VACUUM OPERATED BRAKE BOOSTER

Abstract: A brake booster comprises a casing 21 divided by a diaphragm 24 and support plate 34 into two chambers so that differential pressure can be applied to the movable wall to supplement load from an input rod 15 and provide an increased load at output 33. Mechanical loads between the output region and input region of the booster are transmit­ ted through tie bars 13 which are arranged in re-entrant portions or slots in the casing so as to be near the axis of the loads. The movable wall 34 is correspondingly shaped with re-entrant portions and is strengthened at the roots of these re-entrant portions by a deep flange 37. Casing part 21 is shaped conically to receive the deep flange 37 without restricting the stroke of the movable wall within the casing.

Description: VACUUM OPERATED BRAKE BOOSTER This invention relates to brake boosters and is particularly though not exclusively concerned with direct acting brake boosters for use in motor vehicle braking systems. Brake boosters usually comprise two annular dished pressings joined together at their outer rims to form a hollow casing. The casing is divided into two chambers by a movable wall and contains valve means arranged to provide a controlled differential pressure across the wall to provide boost thrust and thus an output load greater than the input load. A so-called direct acting brake booster is one which is placed between a driver's brake pedal and a hydraulic master cylinder. Installation requirements dictate that, in almost all cases, one end of the booster is mounted against the engine side of the vehicle bulkhead and a hydraulic master cylinder is mounted on the other end of the booster and that consequently reaction load is transmitted by the booster which loads the booster casing in tension. The booster casing is also loaded in bending as a cantilever by the weight of the master cylinder and this load fluctuates with vertical vehicle accelerations. A conventional booster casing has to be sufficiently rigid to avoid significant deformation under these loads and must not be susceptible to fatigue failure. The booster attachment points to the vehicle bulkhead can be spaced out to reduce local loads resulting from the bending moment but the mountings of the master cylinder tend to be dictated by industry standards and the weight disadvantage of an excessively large master cylinder flange. It has already been proposed, for example in U.K. Patent Specification 1070171 to provide two mounting bolts which extend from the master cylinder round the booster casing to mounting points on the bulkhead. To keep the booster assembly compact and to keep the bolts in line with master cylinder mounting points, the bolts are arranged in a narrowed portion of the booster casing which thus has to depart from the conventional circular configuration. This introduces a new difficulty in that the movable wall which corresponds in shape to the casing has individual lobes joined to a central portion and these lobes can be subject to undesirable flexing with respect to or breaking from the central portion. An object of the present invention is to-provide a booster which utilises the advantages of a support structure for the master cylinder without introducing the structural disadvantage associated with the movable wall. According to the invention there is provided a brake booster comprising a casing divided into two chambers by a movable wall, valve means responsive to a booster input load for providing a controlled differential pressure across said wall to provide boost thrust and thus an output load greater than the input load and at least one tie bar for transmitting reaction loads from the output region to the input region of the booster accommodated in a re-entrant portion of the booster periphery wherein the movable wall incorporates a stiffening flange around its periphery and the flange is deeper in the region of the re-entrant portion of the booster periphery than at the outermost parts of the wall. Preferably the wall is sealed to the periphery of the casing by a rolling diaphragm and the flange forms a support for the diaphragm. Preferably one end of the casing is dished so that the casing is longer near its centre than its outer periphery, the deeper part of the flange fitting in the longer part as the movable wall approaches the dished end. Preferably the brake booster incorporates at least two tie bars. Two symmetrically placed tie bars can provide a symmetrical balanced structure but in themselves provide little resistance to bending. In order to provide a structure which is resistant to bending, three tie bars can be advantageous. Other features of the invention are included in the following description of a preferred embodiment shown by way of example only, in the accompanying drawings in which: Figure 1 is a diagrammatic illustration of the installation of a booster according to the invention, also showing other parts of a brake actuation system; Figure 2 is a plan view of the booster; and Figure 3 is an axial section through the booster on the line E-E of Figure 2. Figure 1 shows a brake booster 11 mounted on a vehicle bulkhead 12 and fixed thereto by tie bars 13 extending from the bulkhead to a hydraulic brake master cylinder 14. A booster pushrod 15 is connected to a brake pedal 16 pivoted to the vehicle structure by a pin 17. Figure 2 shows that the booster casing 1 is generally cloverleaf shape with three lobes 2 separated by re-entrant portions or recesses 3. Figure 3 shows that casing 1 comprises a generally cloverleaf bowl shaped plastics moulding 21 closed by a generally plate shaped moulding 22. Integral hooks 23 on the outer periphery of the bowl moulding 21 engage the outer edge of the plate moulding to retain it therewith. The outer edge of the plate moulding 22 also has portions, not shown, which abut the peripheral edge of the bowl moulding 21 to limit insertion of moulding 22 therein. A peripheral bead of a rolling booster diaphragm 24 is trapped in a groove formed by an 'L' shaped flange 25 at the outer edge of the plate moulding by the wall of the bowl moulding. Such a flange 25 is made possible by the adoption of a plastics moulding for the casing construction. Three tie bars 13, threaded at both ends have shoulders 27 to both assist retention of the mouldings 21 and 22 together and to prevent the booster casing being crushed by over-tightening of nuts on the studs. The tie bars are housed in the re-entrant portions 3 of the booster periphery. The mouldings 21 and 22 have projections 4 so that the tie bars snap into grooves formed behind the projections at the base of the re-entrant portions as shown. During assembly the shoulders 27 slide in moulded grooves 28 of the booster casing, and flats on shoulders subsequently prevent the tie bars from turning as the securing nuts are tightened. The booster includes the usual input rod 15 (also shown in Figure 1) and output rod 33 which applies a boosted output load to the master cylinder 14 shown in Figure 1. A valve assembly 32 incorporates a valve body 30 which is formed as an integral plastics moulding with a movable wall 34. The movable wall 34 is guided for axial movement by the valve body 30 sliding through a seal 35 supported in a central aperture in plate 22. The movable wall or diaphragm support plate 34 is sealed against the periphery of the casing by the previously described rolling diaphragm 24. To facilitate the rolling action of the diaphragm 24 and to stiffen the support plate 34, it is provided in the usual way with a flange 36 which merges through a smooth curve with the main part of the support plate. Although not shown, it can be appreciated that the diaphragm support plate 34 must conform to the generally cloverleaf shape of the casing as shown in Figure 2. Thus the diaphragm support plate incorporates three lobes which are joined to a central core, that is the valve body, by comparatively narrow parts. The axial thrust load on the individual lobes tends to bend these lobes or even to break them off at this narrow point. In order to provide a sufficiently stiff and strong structure, the flange 36 has deeper portions 37 in the region of the re-entrant portions 3 of the casing. It is desirable that the whole of the flange 36 should not be increased in size in this way or in other words that the flange should be deeper in the region of the re-entrant portions than at the outermost parts of the diaphragm support plate for a reason which will be explained subsequently. Even for the limited loads carried by the casing, it is desirable in the interests of stiffness to avoid a flat end wall for the moulding 21. Partly for this reason, this end wall has a generally conical portion so that the casing is longer near its centre than it is at its outer periphery. With this arrangement, the deeper part 37 of the flange 36 moves in the longer part of the casing while the shallower part of the flange 36 moves in the shorter part of the casing with the result that travel of the diaphragm support plate is not restricted by the enlarged flange. It is for this reason that outer parts of the flange should not be enlarged. The generally conical end to moulding 21 can also tend to prevent interference between the forward outer part of the casing and another component of a vehicle and assists fitting the booster in a confined space. A recess 38 in the moulding 21 which receives the rear end of a master cylinder also assists in general compactness. The valve 32 is entirely conventional and a detailed explanation of its operation is not relevant to the present invention. It can simply be stated that in the absence of any input load, pressure reduced below atmospheric (hereinafter referred to as vacuum) applied to the forward side of the diaphragm 24 through vacuum connection 39 is also applied to the rear side of the diaphragm. When load is applied to the input rod 15, the valve closes communication between the two sides of the diaphragm and subsequently allows controlled access of air from the atmosphere to the rear of the diaphragm to produce a controlled differential pressure between the two sides of the diaphragm and thus generate a boost thrust. This boost thrust together with the input load itself are applied to the output rod 33 to produce a boosted output load. The re-entrant portions 3 should be so shaped that they reduce the effective area of the movable wall by a minimum amount, subject only to providing a curved shape to permit effective rolling of the diaphragm 24. Despite this, a small part of the effective area must be lost as compared with a conventional circular shape. To provide equivalent boost thrust to that of a circular casing, the diameter of the cloverleaf booster has to be increased. The increase can be small but in itself it is something of a disadvantage. However, a conventional booster casing which has to carry greater mechanical loads than the casing shown in the drawings is normally made from two dished steel pressings which at their junction incorporate outwardly directed flanges. Because with the present invention, a casing of plastics material can be employed, the outwardly directed flange becomes unnecessary and there is a compensating gain in compactness with the present invention which tends to compensate for the increased internal diameter required with the present invention and in some cases can more than compensate for this increase. In use of the booster in an installation as shown in Figure 1, the output load applied to the master cylinder is reacted at the mountings of the master cylinder on the tie bars. Part of this reaction load, equivalent to the input load,is transmitted directly through the tie bars to the mountings on the vehicle bulkhead 12. There are also internal loads within the casing generated by the differential pressure on both sides of the movable wall and the corresponding thrusts on the end faces of the booster. However, the full load applied to the master cylinder is not transmitted through the casing and loads associated with the weight and vertical acceleration of the master cylinder are in general transmitted through the tie bars to the bulkhead.

Claims: CLAIMS 1. A brake booster comprising a casing divided into two chambers by a movable wall (34), valve means (32) responsive to a booster input load for providing a controlled differential pressure across said wall to provide boost thrust and thus an output load greater than the input load and at least one tie bar (13) for transmitting reaction loads from the output region to the input region of the booster accommodated in a re-entrant portion (3) of the booster periphery characterised in that the movable wall (34) incorporates a stiffening flange (36) around its periphery and the flange is deeper (at 37) in the region of the re-entrant portion of the booster periphery than at the outermost parts of the wall. 2. A brake booster as claimed in claim 1 characterised in that the movable wall is sealed to the periphery of the casing by a rolling diaphragm (24) and the flange (36) forms a support for the diaphragm. 3. A brake booster as claimed in claim 1 or claim 2 characterised in that one end (21) of the casing is dished so that the casing is longer near its centre than its outer periphery, the deeper part (37) of the flange fitting into the longer part of the casing as the movable wall approaches the dished end. 4. A brake booster as claimed in any preceding claim characterised in that there are at least two reentrant portions (3) and at least two corresponding tie bars (13) within the re-entrant portions. 5. A brake booster as claimed in claim 4 having three re-entrant portions (3) and three tie bars (13), one within each re-entrant portion. 6. A brake booster as claimed in any preceding claim characterised in that the tie bars (13) have shoulders (27) to assist holding together two parts (21 and 22) of the casing and to prevent crushing of the casing by excessive tightening.

Inventors: BAILEY, WALTER WILLIAM
Applicants: AUTOMOTIVE PRODUCTS PUBLIC LIMITED COMPANY
Application year: 1983
Main classification: B60T13
IPC codes: B60T13