GB2123910A - Sealing hydraulic brake master cylinder assemblies - Google Patents

Abstract

A master cylinder assembly 10 for a hydraulic brake system includes a relatively short primary pressurizing piston 32 that is slidably mounted in a longitudinal bore 14 of a master cylinder housing 12. A static fluid/air seal constituted by an annular elastomeric seal 60 is located in the housing bore in the region of an open rear end of the bore. The seal 60 includes a pair of inwardly directed annular sealing lips 62 which in use engage an output push rod 74 of an associated brake booster 76 to provide a dynamic pressure fluid seal. To maintain a low-pressure fluid seal at the rear end of the master cylinder housing bore during assembly and other handling of the master cylinder, the static seal 60 includes a forwardly directed annular sealing region 64 for co-operation with an annular sealing region 66 provided on the rear end face of the primary piston 32, at a position radially outwardly of a recessed central region 72 for receiving the booster output push rod. Rearward movement of the primary piston under the action of a pair of piston return springs 40 and 46 is limited by a piston stop constituted by a spring ring 50 that is located in a circumferential groove formed in the housing bore immediately forwardly of a circumferential groove accommodating the static seal. <IMAGE>

Description

SPECIFICATION Master cylinder assemblies for hydraulic brake systems This invention relates to master cylinder assemblies for hydraulic brake systems, primarily for use in motor vehicles.

In a conventional form of master cylinder assem buy for a vehicular hydraulic brake system, a pressurizing piston that is slidably mounted in a bore of a master cylinder housing, in the vicinity of an open rear end of the bore, comprises front and rear lands on which are mounted respective annular elastomeric seals providing fluid sealing at these two locations with the wall of the housing bore. The two lands are interconnected by an intermediate piston portion having a smaller diameter than the lands, such that an annular space is formed between this intermediate portion and the wall of the housing bore. This annular space is in fluid communication with an equalization port (compensation hole) that extends through the wall of the housing bore to a reservoir for hydraulic brake fluid.

To maintain fluid-tightness at the rear end of the master cylinder, such an assembly using two dynamic fluid seals requires the use of a pressurizing piston which is long enough to ensure that, during a pressurizing stroke, the rear seal does not pass the equalization port from the fluid reservoir.

By the present invention there is provided a master cylinder assembly for a hydraulic brake system, in which a master cylinder housing includes a bore having an open rear end, a pressurizing piston having a front land axially spaced from a rear land is slidably mounted in the housing bore between the open rear end of the bore and a portion of the bore forming a pressurizing chamberforhyd- raulic fluid, a first annular elastomeric seal is mounted on the front land of the pressurizing piston to provide fluid sealing between the pressurizing piston and the wall of the housing bore, a second annular elastomeric seal is mounted in the housing bore adjacent the open end thereof and is located against movement in a direction towards the open end of the bore, and the second annular elastomeric seal includes a forwardly directed annular sealing region and a generally radially inwardly directed annular sealing lip, a piston return spring is disposed in the pressurizing chamber and is effective to bias the pressurizing piston in a rearward direction towards a position in which the forwardly directed annular sealing region of the second annular elastomeric seal is engaged by a rearwardly directed annular sealing region on an end surface of the rear land of the pressurizing piston, to provide fluid sealing between the annular sealing region on the pressurizing piston and the wall of the housing bore, an annular space formed between the front and rear lands of the pressurizing piston is in fluid communication with a pressure equalization port extending through the wall of the housing bore and also with an annular space formed between the rear land of the pressurizing piston and the second annular elastomeric seal, at a position radially outwardly of the respective annular sealing regions, the end surface of the rear land of the pressurizing piston includes, radially inwardly of the annular sealing region thereof, a central region recessed for engage ment - with minimum space left for hydraulic fluid by a manually operable push rod for imparting pressurization movement to the pressurizing piston, with the annular sealing lip of the second annular elastomeric seal being adapted to provide fluid sealing between such push rod and the wall of the housing bore, and a piston stop is mounted in the housing bore in the region of the open end thereof, such that in the absence of the push rod, rearward movement of the pressurizing piston by the action of the piston return spring is limited by engagement of the pressurizing piston with the piston stop, in such a manner as not to prevent the fluid sealing between the annular sealing regions of the rear land of the pressurizing piston and the second annular elastomeric seal respectively from being effective to inhibit loss of hydraulic fluid through the open rear end of the housing bore.

Such a master cylinder assembly in accordance with the invention thus makes use of a static seal mounted in the master cylinder bore for sealing on to a push rod (for example, in a servo-operated power brake, the push rod of a vacuum-operated brake booster) in normal operating conditions. Also, during assembly operations, or during other handling involving separation of the master cylinder from the push rod, fluid sealing is provided by the static seal sealing against the bore wall and additionally by co-operation of the annular sealing regions of, respectively, the static seal and the rear end surface of the piston rear land, such sealing being potentially fully adequate to accommodate the very low fluid pressures involved under such conditions. The static seal may if required embody an armature of metal or plastics material.

A master cylinder assembly in accordance with the present invention accordingly offers the advantage that the pressurizing piston can be made very short and without an exposed projection, and is effectively subject only to the mechanical requirement that the piston be sufficiently long to slide freely in the housing bore without wedging. Thereby, a significant reduction in the length of the master cylinder, and thus also its weight and cost, is possible.

The pressurizing piston in question may comprise the primary piston (thus, the rearwardly disposed piston) of a dual master cylinder arrangement having primary and secondary pressurizing pistons arranged in tandem in a master cylinder housing bore.

In a power brake utilizing a vacuum-operated brake booster having its own housing shell, separate from the master cylinder housing, favourable sealing conditions can be achieved at the junction between the master cylinder and the booster if the booster incorporates its own air/vacuum seal between the booster push rod and the booster front shell, and if the region between the booster front shell and the master cylinder housing, in the vicinity of the push rod, is provided with an easy vent path to atmosphere. With such a construction, the static seal between the booster push rod and the wall of the master cylinder bore is not exposed to booster vaccum, and can act purely as a fluid/air seal.

In such a power brake construction, also, the airtvacuum seal of the booster itself may if required embody a metal or plastics armature. Irrespective of whetherthe booster air/vacuum seal is reinforced in this way, the air/vacuum seal can serve, in conjunction with the booster push rod, as a guide bearing for the whole power brake assembly, and, in the master cylinder portion of the assembly, effectively reduce side forces on the lands of the pressurizing piston of the master cylinder.

In the drawings: Figure 1 is a fragmentary longitudinal section, with parts in elevation, of one embodiment of a master cylinder assembly in accordance with the present invention, in the form of a dual master cylinder, shown in a condition for assembly or other handling; Figure 2 is a fragmentary longitudinal section to a reduced scale, with parts in elevation, of a complete power brake arrangement comprising a vacuumoperated brake booster having a push rod cooperating with a primary pressurizing piston of a dual master cylinder as shown in Figure 1; and Figure 3 is a view similar to Figure 2, but showing a power brake arrangement having a somewhat different form of brake booster.

In Figure 1 of the drawings, there is shown a master cylinder assembly 10 in accordance with the present invention, for use in a dual-circuit hydraulic brake system of a motor vehicle, the assembly being shown in a condition for assembly or other handling.

As viewed in Figure 1,the left-hand end of the master cylinder assembly 10 will form the front end when the assembly is installed in the vehicle, and correspondingly the right-hand end will form the rear end: references herein to "front" and "rear" are made on this basis.

The master cylinder assembly 10 shown in Figure 1 of the drawings has an elongate master cylinder housing 12 formed with a longitudinally extending bore 14which is closed at the front end by an end wall portion 16 and is open at the rear end. The housing 12 is a metal casting made of cast iron or alternatively aluminium or other light metal alloy or plastics material, and includes first and second hollow bosses 18 and 20 for accommodating a plug-in reservoir 22, shown in interrupted lines, for hydraulic brake fluid. The interior of each of the hollow bosses is in communication with the bore 14 by means of a respective fluid supply port 24 and pressure equalization port 26 passing through the wall of the bore.The housing 12 is also formed with first and second pressure discharge ports 28 and 30 extending from the bore 14 through the wall of the bore to respective fittings (not shown) for connection to respective brake lines of a hydraulic brake circuit.

Within the bore 14 of the master cylinder housing 12 there are slidably mounted a primary pressurizing piston 32 and a secondary pressurizing piston 34, each of these pistons having a rear land connected to a front land by a waist-klike intermediate piston portion of smaller diameter than the two lands. The primary and secondary pressurizing pistons 32 and 34 are arranged in tandem in the bore 14, with the primary piston 32 disposed between the open rear end of the bore and a portion of the housing bore forming a primary pressurizing chamber 36 for hydraulic fluid, and the secondary piston 34 disposed between the primary pressurizing chamber and a secondary pressurizing chamber 38 that extends within the bore to the end wall portion 16 which closes the front end of the bore.

A primary piston return spring 40 precaged between spring retainers 42 and 44 is disposed in the primary pressurizing chamber 36, the spring retainer 42 being mounted on the primary piston 32 and the spring retainer 44 abutting a rear end portion of the secondary piston 34. A secondary piston return spring 46 is disposed in the secondary pressurizing chamber 30, and is seated at its front end in an annular recess in the end wall portion 16 and at its rear end on a spring retainer 48 mounted on the secondary piston 34. The piston return springs 40 and 46 are helical compression springs, and rearward movement of the primary piston 32 under the biasing action of the two piston return springs is limited by a piston stop formed by a spring ring 50 axially located in a circumferential groove that is formed in the housing bore 14 in the region of the open rear end of the bore.

The primary pressurizing piston 32 has a forwardfacing elastomeric cup seal 52 mounted thereon adjacent the front land of the piston, to form a dynamic fluid seal with the wall of the housing bore at the rear end of the primary pressurizing chamber 36. The secondary pressurizing piston 34 has a rearwardly facing elastomeric cup seal 54 mounted thereon adjacent the rear land of the piston, to form a dynamic fluid seal with the wall of the housing bore at the front end of the primary pressurizing chamber 36, and also hasaforwardlyfacing elastomeric cup seal 56 mounted thereon adjacent the front land of the piston, to form a dynamic fluid seal with the wall of the housing bore at the rear end of the secondary pressurizing chamber 38.

The primary and secondary pressurizing pistons 32 and 34 each form an annular space 58 between the waist-like intermediate piston portion and the wall of the housing bore 14, axially between the front and rear lands of the respective piston, this annular space 58 being in fluid communication with the respective pressure equalization port 26.

In conformity with the invention, a static seal is provided for co-operation with the end surface of the rear land of the primary pressurizing piston. For this purpose an annular elastomeric seal 60 is axially located in the housing bore 14 by being mounted in a second circumferential groove formed in the housing bore between the circumferential groove for the spring ring 50 and the open end of the bore, with the outer peripheral region of a front face portion of the seal 60 in abutment with the spring ring either directly as shown or via a spacer (not shown). The annular elastomeric seal 60 provides fluid-tight sealing engagement with the second circumferential groove, and is also formed with first and second annular sealing lips 62 which extend from the main body portion of the seal in a generally radially inward direction.The annular elastomeric seal 60 is additionally formed with a forwardly directed annular sealing region 64 which projects in a forward direction from the front face portion of the séal: in the position of the master cylinder assembly shown in Figure 1,the primary piston 32 is biased by the piston return springs 40 and 46 into an extreme rearward position, with the rear face of the rear land of the primary piston in abutment with the piston stop constituted by the spring ring 50, and with the annular sealing region 64 of the elastomeric seal 60 resiliently deformed by, and forming a fluid seal with, an annular sealing region 66 on the end surface of the rear land of the primary piston 32.

The annular elastomeric seal 60 could if required have embedded therein reinforcement constituted by an armature made of metal or rigid plastics material.

Radially outwardly of the co-operating annular sealing regions 64 and 66, an annular space 68 is formed between the end surface of the rear land of the primary piston 32 and the outer peripheral region of the annular elastomeric seal 60. This annular space 68 is in fluid communication with the annular space 58 around the intermediate portion of the primary piston by way of axially extending through holes 70 in the rear land of the primary piston.

The end surface of the rear land of the primary piston 32 also has a conically recessed central region 72. As is shown in interrupted lines in Figure 1, a manually operable push rod 74, for example an output push rod of a vacuum-operated manually actuable brake booster, can extend through the annular elastomeric seal 60, in sealing engagement with the annular sealing lips 62 of this static seal, with a domed front end portion of the push rod in force-transmitting abutment with the recessed central region 72 of the end surface of the primary piston rear land. Only a minimum space is left for hydraulicfluid between the domed end portion of the push rod 74 and the conically recessed region 72 of the piston rear land.

Figure 2 of the drawings shows a power brake arrangement in which such a push rod constitutes the output push rod of a vacuum-suspended reaction-lever type of brake booster 76 that is actuable by means of an input actuator rod 78 connected to be operated by driver operation of a conventional brake pedal (not shown): the booster output push rod 74 projects through an annular elastomeric air/vacuum seal 80 that is sealingly mounted in a central recessed region of a front shell portion 82 of a housing shell ofthe booster, and the domed front end portion of the push rod co-operates with the master cylinder assembly 10 in the mannerwhich has just been described.

As is also shown in Figure 1 of the drawings, a vent passage 8'4 is provided in an abutment flange portion 86 at the rear end region of the master cylinder housing 12, co-operating with a vent passage 88 in the abutment region of the master cylinder on the seal 80, such that the region between the booster front shell and the master cylinder housing in the vicinity of the booster push rod 74 is thereby placed in communication with the atmosphere.

It is not considered necessary to described the booster 76 in detail, since other types of booster could be used and since the main operating parts and the mode of operating are generally conventional; the internal details of the booster do not themselves form part of the present invention.

The power brake arrangement is shown in Figure 2 in its rest position, with the primary pressurizing piston 32 in contact with the forwardly directed annular sealing region 64 of the annular elastomeric seal 60, but with the piston 32 spaced a small axial distance from the piston stop 50. Operation of the brake pedal by the vehicle driver produces forwardly directed movement of the booster input actuator rod 78, and hence servo-assisted forward movement of the booster output push rod 74, so producing forward movement of the primary and secondary pressurizing pistons 32 and 34, and consequent pressurization of the brake lines in the hydraulic circuit, in a conventional fashion.

The length of the stroke of the primary pressurizing piston 32, in relation to the short overall length of the primary piston, is such that during the forward movement of the primary piston the rear land of the piston can be moved to a position forwardly of the pressure equalization port 26 for hydraulic brake fluid without interference with the normal functions of the master cylinder. At all piston/push rod positions the annular sealing lips 62 of the annular elastomeric static seal 60 co-operate with the external surface of the booster output push rod 74 to provide an effective pressure dynamic seal.

During assembly of the power brake arrangement with the plug-in reservoir 22 in place and containing hydraulic brake fluid, and during other handling of the master cylinder assembly 10 separately from the brake booster assembly 76 and its output push rod 74, the primary pressurizing piston 32 is in abutment with the piston stop 50, with the static seal 60 sealing against the wall surface of its circumferential groove, and with the annular sealing regions 64 and 66 of the static seal 60 and primary piston 32 respectively in interengagement, to provide an effective lowpressure static seal preventing loss of hydraulic brake fluid from the open rear end of the housing bore 14.

The annular elastomeric air/vacuum seal 80 that is sealingly mounted in the central recessed region of the front shell portion 82 of the housing shell of the booster 76 additionally acts as a resilient guide bearing tending to maintain the output push rod 74 of the booster in a centred condition, so effectively contributing to a reduction in side forces acting on the lands of the primary pressurizing piston 32 in the master cylinder portion of the power brake arrangement.

Figure 3 of the drawings shows a power brake arrangement which is generally similar to the Figure 2 arrangement but includes a modified form of brake booster 76' that has a shorter axial length and utilises rubber reaction rather than lever reaction.

The Figure 3 arrangement also includes a slightly different form of plug-in reservoir 22', fitted with a float-operated brake fluid level switch.

In the master cylinder assembly in accordance with the invention which has been described, the very short length of the primary pressurizing piston made possible by the use of the static seal for sealing against the outer surface of the actuating push rod permits a significant reduction in the length of the master cylinder assembly, and a reduction in its weight and cost.

Claims (13)

1. A master cylinder assembly for a hydraulic brake system, in which a master cylinder housing includes a bore having an open rear end, a pressurizing piston having a front land axially spaced from a rear land is slidably mounted in the housing bore between the open rear end of the bore and a portion ofthe bore forming a pressurizing chamber for hydraulic fluid, a first annular elastomeric seal is mounted on the front land of the pressuring piston to provide fluid sealing between the pressurizing piston and the wall of the housing bore, a second annular elastomeric seal is mounted in the housing bore adjacent the open end thereof and is located against movement in a direction towards the open end of the bore, and the second annular elastomeric seal includes a generally radially outwardly directed annular sealing region effective to provide fluid sealing with the wall of the housing bore in all conditions and also includes a forwardly directed annular sealing region and a generally radially inwardly directed annular sealing lip, a piston return spring is disposed in the pressurizing chamber and is effective to bias the pressurizing piston in a rearward direction towards a piston in which the forwardly directed annular sealing region of the second annular elastomeric seal is engaged by a rearwardly directed annular sealing region on an end surface of the rear land of the pressurizing piston, to provide fluid sealing between the annular sealing region on the pressurizing piston and the wall of the housing bore, an annular space formed between the front and rear lands of the pressurizing piston is in fluid communication with a pressure equalization port extending through the wall of the housing bore and also with an annular space formed between the rear land of the pressurizing piston and the second annular elastomeric seal, at a position radially outwardly of the respective annular sealing regions, the end surface of the rear land of the pressurizing piston includes, radially inwardly of the annular sealing region thereof, a central region recessed for engagement - with minimum space left for hydraulic fluid - by a manually operable push rod for imparting pressurization movement to the pressurizing piston, with the annular sealing lip of the second annular elastomeric seal being adapted to provide fluid sealing between such push rod and the wall ofthe housing bore, and a piston stop is mounted in the housing bore in the region of the open end thereof, such that in the absence of the push rod, rearward movement of the pressurizing piston by the action of the piston return spring is limited by engagement of the pressurizing piston with the piston stop, in such a manner as not to prevent the fluid sealing between the annular sealing regions of the rear land of the pressurizing piston and the second annular elastomeric seal respectively from being effective to inhibit loss of hydraulic fluid through the open rear end of the housing bore.

2. A master cylinder assembly according to claim 1, in which the pressurizing piston comprises a primary piston of a dual master cylinder arrangement that additionally includes a secondary pressurizing piston slidably mounted in the housing bore between the primary piston and a second pressurizing chamber extending to a closed end of the housing bore, with the return spring for the primary piston seated at its forward end on the secondary piston, and a secondary piston return spring disposed in the second pulverizing chamber and effective to bias the second pressurizing piston, and thereby also the primary pressurizing piston, in a rearward direction.

3. A master cylinder assembly according to claim 1 or 2, in which a manually operable push rod has a forward region in engagement with the recessed central region of the end surface of the rear land of the pressurizing piston in a manner that leaves minimum space for hydraulic fluid, and also has an external surface sealingly engaged in a fluid-tight manner by the annular sealing lip of the second annular elastomeric seal.

4. A master cylinder assembly according to claim 3, in a power brake arrangement in which the manually operable push rod comprises an output push rod of a vacuum-operated manually actuable brake booster.

5. A master cylinder assembly according to claim 4, in which the brake booster has a housing shell separate from the master cylinder housing, the booster includes an air/vacuum sealing element sealingly mounted on a front shell portion of the housing shell of the booster and sealingly engaging the output push rod of the booster to provide an air/vacuum seal, and the region between the booster front shell and the master cylinder housing, in the vicinity of the booster push rod, is in communication by way of a vent passage with the atmosphere.

6. A master cylinder assembly according to claim 5, in which the air/vacuum seal of the booster comprises a body of elastomeric material having embedded therein reinforcement constituted by an armature made of metal or rigid plastics material.

7. A master cylinder assembly according to any one of claims 1 to 6, in which the piston stop of the master cylinder comprises a spring ring axially located in a circumferential groove in the housing bore of the master cylinder.

8. A master cylinder assembly according to claim 7, in which the second annular elastomeric seal of the master cylinder is axially located in a second circumferential groove formed in the housing bore between the circumferential groove for the spring ring and the open end of the bore, and a front face portion of the second annular elastomeric seal abuts the spring ring either directly or via a spacer.

9. A master cylinder assembly according to any one of claims 1 to 8, in which the second annular elastomeric seal includes first and second annular sealing lips as aforesaid forfluid-sealing engagement with the push rod.

10. A master cylinder assembly according to any one of claims 1 to 9, in which the second annular elastomeric seal has embedded therein reinforcement constituted by an armature made of metal or rigid plastics material.

11. A master cylinder assembly for a hydraulic brake system, substantially as hereinbefore particularly described and as shown in Figure 1 of the accompanying drawings.

12. A master cylinder assembly in a power brake arrangement, substantially as hereinbefore particularly described and as shown in Figure 2 of the accompanying drawings.

13. A master cylinder assembly in a power brake arrangement, substantially as hereinbefore particularly described and as shown in Figure 3 of the accompanying drawings.