GB2270736A

GB2270736A - Spool valve

Info

Publication number: GB2270736A
Application number: GB9219620A
Authority: GB
Inventors: Michael David Baxter
Original assignee: TRANS NORDIC HYDRAULICS LIMITE
Current assignee: TRANS NORDIC HYDRAULICS LIMITE
Priority date: 1992-09-16
Filing date: 1992-09-16
Publication date: 1994-03-23
Anticipated expiration: 2012-09-16
Also published as: GB2270736B; GB9219620D0

Abstract

A spool valve comprises a housing 1 having flow paths defined therein communicating with work ports A, B, pressure source P and a drain. A spool 2 is movable in the housing to control fluid flow through the work ports. Load motion control valve devices 3, 4 associated with the work ports are biassed closed by springs, but can be opened to relieve excess pressure in the load cylinder 6. When the spool 2 is moved from its neutral position (figs 2 and 3, not shown), pressure fluid is supplied to one of the work ports A, B, and to the control valve associated with the other work port, to open it and allow flow from the load cylinder to the drain. Both control devices remain closed during regenerative flow (fig 4, not shown). The flow path for regenerative flow may be formed by recesses and flats on the surface of the spool, or by a passage inside the spool controlled by a further valve. <IMAGE>

Description

SPOOL VALVE This invention relates to directional spool valves for fluid pressure systems, and in particular but not exclusively to a directional spool valve for use with a regenerative hydraulic system, 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 GB 2181519 and GB 2199115, 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 valves includes over centre valves, counter balance valves, and lock valves.

In the valves described in GB 2181519 and 2199115, a pair of motion control valves, one for each load port, is mounted in the valve housing, axially in line with one another. A double acting piston between and coaxial with these valves acts on the pilot plunger of one or the other valve (or neither) in accordance with pressure transmitted to the piston by the spool in accordance with the spool position, so that one or the other valve is opened in synchronizm with movement of the spool.

The presence of the double acting plunger is an undesirable complication, and this and the associated fluid pathways involve extra machining.

According to the present invention, in a directional control spool valve generally as set forth in GB 2181519, the or each motion control valve is actuated directly by pressure fluid transmitted to it from the spool, which is designed to effect this, thereby rendering the actuating piston unnecessary.

The present invention is applicable not only to regenerative directional control valves, but also to three-position control valves with no regenerative position.

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 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 positions in the housing providing four-way directional control of fluid flow in said load circuit, and at least one load motion control valve device disposed in the valve housing and communicating with at least one fluid flow path communicating with a port for fluid flow to or from the external load circuit, arranged to provide load motion control in at least one direction of load motion, the spool being designed to provide a pressure fluid flow path to the motion control valve device, such as to open the said device for flow from the said port to drain, when the spool is positioned for fluid flow from the source to the other load port.

The invention in one preferred embodiment resides in a hydraulic directional control spool valve comprising 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, and 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 normally closed said load motion control valve device in the housing arranged, when open, to permit communication from an associated load port to the drain port, the load motion control valve device being arranged to be opened by fluid pressure present at the source port when the spool is moved to apply such pressure to the other load port.

The invention in a further preferred embodiment resides in a hydraulic directional control spool valve comprising a said load motion control valve device disposed in the housing, a bore in the housing, said 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 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; and a said load motion control valve device arranged, when open, to provide communication from an associated load control chamber and port to a said drain port, the said load motion control valve device is normally closed and is arranged to be opened by 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 load motion control valve device closes the said associated load port from the drain port; 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 control chambers are closed from the source pressure and drain chambers, andin 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 said load motion control valve device disposed in the housing 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.

The invention in yet another embodiment resides in a hydraulic directional control spool valve comprising a bore in the housing, said 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 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; and in which the housing contains at least one said load motion control valve device arranged, when open, to provide communication from an associated load control chamber and port to a said drain port, the load 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; 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 control chambers are closed from the source pressure and drain chambers, and 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 said load motion control valve device disposed in the housing 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.

In one embodiment of the invention, the valve has a regenerative fourth position of the spool displaced beyond a said operating position, in which 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.

In a particularly preferred arrangement at least one said load motion control valve is provided with a pilot element for opening the load motion control valve and with a fluid-pressure connection to the spool for applying fluid pressure to the pilot element, the said connection communicating with the bore at a position which is isolated by the spool from the drain and connected by the sppol to the source port when the spool is in its operating position providing pressure fluid flow to the load port not associated with the said load motion control valve and providing fluid flow to the drain port from the load port associated with the said load motion control valve.

Preferably, the valve includes a motion control valve or counterbalance device disposed in the spool, additional to one or more said motion control valve devices disposed in the housing, arranged to control regenerative fluid flow when the spool is in a regenerative position, and responsive to the source pressure and arranged to prevent regenerative fluid flow in response to insufficient source pressure.

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 modified spool, and Figures 6 to 8 schematically show a non-regenerative spool valve.

Figures 1 to 4 show a spool valve with four operating positions. In the neutral position flow from the pump P 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 thereby locked in position.

In a first operating position 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 left in Figure 1. In the other operating position, the connections are reversed, the piston moving to the right. In a regenerative fourth position, 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 left 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.

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 5a and Sb on opposite sides of the central drain chamber, each communicating with the source pressure inlet port of the housing; respective source pressure chambers Fa, Fb 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 inside them are bilaterally slidable or single-acting plungers 10 and 9 respectively.

Each cartridge comprises a body 36 seated in a corresponding bore in the housing. These bores are coaxial, mirror images of one another, and parallel to the spool. 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 the plungers can open the poppets. The positions of the operating plungers 9, 10 are governed by the pressures in respective pilot chambers M, N acting on the respective counterbalance cartridges. 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 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 spool blocks the passages 7, 8 and 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 plungers 9, 10 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. 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 has a recess 40 and adjoining groove 21 in its land 42, which allow pressure in chamber FB to pass via hole 8 in the housing to chamber N and act on the end of plunger 10 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.The pressures at P and N are similar by inter-connection through hole 8 and if pressure P decays subsequently pressure in chamber .Ndecays 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 the flow to tank T, 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 M via chamber FA, recess 43 and groove 20 in spool land 45, and hole 7 and moves plunger 9 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 plungers 9, 10 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 prevents pressure in S passing via spool grooves 20 and 21 and holes 7 and 8 into chambers M and N. Chamber M is at zero pressure creating zero pressure force on plunger 9 which 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 B1 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 shown in Figure 4 wherein the regenerative flow from chamber A1 to 5A 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, 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 SA.

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 5A and then into the circuit previously described with reference to Figure 4. The flow path from chamber A1 to chamber SA 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 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 SA by occluding holes 22. 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 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.

Figures 6 to' 8 show a directional control spool valve which differs from that of figures 1 to 4 in that it has no regenerative position. This enables the spool to be of essentially symmetrical profile, with a narrow central land 51 providing open-centre operation, broad grooves 52 arranged to permit flow selectively from the source P to the pilot hole 7 or 8 when the spool is moved to an operating position, broad lands 53, and outer grooves 54 providing directional load control.

The manner in which the valve shown in figures 6 to 8 operates will be evident from these figures and from the foregoing description of figures 1 to 3.

The valves set out herein have a number of significant advantages over the valves described in GB 2181519 and 2199115. In these prior designs, the pilot pressure hole for opening a motion control valve is in the opposite half of the valve housing from the motion control valve which it operates. In the present valves each pilot hole (7, 8) is directly adjacent the motion control valve that it operates. This enables the motion control valve to function as independent units, for example one such unit for single acting motion control, two units for double acting motion control. It is relatively simple to machine accommodation for these units into standard multi-spool valve housings, as required. It will be understood that spool valve housings are made to common design standards, incorporating bores or cavities (M, N) of a standard size, for fitting different cartridges. Thus, in the present valves, the only modification needed to a standard valve housing, is the drilling of the pilot pressure holes (7, 8). This leads to a significant reduction in manufacturing costs relative to the valves described in GB 2181519 and 2199115. The assembly operation involved in installing the intervening piston of the prior valves is also eliminated.

The valve of Figure 8 has a motion control valve incorporated in the spool. As an alternative to this, a valve embodying the present invention may have a motion control valve mounted in the valve body, in particular a motion control valve generally as set out in GB 2227295 (8900039. 2).

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 positions in the housing providing four-way directional control of fluid flow in said load circuit, and at least one load motion control valve device disposed in the valve housing and communicating with at least one fluid flow path communicating with a port for fluid flow to or from the external load circuit, arranged to provide load motion control in at least one direction of load motion, the spool being designed to provide a pressure fluid flow.

path to the motion control valve device, such as to open the said device for flow from the said port to drain, when the spool is positioned for fluid flow from the source to the other load port.

2. A hydraulic directional control spool valve as claimed in claim 1, comprising 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, and 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 normally closed said load motion control valve device in the housing arranged, when open, to permit communication from an associated load port to the drain port, the load motion control valve device being arranged to be opened by fluid pressure present at the source port when the spool is moved to apply such pressure to the other load port.

3. A hydraulic directional control spool valve as claimed in claim 1 comprising a said load motion control valve device disposed in the housing, a bore in the housing, said 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 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; and a said load motion control valve de.vice arranged, when open, to provide communication from an associated load control chamber and port to a said drain port, the said load motion control valve device is normally closed and is arranged to be opened by 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 load motion control valve device closes the said associated load port from the drain port; 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 control chambers are closed from the source pressure and drain chambers, and 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 said load motion control valve device disposed in the housing 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.

4. A hydraulic directional control spool valve as claimed in claim 1 comprising a bore in the housing, said spool slidable in the bore, a central drain chamber in the housing communicating with the said bore 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 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; and in which the housing contains at least one said load motion control valve device arr- ged, when open, to provide communication from an associated load control chamber and port to a said drain port, the load 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; 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 control chambers are closed from the source pressure and drain chambers, and 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 said load motion control valve device disposed in the housing 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.

5. A valve as claimed in claim 3 or 4 having a regenerative fourth position of the spool displaced beyond a said operating position, in which 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.

6. A valve as claimed in any of claims 1 to 5 in which at least one said load motion control valve is provided with a pilot element for opening the load motion control valve and with a fluid-pressure connection to the spool for applying fluid pressure to the pilot element, the said connection communicating with the bore at a position which is isolated by the spool from the drain and connected by the spool to the source port when the spool is in its operating position providing pressure fluid flow to the load port not associated with the said load motion control valve and providing fluid flow to the drain port from the load port associated with the said load motion control valve.

7. A valve as claimed in any of claims 1 to 6 which includes a motion control valve or counterbalance device disposed in the spool, additional to one or more said motion control valve devices disposed in the housing, arranged to control regenerative fluid flow when the spool is in a regenerative position, and responsive to the source pressure and arranged to prevent regenerative fluid flow in response to insufficient source pressure.

8 A spool valve assembly substantially as herein described with reference to Figures 1 to 4, Figure 5, or Figures 6 to 8 of the accompanying drawings.