GB2031086A - Brake boosters - Google Patents

Abstract

The invention aims to reduce booster weight by facilitating the use of a lightweight housing, preferably of moulded plastics. The invention provides a tube (10) which extends through the movable wall (8) to transmit forces from the front wall (1) to the rear wall (2) of the housing. The tube is co-axial with the input (30) and output (28) members. Slots (16) are provided in the tube (10), and arms (35) extend radially through the slots for transmitting boost forces from the movable wall to a thrust assembly (27) located within the tube and acting on the output member. A conventional poppet valve assembly is provided in one construction to control the differential pressure applied to the movable wall, but in another construction a reduced number of seals is facilitated by employing a helical cam (58, 70) to operate a valve assembly (Fig. 2) housed in a transverse bore (72, 73, 74) in the rear housing shell (2). <IMAGE>

Description

SPECIFICATION Servo booster for vehicle braking systems This invention relates to servo booster assemblies for vehicle braking systems of the kind comprising a movable wall dividing the interior of the housing into two chambers and adapted to apply a force to an output member when the chambers are subjected to a pressure differential in response to a force applied to an input member.

The current world-wide shortage of fossil fuel reserves coupled with an ever increasing demand highlights the necessity for fuel saving measures. One example of this is the trend towards lighter motor vehicles. Consideration is given to saving every gram of basic vehicle weight in spite of the extra first cost which this often entails. Thus energy is saved by producing lighter parts and even fossil oil by-products are used to achieve weight saving.

In this connection, it has already been demonstrated that vacuum servo housings can be produced from plastics materials. However, with the usual arrangement in which one shell is connected in use to the vehicle bulkhead, and the other shell through which the output member extends is connected to a master cylinder housing, the output force applied to the master cylinder piston by the booster output member is reacted back to the vehicle bulkhead substantially through the shells of the servo housing.

The quantity of plastics material that has therefore been required to afford the requisite stiffness and fatigue strength of the housing with such designs has been so great that the objective of saving weight and scarce raw material has not been realised.

In the Specification of British Patent Application No. 41321/78 (published as No.

2 009 871 and claiming priority from British Patent Application Nos. 43644/77, abandoned 2488/78, 29253/78 and 29255/78) we have disclosed various constructions of servo booster of the kind set forth in which one or more stationary force transmitting means extend through the movable wall from one housing wall to a housing wall on the opposite side of the movable wall, and provided with means sealing the movable wall directly or indirectly to the force transmitting means.

When such a booster is mounted in a vehicle in the usual manner, between a master cylinder and a vehicle bulkhead, the force transmitting means is arranged to convey braking reaction forces form the master cylinder housing to the vehicle bulkhead.

However, such servo boosters may be mounted in vehicles in other ways. The booster and the master cylinder housing may, for example, be secured together on opposite sides of the vehicle bulkhead, a pivotal support for a brake pedal then being carried by the end of the stationary force transmitting means opposite to that which is secured to the master cylinder.

Since the force transmitting means of such a booster is preferably arranged to transmit in use substantially all of the axial forces to which the booster housing would otherwise be subjected, the strength of most of the housing and hence its weight can be reduced and an overall saving in weight may be achieved, even when a metal housing is employed.

In such a booster the force transmitting means at either end may either pass through the respective housing wall for direct connection to an external member, or it may be connected to one region of the housing wall and an adjacent region of the housing wall be provided with means for effecting a connection with an external member. In the later case it will be appreciated that the portion of the housing wall adjacent to the end of the force transmitting means does convey all of the reaction forces between the external member and the force transmitting means, and in that case it is necessary for the housing to be made sufficiently strong in that region. However, the remainder of the housing can still be made relatively thin.

In most of the constructions disclosed in Application No. 41321/78 Serial No 2009 871 a pair of tie rods, which are radially spaced from the booster axis and diametrically spaced with respect to each other, extend axially through the movable wall and are each sealed thereto by either a respective sliding seal or a respective rolling diaphragm.

Although the use of the tie rods provides a convenient means of transmitting the braking reaction forces, the optimum spacing between a pair of tie rods does not correspond to the usual spacing between the mounting studs used on a conventional booster, so that an adaptor plate or a cranked tie rod is required.

The present invention is concerned with an improved form of force transmitting means which does not impose such restrictions on the location of the mounting points.

According to the invention a servo booster assembly for a vehicle braking system comprises a booster housing having front and rear housing walls, a pedal-operated input member axially aligned with an output member, a movable wall dividing the interior of the housing into two chambers and adapted to apply a force to the output member when the chambers are subjected to a pressure differential in response to a force applied to the input member, a force transmitting means extending between the front and rear housing walls for transmitting in use braking reaction forces which would otherwise be transmitted through the housing, the force transmitting means comprising a tube which is co-axial with the input and output members and extends through a movable wall.

Preferably a thrust assembly is interposed between the input and output members and disposed within the tube, and means is provided to transmit forward movement of the movable wall to the thrust assembly.

Preferably the tube is provided with at least one aperture, and the means transmitting forward movement of the movable wall to the thrust assembly comprises an arm extending through the aperture.

Preferably first seal means is arranged to seal the movable wall to the radially outer surface of the tube at a location axially displaced in one direction from said aperture, and second seal means is arranged to seal the tube to the appropriate housing wall at a location further axially displaced from said apertures in the same direction but the first and second seal means may be combined if desired.

Preferably there are a plurality of circumferentially equally spaced apertures in the tube and a corresponding number of arms.

Each aperture preferably comprises an axially extending slot in the tube with each arm extending substantially radially through the slot, but if desired the tube could be of stepped outline and each aperture could be formed in the step with each arm extending substantially axially through the corresponding aperture in the step.

The tube may be integrally formed with one of the housing walls or it may comprise an independent member. When the tube is integral with one housing wall, the integral connection preferably constitutes the second seal means.

When the housing comprises two opposed housing shells the tube is preferably arranged to retain the housing shells in assembled relationship.

The peripheral bead of a diaphragm of the movable wall may then be held clamped between the outer peripheries of the housing shells by the tube.

In one preferred arrangement the movable wall comprises a diaphragm support plate provided with a tubular portion co-axial with and spaced radially outwardly from the outer surface of said tube, and the first seal means comprises a rolling diaphragm which is arranged to roll between the radially outer surface of said tube and the radially inner surface of said tubular portion.

The rolling diaphragm is preferably integral with a main flexible diaphragm of the movable wall located in face contact with the rear of the main part of the diaphragm support plate.

The tubular portion is preferably then arranged to extend forwardly from the main part of the support plate, said arms extending from the front of said tubular portion through said slots which are located in front of the rolling diaphragm.

Preferably the thrust assembly comprises a tubular body housing a reaction means, such as a resilient reaction disc, adapted to apply to a control member a proportion of the force being applied by the thrust assembly to the output member to provide a foot pedal reaction force.

The tubular body is conveniently formed integral with the diaphragm support plate and with said arms, and preferably extends rearwardly from said arms.

The valve assembly of the servo booster may comprise a substantially conventional poppet valve assembly comprising an axially movable valve body sealed, by a sliding seal or rolling diaphragm for example, to said tube with said arms abutting the valve body, but the disadvantage of the need to provide a seal with the tube may be avoided by utilising a valve assembly of the kind disclosed in the Provisional Specification of our earlier U.K.

Application 7599/78 abandoned.

In that Provisional Specification we have described a servo booster in which valve means is located in a stationary part of the booster housing and is operated by an operating assembly located within the housing between an input and an output member, the operating assembly comprising a rotatable actuating member for operating the valve means, an axially movable operating member, and a helical engagement between parts of the actuating member and the operating member constructed and arranged such that relative movement between the actuating member and the operating member in an axial direction towards each other causes rotation of the actuating member to operate the valve means.

In a preferred construction in accordance with the present invention the valve means is located in a bore provided in the rear housing shell of a booster housing comprising opposed housing shells, and the helical engagement comprises co-operating screw threads formed on said operating member and on said actuating member respectively. In one construction the actuating member is provided with axially extending peripheral teeth meshed with a rack formed on a single valve operating member, but in a preferred construction the actuating member is provided with an axially extending radially projecting vane which is adapted to engage at opposite sides with the ends of two oppositely extending valve control rods controlling respective valve members controlling the connection of the rear chamber of the booster respectively with an atmosphere connection and with a vacuum connection.

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 first vacuum servo booster in accordance with the invention, the parts being shown in their retracted positions; Figure 2 is a cross-sectional view on the line 2-2 of Fig. 1; Figure 3 is a cross-sectional view on the line 3-3 of Fig. 1; Figure 4 is a longitudinal cross-sectional view of a second vacuum servo booster in accordance with the invention, the parts being shown in their retracted positions; Figure 5 is a cross-sectional view on the line 5-5 of Fig. 4; and Figures 6 and 7 are longitudinal crosssectional views of third and fourth vacuum servo boosters respectively in accordance with the invention, the parts being shown in their retracted positions.

With reference to Figs. 1 to 3, the first booster comprises opposed lightweight moulded plastics housing shells 1 and 2, the radially outer periphery 3 of the rear shell 2 being stepped to define with a radially outwardly directed flange 4 of front shell 1 an annular recess 5 in which is clamped a peripheral bead 6 of a flexible main diaphragm 7 of an annular movable wall assembly 8. A rearwardly directed flange 9 of front housing shell 1 adjacent to radial flange 4 assists in retaining bead 6 in recess 5.The axial clamping force retaining bead 6 is provided entirely by a tube 10 which is co-axial with the central longitudinal axis of the booster, is integral at its rear end with rear housing shell 2, and is provided at its front end with radially outwardly directed projections 11 having rearwardly facing radial faces 1 2 engaging with a forwardly facing annular face 1 4 on a radially inwardly directed annular lip 1 3 of front housing shell 1, the lip 1 3 being located at the rear end of a rearwardly extending tubular part 1 5 of front shell 1. The tube 10 is provided, as shown in Fig. 3, in its front half with four equi-angularly spaced, axially extending, open-ended, radial slots 1 6 for a purpose to be described in detail hereafter.

Thus the front half of the tube 10 is in the form of four resilient axially extending legs 1 7 separated by slots 16, each leg 1 7 carrying a respective integral projection 11. Each projection 11 is formed with a sloping face 1 8 to provide a snap engagement between the projections 11 and the shell 1 on assembly of the shells together by axial pressure.After assembly of the shells a flanged pressed metal plug 1 9 is inserted into the front end of tube 10 to retain the projections 11, and the plug 1 9 is retained in place in use by its flange 20 being clamped between the rear flange, not shown, of a master cylinder housing and front shell 1 by diametrically spaced threaded studs 21 moulded into thickened portions 21 of front shell 1 which are integral with tubular part 1 5.

Rear housing shell 2 is formed with an integral tubular extension 22 extending rearwardly of rear housing wall 23 and of the same cross-section as tube 1 0. Extension 22 houses a valve actuating assembly 24 for a valve means 25, shown in Fig. 2, located in a stationary vertically extending valve housing 26 integrally moulded with rear housing wall 23.

A thrust assembly 27 is interposed between an axial output rod 28, working through a seal 29 retained within plug 19, and valve control assembly 24, and the assembly 24 is backed by a pedal-operated axial input rod 30.

Movable wall assembly 8, in addition to main diaphragm 7, comprises a moulded plastics annular diaphragm support plate 31 of frusto-conical shape having a forwardly directed peripheral flange 32 for supporting a rolling diaphragm portion 33 of diaphragm 7.

Support plate 31 is integral with a forwardly extending tubular portion 34 co-axial with and spaced radially outwardly from tube 10, and tubular portion 34 is connected by four equiangularly spaced arms 35 to the front of a rearwardly extending tubular body 36 forming part of thrust assembly 27. Arms 35 comprise a first portion 37 which is inclined to the axis of the booster and a second radially extending portion 38, and the arms 35 define between them apertures 39 which are sector shaped when viewed as in Fig. 3 to receive the arms 1 7 of tube 10. The apertures 39 are sized to prevent turning of the diaphragm support plate 31 relative to the valve housing but permit sufficient radial flexing of the arms 1 7 during assembly of the booster.

The movable wall 8 is sealed to the radially outer surface of tube 10 at its unslotted rear end to isolate booster chambers 40 and 41 by a further rolling diaphragm portion 42 of main diaphragm 7, a radially inner peripheral bead 43 of further portion 42 being retained in a radially outwardly facing annular recess 44 in tube 10 by a metal split ring 45, recess 44 being defined between an annular forwardly facing shoulder 46 and a rounded annular projection 47. Rolling diaphragm portion 42 is thus arranged to roll in use between the internal surface of tubular extension 34 and the external surface of tube 10 for support.

The integral connection between the rear end of tube 10 and rear housing wall 23 constitutes a second seal means for the rear booster chamber 41.

Tubular body 36 has a stepped throughbore comprising larger and smaller diameter bore portions 48 and 49 respectively, a rubber reaction disc 50 being located in bore portion 48 between a disc 52 integral with output rod 28 and the front of a stepped cup 53, the smaller diameter portion of which is threadedly secured in bore portion 49. A valve control member 54 comprises a cylindrical body slidable in the bore of cup 53 for engagement with the reaction disc 50 in use.

The valve control member 54, in well known manner, is subjected in use to a fixed proportion of the force being exerted by the disc 50 on the output member 28 to provide a reaction on the input rod 30 which can be felt by the vehicle driver and which is proportional to the force being applied by output member 28.

Valve actuating assembly 24 comprises a piston 55 slidably sealed at its rear in the bore of extension 22 by an annular seal 56 and provided at an intermediate point in its length with an integral cylindrical portion 57 formed with a single helical screw thread 58.

A ball 59 on the front end of input rod 30 is retained in the end of a forwardly extending blind bore 60 of piston 55 by a dimple 61. At its forward end piston 55 is provided with an integral axially extending splined shaft 62 which is axially slidable and keyed against rotation in a splined bore 63 of cup 53, the front of shaft 62 abutting with the rear of valve control member 54. Since cup 53 is fast with tubular body 36 the cylindrical portion 57 is held against rotation.

A rotary valve operating member is constituted by a tubular pinion 64 having external axially extending splines 65 meshed with a rack 66, shown in Fig. 2, formed on the lower portion of a tubular valve member 67.

Pinion 64 is provided at its front end with a radially inwardly directed flange 68 which is rotatably retained on a spigot at the rear end of cup 53 by a circlip 69. Adjacent to its rear end pinion 64 is formed with an internal helical screw thread 70 meshed with thread 58 of piston 55 so that on forward movement of piston 55, which is held against rotation, pinion 64 is rotated anti-clockwise as seen in Fig. 2 to move valve member 67 upwards.

A pre-loaded coiled compression spring 70' acts between cylindrical portion 57 and a washer 71 abutting circlip 62 to bias piston 55 rearwardly relative to pinion 64 to provide a retraction force for valve member 67.

With reference to Fig. 2, valve housing 26 is formed with a stepped bore having three bore portions 72, 73 and 74 of progressively increasing diameters in the upward direction, valve member 67 being slidable in the smallest bore portion 72 and sealed therein by an annular seal 75. A first annular valve seat 76 is formed at the upper end of valve member 67, and a second annular valve seat 77 is formed at the step between bore portions 72 and 73, and both seats are engageable with the annular head 78 of a poppet valve member having an flexible skirt 79 terminating in a bead 80 retained in bore portion 74 by the downwardly directed skirt of a plastics plug 81 welded to housing 26 to effect a seal therewith and provided at its upper end with a vacuum inlet port 82.

Poppet valve head 78 is biassed towards seats 76 and 77 by a coil spring 78 acting between plug 81 and a centrally apertured metal cup 83 supporting skirt 79. Valve member 67 is provided with an axial through-bore 84 and with a transverse bore 85 which provide permanent communication between vacuum inlet port 82 and the interior of extension 22 which is permanently in communication with front chamber 49 by way of apertures 39. A port, not shown, leads directly through rear housing wall 23 from a chamber 86 at the upper end of bore portion 72 to provide permanent communication between chamber 86 and rear booster chamber 41. The simple manner in which such communication can be established is one reason for forming valve housing 26 integrally with rear housing shell 2, the port being formed during moulding of the rear shell 2.A further port, not shown, is directed rearwardly through housing 26 from a bore portion 73 for connection to atmosphere by way of a filter, not shown, that is clamped between housing 26 and the vehicle bulkhead, a suitable aperture being formed in the bulkhead to permit air to enter the filter from the interior of the car, this being preferable to using air from the engine compartment.

Clamping of the booster to a vehicle bulkhead is effected by four equilangularly spaced screw-threaded metal studs 87, the heads of which are moulded into respective cylindrical bosses 88 projecting rearwardly from rear housing wall 23. A respective strengthening rib in the form of a radial vane 89 is integrally connected to each boss 88, to rear housing wall 23 and to extension 22 to provide a path for brake reaction forces transmitted through tube 10 from the central region of the front housing shell 1 to the central region of the rear housing shell 2. Thus axial reaction forces are transmitted in use from the master cylinder housing by way of studs 21, thickened portions 21', tube 20, vanes 89 and studs 87, and the main parts of the shells 1 and 2 may be made relatively thin and therefore light in weight.

The operation of the servo booster of Figs.

1 to 3 will now be briefly described. On initial forward movement of input rod from the retracted position shown in Fig. 1 piston 55 is moved forwards and is held against rotation by the splined engagement between shaft 62 and cap 53. Valve control member 54 is pushed by shaft 62 into engagement with reaction disc 50. The forward movement of piston 55 produces angular movement of pinion 64 through the action of screw threads 58 and 50 to lift valve member 67 and bring valve seat 76 into engagement with valve head 78 to cut off communication between rear chamber 41 and the vacuum inlet 82. On further upward movement of valve member 67 valve head 78 is lifted from seat 77 to provide communication between bore portion 73 and chamber 86 thereby to connect rear chamber 41 to atmosphere.The force applied to the annular movable wall assembly 8 by the pressure differential between chambers 40 and 41 is transmitted to the tubular body 36 of the thrust assembly 27 by the arms 35 and by reaction disc 50 to the output rod 28. The reaction force generated by the reaction disc 28 acting on the valve control member then tends to move valve member 67 downwards to close valve head 78 against seat 77, an equilibrium then being achieved. On increase of pedal pressure on rod 30 valve control member 54 will be forced to the left, and the valve member 67 will be lifted again to admit more air from atmosphere into rear chamber 41 until a new equilibrium position is achieved.

On a reduction of pedal pressure on rod 30 valve control member 54 and piston 55 will be retracted relative to pinion 64 with the effect that the pinion 64 will be moved clockwise in Fig. 2 to disengage valve seat 76 from from valve head 78 to reconnect chamber 41 with the vacuum inlet 82 to enable coil spring 90 to retract the movable wall assembly 8.

The construction of Figs. 1 to 3 will be seen to incorporate a minimum number of seals for isolating the chambers 40 and 41 from each other, seal 56 being the only seal which is independent of the main diaphragm 7.

With conventional servo-boosters the rear end of an axially slidable valve body projects rearwardly through the rear housing wall, and is therefore vulnerable to damage in the droptest to which boosters are now required to be subjected in some countries. The rigid exten sion 22 is much less susceptible to damage in such a test.

Also in such conventional boosters it necessary to protect the external surface of the projecting valve body with a rubber boot since that surface effects a sliding seal with the booster housing. In the above-described construction there is no such vulnerable surface, and a boot is not required.

The second booster shown in Figs. 4 and 5 will now be described. In many respects this booster is similar to that of Figs. 1 to 3, and corresponding reference numerals have been applied to corresponding parts. One important difference is that the tube 10 is not formed integral with either of the housing shells 1 and 2 but is produced as an independent aluminium die casting or pressing. Tube 10 is formed at its rear end with a large radial flange 91 which is provided with stepped holes 92 to receive the heads of studs 87 and which is clamped firmly in engagement with rear housing wall 23 in use by the studs 87 to seal rear booster chamber 41 from the interior of tube 10 which communicates freely with front chamber 40.At the front end of the tube 10 each leg 1 7 is provided with a small radially directed foot 93 which is retained under a corresponding radially inwardly directed projection 94 of a metal retaining ring 95, the projections 94 being spaced apart such that the ring 94 effects a bayonet connection with the projections 93, a key plate 96 being inserted to prevent relative angular movement of the tube 10 and ring 95 after the tube projections 93 have been located angularly in register with and underlying projections 94.

An annular seal 91' seals flange 91 to the rear housing wall 23 and constitutes the second seal means.

Retaining ring 98 supports studs 21, a suitable gasket, not shown, being mounted on studs 21 in face contact with front housing shell 1 to seal the front chamber 40 from atmosphere in use.

The thrust assembly 27 and valve control assembly 24 have been simplified in the construction of Figs. 4 and 5. As compared with the Fig. 1 construction, valve control member 54 has been made integral with piston 55.

Pinion 64 has been replaced by a tubular member 99 provided with two diametrically opposed axially extending vanes 100 the upper one of which is arranged to operate the valve means. Only one vane 100 is strictly necessary, but two are provided to ensure that assembly of the booster with the member 99 turned through 180 about its axis does not affect its operation. At its forward end tubular member 99 is provided with an external screw thread 101 threadedly engaged with an internal thread in bore portion 49 of tubular body 36, and at its rear end member 99 abuts against a step 102 on piston 55, so that on forward movement of piston 55 the tubular member 99 is rotated by the threads 101, tubular body 36 being held against rotation as in the previous embodiment. At its front end member 99 engages a thrust washer 104 retained in position by a circlip 105.

With reference to Fig. 5, the valve means will be seen to comprise two opposed valve housings 106 and 107 extending transversely of the booster axis and slightly displaced above that axis, the housings 106 and 107 both being formed integrally with the rear wall 23 of the rear housing shell 2. Valve mem bers 108 and 109 located in housings 106 and 107 control connections from the rear booster chamber 41 respectively to vacuum inlet port 82 and to an atmosphere port, not shown. The atmosphere port leads into a chamber 110 in housing 107. Housings 106 and 107 are formed with respective valve seats 111 and 11 2 and are closed by plugs 11 3 and 11 4 respectively which are each adapted at 11 5 to have a snap engagement with the respective valve housing.

Chambers 11 6 and 11 7 communicate di rectly with rear booster chamber 41 by way of respective ports, not shown, extending axially of the booster through rear housing wall 23, these ports again being formed during moulding of the rear housing shell 2. A passage 11 8 connects chamber 11 9 permanently to the interior of extension 22 and hence permanently to the front booster chamber 40. Valve actuating rods 1 20 and 1 21 are integrally connected to valve members 108 and 109 respectively and project through opposed bores 1 22 sealed by o-rings 1 23 into the interior of extension 22 for engagement with the upper vane 100.

In the retracted position of input rod 30, the valve members 108 and 109 are in the conditions shown in Fig. 5, the upper vane 100 holding valve member 108 clear of seat 111 against the force of a coil spring 1 24 acting between plug 11 3 and the valve member 108 so that the rear chamber 41 is connected to the vacuum inlet 82 by way of chambers 116 and 11 9. Valve member 109 is held in engagement with its seat 11 2 by a corresponding spring 1 25. On forward movement of input rod 30 and valve control member 54 tubular member 99 is rotated in the clockwise direction, as viewed in Fig. 5, by threads 101 such that upper vane 100 allows valve member 1 98 to engage with seat 111 and then engages with rod 121 to lift valve member 109 from seat 11 2 to connect rear chamber 40 to atmosphere. The lengths of the rods 120 and 121 are chosen such that valve 108 is closed before valve 109 begins to open.

Since the two valve assemblies contained within the housings 106 and 107 are identical and are each of simpler construction than that of the embodiment of Figs. 1 to 3 the overall cost of the valve assembly can be made less despite the fact that two valve assemblies are now employed.

The two boosters of Figs. 1 to 5 may be used with compressed air boosters if desired, minor alterations to the valve assemblies being the only changes necessary.

The booster of Fig. 6 makes use a substantially conventional servo-valve assembly comprising a generally cylindrical valve body 1 26 housing a conventional poppet valve assembly 127. Reaction disc 50 and valve control member 54 are housed in conventional manner is valve body 126.Valve body 1 26 is slidably sealed in conventional manner in an aperture at the rear end of housing shell 2 by seal assembly 1 28. In this booster the housing shells 1 and 2 are formed as metal pressings, tube 10 is a metal pressing having a radially outwardly directed flange 91 at its rear end and a radially inwardly directed flange 1 29 at its front end, the flange 1 29 being welded to a flange 1 30 formed at the rear end of tubular part 1 5 of shell 1. Four slots 1 6 are formed in the front part of tube 10 is in the previous two embodiments, but the slots are not open-ended at their front ends in this case, flange 1 29 being circumferentially continuous.Rear flange 91 is spaced from rear housing wall 23 by a washer 131 on each of the studs 87 to provide permanent air communication between rear chamber 41 and a passage 1 32 in valve body 126 leading to a chamber 1 33 located forwardly of valve seat 134.

In this construction the tubular part 36 of the diaphragm support plate is secured to the valve body 1 26 by a circlip 1 35 which retains an inturned flange 1 36 at the rear end of part 36 against a shoulder 1 37 on the valve body 126.

Valve body 1 26 is axially slidable in tube 10 and is sealed to the internal surface of the unslotted rear part of tube 10 by an annular resilient seal 1 38 located in an external annular recess in valve body 126. The need to provide such a seal is a disadvantage associated with using a conventional valve arrangement as compared with the valve arrangement of the embodiments of Figs. 1 to 5, but the use of a conventional valve assembly has the advantage that parts common with those of other boosters may be used.

The tubular part 36 is provided with four circumferentially equally spaced, axially extending slots 1 39 which are positioned angularly intermediate the slots 1 6 of tube 10 so as to increase the available forward travel of tubular part 26 and thus of the movable wall assembly 8.

As compared with a conventional valve body, body 1 26 is provided with a forwardly extending passage 140 providing permanent air communication between the front housing chamber 40 and a chamber 141 located rearwardly of valve seat 1 34 in body 126. In the retracted condition of the input rod 30 and valve control member 54 poppet valve head 142 is clear of valve seat 1 34 so that chambers 40 and 41 are interconnected by passages 132 and 140, front chamber 40 being permanently connected in use to a vacuum source by way of non-return valve 143. On forward movement of control rod 30 valve head 142 engages with seat 1 34 to isolate chambers 133 and 141, and chamber 1 33 is put in communication with atmosphere by way of a second seat, not shown, of the poppet valve assembly 127, to supply atmospheric air to rear chamber 41.

The booster of Fig. 7 is a modification of the booster of Fig. 6. In the construction of Fig. 7 the rolling diaphragm portion 42 and bead 43 sealing the movable wall assembly 8 of Fig. 6 to the tube 10 have been replaced by an annular resilient sliding seal 144 retained against a radially inwardly directed flange 145 at the front end of tubular portion 34 by a retaining ring 146, the main diaphragm 7 terminating in a bead 147 located in an annular groove 148 defined between the rear of diaphragm support plate 31 and an annular sheet metal member 149 of substantially L-shape in radial cross-seciton welded to plate 31 at 150.

In this construction tube 10 is not slotted at its front end but instead is provided with four circumferentially equally spaced axial slots 1 6 extending rearwardly from a step 141 in the tube to rear tube flange 91, the step 1 51 being located adjacent to the rear of seals 1 44 and 1 38 in the retracted position of the movable wall assembly. The movable wall assembly 8 is connected to the valve housing 1 26 by four radially extending screws 1 52 which pass through respective holes adjacent to the rear end of annular member 149, through respective slots 1 6 and are threadedly engaged with respective radial projections 1 53 slidable longitudinally of slots 16.

Tube 10 is fitted around tubular part 1 5 of front housing shell 1 and is welded thereto, a hole 1 53 providing air communication between front chamber 40 and the interior of tube 10 for communication with passage 140.

Claims (20)

1. A servo booster assembly for a vehicle braking system comprising a booster housing having front and rear housing walls, a pedal operated input member axially aligned with an output member, a movable wall dividing the interior of the housing into two chambers and adapted to apply a force to the output member when the chambers are subjected to a pressure differential in response to a force applied to the input member, a force transmitting means extending between the front and rear housing walls for transmitting in use braking reaction forces which would otherwise be transmitted through the housing, the force transmitting means comprising a tube which is co-axial with the input and output members and extends through the movable wall.

2. A servo booster as claimed in Claim 1 comprising a thrust assembly interposed between the input and output members and means to transmit forward movement of the movable wall to the thrust assembly.

3. A servo booster as claimed in Claim 2 in which the tube is provided with at least one aperture, and the means transmitting forward movement of the movable wall to the thrust assembly comprises an arm extending through the aperture.

4. A servo booster as claimed in Claim 3 in which a first seal means is arranged to seal the movable wall to the radially outer surface of the tube at a location axially displaced in one direction from said aperture, and a second seal means is arranged to seal the tube to the appropriate housing wall at a location further axially displaced from said apertures in the same direction.

5. A servo booster as claimed in Claim 3 or Claim 4 in which the tube is provided with a plurality of circumferentially equally spaced apertures, and a respective arm extends through each aperture.

6. A servo booster as claimed in Claim 5 in which each aperture comprises an axially extending slot, and each arm extends substantially radially though the slot.

7. A servo booster as claimed in any of the preceding claims in which the tube is integral with one of the housing walls.

8. A servo booster as claimed in any of the preceding claims in which the housing comprises two opposed housing shells, and said front and rear housing walls comprise central portions of the respective shells.

9. A servo booster as claimed in Claim 4, or any one of Claims 5 to 8 each as appended to Claim 4, in which the movable wall comprises a diaphragm support plate provided with a tubular portion co-axial with and spaced radially outwardly from the outer surface of said tube, and the first seal means comprises a rolling diaphragm which is arranged to roll between the radially outer surface of the tube and the radially inner surface of the tubular portion.

10. A servo booster as claimed in Claim 9 in which the movable wall comprises a main diaphragm which is sealed to the housing at its radially outer periphery and is integral with the rolling diaphragm.

11. A servo booster as claimed in any of Claims 2 to 6, or any of Claims 7 to 10 each as appended to Claim 2, in which the thrust assembly comprises a tubular body housing a resilient reaction disc.

1 2. A servo booster as claimed in Claim 11 in which the tubular body is integral with the diaphragm support plate and with the arm or arms.

1 3. A servo booster as claimed in any of the preceding claims comprising valve means located in a bore provided in the rear housing wall and controlling the pressure differential of the chambers, a valve operating assembly interposed between the input and output members and comprising a rotatable actuating member for operating the valve means, an axially movable operating member, and a helical engagement between the actuating member and the operating member constructed and arranged such that relative movement between the actuating member and the operating member in an axial direction causes rotation of the actuating member to operate the valve means.

1 4. A servo booster as claimed in Claim 1 3 in which the helical engagement comprises co-operating screw threads formed on the operating member and on the actuating member.

1 5. A servo booster as claimed in Claim 1 3 or Claim 1 4 in which the actuating member is provided with axially extending peripheral teeth meshed with a rack formed on a valve control member.

1 6. A servo booster as claimed in Claim 13 or 14 in which the actuating member is provided with an axially extending radially projecting vane opposite sides of which are adapted to engage with two oppositely directed valve control members controlling respective valve members which control connection of the rear chamber to respective fluid connections.

17. A servo booster assembly for a vehicle braking system substantially as described with reference to Figs. 1 to 3 of the accompanying drawings.

1 8. A servo booster assembly for a vehicle braking system substantially as described with reference to Figs. 4 and 5 of the accompanying drawings.

1 9. A servo booster assembly for a vehicle braking system substantially as described with reference to Fig. 6 of the accompanying drawings.

20. A servo booster assembly for a vehicle braking system substantially as described with reference to Fig. 7 of the accompanying drawings.