage 118. The loaded or partially loaded tipper body then comes down in a controlled manner to prevent uncontrolled freefall of the tipper body, which can cause accidents, injury and possibly death.

To explain operation of the valve assembly in further detail, the area Ai of the piston 136 is set to a calculated amount larger than the area A₂ of the seat of the first check valve. For example, in the embodiment shown in Figures 3-5, an area ratio of Aι:A₂ is set at 4:1. Further, the area Ai is set to be a calculated amount larger than the area A₃ of the seat of the second control valve. In the embodiment shown in Figures 3-5, an area ratio of Aι:A₃ is set to be approximately 40:1. Thus, the approximate area relationships in the embodiments shown in Figures 3-5 are: Aι = 4A₂, and

The force Fi acting on the piston 136 equals AiPi, where Pi_. equals the pressure of the pilot air supplied to port 140. Similarly, when the pressure of hydraulic fluid at inlet 110 is P₂, the force F₂ acting to keep the first valve member closed, equals A P₂ plus F_s, where F_s is the force applied by coil spring 128. Similarly, the force F₃ acting to keep the second valve member 124 in the closed position equals A₃P₂ + F_s. In a typical tipping truck, the pilot air supplied to port 140 is supplied at approximately 80-100psi. An air pilot supply of 80psi will move the first valve member 116 out of engagement with the valve seat of the first check valve provided that the pressure of hydraulic fluid at second port 110 does not exceed about 300psi. An empty tipper body will typically cause the hydraulic pressure in second port 110 to be approximately 200psi. Thus, when the tipper body is empty, both the second valve member 124 and first valve member 116 can open to drain hydraulic fluid from the extendable cylinder of the tipper body. If the pressure P₂ at second inlet 110 exceeds approximately 300psi, for example, as occurs if the tipper body does not empty, the force applied by the pilot air supplied through port 140 is not sufficient to open first valve member 116. However, as the force applied to second valve member 124 is proportional to area A₃, and as area A₃ is only approximately one-fortieth that of area Ai, the force applied to the top of piston 136 by the pilot air is sufficient to unseat second valve member 124. This allows a slow and regulated lowering of the tipper body, even when the tipper body remains loaded. Similarly, should an hydraulic hose connected between the hydraulic pump and first inlet 108 fail during the raising operation, the applied fluid pressures will close both the first valve member 116 and the second valve member 124. The loaded tipper body can then be safely lowered by actuating piston 136 with pilot air to open second valve member 124 to allow the loaded tipper body to be lowered. A further advantage of the valve assembly shown in Figures 3 to 5 is that the flow of hydraulic fluid through the second check valve has been found to occur at a roughly constant rate regardless of the pressure in the hydraulic cylinder (and down to the second inlet 110). This "pressure compensation" feature only occurs when the first valve member 116 is closed and fluid is draining from the hydraulic cylinder through the second check valve. This "pressure compensation" arises because the hydraulic fluid flows through orifice 160 in spring cap 132. When only the second valve member 124 is opened, the second valve member 124 is held off its seat by air pressure on the piston 136. If the oil pressure P₂ in the cylinder (and in second port 110) increases, oil passes through orifice 160 and into cavity C, which is formed between the second valve seat 126 and the upper end of spring cap 132, and out through passage 118. This creates a pressure differential between passage 130 and cavity C (due to the pressure drop of the hydraulic fluid flowing through orifice 160). This allows spring 128 plus the hydraulic pressure P₂ on area (being the area of the top of the spring cap 132) to overcome the air pressure on the top of piston 136 and move the spring cap 132 in an upwards direction. This results in the second ball valve member 124 moving towards the valve seat, thereby restricting the flow of hydraulic fluid through passage 118. As the rate of flow of hydraulic fluid through passage 118 reduces, the flow through orifice 160 also reduces, thereby reducing the pressure differential between cavity 130 and cavity C. This reduction in pressure differential allows valve member 124 to again move downwardly by virtue of the action of piston 136, thereby allowing a larger flow area past second valve member 124. This causes a modulating effect to thereby maintain relatively constant flow through passage 118 regardless of the hydraulic fluid pressure arising from the cylinder. Hence, this maintains relatively constant flow of hydraulic fluid from the cylinder regardless of a load in the cylinder. It has been found that, for typical tipper trucks, this effect is only noticed when the cylinder pressure (and the pressure P₂) is above approximately 400psi. Figure 6 shows a plot of cylinder port pressure versus flow through the valve assembly shown in Figures 3 to 5. As can be seen, the graph designated by "setting A", "setting B" and "setting C", which all relate to use of valve assemblies in accordance with those shown in Figures 3 to 5, show that the rate of fluid flow through the valve is not proportional to the cylinder port pressure, whereas non-pressure compensated designs do show a strong relationship between fluid flow and cylinder port pressure. Figure 4 shows a perspective view, partly in cross-section, of the valve assembly shown in Figures 3 and 5, with the view being an exploded form showing the parts ready for assembly. The reference numerals in Figure 4 coπespond to those used in Figures 3 and 5. Figure 4 also shows plunger O-ring 162 which is used to form a seal between the plunger and the valve body 102. Figure 7 shows a modified version of the valve assembly shown in Figures 3 to 5. In particular, Figure 7 shows a sectional view through the central part of the valve body 102. The embodiment of Figure 7 includes a number of features that are common to those shown in Figure 5. Common features between Figures 5 and 7 are denoted by like reference numerals and need not be described further. However, the embodiment of Figure 7 differs from that shown in Figure 5 in that shoulders 144 of plunger 134 do not come into direct contact with the upper surface 120 of first valve member 116. Instead, a belville washer 164 or other suitable spring arrangement is interposed between the shoulders 144 of plunger 134 and the upper surface 120 of first valve member 116. In this fashion, as the plunger 134 moves downwards, the shoulders 144 contact belville washer 164. Belville washer 164 acts as a spring. The belville washer or other suitable spring means is usefully incorporated into the valve assembly to address a known problem which occurs when a partial load is retained in a tipper body, for example, via "sticky" loads due to wet conditions or similar, and the load retained in the tipper body is still small enough to allow the main check valve (i.e. first valve member 116) to open. If a partial load is retained in the tipper body, opening of first valve member 116 can cause a too rapid rate of fall of the tipper body. Incorporation of the belville washer or other suitable spring means acts as follows: a) the air pilot pressure applied through port 140 acts to fully open the first valve member 116. Because first valve member 116 is open, most of the flow passes through, flow passage 112 and orifice 160 and second valve member 124 become ineffective regarding pressure compensation. As the fluid flow across first valve member 116 increases, the pressure differential between ports 108 and 110 increases. The increase in pressure in port 110 relative to the pressure in port 108 is also reflected in gallery 152 and passage 130. This increase in pressure on the underside of first valve member 116 acts to close first valve member 116 against belville washer 164 (or other spring means) thereby controlling the rate of flow from the extendable cylinder to the oil reservoir. As the pressure differential across first valve member 116 reduces, belville washer 164 again opens first valve member 116 to control the flow rate. This results in a more controlled rate of fall for the unloaded or lightly loaded extendable hydraulics cylinder assembly. Figures 8 and 9 show a further embodiment in accordance with the present invention. In Figures 8 and 9, the valve assembly 200 includes a valve body 202 having a solenoid cap 204 attached to an upper part thereof. A flow controller attachment 206 is attached to a lower part of the valve body 202 and a plug 208 is attached to the lower part of the flow control attachment 206. Each of the various attachments are connected via fluid- tight connections. The valve body 202 includes a first port 210 and a second port 212. A plunger 214 that is of essentially similar construction to plunger 134 shown in Figures 3 to 5 is used to operate a first valve member 216 and a second valve member 218. First valve member 216 and second valve member 218 are arranged in essentially the same manner as the coπesponding first and second valve members shown in Figures 3 to 5. A spring cap 220 fits between the valve members 216, 218 and a coil spring 222. An oil gallery 224 is in fluid communication with the second port 212. Oil gallery 224 is further in fluid communication with second oil gallery 226 formed in flow control attachment 206. Further description of operation of the flow control attachment 206 will follow hereunder. Rather than having actuation of the plunger by pilot air pressure, as is the case in the embodiment shown in Figures 3 to 5, the embodiment shown in Figures 8 and 9 uses a solenoid to operate the plunger. This embodiment will typically be used on smaller vehicles where compressed air is not available. The solenoid cap 204 includes a solenoid coil 228 that is connected at its lower end to the valve body 202. A solenoid end cap 230 further assists in maintaining the solenoid coil in position. A solenoid push pin 232 is moveable in a reciprocating fashion by selective actuation of solenoid coil 228. Solenoid push pin 232 pushes against spring mechanism, indicated generally at 234. Spring mechanism 234 includes opposed spring caps 236 and 238. A shoulder bolt 240 is positioned such that its upper end 242 contacts the inner surface of a blind bore 244 in spring cap 236. The lower end of shoulder bolt 240 passes through an opening in the upper end of spring cap 238. The lower end of shoulder bolt 240 includes shoulders 246 that can engage with coπesponding shoulders 248 on spring cap 238. The lower end of spring cap 238 comes into contact with push pin hat 250 which, in turn, is in contact with push pin 252. Push pin 252 forms part of plunger 214. In order to lower a tipper body using the valve assembly shown in Figures 8 and 9, the solenoid coil 228 is armed. This causes the solenoid push pin 232 to move downwardly, which in turn compresses coil spring 254. As can best be seen in Figure 9, coil spring 254 is fitted between appropriate shoulders 260 and 262 on the respective spring caps 236, 238. Where the tipper body is empty, the force imparted on the plunger 214 by compression of the spring mechanism 234 is sufficient to open both the first valve member 216 and the second valve member 218. However, when the tipper body is still loaded, the pressure acting on the first valve member 216 due to the high pressure hydraulic fluid in the second port 212 is greater than the force that is applied by virtue of movement of the solenoid push pin 232 and compression of the coil spring 254. Thus, first valve member 216 remains closed. However, the force supplied by movement of the solenoid push pin 232 and compression of coil spring 254 is sufficient to open the second valve member 218 to allow a controlled lowering of the tipper body. It will be appreciated that, when this occurs, the shoulder bolt 240 floats through the spring cap 238. Different spring tensions can be achieved by shimming the spring or by using various spring tensions to vary the thrust. The flow control attachment 206 may be used to provide a controlled lowering of full or partial loads in a tipper body. It will be appreciated that controlled lowering of a full or partial loaded body will occur using the embodiment shown in Figures 3 to 5 due to the flow control effects of the orifice 160. Therefore, the flow control attachment 206 shown in Figures 8 and 10 may not be necessary for use in order to obtain flow control. However, the flow control attachment shown in Figures 8 and 10 can be used in embodiments where the orifice 160 as shown in Figures 3 to 5 is not present or it can be used in addition to the orifice 160.1ndeed, the flow control attachment 206 may be used in conjunction with all embodiments of the present invention, if desired. The flow control attachment 206 shown in Figures 8 and 10 includes a recess 256 for receiving and locating a lower end of coil spring 222 (not shown in Figure 10). Recess 256 has a lower passage 258 that has an outlet at its lower end. The flow control attachment 206 also includes a gallery 260 that is fitted with a flow control plug 262. Flow control plug 262 engages with a coil spring 264 which, in turn, engages with an inner shoulder formed on a spool 266. Spool 266 is hollow and includes an orifice 268 and a metering orifice 270. Metering orifice 270 is formed in a side wall of spool 266. In use, when draining hydraulic fluid from the tipper cylinder assembly, high pressure hydraulic fluid flows through gallery 266 and through orifice 268 into spool 266.

The fluid then passes through metering orifice 270 and up through passages 258 and 256 and thereafter through the second check valve. The spring 264 acts to push spool 266 to the right (as drawn in Figures 9 and 10). If a high fluid flow occurs through orifice 268, there will be a higher pressure drop resulting from the flow of fluid through orifice 268. Thus, the pressure of fluid in gallery 226 will be higher than the fluid pressure inside spool 266. This will act to push the spool against the compression spring which, in turn, will close the metering orifice 270. This, of course, reduces the fluid flow through the spool. As a result, the pressure drop through orifice 268 decreases, bringing the fluid pressure in gallery 226 and inside spool 266 closer to equal value. Thus, the spring 264 can then act to move the spool to the right again, thereby opening metering orifice 270. Should the load of fluid pressure rise or fall, the metering orifice will adjust accordingly, ensuring that uniform lowering of the body occurs independent of the load in the body. Figures 11 to 14 show a further embodiment of the valve assembly in accordance with the present invention. The embodiment of Figures 11 to 14 includes a number of features that are common with the embodiment shown in Figures 3 to 5 and, for convenience, like reference numerals are used for like features. These features need not be described further in Figures 11 to 14. The embodiment shown in Figures 11 to 14 differs from that shown in

Figures 3 to 5 in that the plug 106 connected to the bottom of the valve body in Figures 3 to 5 is replaced with plug 300 in the embodiment shown in Figures 11 to 14. Plug 300 includes a recess 302 for receiving and locating coil spring 128. Recess 302 has a passage

304 extending downwardly therefrom. Passage 304 is closed at its lower end by ball valve 306 that is held in sealing engagement with the outlet of passage 304 by plug 308. Plug 308 includes an engagement opening 310 that allows the plug to be screwed in and unscrewed, for example, by using a suitable instrument such as a screwdriver or an alien key. A circlip 312 and a washer 314 prevent over travel of the plug 308 when it is being unscrewed. The circlip is positioned in an annular recess formed in the base of the plug in a fashion that will be known to a person skilled in the art. The plug 300 also includes a further oil passage 316 which has an outlet 318. Outlet 318 is able to be connected to a hydraulic pipe or similar. The plug 300 shown in Figures 11 to 14 is provided to provide for manual lowering of a tipper tray in emergency situations. If it is desired to lower the tray manually, a drain hose having a shut off valve at one end is connected to outlet 318. Plug 308 is then unscrewed until it reaches the limit of its travel as set by circlip 312 and washer 314. Unscrewing the plug releases the ball valve 306 from its sealing engagement with the outlet of passage 304. When the drain valve on the end of the hydraulic hose connected to outlet 318 is opened, hydraulic oil can flow through passage 304 and along passage 316 and into the hydraulic drain hose. Typically, the drain hose is placed with its other end in the hydraulic fluid reservoir so as to not waste the hydraulic fluid. Once the tipper body has been fully lowered, plug 308 is screwed in, which again places ball valve 306 into sealing engagement with the outlet of passage 304. The plug 300 shown in Figures 11 to 14 allows emergency lowering of the tipper tray from a safe distance away from the tipper tray, thereby minimising the likelihood of accidents from emergency lowering of the tipper tray. Figure 15 shows a safety control valve for use with the valve assembly of the present invention. The safety control valve shown in Figure 15 is intended to prevent the valve assembly of the present invention from lowering the tipper body if the safety control valve detects that there is no pressure in the hydraulic cylinder assembly. Pot holes and uneven work sites can result in twisted truck bodies causing binding of the hydraulic cylinder assembly during lowering. If binding occurs, hydraulic fluid can drain from the hydraulic cylinder leaving an air void in the cylinder. Typically, the driver is totally unaware that the hydraulic fluid has drained from the cylinder even though the cylinder has remained in an extended position. Moving the vehicle and releasing the twist can result in the body going into freefall with potentially dangerous consequences. If oil velocity fuses or rupture valves are fitted which close when a set flow is achieved, then the combination of oil and air can result in the body reaching much greater falling speeds than are safe and when loaded can destroy the tipping cylinder and mountings. The safety control valve shown in Figure 15 ensures at all times that the valve assembly of the present invention can not be operated to lower the tipper body unless there is positive hydraulic pressure in the fluid chamber of the hydraulic cylinder. The safety control valve shown in Figure 15 includes a valve body 320. End 321 of valve body 320 is adapted to be mounted to the valve assembly of the present invention. The safety control valve includes an air spool 324 that is exposed to the pressure of hydraulic fluid in the fluid chamber of the tipping cylinder. The valve body further includes port 322 that is connected to the air pilot line for the directional control valve. Port 323 is connected to the main pilot air piston port 140 of the valve assembly shown in Figure 3 (for example). Thus, to activate the piston 136 of the valve assembly shown in Figure 3, pilot air must flow into port 322, through valve body 320 and out of port 323. As spool 324 is mounted in passage 325, spool 324 can move along the passage 325 in a reciprocating motion. The end 326 of spool is exposed to pressure P₂, as shown in Figure 3. Pressure P₂ equates to the oil or hydraulic fluid pressure in the fluid chamber of the hydraulic cylinder. The other end 327 of spool 324 is exposed to the pressure of the pilot air. The area Ai of the end 326 of spool (which is exposed to the hydraulic fluid pressure) is chosen to be, for example, approximately four times larger than the area A₂ of the end 327 of spool that is exposed to the pilot air pressure. In a typical truck, the pilot air pressure is approximately lOOpsi. Therefore, due to the ratio of areas A₅:A₂ being, for example, approximately 1:4, if the pressure of hydraulic fluid drops below about 25psi, the pilot air pressure will force the spool 324 to the position shown in Figure 15. In this position passage 328 will be arranged such that its outlet 329 will not be in register with port 323. Thus, pilot air cannot be provided to the piston chamber of the valve assembly shown in Figure 3. Instead, the pilot air drains via passage 330 and port 331. As can be seen, port 331 is fitted with a breather 332. However, if the oil pressure at end 326 is greater than about 25psi (which indicates that there is still hydraulic fluid in the fluid chamber of the extendable cylinder assembly), the spool 324 moves towards the left as shown in Figure 15 such that the passage outlet 329 of passage 328 is in register with port 323. Pilot air can then flow in through port 322, along passage 328, out passage outlet 329 and out through port 323 to thereafter be supplied to the piston chamber of valve 100 via port 140 (see Figure 3). Suitably, an audible alarm is also provided to alert the driver to the danger when the vehicle body is twisted and the cylinder is subject to binding. The audible alarm includes a pressure switch fitted into port 323. Figure 16 shows a flow diagram incorporating the valve assembly 100 shown in Figure 3 and the safety control valve body 320 shown in Figure 15. As can be seen from Figure 16, the valve body 320 is joined to valve assembly 100, as is indicated by dotted outline 350. The extendable hydraulic cylinder assembly 352 and the directional control valve 354 are also shown with appropriate connections to the valve assembly, safety control valve and hydraulic cylinder. In the flow sheet shown in Figure 16, air supply lines are shown as dotted outlines whilst oil supply lines are shown as solid lines. The hydraulic and pneumatic circuit shown in Figure 16 enables the safety valve to operate as described with reference to Figure 15. Figure 17 shows a flow diagram of a conventional hydraulic circuit used to control the operation of an extendable hydraulic cylinder for raising and lowering the tipper body of a tip truck or the like. The circuit shown in Figure 17 includes a reservoir 400 of hydraulic fluid. A hydraulic line 402 connects reservoir 400 with hydraulic pump 404. Hydraulic pump 404 is a conventional hydraulic pump that is driven from a gear box power takeoff with a mechanism to engage or disengage the pump. Although such driving mechanisms for driving hydraulic pumps are well known, they do constitute expensive items. A conventional directional control valve 406 is connected to hydraulic line 408. Directional control valve 406 is connected via appropriate pilot pneumatic lines 410, 412 to a conventional raising and lowering controller 414. A hydraulic line 416 passes from the directional control valve 406 to the extendable hydraulic cylinder 418. The hydraulic circuit also includes a system relief valve 420. In order to extend the cylinder 418 in order to raise the tipper body, the controller 414 is operated to provide a "raise" signal. It also actuates the directional control valve such that pressurised hydraulic fluid from pump 404 travels through lines 408, through the directional control valve 406, along hydraulic line 416 and into the hydraulic cylinder 418 to thereby extend the cylinder. In order to lower the tipper body, the controller 414 is operated to activate the directional control valve to allow fluid in the cylinder 418 to drain. The weight of the tipper body and any remaining load in the tray combine to force the hydraulic fluid in cylinder 418 out of the cylinder, through the directional control valve and returns to the reservoir 400. On vehicles where there is no air supply, a DC solenoid valve is used to pilot and control the directional control valve. The speed of the extension of the hydraulic cylinder is governed by the cylinder size, the pump capacity, the gearing of the power take off and the engine speed of the truck. Figure 18 shows a flow diagram for an hydraulic circuit in accordance with an embodiment of the third aspect of the present invention. The hydraulic circuit of Figure 18 shows a reservoir 430 of hydraulic fluid. A hydraulic line 432 that has a filter 434 therein is connected to the inlet of a pump 436. The outlet of the pump 436 is connected to hydraulic line 438. Hydraulic line 438 is in fluid communication with a first port of valve assembly 440. Valve assembly 440 is suitably as described with reference to any one of Figured 1 to 16 of the specification. The hydraulic line 438 also has a bypass line 442 extending therefrom. A bypass valve 444 is included in bypass line 442. The hydraulic cylinder 446 is connected to the second port of valve assembly 440 by hydraulic line 448. A system relief valve 450 is also provided in the circuit. A controller 452, which suitably sends control signals via pneumatic pilot lines 454, 456, is provided for manual operation by the operator of the vehicle. When it is desired to extend the cylinder 446, the controller 452 is operated by the operator to send a "raise" signal. The raise signal closes bypass valve 444 such that pressurised fluid from pump 436 cannot pass through bypass line 442. Therefore, pressurised fluid from pump 436 must pass into the first port of the valve assembly 440 and thereafter through line 448 and into cylinder 446 to thereby extend the cylinder. When it is desired to lower the tipper body, a "lower" signal is sent from the controller 452. The lower signal operates the bypass valve 444 such that pressurised fluid from the pump 436 passes through bypass line 442. In this fashion, pressurised fluid from the hydraulic pump is not supplied to the cylinder 446. At the same time, the controller 452 sends a pneumatic signal to the valve assembly 440 to open the valve assembly and thereby allow hydraulic fluid in the cylinder 446 to drain out of the cylinder and through the valve assembly 440. The hydraulic fluid from the cylinder flows back to the reservoir via the bypass valve 444. The hydraulic circuit shown in Figure 18 is advantageous because it can do away with the directional control valve required in conventional circuits. Such directional control valves are expensive items. In particular, the valve assembly 440 used in the circuit shown in Figure 18 controls the rate of fall of the tipper body and thereby removes any requirement for control of the rate of fall by the directional control valve or any flow restriction provided by additional hoses and/or fittings. The bypass valve is a relatively cheap valve (when compared to a directional control vale), thereby reducing the cost of the circuit. It will be appreciated that the circuit shown in Figure 18 may still require the power take off and associated clutch mechanisms in order to control the operation of the pump 436. Figure 19 shows a flow diagram of an hydraulic circuit in accordance with an embodiment of the fourth aspect of the present invention. The circuit shown in Figure 19 includes a reservoir 460 for holding hydraulic fluid. An hydraulic line 462 that includes a filter 464 is connected to the inlet of a hydraulic pump 466. Hydraulic pump 466 is a hydraulic pump as described in my co-pending international patent application no. PCT/AU2004/000951, the entire contents of which are herein incorporated by cross- reference. By way of brief description, hydraulic pump 466 includes extendable and retractable sliding vanes mounted in a rotor inside the pump housing. The pump also includes retaining means for selectively holding the vanes in the retracted position. When it is desired to pump hydraulic fluid using pump 466, the pump is operated such that the vanes can extend and retract as they rotate along their path of rotation. As the vanes can move to the extended position, they pump the hydraulic fluid in this mode of operation. If it is desired to stop pumping fluid, the retaining means can be actuated such that the vanes are retained in the retracted position. In this mode, the hydraulic fluid in the pump is effectively exposed to a spinning rotor, which does not work (and therefore does not pump) the hydraulic fluid. Actuation of the retaining means in pump 466 occurs via pressurised pilot hydraulic fluid, which is supplied through line 468. Pilot hydraulic fluid may come from the hydraulic steering pump or any other hydraulic pump on the vehicle. Control valve 470, which is operated by a pneumatic signal through line 472, controls the flow of hydraulic pilot fluid to the pump 466. The outlet of pump 466 is connected via hydraulic line 474 to the first port of valve assembly 476. Valve assembly 476 may be as described with reference to any of

Figures 1 to 16 of the present application. The second port of valve assembly 476 is connected via hydraulic line 478 to hydraulic cylinder 480. Hydraulic cylinder 480 is used to raise and lower a tipper body of a tip truck, for example. The circuit also includes a controller 482. Controller 482 is suitably manually actuated by an operator of the vehicle. The circuit also includes a pressure relief valve 484. In order to raise the tipper body, the controller 482 is operated to send a raise signal. The raise signal sends a pneumatic signal via pneumatic line 472 to control valve 470. This allows appropriate flow of pilot hydraulic fluid through line 468 to actuate pump 466 such that hydraulic fluid is worked by pump 466. As a result, pressurised hydraulic fluid passes from the outlet of pump 466, through hydraulic line 474, valve assembly 476 and hydraulic line 478 into hydraulic cylinder 480 to thereby extend the cylinder. To lower the cylinder, the control 482 is actuated to send a "lower" signal. This lower signal causes the vanes of pump 466 to be retained in the retracted position (thereby stopping the supply of pressurised fluid from pump 466). The lower signal also operates valve assembly 476 such that a flow passage between the second port and the first port is opened so that the hydraulic fluid in the cylinder 480 can drain therefrom. The weight of the tipper body and any load remaining in the tipper body acts to lower the cylinder. The hydraulic fluid draining from the cylinder 480 returns to the reservoir 460 via the pump 466. The hydraulic circuit shown in Figure 19 is less expensive, in both the cost of components and the installation thereof, than the conventional circuit shown in Figure 17. In particular, the directional control valve of Figure 17 can be omitted and replaced by small control valve 470. Control valve 470 is suitably a two position pneumatically or electrically piloted valve Similarly, use of the pump 466 in Figure 19 means that a power take off and associated clutch to engage and disengage the pump is no longer required. This represents a significant saving in the cost of components in the hydraulic circuit. Those skilled in the art will appreciate that the present invention may be subject to variations and modifications other than those specifically described. It will be appreciated that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A valve assembly for controlling flow of fluid to and from a fluid chamber of a telescopic hydraulic cylinder assembly, the valve assembly having: a valve body; - a first port in the valve body; a second port in the valve body, the second port being in fluid communication with the fluid chamber; a first check valve comprising a first valve seat located in a flow passage between the first port and the second port and a first valve member for seating on the first valve seat; a second check valve comprising a second valve seat and a second valve member for seating on the second valve seat; actuation means for unseating the first valve member and for unseating the second valve member; - said valve assembly being operable to allow fluid to drain from the fluid chamber when fluid pressure at the second port is greater than fluid pressure at the first port, wherein wherein said actuation means operates to unseat the first valve member to drain fluid from the fluid chamber if fluid pressure at the second port is less than a predetermined maximum pressure, and wherein said actuation means operates to unseat only the second valve member to drain fluid from the fluid chamber if the fluid pressure at the second port exceeds the predetermined maximum pressure.

2. A valve assembly as claimed in claim 1 wherein the actuation member operates to unseat both the first valve member and the second valve member if the fluid pressure at the second port is less than the predetermined maximum pressure.

3. A valve assembly as claimed in claim 1 or claim 2 wherein the actuation means operates to unseat the second valve member prior to unseating the first valve member.

4. A valve assembly as claimed in any one of the preceding claims wherein the second check valve operates to open and close a second flow passage and wherein at least a part of the second flow passage extends through the first valve member.

5. A valve assembly as claimed in any one of the preceding claims wherein the first valve member is biased to a closed position in which the first valve member is sealed against the first valve seat and the second valve member is biased to a closed position in which the second valve member is seated against the second valve seat.

6. A valve assembly as claimed in any one of the preceding claims wherein the second check valve allows a lesser flow of fluid therethrough relative to the first check valve.

7. A valve assembly as claimed in any one of the preceding claims wherein the actuation means comprises a plunger adapted for reciprocal movement, the plunger having a shoulder that abuts on the first valve member to unseat the first valve member and a projection extending beyond the shoulder, the projection extending through a passage in the first valve member and contacting the second valve member, the actuation means further comprising force applying means to apply a force to the plunger.

8. A valve assembly as claimed in claim 7 wherein the force applying means comprises a piston moveable by pneumatic pressure such that applying pneumatic pressure applies a force to the piston which causes the piston to move and apply a force to the plunger.

9. A valve assembly as claimed in claim 7 wherein the force applying means comprises a solenoid that moves an arm or a camming member to apply a force to the plunger.

10. A valve assembly as claimed in claim 9 wherein a spring is interposed between the force-applying arm or camming member and the plunger.

11. A valve assembly as claimed in claim 5 wherein biasing means for biasing the first valve member to a closed position and biasing means for biasing the second valve member to a closed position comprises a biasing spring that acts on both the first valve member and the second valve member.

12. A valve assembly as claimed in claim 5 wherein each of the first valve member and the second valve member are provided with separate biasing springs.

13. A valve assembly as claimed in any one of the preceding claims wherein the second flow passage includes an orifice for causing a pressure drop as fluid flows therethrough, said orifice being located between the second valve member and the second port.

14. A valve assembly as claimed in any one of the preceding claims further including a spring located between the actuating means and the first valve member.

15. A valve assembly for controlling flow of fluid to and from a fluid chamber of a telescopic hydraulic cylinder assembly, the valve assembly having: a valve body; - a first port in the valve body; a second port in the valve body, the second port being in fluid communication with the fluid chamber; a first check valve comprising a first valve seat located in a flow passage between the first port and the second port and a first valve member for seating on the first valve seat; a second check valve comprising a second valve seat and a second valve member for seating on the second valve seat; actuation means for unseating the first valve member and for unseating the second valve member; - said valve assembly being operable to allow fluid to drain from the fluid chamber when fluid pressure at the second port is greater than fluid pressure at the first port, wherein the actuating means unseats the second valve member prior to unseating the first valve member

16. A valve assembly as claimed in any one of the preceding claims further including a safety control means comprising a pressure sensing means for sensing pressure at the second port and control means for stopping flow of fluid from the second port to the first port of the sensing means senses that the pressure at the second port has dropped below a predetermined level.

17. An hydraulic circuit for moving an hydraulic cylinder between extended and non-extended positions comprising a source of hydraulic fluid, a hydraulic pump receiving hydraulic fluid from the source of hydraulic fluid, a valve assembly in accordance with the first or second aspect of the present invention, a first hydraulic line connecting the hydraulic pump to the first port of the valve assembly, an extendable hydraulic cylinder in fluid communication with a second port of the valve assembly, a bypass in the first hydraulic line having a bypass valve for selectively passing pressurised hydraulic fluid from the pump to the first port of the valve assembly or to pass pressurised hydraulic fluid from the hydraulic pump to a bypass line, and control means for controlling fluid flow through the hydraulic circuit, said control means being operable in a first mode to extend the hydraulic cylinder and operable in a second mode to allow the hydraulic cylinder to move from the extended position to the retracted position, wherein in the first mode the control means actuates the bypass valve to prevent flow of pressurised fluid from the pump through the bypass line such that pressurised fluid from the pump passes to the first port of the valve assembly and thereafter into the extendable cylinder to thereby extend the cylinder, and wherein in the second mode the control means actuates the bypass valve to allow pressurised fluid from the pump to pass through the bypass line and actuates the valve assembly such that pressurised hydraulic fluid in the extendable cylinder drains from the extendable cylinder.

18. A hydraulic circuit including a source of hydraulic fluid, a hydraulic pump having vanes that can be selectively retained within a pump rotor so that the pump does not pump hydraulic fluid and selectively released so that the pump pumps hydraulic fluid, the hydraulic pump being in fluid communication with the source of hydraulic fluid, a valve assembly in accordance with the first or second aspect of the present invention, the first port of the valve assembly receiving pressurised hydraulic fluid from the hydraulic pump, an extendable hydraulic cylinder in fluid communication with the second port of the valve assembly, and control means for controlling fluid flow through the hydraulic circuit, wherein the control means operates in a first mode in which the vanes of the hydraulic pump are not retained in the retracted position such that the hydraulic pump delivers pressurised hydraulic fluid to the valve assembly and thereafter to the extendable hydraulic cylinder to thereby extend the hydraulic cylinder, and a second mode in which the vanes are retained in the retracted position such that the hydraulic pump does not supply pressurised hydraulic fluid to the valve assembly, whereby in the second mode the control means also actuates the valve assembly such that pressurised hydraulic fluid in the extendable hydraulic cylinder can drain from the hydraulic cylinder and through the valve assembly such that the hydraulic cylinder moves from the extended position to the retracted position.