GB2199115A

GB2199115A - Spool valve

Info

Publication number: GB2199115A
Application number: GB08628335A
Authority: GB
Inventors: Michael David Baxter
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-11-27
Filing date: 1986-11-27
Publication date: 1988-06-29
Also published as: GB8628335D0

Abstract

A directional control spool valve has a motion control valve 26 in the spool 2 controlling the regenerative flow path provided in the spool, to lock the load if source pressure drops during regenerative operation. A motion control valve 3, 4 for each load port A, B may be provided in the housing, arranged to act as a relief valve and also to cut off drain flow from the load (6) if source pressure P drops. The regenerative flow path is isolated from the normal flow path and includes a bypass chamber SA; and the normal path includes check valves 40, 41. <IMAGE>

Description

SPOOL VALVE This invention relates to spool valves for fluid pressure systems, and in particular to a directional spool valve for use with a regenerative hydraulic system1 that is to say, a hydraulic system having an operative condition in which pressure fluid recirculates, without return flow to the tank or reservoir. for example from one end to another of a hydraulic cylinder under application of an external load.

According to the present invention, a spool valve having a "regenerative" position has a motion control valve, such as a counterbalance valve, in the valve housing containing the spool, and/or in the spool, in a fluid flow path communicating with a load.

The term "motion control valve" includes over centre valves, counter balance valves. and lock valves.

More specifically, the present invention in one aspect resides in a hydraulic directional control spool valve comprising a housing. flow paths defined in the housing and communicating with ports for coxlnection to a pressure source, a drain. and an external load circuit, a spool movable in the housing and so related to the flow paths that the spool has three operating positions in the housing providing four-way directional control of fluid flow and a fourth operating position providing for regenerative fluid flow in said load circuit , and at least one motion control valve device disposed in the valve housing and/or in the spool and communicating with at least one fluid flow path communicating with a port for connection to the external load circuit. arranged to provide load motion control in at least one direction of fluid flow.

The invention in another aspect resides in a hydraulic directional control spool valve comprising a housing1 a bore in the housing, a spool slidable in the bore, a source port for connection to a source of fluid under pressure. a drain port, and two load ports for connection to an external load circuit; the spool having a neutral position in which it allows communication from the source port to the drain port and blocks flow to and from the load ports. a first operating position in which it allows communication from the source port to a first load port and from the second load port to the drain port. a second operating position in which it allows communication from the source port to the second load port and from the first load port to the drain port, and a regenerative position in which it allows communication between the source and load ports; and a normally closed motion control valve device in the housing arranged, when open, to permit communication from one load port to the drain port, the motion control valve device being arranged to be opened in response to fluid pressure present at the other load port.

The invention in a further aspect resides in a hydraulic directional control spool valve comprising housing, a bore in the housing, a spool slidable in the bore, a central drain chamber in the housing communicating with the said bore and with a drain port of the housing, bypass chambers in the housing at respective opposite sides of the drain chamber and communicating with the bore and with a source pressure part of the housing. source pressure chambers in the housing respectively at opposite sides of the bypass chambers, communicating with the bore and with the source pressure port of the housing, load control chambers in the housing respectively at opposite sides of the source pressure chambers. each communicating with a respective load port of the housing and with the bore, and further drain chambers respectively at oppositesides of the load control chambers. communicating with the said bore and with the or a drain port of the housing; the said chambers and spool being so arranged that in a neutral first position of the spool the bypass chambers communicate via the bore with the central drain chamber and both load chambers are closed from the bore, in each of two operating positions of the spool displaced in respective opposite directions from the neutral position a respective source pressure chamber is connected via the bore to a respective load control chamber, the other load control chamber is connected via the bore to the adjacent further drain chamber. and the bypass chambers are closed from the central drain chamber, whereby said load ports are connected respectively to the source pressure port and drain port, and in a "regenerative" position of the spool displaced beyond a said operating position both load control chambers are connected via the bore to the adjacent source pressure chambers and thus to each other, and the drain chambers are closed; and in which the housing contains at least one motion control valve device arranged, when open, to provide communication from an associated load control chamber and port to a said drain port, the motion control valve device being normally closed and being arranged to be opened in response to operating fluid pressure on movement of the spool to a said operating position in which the other load control chamber is connected to the source pressure port, whereby in the absence of source pressure said motion control valve device closes the said associated load port from the drain port.

The invention in yet another aspect resides in a hydraulic directional control spool valve comprising housing, a bore in the housing, a spool slidable in the bore, a central drain chamber in the housing communicating with the said bore and with a drain port of the housing, bypass chambers in the housing at respective opposite sides of the drain chamber and communicating with the bore and with a source pressure part of the housing, source pressure chambers in the housing respectively at opposite sides of the bypass chambers, communicating with the bore and with the source pressure port of the housing, load control chambers in the housing respectively at opposite sides of the source pressure chambers, each communiating with a respective load port of the housing and with the bore.

and further drain chambers respectively at opposite sides of the load control chambers, communicating with the said bore and with the or a drain port of the housing; the said chambers and spool being so arranged that in a neutral first position of the spool the bypass chambers communicate via the bore with the central drain chamber and both load chambers are closed from the bore, in each of two operating positions of the spool displaced in respective opposite directions from the neutral position a respective source pressure chamber is connected via the bore to a respective load control chamber, the other load control chamber is connected via the bore to the adjacent further drain chamber, and the bypass chambers are closed from the central drain chamber1 whereby said load ports are connected respectively to the source pressure port and drain port.

and in a "regenerative" position of the spool displaced beyond a said operating position both load control chambers are connected via the bore to the adjacent source pressure chambers and thus to each other. and the drain chambers are closed: and in which the housing contains at least one motion control valve device arranged, when open, to provide communication from an associated load control chamber and port to a said drain port. the motion control valve device being normally closed and being arranged to be opened in response to excessive fluid pressure in the associated load control chamber and port. for relieving said excessive pressure.

Preferably, the valve includes a motion control valve or counterbalance device disposed in the spool, additional to or instead of one or more motion control valve devices disposed in the housing, arranged to control the regenerative fluid flow when the spool is in its regenerative position, and responsive to the source pressure and arranged to prevent regenerative fluid flow in response to insufficient source pressure. More preferably, the motion control valve or counterbalance device in the spool provides a regenerative flow path that does not communicate with the flow paths(s) of the directional valve for pressure fluid flow from source to load; for example the regenerative flow path is via bypass pressure chambers of the directional valve.

This enables oad check means to be installed on the source side of the spool valve.

In the accompanying drawings: Figures 1 to 4 are longitudinal sectional views of a spool valve embodying the invention, showing the spool in different positions corresponding respectively to neutral state, raising load, lowering load, regenerative lowering, Figure 5 is a view corresponding to Figure 4 showing a spool containing a counterbalance or motion control device.

Figure 6 schematically shows a regenerative hydraulic circuit. and Figure 7 is a view corresponding to Figure 5 showing a spool containing a modified counterbalance or motion control device Figure 6 shows a spool valve S with four operating positions numbered I-IV respectively. The valve is shown in neutral position II in which flow from the pump is recirculated to the tank T, and flow to and from both ends of the hydraulic operating cylinder 6 controlled by the valve. is blocked by the spool. The cylinder is In position III, the pump is connected to the piston chamber 30 of the cylinder, and the piston rod chamber 31 of the cylinder is connected to the tank, and the piston moves to the right in Figure 6. In position I, the connections are reversed, the piston moving to the left.In position IV, the pump is connected to the piston chamber 30, and the piston rod chamber 31 is also connected, within the spool valve, to the piston chamber 30. The piston moves to the right at a speed proportional to the sum of the fluid flow from the pump and the flow from the piston rod chamber 31. This is known as regenerative operation and is used for example when a load attached to the piston rod is being lowered.

A spool valve embodying the invention is shown in detail in Figures 1 to 4.

The valve comprises a spool 2 in a housing 1 provided with internal galleries. and external ports, for connection to a pressure source or pump P, a drain or tank, and a load cylinder by way of load ports A, B.

The spool itself, and the associated galleries and ports of the valve housing. are arranged to provide four-way directional control.

The valve housing has, communicating with the bore in which the spool slides: a central drain chamber T1. connected to a drain port; first and second bypass chambers SA and SB on opposite sides of the central drain chamber, each communicating with the source pressure inlet port of the housing and hence with each other; respective source pressure chambers FA, F B at opposite sides of the bypass chambers. each communicating with the source pressure port of the housing: between each bypass chamber and the adjacent source pressure chamber, a hole or passage 7, 8 extending from a minimum-diameter region of the bore to an adjacent chamber M, N in the housing: a respective load control chamber A1, B1 on opposite sides of the source pressure chambers, each communicating with a respective load port A, B of the housing: and further drain chambers T2 at opposite sides of the load control chambers.

The associated arrangement of lands. grooves and passages on the spool will appear from the drawings and from the following description of operation of the valve.

The spool and its bore are designed to provide a metering action between the spool lands and the bore chambers as the spool is moved, thereby providing progressive control of the fluid flow on movement of the spool.

The load pressure supply galleries or chambers Al, B1 communicate directly with the load ports A, B. In each case the fluid flow path communicates with a respective motion control or counterbalance valve cartridge 3 or 4 mounted in the valve housing. These two cartridges are coaxial and mounted between them in the housing is a bilaterally slidable or double-acting piston 5 between chambers M, N.

Each cartridge comprises a body 36 seated in a corresponding bore in the housing. Each cartridge body has an internal bore which accommodates a movable poppet 11 on which is a valve face 37. urged against a valve surface in the cartridge body by a spring 16. The poppet in turn contains a blind bore 32 which accommodates a stem 33 of a pilot plunger 9, 10. The diameter of this bore is greater than that of the plunger stem, so that an annular passage extends within the poppet 11 from its end face. and communicates with lateral holes 15 in the poppet. The end of the plunger stem seats against the blind end of the bore and, when the poppet is closed on its valve surface, the end of the plunger projects from the inner end of the cartridge body.A hole 13 extends from the end of the plunger. axially within the plunger to a lateral opening in the side of the plunger stem.

In concept, each counterbalance cartridge 3, 4 is a relief valve cartridge converted to pilot operation by the insertion of the plunger 9, 10, arranged so that it can be operated by the piston 5 to open the poppet. The position of the operating piston 5 is governed by the pressure balance between chambers M, N between the respective counterbalance cartridges, and opposite ends of the piston 5. These chambers can receive fluid pressure through holes 7, 8 from the valve spool bore.

A chamber 14 between the pilot plunger and the end of the poppet communicates permanently with the tank galleries T2 through the holes 15.

Axially spaced from its end and its valve face, each poppet has an annular shoulder 35, which is exposed to the fluid pressure in port A and chamber A1, through lateral holes 34 in the cartridge body. Consequently, pressure in the load port or chamber can move the poppet against the spring 16 to open the poppet and connect the load port and chamber to the tank by way of the holes 15 and the gallery T2, regardless of the position of the piston 5 and pilot plunger.

Figure 1 shows the valve with the spool 2 in the neutral position. In this position of the spool, fluid flows from the pump P by way of bypass galleries S in the valve housing. to the tank by way of the central gallery T1. A load cylinder 6 is connected to the load ports A, B. The spool blocks communication between these ports and the tank galleries T2, so that the fluid in the load cylinder is trapped and cannot flow from the cylinder to the tank. In this neutral position, the piston 5 is centered so that neither of the pilot plungers 9, 10 is actuated, and the poppets 11, 12 are held closed by the respective springs 16.

Any pressure in the chambers M and N is dispersed through bleed holes 13 in the plungers 9 and 10 via chambers 14 and holes 15 in poppets 11 and 12 to galleries T2 in the body and thence to tank at tank pressure. Any excessive fluid pressure in chamber A1 will cause the poppet 11 to compress spring 16 and fluid will pass from chamber A1 through the hole 15 in the poppet and thence to tank via chamber T2. Likewise any excess pressure in chamber B1 will similarly cause counterbalance cartridge 3 to act as a relief valve and pass fluid from chamber B1 to chamber T2. Both counterbalance cartridges 3 and 4 can act as pressure relief valves. The pressures at which they operate are governed by the force created by springs 16.

The springs 16 in cartridges 3 and 4 centre piston 5 by creating a force on poppet 12 and plunger 10 in cartridge 3 and a force on poppet 11 and plunger 9 in cartridge 4 which forces act on opposite sides of piston 5, to push piston 5 to a central equilibrium position.

Any fluid pressure trapped in chambers M and N will fall to tank pressure because of drainage through the hole system 13-15 previously described.

Figure 2 depicts the spool valve in the "raise" position. Spool 2 has moved to the right to close the flow path between S and T1. A spool gap 17 allows fluid to flow from P via load control chamber F A to A into the annular chamber 31 of cylinder 6 to move the piston into the cylinder and eject the fluid from the large bore chamber 30 of the cylinder 6 into port B of the valve. The pressure of this fluid is insufficient to open the poppet 12 of cartridge 3. The spool allows pressure in chamber F to pass via hole 7 in the housing to chamber M and act on the left end of piston 5 creating a force which acts to the right on plunger 10 and thereby on poppet 12 to open cartridge 3 and allow oil to pass from chamber B1 through the poppet 12 and hole 15 to tank gallery T2.Any counter pressure in chamber N is bled via the small hole 13 in plunger 10 to tank gallery T2. The pressures at P and M are similar by inter-connection through hole 7 and if pressure P decays subsequently pressure in chamber M decays and the spring 16 will force the closure of counterbalance cartridge 3 thus preventing flow from B to T2. This counterbalance feature is a fail-safe device preventing movement of cylinder 6 in the event of failure in the pressure supply P. This counterbalance technique also conditions the movement of cylinder 6 to be proportional to the flow generated by supply pressure P.

In particular. if the load when moving tries to run away. that is. to run ahead of the pressure fluid pump, the pump pressure will be reduced, causing the appropriate counterbalance or motion control valve cartridge to throttle or close. thereby preventing the load from running away.

Figure 3 depicts the spool valve in the "lower" position, that is to say in the reverse mode to that described for Figure 2. The spool 2 has been moved to the left to a position to prevent fluid passing from P via S to T. The fluid passes from pressure source P via control chamber F B across gap 18 in spool 2 to chamber B1 and port B and thence to the large bore area 30 of cylinder 6. This flow forces the piston in cylinder 6 to move out and displace the fluid in the annular chamber 31 into port A. The pressure from the pressure source P enters chamber N via chamber p and hole 8 and moves piston 5 to open cartridge 4 in an identical way to that already described for the opening of cartridge 3, Fig. 2. The fluid passes out of the chamber A1 through the open cartridge 4 to tank gallery T2.In the event of a failure in pressure source P the cartridge 4 will close as already described for cartridge 3 and this will prevent the further flow of fluid from the cylinder. Cartridge 4 behaves as a counterbalance means to condition the movement of cylinder 6 in the outwards going direction of the piston at a speed which is proportional to the flow from pressure source P.

Cartridges 3 and 4 prevent any movement of fluid into or out of cylinder 6 in the event of insufficient pressure being available from pressure source P to move piston 5 in a direction to open cartridges 3 or 4 when the spool is in the neutral. raise or lower position or any position between these positions. This is an absolute double acting counterbalance protective system. It can be reduced to an absolute single acting counterbalance protective system by replacing one of the cartridges 3 or 4 by a simple plug shaped externally to match that cartridge.

Figure 4 shows the spool in the "regenerative lower" position which is the fourth position beyond the "lower" position of the spool. In this position the fluid flows from the pressure source P around the circuit in which the cylinder 6 is connected and there is no flow to tank gallery T2. In this condition both cartridges 3 and 4 are shut. In this position the spool maintains the closed position between S and T1 that was established in the "lower" position (Fig. 3) but allows pressure in S to pass via spool grooves 20 and 21 and holes 7 and 8 into chambers M and N to create an equal pressure force on each side of piston 5. Piston 5 is in pressure force equilibrium. and can no longer hold open cartridge 4 which closes under the influence of spring 16. Both cartridges 3 and 4 are closed and the fluid is contained within the closed circuit P,A,B.

Pressure in B creates a force on the piston of cylinder 6 which intensifies the fluid in the annular side of the cylinder 6 creating a larger pressure in A than in B which creates a movement of fluid flow in the direction A to B via a gap 19 in the spool which communicates this fluid to FA and around the internal gallery system of the valve. joining with flow from the pressure source P to pass through gap 18 of spool 2 to chamber B and thence to cylinder 6, causing the piston to continue to move at a rate proportional to the sum of the flows from the annular side of cylinder 6 via port A and the flow from the pressure source P into the system. This is in accordance with the classical regenerative circuit concept.

The spool can be moved back to the neutral position instantaneously thereby creating a pressure transient in either port A or port B which is restored to equilibrium by the action of the counterbalance cartridges 3 and 4 operating as self-regulating pressure relief valves.

The described spool design and arrangement of counterbalance cartridges in the valve housing provides a self-regulating regenerative overcentre system in which the movement of the spool takes the system from ',lower" to regeneration in a progressive manner without any interruption or inertial problems and has within it the fail-safe feature of load retardation and load stop in the event of a failure in the pressure supply.

Figure 5 shows a modification of the valve of Fig 4 wherein the flow from chamber A1 to FA passes through a motion control or counterbalance valve device inside the spool 2.

This device is formed by a piston 26 located in a bore in the end of the spool adjoining chamber Al. This piston is urged to the right by a spring 27 in a first a left hand, end chamber 28. At the opposite end of the bore is a right hand end chamber 25. The piston 26 is double-ended. with a groove between respective lands.

The bore has lateral holes 22, 23, of which the hole 22 is closed by the piston 26 when the piston is at its right-hand limit position, i.e. when pressure in chamber 25 cannot overcome the force of spring 27. In the regenerative position of the spool. holes 22. 23 communicate respectively with chambers Al and FA.

Pressure in chamber 25 for moving the piston to open the hole 22 is provided through a hole 24 in the spool, from chamber S.

In Figure 5 flow from A1 passes through holes 22 and 23 in the spool via the groove in piston 26 to chamber F A and then into the circuit previously described with reference to Figure 4. The flow path from chamber A1 to chamber FA can only be open when a pressure is achieved in chamber 25 to create a force which pushes the piston 26 into an open position relative to holes 22 and 23. This pressure is achieved in chamber 25 by pressure passing from chamber S through hole 24 into chamber 25. When this pressure is not present spring 27 exerts a force against piston 26 to close the flow between A1 and F A by occluding holes 22 and 23. Hole 29 connect8 chamber 28 to chamber T2 to enable any pressure in chamber 28 to escape.This is necessary in order to allow piston 26 to move against spring 27 into the open position. Normally piston 26 is in repose in the closed position. Piston 26 and spring 27 form a counterbalance or motion control device which prevents flow out of chamber A1 in the event of loss of pressure in the system and thereby prevents any load movement dependent upon the flow from chamber A1, thus, holding the load in position, which is an ideal fail safe characteristic.

The valve of Figure 5 can be further modified by removal of one of the counterbalance cartridges and substitution of a correspondingly shaped plug, as mentioned above.

The in-spool motion control valve device comprising the piston 26 can be used alone, viz. in the spool of a spool valve which is not provided with any motion control or counterbalance devices in the valve housing.

to control regenerative flow.

Figure 7 shows a modification of the valve shown in Figure 5 wherein the flow from chamber A1 to F A passes through a motion control or counterbalance valve device inside the spool 2.

This device is formed by a piston 26 located in a bore in the end of the spool adjoining chamber A1.

This piston is urged to the right by a spring 27 in a first. left hand, end chamber 28. At the opposite end of the bore is a right hand end chamber 25. The piston 26 is double-ended, with a groove between respective lands. The bore has lateral holes 22, 23, of which the hole 22 is closed by the piston 26 when the piston is at its right-hand limit position, i.e. when pressure in chamber 25 cannot overcome the force of spring 27. In the regenerative position of the spool, holes 22, 23 communicate respectively with chambers A1 and SA.

Pressure in chamber 25 for moving the piston to open the hole 22 is provided through holes 24 in the spool, from bypass chamber SA In Figure 7 flow from A1 passes through holes 22 and 23 in the spool via the groove in piston 26 to chamber SA and then into the circuit previously described with reference to Figure 4, i.e. via source pressure chamber F B to chamber B1 and port B.

The flow path from chamber A1 to chamber F A can only be open when a pressure is achieved in chamber 25 to create a force which pushes the piston 26 to the left into an open position relative to holes 22 and 23. This pressure is achieved in chamber 25 by pressure passing from chamber SA through hole 24 into chamber 25. When this pressure is not present spring 27 exerts a force against piston 26 to close the flow between A1 and 5A by occluding holes 22 and 23. Hole 29 connects chamber 28 to chamber T2 to enable any pressure in chamber 28 to escape.

This is necessary in order to allow piston 26 to move against spring 27 into the open position. Normally piston 26 is in repose in the closed right-hand position. Piston 26 and spring 27 form a counterbalance or motion control device which prevents flow out of chamber A1 in the event of loss of pressure in the system and thereby prevents any load movement dependent upon the flow from chamber A1, thus, holding the load in position, which is an ideal fail safe characteristic.

The regenerative flow path is through the bypass chambers. and therefore does not communicate with the flow path used for normal load operation. Consequently, it is possible to install a load check device 40 in the fluid flow path to the source pressure chamber F A used for raising the load. This prevents the possibility of the hydraulic fluid flowing back from the load cylinder through the valve during "raise" operation, for example.

if the pump were to fail. It would be difficult or impossible to install such a load check device, if the regenerative flow path were through the chamber FA.

Alternatively or additionally, a load check device 41 can be provided between the pump and the other source pressure chamber F B to prevent reverse fluid flow from the load control chamber B1 to the pump.

The valve of Figure 7 can be modified by removal of one of the counterbalance cartridges and substitution of a correspondingly shaped plug, as mentioned above.

The in-spool motion control valve device comprising the piston 26 can be used alone, viz. in the spool of a spool valve which is not provided with any motion control or counterbalance devices in the valve housing, to control regenerative flow.

Claims

1. A hydraulic directional control spool valve comprising a housing, flow paths defined in the housing and communicating with ports for connection to a pressure source, a drain, and an external load circuit, a spool movable in the housing and so related to the flow paths that the spool has three operating positions inthe housing providing four-way directional control of fluid flow and a fourth operating position providing for regenerative fluid flow in said load circuit, and at least one motion control valve device disposed in the valve housing and/or in the spool and communicating with at least one fluid flow path communicating with aport for connection to the external load circuit, arranged to provide load motion control in at least one direction of fluid flow.

2. A spool valve as claimed in claim 1, so arranged that changes in direction of fluid flow which occur at each working position take place progressively as the spool moves between the operating positions.

3. A spool valve as claimed in claim 1 or 2 in which the motion control valve device functions or devices function to stop the flow of fluid from the load circuit when insufficient pressure is available from the source.

4. A spool valve as claimed in claim 3 in which the motion control valve device or devices stop the flow of fluid in response to insufficient source pressure5 for any position of the spool.

5. A spool valve as claimed in any of the preceding claims in which at least one said motion control valve device is arranged and adapted to act as a relief valve for passing to the drain hydraulic fluid at excessive pressure in the load circuit.

6. A hydraulic directional control spool valve comprising a housing. a bore in the housing. a spool slidable in the bore, a source port for connection to a source of fluid under pressure, a drain port, and two load ports for connection to an external load circuit; the spool having a neutral position in which it allows communication from the source port to the drain port and blocks flow to and from the load ports, a first operating position in which it allows communication from the source port to a first load port and from the second load port to the drain port, a second operating position in which it allows communication from the source port to the second load port and from the first load port to the drain port. and a regenerative position in which it allows communication between the source and load ports; and a normally closed motion control valve device in the housing arranged, when open, to permit communication from one load port to the drain port. the motion control valve device being arranged to be opened in response to fluid pressure present at the other load port.

7. A hydraulic directional control spool valve comprising housing. a bore in the housing, a spool slidable in the bore. a central drain chamber in the housing communicating with the said bore and with a drain port of the housing, bypass chambers in the housing at respective opposite sides of the drain chamber and communicating with the bore and with a source pressure part of the housing. source pressure chambers in the housing respectively at opposite sides of the bypass chambers. communicating with the bore and with the source pressure port of the housing, load control chambers in the housing respectively at opposite sides of the source pressure chambers, each communicating with a respective load port of the housing and with the bore, and further drain chambers respectively at opposite sides of the load control chambers. communicating with the said bore and with the or a drain port of the housing; the said chambers and spool being so arranged that in a neutral first position of the spool the bypass chambers communicate via the bore with the central drain chamber and both load chambers are closed from the bore, in each of two operating positions of the spool displaced in respective opposite directions from the neutral position.a respective source pressure chamber is connected via the bore to a respective load control chamber, the other load control chamber is connected via the bore to the adjacent further drain chamber, and the bypass chambers are closed from the central drain chamber, whereby said load ports are connected respectively to the source pressure port and drain port, and in a "regenerative" position of the spool displaced beyond a said operating position both load control chambers are connected via the bore to the adjacent source pressure chambers and thus to each other, and the drain chambers are closed: and in which the housing contains at least one motion control valve device arranged. when open, to provide communication from an associated load control chamber and port to a said drain port, the motion control valve device being normally closed and being arranged to be opened in response to operating fluid pressure on movement of the spool to a said operating position in which the other load control chamber is connected to the source pressure port, whereby in the absence of source pressure said motion control valve device closes the said associated load port from the drain port.

8. A hydraulic directional control spool valve comprising housing. a bore in the housing, a spool slidable in the bore, a central drain chamber in the housing communicating with the said bore and with a drain port of the housing. bypass chambers in the housing at respective opposite sides of the drain chamber and communicating with the bore and with a source pressure part of the housing, source pressure chambers in the housing respectively at opposite sides of the bypass chambers, communicating with the bore and with the source pressure port of the housing, load control chambers in the housing respectively at opposite sides of the source pressure chambers, each communicating with a respective load port of the housing and with the bore, and further drain chambers respectively at opposite sides of the load control chambers, communicating with the said bore and with the or a drain port of the housing: the said chambers and spool being so arranged that in a neutral first position of the spool the bypass chambers communicate via the bore with the central drain chamber and both load chambers are closed from the bore, in each of two operating positions of the spool displaced in respective opposite directions from the neutral position a respective source pressure chamber is connected via the bore to a respective load control chamber, the other load control chamber is connected via the bore to the adjacent further drain chamber, and the bypass chambers are closed from the central drain chamber, whereby said load ports are connected respectively to the source pressure port and drain port, and in a "regenerative" position of the spool displaced beyond a said operating position both load control chambers are connected via the bore to the adjacent source pressure chambers and thus to each other, and the drain chambers are closed; and in which the housing contains at least one motion control valve device arranged, when open. to provide communication from an associated load control chamber and port to a said drain port, the motion control valve device being normally closed and being arranged to be opened in response to excessive fluid pressure in the associated load control chamber and port, for relieving said excessive pressure.

9. The spool valve of claim 7 in which the motion control valve device is additionally arranged to open in response to excessive fluid pressure in the associated load control chamber and port. for relieving said excessive pressure.

10. The spool valve of claim 7 or 9 which in the last-mentioned operating position of the spool, the spool allows communication via the said bore from the said other load control chamber to means for opening the motion control valve device.

11. The spool valve of any of cl.aims 6 to 10 in which the motion control valve device comprises a valve member spring-loaded to a closed position, and an operating member movable to open the valve member and accessible from outside the motion control valve device, and in which the housing is provided with a cylinder and a piston therein adjacent the operating member, this piston being arranged to be moved and thereby to move the operating member for opening the said valve member when the cylinder is under pressure, and the spool is arranged to open a fluid path from the source port to the said cylinder when the spool is in an operating position such as to connect the source port to the load port not associated with the motion control valve device.

12. The spool valve of claim 11 having a said motion control valve device asociated with each load chamber and port, the motion control valve devices being arranged symmetrically relative to the said cylinder and poston, and the cylinder and piston being double-acting for opening one or the other motion control valve device when the spool is in one or the other of its said operating positions.

13. A spool valve as claimed in any of the preceding claims in which a motion control valve device is disposed in the spool, arranged to control the regenerative fluid flow when the spool is in its regenerative position, and responsive to the source pressure and arranged to prevent regenerative fluid flow in response to insufficient source pressure.

14. The spool valve of claim 13 when dependent on claim 7 or 8 in which the motion control valve device in the spool comprises a bore in the spool, a valve member in the bore, an operating member therefor in the bore, passage means providing a path from a load control chamber receiving inflowing regenerative fluid via the bore in the spool to the adjacent source pressure chamber when the spool is in its regenerative position.

the valve member being spring-loaded to close the said path, and a passage connecting the adjacent bypass chamber to the operating member whereby source pressure will move the operating member to open the valve member.

15. A hydraulic directional control spool valve comprising a housing. flow paths defined in the housing and communicating with ports for connection to a pressure source. a drain. and an external load circuit, a spool movable in the housing and so related to the flow paths that the spool has three operating positions in the housing providing four-way directional control of fluid flow and a fourth operating position providing for regenerative fluid flow in said load circuit, and a motion control valve disposed in the spool, arranged to control the regenerative fluid flow when the spool is in its regenerative position, and responsive to the source pressure and arranged to prevent regenerative fluid flow in response to insufficient source pressure.

16. A spool valve as claimed in claim 15 in which the motion control valve provides a regenerative flow path isolated from the flow path(s) providing for pressure fluid in-flow from the source port towards the load circuit.

17. A hydraulic directional control spool valve comprising housing. a bore in the housing. a spool slidable in the bore, a central drain chamber in the housing communicating with the said bore and with a sn r drain port of the housing, bypass chambers'in the housing at respective opposite sides of the drain chamber and communicating with the bore and with a source pressure port of the housing, source pressure chambers in the housing respectively at opposite sides of the bypass chambers, communicating with the bore and with the source pressure port of the housing, load control chambers in the housing respectively at opposite sides of the source pressure chambers, each communicating with a respective load port of the housing and with the bore. and further drain chambers respectively at opposite sides of the load control chambers, communicating with the said bore and with the or a drain port of the housing; the said chambers and spool being so arranged that in a neutral first position of the spool the bypass chambers communicate via the bore with the central drain chamber and both load chambers are closed from the bore, in each of two operating positions of the spool displaced in respective opposite directions from the neutral position a respective source pressure chamber is connected via the bore to a respective load control chamber, the other load control chamber is connected via the bore to the adjacent further drain chamber. and the bypass chambers are closed from the central drain chamber. whereby said load ports are connected respectively to the source pressure port and drain port, and in a "regenerative" position of the spool displaced beyond a said operating position both load control chambers are connected via the bore to the adjacent source pressure chambers and thus to each other, and the drain chambers are closed; and in which the spool contains a motion control valve device responsive to fluid pressure and arranged when actuated by such pressure in the regenerative position, to open a fluid flow path within the spool from a load control chamber to the nearer bypass chamber for regenerative fluid flow.

18. The spool valve of claim 16 or 17 further provided with at least one load check device connected in the flow path for load-actuating pressure flow on the upstream side of the spool valve.

19. A spool valve substantially as herein described with reference to Figures 1 to 4, 5 or 7 of the accompanying drawings.