GB2266752A - Master cylinder - Google Patents

Abstract

A tandem master cylinder has a body (1) of which an internal bore (2) is divided into a pair of pressure chambers (A, B) by a piston (16) which moves to pressurise the chamber (A), and a further piston (40) connected to a force input rod (41) moves to pressurise the chamber (B). Each piston (16, 40) has a respective pressure seal (21, 43), stationary in the body, which, when the master cylinder is not actuated, permits flow between the associated chamber and low pressure fluid source (7, 8) via a port (23, 46) in the piston, and respective fluid recuperation paths are provided for the chambers (A, B). Each recuperation path extends between the low pressure fluid source (7, 8) and its associated one of the chambers, is controlled by a respective valve (32A, 33A) and by-passes its associated seal. In alternative embodiments only one of the pistons has a port and seal permitting flow between the associated chamber and the low pressure fluid source, and a valve controlled recuperation path. <IMAGE>

Description

MASTER CYLINDER This invention relates to a master cylinder of the general kind in which a body defines a pressure cylinder within which a piston is slidable in order to pressurise a fluid for actuation of a brake to which the master cylinder is connected in use, the piston being provided in a wall thereof with a port which co-operates with a surrounding seal held stationary by the body, such that, with the piston in its non-operative position, the port is clear of the seal and the pressure cylinder is ventilated via the port to a low pressure fluid source, and as the piston is moved away from said position the port is covered by the seal to permit pressurisation of the pressure cylinder for brake application.

In a master cylinder of the aforesaid general kind, it is necessary to provide means to enable recuperation to occur by flow of fluid from a fluid source into the cylinder when return movement of the piston creates a low pressure condition therein. Because the pressure seal is tensioned by passage of the piston therethrough, it is difficult for fluid to displace the seal from the piston in order to recuperate the cylinder and the tendency is for the fluid to pass around the outer periphery of the seal in order to gain access to the cylinder via a recuperation port. Since the seal is stationary, its outer surface is not wiped clean by movement of the piston and there is a resultant risk of debris accumulating behind the seal and leading to possible fluid leakage and other problems.

An object of the present invention is to provide a master cylinder of the aforesaid kind in which the aforesaid problem is avoided.

According to the present invention, a master cylinder of the aforesaid kind is provided with a valvecontrolled recuperation path between the pressure cylinder and a fluid source by-passing the pressure seal.

With such an arrangement, recuperation can take place without passage of fluid past the pressure seal and the aforesaid problem found in some conventional cylinders is thereby overcome.

In a preferred arrangement, the recuperation path includes a passage providing communication between the pressure cylinder and a valve chamber within which a valve element co-acts with a port providing access to the low pressure fluid source.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a view, in longitudinal cross-section, of one form of the master cylinder of the invention, and Figures 2 and 3 are views similar to Figure 1 illustrating respectively alternative embodiments of the invention.

Referring to Figure 1, the master cylinder illustrated therein is a tandem master cylinder having a body 1 with a cylindrical through bore 2, one end of which is closed by an end member 3 secured thereto by suitable means (not shown) such as bolts, a seal 4 being provided between the end member and body in order to ensure a fluid-tight structure. The body is provided with a radially extending mounting flange 5, which would normally be provided with holes (not shown) through which fixing bolts may be passed to secure the master cylinder to a fixed support, such as a vehicle bulk head. The to a fixed support, such as a vehicle bulk head.The body 1 has an upstanding boss 6 which forms a pair of inlet connections 7, 8 arranged longitudinally of the cylinder and adapted to receive, in use, complementary connections of a fluid reservoir (not shown) inserted into the connections 7, 8 in fluid tight manner.

The bore 2 is stepped internally to provide a shoulder 9 in an intermediate region thereof and this is engaged by a radial flange 10 of an inner cylindrical member 11 which engages the internal surface of the cylinder and extends to a region adjacent the rear end portion of the latter. Disposed against the forward end surface of the flange 10 is a first annular bearing member 12 and a second annular bearing member 13 is arranged between the member 12 and the annular end surface 14 of the end member 3. The bearing member 13 is dimensioned to be spaced from the surface 14 and held in spaced relationship therewith by a resilient wavy washer or similar device 15, the resulting space forming a passage 17, for the purpose to be described.The internal diameter of the bearing members 12, 13 is smaller than that of the bore 2 and the internal surfaces of the bearing members support a first piston 16 for axial sliding movement within the bore. The retracted position of the piston 16 is determined by engagement of a radial flange 17A thereof against the forward facing end surface of the bearing member 13 and the piston is urged into its retracted position by a spring 18 acting between the end member 3 and an internal end surface 19 of the piston which, in the embodiment described, is hollow over approximately half of its length and houses a corresponding part of the length of the spring 18.

Both of the members 12 and 13 are sealed against the internal cylinder wall and the member 12 carries a seal 20 which engages the outer surface of the piston 16 and effectively divides the bore 2 into pressure chambers A and B, the seal 20 providing the necessary isolation between these chambers.

The bearing member 13 carries a further seal 21 housed therein and this engages the piston 16 forwardly of the seal 20 in the direction towards the end member 3, by way of a lip 22 so as to serve as a pressure seal.

The piston 16 is provided with a plurality of radial recuperation ports 23 formed through the wall of the hollow part of the piston and which, in the rest position of the piston as illustrated, are disposed rearwardly of the lip 22 and communicate with a ventilation path formed by an unsealed internal cylindrical surface portion 24 of the member 12, a radial passage 25 in that member and a passage 26 formed through the boss 6 and leading into the reservoir connection 7 and thence, in use, to the interior of the reservoir.

The connectors 7 and 8 contain respective identical valve bodies 30, 31, each having a longitudinal passage 30A, 31A which communicates with the respective connection 7, 8, by a respective radial passage 30B, 31B.

Valve chambers 32, 33 are formed beneath the respective valve bodies 30, 31 and each contains a valve element 32A, 33A which is normally urged lightly into engagement against the underside of the corresponding valve body by a light spring 32B, 33B, the valve members, when in such position, isolating the passages 30A, 31A from the valve chambers 32, 33. Each of the chambers 32, 33 is provided with an outlet port 34, 35, of which the port 34 communicates with the passage 17.

A further piston 40 moves within the chamber B and is sealed against the cylinder body 1 by way of a seal 42 and is engaged around its outer surface by a lip 43 of a pressure seal 44 retained between the inner member 11 and a retainer 45. The piston 40 is provided with ventilation holes 46 arranged in relation to the seal in similar manner to the corresponding holes 23 of the piston 16, a ventilation path being provided internally of the retainer 45 and by way of a radial passage 47, an axial passage 48 in the body 1 and a further radial passage 49 through the boss 6 communicating with the connection 8 and thence, in use, with the interior of the reservoir. The piston 40 has an extension 41, of which the outer surface engages the seals 42, 43 during inward movement of the piston.In the illustrated embodiment, the extension 41 is hollow and receives a force input rod (not shown) for actuating the cylinder from the driver's pedal for example.

The piston 40 is hollow and houses a cup 50 which rests against the inner end of the piston and provides a radial flange 51 which acts as an abutment for one end of a return spring 52, of which the other end engages the piston 16. A bolt 53 extends through an opening in the inner end 54 of the cup and is screwed into a corresponding bore in the facing end of the piston 16. A head 55 of the bolt lies against the inner surface of the wall 54 and provides an abutment setting the retracted position of the piston 40. The inner member 11 forms a passage 1lA providing communication between the chamber B and valve chamber 33 via the port 35. Slots 56 in the flange end of the cup 50 encourage trapped air in the cup to exit at the periphery thereof to facilitate bleeding of the cylinder.

With the components of the master cylinder in their illustrated positions, the chambers A and B are in communication with the reservoir via the ventilation holes 23, 46, enabling any changes in pressure occurring, for example, as a result of fluid expansion, to be equalised by flow of fluid in the appropriate direction between the chambers and reservoir. When actuating force is applied by the input rod to the piston extension 41, the piston 40 moves forwardly and this movement is communicated by the relatively stiff spring 52 to the piston 16 so that movement of both pistons is initiated simultaneously.Once the openings 23, 46 have moved on to the seal lips 42, 43, continued movement of the pistons results in a build-up of pressure in the chambers A and B and pressure fluid is expelled via the usual pressure outlets (not shown) of these chambers for actuation of one or more brakes to which the master cylinder is connected, in use. High pressure is applied via the passage 17 to the valve chamber 32 and reinforces the closure of the passage 30A by the valve element 32A.

Since the chamber 32 is at system working pressure, it is convenient to provide the pressure fluid outlet to the brake from this chamber, although this is not visible in the drawings. A similar action occurs in relation to the valve element 33A as a result of pressure fluid from the chamber B passing through passages 11A and 35 and acting on the valve element 33A. Again, the pressure outlet for the chamber B may convenivently be from the chamber 33.

When the input force is removed, the pistons are initially retracted somewhat by the springs 18 and 52 and this can cause a negative pressure to arise in the chambers A and B. When this happens, the valve elements 32A, 33A are urged away from their illustrated sealing positions around the passages 30A, 31A, thereby establishing communication between the respective reservoir chambers and the cylinder chambers A, B to permit a recuperating flow of fluid into the cylinders to take place via respective passages 17 and 11A in order to neutralise the negative pressure. It will be seen that, because the valve-controlled recuperation paths by-pass the static pressure seals 21, 43, there is no tendency for fluid to pass around these seals during the recuperation process, so that the risk of debris accumulating behind the pressure seals is substantially avoided.

In the alternative embodiment illustrated in Figure 2j the cylinder body 1 is again divided by a piston 60 into chambers A and B and the ventilation and recuperation arrangements for the chamber A, as well as the isolation between the two chambers is identical to that described in Figure 1, with the valve-controlled recuperation path 17 for the chamber A bypassing a fixed seal 22, providing the advantage enumerated previously.

Moreover, the connection between the piston 60 and the primary piston 61 is again effected by means of a bolt 62, but the head 63 thereof cooperates, in this embodiment, with the piston 61 itself, rather than with a separate component. The principal difference in this embodiment is that the cylindrical member 11 of Figure 1 is omitted and the seal 64 of the piston 61 is carried by the piston and moves therewith along the cylinder, being retained in position thereon by a seal retainer 65 held in position by the piston return spring 66. The recuperation hole 67 for the chamber B is formed directly through the wall of the body 1 and the seal 64 slides against the internal wall proper of the body, for which purpose, this piston is of slightly greater diameter than the secondary piston 60.Ventilation of the chamber B occurs via the port 67 when the piston is in its illustrated retracted position and recuperation occurs, upon piston retraction after actuation, via a port 68, fluid leaking past the seal 64 for this purpose. Since the seal 64 moves with the piston, and is therefore continuously wiped, recuperation past the seal should not lead to debris accumulation. Operation of chamber A is identical to that previously described.

In the embodiment illustrated in Figure 3, the primary chamber B and its associated components are substantially the same as those for the corresponding chamber in Figure 1 including a fixed pressure seal 44, but recuperation occurs via a path by-passing the seal 44 and including a radial port 70 formed through the wall of the inner member 71 leading into an axial passage 72 communicating with the recuperation valve. The secondary piston has a lip seal 73 mounted for movement therewith and operating, in conventional manner, in conjunction with a radial port 74 of the body to permit ventilation in its illustrated retracted position. Recuperation takes place via a port 75 behind the seal and fluid flows around the seal into the chamber A. Since the seal 73 is movable, the problem of debris build-up does not occur.

The Figure 1 embodiment is particularly suitable for use with anti-lock braking systems (ABS), in which the action of the modulator pump may force the master cylinder pistons back to their ventilating positions with an input force still applied by the driver to the input rod 41. In this event, fluid pressure in the radial ventilation holes 23, 46 balances the same pressure acting on the associated seals and seal damage is thereby avoided. Advantages can also accrue if the master cylinder is used with a traction control system in which a solenoid valve connects the master cylinder to the modulator pump inlet and isolates the brake. The large recuperation passages 17, l1A enable the pump readily to draw fluid from the reservoir via the recuperation valve and to return the fluid upon wheel recovery and may contribute to a more rapid response time than is possible in some conventional arrangements.

Although the invention has been described exclusively in connection with a tandem master cylinder, it may equally be applied to a master cylinder having only a single pressure cylinder and piston.

Claims (11)

1. A master cylinder comprising a body defining a cylinder within which a piston is slidable in order to pressurise a fluid for actuation of a brake to which the master cylinder is connected in use, the piston being provided in a wall thereof with a port which co-operates with a surrounding seal held stationary by the body, such that, with the piston in its non-operative position, the port is clear of the seal and the pressure cylinder is ventilated via the port to a low pressure fluid source, and as the piston is moved away from said position the port is covered by the seal to permit pressurisation of the pressure cylinder for brake application, and a valvecontrolled recuperation path between the pressure cylinder and a fluid source by-passing the pressure seal.

2. A master cylinder according to Claim 1, wherein the recuperation path includes a passage providing communication between the pressure cylinder and a valve chamber within which a valve element co-acts with a port providing access to the low pressure fluid source.

3. A master cylinder according to Claim 1 or Claim 2, wherein the stationary seal is carried by an annular body fixed to the cylinder body.

4. A master cylinder according to Claim 3, wherein the recuperation path includes a passage formed by a space between facing surfaces of the annular body and cylinder body.

5. A master cylinder according to Claim 4, wherein the space is maintained by a resilient device urging apart the annular and cylindrical bodies.

6. A master cylinder according to any one of the preceding claims, wherein a ventilation path is provided between the port in the piston wall and the low pressure source, the ventilation path being at the opposite side of the stationary seal from the recuperation path.

7. A master cylinder according to Claim 3, wherein an end of the cylinder is closed by a closure member secured to the remaining part of the body and the recuperation path includes a passage formed by a space between facing surfaces of the annular body and closure member.

8. A master cylinder according to any one of the preceding claims in the form of a tandem master cylinder having a plurality of co-axially arranged pressure cylinders containing respective pistons, at least one piston cooperating with a fixed seal by-passed by a valve-controlled recuperation path.

9. A master cylinder according to Claim 8, wherein the piston of each pressure cylinder cooperates with a respective fixed seal, each seal being by-passed by a respective valve-controlled recuperation path.

10. A master cylinder according to Claim 8, wherein one of the pistons carries a pressure seal in sliding engagement with the cylinder internal wall.

11. A master cylinder substantially as hereinbefore described with reference to Figure 1, Figure 2 or Figure 3 of the accompanying drawings.