GB2093136A

GB2093136A - Servo boosters for vehicle braking systems

Info

Publication number: GB2093136A
Application number: GB8203296A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1981-02-12
Filing date: 1982-02-05
Publication date: 1982-08-25
Also published as: AR227086A1; BR8200742A; FR2499488B1; ES8301786A1; GB2093136B; ES509527A0; DE3204549A1; FR2499488A1

Abstract

In a servo booster of the kind in which a stationary force transmitting member 13 extends through the movable wall, which comprises a diaphragm 9 and a support plate 6, and the diaphragm is sealed to the member 13 by a flexible boot located about the member 13 with one end portion 21 sealed to the diaphragm 9, the support plate is deliberately made capable of parting from the diaphragm 9. This has been found to reduce substantially the risk of air becoming trapped between the plate 6 and diaphragm 9 during bleeding of the brakes, when a pressure differential is created between the front and rear chambers of the booster. Forward displacement of the plate 6 relative to the diaphragm 9 and boot end portion 21 is preferably limited by an abutment 25' on the boot. In a preferred arrangement excessive pressure build-up is prevented by venting between a surface 12' carried with the plate and a sealing surface, e.g. of a lip 47, carried with the diaphragm. <IMAGE>

Description

SPECIFICATION Servo boosters for vehicle braking systems This invention relates to servo boosters for vehicle braking systems of the kind comprising a booster housing, a movable wall dividing the interior of the housing into front and rear chambers and comprising a diaphragm support plate and a diaphragm sealingly connected to the booster housing, the support plate having an operative connection with an output member, a stationary force transmitting member extending through an aperture in the diaphragm support plate, and a flexible sealing boot which extends substantially coaxially of the force transmitting member, one end portion of the boot being sealingly connected to the force transmitting member, and the other end portion extending through the aperture in the support plate and being sealingly connected to the diaphragm.

Several examples of boosters of this kind are disclosed in U.K. Patent Specification No.

2009871 A.

The invention is concerned with a problem which arises with many vacuum servo boosters, that is, boosters in which the chamber in front of the movable wall (the front end of the booster being taken as that end through which the output member is operative) is connected to a vacuum source, and a control valve of the booster admits -atmospheric air into the rear booster chamber to generate a boost force on the output member.

When a braking system provided with a vacuum servo booster is being bled the diaphragm support plate is moved forwards and backwards in response to pumping of the brake pedal. Since the front chamber of the booster and its vacuum supply conduit are sealed, and the diaphragm is carried forwards with the support plate, pressure builds up in the front chamber during forward movement of the support plate. It has been known for this pressure build-up to cause the diaphragm to part from its support plate with the result that air can become trapped between the support plate and the diaphragm and cause damage to the diaphragm. Similar problems can arise during normal application of the brakes for certain failure conditions of the vacuum source.

In a booster of the kind to which this invention relates, an additional problem may arise in that the flexible boot, such as a bellows or rolling diaphragm, may become permanently unseated from the support plate, to which it is usually attached.

According to the invention, a booster of the kind referred to in the opening paragraph above is distinguished by the fact that the movable wall is so designed as to permit forward displacement of the diaphragm support plate relative to the diaphragm and the end portion of the boot to which the diaphragm is connected.

It has been found that by deliberately arranging for the support plate to part from the diaphragm during the bleeding operation the chance of air becoming trapped between the plate and the diaphragm can be significantly reduced. On the other hand, such an arrangement has no discernible effect upon the normal operation of the booster. Furthermore the change in volume of the front chamber is reduced over that which would have occurred if the diaphragm had been firmly secured to the diaphragm support plate, so the pressure differential between the front and rear chambers is correspondingly reduced.

There is preferably an abutment which limits the forward displacement of the support plate to a predetermined amount so that, after the support plate has parted from the diaphragm, the diaphragm and the boot end portion are carried forwards with the support plate. Preferably the abutment is carried on the end portion of the boot to which the diaphragm is connected. Thus, the boot may be arranged to exert a rearward force on the diaphragm to assist in its separation from the support plate, but when the plate has become displaced by the predetermined amount it contacts the abutment and, on further forward movement, carries the end portion of the boot with it so that the boot end portion now serves to urge the diaphragm forwards.

The diaphragm and the boot may be integrally formed, but it will normally be easier to manufacture them as separate items.

The end portion of the boot is preferably provided with a forwardly facing annular shoulder which abuts the rear face of the diaphragm to prevent the diaphragm from becoming disengaged from the boot end portion in response to a positive pressure differential between the front and rear booster chambers. The shoulder is conveniently provided by an annular radially outwardly (with respect to the boot axis) directed rib.

Preferably the annular shoulder forms one wall of an outer circumferential recess in the boot end portion in which is sealingly seated an annular bead of the diaphragm.

An annular portion of the front wall of the diaphragm is preferably exposed through the aperture in the support plate so that a pressure build-up in the front chamber acts on this exposed portion of the diaphragm to initiate the separation of the diaphragm from the support plate. This helps the diaphragm to peel away smoothly in a controlled manner.

Preferably the radially outer part of the front face of the diaphragm rib contacts the rear face of the diaphragm support plate around the aperture, when the support plate is in its normal rearward position relative to the boot end portion, so that normally the diaphragm rib is retained between the support plate and the forwardly facing annular shoulder.

The boot is preferably of the concertina bellows type but it may be of any convenient form.

According to a preferred feature, a valve body carried by the diaphragm support plate presents a radially outwardly facing surface with which cooperates a radially inwardly facing sealing surface carried by the diaphragm, the arrangement being such that forward displacement of the support plate relative to the diaphragm and boot end portion allows venting of excess pressure from the front chamber to the rear chamber between the two surfaces.

The sealing surface of the valve body may be a surface which is parallel to the axis of movement of the valve body, e.g. a plain cylindrical surface, but preferably the outwardly facing surface of the valve body tapers rearwardly so that the relative rearward displacement of the sealing surface carried by the diaphragm lessens the sealing contact between the two surfaces.

Preferably the sealing surface carried by the diaphragm is provided by an annular sealing lip which would normally extend radially inwards but which is resiliently deflected rearwardly by contact with the outwardly facing surface of the valve body. The sealing lip allows venting of pressure from the front to the rear chamber during brake bleeding, yet provides a sealing force which is enhanced by a positive pressure differential between the rear and front chambers during normal booster operation.

Preferably there is an annular bead carried by the diaphragm rearwardly of the sealing lip and the bead is co-operable with the outwardly facing surface of the valve body to guide the lip over the valve body and so relieve the lip of lateral forces which could otherwise adversely affect its designed sealing characteristics.

Several vacuum servo boosters in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a longitudinal cross-sectional partial view of a first booster, with the diaphragm plate being shown in its fuily retracted position; Figure 2 is a similar view of a second booster; Figure 3 is a portion of Figure 2 showing a modification; and Figure 4 is a view similar to Figure 3 but showing the valve body and diaphragm support plate in an advanced position.

With reference to Figure 1, the booster comprises front and rear metal housing shells 1 and 2 respectively, the rear shell 2 being provided with a rearwardly directed tubular extension 3 which slidably guides a moulded plastics servo valve body 4. A movable wall 5 comprises a dished metal diaphragm support plate 6 which is rigidly secured at its radially inner margin to the valve body 4 by indentations 7 locked in an annular recess 8 of the valve body 4, and a flexible diaphragm 9. The support plate 6 is operatively connected to an output member (not shown) via the valve body 4. The diaphragm 9 is sealed to the housing at its radially outer periphery by a first peripheral bead 10 clamped between the shells 1 and 2, and the radially inner margin of the diaphragm 9 is sealed to the valve body 4 by a second peripheral bead 11 seated in a further annular recess 12 of the valve body 4.The diaphragm normally lies in contact with the rear face of support plate 6, as shown.

A pair of force transmitting members in the form of diametrically opposed tie rods 1 3 extend axially through the housing shells 1 and 2 and through respective circular apertures 14 in the support plate 6. The front and rear ends 1 5, 1 6 of the tie rods are threaded to receive locking retainers, and in use the front ends 1 5 would normally be secured to a master cylinder housing, and the rear ends 1 6 to a vehicle bulkhead.

Braking reaction forces from the master cylinder are transmitted through the tie rods 13 to the bulkhead, substantially to relieve the housing shells 1,2 of such reaction forces.

The movable wall 5 is sealed to the tie rods by identical respective resilient concertina boots 1 7.

The front end 1 8 of each boot abuts an adjuster ring 1 9 and is provided with an inwardly directed skirt 20 to effect a seal with the respective rod 13.

(The purpose of the adjuster ring 1 9 will be described below.) The rear end portion 21 of each boot 17 is of substantially cylindrical shape, but its radially outer surface is provided with three shoulders 23, 24 and 25, the shoulder 23 facing forwardly of the booster, and the shoulders 24 and 25 facing rearwardly. Forwardly facing shoulder 23 is provided by an annular radially outwardly (with respect to the respective tie rod) projecting rib 26 at the extreme rear end of the boot end portion 21. The shoulder 23 abuts the rear face 27 of an annular bead 28 of the diaphragm 9, which is sealingly seated on an outer cylindrical surface 29 of end portion 21. The diaphragm bead 28, as shown, is normally contacted on the radially outer part of its front face 30 by the rear face 31 of the margin of the support plate 6 around aperture 14.

Rearwardly facing shoulder 24 is of lesser radial height than that of shoulder 23 to enable it to be located within the aperture 14 and to enable the plate 6 to move forwards over the end portion 21 until it comes into contact with shoulder 25 which is provided by a radially outwardly projecting rib 32 at the front end of the end portion 21. The shoulder 24 is closely spaced axially from the exposed radially inner part of the front face 30 of the bead 28 to define with the shoulder 23 and face 29 an annular recess in which the bead 28 is located.

During bleeding of the brakes the valve body 4 and support plate 6 are urged forwards and backwards in response to pumping of the brake pedal. Initial forward movement of the diaphragm support plate 6 from the retracted position shown tends to carry the diaphragm 9 and boot end portion 21 with it, but this results in an increase in pressure in front chamber 33 relative to that in rear chamber 34, since the front chamber 33 is connected to an inoperative, and sealed, vacuum source. The increased pressure in chamber 33 acts through the apertures 14 on the exposed part of the front face 30 of the beads 28 urging beads 28 to separate from support plate 6, and on further forward movement of the plate 6 relative to the housing 1,2 the diaphragm beads 28 are left behind as the diaphragm peels away from the plate 6 in a controlled manner.Separation of the diaphragm from the plate is aided by the inherent stiffness of the boots 1 7 which tend to exert a rearward force on the diaphragm by virtue of the engagement between the beads 28 and the end portions 21. Once the plate has separated from the diaphragm the plate moves forwards over the end portion 21 until it contacts the shoulders 25.

Further forward movement of the plate carries the boot end portions 21 forwards, relieving the diaphragm of the rearward force of the boots. In fact the end portions 21 now urge the diaphragm forwards through the contact between the rear surfaces 27 or beads 28 and shoulders 23, which tends to alleviate the risk of the beads 28 being forced out of engagement with the boot end portions 21 by rearward ballooning of the diaphragm. The limited range of travel of the plate 6 relative to the boots provided by the shoulders 25 also serves to prevent the plate from damaging the concertina portion of the boots.

During retraction of the support plate 6, the reverse of the foregoing sequence of events takes place so that the diaphragm is progressively restored to its original position. This progressive control reduces the chance of air being trapped between the diaphragm 9 and the support plate, which might otherwise cause damage to the diaphragm.

Thus, throughout the full range of travel of the support plate 6 the diaphragm beads 28 are retained in sealing engagement with the end portions 21 of the respective boots, and no air is lost from chamber 33 to chamber 34 during the bleeding operation. Such a loss of air could be undesirable in some circumstances since air which is lost to the rear chamber will be replenished with air from the vacuum source, and this air will often be contaminated with petrol fumes. The petrol fumes could then be sucked, during further bleeding and the pumping actions, into the rear chamber where the poppet valve, which controls the inlet of air under atmospheric pressure into the rear chamber, is positioned. The poppet valve may not be resistant to petrol and could therefore be damaged.

A further feature of the booster of Figure 1 is the manner in which provision is made for adjustment of the effective length of the tie rods 13 between the front and rear walls 35, 36 of the shells 1,2 respectively. The rods 13 are each provided with an integral radial flange 37 provided in its rear face with an annular recess 38 into which the rear wall 36 is sealably deformed. A cylindrical adjuster ring 19 is threadedly engaged with the threaded portion 1 5 of each rod 13, and the rings 1 9 are initially adjusted, by screwing, to provide the desired axial spacing between the front face 39 of the rings 19 and the rear face of flanges 37 to ensure that the housing shells 1, 2 are not distorted when the booster is mounted between a master cylinder housing and a vehicle bulkhead.Once the adjuster rings 1 9 have been adjusted they are secured in position and sealed to the rods 13 by a suitable adhesive such as LOCTITE. Each ring 1 9 is provided at its front end with a counterbore 40 which receives a rearwardly extending coaxial flange 41 on the front wall 35, and an O-ring 42 is sandwiched radially between the wall of counterbore 40 and the flange 41 to seal the front ends of tie rods 13 to housing shell 1.

In Figure 2 parts corresponding to those of the booster of Figure 1 have been given corresponding reference numerals. In this embodiment the abutment face 25', instead of being provided on a rib, is provided on a stepped portion 32' of the boot wall. The boot 1 7 is provided with radially inwardly directed reinforcing and guide ribs 43.

Instead of the annular bead 11 the inner margin of the diaphragm 6 is provided with a bead 11', of wedge shape in transverse cross-section, in sealing contact with a complementarily inclined frusto-conical face 12' on valve body 4. On forward displacement of the support plate 6 relative to boot end portion 21 during bleeding, the bead 28 urges the bead 11' rearwardly relative to valve body 4 by means of a relatively inflexible connecting web 44. This reduces the sealing force between bead 11' and surface 12', and allows some venting of chamber 33 to chamber 34 when the pressure difference builds up sufficiently, in order to prevent excessive ballooning of the diaphragm 9 and possible unseating of the bead 28 from the boot end portion 21.On retraction of the valve body, the cooperating tapers of the bead 11' and surface 12' help to centralise the bead 11'. The rear face 45 of bead 11' abuts the rear shell 2 to determine the retracted position of the movable wall.

In the embodiment just described the annular bead 11' is of relatively large dimensions in comparison with the thickness of the main diaphragm, in part in order to provide a backstop for the movable wall 5, which may make it difficult to obtain a defined sealing characteristic without close attention to tolerances. The modification shown in Figures 3 and 4 provides a marked improvement in this respect.

In Figures 3 and 4, parts corresponding to those of Figures 1 and/or 2 have the same reference numerals. In this embodiment the bead 11' is replaced by a somewhat shorter wedge-shaped bead 11" connected to the bead 28 by a relatively inflexible connecting web 44'. A radially inwardly directed sealing lip 47 depends from the web 44", adjacent to the bead 11". Lip 47 is of substantially smaller cross-sectional area than bead 11". Since the web 44' is connected to the radially outer part of bead 11", an internal annular recess 48 is formed between the sealing surface 12', bead 11" and web 44', in which the lip 47 is located.

Lip 47 in its undeformed shape would extend substantially in a radial plane but, as shown, it is deflected rearwardly by contact with the frustoconical outer surface 12' of valve body 4. When the movable wall 5 is in the retracted position the inner surface of the bead 11" engages the surface 12' and so centralises the lip 47.

In normal operation of the booster the free inner margin of the lip 47 effects a seal with the surface 12' and that seal is enhanced by the application of a positive pressure differential between booster chambers 34 and 33 since the lip is pressed more firmly against surface 12'.

Figure 4 shows the result of advancing the valve body 4 by operation of the brake pedal (not shown) during bleeding of the brakes, in which condition the front chamber 33 is connected to the sealed vacuum supply. The pressure increase in chamber 33 resulting from forward movement of the movable wall 5 acts to separate the diaphragm 9 from the support plate 6 as described above, and plate 6 comes into engagement with steps 25' on the respective boots 1 7. The rearward displacement of diaphragm 9 produces a corresponding rearward displacement of bead 11" and lip 47 relative to valve body 4 and, since surface 12' is frusto-conical and has its smaller radial dimension directed towards the rear of the valve body 4, this results in a reduced radial deflection of lip 47 and thereby a reduced sealing force between the free margin of the lip and sealing surface 12'. If sufficient pressure builds up in chamber 33 relative to atmospheric pressure in chamber 34 the lip will be forced away from surface 12' to vent the excess pressure to the rear chamber 34.

In a further modification (not shown) the sealing surface 12' extends parallel to the axis of movement of the valve body, or is only slightly inclined with respect to that axis, and the radially inner face of the bead 11" is provided with a series of axially extending grooves to prevent a seal being produced between bead 11" and surface 12'.

Claims

1. A servo booster for a vehicle braking system, comprising a booster housing, a movable wall dividing the interior of the housing into front and rear chambers and comprising a diaphragm support plate and a diaphragm sealingly connected to the booster housing, the support plate having an operative connection with an output member, a stationary force transmitting member extending through an aperture in the diaphragm support plate, and a flexible sealing boot which extends substantially co-axially of the force transmitting member, one end portion of the boot being sealingly connected to the force transmitting member, and the other end portion extending through the aperture in the support plate and being sealingly connected to the diaphragm, distinguished by the fact that the movable wall is so designed as to permit forward displacement of the diaphragm support plate relative to the diaphragm and the end portion of the boot to which the diaphragm is connected.

2. A servo booster according to Claim 1, including an abutment which limits the forward displacement of the support plate, relative to the diaphragm and boot end portion, to a predetermined amount.

3. A servo booster according to Claim 2, in which the abutment is carried on the end portion of the boot to which the diaphragm is connected.

4. A servo booster according to any preceding claim, in which the end portion of the boot is provided with a forwardly facing annular shoulder which abuts the rear face of the diaphragm.

5. A servo booster according to Claim 4, in which the annular shoulder forms one wall of an outer circumferential recess in the boot end portion in which is sealingly seated an annular bead of the diaphragm.

6. A servo booster according to any preceding claim, in which an annular portion of the front wall of the diaphragm is exposed through the aperture in the support plate.

7. A servo booster according to any preceding claim, in which a valve body carried by the diaphragm support plate presents a radially outwardly facing surface with which co-operates a radially inwardly facing sealing surface carried by the diaphragm, the arrangement being such that forward displacement of the support plate relative to the diaphragm and boot end portion allows venting of excess pressure from the front chamber to the rear chamber between the two surfaces.

8. A servo booster according to Claim 7, in which the outwardly facing surface of the valve body tapers rearwardly.

9. A servo booster according to Claim 7 or 8, in which the sealing surface carried by the diaphragm is provided by an annular sealing lip which would normally extend radially inwards but which is resiliently deflected rearwardly by contact with the outwardly facing surface of the valve body.

10. A servo booster according to Claim 9, in which there is an annular bead carried by the diaphragm rearwardly of the sealing lip and the bead is co-operable with the outwardly facing surface of the valve body.

11. A servo booster which is substantially as described with reference to Figure 1 of the drawings.

12. A servo booster which is substantially as described with reference to Figure 2 of the drawings.

13. A servo booster according to Claim 12, modified substantially as described with reference to Figures 3 and 4 of the drawings.