GB2200419A

GB2200419A - Vehicle hydraulic systems

Info

Publication number: GB2200419A
Application number: GB08729469A
Authority: GB
Inventors: Glyn Philip Reginald Farr
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1987-01-09
Filing date: 1987-12-17
Publication date: 1988-08-03
Also published as: GB8729469D0

Abstract

In a vehicle hydraulic clutch or braking system, with a remote booster 5 a master cylinder 63 (1; 51) Figs 1, 2 not shown is operative is response to an input from a pedal to produce a fluid pressure output force to operate at least one fluid pressure operated device 62 (4, 61), and the booster 5 augments the output force from the master cylinder 1. The master cylinder comprises a housing in which one or more pistons 52, 53(2) work to pressurise respective pressure spaces 54, 55(3), the housing also having a control chamber 64 (3; 54) and an hydraulic servo chamber 6 defined behind the pistons. The booster 5 is remote from the master cylinder and is operative to pressurise the servo chamber in response to pressurisation of the control chamber. This arrangement means that the hydraulic circuit containing the booster can be separate from the circuit or circuits containing the device, so that failure of one will not affect the other. The control chamber may comprise the or one of the pressure spaces, (Figs 1, 2 not shown) or be a separate chamber. The booster is preferably of the vacuum-suspended type. <IMAGE>

Description

VEHICLE HYDRAULIC SYSTEMS This invention relates to vehicle hydraulic systems of the kind in which a master cylinder is operative in response to an input force from a pedal to produce a fluid pressure output force to operate at least one fluid-pressure operated device, and a booster is operative to augment the output force from the master cylinder.

In most passenger vehicles the booster in an hydraulic system of the kind set forth is vacuum-suspended, and is located between the pedal and the master cylinder. This arrangement has two major disadvantages. Firstly, as the booster is relatively large, it is often difficult to find space for it, particularly when the vehicle is being converted from left hand to right hand drive, or vice versa.

Secondly, as the booster is responsive to pedal movement, it operates even when the clearances in the system are being taken up, so it uses more power than is necessary. If the booster is remote from the master cylinder this overcomes not only the first problem, but also the second, since the booster can be operated in response to pressure in the system, and so will not operate until the clearances have been taken up.

However, known remote boosters have their disadvantages. British Patent No. 786 524 shows a vehicle braking system of the kind set forth, with a remote vacuum-suspended booster providing servo assistance between the master cylinder and the brakes.

This has the disadvantages that if the hydraulic seals in the booster fail, the brake circuit as a whole fails, and that a single booster cannot augment the output force in both circuits of a dual circuit braking system.

According to our invention, in a vehicle hydraulic system of the kind set forth, the master cylinder comprises a housing having a bore in which piston means works, pressure space means defined in the bore and pressurised by operation of the piston means in response to the input force to produce the output force, a control chamber defined in the bore and pressurised in response to the input force and an hydraulic servo chamber defined in the bore behind the piston means; and the booster is remote from the master cylinder and comprises a separate housing, in which works a movable wall operated by pressurisation of a boost chamber, control valve means for controlling pressurisation of the boost chamber in response to pressure in a booster control chamber, an output member operated by the movable wall to pressurise an hydraulic output chamber, the booster control chamber being connected to the master cylinder control chamber, and the output chamber being connected to the hydraulic servo chamber.

This arrangement means that the hydraulic circuit containing the booster can be separate from the circuit or circuits containing the fluid-pressure operated devices, so that failure in the one will not affect the others. Furthermore, if the system has dual hydraulic circuits supplied by a tandem master cylinder, a single booster can act on both circuits.

The hydraulic system may be a vehicle clutch system, in which case a single master cylinder is sufficient. The piston means and pressure space means therefore comprise a single piston and pressure space for operating the clutch. The pressure space preferably also comprises the control chamber, which is pressurised by the piston on which the input force acts.

Alternatively, the system may be a dual circuit braking system, and a tandem master cylinder is used.

In this case the piston means and pressure space means comprise primary and secondary pistons acting on respective primary and secondary pressure spaces which operate the brake circuits. The primary pressure space may also comprise the control chamber, pressurised by the primary piston on which the input force acts. In an alternative construction the control chamber is separate from the pressure spaces. It is conveniently defined in the bore behind the servo chamber, and is pressurised by an input member.

The booster is preferably of the vacuum-suspended type, but may alternatively be operated by compressed air. In either case the boost chamber is pneumatic.

Operation of the valve means is preferably controlled by the hydraulic pressure in the servo chamber, as well as the control chamber. The valve means conveniently comprises a piston, with the control and servo pressures acting on opposite ends of the piston.

The servo chamber is connected to a reservoir for hydraulic fluid through a normally-open isolating valve, which is controlled by the output member. Thus, on operation of the booster, movement of the output member first closes the isolating valve, and then acts to pressurise the servo chamber through an hydraulic strut.

Some embodiments of our invention are illustrated in the accompanying drawings, in which: Figure 1 is a diagram of a vehicle hydraulic clutch system; Figure 2 is a diagram of a vehicle hydraulic braking system; and Figure 3 is similar to Figure 2, but shows a modified master cylinder.

The vehicle hydraulic clutch system shown in Figure 1 comprises a pedal-operated master cylinder 1 having a piston 2 operative to pressurise a pressure space 3 to generate a fluid pressure output force to operate a clutch 4, and a vacuum-suspended booster 5 remote from the master cylinder 1 providing servo assistance to a servo chamber 6 in the master cylinder 1 behind the piston 2.

The master cylinder 1 has a housing 7 with a longitudinal stepped bore 8. The piston 2 works in the bore 8, and the servo chamber 6 and pressure space 3 are defined in the bore 8. The piston 2 is of stepped outline, and at its rearward end works through a seal 9 carried in a closure member 10 for the bore. The seal 9 forms the rearward seal for the servo chamber 6, the forward seal 11 of which is carried by the piston 2. At its forward end the piston 2 is of reduced diameter, and carries a further seal 12, which is retained on the piston by a stud 13. A return spring 14 acting between the stud 13 and the forward end of the housing biasses the piston 2 rearwardly.

The seal 12 co-operates with a radial recuperation port 15 in the housing 7 to control communication between the pressure space 3 and a fluid reservoir (not shown). An annular chamber 16 defined in the bore 8 round the piston 2 between the seals 11 and 12 is permanently connected to the reservoir through a further radial port 17.

An outlet 18 from the pressure space 3 is connected to a slave cylinder (not shown) for the clutch 4. As the outlet 18 is also connected to a booster control chamber 19 in the booster 5, where it acts on a control valve means 20 of the booster 5, the pressure space 3 also acts as a control chamber. The servo chamber 6 is connected to a fluid reservoir 21 through an output chamber 45 and an isolating valve 22 in the booster 5, and the pressure in the servo chamber 6 also acts on the control valve means 20.

The booster 5 has a housing 23, in which are defined a vacuum chamber 24 connected to a vacuum source (not shown) and a pneumatic boost chamber 25.

The chambers 24 and 25 are separated by movable wall in the form of a elastomeric diapnragm 26, which is connected to an output member 27 and biassed into the position shown by a booster return spring 28.

Pressurisation of the boost chamber 25 is controlled by the valve means 20, which is located in a bore 29 in the housing 23.

The valve means 20 has an inlet valve 30 controlling communication between an air inlet 31 and a passage 32 leading to the boost chamber 25, and an exhaust valve 33 controlling communication between the passage 32 and a passage 34 leading to the vacuum chamber 24. The valve means comprises a piston 35 and an elastomeric seating member 36. The inlet valve 30 is defined by a valve part 37 on the piston 35 which co-operates with a seating portion on the member 36, the two parts being biassed into engagement by springs 38 and 42. The spring 38 acts between the piston 35 and the member 36, while the spring 42 biasses the piston 35 towards the booster control chamber 19. The exhaust valve 33 is defined by a valve part 39 in the housing 23 which co-operates with a further seating portion on the member 36.In the retracted position shown, the inlet valve 30 is closed and the exhaust valve 33 is open. Pressure in the booster control chamber 19 acts on one end of the piston 35 through a flexible sealing diaphragm 40. A seal 41 is provided on the other end of the piston 35, on which the pressure in the servo chamber 6 acts.

The output member 27 of the booster comprises a piston which works through a seal 43 in the housing 23 and terminates in an hydraulic piston 44 working in the output chamber 45, which is located in the line between the servo chamber 6 and the reservoir 21. The piston 44 carries a seal 46 and the seat 47 of the isolating valve 22. The valve member 48 of the isolating valve comprises a poppet carried by a clip 49. A spring 50 biasses the valve member 48 towards its seat 47, but in the retracted position shown the engagement of the clip 49 with the housing holds the isolating valve 22 open.

It will be noted that the hydraulic part of the system is divided into two separate circuits; a clutch circuit containing the master cylinder space 3 and the clutch 4, and a booster circuit containing the servo chamber 6 and the booster 5.

In operation, an input force from the pedal is applied to the piston 2, moving it against the return spring 14. The movement firstly closes the recuperation port 15, and then, after the clearances in the system have been taken up, starts to generate pressure in the pressure space 3. As the piston 2 advances, hydraulic fluid is drawn into the servo chamber 6 from the reservoir 21 through the open isolating valve 22. The pressure in the pressure space 3 -acts on the slave cylinder to start operation of the clutch. The pressure is also present in the booster control chamber 19, and when it reaches a given value, it moves the piston 35 against the force in the spring 42. This firstly closes the exhaust valve 33, and then opens the inlet valve 30, allowing air to flow to the boost chamber 25. Pressurisation of the boost chamber 25 moves the diaphragm 26 and the output member 27.Movement of the piston 44 first allows the isolating valve 22 to close thus creating an hydraulic strut in the chamber 45 and the line leading to the servo chamber 6. Further movement of the piston 44 then acts to pressurise this strut, to increase the pressure in the servo 6, where it acts onthe piston 2 to augment the master cylinder output. The hydraulic servo pressure also acts on the piston 35 in opposition to the pressure in the booster control chamber 19.

Once the hydraulic servo pressure is substantially equal to the control pressure, it moves the piston 35 into its balanced position, where both valves 30, 33 are closed.

When the input force is reduced, the pressure in the space 3 is reduced, allowing the master cylinder return spring 14 to return the piston 2 towards its retracted position. At the same time, the pressure in the booster control chamber 19 is reduced, allowing the spool 35 to move to open the exhaust valve 33 to reduce the pneumatic boost pressure. The output member 27 will therefore return towards its retracted position under the influence of the spring 28. During this movement the isolating valve 22 remains closed, and the -hydraulic servo pressure is reduced. If the input force has been removed altogether, the parts will return to their retracted positions shown, with the isolating valve 22 opening again when the clip 49 engages the housing. Otherwise, the parts will find a new balanced position.

If the booster fails, the master cylinder 1 can simply be operated manually. Because the clutch circuit is separate from the booster circuit, the clutch can still be operated even if the seals 9, 11, 41, 43 or 46 in the booster circuit fail.

Further advantages of a remote booster over a direct-acting booster - located between the pedal and the master cylinder - are also apparent from the system. Firstly, it can be located at any convenient position with any suitable dimensions, and the master cylinder, although it has the extra servo chamber 6, is not appreciably longer than a normal master cylinder.

Secondly, as the booster is responsive to hydraulic pressure in the system, it does not start to operate until the clearances in the system have been taken up, which reduces the amount of vacuum used. Lastly, on failure of the booster it is not necessary to operate any booster parts and to overcome the booster return spring, as it is with a direct-acting servo, so the pedal effort in the failed case is reduced. This also means that the housing of a remote booster can be lighter in construction, as it does not need to withstand the forces generated by manual operation.

The hydraulic system of Figure 2 is a dual circuit vehicle braking system, and is provided with a tandem master cylinder 51 rather than the single master cylinder 1 of Figure 1. Corresponding reference numerals have been applied to corresponding parts.

The master cylinder 51 has primary and secondary pistons 52, 53 operative to pressurise respective primary and secondary pressure spaces 54, 55, with the servo chamber 6 defined behind the primary piston 52.

The primary piston 52 is similar in construction to the piston 2 of Figure 1, and has a similar recuperation port. The secondary piston 53 works through a seal assembly 56 located in the bore, and has a radial port 57 cooperating with the assembly 56 to form the recuperation port. The secondary piston 53 has a smaller pressure-effective area than the primary piston 52, their areas being so arranged that in operation the travel of the pistons is substantially equal. Thus, in the retracted position shown they are in abutment, and a single master cylinder return spring 58 can be used, acting between the housing and the secondary piston 53. Outlets 59, 60 lead from the pressure spaces to primary and secondary brake circuits 61, 62, and the pressure in the primary space 54 also acts in the booster control chamber 19, so that the primary space 54 operates as a control chamber.

The construction of the system is otherwise the same as in Figure 1, and operation is very similar as well. Thus in normal operation both master cylinder spaces are pressurised, with the pistons 52, 53 moving substantially together, and the booster is operative, under the control of valve means 20, to pressurise the servo chamber 6.

On failure of the booster the master cylinder 51 is of course operated manually. If the pressure in the primary space 54 fails, the booster is unable to operate, but the secondary brake circuit 62 can be applied relatively easily. As the pistons 52, 53 are in abutment the input force is applied immediately to the secondary piston 53 through the primary piston 52.

There is therefore no extra pedal travel, and because of the reduced area of piston 53 relatively less pedal effort is required to produce a given braking force.

On failure of the secondary brake circuit, the pistons 52, 53 move until the secondary piston 53 engages the end of the bore, and then the primary piston 52 operates to pressurise the primary space 54 with booster assistance, as usual.

In these cases of failure, the master cylinder 51 and remote booster 5 operate in a similar way to a master cylinder with direct-acting booster, but of course have all the advantages pointed out above. A further advantage of the system of Figure 2 is that one remote booster can operate a tandem master cylinder satisfactorily.

Figure 3 shows a dual circuit braking system similar to that of Figure 2, but with a modified tandem master cylinder. Again, corresponding reference numerals have been applied to corresponding parts.

The master cylinder 63 of Figure 3 has primary and secondary pistons 52, 53 to pressurise respective spaces 54, 55, and a servo chamber 6 behind the primary piston 52, but the pressure in the primary space 54 acts only in the brake circuit 61, and not in the booster control chamber 19. Instead, a separate control chamber 64 is provided in the bore behind the servo chamber 6, between the primary piston 52 and a separate input member 65 on which the pedal acts.

The primary and secondary pistons 52, 53 are of substantially equal area, so are not in abutment in their retracted positions, and a second master cylinder return spring 66 is provided between them. It will be noted that the recuperation arrangements for the pressure spaces are opposite to those shown in Figure 2. The primary piston 52 extends rearwardly into the control chamber 64, and works through spaced seals 67, 68 in the housing. The servo chamber 6 is defined between the seal 11 on the primary piston 52 and the seal 67, while an annulus 69 provided in the housing between seals 67, 68 permits communication between two parts of a line 70 connecting the output chamber 45 of the booster 5 to the reservoir 21. The control chamber 64 is defined between the seal 68 and a seal 71 on the input member 65.A branch passage 72 from the line 70 leads to the control chamber 64 through a tipping valve 73, and an outlet 74 from the chamber 64 is connected to the control chamber 19.

In the retracted position shown the master cylinder springs 58, 66 hold the pistons 52, 53 in inoperative positions with the recuperation ports open, and the input member 65 holds the tipping valve 73 open against the force in a spring 75, so that the control chamber 64 is connected to the reservoir 21 through an annulus -76 round the member 65, and diametral and axial ports 77, 78 in it.

In operation, when the input from the pedal is applied to the member 65 it moves in the bore, permitting the tipping valve 73 to close to trap fluid in the control chamber 64, and then after the clearances have been taken up starting to generate pressure in the control chamber. This will operate the primary and secondary pistons 52, 53, and then the control valve means 20 to operate the booster 5, pressurising the servo chamber 6. Thereafter the system operates as the system of Figure 2.

On failure of the boost pressure, the master cylinder can still be operated manually. If the pressure in the control chamber 64 fails, the booster cannot operate, but the master cylinder can still be operated manually, with the input member 65 engaging the end of the primary piston 52.

If the pressure in the primary space 54 fails, the primary piston 52 moves into engagement with the secondary piston 53, which is then applied with booster assistance, as the booster still operates. On failure of the secondary brake circuit the pistons 52, 53 move until the secondary piston 53 engages the end of the bore, and then the primary piston 52 pressurises the primary space 54 with booster assistance.

The characteristics of the system in these failed cases are in fact similar to those of Figure 2, although the provision of the control chamber 64 separate from the primary pressure space 54 has the advantage that servo assistance is not lost if the primary circuit fails.

Claims

1. A vehicle hydraulic system of the kind set forth, in which the master cylinder comprises a housing having a bore in which piston means works, pressure space means defined in the bore and pressurised by operation of the piston means in response to the input force to produce the output force, a control chamber defined in the bore and pressurised in response to the input force and an hydraulic servo chamber defined in the bore behind the piston means; and the booster is remote from the master cylinder and comprises a separate housing, in which works a movable wall operated by pressurisation of a bdost chamber, control valve means for controlling pressurisation of the boost chamber in response to pressure in a booster control chamber, an output member operated by the movable wall to pressurise an hydraulic output chamber, the booster control chamber being connected to the master cylinder control chamber, and the output chamber being connected to the hydraulic servo chamber.

2. A vehicle hydraulic system as claimed in claim 1, in which the master cylinder is a single master cylinder, with the pressure space means comprising a single pressure space and the piston means comprising a single piston.

3. A vehicle hydraulic system as claimed in claim 2, in which the pressure space also comprises the control chamber, which is pressurised by the piston, on which said input force acts.

4. A vehicle hydraulic system as claimed in claim 1, in which the master cylinder is a tandem master cylinder, with the pressure space means comprising primary and secondary pressure spaces and the piston means comprising respective primary and secondary pistons acting on the pressure spaces.

5. A vehicle hydraulic system as claimed in claim 4, in which the primary pressure space also comprises the control chamber, and is pressurised by the primary piston, on which the input force acts.

6. A vehicle hydraulic system as claimed in claim 4, in which the control chamber is separate from the pressure spaces, and is pressurised by a separate input member.

7. A vehicle hydraulic system as claimed in claim 6, in which the control chamber is defined in the bore behind the servo chamber, and is pressurised by an input piston on which the input force acts.

8. A vehicle hydraulic system as claimed in any preceding claim, in which the control valve means is operated by hydraulic pressure in the servo chamber and the booster control chamber.

9. A vehicle hydraulic system as claimed in claim 8, in which the valve means comprises a valve piston, with the control and servo pressures acting on opposite ends of the valve piston.

10. A vehicle hydraulic system as claimed in any preceding claim, in which the booster has a normally-open isolating valve for controlling communication between a reservoir for fluid and the output chamber, the isolating valve being controlled by the output member.

11. A vehicle hydraulic system as claimed in any proceding claim, in which the booster is of the vacuum-suspended type.

12. A vehicle hydraulic system as claimed in any of claims 1 to 10, in which the booster is operated by compressed air.

13. A vehicle hydraulic system substantially as described herein with reference to and as illustrated in Figure 1 of the accompanying drawings.

14. A vehicle hydraulic system substantially as described herein with reference to and as illustrated in Figure 2 of the accompanying drawings.

15. A vehicle hydraulic system substantially as described herein with reference to and as illustrated in Figure 3 of the accompanying drawings.