GB2034429A

GB2034429A - Master cylinder assemblies for vehicle braking systems

Info

Publication number: GB2034429A
Application number: GB7936123A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1978-10-18
Filing date: 1979-10-18
Publication date: 1980-06-04
Also published as: ES485171A1; IT7926594A0; DE2942024A1; BR7906698A; GB2034429B; FR2439113A1; ZA795365B; KR830001071A; AR218172A1; IT1124617B; AU5170779A; JPS5594006A

Abstract

In a boosted master cylinder assembly the booster output rod (38) is connected to the primary piston (23) of a tandem master cylinder (1) and a spring (39') in the booster (2) biases both the primary piston and the output rod rearwardly. A single stop in the form of an annular seal assembly (31) is engaged by the retracted primary piston to determine the retracted position of both the primary piston and the output rod so that no adjustment is required. A substantial part of the length of the master cylinder is located within the booster housing (4), and the master cylinder reservoir 81 is mounted on the booster housing. <IMAGE>

Description

SPECIFICATION Master cylinder assemblies for vehicle braking systems This invention relates to a boosted master cylinder assembly for a vehicle braking system and of the kind comprising a hydraulic master cylinder assembly and a servo booster assembly, the master cylinder assembly comprising a master cylinder housing provided with a bore in which is slidable a master cylinder piston, and the servo booster assembly comprising a servo booster housing and an output member which is adapted to apply a force to the master cylinder piston, the master cylinder housing being connected to the servo booster housing, and the master cylinder assembly including a recuperation valve responsive to the axial position of the master cylinder piston in said bore.

It is important that in the retracted position of the master cylinder piston the recuperation valve should be open to provide fluid communication between a hydraulic reservoir and the pressure space of the master cylinder assembly. In the usual arrangement the master cylinder assembly incorporates a spring which biases the master cylinder piston rearwardly against a stop in the master cylinder, and the booster incorporates a further spring which biases the booster output rod rearwardly against a stop in the booster housing. It is necessary to ensure that the front end of the output rod in its retracted position does not hold the master cylinder piston off its stop, and also to ensure that there is not, on the other hand, a large axial spacing between the retracted piston and output rod.Since the retracted position of the front end of the output rod relative to the master cylinder housing is determined by the tolerances in a large number of dimensions in the master cylinder assembly and booster assembly it has usually been necessary to provide some means of adjusting the retracted position of the front end of the output rod, and the adjustment means, which often comprises a screw or shims, of each booster has required individual setting. This is a costly operation.

Often the master cylinder assembly is of the tandem kind incorporating primary and secondary pistons working in tandem, the secondary piston being arranged in front of the primary piston which is engaged by the output member of the booster. In the most frequently employed arrangement the primary and secondary pistons are connected together by a lost-motion connection, and the spring for retracting the primary piston is located in the cylinder bore in front of the secondary piston, the force of the spring being transmitted from the secondary piston to the primary piston by a stronger spring located for that purpose between the pistons.Disadvantages of this arrangement are that the master cylinder housing needs to be made sufficiently long to accommodate the spring in front of the secondary piston, and that the strong spring located between the pistons inhibits the attainment of a pressure balance between the two pressure spaces of the master cylinder.

It has been proposed in a master cylinder with only one piston to form that piston integrally with the output rod of a servo booster and to dispense with the usual return spring for the master cylinder piston, the return spring for the booster output rod being made sufficiently strong to overcome the seal friction forces of the master cylinder piston. However, such a construction wouid need in practice to be provided with means for adjusting the retracted position of the output rod and thus of the master cylinder piston.

According to one aspect of the invention in a boosted master cylinder assembly of the kind set forth the output member of the servo booster assembly is connected to the master cylinder piston, and resilient means is arranged to urge both the output member and the master cylinder piston rearwardly, the retracted position of the piston and output member being determined by a stop carried by the master cylinder housing.

Since the axial location of the stop and the axial position of the piston on opening of the recuperation valve are determined solely by the dimensions of the master cylinder assembly, it is unnecessary to provide the booster assembly with means to adjust the output member to accommodate tolerances in the dimensions of the components of the booster assembly, and the costly adjustment procedure may be avoided.

Preferably the resilient means comprises a spring located within the booster housing. This enables a substantial portion of the length of the master cylinder housing to be accommodated within the booster housing if desired.

The front end of the spring may be arranged to bear upon the booster housing or it may bear upon the rear end of the master cylinder housing.

The stop conveniently comprises an annular seal assembly retained against axial movement in the bore of the master cylinder housing and adapted to be engaged by a rearwardly facing step on the master cylinder piston to determine the retracted position of the piston and booster output member.

Preferably a further annular seal is carried by the piston for sliding engagement with the cylinder bore, said further seal co-operates with a recuperation port in the wall of the master cylinder housing to define, in well known manner, the recuperation valve, and said seal assembly effects a seal with a reduced diameter portion of the piston extending rearwardly from the step, and a further port in the cylinder wall located between the recuperation port and the seal assembly is permanently connected to the hydraulic reservoir.

Preferably the front portion of the seal assembly that is engaged by the step on the piston is formed of elastomeric material to avoid a noise being produced when the piston is arrested by the seal assembly.

The seal assembly is preferably retained against rearward axial movement by a substantially U-shaped wire clip the arms of which are received in transverse bores in the master cylinder housing that intersect said bore.

When the master cylinder is a tandem master cylinder the piston connected to the booster output member is the primary piston of the master cylinder, and the secondary piston, which is in front of the primary piston, is preferably then connected to the primary piston by a lost-motion connection, a further resilient means being arranged to bias the pistons apart. Since the further resilient means does not have to transmit the force for retracting the primary piston it does not require to be as strong as the resilient means located between the pistons in the usual arrangement previously described. This results in an improved pressure balance in use between the primary and secondary pressure spaces.

According to another aspect of the invention in a boosted master cylinder assembly of the kind set forth a substantial portion of the length of the master cylinder housing is accommodated within the servo booster housing, and a fluid reservoirforthe master cylinder comprises a body which is integrally formed with one wall of the servo booster housing.

The reservoir body may, for convenience, be formed open on one side, the opening subsequently being closed by a suitable closure member.

When the master cylinder is a tandem master cylinder, and the fluid reservoir defines two fluid compartments, the compartments are preferably disposed transversely of each other with respect to the longitudinal axis of the master cylinder housing.

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-section of a booster master cylinder assembly in accordance with the invention with the parts being shown in their retracted positions; Figure 2 is a section on the line 2-2 of Figure 1 showing a wire clip for retaining the rear seal assembly of the master cylinder; Figure 3 is a longitudinal cross-section of a further boosted master cylinder assembly in accordance with the invention and incorporating a tandem booster, the upper tie assembly having been brought into the vertical plane for the purposes of illustration; and Figure 4 is a view looking from the left in Figure 3 with the upper part of the booster front housing shell shown in cross-section to show the details of a hydraulic reservoir incorporated in that shell, and with the tie assemblies being shown in their actual positions.

With reference to Figure 1 the boosted master cylinder assembly comprises a tandem master cylinder assembly 1 connected to a pedal-operated servo booster assembly 2. The booster assembly 2 comprises a booster housing 3 formed of opposed moulded plastics shells 4 and 5 having a snapengaged with each other at 6 around their outer peripheries and trapping there between the outer peripheral bead 7 of the main diaphragm 8 of a single movable wall assembly 9 located within booster housing 3. The movable wall assembly 9 further comprises a diaphragm support plate 10 formed as an integral plastics moulding with a generally cylindrical valve body 11 housing an annular poppet valve assembly 12, a rearwardly extending tubular extension 13 of the valve body 11 being slidably guided by an annular seal 14 located in a central aperture in rear housing shell 5.Seal 14 is integral with a boot 14', and is the subject of our U.K. Patent No.29265/78.

The booster housing shells 4 and 5 are integrally formed with rearwardly and forwardly extending tubular members 15 and 16 respectively which house diametrically opposed ties in the form of metal studs 17 provided with threaded end portions 18 and 19forconnection respectivelytothe rear flange 20 of the master cylinder housing and to a vehicle bulkhead, not shown. The studs 17 are the subject of our U.K. Patent Application Nos. 43644/77, 2488/78, and 29255/78 and transmit in use braking reaction forces from the master cylinder housing 21 to the vehicle bulkhead to relieve the shells 4 and 5 of such forces and thereby enable the shells to be made relatively thin.

Movable wall 9 is sealed to the studs 17 by means of rolling diaphragms 22 formed integrally with main diaphragm 8. The arrangement of the various beads of the diaphragms 8 and 22 is the subject of our U.K.

Patent Application No. 29253/78.

The master cylinder assembly 1 is of the tandem kind and comprises primary and secondary pistons 23 and 24 respectively slidable axially in longitudinal bore 25 of housing 21 to define primary and secondary pressure spaces 26 and 27 respectively. The pistons 23 and 24 are connected together by a lostmotion connection comprising a headed spigot 28 extending forwardly from and integral with primary piston 23, and a rearwardly extending half-cup member 29 extending rearwardly from and integral with secondary piston 24. The half-cup member 29 is formed in its base with a transverse slot of sufficient width to receive the stem of spigot 28 but of less width than head 30.The pistons 23 and 24 are urged apart by a coil spring 32 so that in the retracted position of the primary piston 23 the secondary piston 24 is held forwards relative to piston 23 with a primary seal 33 on the secondary piston located just clear of a recuperation port 34 for the secondary pressure space 27.

Master cylinder housing 21 is formed at its rear end with a tubular spigot 35 which is received within a rearwardly extending tubular socket 36 integral with booster housing shell 4 and is sealed thereto by an O-ring 37 located in an external annular groove in spigot 35. The axial position of master cylinder housing 21 relative to booster housing 3 is determined by the engagement between the rear face of flange 20 and eitherthe front face of booster shell 4 or preferably shoulders 18' of studs 17, the shoulders 18' being axially spaced from shell 4.

The primary piston 23 is integral at its rear end with an output rod 38 of the booster assembly 2 ancl is sealed in counterbore 40 to bore 25 by a primary annular seal 39 carried by the piston 23 and by an annular seal assembly 31 located rearwardly of the seal 39 and retained against axial movement relative to the housing 21 in counterbore 40 by a washer 41 which is itself retained by a substantially U-shaped wire clip 42 shown in Figure 2. Wire clip 42 has its arms 43 extending through transverse bores in housing 21 which intersect the counterbore 40 such that the central portions of the arms 43 are engaged by the washer 41. The base part 44 of the clip 42 is located within an external annular groove 45 formed in a reduced portion 46 of spigot 35, and the free ends of the arms 43 are sheared off at an angle.

When the said arms are bent to prevent the clip from becoming displaced from the retention position shown in Figure 2, this angular shearing provided a "lead-in" for ease of assembly, and said ends may also be readily peened over to provide more positive retention means.

Seal assembly 31 comprises two axially spaced annular resilient seals sealing against the surface of output rod 38 and bonded to the internal surface of a cup-shaped metal abutment member 47 the outer surface of which is coated with rubber to effect a seal with the counterbore 40. The abutment member 47 abuts with the washer 31 at its rear end, and its coated front end is engaged buy a rearwardly facing step on the primary piston provided by the rear face of a guide flange 38 on the piston to determine the retracted position of the primary piston 23 and that of the output rod 38. In the retracted position of the primary position 23 the primary seal 39 is located just clear of a recuperation port 49 for the primary pressure space 26.

Primary piston 23 and output rod 38 are urged to their retracted positions by a single coil spring 39' located within the booster housing 3 co-axial with rod 38. The spring 39' bears at its front end against a step in the spigot 35 of the master cylinder housing 21, and at its rear end against a circular pressedmetal abutment plate 50 mounted on output rod 38 and abutting a stepped head 51 formed at the rear end of rod 38. Abutment plate 50 is formed with circumferentially spaced forwardly extending tabs 52 which locate radially the rear end of spring 39'.

Valve body 11 is formed at its front end with a cylindrical bore 53 in which is housed a rubber reaction disc 54 for providing the operator, in well-known manner, with a reaction to an increased force applied by the booster output rod 38. The full area of the front of the disc 54 is engaged by rear face of the head 51 of equal area, but the effective area of the rear of the disc 54 for engagement by the front face of a valve control member 55 is reduced by a sleeve 56. Valve control member 55 is secured to the ballhead of pedal-operated input rod 57 by a dimple 58 which is made after rearward insertion of the control valve member against a step 59 by inserting a suitable tool through a radial port 60 providing communication between rear booster chamber 61 and the poppet valve assembly 12.The valve control member 55 and the valve body 11 are provided in conventional manner with co-axial rearwardly facing annular valve seats for engagement by the front face of poppet valve member 62. The operation of poppet valve assembly 12 and of valve control member 44 is conventional, the valve assembly controlling the admission of vacuum or atmospheric air to the rear booster chamber.

Since the head 51 of output rod 38 acts on sleeve 56 through reaction disc 54 the spring 39' is effective to bias the movable wall 9 to a retracted position.

In the construction of Figure 1 the retracted position of the primary piston is determined entirely by the dimensions of the master cylinder assembly and is unaffected by tolerances in the booster assembly 2. The retracted position of the primary piston relative to the master cylinder housing 21 is determined by the location of seal assembly 31, and its location is determined by the position of the step between the counterbore 40 and bore 25. The axial position of the step relative to the recuperation port 49 can be accurately controlled in manufacture without the need to provide adjustment means. Also, the axial position of the rear face of the master cylinder housing flange 20 can be accurately controlled in manufacture to determine precisely the retracted position of the movable wall 9 relative to the booster housing 3.

Ports 63 and 64 provide permanent communication between the respective hydraulic reservoirs and the spaces behind the primary seals 39 and 33 respectively in the conventional manner.

It should be noted that the spring 39' is entirely housed within the booster housing 3 so that the length of the master cylinder assembly 1 does not need to accommodate the spring and may therefore be made shorter than normal. Also, the spring 32 between the master cylinder pistons does not have to bear the force of the return spring for the primary piston 23 and thus may be made of less strength than normal to improve the pressure balance in use between the primary and secondary pressure spaces 26 and 27 through shuttling of the secondary piston 24.

The coating of rubber on the front end of the cup member 47 prevents an unpleasant noise being produced when the primary piston is retracted quickly against the member 47.

In Figures 3 and 4 reference numerals corresponding to those of Figures 1 and 2 have been applied to corresponding parts. The boosted master cylinder assembly of Figure 3 differs from that of Figure 1 in its use of a tandem booster assembly 2, the booster assembly 2 being connected to a tandem master cylinder assembly 1 having an arrangement of pistons identical to that of Figure 1 but having a modified arrangement of the recuperation ports 49 and 34 and of the ports 63 and 64. The seal assembly 31 of the master cylinder assembly 1 is identical to that of the Figure 1 embodiment and performs an identical function. The greater length of the tandem booster assembly 2 enables substantially half of the length of the master cylinder housing 21 to be accommodated within the booster having 3, and the flange 20 on the master cylinder housing for engagement with the front face of the booster housing is consequently located approximately halfway along the master cylinder housing 21.

The front and rear housing shells 4 and 5 of the booster assembly 2, the arrangement of the valve assembly 12, and that of the rear movable wall 9a are similar to those of the booster assembly of Figure 1, but a substantially cylindrical moulded plastics intermediate housing member 65 provided with an integral partition wall 66 is clamped between shells 4 and 5 to define with a front movable wall 9b the additional pressure chambers required bythetan- dem booster. Four chambers 67,68,69 and 70 are defined with the booster housing 3. As compared with the Figure 1 construction the ties 17 each comprise a tubular central portion 71 provided with axially spaced transverse drillings 72 and 73.The drillings 72 and 73 of the tie shown in Figure 3 provide permanent fluid communication between the chambers 68 and 70, and the corresponding drillings 72 and 73 provided in the other tie, not shown in Figure 3, are axially staggered to provide permanent fluid communication between chambers 67 and 69, the position of these holes being indicated in dotted outline in the tie shown in Figure 3. The use of tubular ties to provide communication between the booster chambers is the subject of our U.K. Patent Application No.20992/78.

In the assembly of Figure 3 the return spring 39' is effective to retract both movable walls 9a and 9b together with both pistons 23 and 24. The front end of spring 39' in this case abuts against the front housing shell 4, and its rear end abuts against the front end of abutment member 50 which, unlike Figure 1, is of generally elongate cup shape comprising a portion 74 of constant diameter slidably sealed to the partition wall 66 by annular seal 75, and a doubly-stepped mouth portion 76 comprising two axially spaced steps 77 and 78, of which step 77 is engaged by spring 39. Step 77 permanently abuts the front diaphragm support plate 1 Ob to provide the return force for the front movable wall 9b, and the force for returning the rear movable wall 9a is transmitted by way of the portion 74, base 79 of member 50, head 51 and reaction disc 54.

It will be noted that the two movable walls 9a and 9b are not connected together by a rigid connection as is usual with tandem servo boosters but that the reaction disc 54 is interposed between the two movable walls. This is the subject of our U.K. Patent Application No. of even date entitled 'Servo Boosters for Vehicle Braking Systems'.

A washer 80 mounted on output rod 38 between threaded head 50 and the main part of rod 38 provides an abutment for transmitting a forward force generated by the front movable wall 9b directly to the output rod 38.

In a modification both ties 17 are arranged to provide fluid communication between chambers 67 and 69, and the rear end of the portion 74 is provided with radial supports to provide permanent fluid communication between chambers 68 and 70.

As shown in Figure 4, the front booster housing shell 4 is integrally moulded with the body 81 of a double-chamber hydraulic reservoir for the tandem master cylinder assembly. End caps 82 for the two reservoir chambers 83 and 84 are independently moulded parts, preferably formed from transparent plastics material, which are subsequently welded at 85 to the body 81 to complete the reservoir. As is conventional, the reservoir is provided with a single filler opening 86 closed by cap 87 provided with a relief opening, and each chamber 83,84 is provided with a respective fluid level sensor 88,89 respectively for detecting loss of hydraulic fluid from either chamber through a leak in the respective hydraulic brake circuit. An apertured partition wall 90 extending longitudinally of the master cylinder assembly separates the two chambers.

Since the chambers 83 and 84 are disposed transversely with respect to the longitudinal axis of the booster assembly, and since that axis is normally arranged to extend longitudinally of the vehicle there will be little transfer of hydraulic fluid between the chambers 83 and 84 through the aperture in partition wall 90 during vehicle braking. This is an advantage over the usual arrangement where the two chambers of the hydraulic reservoir are disposed longitudinally of each other and the consequent temporary fluid transfer due to vehicle deceleration forces can lead to unnecessary actuation of the driver warning system connected to the sensors 88 and 89.

Chambers 83 and 84 are provided with outlet ports 91 and 92 respectively which are connected by flexible tubes 93 and oppositely directed elbows 94 to respective inclined passages 96 and 95 communicating respectively with recuperation ports 34 and 49.

Figure 4 also shows that the two ties are disposed in a plane inclined at approximately 20 to the horizontal to provide some resistance to bending of the booster in the cantilever modes.

Claims

1. A boosted master cylinder assembly of the kind set forth in which the output member of the servo booster assembly is connected to the master cylinder piston, and resilient means is arranged to urge both the output member and the master cylinder piston rearwardly, the retracted position of the piston and output member being determined by a stop carried by the master cylinder housing.

2. An assembly as claimed in Claim 1 ifl which the resilient means comprises a spring located within the booster housing.

3. An assembly as claimed in Claim 2 in which the front end of the spring is arranged to bear upon the booster housing.

4. An assembly as claimed in Claim 2 in which the front end of the spring bears upon the rear end of the master cylinder housing.

5. An assembly as claimed in any of the preceding claims in which the stop comprises an annular seal assembly retained against axial movement in the bore of the master cylinder housing and adapted to be engaged by a rearwardlyfacing step on the master cylinder piston.

6. An assembly as claimed in Claim 5 in which a further annular seal is carried bythe piston for sliding engagement with the cylinder bore, said further seal co-operates with a recuperation port in the wall of the master cylinder housing to define the recuperation valve, and said seal assembly effects a seal with a reduced diameter portion of the piston extending rearwardly from the step, and further port in the cylinder wall located between the recuperation port and the seal assembly is permanently connected to the hydraulic reservoir.

7. An assembly as claimed in Claim 5 or Claim 6 in which the front portion of the seal assembly that is engaged by the step on the piston is formed of elas tomeric material.

8. An assembly as claimed in any of Claims 5 to 7 in which the seal assembly is retained against rearward axial movement by a substantially U-shaped wire clip the arms of which are received in transverse bores in the master cylinder housing that intersect said bore.

9. An assembly as claimed in any of the preceding claims in which the master cylinder is a tandem master cylinder having primary and secondary pistons, the secondary piston being that piston remote from the booster output member which is connected to the primary piston, and comprising a lost-motion connection between the pistons and further resilient means biassing the pistons apart.

10. A boosted master cylinder assembly of the kind set forth in which a substantial portion of the length of the master cylinder housing is accommodated within the servo booster housing, and a fluid reservoir for the master cylinder comprises a body which is integrally formed with one wall of the servo booster housing.

11. A boosted master cylinder assembly substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

12. A boosted master cylinder assembly substantially as described with reference to Figures 3 and 4 of the accompanying drawings.