GB2220040A - Vacuum brake servo - Google Patents

Abstract

A direct acting vacuum servo has a differential pressure responsive piston (23) slidably and sealingly located within a casing (20) by means of a tubular piston extension (24), extending through an aperture in the casing end wall. The piston (23) and sealing diaphragm (30) divide the casing (20) into two chambers (33 and 34) interconnected via a first annular valve seat (43) which is in fixed relationship to the piston (23), a plunger (50) being mounted coaxially of the first annular valve seat (43) and defining a second annular valve seat (60) by which chamber (34) is connected to an air inlet. A valve member (65) is mounted coaxially of the valve seats (43 and 60) and is urged axially towards said seats (43 and 60) by spring (66). The annular valve seat (43), plunger (50) and valve member (65) are disposed within an enlarged diameter portion of the tubular extension (24) which is located wholly within the casing (20), said plunger (50) being mounted for movement axially of the piston (23) on an input rod (51) and the plunger (50) and piston (23) acting on an output rod (75) via a reaction disc (86) of elastically deformable material, stop means (57) being provided to limit movement of the plunger (50) away from the piston (23). <IMAGE>

Description

DIRECT ACTING VACUUM SERVO The present invention relates to a direct acting vacuum servo of the type used to operate power assisted brakes.

In a typical direct acting vacuum servo, a piston is slidingly mounted within a casing, the piston being sealed to the wall of the casing by means of a diaphragm to divide the casing into a pair of fluid tight chambers. One chamber provides a reservoir which is connected to vacuum and the other provides a working chamber which may be connected to the reservoir or to atmosphere. The working chamber is interconnected with said reservoir via a first annular valve seat. A plunger is mounted coaxially of the first valve seat and defines a second annular valve seat via which the working chamber may be connected to atmosphere. A valve member is mounted coaxially of the first and second valve seats and is urged axially towards said seats by spr-ing means. The plunger is mounted for movement axially with respect to the piston on an input rod which is controlled by the brake actuation means.An output rod is mounted coaxially of the input rod and acts against one face of a rubber disc, the piston and plunger acting against the other face of the rubber disc. The output rod extends into engagement with the piston of the master cylinder of the brake system.

When the brakes are released, the second seat is disposed axially further from the piston than the first seat The valve member will consequently engage and close the second seat while the first seat will remain open. The working chamber is consequently connected to the reservoir so that there will be no pressure differential across the piston.

Movement of the input rod and plunger will cause the valve member to move axially with the plunger, such movement being accommodated by a clearance between the plunger and rubber disc, until the first seat is closed. Continued movement of the plunger will then open the second seat connecting the working chamber to atmosphere. A pressure differential will then be established across the piston thus producing a force which acts through the rubber disc and on to the output rod.The piston and valve assembly will then move to actuate the master cylinder and apply the brakes. Slhen the vacuum servo reaches equilibrium, movement of the piston relative to the plunger will allow the valve member to close the second valve seat, thereby restricting the pressure differential across the piston, so that the servo effect thereof will be proportional to the load applied by the brake actuator.

In order to achieve maximum efficiency, it is necessary to ensure that the relative axial position of the first and second seats is accurately set with the brakes released, so that movement of the brake actuator necessary to open the second seat is minimised. This requires means to limit rearward movement of both the piston and the plunger by which the first and second seats respectively are defined.

One method of achieving this is disclosed in UK Patent GB 2064690B in which a key engages in a groove in the forward end of the plunger so that it will engage the rear-face of the casing to limit backward movement of' the plunger. This arrangement has the disadvantage that the valve assembly is located in a tubular extension of the piston which is slidingly located through the rear wall of the casing. As a result, the tubular extension is of relatively large cross section and air pressure acting on this cross section must be balanced by a relatively strong return spring acting on the piston. The effective area of the piston exposed to the working chamber is also reduced by the cross section of the tubular extension and consequently the servo effect of a pressure differential across the piston will also be reduced in proportion to this cross section.

This invention provides direct acting vacuum servo with means for limiting rearward movement of the plunger while the cross sectional area of the tubular extension of the piston is minimised thereby permitting the use of a light return spring and maximising the servo effect.

According to one aspect of the present invention a direct acting vacuum servo comprises; a casing, a piston slidingly located within said casing by means of a tubular extension which extends through one end wall of the casing and is slidingly sealed with respect thereto, said piston being sealed with respect to the casing by means of a diaphragm to divide the casing into first and second fluid tight chambers, the first and second chambers being interconnected via a first annular valve seat which is in fixed relationship to the piston, a plunger being mounted coaxially of the first annular valve seat and defining a second annular valve seat by which said second fluid chamber is connected to an inlet, a valve member being mounted coaxially of the first and second valve seats is urged axially towards said seats by spring means, said first annular valve seat, plunger and valve member being disposed within an enlarged diameter portion of the tubular extension which is located wholly within the casing, said plunger being mounted for movement axially of the piston on an input rod and the plunger and piston acting on an output rod via a disc of elastically deformable material, stop means being provided to limit movement of the plunger away from the piston.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a sectional elevation of a direct acting vacuum servo formed in accordance with the present invention and Figure 2 shows a sectional elevation on a plane at 900 to that illustrated in Figure 1 of the valve assembly of the direct acting servo shown in Figure 1.

The direct acting vacuum servo illustrated in Figure 1 has a casing 20 formed from two pressings 21 and 22 which are clamped together in known manner. A piston 23 is located within the casing 20, the piston 23 having a tubular extension 24 which extends through an aperture in the end wall defined by pressing 22 and a flange formation 25 which is secured to the piston in suitable manner, for example by means of bolts as illustrated or by sonic welding. The tubular extension 24 is slidingly sealed through the aperture by means of a seal 26, which is located in a retaining ring 27 by a rubber gasket 28.

An elastomeric diaphragm 30 is located by an inner peripheral bead 31 to the piston 23, the bead 31 being clamped between the piston 23 and flange formation 25 of tubular extension 24. An outer peripheral bead 32 of the diaphragm 30 is clamped between the pressings 21 and 22 to provide a seal therebetween and to divide the casing 20 into two fluid tight chambers 33 and 34. Chamber 33 is connected via an outlet 35 to a source of vacuum, for example the inlet manifold of an internal combustion engine.

An annular valve body 40 is defined by the central portion of the piston 23. The valve body 40 extends from the piston 23 towards the end wall of the casing defined by the pressing 22 and engages in an enlarged diameter portion 41 of the tubular extension 24, said portion 41 being disposed wholly within the casing 20.

The valve body 40 defines an annular valve seat 43 which is spaced radially inside an internal cylindrical surface 67 of the valve body 40 to define an annular recess 42 therebetween. The annular recess 42 is connected to chamber 33 by means of a pair of diametrically opposed passages 45.

A plunger 50 is mounted in sliding relationship within a central bore 44 of the valve body 40. The plunger 50 is mounted on a input rod 51 by means of a ball and socket joint 52, by which it is staked thereto. The input rod 51 extends coaxially of the tubular extension 24 of piston 23 and is connected to the brake actuating pedal. A return spring 53 acts between an internal shoulder 54 on tubular extension 24 and a circlip 55 on input rod 51, to urge the input rod 51 and plunger 50 mounted thereon towards the end wall of casing 20 defined by pressing 22.

The plunger 50 defines a further annular valve seat 60 which is concentric of seat 43 but is spaced radially inwardly thereof, so that an annular gap 61 is provided between the plunger 50 and valve body 40.

A pair of diametrically opposed radially extending slots 56 extend through the valve body 40, intermediate of the passages 45, to interconnect the central bore 44 of the valve body 40 to chamber 34. A key 57 extends through the slots 56 and engages the plunger 50. The key 57 may, for example, be in the form of a bifurcated spring clip which engages in a circumferential groove 58 in the plunger A clearance is provided between the key 57 and slots 56, axially of the plunger 50 in order to provide for limited axial movement between the plunger 50 and piston 23.

A valve member 65 is trapped at its outer periphery between the valve body 40 and portion 41 of tubular extension 24.

The inner periphery 68 of the valve member 65 is slidingly located against surface 67. Spring means 66 acts between a shoulder portion 69 of tubular extension 24 and the inner periphery of valve member 65 to urge it towards the valve seats 43 and 60.

An open pored sponge filter element 70 is provided at the end of the tubular extension 24 of piston 23 and the filter element 70 and extension 24 are covered by a rubber bellows 71 which engages between the input rod 51 and pressing 22.

Apertures are provided in the input rod end of the bellows 71 to permit air to enter the extension 24 through filter element 70.

An output rod 75 extends coaxially of the input rod 51, through an aperture 76 in the end wall of casing 20 defined by pressing 21 and engages the piston of a brake master cylinder of conventional design. The casing 20 is secured to the brake master cylinder in a manner which will seal the aperture 76.

The end of the output rod 75 adjacent piston 23 has a flange formation 78, which defines a central circular recess 85 in which is located a rubber disc 86. The face of the disc 86 remote from the output rod 75 is abutted by a central portion 87 of the valve body 40 which also engages in the circular recess 85. A return spring 89 acts between the end of the casing 20 defined by the pressing 21 and the flange formation 78 to urge the piston 23 and valve body 40 towards the end wall of casing 20 defined by pressing 22.

Initially and without vacuum spring 89 will urge the piston 23 and plunger 50 respectively towards the end wall of casing 20 defined by pressing 22, where the spring load is reacted by key 57. Spring 53 also reacts a load on the key 57 and pushes on the valve body 40 in opposition to spring 89. The fitted load of spring 89 is however greater than that of spring 53 so that the plunger 50 is retracted back until the key 57 engages pressing 22 to limit movement of the plunger 50. In this position seat 43 will be closed by valve member 65 while seat 60 will be open. The chamber 34 will consequently be connected to atmosphere via slots 56, past the seat G0 and the tubular extension 24.

Upon establishment of vacuum in chamber 33, the pressure differential across piston 23 will balance the load applied by spring 89, less the load of spring 53, and will move piston 23 away from the wall of the casing 20 defined by pressing 22 until seal member 65 engages seat 60. This movement is permitted by the clearance between key 57 and slots 56.

Upon actuation of the brake, movement of the input rod 51 will first move plunger 50 to reopen the valve seat 60, connecting chamber 34 to atmosphere. A pressure differential is thus established across the piston 23. The forces applied directly by the input rod 51 and that generated by the pressure differential across the piston 23 are then transmitted to the output rod 75 via the rubber disc 86. The rubber disc 86 is totally enclosed and the forces transmitted thereby from the input rod 51 and piston 23 will be in proportion to the areas of the rubber disc 86 engaged by the plunger 50 and portion 87 of valve member 40 respectively. At equilibrium, the piston 23 will move forward relative to the -plunger 50, so that the seat 60 will again be closed by valve member 65.

Upon release of the brakes, input rod 51 will move back moving plunger 50 towards the end wall defined by pressing 22. Engagement of the valve member 65 by the seat 60 will also cause the valve member 65 to move, opening seat 43 so that the pressure differential across the piston 23 can be reduced. The return spring 89 will then move the piston 23 back towards the end wall defined by pressing 22 until key 57 again engages the pressing 22. The output rod 75 and the piston of the master cylinder will move with piston 23 to release the brakes.

Claims (8)

1. ' A direct acting vacuum servo comprising a casing, a piston slidingly located within said casing by means of a tubular extension which extends through one end wall of the casing and is slidingly sealed with respect thereto, said piston being sealed with respect to the casing by means of a diaphragm to divide the casing into first and second fluid tight chambers, the first and second chambers being interconnected via a first annular valve seat which is in fixed relationship to the piston, a plunger being mounted coaxially of the first annular valve seat and defining a second annular valve seat by which said second fluid chamber is connected to an inlet, a valve member being mounted coaxially of the first and second valve seats is urged axially towards said seats by spring means, said first annular valve seat, plunger and valve member being disposed within an enlarged diameter portion of the tubular extension which is located wholly within the casing, said plunger being mounted for movement axially of the piston on an input rod and the plunger and piston acting on an output rod via a disc of elastically deformable material, stop means being provided to limit movement of the plunger away from the piston.

2. A direct acting vacuum servo according to Claim 1 in which the first annular valve seat is formed by an annular valve body defined by a central portion of the piston, the first valve seat and a cylindrical surface of the valve body defining an annular recess around the outside of the first valve seat, said recess being connected to one chamber by an axially extending passageway, a diametral slot being provided through the valve body to connect a central bore thereof to said other chamber, said plunger being slidingly located in the central bore.

3. A direct acting vacuum servo according to Claim 2 in which the valve member is mounted with its outer periphery trapped between-the valve body and the enlarged diameter portion of the tubular extension to form a seal therebetween, the inner periphery of the valve member being resiliently loaded towards the first and second valve seats.

4. A direct acting vacuum servo according to Claim 3 in which an annular formation on the inner periphery of the valve member engages the cylindrical surface of the valve body and slidingly guides the inner periphery of the valve member for movement axially of the valve body.

5. A direct acting vacuum servo according to any one of Claims 2 to 4 in which a key is located through the diametral slot in the valve body and engages the plunger, so that the key will engage a formation on the end wall of the casing to limit movement of the plunger, a clearance being provided between the key and the diametral slot, axially of the plunger, to permit limited relative movement between the plunger and piston.

6. A direct acting vacuum servo according to Claim 5 in which the key is in the form of a bifurcated spring clip which engages in a circumferential groove in the plunger.

7. A direct acting vacuum servo according to any one of the preceding claims in which the end of the output rod adjacent the piston is provided with a flange formation which defines a circular recess for location of the disc of elastically deformable material, a cylindrical portion of the piston engaging in said circular recess to abut the disc.

8. A direct acting servo substantially as described herein with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.