GB2078325A

GB2078325A - Control valve assemblies for dual hydraulic braking systems

Info

Publication number: GB2078325A
Application number: GB8117556A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1980-06-17
Filing date: 1981-06-09
Publication date: 1982-01-06
Also published as: GB2078325B

Abstract

In a dual hydraulic braking system in which brakes (6a) on opposite sides of a vehicle can be applied simultaneously for retarding or independently for steering the vehicle, a dual control valve assembly (6) ensures that the brakes on one side cannot be applied before the brakes on the opposite side. When the brakes on one side are to be applied independently, a stem (20) carrying inlet and exhaust valve heads (19 and 21) moves with a pedal-operated piston (12) so that the exhaust valve head (21) engages with a valve spool (25) to close an exhaust passage (28). The spool (25) moves with the piston (12) and head (21) until it is arrested by a stop face (32), whereafter a seating (18) on the piston (12) moves away from the inlet valve head (19) to allow pressurised fluid to be supplied to the outlet (24) for the brakes. When the brakes are to be applied simultaneously, a volume of fluid trapped in a transfer connection (31) between the exhaust valve heads (21) arrests movement of the spools (25) to ensure that the inlet valves open simultaneously. <IMAGE>

Description

SPECIFICATION Improvements in control valve assemblies for hydraulic braking systems This invention relates to improvements in control valve assemblies for vehicle dual hydraulic braking systems of the kind commonly used on agricultural tractors and like vehicles in which separate brakeapplying valves are operable independently or simultaneously by independently operable pedals to cause hydraulic fluid under pressure to be supplied to one or both of a pair of outlets for connection to brakes on wheels on opposite sides of a vehicle.

The brake-applying valves may be housed in a common housing and can be operated simultaneously for retarding the vehicle or separately for steering.

Known control valve assemblies of the kind set forth are usually operated by a yoke comprising a one-piece member which is pivotally connected at substantially the mid-point in its length to a valveoperating mechanism for controlling the hydraulic fluid under pressure and, at opposite ends, is coupled to different pedals, the yoke also being provided with spaced abutments for operating the brake-applying valves. When both pedals are operated simultaneously the yoke is moved bodily away from the housing to withdraw and operate the valve operating mechanism and allow both brakeapplying valves to open or remain open so that the hydraulic fluid under pressure is supplied to both outlets.When one pedal is operated on its own, initially the yoke pivots about the pivotal connection with the valve-operating mechanism with the abutment for the brake-applying valve corresponding to that pedal moving away from the said brakeapplying valve which can open or remain open and the other abutment closing the other brake-applying valve. Further movement of that pedal in the same direction acts through the yoke to close or increase a closing force applied to the said other brakeapplying valve, and withdraws the valve-operating mechanism from the housing so that pressurised fluid is supplied through the open brake-applying valve to the brakes on that side of the vehicle.

According to our invention a control valve assembly of the kind set forth comprises a pair of pedal-operated brake-applying pistons working in separate bores to control the supply of brakes on opposite sides of a vehicle of pressure fluid in pressure spaces in the bores in advance of the pistons, a normally closed inlet valve associated with each piston for isolating a source of high pressure fluid from a respective pressure space, a normally open exhaust valve associated with each piston for providing communication between a respective pressure space and a reservoir for fluid, movement of a piston in a brake-applying direction closing the respective exhaust valve to isolate the pressure space from the reservoir and thereafter opening the inlet valve whereby pressure fluid from the source is supplied to the pressure space, and balancing means for ensuring that following closure of both exhaust valves, both inlet valves can only open substantially simultaneously when both pedals are operated simultaneously.

This ensures that the brakes on one side of a vehicle cannot be applied before the brakes on the opposite side when both pedals are operated simultaneously.

The balancing means may comprise a transfer connection which interconnects the pressure spaces through transfer passages in valve spools when the exhaust valves are open. Closure of an exhaust valve by engagement of a head with an adjacent end of a respective spool to close an exhaust passage in the spool also closes the transfer passage, but the respective inlet valve cannot open until movement of the spool in a brake-applying direction with the piston and the head has been arrested. When one pedal is operated on its own this is achieved by the engagement of the spool with a stop face. However, when both pedals are operated together this is achieved by the column of fluid which is trapped in the transfer passages and the transfer connection, between the heads of the exhaust valves.

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

In the braking system illustrated in the drawing a pump 1 is arranged to draw fluid from a reservoir 2 and pump it through a one-way valve 3 to an hydraulic accummulator 4. The accummulator 4 supplies fluid under pressure to an inlet 5 of a dual control valve assembly 6 which controls the supply of fluid to brakes 6a on wheels on opposite sides of the vehicle depending upon whether one or both of a pair of pedals are operated independently or simultaneously. The pump 1 is driven continuously and, when the accummulator 4 is fully charged, fluid is returned from the pump 1 to the reservoir 2 through a by-pass passage 7 which includes a one-way valve 8to prevent reverse circulation.

The dual control valve 6 comprises a housing 9 provided with two parallel stepped bores 10,11, each incorporating a single pedal-operated control valve.

Since both control valves are identical only one need be described.

A piston 12 working in the portion of the bore 10 which is of greater diameter is engaged a its rear end by a pedal-operated push-rod 13. A return spring 14 acting between an apertured stop member 15 in abutment with a shoulder at a step at the change in diameter normally urges the piston 12 at its outer end against a stop 16 in the housing 9.

The piston 12 has a central blind bore 17 formed at its inner end with a radial flange 18 forming a seating for a inlet valve head 19 which is carried by a stem 20 projecting axially through the stop member 15 and carrying an exhaust valve head 21. A spring 22 normally urges the head 19 into engagement with the seating 18 to isolate the inlet 5 from a pressure space 23 in advance of the piston 12, which pressure space is connected to the wheel brake 6a on one side of the vehicle through an outlet port 24.

A spool 25 is slidably guided in the portion of the bore 10 which is of smaller diameter and is normally held in engagement with a stop face 26 surrounding the head 21 by means of a spring 27. The spool 25 has an exhaust passage 28 formed by an axial passage and a radial port which provides communication between the pressure space 23 and a low pressure outlet 29 in the housing 9 which is connected to the reservoir 2. The spool 25 also has an axial through-bore 30 defining a transfer passage which is connected to the transfer passage of the other control valve through a transfer connection 31.

In the inoperative "brake-off" position shown in the drawing the inlet heads 19 are in engagement with the seatings 18 to isolate the high pressure inlet 5 from the pressure spaces 23, and the pressure spaces 23 are in communication with the reservoir 2 through the exhaust passages 28 and the low pressure outlet 29.

When one pedal is operated on its own, say the pedal acting on the piston 12 in the bore 10, initial movement of the piston 12 causes the head 21 to engage the adjacent end of the spool 25, thereby isolating the pressure space 23 from the reservoir 2 and from the other pressure space by closing the inner ends of the exhaust passage 28 with the transfer passage 30. Since the spring 22 is stronger than the spring 27 further movement of the piston 12 in the same direction carries the spool 25 with it with the head 19 remaining in engagement with the seating 18 until movement of the spool 25 and the head 21 is arrested by engagement with a stop face 32 at the adjacent end of the bore 10.Additional movement of the piston 12 then causes the seating 18 to move away from the inlet head 19 so that fluid under pressure can be supplied from the inlet 5 and to the outlet port 24 through the pressure space 23.

When both pedals are operated simultaneously, the exhaust valve heads 21 engage with the spools 25 and the spool 25 are advanced in their bores as described above. However, in this instance, movement of the spools 25 with the pistons 12 is arrested by the volume of fluid in the transfer passages 30 and the transfer connection 31 which is trapped between the exhaust valve heads 21. Thereafter both inlet valve seats 18 move away from the heads 19 simultaneously so that fluid from the inlet 5 is supplied through both pressure spaces 23 to the brakes on the wheel on both sides of the vehicle.

The distance through which one spool 25 can move before engaging the stop 32 when one pedal is operated on its own is chosen to be greater than the distance through which the exhaust valve head 21 of the other control valve must move before it can engage within its respective spool 25. This ensures that, when both pedals are operated together, movement of the spool 25 can be arrested only by the trapped volume of fluid.

Should the pump 1 fail or the accumulator 4 because exhausted, or both, the brakes can be operated hydrostatically with the or each piston 12 acting to pressurise fluid trapped in a respective pressure space 23 by the one-way valve 3 which prevents reserve flow to the pump 1 and the by-pass passage 7.

Claims

1. A control valve assembly of the kind set forth comprising a pair of pedal-operated brake-applying pistons working in separate bores to control the supply of brakes on opposite sides of a vehicle of pressure fluid in pressure spaces in the bores in advance of the pistons, a normally closed inlet valve associated with each piston for isolating a source of high pressure fluid from a respective pressure space, a normally open exhaust valve associated with each piston for providing communication between a respective pressure space and a reservoir for fluid, movement of a piston in a brake-applying direction closing the respective exhaust valve to isolate the pressure space from the reservoir and thereafter opening the inlet valve whereby pressure fluid from the source is supplied to the pressure space, and balancing means for ensuring that following closure of both exhaust valves, both inlet valves can only open substantially simultaneously when both pedals are operated simultaneously.

2. A control valve assembly as claimed in Claim 1, in which the balancing means comprises a transfer connection which interconnects the pressure spaces when the exhaust valves are open.

3. A control valve assembly as claimed in Claim 2, in which the transfer connection interconnects the pressure spaces through transfer passages in valve spools.

4. A control valve assembly as claimed in Claim 3, in which each exhaust valve comprises an exhaust valve head which is engageable with an adjacent end of a respective valve spool to close an exhaust passage in the spool and to also close the transfer passage.

5. A control valve assembly as claimed in Claim 4, in which each spool is movable with the piston and the exhaust valve head in a brake-applying direction upon engagement of the said head with the adjacent end of the spool, and each inlet valve cannot open until movement of the spool in the brake-applying direction is arrested.

6. A control valve assembly as claimed in Claim 5, in which the movement of the spool in the brake-applying direction is arrested by engagement of the spool with a stop face when only one pedal is operated and the volume of fluid trapped in the transfer passages and the transfer connection arrests movement of the spool in the brake-applying direction when both pedals are operated together.

7. A control valve assembly as claimed in Claim 6, in which the distance through which each spool moves before it engages with its respective stop face is greater than the distance through which the exhaust valve head of the other exhaust valve moves before it engages with the adjacent end of the respective spool.

8. A control valve assembly as claimed in any of the Claims 5 to 7, in which the inlet valve comprises an inlet valve head which normally engages in a closed position with an inlet valve seating, said head and said seating being relatively movable to open the inlet valve only when movement of the spool in the brake-applying direction is arrested.

9. A control valve assembly as claimed in Claim 8, in which each piston has a central blind bore formed at its inner end with a radial flange which comprises the inlet valve seating, and the inlet valve head is carried by a stem which also carries the exhaust valve head, the inlet valve head being normally urged into engagement with the radial flange by a spring within the central blind bore and the exhaust valve head being normally spaced from the adjacent end of the valve spool.

10. A control valve assembly of the kind set forth substantially as described herein with reference to and as illustrated by the single Figure of the accompanying drawings.