GB2121898A

GB2121898A - Vacuum brake booster

Info

Publication number: GB2121898A
Application number: GB08311580A
Authority: GB
Inventors: Kaoru Tsubouchi
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1982-04-30
Filing date: 1983-04-28
Publication date: 1984-01-04
Also published as: GB2121898B; GB8311580D0

Abstract

A vacuum brake booster for an automobile braking system comprises a power piston 12 carrying a control valve piston member 22 that is axially slidable therein to control a pressure differential across the power piston. The control valve member 22 is keyed to the power piston 12 by a key 26 which is of generally W-shaped configuration. A central leg 26a of the key 26 extends through a radial slot 12d in the power piston to straddle a stem portion of the control valve member 22. The outer legs 26c extend around the periphery of the power piston 12 over an angle of slightly more than 180 DEG , and straddle shoulders of the power piston and housing member so that they overlap with the power piston by an amount e and with the housing by an amount f. Strain on the key 26 is thus taken by the outer legs 26c, rather than by the central leg 26a which limits axial movement of the control valve member 22. Actuation of the control valve member 22 is therefore not affected by distortion of the key 26. <IMAGE>

Description

SPECIFICATION Brake power booster This invention relates to brake power boosters for use with brake master cylinders of motor vehicles.

Conventionally, brake power boosters of this type include a housing, a movable power piston and diaphragm which divide the housing into a vacuum chamber and a control pressure chamber.

The diaphragm inner periphery is secured to the power piston and its outer periphery is secured to the housing. A valve member is located in a recess in the power piston and is axially movable, relative to the power piston, to control the pressure differential between the two chambers. The brake power booster includes a key member for restricting the axial movement of the valve member relative to the power piston. The key member is a W-shaped metal plate and is mounted in a radial recess in the power piston. In the rest condition of the power piston, the power piston contacts the central leg of the W-shaped metal plate, and the outer legs of the W-shaped metal plate contact an inner wall of the housing.

The central leg is thus biased by the force of the returning spring of the piston, and is slightly displaced from the plane of the outer legs.

This results in disadvantages such as an unsteady motion of the key member and a poor reliability of the brake booster. Also it has been proposed to adhere the key member to the power piston with rubber. This method was employed as the most rational in the circumstances, but has the disadvantage that the rubber is weakened by heat and stress, and its supporting strength is reduced.

The invention provides a brake power booster for an automobile braking system, comprising a power piston which is axially movable in a brakeactuating direction in response to a pressure differential thereacross, wherein a control valve member is axially movable in the power piston to control the pressure differential and is keyed to the power piston by a key member having a central portion which extends through a radial slot in the power piston to limit the permitted axial movement of the control valve member in the power piston, and two side portions each of which straddles an external shoulder of the power piston and a shoulder of a housing to take the strain of the power piston against the housing when the power piston is in a rest condition.

Preferably the external shoulder of the power piston is a shoulder defining a forward end of an arcuate slot formed around a part of the power piston. Advantageously the key member is retained on the power piston by a retainer member which passes around the power piston to the side remote from the radial slot in the power piston.

The retainer is preferably resiliently attached to the key member to prevent unsteadiness of the key member. The present invention can eliminate the use of rubber for combining the key member and the power piston, which costs a lot, therefore the cost of the invention is low. The present invention can also exclude the reduction of the supporting strength of the rubber caused by heat and stress.

Drawings: Figure 1 is an axial section through a brake booster system of the invention; Figure 2 is a sectional view along line Il-lI of Figure 1; and Figure 3 is a perspective view showing the connection between the output rod retainer 33 and detent housings 34.

In Figure 1, a housing 11 of a brake booster 10 includes a front shell section 11 a and a rear shell section 11 b. The housing 11 is divided into a vacuum chamber 14 and a control chamber 15 by a power piston 12 and a diaphragm 13. The power piston 1 2 comprises a centre portion 1 2a which is formed of a moulded resin material, and a disc-like metal plate 1 2b. The outer peripheral bead of the diaphragm 1 3 is interposed air-tightly between the housing 11 and a curved portion 11 b' of the rear shell section 11 b. The inner peripheral bead of the diaphragm 13 is interposed air-tightly between the inner circumference of the plate 12b and the centre portion 1 2a of the power piston 12.The vacuum chamber 14 is connected to an intake manifold of an engine (not shown) from a port 1 7 disposed in the housing 11 through a check valve (not shown).

Thus constructed, subatmospheric pressure is always established in the vacuum chamber 14 while the engine is running. The centre portion 1 2a of the power piston 12 is provided with a cylindrical extension 1 2c which extends through the rear shell section 11 b of the housing 11.

Between the rear shell section 11 b and the cylindrical extension 1 2c there are located a sealing member 19 and a bearing member 1 8.

The centre portion 1 2a of the power piston 12 carries a conventional control valve assembly 20 including a control valve member 22 and a valve closure member 21. A rear end of the valve closure member 21 is slidably received in the cylindrical extension 1 2r via a spring retainer 23 on which a spring 45 is seated. The valve closure member 21 is formed in a shape of tubes and is flexible, with its front face being biased forwardly by a spring 24 located in an internal chamber 47.

The control valve member 22 is integrally combined with the end of a push-rod 25 which is moved in response to movement of a brake pedal (not shown). An annular groove 22b is defined adjacent to a front portion 22a of the control valve member 22. A key 26 passes through a radial groove in the power piston 12 as is described in greater detail hereinbelow, and at its centre engages the annular groove 22b. This engagement restricts the axial movement of the control valve member 22 within the power piston 12.

A valve seat 27 is formed at the inner circumference of the power piston 12, for cooperation with the valve closure member 21.

Figure 1 shows the condition wherein a space is left between the front face of the valve closure member 21 and the seat 27 so that a chamber 28 in the power piston 12 is connected to ports 29 and 30. The port 30 is always connected to an annular chamber 31 outwardly of the power piston 12, so that in this rest condition the same subatmospheric pressure is established in both the vacuum chamber 14 and the control pressure chamber 1 5.

A spring retainer 33 supporting one end of à return spring 32 is attached to the front face of the centre portion 1 2a of the power piston 12. The retainer 33 is fastened by fitting three foot portions 33a of the retainer 33 beneath three detent housings 34 welded around the circumference of the metal plate 12b. It will be apparent from Figure 3 how this is achieved with a press-and-turn movement. Advantageously the overhanging surfaces 34a of the detent housings 34 are pressed around an upstanding profile 33b of each foot portion 33a after assembly to secure the retainer 33 against reverse rotational movement. Alternatively a cooperating profile could be formed in the overhanging surfaces 34a of the detent housings 34 to resist reverse rotational movement.

The forward end of the return spring 32 is supported by a spring retainer plate 35 which is welded to the inner surface of the front shell section 11 a, so that the spring 32 always pushes the piston 12 in the axial direction. An output member 36 is positioned along the axis of the spring 32, and comprises a rod 37 which passes through an air seal through the front shell section 11 a of the housing 11, and a cylindrical body 38 which is integrally combined with the back end of the rod 37. The body 38 is slidably inserted into a central opening 39 formed in the centre portion 1 2a of the piston 12. The central opening 39 communicates with a smaller hole 40 which receives the front portion 22a of the control valve member 22. In the central opening 39 between the body 38 and the smaller hole 40 is a resilient disk 41 of rubber.

The cylindrical extension 1 2c of the power piston 12 is covered with an elastic boot 42, a front end of which is secured around a back end of the rear shell section 11 b, and the back end of which is secured to an outer circumference of the push rod 25. The boot 42 may be extended or contracted according to the moving of the push rod 25 or the power piston 12 in the axial direction. Two kinds of air filters 43 and 44 are fixed over a back end opening of the cylindrical extension 12c. A spring retainer 46 which supports the spring 45, is placed around the push rod 25 in front of a front end surface of the forward filter 43. The spring retainer 46 is engaged by a shoulder of the push rod 25 on forward movement of the push rod, so that the push rod 25 is given a return bias by the spring 45 through the retainer 46.

Attachment bolts 48 and 49 are disposed on the front and rear shell sections 11 a, 11 L; of the housing 11 respectively, in order to mount a master cylinder on the brake booster and the brake booster on the-vehicle.

Referring next to Figure 2, there is shown a detail of the key 26 and the power piston 12. It will be seen that the key 26 is formed in the general shape of a letterW, and comprises a central portion 26a and two side portions 26c. A U-shaped recess 26b is formed at the distal end of the central portion 26a. Three radial projections 26d are formed around the outer periphery of the key 26. Also each side portion 26c terminates in a distal portion 26e, with aligned transverse bores 26f of very small diameter being formed in the two distal portions. The bores 26f receive the inturned ends 50a of a wire key retainer 50.A central portion 50b of the retainer 50 resiliently engages a top surface of the power piston 12 and is prevented from moving axially along the power piston for more than a small permitted axial extent by an upward projection 121 formed on the power piston 12.

It will be appreciated that other shapes of retainer 50 are possible, although it is advantageous that there is resilient engagement between the retainer and the top of the power piston 12 (that is, the portion diametrically opposite to the radial slot 1 2d receiving the central portion 26a of the key 26). It will be appreciated that the key 26 may be in any angular orientation and not merely that shown in Figure 2.

Considering the otherwise circular section of the power piston 12, it will be seen that in the plane of the key 26 it is formed with an arcuate exterior groove 122 which extends over a major arc and communicates with the radial slot 12d. The groove 122, shown in Figure 2 as being of depth e, receives internal edges of the side portions 26c of the key 26. Thus when the power piston is in its rest position shown, the outer edges of the side portions 26c of the key rest on the inner wall of the rear housing section 11 b and the inner edges of the side portions 26c resist axial movement of the power piston 12 by engagement with the shoulder of the power piston defining a forward face of the groove 122. The arc 111 in Figure 2 designates the limit of the contact between the key 26 and the inner wall of rear shell section 1 16.

and fdesignates the width of that contact area.

Operation of a brake power booster generally as shown in Figure 1 is well understood by those skilled in the art. On brake release, the spring 45 urges the push rod 25 to the right, bringing the front portion 22a of the control valve member 22 into engagement with the central portion 26a of the key 26. The control valve member 22 and key 26 are thus moved together to the right until the space (shown in Figure 1 between the key 26 and the right hand extremity of the slot 12bJ is closed to zero. Thus there is an immediate initial backward movement of the control valve member 22, which is sufficient to close the valve seat portion of the control valve member against the valve closure member 21 to terminate the supply of air at atmospheric pressure to the control pressure chamber 1 5.

The valve closure member 21 is then lifted away from the valve seat 27 to reestablish the vacuum in the control pressure chamber 1 5.

During the whole of the return movement of the power piston 1 2 the separation between the valve closure member 21 and the seat 27 is maintained at its maximum valve of (a + b), which establishes the most rapid release of braking feasible with such a power booster.

When the periphery of the key 26 comes into contact with the housing rear section 11 b, the movement of the key to the left is arrested but the power piston 12 and control valve member 22 continue to move for the distance b. The separation of the valve seat 27 and valve closure member 21 is thus again reduced to a, which is a condition for optimum response time on recommencement of braking.

The improvement which is provided by the present invention is that when the key 26 is arrested in its rightward movement by the housing section 1 16, the power piston 12 bears on the inner peripheries of the outer legs 26c of the key rather than exclusively on the central leg 26a. The zone of engagement is the recess 122 in the power piston 12, and there is little if any distortion or flexure of the central leg 26a of the key.

Claims

1. A brake power booster for automobile braking system, comprising a power piston which is axially movable in a brake-actuating direction in response to a pressure differential thereacross, wherein a control valve member is axially movable in the power piston to control the pressure differential and is keyed to the power piston by a key member having a central portion which extends through a radial slot in the power piston to limit the permitted axial movement of the control valve member in the power piston, and two side portions each of which straddles an external shoulder of the power piston and a shoulder of a housing to take the strain of the power piston against the housing when the power piston is in a rest condition.

2. A brake power booster according to claim 1, wherein the external shoulder of the power piston is a shoulder defining a forward end of an arcuate slot formed around a part of the power piston.

3. A brake power booster according to claim 1 or claim 2, wherein the key member is retained on the power piston by a retainer member which passes around the power piston to the side remote from the radial slot in the power piston.

4. A brake power booster according to claim 3, wherein the retainer member resiliently engages the power piston at the side thereof remote from the radial slot.

5. A brake power booster for an automobile braking system, substantially as described herein with reference to the drawings.