GB2097080A - Deceleration-sensing pressure proportioning or limiting valve assembly for vehicle braking systems - Google Patents

Abstract

A valve assembly has a stepped control piston (8) which normally abuts a stepped adjustment piston (23) working in a common bore (2), and the pistons are biassed sway from an inlet end of the bore by a control spring (38). A normally open valve seat (15) controlling communication between an inlet port (22) and an outlet port (43) is carried by the control piston (8) and is closed by a valve member (14) on separation of the pistons (8, 23). The adjustment piston (23) houses a deceleration sensing ball (26) which, on excessive deceleration of the vehicle or during rapid brake applications, moves to close off a passage (33) to isolate an adjustment chamber (34), the valve (14, 15) thereafter providing a metering action as pistons (8, 23) move apart. In other constructions the adjustment chamber is a chamber which is connected to the outlet port, and in some of these constructions (Figures 3 and 4) the deceleration ball is housed in the control piston and cooperates with a valve tube carried by the adjustment piston. <IMAGE>

Description

SPECIFICATION Deceleration-sensing modulating valve assembly for vehicle braking systems This invention relates to deceleration-sensing modulator valve assemblies for vehicle braking systems of the kind comprising a housing provided with a brake pressure inlet and a brake pressure outlet, a control piston working in the housing which controls the relationship between the inlet and outlet pressures, and a deceleration sensing element located in the housing and adapted to operate a deceleration valve for decelerations of the housing above a deceleration threshold, operation of the deceleration valve being arranged to influence the control exerted by the control piston. Such a valve assembly will hereinafter be referred to as 'a valve of the kind set forth'.

There have been many proposals for valve assemblies of this kind. Usually the brake pressure inlet of such an assembly is connected to a brake pressure supply which is also connected to the front wheel brakes, the brake pressure outlet is connected to the rear wheel brakes, and the arrangement of the control piston is such that for inlet pressures above an inlet pressure threshold value the rate of increase of the outlet pressure is reduced compared with the rate of increase of the inlet pressure, in order to provide a proportionally reduced pressure to the rear wheel brakes to take account of the changes in weight distribution during deceleration of the vehicle.

It is known to arrange the deceleration valve in various ways in order to modify the relationship between inlet and outlet pressures. The deceleration valve is provided to take account of the fact that a larger brake pressure is required to produce a given deceleration in a loaded vehicle than for the unloaded condition of the vehicle, and it has therefore been proposed to modify the inlet pressure threshold value by making the threshold value dependent upon the closure of the deceleration valve.

In one type of such valve assemblies closure of the deceleration valve is arranged to trap a volume of fluid in an adjustment chamber bounded by an adjustment piston which control the spring biassing of the control piston. Usually a control spring for the control piston acts between the two pistons but such an arrangement leads to a relativeiy long valve assembly and its operating characteristics are not entirely satisfactory.

One aim of the present invention is to provide a valve assembly of the kind set forth which is compact yet which may be arranged to have satisfactory operating characteristics.

In Patent Specification G..B. No. 2 010 425 A a valve assembly of the kind set forth has been proposed in which a modulator valve seat carried by a stepped adjustment piston is arranged to be engaged by a valve head on a control piston on relative movement apart of the piston, but the pistons are normally biassed together to maintain the modulator valve open. A deceleration sensing element is provided in a separate bore of the valve housing from the bore in which the adjustment and control pistons are located, and the deceleration valve controls fluid communication between the brake pressure inlet and an adjustment chamber bounded by the rear of the adjustment piston.During brake pressure increase the control piston and adjustment piston move relative to the housing in the same direction to increase the biassing force of a control valve spring acting on the control piston, so that when the deceleration valve closes to lock the adjustment piston the loading of the control valve spring has been set in accordance with the inlet pressure at which the deceleration threshold of the deceleration valve was reached.

Whilst the proposed arrangement of the adjustment and control pistons assists in reducing the length of the valve assembly, the assembly is still of substantial bulk and complexity.

According to our invention a valve assembly of the kind set forth comprises an adjustment piston arranged in a bore of the housing in which the control piston is also located, resilient means biassing the pistons together into a normal relative position in which they engage each other, a modulator valve comprising a moduiator valve seat carried by the adjustment piston or control piston, and a modulator valve member carried by the control piston or adjustment piston respectively, the modulator valve being arranged to be in one condition when the pistons are in the normal relative position and to be operated to a second condition on relative movement apart of the pistons, an adjustment chamber defined adjacent to the adjustment piston, a control spring of which the loading is increased by movement of the control piston relative to the housing from a retracted position of the control piston in the direction away from the adjustment chamber, the arrangement being such that before operation of the deceleration valve and modulator valve the pistons move in unison relative to the housing, thereby maintaining said relative normal position, in response to increasing inlet pressure to increase the loading of the control spring, and the deceleration sensing element is located at least substantially in a cavity defined in one of the pistons.

Since the deceleration sensing element is housed in one of the pistons it is possible to arrange all of the movable components in a single bore of the valve housing, and it may be arranged that the length of the bore is not substantially increased by the incorporation of the sensing element. Thus the bulk and weight of the assembly, and often its cost of manufacture, may be reduced.

It is also a significant advantage to have the deceleration sensing element arranged in the path of fluid flow between inlet and outlet, in order that during rapid brake applications the sensing element is urged by flow forces onto its seat, anci such an arrangement is enabled by arranging the sensing element in one of the pistons.

Said fluid flow path may be arranged such that fluid passes around the sensing element from one side to the opposite side thereof, or the arrangement may be such that the flow impinges on one side of the sensing element and is thereby substantially reversed in direction. It will be appreciated that intermediate arrangements of the flow between these two extremes are also envisaged.

The resilient means may be an independent spring, but preferably the control spring also constitutes said resilient means.

Preferably the modulator valve is arranged to be open when the pistons are in engagement and to close when the pistons move apart by a preset valve clearance.

Preferably the pressure responsive areas of the pistons and the resilient biassing of the pistons are arranged such that for a substantially unioaded vehicle only a small increase in inlet pressure following closure of the deceleration valve by deceleration forces will result in sufficient movement apart of the pistons to close the moduiator valve.

For deceleration of the vehicle in a loaded condition the inlet brake pressure is allowed to rise by a significant amount following operation of the deceleration valve before the modulator valve is operated. This effectively displaces the cut-in point of the modulator valve further up the input versus output brake pressure curve, which is generally considered to be desirable with such valve assemblies.

The deceleration sensing element may cooperate with a deceleration valve seat provided on the piston in which the element is located, in which case the seat is preferably provided around one end of a bore extending through an end wall of that piston, or it may be arranged to cooperate with a valve seat carried by the other piston, in which case the valve seat is preferably provided around one end of a bore which extends right through said other piston.

This last-mentioned bore may be provided by a tube extending through said other piston, and said tube may be capable of movement against resilient biassing to assist in releasing the rear wheel brakes.

The deceleration sensing element conveniently comprises a ball which can roll in said cavity of which the base is arranged in known manner in use to be inclined at a small angle to the horizontal. This angle determines the level of deceleration at which the deceleration sensing element moves towards the deceleration valve seat.

The adjustment chamber may be arranged to be isolated from the inlet and outlet when the deceleration valve is closed, but in some embodiments of the invention the adjustment chamber is permanently connected to the brake pressure outlet, and fluid communication between the inlet and outlet is normally through the open deceleration valve. Thus, in those embodiments of the invention increase of the outlet brake pressure following closure of the deceleration valve is accompanied by movement of the adjustment piston relative to the housing to decrease the volume of the adjustment chamber.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-section of a valve assembly in accordance with the invention, with the pistons being shown in their normal, retracted positions; Figure 2 is a graph to illustrate the characteristics of the assembly of Figure 1; Figure 3 is a view similar to Figure 1 of a further valve assembly in accordance with the invention in which the ball deceleration sensor is housed in the control piston, the pistons being shown in their retracted position; Figure 4 is a view similar to Figure 3, but of a modification of the assembly of Figure 3; Figure 5 is a graph to illustrate the characteristics of the assembly of Figure 4; and Figure 6 is a longitudinal cross-section of a modification of the assembly of Figure 1.

The terms 'front' and 'rear' will be used in the following description in the sense that the right hand end of the assemblies shown in the drawings is the front end, that end which faces forwards when the assembly is mounted in a vehicle.

With reference to Figure 1, the valve assembly comprises a generally cylindrical housing 1 provided with a longitudinal stepped bore 2 having bore portion 3 to 7 of progressively larger diameter. It will be appreciated that the single stepped bore 2 may be produced at relatively low costs by well-known methods.

A stepped control piston 8 comprises a larger diameter portion 9, of cross-sectional area A3, sealed by an annular seal 10 in bore portion 4, a rearwardly directed spigot 11, of cross-sectional area A1, sealed by annular seal 12 in blind bore portion 3, and a forwardly extending tubular projection 1 3 in which is housed a ball modulator valve member 14 engageable with a frustoconical modulator valve seat 1 5 provided on a plug 1 6 firmly secured in the front end of projection 1 3. The bore of projection 1 3 leads into an axial blind bore 1 7 of the control piston in which is housed a coiled compression spring 1 8 biassing the modulator valve member 14 towards seat 1 5. Valve member 14 is held off its seat, as shown, by a valve control rod 1 9 extending with clearance through an axial bore of plug 1 6.

Transverse passages 20 are provided in piston spigot 11 to provide fluid communication between bore 17 and an annular inlet chamber 21 communicating permanently with a brake pressure inlet port 22 of the housing 1. In use the inlet port 22 is connected to a master cylinder outlet which is also connected to the front wheel brakes.

An adjustment piston 23 comprises a cup shaped piston body 24 of stepped outline and a perforated abutment plate 25 which is rigidly secured in the mouth of the piston body 24 to hold captive a deceleration sensing ball 26 housed in the chamber 27 defined by the internal wall of the piston body 24. The piston body is sealed in housing bore portions 5 and 6 respectively by axially spaced annular piston seals 28 and 29 respectively, and an annular chamber 30 defined between seals 28 and 29 is vented to atmosphere by a tubular member 31. The forward retracted position, shown, of the adjustment piston 23 is defined by engagement of piston 23 with a plug 35 provided with radial flow recesses 35'.

The base of the cup shaped piston body 24 is provided with a stepped bore in which is secured a correspondingly stepped resilient seat member 32 provided with a central passage 33 which is sealed when ball 26 is caused to roll into engagement with seat 32. Passage 33 communicates with an adjustment chamber 34 defined between the piston 23 and plug 35 which is sealed in bore portion 7 and retained by a circlip 36. A bleed port 37 leads into adjustment chamber 34 and is normally closed by a bleed nipple.

A valve control spring 38 engages at one end with an abutment washer 39 which engages the step between bore portions 4 and 5, and at its other end with the base of a cup shaped spring retainer 40, which base is retained between a head of plug 16 and the free end of tubular extension 13, and the control spring 38 is preloaded to bias the control piston 8 forwardly relative to the housing into engagement with the adjustment piston 23. The actual engagement between the pistons 8 and 23 takes place through abutment of an annular rib 16' of the head of plug 1 6 with the abutment plate 25. An annular chamber 42 is defined generally between the rear end of adjustment piston 23 and the washer 39, chamber 42 communicating directly with a brake pressure outlet port 43 connected in use to the rear wheel brakes.

A further coiled compression spring 44 of lighter strength than control spring 38, is fitted over the spring retainer 40 to abut at its rear end with a radially outwardly directed flange 45 on the retainer 40 and at its front end with abutment plate 25. thereby to provide a light force tending to bias the pistons 8 and 23 apart. In addition to spring 38 a light compression spring 46 acts between a retaining washer 47 for seal 12 and the step of piston 8 between spigot 11 and larger diameter piston portion 9 and assists spring 38.

The operation of the valve assembly of Figure 1 will now be described, with reference being made to the graph of Figure 2. The valve assembly is mounted in a vehicle with its horizontal axis inclined upwardly by a predetermined small angle so that the deceleration sensing ball 26 normally rests in engagement with abutment plate 25, and passage 33 is open. In the absence of applied brake pressure to inlet port 22 the control piston 8 and adjustment piston 23 are in abutting engagement as shown, with modulator ball valve 14 held clear of seat 15 by the valve rod 19 engaged with abutment plate 25.Thus, on initial increase of the brake pressure applied by the master cylinder to inlet port 22 fluid flows to the rear brakes by way of chamber 21, passages 20, bore 17, valve seat 15, through the central perforations of plate 25 into chamber 27, out of chamber 27 through the radially outer perforations of plate 25 into chamber 42, and outlet port 43. During the initial rise in inlet pressure between points 0 and A on the graph the pressure applied to the rear wheel brakes will equal the master cylinder pressure applied to inlet port 22.

It will be seen that the engagement of the annular rib 16' of the plug 1 6 with the plate 25 directs the fluid flow between inlet 22 and outlet 43 through the central perforations of plate 25 against ball 26. During a rapid increase of inlet pressure impingement of the flow on the rear side of ball 26 will move the ball 26 against valve seat 32, irrespective of whether the deceleration threshold of the ball 26 has been reached, and the flow will be reversed by the ball as it proceeds to the radially outer perforations of plate 25. The early closing of the valve seat 32 during a rapid increase of inlet pressure is a very desirable characteristic of this valve assembly.

In the driver-only condition of the vehicle, that is without an additional load, the braking force resulting from the outlet pressure at point A causes a vehicle deceleration equal to the threshold deceleration necessary to cause the deceleration ball 26 to roll into engagement with valve seat 32 and close off port 33, thereby trapping a volume of fluid at the current inlet pressure in adjustment chamber 34 to lock adjustment piston 23 relative to the housing 1.

During the initial rise in inlet pressure from 0 to A in Figure 2 the control piston is subjected to the effect of outlet pressure (equals inlet pressure) acting over its front end, and to inlet pressure acting in chamber 21, so that the net force is a leftward force on the control piston 8 acting over the area A1, of spigot 11. Since adjustment chamber 34 communicates freely with inlet port 22 during this initial pressure increase, adjustment piston 23 is subjected to a leftward force resulting from inlet pressure acting over the annular area A2 corresponding to the difference in diameters between bore pistons 5 and 6. Thus the total leftward force on the pistons 8 and 23 is equal to the inlet pressure acting over the area (A1 + A2), and this is opposed by the load of control spring 38.

The fitted load of control spring 38 is preferably arranged such that said total leftward force on the pistons 8 and 23 due to fluid pressure forces balances the fitted load of spring 38 at the inlet pressure at which the ball rolls due to the vehicle deceleration threshold being reached in the driveronly condition. That is, the arrangement is such that the pistons 8, 23 are just about to move rearwardly against the force of spring 38 when the deceleration ball closes port 33.

On a further slight rise in inlet pressure adjustment piston 23 remains stationary and control piston 8 moves to the left to bring valve seat 1 5 into engagement with ball valve member 14. On further increase of inlet pressure the control piston 8 will act in well known manner as a proportioning or modulator piston to increase the outlet pressure at port 43 at a reduced rate to the increase of pressure at inlet port 22, valve 14 oscillating into and out of sealing engagement with valve seat 1 5. It will be appreciated that during this modulation phase,indicated by line B in Figure 2, the inlet pressure acts rightwardly on control piston 8 over an area (A3 - A1), and the outlet pressure acts leftwardly over the area A3.

When the vehicle is braked in a loaded condition the line 0 to A of Figure 2 will be retraced, but the outlet pressure at port 43 will continue to rise at the same rate as the inlet pressure beyond point A, as shown by the dotted line C. The threshold deceleration is not reached until the inlet pressure has reached point D.

However, as previously explained the net fluid pressure force acting rearwardly on pistons 8 and 23 at point A is equal to the fitted load of control spring 38, so that during the pressure rise from point A to point D the pistons 8 and 23 move in unison rearwardly relative to the housing 1 to increase the load of control spring 38. During this movement the valve seat 1 5 remains open due to the continuing engagement of the pistons 8, 23.

When point D is reached and valve seat 32 is engaged by ball 26 to isolate chamber 7 and lock piston 23, the rearward force exerted by piston 23 on piston 8, due to the fluid pressure acting over the area difference A2 of piston 23, is no longer availabie to assist further rearward movement of piston 8 during further rise of inlet pressure. Thus, a further substantial rise in inlet pressure must take place, from point D to E, before the piston 8 is caused to move rearwardly once again. Once piston 23 has been locked in position, the spring loading of piston 8 opposing its movement is equal to the load of control spring 38 less the fitted load of light spring 44, and the effective area of piston 8 subject to rearward pressure forces iso1.

Thus the cut-in point of the modulator valve is displaced significantly up the pressure curve to point E from the point D at which the threshold deceleration is detected in the vehicle-loaded condition.

In Figure 3 parts corresponding to those of the construction of Figure 1 have been given corresponding reference numerals. In this construction the deceleration ball 26 is housed in the control piston 8, which is made of generally similar shape to that of the adjustment piston 24 of Figure 1. In this construction an annular resilient member 32 of channel section in a radial plane acts both as the valve seat for cooperation with the deceleration ball 26 and as an annular valve member for co-operation with the modulator valve seat 15, which is defined by a rearwardly directed rib of piston 8 surrounding an aperture through which a valve support tube 48 extends with clearance.The eiastomeric valve member 32 embraces a radially outwardly directed flange on the rear end of tube 48, and its rearwardly facing flange 50 is of greater radial depth than its forwardly facing flange 51 so as to define an adequate seating area for sealing engagement by ball 26. Valve support tube 48 extends slidably through the rear end wall 52 of adjustment piston 23 and is normally biassed by a spring 55 against a centrally apertured abutment washer 53 retained in the mouth of a piston bore 54 by a circlip 56. Spring 55 is a light coiled compression spring fitted over tube 48 and abutting against the blind end of bore 54 at its rear end and at its front end against a circlip 57 located in an external annular recess of tube 48. Tube 48 is slidably sealed to piston end wall 52 by an O-ring 58 which is retained in position by a cup shaped abutment member 25.Abutment member 25 has a radially outwardly directed flange against which the front end of spring 44 bears, and this flange is located in an annular recess 60 provided in the rear face of piston end wall 52. Abutment member 25 is normally engaged with the front face of control piston 8 which face is provided with radially directed recesses 59 to provide permanent fluid communication between the interior of valve seat 1 5 and chamber 42. In this case control spring 38 is located in bore portion 3 and acts between the blind end of bore portion 3 and an apertured, dished plate 61 which holds the deceleration ball 26 captive in piston 8. A further light compression spring 62 retains in place an annular seal 63 which seals the rear end of annular chamber 42.

Annular chambers 30 and 63 are vented to atmosphere.

When no brake pressure has been applied to the inlet 22 the pistons 8 and 23 are biassed into engagement with each other and with adustment piston 23 in engagement with plug 35 as shown in Figure 3. Valve tube 48 is biassed fully rightwards relative to piston 23, and valve member 32 is clear of both the ball 26 and seat 1 5, so that on initial application of the master cylinder brake fluid can flow freely from inlet port 22 to outlet port 42 by way of the aperture in plate 61, between valve member 32 and seat 15, recesses 59 and chamber 42. Also fluid can pass down the bore of valve tube 48 to adjustment chamber 7. Thus, as with the Figure 1 construction, the pistons 8 and 23 will move rearwardly relative to the housing in unison in the laden condition.

The pistons 8 and 23 remain in mutual engagement until a sufficient deceleration is produced to move ball 26 into engagement with flange 50 to close the bore of tube 48, and thereafter, whilst the inlet pressure at 22 continues to rise, adjustment piston 23 is locked in position by the trapped fluid in chamber 7.

Control piston 8 will move to the left on further increase of inlet pressure, almost immediately for the driver-only condition but after a delay for the loaded condition, due to the difference in area between bore portions 4 and 3 to bring valve seat 1 5 into engagement with flange 51 of valve member 32, and thereafter a modulating action will be performed by oscillation of control piston 8.

Since ball 26 is arranged directly in the fluid path between the aperture in plate 61 and seat 15, a sudden increase in inlet pressure will cause the ball 26 to be moved by flow forces onto seat member 32 before the deceleration force produced by braking has had sufficient time to rise to the deceleration threshold. This, again, substantially improves the response time of the valve assembly to sudden rise in inlet pressure.

Tube 48 is permitted to move rearwardly against spring biassing to assist in release of the brakes. There might otherwise be a danger that ball 26 would be held on sealing flange 50 by the trapping of fluid in chamber 7, and the valve seat 1 5 held closed by pressure in chamber 42. Since tube 48 can move rearwardly relative to piston 23 on release of inlet pressure, the trapped pressure in chamber 7 can be relieved.

It is to be noted that the construction of Figure 3 is particularly compact, largely as a result of the reduced length of the control piston 8 as compared with that of Figure 1.

The operating characteristics of the Figure 3 assembly as similar to those shown in Figure 2.

Figure 4 shows a construction which is a modification of that of Figure 3, and corresponding reference numerals have been applied to corresponding parts. An essential difference is that the bleed port 37 of the Figure 3 construction is now used as an outlet port for connection to the rear wheel brakes, and this difference brings about significant changes in the characteristics of the valve, as shown in Figure 5.

Control piston 8 is of similar shape to that of Figure 3, but in this case its front end is slidably sealed to a rearwardly directed skirt 64 provided on the body 23' of adjustment piston 23. This enables the housing bore to have only two different diameter portions 3 and 6, and also defines a chamber 65 between the pistons 8 and 23 which is sealed on engagement of seat 1 5 with valve member 32. A further advantage of this arrangement is that the number of sliding seals is reduced by one.

In this case valve tube 48 is threadedly and sealingly secured at its front end to piston body 23' and the tube bore communicates permanently with adjustment chamber 7 by way of a hole 66.

Adjustment piston 23, unlike that of Figure 3, has a plain outer wall working in a plain bore portion, 6. In the normal condition of the assembly shown, in which no master cylinder pressure is applied to the inlet port 7, adjustment piston 23 engages plug 35, and the front end of control piston 8, which is provided with rece,sses 67, engages with piston body 23' under the force of control spring 38.

When master cylinder pressure is applied to port 22 of the Figure 4 assembly fluid initially passes to the outlet port 37 by way of the aperture in plate 61, the bore of valve tube 48, hole 66 and adjustment chamber 7. The outlet pressure at port 37 thereby rises at the same rate as the inlet pressure at port 22 for the portion 0 to A of the graph shown in Figure 5. At point A in the case of the driver-only condition, or point F in the case of a laden condition, a sufficient vehicle deceleration is produced to cause deceleration ball 26 to roll forwards to engage valve seat member 32 and thereby close the bore of tube 48.During the inlet pressure rise from O to A (or F) the pistons 8, 23 will have moved in unison rearwards relative to the housing to increase the loading of control spring 38, as distinct from what happens in the driver-only condition for Figures 1 and 3. Since the pistons remain in engagement during this movement valve seat 1 5 remains clear of valve member 32.

Following closure of tube 48 by ball 26 and on further increase of inlet pressure pistons 23 and 8 will be urged forwards relative to the housing.

Since the front end of piston 23 exposed to adjustment chamber 7 is of larger diameter than the cross-sectional area of chamber 65 the resulting increase in pressure in adjustment chamber 7, and therefore the outlet pressure, will proceed at a proportionally reduced rate to that o-i the increase in inlet pressure in chamber 65. This gives the portion A to B (F to G) of the response curve. When point B (G) is reached the relative movement apart of the pistons 8, 23 brings valve seat 1 5 into engagement with valve member 32 to seal chamber 65, and on further increase in inlet pressure the pistons 8, 23 move forwards in unison relative to the housing with valve seat 1 5 engaging member 32, as fluid is expelled from adjustment chamber 7 to the outlet port 37.In this condition the pistons 8, 23 act as a single modulator piston, since on further increase in pressure to inlet port 7 the piston assembly 8, 23 moves forwards to expell further fluid from chamber 7 to the outlet port 37, and the rate of pressure increase is determined by the ratio of the areas of the front end of piston 23 to the rear end of piston 8. Since the rear end of piston 8 is of less diameter than its front end, a further reduced rate of outlet pressure results, to give the portion B to C (G to H) of the characteristic. Eventually. at point C (H) the piston 23 has moved into engagement with plug 35, so that no further increase in outlet pressure is possible, as indicated by the curve portion C to D (H to J).

Thus, the assembly of Figure 4 provides a more progressive change in the relationship of output pressure to input pressure which more nearly approaches the ideal curve.

As with the Figure 3 assembly, the ball 26 of the Figure 4 assembly is arranged directly in the fluid flow path between inlet 22 and outlet 37.

Figure 6 shows a further modification which is essentially the assembly of Figure 1 but with the port 37 used as the outlet port for connection to the rear wheel brakes, and port 43 as a bleed port closed by a bleed nipple, not shown. In most respects the assembly therefore operates in a similar manner to the assembly of Figure 4, and this assembly has similar characteristics to those shown in Figure 5. The portion of the characteristic A to B (F to G) is similarly produced as a result of the modulator valve 14 being open, deceleration ball 26 engaged with seat 32, and in response to the pressure differential acting across piston 23 due to its different front and rear areas.

However, the portion of the curve B to C (G to H) is produced in a different way to that of Figure 4. In the construction of Figure 4 the valve seat 1 5 remained in engagement with seat 32 during this portion of the curve, but in the Figure 6 assembly the modulator valve 14 progressively meters fluid into chamber 27 at a reduced pressure compared with the inlet pressure, by oscillation of piston 8 relative to valve member 14. Thus a first reduction in pressure is performed by the modulator valve 14 and a second pressure reduction results again from the modulating effect of piston 23 itself. The two stages of pressure reduction therefore produce the curve portion B to C (G to H). Again, at point C (H) the piston 23 engages plug 35 to prevent further increase of outlet pressure.

Claims (3)

1. A valve assembly of the kind set forth comprising an adjustment piston arranged in a bore of the housing in which the control piston is also located, resilient means biassing the pistons together into a normal relative position in which they engage each other, a modulator valve comprising a modulator valve seat carried by the adjustment piston or control piston, and a modulator valve member carried by the control piston or adjustment piston respectively, the modulator valve being arranged to be in one condition when the pistons are in the normal relative position and to be operated to a second condition on relative movement apart of the pistons, an adjustment chamber defined adjacent to the adjustment piston, a control spring of which the loading is increased by movement of the control piston relative to the housing from a retracted position of the control piston in the direction away from the adjustment chamber, the arrangement being such that before operation of the deceleration valve and modulator valve the pistons move in unison relative to the housing, thereby maintaining said relative normal position, in response to increasing inlet pressure to increase the loading of the control spring, and the deceleration sensing element being located at least substantially in a cavity defined in one of the pistons.

2. A valve assembly as claimed in claim 1 in which the modulator valve is arranged to be open when the pistons are in engagement and to close when the pistons move apart by a preset valve clearance.

3. A valve assembly substantially as described with reference to Figure 1 or Figure 3 or Figure 4 or Figure 6 of the accompanying drawings.