GB2108610A - Servo boosters for vehicle braking systems - 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 to the rear of the housing (1, 2). 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. The thrust assembly incorporates a reaction disc (50) housed in a tubular body (36) which is formed integrally with arms (35) and diaphragm support plate (31). The booster control valve assembly employs a helical cam (58, 70) to operate a valve member (78) housed in a transverse bore (72,73, 74) in the rear housing shell (2). <IMAGE>

Description

SPECIFICATION Servo boosters 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 Specification G.B. No. 2 009 871 are 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 arid a vehicle bulkhead, the force transmitting means is arranged to convey braking reaction forces from 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 wail 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 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 Specification G.B. No. 2 009 871 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, a tubular 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, axially aligned input and output members, a movable wall dividing the interior of the housing into two chambers, the movable wall comprising a diaphragm and a diaphragm support plate, a control valve assembly operated by the input member and arranged to control the pressure difference between the chambers, a force transmitting tube arranged co-axially of the input and output members and connecting the front and rear housing walls, a thrust assembly interposed between the input and output members and comprising a resilient reaction disc, the tube being formad with a plurality of slots which extend axially from one end thereof, and a plurality of arms which integrally connect the diaphragm support plate with a tubular body in which the reaction disc is housed, each arm extending through a respective one of the slots.

The control valve assembly 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.

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 arrangement in accordance with the present invention the control valve assembly comprises valve means located in a bore provided in the rear housing wall, 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.

Preferably the helical engagement comprises co-operating screw threads formed on the operating member and on the actuating member.

In one preferred embodiment the actuating member is provided with axially extending peripheral teeth meshed with a rack formed on a valve control member.

In another preferred embodiment 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.

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 Figure 1; Figure 3 is a cross-sectional view on the line 3-3 of Figure 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; and Figure 5 is a cross-sectional view on the line 5-5 of Figure 4.

Figures 1 to 3 of these drawings also appear in Application No. 8223904 (Agents ref: R712).

With reference to Figures 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 on 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 12 engaging with a forwardly facing annular face 14 on a radially inwardly directed annular lip 13 of front housing shell 1, the lip 1 3 being located at the rear end of a rearwardly extending tubular part 15 of front shell 1. The tube 10 is provided, as shown in Figure 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 10. Extension 22 houses a valve actuating assembly 24 for a valve means 25, shown in Figure 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 frustoconical 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 equi-angularly 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 Figure 3 to receive the arms 1 7 of the tube 1 0. 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 through-bore 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 cut 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 porportion 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 Figure 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 Figure 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 Figure 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 a 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 throughbore 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 equiangularly spaced screwthreaded 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 ways 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 Figures 1 to 3 will now be briefly described. On initial forward movement of input rod from the retracted position shown in Figure 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 and 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 the 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 control member 54 rearwardly to retract piston 55 rearwardly relative to the pinion 64 and thereby 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 Figure 2 to disengage valve seat 76 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 Figures 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 drop-test to which boosters are now required to be subjected in some countries. The rigid extension 22 is much less susceptible to damage in such a test.

Also in such conventional boosters it is 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 Figures 4 and 5 will now be described. In many respects this booster is similar to that of Figures 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 95 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 Figures 4 and 5. As compared with the Figure 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 1 80O 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. As its front end member 99 engages a thrust washer 104 retained in position by a circlip 105.

With reference to Figure 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 wail 23 of the rear housing shell 2. Valve members 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 112 and are closed by plugs 113 and 114 respectively which are each adapted at 1 15 to have a snap engagement with the respective valve housing.

Chambers 11 6 and 11 7 communicate directly 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 119 permanently to the interior of extension 22 and hence permanently to the front booster chamber 40. Valve actuating rods 1 20 and 121 are integrally connected to valve members 108 and 109 respectively and and project through opposed bores 1 22 sealed by o-rings 123 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 Figure 5, the upper vane 100 holding valve member 108 clear of seat 111 against the force of a coil spring 124 acting between plug 113 and the valve member 108 so that the rear chamber 41 is connected to the vacuum inlet 82 by way of chambers 116 and 119. Valve member 109 is held in engagement with its seat 112 by a corresponding spring 125. On forward movement of input rod 30 and valve control member 54 tubular member 99 is rotated in the clockwise direction, as viewed in Figure 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 112 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 Figures 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 Figures 1 to 5 may be used with compressed air if desired, minor alterations to the valve assemblies being the only changes necessary.

Claims (7)

1. A servo booster assembly for a vehicle braking system comprising a booster housing having front and rear housing walls, axially aligned input and output members, a movable wall dividing the interior of the housing into two chambers, the movable wall comprising a diaphragm and a diaphragm support plate, a control valve assembly operated by the input member and arranged to control the pressure difference between the chambers, a force transmitting tube arranged co-axially of the input and output members and connecting the front and rear housing walls, a thrust assembly interposed between the input and output members and comprising a resilient reaction disc, the tube being formed with a plurality of slots which extend axially from one end thereof, and a plurality of arms which integrally connect the diaphragm support plate with a tubular body in which the reaction disc is housed, each arm extending through a respective one of the slots.

2. A servo booster as claimed in claim 1 in which the control valve assembly comprises valve means located in a bore provided in the rear housing wall, 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.

3. A servo booster as claimed in claim 2 in which the helical engagement comprises cooperating screw threads formed on the operating member and on the actuating member.

4. A servo booster as claimed in claim 2 or claim 3 in which the actuating member is provided with axially extending peripheral teeth meshed with a rack formed on a valve control member.

5. A servo booster as claimed in claim 2 or 3 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.

6. A servo booster assembly as claimed in claim 1 and substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings.

7. A servo booster assembly substantially as described herein with reference to Figures 4 and 5 of the accompanying drawings.