GB2086512A - Venting and replenishing vehicle hydraulic master cylinders - Google Patents

Abstract

In an hydraulic master cylinder (Figure 1) a recuperation connection (13), through which a pressure space (12) communicates with a reservoir for fluid when a piston (4) is in a retracted position, is formed by a plug (15) of porous material, suitably sintered metal. The plug (15) is housed in an opening (16) in the wall of a body (1) and the peripheral edge of the inner end of the plug (15) conforms to the curvature of a bore (2) in the body (1) at the inner end of the opening (16) and in which the piston (4) works. In a further embodiment (Figure 4, not shown) the porous plug (13) spans the piston seal (9) in such a way that the single plug performs the function of vent port and recuperation port. <IMAGE>

Description

SPECIFICATION Improvements in hydraulic master cylinders for vehicle hydraulic systems This invention relates to hydraulic master cylinders for vehicle clutch or brake systems of the kind in which a pressure space is defined in a bore of a body in advance of a piston which works in the bore, and the body is provided with an outlet port leading from the pressure space, and a recuperation connection through which the pressure space is in communication with a reservoir for fluid at least when the piston is in an inoperative retracted position, initial move mentofthe piston in the bore in an operating direction closing the recuperation connection to isolate the pressure space from the reservoir, and subsequent movement of the piston in the same direction being operative to pressurise fluid in the pressure space for supply to the outlet port.

In some known master cylinders of the kind set forth the recuperation connection comprises a recuperation port in the wall of the body which is countersunk at its inner end. When the piston is moved in the operating direction, a seal carried by the piston, suitably a lip seal, closes the recuperation port to isolate the pressure space from the reservoir.

Theoretically, when the piston is operated, the seal must be moved to close the recuperation port before fluid in the pressure space can be pressurised. In practice, however, very fast movement of the piston in the operating direction can cause pressure to be generated in the pressure space, before the recuperatrion port is completely closed. The seal lip is, at this time, unsupported and is liable to be torn when subjected to such pressure. The axial spacing of the recuperation port from the seal lip has to be very carefully controlled in production to limit stroke loss to an acceptable value. This involves not only accurate and expensive control of the machining of the body, but also of the piston and the seal carried by the piston, which together comprise a piston sub-assembly.Typically, in order to obtain a recuperation port which is countersunk at its inner end, the port is punched through the wall of the body from the inside, using a tool which "coins" the required amount of countersinking. This operation has to be performed as a precision operation at a very late stage in the machining sequence of the body and, at that stage, any scrap represents a substantial financial loss because of the cost of the body itself plus the cost of machining. Also, in such known master cylinders, in order to avoid an hydraulic pressure lock, a second vent port of substantial diameter is provided in the body to place the reservoir in communication with a portion the bore to the rear of the seal, for all positions of the piston in the bore.

In most master cylinders of the said known type referred to above the recuperation port provides direct, unrestricted, communication between the reservoir and the pressure space. However, in the master cylinder described in US Patent No 3199 299, a plug of porous material is disposed in the wall of the body, between the recuperation port andthe reservoir. This material of the plug permits fluid to pass through it at low outlet pressures, but will restrict pressure loss during such flow so that when high pressure exists in the master cylinder a pressure build up will occur on that side.Although this enables pressure to be built up quickly in the master cylinder, such pressure still acts on a lip seal carried by the-piston and may still cause the lip to tear since the lip seal is, at this time, unsupported due to the presence of the recuperation port, as described above.

According to our invention, in an hydraulic master cylinder of the kind set forth the recuperation connection comprises a plug of porous material which is housed in an opening in the wall of the body with the peripheral edge of the inner end of the plug conforming to the curvature of the bore at the inner end of the opening, and the outer end of the plug being in direct contact with hydraulic fluid in the reservoir.

The porous plug allows fluid to pass through it at low pressures but restricts pressure loss during such flow so that, when the master cylinder is operated, fluid pressure can build-up quickly in the pressure space even before the leading edge of a seal carried by the piston has traversed the portion of-the area of the plug which is disposed in front of the piston when the piston is in its retracted position.

The elimination of a recuperation port reduced wear of the seal carried by the piston and the plug provides a support forthe seal in all positions in which the seal overlies the inner end of the plug.

Since the inner end of the plug is flushed with the bore, the seal can ride over it without damage and is supported by it to prevent damage when the seal is subjected to pressure in the pressure space, before the leading edge of the seal has traversed the plug as described above.

Supporting the seal by the porous plug reduces wear of the seal and, in particular, serves to reduce wear should the piston be forced back towards the retracted position in service when subjected to pressure, for example due to an increase in the pressure in the pressure space which may occur when the master cylinder is installed in an anti-skid- braking system.

Since the porous plug itself replaces a conventional recuperation port, a precision operation to form the recuperation port is replaced by a simple noncritical operation to form the opening which receives the plug.

Preferably the plug is sufficiently large to provide a connection between the portions of the bore on both sides of the seal and the reservoir when the piston is in its inoperative retracted position.

This eliminates the need for providing a second vent port in the wall of the body since the plug will provide communication between the reservoir and the portion of the bore to the rear of the seal. In consequence, elimination of the second vent port simplifies manufacture and reduces the length of the piston since the seal can stroke, at least partially, over the plug.

Conveniently the plug is constructed from a sintered material, suitably metal.

Preferably the porosity of the sintered material is substantially 20%.

Preferably the central portion of the inner end of the plug is at concave outline of which the radius of curvature is smaller than that of the bore of the body with the peripheral edge of the inner end conforming to the profile of the edge of the wall of the body which surrounds the opening in which the plug is received.

The plug may be pressed with interference into the opening in the wall of the body such that the cylindrically radiused peripheral edge stands proud of the pre-finished bore of the body. During subsequentfinish machining, suitably ironing or rolling, only the peripheral edge is affected, thereby leaving substantially all the pores open.

Preferably the plug is of circular cross-section and, to ensure that is is corrected orientated in the opening, the outer end of the plug is provided with a screw-driver slot by means of which the plug can be orientated in the opening to ensure correct positioning. For example the screw-driver slot can be aligned with the longitudinal axis of the bore.

The slot may be provided in an enlarged head at the outer end of the plug. In such a construction the enlarged head acts as a filter and enables a maximum surface area of material to be exposed to the fluid in the reservoir.

When the master cylinder is of tandem construction having a primary piston working in the bore and a secondary piston working in a portion of the bore which is disposed between the primary piston and a closed end of the body, at least both recuperation connections between a reservoir and a primary pressure space between the pistons, and a reservoir and a secondary pressure space between the secondary piston and the closed end, are each formed by a plug of porous material.

Two embodiments of our invention are illustrated in the accompanying drawing in which: Figure 1 is a longitudinal section through an hydraulic master cylinder; Figure 2 is a view on an enlarged scale of a plug suitable for use in the master cylinder of Figure 1, with the view taken along the main longitudinal axis of the body; Figure 3 is a view of the plug in a duration normal to view of Figure 2; and Figure 4 is a longtitudinal section through a tandem master cylinder.

The master cylinder illustrated in Figure 1 of the accompanying drawings comprises a body 1 having a longitudinally extending bore 2 which is closed at one end by a closure wall 3 and in which works a pedal-operated piston 4. The pedal acts on the piston 4 through a piston-rod 5 which works through axially spaced seals 6 for the opposite open end of the bore 2.

A compression return spring 7 acts on the inner end of the piston 4, normally to hold it in an inoperative retracted position in engagement with a stop ring 8.

A lip seal 9 is mounted on a reduced diameter portion at the inner end of the piston 4, and the spring 7 abuts against a retainer 10 on the inner end of the piston 4 so that a rearwardly directed finger 11 on the retainer 10 acts as a stop to limit movement of the seal 9 relatively away from the piston 4.

A pressure space 12 is defined in the bore 2 between the seal 9 and the closure wall 3. Normally the pressure space 12 is in communication through a recuperation connection 13 with an enlarged outlet 14formed in a radial projection on the wall of the body 1 and with which a reservoirforfluid (not shown) is in open communication. An outlet port (not shown) connects the pressure space 12 at all times to the clutch or the brake or brakes.

The recuperation connection 13 comprises a plug 15 of porous material, suitably sintered metal, which is housed in a radial opening 16 extending through the wall of the body 1. The plug 15 permits fluid and air to pass through it at low flow rates without substantial pressure drop. Return flows to the reservoir, caused for instance by volumetric expansion of heated brake fluid, can take place over long periods, for example up to one minute. However, when the piston 4 is stroked at reasonable speeds the restriction offered by the plug 15 is such that substantially no fluid is lost through it to the reservoir during the extremely-short period of time it takes for the seal 9 to traverse the plug 15 completely. The whole piston stroke remains effective to displace fluid to the brakes.

Test have shown that good results are obtained using a plug of sintered material having a porosity of 20%.

The plug 15 may be of the outline shown in Figures 2 and 3 with a stem 17 of circular crosssection extending through the opening 16 and carried by an enlarged head 18.

The inner end of the stem 17 is provided with a concave recess 19 of a radius r smaller than that of the radius R of the bore 2 so that when the bore 2 is finished, suitably by honing or rolling, only a small area of the stem 17 at the peripheral edge of its inner end, which in any case conforms closely to the profile at the inner end of the opening 16, will be affected. This ensures that substantially all the pores remain open.

The head 18 is provided with a screw-driver slot which, when aligned with the main longitudinal axis of the bore 2, determines the correct orientation of the plug 15 in the opening 16.

The head 18 acts as afilterforfluid.

In the inoperative retracted position shown the seal 9 is located at the rear of the plug 15 and a vent port 21 is in the wall of the body 1 provides -unrestricted communication at all times between the reservoir and a portion of the bore 2 which is disposed rearwardly of the seal 9 to prevent the formation of a pressure lock when the piston 4 is advanced in the bore.

In operation movement of the piston 4 in an operating direction pressurises fluid in the pressure space 12 for supply to the clutch or brake and pressurisation occurs even before the seal 9 has completely covered the plug 15 to isolate the reservoir fully from the pressure space. During this movement the seal 9 is supported fully by the plug 15 and moves smoothly over the inner end of the stem 17 since at least a region terminating at its peripheral edge is flush with the cylindrical wall of the bore 2.

By increasing the size of the plug 15, both sides of the seal 9 can communicate with the reservoir when the piston 4 is in the retracted position. This is possible since the lug 15 supports the seal 9. This has the advantage that the vent port 21 can be omitted, thereby reducing the stroke and the effective length of the master cylinder.

A tandem master cylinder is illustrated in Figure 4 of the drawings. In this master cylinder a secondary piston 30 is disposed between the piston 4 and the closure 3, one of the seals 6 is carried by the rear end of the piston 4, the spring 7 acts between the piston 4 and an extensible abutment 31 engagable with the secondary piston 30, and a second spring 32 is housed in a secondary pressure space 33 between the piston 30 and the closure 3, the secondary pressure space 33 communicating with an outlet port 34. The spring 33 abuts against a retainer 35, and the retainer 35 has a rearwardly directed finger 36 for limiting relative movement between the piston 30 and a lip seal 37 mounted on an extension 38 at the forward end of the piston 30. A second lip seal 39 is also mounted on an extension 40 at the rear end of the piston 30.

The tandem master cylinder of Figure 4 incorporates a further modification of the invention. The plug 15 is increased in size to extend both forwardly and rearwardly of the position assumed by the forward piston seal 9 at least when the piston 4 is in its retracted position. This is achieved by constructing the plug 15 to be of a diameter greater than the axial length of the seal 9 so that, in the retracted position shown, portions of the bore 2 on both sides of the seal 9 are in communication with the reservoir. Similarly a similar plug 41 of porous material which forms a recuperation connection between the secondary pressure space 33 and an outlet 42 to the reservoir is of sufficient diameter to provide communication between the reservoir and both sides of the seal 37 when the secondary piston 30 is the retracted position shown.

Increasing the relative size of the plug 15 (and 37) eliminates the need for a second vent port (or ports), corresponding to the vent port 21 in the master cylinder of Figure 1, thereby simplifying machining.

It also facilitates the shortening of both pistons 4 and 30 since each rearward seal 6 and 39 can stroke, at least partially, over a respective plus 15,41 whereas, in a conventional design, a seal could not be permitted to stroke over a vent port. Since the effective lengths of both pistons 4 and 30 can be relatively shorter, the master cylinder itself can be shortened, but the stroke will remain the same. This is possible only because the seals 6 and 39 can touch parts of the plugs 15 and 41 which are equivalent to the vent ports, without damage to the seals 6 and 39 themselves.

Upon operation of the pedal both pistons 4 and 30 are advanced in the bore 2 to pressurise the fluid in the pressure spaces 12 and 33, each as described above with reference to the piston 4 of Figure 1.

The construction and operation of the master cylinder of Figure 4 is otherwise the same as Figure 1 and corresponding reference numerals have been applied to corresponding parts.

Claims (14)

1. An hydraylic master cylinder of the kind set forth in which the recuperation connection comprises a plug of porous material which is housed in an opening in the wall of the body with the peripheral edge of the inner end of the plug conforming to the curvature of the bore at the inner end of the opening, and the outer end of the plug being in direct contact with hydraulic fluid in the reservoir.

2. A master cylinder as claimed in Claim 1, in which the plug is sufficiently large to provide a connection between portions of the bore on both sides of the seal and the reservoir when at least the piston is in its inoperative retracted position.

3. A master cylinder as claimed in Claim 1 or Claim 2, in which the plug is constructed from a sintered material.

4. A master cylinder as claimed in Claim 3, in which the material has a porosity of substantially 20%.

5. A master cylinder as claimed in any preceding claim in which the porous material comprises a sintered metal.

6. A master cylinder as claimed in any preceding claim in which the central portion of the inner end of the plug is at concave outline of which the radius of curvature is smaller than that of the bore of the body with the peripheral edge of the inner end conforming to the profile of the edge of the wall of the body which surrounds the opening in which the plug is received.

7. A master cylinder as claimed in Claim 6, in which the plug is pressed with interference into the opening in the wall of the body such that the cylindrically radiused peripheral edge stands proud of the prefinished bore of the body, whereby during subsequent finish machining only the peripheral edge is affected, thereby leaving substantially all the pores open.

8. A master cylinder as claimed in any preceding claim in which the plug is of circular cross-section and the outer end of the plug is provided with a screw-driver slot by means of which the plug can be orientated in the opening to ensure correct positioning.

9. A master cylinder as claimed in Claim 8, in which the slot is aligned with the longitudinal axis of the bore when the plug is correctly orientated.

10. A master cylinder as claimed in Claim 8 or Claim 9, in which the slot is provided in an enlarged head at the outer end of the plug, the enlarged head acting as a filter and enabling a maximum surface area of material to be exposed to the fluid in the reservoir.

11. A master cylinder as claimed in any preceding claim in which the master cylinder is of tandem construction having a primary piston working in the bore and a secondary piston working in a portion of the bore which is disposed between the primary piston and a closed end of the body, and both recuperation connections between a reservoir and a primary pressure space between the pistons, and between a reservoir and a secondary pressure space between the secondary piston and the closed end, are each formed by a plug of porous material.

12. An hydraulic master cylinder substantially as described with reference to and as illustrated in Figure 1 ofthe accompanying drawings.

13. An hydraulic master cylinder substantially as described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.

14. An hydraulic master cylinder substantially as described with reference to and as illustrated in Figure 4 of the accompanying drawings.