GB2162263A

GB2162263A - Improvements in dual hydraulic braking systems for vehicles

Info

Publication number: GB2162263A
Application number: GB08515432A
Authority: GB
Inventors: Roy Campbell; Anthony George Price
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1984-06-27
Filing date: 1985-06-18
Publication date: 1986-01-29
Also published as: FR2566730A1; DE3522211A1; FR2566730B1; GB2162263B; GB8416367D0; GB8515432D0; IT8521205A0; IT1209640B

Abstract

An hydraulic braking system incorporates a pair of tandem master cylinders (1, 2), with primary pressure spaces (18) connected to brakes (41 and 42) on the rear wheels of the vehicle, and secondary pressure spaces (20) both connected to an isolator valve assembly (50) having an outlet (61) connected to brakes (63, 64) on both front wheels of the vehicle. The isolator valve (50) has a flow- conscious valve member (67) provided with opposed heads (68, 69) for alternative engagement with spaced seatings (65, 66) on opposite sides of the outlet (61) to isolate the secondary pressure space (20) of one master cylinder from the brakes (63, 64) when that one master cylinder is operated on its own and the valve member (67) is provided with flow paths (73) permitting flow past the heads (68, 69) to the brakes (63, 64) when both master cylinders are operated simultaneously. Since only a relatively small volume of fluid is expelled from a secondary pressure space (20) to affect operation of the isolator valve assembly (50), pedal travel is reduced to a minimum. <IMAGE>

Description

SPECIFICATION Improvements in dual hydraulic braking systems for vehicles This invention relates to dual hydraulic brak ing systems for vehicles of the kind in which a pair of hydraulic master cylinders are adapted to be operated simultaneously to effect retar dation of a vehicle and independently to assist steering, each master cylinder comprising 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 pistons, and a secondary pressure space defined between the secondary piston and the closed end of the bore, separate connections are provided between the pressure spaces and the brakes on one set of wheels on opposite sides of the vehicle, and a common connection is provided between both secondary pressure spaces and auxiliary braking means.

Off highway vehicles, such as agricultural tractors, which have the capability of exceed ing a predetermined speed over the ground must be provided with a braking system which is capable of achieving a certain braking performance. Thus, it is essential to provide an effective auxiliary, suitably, front wheel braking facility. In addition the perfor trance requirement of such a vehicle makes it essential for brakes on wheels on opposite sides of the vehicle to be applied independently in order to assist steering, for example to enable the vehicle to execute a "spin-turn".

Normally the brakes on the rear wheels alone of the vehicle are applied independently when a spin-turn is performed and any auxiliary braking means is rendered inoperative.

Systems of the kind set forth are disclosed in our GB-A-2098687 and GB-A-2121901.

In these systems the two secondary pressure spaces are interconnected through a direct open connection, which also leads to the auxiliary braking means, suitably to apply brakes on a trailer. When one of the master cylinders is operated on its own, fluid expelled from the secondary pressure space of that master cylinder is returned to a reservoir for fluid through the secondary pressure space of the other, inoperative, master cylinder and an open recuperation valve between the reservoir and that secondary pressure space. Fluid in the primary pressure space of the said one master cylinder is pressurised to operate the brake on the corresponding (rear) wheel on one side of the vehicle such that the vehicle can execute a spin turn.A pedal travel penalty is experienced whilst fluid is expelled from the secondary pressure space and before the fluid in the primary pressure space can be pressurised. However, in the systems of GB-A2098687 and GB-A-2121901, such a pedal travel penalty is minimal. This is because only a relatively small volume of fluid needs to be expelled from the secondary pressure spaces in order to actuate the auxiliary braking means and, in consequence, the effective volumes of two pressure spaces are relatively small in comparison with those of the primary pressure spaces.

Should it be desired to adapt the systems of GB-A-2098687 and GB-A-2121901 to utilise hydrostatic pressure generated in the two secondary pressure spaces to operate directly brakes on corresponding (front) wheels on opposite sides of the vehicle, it will be necessary to increase the effective volumes of the two pressure spaces in order to be able to generate brake-applying forces capable of achieving the effective braking performance discussed above. This, in turn, will increase the degree of pedal travel necessary to pressurise the fluid in a primary pressure space when the respective master cylinder is operated on its own.

According to our invention in a dual hydraulic braking system of the kind set forth an isolator valve assembly incorporated in a connection between the two secondary pressure spaces has an outlet leading to the auxiliary braking means, the isolator valve assembly being so constructed and arranged as to provide communication between at least one of the secondary pressure spaces and the auxiliary braking means when both master cylinders are operated simultaneously but to isolate the secondary pressure space of a master cylinder from the auxiliary braking means when that master cylinder is operated on its own.

When one master cylinder is to be operated on its own, only a relatively small volume of fluid need be expelled from the secondary pressure space to affect operation of the isolator valve assembly to isolate the secondary pressure space from the auxiliary braking means. As a consequence the pedal travel can be reduced to a minimum.

The isolator valve assembly may comprise a housing in which is located a flow-conscious valve member carrying opposed valve heads for alternate engagement with one of a pair of spaced seatings in the housing with the seatings disposed on opposite sides of the outlet to the auxiliary braking means to isolate the secondary space from the auxiliary braking means when the respective master cylinder is operated on its own, a flow path being defined between each head and the housing to permit fluid to flow from at least one of the secondary pressure spaces to the auxiliary braking means when both master cylinders are operated simultaneously.

When both master cylinders are operated simultaneously normally the valve member will be held in a neutral position with the heads spaced from both seatings. However should for any reason one of the heads remain in contact with its respective seating, then the auxiliary braking means will be operated by flow through the isolator valve assembly between the other head and its seating and from the secondary pressure space of the respective master cylinder. Should such a condition occur, the pressure applied to the brakes from the two primary pressure spaces will be equalised by the provision of a transfer means which provides open communication between the two primary pressure spaces when both master cylinders are operated simultaneously.

When the system is incorporated in an agricultural tractor or like vehicle, the outlet from the isolator valve assembly will be connected to brakes on both front wheels of the vehicle, and the primary pressure spaces will be connected to the brakes on different rear wheels of the vehicle. Thus all four wheels will be braked simultaneously when both master cylinders are operated together, but only the brake on one rear wheel will be operated when the corresponding master cylinder is operated on its own to assist steering.

In a modification, the outlet from the isolator valve assembly may operate a power valve, in turn for operating a trailer braking system.

One embodiment of our invention is illustrated in the single Figure of the accompanying drawings which is a layout of a dual hydraulic braking system for a four-wheel vehicle.

The layout illustrated in the drawings incorporates a dual master cylinder assembly comprising two master cylinders 1 and 2 which are similar to those described in GB-A2098687. Since the master cylinders are identical in construction only the master cylinder 1 will be described in detail.

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 pedaloperated 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.

The primary piston 5 is provided with a forward extension 1 2 of reduced diameter.

The secondary piston 8 has a longitudinally extending through bore 1 3 which is in communication at all times with a reservoir 14 for hydraulic fluid through recuperation port 1 5 in the wall of the body 3, an annular passage 1 6 in the wall of the piston 8, and a diametral passage 1 7 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 of known construction, 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 of known construction.

Outlet ports 40 and 41 leading from the primary pressure spaces 1 8 of the master cylinders 1 and 2 are connected to the brakes 42 and 43 on wheels on opposite sides of the vehicle. Outlet ports 44 and 45 leading from the secondary pressure spaces 20 are connected by an isolator valve assembly 50 to be hereinafter described.

The two primary pressure spaces 1 8 of the master cylinders 1, 2 are interconnected by a transfer passage comprising a pipe-line 49 which is connected at each opposite end to a radial outlet passage 51 in the wall of the body 3 through a suitable union 52, a transfer valve 53 of known construction and a transfer port 54.

Each passage 51 extends upwardly from its respective body 3 when the master cylinders are installed in a vehicle in their positions of use.

A thrust member 59 in the form of a ball is guided in the transfer port 54. The thrust member 59 is spaced from the valve member 55 of the valve 53 and, in the retracted position shown, engages with a stepped portion of the piston 5.

The isolator valve assembly 50 comprises a housing 60 having an axial through-bore from which projects a single radial outlet port 61 connected through a pipe-line 62 to brakes 63, 64 on the two front wheels of the vehicle.

The housing 60 is provided on opposite sides of the outlet port 61 with axially spaced seatings 65, 66, and a valve member 67 located in the housing 60 is provided with axially spaced valve heads 68, 69 for alternate engagement with the respective seatings 65 and 66. The valve heads 68 and 69 are interconnected by a common stem 70. Opposite ends of the housing 60 are provided with unions 71, 72, each connected to a respective one of the ports 44, 45 so that the outer ends of the heads 68, 69 are exposed to fluid in the secondary pressure spaces 20. Each head 68, 69 is provided with a flat or other reduced diameter portion extending between its opposite ends to define a flow path 73 past the head when the head is spaced from its seating.

When one of the master cylinders, say the master cylinder 1, is operated, a short forward movement of the piston 5 is sufficient to close the recuperation valve 1 9 to isolate the reservoir 14 from the pressure space 18. Simultaneously, or almost immediately thereafter, the piston 5 also urges the thrust member 59 radially outwardly by the engagement therewith of an inclined annular face on the piston 5. This movement urges the valve member 55 away from the adjacent seating 57 and into a fully open-position.Further movement of the piston 5 in the same direction causes fluid to flow through the outlet port 40 to the brake, to the transfer passage 49 through the open transfer valve 53 which, in turn, creates a pressure drop across the valve member in the transfer valve of the other master cylinder, urging that valve member towards the adja cent seating in the body of that master cylin der to close the transfer valve, and advancing the secondary piston 8 towards the closed end of the bore 4.

Movement of the secondary piston 8 towards the closed end of the bore 4 closes the recuperation valve 21 to isolate the secondary pressure space from the reservoir 14. Fluid displaced from the secondary pressure space 20 by further movement of the piston in the same direction acts on the adjacent end of the valve member 67 to cause the head 68 to engage with the seating 66 and isolate the secondary pressure space from the brakes 63 and 64.

Since the master cylinder 2 is inoperative, only the brake 42 on the corresponding rear wheel of the vehicle is operative to assist in steering the vehicle, for example to enable it to execute a spin-turn.

When the two master cylinders 1 and 2 are operated simultaneously the thrust members 59 both act in opposite directions to open both transfer valves 53 so that. the pressure spaces 1 8 are in free communication to com pensate for differential wear of the linings of the rear wheel brakes 42 and 43. Closure of the recuperation valves 19 and 21 of each master cylinder 1, 2 enables the two pressure spaces 1 8 and 20 to be pressurised in order to apply the rear wheel brakes 42 and 43 simultaneously, and fluid from the secondary pressure spaces is applied to the front wheel brakes 63 and 64 through the isolator valve assembly 50. Thus the brakes on all four wheels of the vehicle are applied simultaneously.

When both secondary pressure spaces 20 are pressurised, the valve member 67 is nor mally held in a neutral position with both heads 68, 69 spaced from the seating 65, 66 so that flow to the front wheel brakes 63 and 64 takes place through both flow paths 73.

However, should one of the heads, say the head 69, by in engagement with its seating 66, when both master cylinders are operated simultaneously, then the front wheel brakes 63 and 64 will be applied by the fluid from the secondary pressure space 20 of the master cylinder 1 which passes through the flow path 73 between the head 68 and the housing 60. In such a situation fluid is transferred between the primary pressure spaces 1 8 through the connection 49 to equalise the pressures.

In a modified construction the outlet port 61 from the isolator valve assembly 50 could be connected to a power valve in order to operate a braking system of a trailer or the like.

Claims

1. A dual hydraulic braking system of the kind set forth in which an isolator valve assembly incorporated in a connection between the two secondary pressure spaces has an outlet leading to the auxiliary braking means, the isolator valve assembly being so constructed and arranged as to provide communication between at least one of the secondary pressure spaces and the auxiliary braking means when both master cylinders are operated simultaneously but to isolate the secondary pressure space of a master cylinder from the auxiliary braking means when that master cylinder is operated on its own.

2. A dual hydraulic braking system as claimed in claim 1 in which the isolator valve assembly comprises a housing in which is located a flow-conscious valve member carrying opposed valve heads for alternate engagement with one of a pair of spaced seatings in the housing with the seatings disposed on opposite sides of the outlet to the auxiliary braking means to isolate the secondary space from the auxiliary braking means when the respective master cylinder is operated on its own, a flow path being defined between each head and the housing to permit fluid to flow from at least one of the secondary pressure spaces to the auxiliary braking means when both master cylinders are operated simultaneously.

3. A dual hydraulic braking system as claimed in either claim 1 or claim 2, in which the outlet from the isolator valve assembly is connected to brakes on both front wheels of the vehicle, and the primary pressure spaces are connected to the brakes on different rear wheels of the vehicle.

4. A dual hydraulic braking system as claimed in any of claims 1, 2 or 3 in which the outlet from the isolator valve assembly operates a power valve, in turn for operating a trailer braking system.

5. A dual hydraulic braking system substantially as herein described with reference to the accompanying drawing.