A VEHICLE HYDRAULIC BRAKING SYSTEM

Abstract: A pair of tandem master cylinders (1, 2) has primary pres­ sure spaces (18) connected to the brakes on wheels on oppo­ site sides of the vehicle through separate pipe-lines (63, 64), each connected to a different radial port (60, 61) of a control valve assembly (50), and secondary pressure spaces (20) both connected through a common pipe-line (65) to an auxiliary braking system (46) and an axial inlet port (62) of the control valve assembly (50). When both master cylinders (1, 2) are operated simultaneously, pressure from the primary pressure spaces (18) is applied to both brakes (42 and 43), and a signal pressure from the secondary pressure spaces (20) opens the control valve assembly (50) to equalise the pressures applied to the brakes (42 and 43) and apply that, equalised pressure, to the auxiliary baling system (46). When one master cylinder is operated on its own fluid from its secondary pressure space (18) is displaced to the reservoir (14) through the secondary pressure space (18) of the other master cylinder and the re­ cuperation valve (21) which is open. No signal pressure is generated so the control valve assembly (50) remains closed to isolate the actuated brake (42 or 43) from the non-actuated brake (43 or 42).

Description: A VEHICLE HYDRAULIC BRAKING SYSTEM This invention relates to vehicle hydraulic braking systems of the kind in which separate master cylinders are operable simultaneously to apply brakes on both sides of a vehicle to retard the vehicle and operate auxiliary braking means, for example brakes on a trailer, and are operable independently to apply the brakes on one side of the vehicle to facilitate steering of the vehicle, means being provided for preventing operation of the auxiliary braking means when one master cylinder is operated on its own. Where used herein the term "master cylinder" is intended to cover constructions in which a pedal operates a piston to pressurise hydraulic fluid in a pressure space and to constructions in which a pedal is operative to operate a valve for controlling the supply of pressurised fluid to a pressure space, either from a source of high pressure fluid, or by throttling a supply of fluid which is normally circulated through the pressure space in a closed circuit at low pressure. In our GB-A-2098687 we have described an hydraulic system of the kind set forth in which each master cylinder is of tandem construction having a primary pressure space in front of a pedal-operated primary piston and which is connected to the brakes on wheels on one side of the vehicle, a secondary pressure space in front of a secondary piston and which is connected to the auxiliary braking means through a common connection with -tthe secondary pressure space of the other master cylinder, and normally-open recuperation valves :connecting the respective pressure spaces to a reservoir for fluid, movement of the primary and secondary pistons in a brake-applying direction being operative to close the two recuperation valves whereafter to pressurise the fluid in the¯pressure spaces. When both master cylinders are operated simultaneously, the pressure spaces of both master cylinders are pressurised and the primary pressure spaces are interconnected by transfer valve means to equalise the pressure applied to the brakes on opposite sides of the vehicle in order to compensate for wear of the friction. linings but when one master cylinder is operated on its own fluid from the secondary pressure space of that master cylinder is returned to the reservoir through the secondary recuperation valve of the other master cylinder, which is open, and the transfer valve means is operative to isolate the two primary pressure spaces from each other. In the specific construction of our GB-A-2098687 the auxiliary braking means, which comprises brakes on a trailer, is operated by the direct displacement of fluid from the secondary pressure spaces, which must therefore be of sufficient volume to accommodate an adequate quantity of hydraulic fluid. The transfer valve means comprises a pair of separate transfer valves which are interconnected by a transfer passage and each transfer valve is housed in a wall of a respective master cylinder and comprises a valve member for co-operation with a seating surrounding a radial port in the wall. When both master cylinders are operated simultaneously, the primary pistons are operative to urge the two valve members away from the seatings so that the two pressure spaces are placed in open communication through the transfer passage, and when one master cylinder is operated on its own the pressure from the primary pressure space is operative to urge the valve member of the transfer valve of the#other master cylinder into ¯engagement with its seating to ensure that the two primary pressure spaces are isolated from each other. The primary pistons must therefore be of sufficient length to co-operate with the valve members in order to open the respective transfer valves. According to our invention in a master cylinder assembly for a vehicle hydraulic braking system comprising a pair of master cylinders which are adapted to be operated simultaneously to effect retardation of a vehicle and independently to assist steering, each master cylinder comprises a pedal-operated primary piston working in a bore, a secondary or floating piston working in a portion of the bore between the primary piston and a closed end of the bore, a primary pressure space defined in the bore between the primary and secondary pistons, and a secondary pressure space defined in the bore between the secondary piston and the closed end of the bore, the primary pressure spaces are adapted to be connected to brakes on wheels on opposite sides of the vehicle, the secondary pressure spaces are operative to control operation of a common auxiliary braking system, which is adapted to be operated only when the two master cylinders are operated simultaneously, each secondary piston being provided with passage means in communication at all times with a reservoir for fluid, and normally open primary and secondary recuperation valves provide communication between the passage means and the primary and secondary pressure spaces when the pistons are in their retracted positions, simultaneous operation of both master cylinders causing both pairs of primary and secondary recuperation valves to close followed by pressurisation of the fluid in both pairs of primary and secondary pressure spaces whereby to apply simultaneously the brakes on opposite sides of the vehicle and to operate the auxiliary braking system, and operation of one of the master cylinders on its own causing the primary and secondary recuperation valves of that master cylinder to close followed by pressurisation of the fluid in the primary pressure space but with fluid in the secondary pressure space being displaced to the reservoir through the open secondary recuperation valve of the other, inoperative, master cylinder, whereby to prevent operation of the auxiliary braking system, and a control valve assembly is provided to control communication between the brakes on opposite sides of the vehicle, and between the said brakes and the auxiliary braking system, the valve assembly being normally closed to isolate the brakes on apposite sides of the vehicle from each other when one master cylinder is operated on its own, but being opeted in response to pressure generated in the secondary pressure spaces when both master cylinders are operated simultaneously, whereby to place the brakes on opposite sides of vehicle in communication both with each other and with the auxiliary braking system. The control valve assembly acts as a transfer valve to provide communication between the brakes on wheels on opposite sides of the vehicle to compensate for differential wear of the frictions linings, and enables the aUxiliary braking system to be operated by fluid from the priiiary pressure spaces of the two master cylinders. The provision of the control valve assembly has the advantages of enabling us to reduce the effective volumes of the two secondary pressure spaces since only a relative small volume of fluid has to be displaced in order to open the control valve assembly, and the volumes of the secondary pressure spaces can be standardised to cater for the requirements of a specific control valve assembly. In other words the capacity of the two secondary pressure spaces need only be sufficient to generate a signal pressure of a magnitude sufficient to open the control valve assembly, whereafter the auxiliary braking system is operated by pressure from the two primary pressure spaces. Since no transfer valves operated by the primary pistons are provided, the effective lengths of the primary pistons can be correspondingly reduced. The conservation of space bythe reduction in the volume of the two secondary pressure spaces and the reduction in the lengths of the primary pistons means that the overall lengths of the master cylinders are of similar complementary lengths. The assembly of the two master cylinders is relatively more compact and therefore lighter and cheaper to produce. Conveniently the control valve assembly comprises a housing having an axial bore, a valve member, suitably in the form of a spool or piston, working in the axial bore, the housing also being provided with a pair of radial ports which communicate with the bore and each of which is connected to the primary pressure space of a respective master cylinder and to the brake on the wheel on the corresponding side of the vehicle, and with an axial inlet port connected to both secondary pressure spaces and to the auxiliary braking system, and a spring for urging the valve member in a first direction to close both radial ports and isolate the radial ports from the inlet port in opposition to pilot pressure in the secondary pressure spaces which, when both master cylinders are operated simultaneously, is operative to urge the valve member in the bore in a second direction opposite the first to place the three ports in communication with each other. One embodiment of our invention is illustrated in the single Figure of the accompanying drawings which is a layout of an hydraulic braking system for a vehicle having an auxiliary braking system. The braking system shown in the layout incorporates a dual master cylinder assembly which comprises two master cylinders 1 and 2. Since the master cylinders are identical in construction only the master cylinder 1 will be described in detail although, for convenience, corresponding reference numerals have been applied to corresponding parts of the two master cylinders 1 and 2. The master cylinder 1 comprises a body 3 provided with a longitudinal bore 4 in which works a primary piston 5 engaged at its rear end by a part-spherical head 6 on a pedal-operated push-rod 7, and a secondary or floating piston 8 disposed between the piston 5 and the closed end of the bore 4. A stop for the head 6 is formed by an annular collar 9 located by a spring ring or circlip 10 received in an annular groove 11 in the bore 4. The primary piston 5 is provided with a forward extension 12 of reduced diameter. The secondary piston 8 has a longitudinally extending through bore 13 which is in communication at all times with a reservoir 14 for hydraulic fluid through a recuperation port 15 in the wall of the body 3, an annular passage 16 in the wall of the piston 8, and a diametral passage 17 in the piston 8 which traverses the bore 13. A primary pressure space 18 defined in the bore 4 between the pistons 5 and 8 communicates with the reservoir 14 through a normally open primary recuperation valve l9 and a secondary pressure space 20 defined in the bore 4 between the secondary piston 8 and the closed end of the bore 4 communicates with the reservoir through a normally open secondary recuperation valve 21. The recuperation valve 19 comprises a head 22 for engagement with a seating 23 surrounding the bore 13 at the adjacent end of the piston 8. The head 22 is carried by the forward end of an axially extending stem or rod 24 of which the opposite end carries an enlarged head 25 guided to slide in a longitudinally extending bore 26 in the adjacent end of the extension 12. The head 25 is retained within the bore 26 by a thimble 27 which fits over the extension 12. A flange 28 on the thimble 27 forms an abutment for one end of a compression return spring 29 of which the opposite end acts on a cage in which the head 22 is housed. In the position shown in the drawings the spring 29 urges the piston 5 into a retracted position with the head 22 spaced from the seating 23 by the engagement of the thimble 27 with the head 25. The recuperation valve 21 comprises a head 30 for engagement with a seating 31 surrounding the opposite end of the bore 13 in the piston 8. The head 30 is carried by the rear end of a stem 32 of which the front end has an enlarged head 33 working in a portion of the bore 4 which is of reduced diameter. The head 33 is disposed on the opposite side of a plate 34 which is in abutment with a shoulder 37 at the step in diameter. The plate 34 engages with the head 33 and forms an abutment for one end of a compression spring 35 of which the opposite end engages with a cage 36 in which the head 30 is housed. In the position shown the spring 35 acts through the plate 34 to urge it into engagement with a shoulder 37 at the step in diameter and to hold the head 30 in a retracted position spaced from the seating 31. Outlet ports 40 and 41 leading from the primary pressure spaces 18 of the respective master cylinder 1 and 2 are connected to the brakes 42, 43 on wheels on opposite sides of the vehicle. Outlets ports 44 and 45 leading from the secondary pressure spaces 20 are connected to an auxiliary braking system 46. A control valve assembly 50 is incorporated in the system. The valve assembly 50 comprises a housing 51 having a longitudinally extending stepped through-bore 52. The bore 52 is closed at its end of greater diameter by a plug 53 which is retained in position by means of a circlip 54 and has a through-bore 55. That end of the housing 51 is enclosed with sealing boot 56. A valve member comprising a spool or piston 57 works in the bore 52 and is normally urged by means of a compression spring 58 in abutment with the plug 53 into an advanced closed position defined by the engagement with a shoulder 59 at a step at the change in the diameter of the bore 52. In this position the valve member 57 isolates each of a pair of diametrically opposed radial ports 60, 61 in the housing 51 and communicating at their inner ends with the bore 52 from an axial inlet port 62 defined by the end of the bore 52 which is of smaller diameter. The radial port 60 is connected to a pipe-line 63 leading both to the outlet port 40 of the master cylinder l and the brake 42, the radial port 61 is connected to a pipe-line 64 leading both to the outlet port 41 of the master cylinder 2 and the brake 43, and the inlet port 62 is connected to a pipe-line 65 leading both to both outlet ports 44 and 45 and the auxiliary braking system 46. In the inoperative position shown in the drawings the pressure spaces 18 and 20 of both master cylinders are in coii¯mnication with the reservoir 14 through the respective recuperation valves 19 and 21 which are open, and the control valve assembly 50 is in a closed position with the valve member held by the spring 58 against the shoulder 59 to close the inner ends of the three ports 60, 61 and 62, thereby isolating them from each other. When both master cylinders are operated simultaneously a short forward movement of the pistons 5 and 8 with corresponding compression of the springs 29 and 35 is sufficient to cause the heads 22 and 30 to engage with the seatings 23 and 31 to isolate the reservoir 14 from both pressure spaces 18 and 20. Further movements of the pistons 5 in the same directions cause fluid to flow through the outlet ports 40, 41 to the brake on the wheels 42 and 43 and through the outlet ports 44 and 45 to the auxiliary braking system 46 and simultaneously to the inlet port 62 where that pressure urges the valve member 57 axially against the loading in the spring 58 to place the three ports 60, 61 and 62 in open communication with one other. Placing the two radial ports 60, 61 in communication with each other equalises the applied pressures to compensate for differential wear of the friction linings of the two brakes 42 and 43, and placing the three ports 60, 61 and 62 in communication with each other enables the auxiliary braking system 46 to be operated, or for the pressure to be augmented substantially, by pressure from the primary pressure spaces 18 of the two master cylinders 1 and 2. Once the control valve assembly is in an open position with the three ports 60, 61 and 62 in communication, the valve assembly 50 will remain in this position so long as pressure is available in the brake circuits. When one master cylinder is operated on its own, say the master cylinder 1, as before forward movement of the pistons 5 and 8 with corresponding compression of the springs 29, 35 is sufficient to cause the heads 22 and 30 to engage with the seatings 23 and 31 to isolate the reservoir 14 from both pressure spaces 18 and 20. Further movement of the pistons 5 and 8 in the same direction causes pressurised fluid to flow from the primary pressure space 18 to the brake 42 through the outlet port 40, but fluid from the secondary pressure space 20 is displaced to the reservoir 14, which is at atmospheric pressure, through the pressure space 20 and the open recuperation valve 21 of the other, non-actuated, master cylinder 2. Thus no signal pressure is generated and the control valve assembly 50 remains in its closed position with the valve member 57 held against the shoulder 59. The two radial ports 60 and 61 are therefore isolated from each other to prevent fluid from being transferred between the two brakes 42 and 43, and the radial ports 60 and 61 are isolated from the port 62 which prevents the auxiliary braking system 46 from being operated when one master cylinder 1 or 2 is operated on its own. When the auxiliary braking system comprises brakes on opposite sides of a trailer, these brakes are inoperative when either of the brakes 42 or 43 on the vehicle is operated to facilitate steering or to enable the vehicle to execute a "spin-turn". In a modificatibn the auxiliary braking systejrmay comprise auxiliary brakes on wheels on opposite sides of the vehicle itself.

Claims: CLAIMS 1. A master cylinder assembly for a vehicle hydraulic braking system comprising a pair of master cylinders (1, 2) which are adapted to be operated simultaneously to effect retardation of a vehicle and independently to assist steering, in which each master cylinder (1, 2) comprises a pedal-operated primary piston (5) working in a bore (4), a secondary or floating piston (8) working in a portion of the bore (4) between the primary piston (5) and a closed end of the bore (4), a primary pressure space (18) defined in the bore (4) between the primary and secondary pistons (5, 8), and a secondary pressure space (20) defined in the bore (4) between the secondary piston (8) and the closed end of the bore (4), the primary pressure spaces (18) are adapted to be connected to brakes (42, 43) on wheels on opposite sides of the vehicle, the secondary pressure spaces (20) are operative to control operation of a common auxiliary braking system (46), which is adapted to be operated only when the two master cylinders (1,2) are operated simultaneously, each secondary piston (8) being provided with passage means (17) in communication at all times with a reservoir for fluid (14), and normally open primary and secondary recuperation valves (19, 21) provide communication between the passage means (17) and the primary and secondary pressure spaces (18, 20) when the pistons (5, 8) are in their retracted positions, simultaneous operation of both master cylinders (1, 2) causing both pairs of primary and secondary recuperation valves (19, 21) to close followed by pressurisation of the fluid in both pairs of primary and secondary pressure spaces (18, 20) whereby to apply simultaneously the brakes (42, 43) on opposite sides of the vehicle and to operate the auxiliary braking system (46), and operation of one of the master cylinders (1, 2) on its own causing the primary and secondary recuperation valves (19, 21) of that master cylinder to close followed by pressurisation of the fluid in the primary pressure space (18) but with fluid in the secondary pressure space (20) being displaced to the reservoir (14) through the open secondary recuperation valve (21) of the other, inoperative, master cylinder (1, 2), whereby to prevent operation of the auxiliary braking system (46), and a control valve assembly (50) is provided to control communication between the brakes (42, 43) on opposite sides of the vehicle, and between the said brakes (42, 43) and the auxiliary braking system (46), characterised in that the valve assembly (50) is normally closed to isolate the brakes (42, 43) on opposite sides of the vehicle from each other when one master cylinder (1, 2) is operated on its own, but is opened in response to pressure generated in the secondary pressure spaces (20) when both master cylinders (1, 2) are operated simultaneously, whereby to place the brakes (42, 43) on opposite sides of vehicle in communication both with each other and with the auxiliary braking system (46). 2. A master cylinder assembly according to claim 1 characterised in that the control valve assembly (50) acts as a transfer valve to provide communication between the brakes (42, 43) on wheels on opposite sides of the vehicle to compensate for differential wear of frictions linings, and enables the auxiliary braking system (46) to be operated by fluid from the primary pressure spaces (18) of the two master cylinders (1, 2). 3. A master cylinder assembly to claim 1 or 2 characterised in that the control valve assembly (50) comprises a housing (51) having an axial bore (52), a valve member (57) working in the axial bore (52), the housing (51) also being provided with a pair of radial ports (60, 61) which communicate with the bore (52) and each of which is connected to the primary pressure space (18) of a respective master cylinder (1, 2) and to the brake (42, 43) on the wheel on the corresponding side of the vehicle, and with an axial inlet port (62) connected to both secondary pressure spaces (20) and to the auxiliary braking system (46), and a spring (58) for urging the valve member (57) in a first direction to close both radial ports (60, 61) and isolate the radial ports (60, 61) from the inlet port (62) in opposition to pilot pressure in the secondary pressure spaces (20) which, when both master cylinders (1, 2) are operated simultaneously, is operative to urge the valve member (57) in the bore (52) in a second direction opposite the first to place the three ports (60, 61, 62) in communication with each other. 4. A master cylinder assembly according to any preceding claim characterised in that the valve member (59) comprises a spool. 5. A master cylinder assembly according to any preceding claims characterised in that the valve member (57) comprises a piston. 6. A master cylinder assembly according to any preceding claim characterised in that the auxiliary braking system (46) comprises auxiliary brakes on wheels on opposite sides of the vehicle itself.

Inventors: PRICE, ANTHONY GEORGE
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1985
Main classification: B62D11
IPC codes: B62D11, B60T11