GB2024967A

GB2024967A - Servo Booster for Vehicle Braking Systems

Info

Publication number: GB2024967A
Application number: GB7829267A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1978-07-08
Filing date: 1978-07-08
Publication date: 1980-01-16
Also published as: GB2024967B

Abstract

A problem which arises with conventional poppet valve assemblies for servo boosters is that a strong spring for retracting the input rod and locating a bead of the valve member can damage the resilient valve head prior to insertion of the valve assembly into the valve body. The problem is overcome by providing an abutment (14) on the valve body (1) for the spring (13), and by locating the valve member bead (19) by a retaining ring (20) having a snap-engagement with the valve body. The abutment (14) is spaced rearwardly from the retaining ring (20) so that prior to insertion of the valve assembly into the valve body the spring (13) is under a reduced load and exerts a reduced force on the valve head (17) which is less likely to damage it. <IMAGE>

Description

SPECIFiCATION Servo Booster for Vehicle Braking Systems This invention relates to servo boosters for vehicle braking systems of the kind comprising a housing in which is located a movable wall for augmenting the force applied to an output member by a pedal-operated input rod, valve means controlling the differential pressure applied to the movable wall and comprising a valve body provided with an axial bore, a valve control member slidable in the forward part of the bore, the valve control member and the valve body being formed with co-axial rearwardly facing valve seats, and a poppet valve member comprising an annular valve head engageable with the valve seats and connected by an axially compressible skirt to a bead held in sealing engagement with the wall of the bore, the input rod extending through the poppet valve member to engage with the valve control member, first resilient means biassing the input member rearwardly relative to the valve body, and second resilient means biassing the valve head forwardly relative to the valve body.

Usually the first resilient means comprises a first coil spring which bears at its forward end against a metal retaining ring for the bead of the poppet valve member and at its rear end against an abutment on the input rod.

It is also usual for the control rod to be permanently connected to the valve control member so that the valve control member, poppet valve, first and second resilient means, and the input rod constitute a self-contained subassembly which is assembled before being inserted into the valve body.

A problem which arises with such a subassembly is that prior to insertion of the subassembly into the valve body, the poppet valve member is subject to the full force of the first coil spring which holds the valve head in firm engagement with the valve seat of the valve control member. Since the first coil spring is a strong spring this results in the valve head being formed with an annular recess. Since no means is provided in the subassembly for locating the poppet valve member transversely with respect to the valve control member, the recess formed in the valve head does not always coincide with the annular part of the valve head that is engaged by the seat of the valve control member when the booster is fully assembled. Consequently an imperfect seal is sometimes achieved between the seat of the valve control member and the poppet valve member leading to failure of the booster.Also, excessive overlap between the opening of one valve and the closing of the other is produced.

According to the invention in a servo booster of the kind set forth the bead of the poppet valve is held in position by a retaining ring having a snapengagement with a formation provided on the bore wall of the valve body, and the first resilient means engages at its front end with an abutment provided on the bore wall and spaced rearwardly from the retaining ring.

Since the front of the first resilient means in the fully assembled booster is spaced from the retaining ring, the first resilient means will exert a much reduced load on the retaining ring, and thus on the valve head, in the sub-assembly before insertion of the subassembly into the valve body.

Preferably the retaining ring comprises a resilient plastics member, and the bore wall is provided with an annular recess adapted to receive the periphery of the ring.

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-sectional partial view of the valve means and associated components of a servo booster; Figure 2 is an enlarged view of the retaining ring for the poppet valve bead; and Figure 3 is a view similar to Figure 2 but of a modification.

A substantially cylindrical moulded plastics valve body 1 is provided with a multi-stepped axial bore essentially comprising three bore portions 2, 3 and 4 of progressively increasing diameter. A cylindrical valve control member 5 is slidable in the front bore portion 2 and is provided at its rear end with a first annular valve seat 6 coaxial with a second annular valve seat 7 formed on the valve body 1 at the step between bore portions 2 and 3. A poppet valve assembly 8 is located in bore portion 3, and a pedal-operated input rod 9 extends axially through an air filter 10 located in the rear of bore portion 5 and through the poppet valve to engage with the valve control member 5 by means of a ball-end 11. The input rod 9 is held captive to the valve control member 5 as is usual by a deformed portion 12 of the valve control member 5.

The input rod is biassed to a retracted position by a strong coil spring 1 3 which bears at its front end against a step 14 in the bore at the front end of the bore portion 4 and at its rear against a washer 1 5 engaging with a shoulder 1 6 on the rod.

The poppet valve assembly 8 comprises an annular valve head 1 7 integral with the rearwardly extending axially compressible skirt 1 8 terminating in a bead 1 9 retained in position in sealing engagement with the wall of bore portion 3 by a plastics retaining ring 20. The valve head 17, skirt 18 and bead 19 comprise a single rubber moulding except that the valve head is reinforced by a flat metal ring 21.

Valve head 1 7 is adapted to co-operate in wellknown manner with valve seats 6 and 7 in dependence upon the axial position of valve control member 5 relative to the valve body 1.

Retaining ring 20 comprises a portion 22 in the shape of a washer integral with a forwardly extending tubular portion 23 which terminates in a radially outwardly directed flange 24. The outer periphery of the portion 22 has a snapengagement with an internal annular recess 25 in valve body 1 at the rear end of bore portion 3 but spaced forwards from shoulder 14 by a sloping face 26. As shown in Figure 2 the outer periphery of the portion 22 of the retaining ring 20 is provided with a sloping facing 27 of the same slope as face 26 for engagement therewith during insertion of the ring 20 into position, with a plain cylindrical face 28 and with a face 29 sloping in the opposite direction to face 27 and more steeply for co-operation with a complementary face 30 at the rear end of recess 25.When the retaining ring 20 is urged forwards into the valve body during assembly the faces 26 and 27 cooperate to compress radially the retaining ring, and on further forward movement of the retaining ring 20 the faces 29 and 30 then co-operate to drive the retaining ring into the assembled position shown in the drawings. Since the faces 28 and 29 are relatively steep there is no danger of'the retaining ring becoming dislodged in use from the groove 25.

A second coil spring 31 constituting the second resilient means acts between the valve head 17 and the radially inner part of portion 22 of the retaining ring to bias the valve head forwardly for engagement with the valve seats.

Bead 19 is held radially outwards against the wall of bore portion 3 to effect a seal therewith by the tubular portion 23 of the retaining ring 20 and is held against axial movement by flange 24 and portion 22 of the ring 20.

During assembly of the booster the washer 1 5, spring 13, and poppet valve assembly 8 are first mounted on input rod 9 before the valve control member is permanently attached to the rod by effecting the deformation 12. The subassembly so formed is introduced into the valve body 1 at a convenient time, but before assembly into the valve body the front end of spring 13 will not exert a strong force on the retaining ring 22 as with previous constructions since it will be more expanded than in its working condition. Thus valve head 17 will not be held in firm engagement with the valve seat 6 as has been the case with previous constructions.

The novel retaining ring 20 helps to provide consistently accurate centering of valve head 1 7 relative to the valve seats 6 and 7 by its accurate location of the bead 1 9. Previously the bead 1 9 has had to be made a relatively massive member in order to try to achieve accurate centering of the valve head.

Figure 3 shows a modification in which the solid bead 19 of the Figure 2 embodiment has been replaced by a more flexible bead of generally U-shape in transverse cross-section.

The retaining ring 20 in both embodiments is preferably formed of any polymeric semi-rigid material such as glass-filled Nylon, Nylon 66, Delrin, Acetal or Kemetal. However, the resilience of the valve body 1 may permit the use of a metal ring 20.

Claims

1. A servo booster of the kind set forth in which the bead of the poppet valve is held in position by a retaining ring having a snap-engagement with a formation provided on the bore wall of the valvebody, and the first resilient means engages at its front end with an abutment provided on the bore wall and spaced rearwardly from the retaining ring.

2. A servo booster as claimed in Claim 1 in which the retaining ring is formed of resilient plastics material, and the formation comprises an annular recess which receives the periphery of the ring.

3. A servo booster as claimed in Claim 2 in which the ring is of generally U-shape in radial cross-section, but with one limb of the U shorter than the other limb, the free end of the longer limb being received within the annular recess, and the bead being held between the limbs and between the base of the U and the bore wall.

4. A servo booster of the kind set forth and substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

5. A servo booster as claimed in Claim 3 and modified substantially as described with reference to Figure 3 of the accompanying drawings.