GB2036216A - Hydraulic brake priority system - Google Patents

Abstract

A hydraulic power brake system for a tractor trailer combination includes a pump (12), a demand valve (13), a brake valve (16) and auxiliary hydraulic apparatus (21), e.g. a loading shovel. The demand valve (13) is arranged to open to supply the auxiliary apparatus (21) from the pump 12 at a pre-determined fluid pressure, but close in response to a fluid pressure signal from the brake valve (16) when actuated; the fluid pressure signal ceasing as the brake valve demand is satisfied. The fluid pressure signal constantly vents through a fluid restriction (27) to allow the demand valve (13) to reopen as the signal decays. <IMAGE>

Description

SPECIFICATION Improvements in hydraulic brake systems and control valves therefor This invention relates to apparatus for dividing a supply of pressurised hydraulic fluid between a hydraulic brake system and other hydraulic apparatus, being particularly though not exclusively for tractors equipped with means for supplying hydraulic fluid under pressure for auxilliary services and, on demand, preferentially diverting that pressurised fluid to actuate the tractor wheel brakes. The invention further relates to actuation of trailer wheel brakes.

According to the invention there is provided a hydraulic brake system for a tractor having a pump to supply hydraulic fluid under pressure through a demand valve to a brake valve for wheel brake application and selectively to auxilliary hydraulic apparatus in response to driver actuation of the brake valve, characterised thereby that the demand valve includes means which, in response to increase of fluid pressure in the demand valve as a consequence of initiation of manual actuation of the brake valve, cuts off the supply of hydraulic fluid to the auxilliary apparatus so that the hydraulic fluid supply is wholly available for brake application.

Preferably the demand valve comprises a piston movable in a closed bore to and from a closed condition, in which communication between the pump and the auxilliary apparatus is closed, the piston being loaded towards the closed condition by a spring in its bore and fluid connection being provided to the bore at both sides of the piston so that, when the pump is not running the spring moves the piston to the closed condition, when the pump is running and the brake valve is not operated the piston is moved against the spring from the closed condition, and when the pump is running and the brake valve is operated by the driver the piston moves to the closed condition.

Other features of the invention are included in the following description of three preferred embodiments shown, by way of example, on the accompanying drawings in which: Figure 1 is a schematic representation of a hydraulic fluid circuit incorporating one embodiment of the invention; Figure 2 is a schematic representation of a hydraulic fluid circuit incorporating a second embodiment of the invention; and Figure 3 is a schematic representation of a hydraulic fluid circuit incorporating a third embodiment of the invention.

The apparatus shown is suitable for an agricultural tractor having a pump for the supply of hydraulic fluid to auxilliary tractor apparatus, e.g. a loading shovel; the tractor having hydraulically actuable wheel brakes and being capable of towing a trailer also having hydraulically actuable wheel brakes.

Referring to Fig. 1, in which arrows indicate the direction of fluid flow, there is shown a hydraulic fluid reservoir 11 from which a pump 1 2 delivers fluid under pressure to the inlet port 22 of a demand valve 13, and through a non-return valve 14 to the feed port 15 of a brake valve 16.

The demand valve 1 3 has piston 1 7 biased by a spring 1 8 to isolate the inlet port 22 from an outlet portal 9 which is connected through the auxilliary tractor apparatus 21 to the reservoir 11. The spring chamber 20 of the demand valve 1 3 is vented through a fluid restrictor 27, also to the reservoir 11.

The brake valve 1 6 has a signal port 23 connected to the spring chamber 20 of the demand valve 13, a trailer brake port 24 connected to the trailer brakes 25 and a drain port 26 vented to the reservoir 11.

A conventional brake master cylinder 30, having a spring-returned piston 31 and a fluid reservoir 32 forms part of the brake valve 1 6 and is connected to the tractor brakes 33.

Co-axial with the master cylinder 30, a stepped bore 40 within the brake valve 16, houses a sleeve valve 41 which is operable through a spring box 42 by the thrust rod 43 of a tractor brake pedal (not shown) to place the feed port 1 5 sequentially in communication with the trailer brake port 24 and the signal port 23. The spring box 42 constitutes the brake valve "feel" device.

A push rod 44 which passes axially through the sleeve valve 41 can mechanically transmit pedal load from the thrust rod 43 to the master cylinder piston 31 for emergency tractor brake operation, as will be subsequently explained. In the return position of the brake valve 16, as shown the push rod 44 serves to mechanically connect the master cylinder piston 31 and the spring box 42 to return the sleeve valve 41 to its inactive position by virtue of the master cylinder spring 34.

A fluid passage 45 connects the trailer brake port 24 through a booster chamber 35, of the master cylinder, to a reaction chamber 36 of the brake valve 16, as shown.

Operation of the apparatus will now be described.

When the brake pedal is not applied the tractor brakes 33 are vented to the master cylinder reservoir 32, the trailer brakes 25 are vented to the reservoir 11 and the spring chamber 20 of the demand valve 1 3 is unpressurised. The pump 1 2 is running and the demand valve piston 1 7 is held by the influence of fluid under pressure to connect the inlet port 22 with the outlet port 1 9 and hence the auxilliary apparatus 21. The sleeve valve 41 of the brake valve 16 in covering the port 15.

Application of the tractor brake pedal causes the thrust rod 43 to advance the push rod 44 and the sleeve valve 41 in unison. The master cylinder piston 31 isolates the tractor brakes 33 from the master cylinder reservoir 32 add the sleeve valve 41 isolates the trailer brakes 25 from the reservoir 11.

Further travel of the thrust rod 43 under pedal pressure causes the sleeve valve 41 to uncover the feed port 1 5 and admit pressurised fluid through the trailer brake port 24 to the trailer brakes 25. Simultaneously, through the passage 45, pressurised fluid is admitted to the booster chamber 35 of the master cylinder 30 and to the reaction chamber 36 of the brake valve 16.

Fluid pressure in the booster chamber 35 causes the master cylinder piston 31 to move away from the push rod 44 and the master cylinder pressure so generated actuates the tractor brakes 33. This same fluid pressure in the interconnected reaction chamber 36 loads the thrust rod 43 through the sleeve valve 41 and spring box 42 to resist further advance of the thrust rod 43. The push rod 44 is not inoperative.

Still further travel of the thrust rod 43 causes the sleeve valve 41 to connect the feed port 1 5 to the signal port 23 and pressurised fluid passes into the spring chamber 20 of the demand valve 1 3 to balance the fluid pressure at the inlet port 22. The spring 1 8 now moves the piston 1 7 to isolate the outlet port 1 9 from the inlet port 22 and the entire fluid flow from the pump 1 2 is diverted through the non-return valve 1 4 to the brake valve 16. The fluid restrictor 27 retains sufficient fluid pressure in the demand valve spring chamber 20 to ensure that the piston 1 7 maintains isolation of the outlet port 1 9.

As a result of the above operation the tractor and trailer brakes 33 and 25 are applied in accordance with the pedal load on the thrust rod 43 and as the brake valve 1 6 becomes balanced the sleeve valve 41 returns to cover again the signal port 23, the brakes are held applied.

Fluid pressure in the demand valve spring chamber 20 decays through the fluid restrictor 27 and the piston 1 7 returns under the influence of fluid pressure from the pump 1 2 to re-connect the inlet port 22 and the outlet port 19.

If the fluid pressure loading on the sleeve valve 41 is exceeded by the brake applying pedal load acting through the thrust rod 43, the spring box 42 will collapse and mechanically connect the thrust rod 43 with the master cylinder piston 31 to transmit the excess load.

Release of load on the thrust rod 43 causes the master cylinder piston 31 and sleeve valve 41 to return to the inactive position under the influence of the master cylinder spring 34.

The tractor and trailer brakes 33 and 25 will exhaust to their respective reservoirs 32 and 11.

Should the pump 1 2 fail, the tractor brakes 33 will remain operable by mechanical connection of the thrust rod 43 with the master cylinder piston 31 through the push rod 44.

Thrust rod loads will be appreciably higher, there being no boost assistance, and the trailer brakes 25 will be inoperable.

An alternative embodiment, shown in Fig.

2, incorporates a brake valve 50 without a separate fluid reservoir for supplying the tractor brakes 33. The hydraulic fluid circuit is identical to that shown in Fig. 1 and the common features carry the same reference numerals. The brake valve 50 is shown in the inactive position with both tractor and trailer brakes vented through the drain port 26 to the fluid reservoir 11.

The brake valve 51 has a stepped bore 51 housing, in a larger end a sleeve valve 52 which is biased away from the larger end of the bore 51 by a spring 53. The spring chamber 54 so formed is connected to the tractor brakes 33 through the end port 37.

At the other end of the brake valve 50 the sleeve valve 52 has an annular extension 55 passing into a smaller diameter portion of the bore 51. An annular booster chamber 56, formed between the sleeve valve 52 and a major step in the bore 51, communicates directly with the trailer brakes 25 through the port 24.

The sleeve valve 52 houses a valve spool 59, which acts through a spring box 57, on a cup shaped piston 58, the base of which carries a resilient pad 60 to close, under pedal pressure, a return port 61 formed in a closed end of the sleeve valve 52, as shown.

The cup piston 58 is urged away from the closed end of the sleeve valve 52 by a light return spring 62 and a fluid return passage 63 is provided through the base of the cup piston 58.

The thrust rod 42 is connected to the brake pedal, (not shown), and acts on a push rod 64 which is slidable in the open end of the brake valve bore 51.

The push rod 64 passes into the annular extension of the sleeve valve 52 to act against the spool 59, but the push rod 64 can act against the sleeve valve extension under certain circumstances as will be explained.

The sleeve valve 52 and spool 59 have fluid ducts, cross ports and annular chambers, as shown, which provide for interconnection of the various parts of the hydraulic circuit.

Operation of the second embodiment is as follows:- When the brake pedal is not applied and the pump 1 2 is running, the feed port 1 5 of the brake valve 50 is pressurised and the spring chamber 20 of the demand valve 1 3 is vented.

Application of the tractor brake pedal causes the thrust rod 43 to advance the push rod 64 and the spool 59 in unison. The spool 59 covers the sleeve valve drain passage 65 to isolate the trailer brake port 24 and the light return spring 62 yields and allows the pad 60 carried by the cup piston 58 to close the return port 61, and so isolate the tractor brake port 37 from the drain passage 65. At the same time the spool 59 places the feed port 1 5 and the signal port 23 in communication, so pressurising the spring chamber 20 of the demand valve 1 3 and diverting the entire output flow from the pump 1 2 to the feed port 1 5 as previously described.

Further travel of the thrust rod 43 advances the spool 59 to place the feed port 1 5 in communication with the booster chamber 56 and hence the, trailer brake port 24, also the spring box 57, between the spool 59 and the cup piston 58 is partially collapsed.

Fluid pressure in the booster chamber 56 urges the sleeve valve 52 against the effect of the return spring 53 to pressurise fluid in the spring chamber 54 and hence, through tractor brake port 37 apply the tractor brakes 33.

This pressure also acts in the spring chamber 57 against the spool 59 to provide resistance against further advance of the thrust rod 43 and provide a "feel" indication of the degree of application of the tractor and trailer brakes, 33 and 36.

As the brake valve 50 becomes balanced and the sleeve valve 52 advances relatively to the spool 59, the feed port 1 5 and signal port 23 are re-isolated, and the demand valve 1 3 acts as previously described to re-supply the auxilliary apparatus 21.

If the fluid pressure loading on the spool 29 is exceeded by the brake applying load acting through the thrust rod 43, the spring box 57 will collapse still further until the thrust rod 43 acts mechanically, through the push rod 64, on the sleeve valve 52 to transmit the excess thrust.

Release of the brake pedal removes the load from the thrust rod 43 and the sleeve valve 52, the spool 59 returns under the action of springs 54 and 62 and the spring box 52 to the position shown. Tractor and trailer brakes 33 and 25 vent to the fluid reservoir 11.

Should the pump 1 2 fail, tractor brakes 33 will be operable through direct mechanical connection of the thrust rod 43 and sleeve valve 52 through the push rod 64.

The third embodiment, shown in Fig. 3, incorporates the brake valve and demand valve in the same housing. The hydraulic fluid circuit is the same as that of previous embodiments and the common features carry the same reference numerals.

No fluid connection is shown to the tractor brakes, the valve being responsive to a fluid signal from a tractor brake master cylinder 17.

The demand valve 1 3 comprises a piston 17, spring biased to close communication from the inlet port 1 3 to the auxilliaries outlet port 19, Pressurised fluid supplied from the demand valve through a brake valve feed port 72 and a fluid duct 73 to the brake valve 16, as previously described.

The brake valve 1 6 has a spool 74 operatively connected through a spring box 75 to the tractor brake pedal by a hydraulic master cylinder 71 and co-operating slave cylinder 76.

The non-return valve 1 4 is situated downstream of the brake valve 16, between an outlet port 77 and the trailer brake port 24.

While the brake valve is in the return position, the trailer brakes 25 are vented through fluid duct 78, cross port 79, fluid duct 81, spring box 75 and drain port 26.

A feedback piston 82, responsive to trailer brake pressure, acts through the spool 74 and spring box 75, when the trailer brakes are applied, to oppose further brake applying movement of the tractor brake pedal and so provide an indication of the degree of trailer braking effected.

The outlet port 77 also supplies a fluid pressure signal through a fluid duct 83 to the blind face of the demand valve piston 1 7. The duct 83 is connected through fluid restrictor 27 and port 84 to the fluid reservoir 11.

The operation of this embodiment will now be described: When the tractor brake pedal is not applied and the pump 1 2 is running, the feed port 72 of the brake valve is pressurised and the spring chamber 20 of the demand valve 1 3 is vented. At a threshold pressure level, determined by the pre-load of spring 1 8 the piston 1 7 moves to open communication from the inlet port 22 to the auxilliary outlet port 1 9 so as to supply the auxilliary apparatus 20.

When the tractor brake pedal is applied, the piston of the slave cylinder 76 acts through the spring box 75 to move the brake valve spool 74 and place the outlet port 77 in communication with the pressurised fluid duct 73. Fluid passes through the non-return valve 14, duct 78 and port 24 to actuate the trailer wheel brakes 25. Fluid also passes into duct 83 and as the pressure drop across the fluid restriction 27 increases, so the pressure acting on the blind face of the demand valve piston 1 7 increases, in opposition to the pressure at the inlet port 22.

Should the fluid requirement of the brake valve 1 6 be such that the pressure across the demand valve spool 1 3 becomes substantially equalized, then the piston 2x will move under the action of spring 1 8 to close the auxilliaries outlet port 1 9. Thus the entire output of the pump 1 2 is available for trailer wheel brake actuation as previously described.

Trailer wheel brake pressure acts in the known manner through feedback piston 82 on brake valve spool 74 and when the required degree of braking is achieved the spool 74 moves to isolate the outlet port 77 and so trap the fluid pressure in the trailer brakes.

Isolation of outlet port 77 causes the fluid pressure in duct 83 to decay to drain through the fluid restriction 27, and the demand valve spool 1 7 moves under the action of pump pressure to reconnect the inlet port 22 and the auxilliaries outlet port 1 9.

Further brake pedal application will repeat the sequence of events detailed above until the brake valve spool 74 is again balanced.

Removal of brake pedal load causes the spool 74 to return to the inactive position under the action of spring 85 and the trailer brakes are vented through port 79 and spring chamber 75.

Claims (9)

1. A hydraulic brake system for a tractor having a pump to supply hydraulic fluid under pressure through a demand valve to a brake valve for wheel brake application and selectively to auxilliary hydraulic apparatus in response to driver actuation of the brake valve, characterised thereby that the demand valve includes means which, in response to increase of fluid pressure in the demand valve as a consequence of initiation of manual actuation of the brake valve, cuts off the supply of hydraulic fluid to the auxilliary apparatus so that the hydraulic fluid supply is wholly available for brake application.

2. A hydraulic brake system according to Claim 1, characterised thereby that the demand valve comprises a piston movable in a closed bore to and from a closed condition, in which communication between the pump and the auxilliary apparatus is closed, the piston being loaded towards the closed condition by a spring in its bore and fluid connection being provided to the bore at both sides of the piston so that, when the pump is not running the spring moves the piston to the closed condition, when the pump is running and the brake valve is not operated the piston is moved against the spring from the closed condition, and when the pump is running and the brake valve is operated by the driver the piston moves to the closed condition.

3. A hydraulic brake system according to Claim 2, characterised thereby that fluid connection is constantly provided to one side of the piston from the pump and from the other side of the piston through a fluid restriction to the reservoir.

4. A hydraulic brake system according to any preceeding claim, characterised thereby that the brake valve includes means for regulation of the pressure of fluid from the pump in accordance with brake pedal actuation and means for supplying the fluid at the regulated pressure for brake application.

5. A hydraulic brake system according to Claim 4, characterised thereby that means are provided for distribution of the regulated pressure fluid to more than one hydraulic brake circuit.

6. A hydraulic brake system according to Claim 5, characterised thereby that means are provided for distribution of regulated pressure fluid to one brake circuit in a constant ratio to that supplied to another brake circuit.

7. A hydraulic brake system according to Claim 5 or Claim 6, characterised thereby that supplementary means are provided for generation of hydraulic fluid pressure for brake application, in accordance with brakeppedal actuation, in at least one brake circuit when the pump is not running.

8. A hydraulic brake system according to any preceeding claim, characterised thereby that a non-return valve is provided in the fluid supply line between the pump and the brake valve.

9. A hydraulic brake system substantially as described herein and as illustrated by any of Figs. 1 to 3 of the accompanying drawings.