DIRECT-ACTING VACUUM SERVO

Abstract: A direct-acting vacuum servo incorporates a casing formed by front and rear pressings (11 and 12) and a body (15) forming a movable wall in the casing sealed by a diaphragm (13). A front chamber (24) is connected to a vacuum source. A valve assembly constitutied by valve body (15), valve member (19) and valve seat (21) controls the pressure in a rear chamber (25) by sealing it off (as shown) or connecting it to atmosphere or to the front chamber (24). A reaction member (34) relates relative movement between valve member and valve body to an input load and servo load applied to the valve member and valve body respec­ tively. Axial transfer passages (23) and inlet passages (26) communicate with transfer ports (29) and inlet ports (28) which are arranged to occupy alternating discrete sectors within an annular area covered by the valve seat (21).

Description: Direct-Acting Vacuum Servo This invention relates to direct-acting vacuum servos. A known vacuum servo has a valve body moulded in a plastics material and has one set of integrally moulded passages to provide connections between a reservoir or front chamber (which is connected to vacuum) and a variable pressure or rear chamber and another set of passages between the rear chamber and atmosphere. The known servo has a valve mechanism in which concentric annular ports are selectively opened and closed by an annular valve seat. The valve body of the known servo can be difficult and expensive to manufacture since radial and angled core dies are usually required to form the passages. An object of the present invention is to provide a vacuum servo with a new valve arrangement which enables components of the servo to be manufactured in a simpler and less expensive way. According to the present invention there is provided a direct-acting vacuum servo comprising a casing, a movable wall in the casing, a reservoir or front chamber in the casing on one side of the movable wall for permanent connection to a vacuum source, a variable pressure or rear chamber in the casing on the other side of the movable wall and a valve mechanism which controls the admission of air between atmosphere and the rear chamber and between the rear chamber and the front chamber, the valve mechanism comprising a valve body which is connected to or forms part of the movable wall and incorporates at least one transfer passage for inter-connecting the front chamber and the rear chamber, a valve member which is slidable in the valve body, incorporates at least one inlet passage for connecting the rear chamber to atmosphere and can transmit a load from an input rod to a reaction device for transmitting load from the movable wall to an output rod, and a valve seat which is slidable relative to the valve member to selectively block and open the inlet and transfer passage in response to relative movement between the valve member and the valve body, the transfer passage extending through a projection on the valve body which projection itself extends axially through the valve member such that inlet and transfer ports formed at one end of the inlet and transfer passages respectively occupy discrete sectors within an annular area covered by the valve seat. In manufacturing the valve body of the servo according to the invention, the transfer passages can be formed axially using the main dies, as also can the inlet passages in the valve member. One embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a section on line I-I of figure 2 showing a direct-acting vacuum servo according to the invention; and Figure 2 is a cross-section on line II-II of figure 1 showing certain components only Referring lo the drawings, the servo includes a casing formed by a front pressing 11 and a rear pressing 12 which are clamped together using a known crimping technique to grip a diaphragm 13 at its outer peripheral bead 14. The diaphragm 13, a valve body 15, and a diaphragm support plate 16 form a movable wall. An inner peripheral bead 17 of the diaphragm 13 is retained in a groove formed by the valve body 15 and a retainer 18. The valve body 15 and retainer 18 are moulded in thermoplastics and these parts may be assembled as a push fit or secured by adhesive, solvent welding or ultrasonic welding. The valve body 15 forms part of a valve mechanism which also includes a valve member 19 which is slidable in the valve body 15 and valve seat 21 which is slidable in a tubular extension 22 of the valve member 19. The tubular extension 22 is united with the main part of the valve member by webs 20. Transfer passages 23 are moulded into the valve body 15 for connecting a reservoir or front chamber 24 to a variable-pressure or rear chamber 25 and inlet passages 26 are moulded into the valve member 19 for connecting the rear chamber 25 to atmosphere through the tubular extension 22 (see Figure 2). The transfer passages 23 extend through projections 27 on the valve body 15 and these projections themselves extend axially through two of the four inlet passages 26 in the valve member 19. These two inlet passages are almost completely filled by the projections and so, apart from a small leakage area, do not act as inlet passages as such. Inlet and transfer ports 28 and 29 are formed at one end of the inlet and transfer passages 26 and 23 respectively. As can be seen most readily from figure 2, the end portions of the passages 23 and 26, and thus the ports 28 and 29, occupy discrete sectors within an annular area covered by the valve seat 21. The axial orientation of the passages, made possible by this arrangement of the ports, simplifies the tools required to mould the valve member and the valve body and also simplifies the moulding operation. The valve seat is faced with rubber to enhance sealing but, because it is arranged so that it can completely close off the ports 28 and 29, does not require any form of sliding seal or diaphragm to seal against the tubular extension 22. The valve member 19 is connected to an input rod 31 by means of a socket member 32 which is snapfitted onto a ball head 33 before insertion into a bore in the valve body 15. The wall of the socket member 32 is slotted to provide resilience and its open end has an external lip which engages in a groove in the bore of the valve body 15 to retain the socket member in the bore. When installed in the braking system of a motor vehicle, the input rod 31 can transmit a load from a driver's brake pedal to the rubber block 34 a reaction device through a disc 35. The rubber block 34 can also receive a load from the valve body 15 through a washer 36 which engages in a recess in the valve body 15, the sum of the loads from the input rod 31 and the valve body being transmitted by the rubber block 34 to a disc 37 which is formed as a head on an output rod 38. In use, the output rod 38 would apply its load to a hydraulic master cylinder. The servo is shown in figure 1 with the valve mechanism in the lap position, that is with the valve seat 21 blocking both ports 28 and 29. In this position the input rod 31 has advanced the valve member 19 against a compression valve spring 39 which acts between the valve body 15 and the valve member 19. This lap position corresponds to a steady state with an input load on rod 31 and a proportionate pressure differential load from the movable wall, both acting on the output rod 38 through the reaction device. In the no-lod condition the valve spring 39 biasses the valve member 19 into a retracted position to the right of the position shown in which the valve seat 21 is lifted off the ends of tubular extensions 27 and so opens communication between the chambers 24 and 25 through the transfer passages 23 and transfer ports 29 at the ends of the projections 27. Valve seat 21 then rests on the inlet ports 28 under the action of a compression pushrod return spring 41 which bears on the input rod 31, blocking the inlet passages 26 from atmosphere. The front chamber 24, when the servo is installed as described, is connected to a source of vacuum so that in the retracted position both chambers 24 and 25 are at a reduced pressure. A compression diaphragm return spring 42 urges the valve body 15 away from the front pressing 11 such that the diaphragm 13 abuts the rear pressing 12. The diaphragm return spring 42 has to balance the pressure loading caused by the difference in sectional area presented to atmosphere by the output rod 38 and the tubular extension 22 and is of greater preload than the valve spring 39. The pushrod return spring 41 has a lesser preload than the valve spring 39. In the retracted position, the valve member 19 is spaced from the disc 35 which is constrained to abut the rubber block by ribs 43 in the bore of the valve body. The retracted position is set by inter-engaging abutment means comprising a flange 44 on the valve member 19 and pegs 45 formed at the end of fingers 46 which project axially from the valve body 15. The pegs 45 have inclined radially inner faces and the fingers 46 are resilient to allow the pegs 45 to latch over the flange 44 during assembly. After moving forward from the retracted position described into the lap position shown in figure 1, the valve member 19 continues to move towards the output rod. Corresponding movement of the valve seat 21 is prevented by abutment against the transfer ports 29 so the inlet ports 28 are moved away from the valve seat 21 and allow air from atmosphere into the rear chamber 25. During this opening of the inlet ports there is only just perceptible movement of disc 35 to compress the rubber block 34. However, on allowing air into chamber 25, there is a pressure differential across the diaphragm 13 resulting in a servo thrust load applied by valve body 15 through washer 36 to exert a load on the rubber block 34. The discs 35 and 37 and the washer 36 act as piston areas on an effectively hydraulic medium so that the input loads on disc 35 and servo load on washer 36 are apportioned according to the area of disc 35 and the annular area of washer. When the servo load on washer 36 balances the input load on disc 35, the valve body 15 moves forward to the same extent .Is the initial movement of the valve member 19, again closing off both ports 28 and 29 to reestablish the lap position. In this way, a servo load proportional to the inlet load in a ratio determined by the relative areas of washer 36 and disc 35 is applied to the output rod 38 in addition to the direct application of input load to the output rod. Since disc 35 and washer 36 are small components, their relative areas and thicknesses can be selected to suit a particular vehicle installation without changing the tooling required for the valve body 15 or valve member 19.

Claims: CLAIMS 1. A direct-acting vacuum servo comprising a casing (11, 12), a movable wall (13, 15, 16) in the casing, a reservoir or front chamber (24) in the casing on one side of the movable wall for permanent connection to a vacuum source, a variable pressure or rear chamber (25) in the casing on the other side of the movable wall and a valve mechanism (15, 19, 21) which controls the admission of air between atmosphere and the rear chamber and between the rear chamber and the front chamber, the valve mechanism comprising a valve body (15) which is connected to or forms part of the movable wall and incorporates at least one transfer passage (23) for interconnecting the front chamber and the rear chamber, a valve member (19) which is slidable in the valve body, incorporates at least one inlet passage (26) for connecting the rear chamber to atmosphere and can transmit a load from an input rod (31) to a reaction device (34, 35, 36, 37) for transmitting load from the movable wall to an output rod, and a valve seat (21) which is slidable relative to the valve member to selectively block and open the inlet and transfer passage in response to relative movement between the valve member and the valve body, characterised in that the transfer passage extends through a projection on the valve body which projection itself extends axially through the valve member such that inlet and transfer ports (28 and 29 respectively) formed at one end of the inlet and transfer passage respectively occupy discrete sectors within an annular area covered by the valve seat. 2. A servo 'according to claim 1, characterised by provision of two circumferentially spaced transfer ports (29) and two inlet ports (28) interposed between the transfer ports. 3. A servo according to claim 1 or claim 2, characterised in that the valve member has a tubular extension (22) which is slidable in the servo casing and closes the rear chamber (25) to atmosphere. 4. A servo according to claim 1 or claim 2 or claim 3, characterised in that a valve spring (39) acts between the valve body and the valve member to bias the valve member into a retracted position in which the valve seat rests on the inlet ports and the transfer ports are open. 5. A servo according to claim 4, characterised in that the valve body and valve member incorporate interengaging abutment means (45, 44) which together limit the movement of the valve member to set said retracted position and which latch together during assembly. 6. A servo according to any preceding claim, characterised in that the reaction device comprises a rubber block (34) bounded on one side by a piston area (piston 37) on the output rod and on the opposite side by an annular piston area (washer 36) connected to the valve body and a piston area (disc 35) connected to the valve member and surrounded by the annular area, the annular area being on a washer (36) which engages in a recess in the valve body. 7. A servo according to claim 6, characterised in that the piston area connected to the valve member is on a disc (35) which is slidable within tl.. washer (36) and is movable towards the rubber block by the valve member. 8. A servo according to claim 7, characterised in that the valve body provides an abutment (43) for the disc to limit movement away from the rubber block.

Inventors: ERICSON, ANTHONY HENWOOD
Applicants: AUTOMOTIVE PRODUCTS PUBLIC LIMITED COMPANY
Application year: 1983
Main classification: B60T13
IPC codes: B60T13