GB2123503A - Master cylinders - Google Patents

Abstract

A variable displacement master cylinder includes a first fluid connection (22, 23) between a high volume displacement chamber (15) and a low volume displacement chamber (14) and a second fluid connection (23, 24) between the high volume displacement chamber and a fluid reservoir (17). Each fluid connection includes a one-way valve device (26 and 28) so that after a predetermined piston displacement the valve device (28) in the second fluid connection opens to allow flow from the high volume displacement chamber to the reservoir whilst the valve device (26) in the first fluid connection prevents flow from the low volume displacement chamber to the high volume displacement chamber. Alternative arrangements with relief valves opened at a set piston displacement are shown. <IMAGE>

Description

SPECIFICATION Master cylinders This invention relates to master cylinders and in particular but not exclusively to master cylinders for automobile clutch hydraulic actuation systems.

When a vehicle clutch is operated through a hydraulic master cylinder by the vehicle driver depressing the control pedal, the initial pedal travel takes up various deflections in components in the vehicle before the clutch begins to operate.

These deflections are present for example in the vehicle bulkhead to which the master cylinder is secured, the master cylinder body, slave cylinder body, release forks, the clutch cover, and the diaphragm spring fingers in the case of a diaphragm spring clutch. The initial pedal movement to take out these deflections imparts to the driver a nonlinear 'feel' to the system as the driver begins to control the clutch.

It is the object of the present invention to provide a hydraulic master cylinder for which the initial pedal travel required to take out known deflections is reduced.

Accordingly there is provided a master cylinder having a body with a stepped bore therein having a mating stepped piston reciprocable within the bore and dividing the bore into a smaller diameter low volume displacement chamber, and a larger diameter high volume displacement chamber, a fluid connection connecting the high volume chamber with the low volume chamber and a second fluid connection connecting the high volume chamber with a fluid reservoir, each of the fluid connections having a one way valve device therein so that after a predetermined displacement of the piston the valve device in the second fluid connection opens to allow fluid to flow from the high volume chamber to the reservoir, and the valve device in the first fluid connection prevents fluid from flowing from the low volume chamber into the high volume chamber.

An advantage of this master cylinder is that because the required pedal travel is reduced the lever ratio of the pedal can be increased and thereby reduce the pedal loads applied by the drive during operation of the clutch.

The invention will be described by way of example and with reference to the accompanying drawings in which: Fig. 1 is a longitudinal section through a hydraulic master cylinder according to this invention; Fig. 2 is a section on the line Il-Il of Fig. 1; Fig. 3 is a longitudinal section through another master cylinder also according to this invention; and Fig. 4 is yet another embodiment of the invention.

With reference to Fig. 1 and Fig. 2 of the accompanying drawings, a hydraulic master cylinder body 11 has a stepped bore 12 thereon having a mating stepped piston 1 3 reciprocable within the bore 12. The piston 13 divides the bore up into three chambers, a smaller diameter low volume displacement chamber 14, a larger diameter high volume displacement chamber 15, and a recuperation chamber 1 6. The high volume chamber 1 5 is connected to a fluid reservoir 1 7 by a breather port 1 8 and the recuperation chamber 1 6 is connected to the reservoir 17 by a recuperation port 19.Whilst the reservoir is shown integrally with the master cylinder it could be assembled onto the master cylinder, or remote from the master cylinder and connected to the ports 18 and 19.

The low volume chamber 14 has an outlet port 21 connectable to a hydraulic slave motor and is connected to the high volume chamber 1 5 by a co-axial passageway 22 and diametral passageway 23. The passageway 22 is extended axially along the length of the piston 1 3 and together with a second diametral passageway 24 interconnects the chamber 1 5 and 1 6. For ease of manufacture the bore 22 is a through bore subsequently closed at the second valve end by a plug 20.

The end portion of the passageway 22 adjacent the low volume chamber 14 has a shut off valve 25 therein. The shut off valve 25 has a spring loaded ball 26 that when loaded is biassed to close the passageway 22 against fluid flow from the chamber 14 to the chamber 1 5.

The other end portion of the passageway 22 adjacent to recuperation chamber 16 has a second shut off valve 27 therein which has a spring loaded ball 28 which is biassed to close the passageway 22 to fluid flow from the recuperation chamber 16 to the high volume chamber 1 5. The two balls 26 and 28 are interconnected by a pin 29 of crusiform cross-section (see Fig. 2) which does not interfere with fluid flow along the passageway 22, the pin being slightly longer than the distance between the balls 26 and 28 when both valves are closed so that if one valve is closed the ball of the other is lifted off its valve seat by the pin 29.

The ball 26 of valve 25 is biassed into its closed position by a spring 30 which has a free length which is less than the displacement length of the piston 13 in chamber 14 so that the spring 30 is only compressed after a predetermined axial displacement of the piston. A moulded plastic plate 41 with a raised co-axial spigot 42 thereon provides a guide and support for the spring 30.

The spring 30 is of a greater strength than the spring 32 in the second valve 27 so that when the spring 30 becomes compressed the valve 27 is opened.

The piston 1 3 is biassed towards the mouth 33 of the bore 12 by return springs 35 and 36 in chambers 14 and 15 respectively and is held in the bore by circlip 34 in the mouth 33.

The master cylinder functions as follows:- When an input load 'L' is applied to the piston 13 to operate the clutch, the piston 1 3 is pushed down the bore 12 and fluid is passed from the large volume chamber 15, via passageways 23 and 22, past shut off valve 25, chamber 14, and port 21 into the hydraulic system. The spring 32 ensures that valve 27 is closed and consequently that valve 25 is open.

After sufficient piston travel the spring 30 becomes compressed against the end of the master cylinder bore and closes the valve 25 causing valve 27 to open. Thereafter further inwards displacement of the piston caused fluid to be displaced from chamber 14 into the hydraulic system and fluid from chamber 1 5 is passed via passageways 23, 22 and 24, to the chamber 16 and back into the reservoir 1 7.

When the clutch re-engages seals 38 and 39 in the low volume displacement and high volume displacement chambers 14 and 1 5 respectively collapse allowing fluid to flow into the system via the recuperation chamber 1 6 and open valve 27.

In the at-rest position the hydraulic system can breath through the port 18.

For a given hydraulic system the relative bore sizes and amount of piston displacement before valve 25 closes and valve 27 opens would have to be determined to produce the maximum saving in pedal travel and hence pedal effort.

With reference to Fig. 2, those components which have equivalent components as described for Fig. 1 will be given the same reference numerals. In this master cylinder the stepped piston 13 has a longitudinal diametral slot 51 therein which accommodates a diametral pin 52 which is held secure in place, against the shoulder between the chambers 14 and 15, by a washer 53 and the return spring 36.

A first co-axial passageway 55 in the piston 13 connects the chambers 14 with the slot 51 and hence chamber 1 5. A poppet valve 56 is spring biassed to the closed position and has a stem portion 57 that extends through the passageway 55 for abutment against the pin 52 when the piston 13 is in the at-rest position.

A second co-axial passageway 58 connects the slot 51 with the recuperation chamber 16, and has a second valve 59 therein spring biassed to its closed position. The valve 59 has a stem that extends through the passageway 58 into the slot 51 towards the pin 52.

When the piston 13 is pushed inwards by an operating load 'L' fluid is displaced from the high volume chamber 1 5 through the valve 56 into the hydraulic system. The valve 56 is held open by the abutment of its stem 57 on the pin 52, until the piston 1 3 has been moved sufficiently to allow the stem 57 to separate from the pin and hence the valve to close. At this moment or shortly thereafter the pin 52 is brought into contact with the stem 59 of valve 58 hence connecting the high volume chamber with the reservoir 1 7 via chamber 1 6.

Fig. 4 shows a third embodiment of the invention and again those components that are equivalent to those used in the previous embodiment will have the same reference numerals.

In this master cylinder there is no separate recuperation chamber, the stepped piston 13 dividing the bore into the low volume and high volume displacement chambers 14 and 15 only.

The seal 1 38 is a collapsable seal which allows fluid to flow from chamber 1 5 to chamber 14 so that when the piston 13 is moved inwardly to operate the clutch fluid will be displaced from the high volume displacement chamber 15, around the collapsing seal and into the low volume displacement chamber 14.

This is because on the initial movement the pressure build up in the now closed chamber 1 5 will be faster than in the open-to-system in chamber 14.

After a predetermined piston travel a cam surface 140 on the piston 1 3 opens a spring loaded poppet valve 141 to allow fluid to drain back to the reservoir 1 7. Further inwards movement of the piston then causes fluid to be displaced into the system from chamber 14 only.

On the return stroke a recuperation one way valve 142 in a connecting port 143 between the reservoir and chamber 1 5 allows the hydraulic system to recuperate. The valve 142 operates to prevent flow from the chamber 1 5 to the reservoir 1 7 but permits the reverse flow.

Claims (10)

1. A master cylinder having a body with a stepped bore therein having a mating stepped piston reciprocable within the bore and dividing the bore into a smaller diameter low volume displacement chamber, and a larger diameter high volume displacement chamber, a first fluid connection connecting the high volume chamber with the low volume chamber and a second fluid connection connecting the high volume chamber with a fluid reservoir, each of the fluid connections having a one way valve device therein so that after a predetermined displacement of the piston the valve device in the second fluid connection opens to allow fluid to flow from the high volume chamber to the reservoir, and the valve device in the first fluid connection prevents fluid from flowing from the low volume chamber into the high volume chamber.

2. A master cylinder as claimed in Claim 1, wherein the valve device in the first fluid connection is a collapsable seal in the stepped piston, and which is the pressure seal for the low volume displacement chamber, the seal being collapsable to allow fluid to flow around its periphery from the high volume chamber to the low volume chamber.

3. A master cylinder as claimed in Claim 1 or Claim 2, wherein the valve device in the second fluid connection is a poppet valve which can be opened by a cam surface on the piston to allow fluid flow from the high volume chamber to the fluid reservoir.

4. A master cylinder as claimed in Claim 1, wherein the stepped piston divides the stepped bore into three chambers, the low volume displacement chamber, the high volume displacement chamber and a recuperation chamber, and the first fluid connection is constituted by a first passageway in the piston and the second fluid connection is constituted by a second passageway in the piston connecting the high volume chamber with the recuperation chamber and a port in the body connecting the recuperation chamber with the reservoir, and the two valve devices are located one in each of the passageways in the piston.

5. A master cylinder as claimed in Claim 1, wherein the first and second passageways form a single continuous passageway, and the valve devices are interconnected by a pin located in the continuous passageways so that as the valve device in the first passageway seats the pin is displaced to open the valve in the second passageway.

6. A master cylinder as claimed in Claim 4 and Claim 5, wherein the valve device in the first passageway is closed by a spring which acts to bias the valve closed only after a predetermined inwards displacement of the piston.

7. A master cylinder as claimed in Claim 4, wherein the first and second passageways are each connected to a longitudinal slot in the piston.

8. A master cylinder as claimed in Claim 7, wherein the two valve devices are each operated by abutment with a pin held fast with the cylinder body and accommodated in said slot for movement of the piston.

9. A master cylinder as claimed in Claim 8, wherein the first and second passageways are coaxial and the valve devices each have an elongated stem that extends through its respective passageway to project into the slot for abutment with the pin.

10. A master cylinder substantially as described herein and as illustrated in one of Fig. 1, Fig. 3 or Fig. 4 of the accompanying drawings.