WO1997033101A1

WO1997033101A1 - Braking system incorporating a liquid-cooled brake

Info

Publication number: WO1997033101A1
Application number: PCT/GB1997/000629
Authority: WO
Inventors: Daniel James Philpott; Michael Williamson
Original assignee: Lucas Industries Public Limited Company
Priority date: 1996-03-07
Filing date: 1997-03-06
Publication date: 1997-09-12
Also published as: AU2101497A

Abstract

A braking system has a liquid-cooled brake (1) having a housing (2) containing at least one rotary braking element (3) coupled to a rotary member (3A) to be braked, and a relatively fixed surface for engagement by the braking element to effect braking thereof. The system includes actuator means (4) operable to bring the braking element and fixed surface into braking engagement, and fluid flow means (9) for supplying cooling liquid to the brake during actuation thereof. The fluid flow means serves both to charge the brake with cooling liquid during brake actuation and to withdraw liquid from the brake for cooling and may be arranged to act in proportion to brake actuation energy input.

Description

BRAKING SYSTEM INCORPORATING A LIQUID-COOLED BRAKE

This invention relates to a braking system incorporating a liquid-cooled brake, primarily for a vehicle, the brake being of the kind having a housing containing at least one rotary braking element coupled to a rotary member to be braked, and a relatively fixed surface for engagement by the braking element to effect braking thereof, the system including actuator means operable to bring the braking element and fixed surface into braking engagement, and means for supplying cooling liquid to the brake during actuation thereof.

Brakes of the aforesaid general kind are primarily used in tractors and other off-highway vehicles and the vehicle gearbox oil is then conveniently used as the cooling liquid. Although a relatively large quantity of oil is required for cooling the brake during prolonged and/or heavy periods of braking, it is desirable to reduce the amount of oil present in the housing at other times in order to minimise the well- known parasitic oil drag effects and thus avoid excessive brake running clearances and the consequent long brake pedal travel and/or high actuation input forces.

An object of the invention is to provide a system for actuating a liquid- cooled brake, particularly of the multi-disc type, incorporating improved means for controlling the flow of cooling liquid to and from the brake housing.

According to a first aspect of the present invention, a system of the aforesaid kind includes fluid flow means serving both to charge the brake with cooling liquid during brake actuation and to withdraw liquid from the brake for cooling.

Preferably, the cooling liquid is contained in a chamber, and fluid pressure is applied to the liquid, conveniently by way of an intervening element, such as a piston, for urging the liquid into cooling engagement with the brake.

The intervening element may conveniently be a piston, of which one side partially defines the chamber containing the cooling liquid, and to the other side of which the fluid pressure is applied, the piston being subject to a force which opposes the pressure and acts to retract the piston upon reduction of the pressure, thereby to withdraw cooling liquid from the brake.

The fluid flow means may be arranged to operate so as to supply cooling liquid to the brake whilst simultaneously withdrawing liquid from the brake for cooling.

In a typical practical arrangement, the fluid flow means is a double- acting device having a pair of pistons arranged to perform alternating movements in respective cylinders, such that one piston moves to charge the brake with liquid while the other moves to withdraw fluid from the brake for cooling, and vice versa.

The fluid flow means may conveniently be arranged to act in response to brake actuation energy input, and conveniently in direct proportion to the latter. Preferably, a housing of the fluid flow means is provided with an extended external surface, conveniently in the form of projecting fins, to promote cooling of the housing.

According to a second aspect of the invention, a system of the aforesaid kind includes fluid flow means serving to supply cooling liquid to the brake during brake actuation, the cooling liquid supply being provided in proportion to the actuating force applied to the brake.

Preferably, the cooling liquid is contained in a chamber, and fluid pressure is applied to the liquid, conveniently by way of an intervening element, such as a piston, for urging the cooling liquid into cooling engagement with the brake.

The intervening element may conveniently be a piston, of which one side partially defines the chamber containing the cooling liquid, and to the other side of which the fluid pressure is applied, the piston being subject to a return force which opposes the pressure and acts to retract the piston upon reduction of the pressure thereby to withdraw cooling liquid from the brake.

Preferably, a housing of the fluid flow means is provided with an extended external surface, conveniently in the form of projecting fins, to promote cooling of the housing.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a partly schematic representation of one form of the braking system of the invention;

Figure 2 illustrates a valve for use in evacuating accumulated air from a liquid reservoir of the braking system;

Figure 3 illustrates another valve arrangement;

Figure 4 is a diagrammatic representation of part of an alternative braking system of the present invention;

Figure 5 is a diagrammatic representation of another alternative form of the braking system of the invention;

Figure 6 is a plan view, partly in longitudinal cross-section of a practical arrangement of part of the system of Figure 5;

Figure 7 is a diagrammatic end view of the arrangement of Figure 6, and

Figure 8 is a schematic illustration of one form of control circuit for the system of Figures 5 to 7.

Referring to Figure 1 , this shows a braking system incorporating a multi- disc oil-immersed brake, indicated generally at 1 , of conventional type having a housing 2 containing a multiple brake disc assembly 3 surrounding a shaft 3A coupled to a device to be braked (not shown), the brake being actuated by an air actuation device 4. The construction of the brake will be well understood by a person skilled in the art and a detailed description thereof is not required for an understanding of the present invention. The supply of air to the actuator 4 from a main air supply 5 is controlled by a relay valve 6 in response to a signal emanating from a driver-operated control 7 which may be, for example, a foot pedal or hand lever device. Surplus air from the relay valve 6 is dumped at an exhaust outlet 7A. Air is supplied to the actuator 4 via a first part 8A of a main air supply line, a further part 8B of which supplies air to a reservoir 9 containing a cooling liquid, such as axle or gearbox oil from the vehicle in which the brake is installed.

The reservoir 9 forms a cylindrical chamber 10 containing the cooling liquid and within which slides a piston 1 1 urged by a spring 12 towards an upper fixed end member 13 of the reservoir. Arranged between the air supply line 8B and a fluid inlet 8C to the reservoir 9 is a one-way valve 14 permitting air flow only in the direction towards the reservoir and this is arranged in parallel with an adjustable restrictor device 15, the purpose of which will be referred to hereafter. An oil outlet 16 from an end region of the chamber 10 is connected by a line 17 to an oil inlet 18 of the brake housing 2, the line 17 including a non-return valve 19 preventing return flow of oil to the chamber along this line. An oil outlet 20 of the brake housing is arranged at a level such as to permit a minimum level of cooling liquid to remain within the brake in the non- actuated condition thereof, the outlet 20 being connected by a line 21 to a return port 22 of the reservoir 9 by way of a further non-return valve 23 permitting liquid flow only in a direction towards the chamber 10.

When the relay valve 6 is actuated by a signal from the driver-operated control 7 to allow actuating fluid, in this case air, to be fed into the supply line 8A, 8B, air pressure is supplied simultaneously to the actuator 4 for brake actuation and, via the non-return valve 14 and inlet 8C, to the upper side of the piston 1 1. When the pressure applied to the piston 1 1 is sufficiently high to cause compression of the spring 12, the piston 1 1 will move along the reservoir chamber 10, expelling cooling liquid through the outlet port 16, along supply line 1 7 and through inlet 18 into the brake to cool the latter during actuation thereof. The cooling liquid flows from the brake inlet 18 through an axial passage formed in the brake shaft and is then thrown centrifugally onto the friction linings through radial passages in the shaft. It will be understood that alternative conduit arrangements may be employed for ingress of cooling liquid to the brake.

The higher the actuating pressure and thus braking load applied to the brake, the further and more rapidly the piston 1 1 will be moved by this pressure along the chamber 10, expelling further liquid into the brake. The amount of cooling liquid supplied to or withdrawn from the brake is continuously variable with the braking load as the piston 1 1 moves in one direction or the other along the chamber 10 under the influence of pressure fluid at one side thereof and the spring 12 at the other. Since the braking load is proportional to the actuating pressure, and since it is this pressure which controls expulsion of liquid from the chamber 10, it can be seen that the amount of cooling liquid applied to the brake will vary directly in proportion to the braking load, so that the increasing heat generated at higher braking loads may readily be dissipated by a su^fficiently increased amount of cooling liquid, and vice versa.

When the piston is moved by the spring 12 towards its illustrated rest position away from the outlet 16, liquid is withdrawn from the brake through the brake port 20 along the line 21 , through the non-return valve 23 and return port 20 for reintroduction into the chamber 10. The housing 9 is provided with integral or attached cooling fins 24 which facilitate cooling of the relatively hot liquid returning from the brake. The height of the liquid outlet port 20 in the brake housing 2 is chosen so that a sufficient minimum amount of oil will always remain in the brake housing for lubrication purposes when the brake is in its non- operative condition. It will be seen that the liquid inlet port 18 of the housing is positioned higher than the outlet port 20 to ensure that cool fluid is allowed to flow over the brake components prior to reaching the port 20.

Reduction or cessation of the actuating air pressure will result in the spring 12 urging the piston 1 1 towards the end member 13. Actuating air expelled from the housing 9 by such piston movement will be unable to pass the non-return valve 14 and must pass through the throttling device 15 which is adjusted to permit a gradual decay of the air pressure in the reservoir 9. This controls the aforesaid movement of the piston and thereby withdrawal of liquid from the brake housing to ensure that sufficient liquid remains in the housing for long enough to provide adequate cooling of the brake.

The cooling liquid returning from the brake to the chamber 10 is likely to contain a quantity of entrained air, due to the agitation to which it is subjected within the brake. This air will rise through the chamber 10 and can be transferred to the far side of the piston 11 through ports 1 1 A into a space 11 B, whence it may be evacuated to atmosphere. In order to ensure adequate sealing of the passages 1 1A during application of actuating pressure to the upper side of the piston 1 1 , a valve such as that illustrated in Figure 2 may be embodied in the or each port 1 1A. In an alternative arrangement, the reservoir 9 may be inverted relative to its position shown in Figure 1 so that the piston 1 1 is moved upwardly by the actuating pressure to expel cooling liquid to the brake. With such an arrangement, trapped air would rise to the top of the cylinder and be displaced through line 1 7 to the brake, together with the cooling fluid. The passages 1 1A and associated valves in the piston may then be dispensed with.

Figure 2 illustrates the piston 1 1 provided with a single central passage

40 of which the upper end portion is widened to receive a port element

41 screwed therein. A check valve in the form of a ball 42 lies in the passage 40 and is urged by a spring 43 away from a conical seat 44 within the passage. Upon application of actuating pressure to the piston 1 1 , the ball is forced into sealing engagement with the seat 44 to preclude leakage of actuating fluid through the passage 40. Upon release of actuating pressure, the ball resumes its illustrated position under the action of the spring 43 and trapped air can then bleed past the valve to atmosphere.

Referring again to Figure 1 , the dotted line A represents an optional passage connecting the brake housing 2 to the reservoir 9 at two locations via further passages A1 , A2 leading into the chamber 10 at different levels. This arrangement may usefully be employed when brake actuation is by way of a powered hydraulic supply. A non-return valve A3 is provided between the passage A and passages A1 , A2, arranged to prevent leakage of liquid from the chamber 10 when pressurized by the piston 1 1. The non-return valve A3 is illustrated in more detail in Figure 3 and can be seen to include a floating ball valve 50 illustrated in its closed position against a seat 51. The valve assumes this position during application of actuating pressure to the piston 1 1 to preclude leakage of cooling liquid along the passage A. The passage A1 is connected to the chamber 10 just below the level assumed by the underside of the piston 1 1 in its fully raised position, the normal oil level within the reservoir, illustrated by line L, being level with the underside of the piston. The passage A2 is provided to ensure that no air lock occurs and, with the valve in its open position, as illustrated in dotted lines in Figure 3, air from the underside of the piston can bleed through the passage A to the brake housing. The non-return valves A3, 19 and 23 may be separate units or incorporated in the reservoir housing 9 or brake housing 2.

It will be understood that the reservoir 9 may be connected to a plurality of brakes for the supply of cooling fluid to each brake when actuated simultaneously with another of the brakes, or otherwise.

Referring now to the alternative arrangement illustrated in Figure 4, the illustrated system has a pair of fluid pumps, designated respectively by the reference numerals 61 , 62 and a fluid control circuit indicated generally at 63. Each pump has a cylinder body 61 A, 62A, each containing a pair of opposed pistons 64 urged towards the outer ends of the cylinder bodies by respective springs 65 each acting between the inner end of the associated piston and a valve block 66, which divides the cylinder body into two separate chambers 64A, 65A and contains ports and valves contributing to the operation of the system, in the manner to be described. When the pistons are in their illustrated outermost positions the chambers 64A, 65A will all be full of stored cooling fluid drawn in through non-return valves 68A from the brakes. The cylinder body 61A has an outlet port 67 for the delivery of cooling liquid to a first brake, designated B 1 and a corresponding outlet 67 is provided in the cylinder body 62A for a second brake designated B2. Fluid flow through the ports 67 is controlled by respective pairs of non¬ return valves 67A in the valve blocks 66, each associated with a respective one of the pistons 64. Fluid return ports 68 are provided in the valve blocks 66 of the cylinder bodies, each such port receiving oil returning from the associated brake and delivering it to the chambers 64A, 65A under the control of respective ones of the non-return valves 68A, in the manner to be described.

Communication ports 69 for the respective chambers 64A, 65A are connected respectively by lines 70 to a pair of "OR" gates 71 , 72, and pairs of the ports 69, each containing respectively one such port from each cylinder body, are connected to opposed ends of the respective "OR" gates. Each "OR" gate contains a pair of opposed non-return valves 71 A, 71 B, 72A, 72B and is connected by a line 73 to one side of a respective "NOT" gate indicated generally at 74, 75, the respective connections being indicated at a and b. A main air input valve (not shown), producing, in the illustrated embodiment, an air input signal proportional to the brake air actuating pressure, supplies a four-way connector device 76 having branches 76A, 76B, connected respectively to the "NOT" gates at locations p, q. Each such branch has a by-pass 76C connected at c to its associated "NOT" gate at the opposite end thereof to connection a, b. Respective ports d of the "NOT" gates are connected to opposite ends of a changeover valve, indicated generally at 77, at locations e and f. Ports g and h of the changeover valve are connected respectively to delivery lines 78, 79, of which line 78 is connected to corresponding inlet ports 80 of the respective cylinder bodies 61 A, 62A and line 79 is connected to corresponding inlet ports 81 of the cylinder bodies. Each liquid-cooled brake in the system, when inoperative, contains a relatively small amount of liquid, which is conveniently the minimum required for proper lubrication of the brake components, so that parasitic drag effects within the brake during non-operative periods thereof, are minimised. When the brake is to be operated, a considerable additional amount of cooling liquid needs to be injected to absorb heat generating during braking and, since the brake is normally enclosed within a vehicle chassis and substantially shielded from cooling air flow, it is desirable to withdraw heated fluid from the brake for cooling and reintroduce the cooled fluid back into the brake. The system of the invention enables this to be done in a continuous manner during braking, as will be described hereafter.

It will be understood that the pump bodies have a high ratio of surface area to fluid volume, which may conveniently be achieved by the provision of external fins, enabling rapid heat dissipation to take place from heated fluid from the brakes entering the pump bodies. The pump bodies are located, in use, in a vigorous air stream created by the moving vehicle, in order to enhance the cooling effect.

When there is no air signal from the main air supply (not shown) through the connector device 76, the system components are in the positions illustrated in the drawing. All of the pistons 64 are at the outer ends of thr; chambers 64A, 65A, which latter are full of cooling fluid. If each chamber 64A, 65A is regarded as containing a single unit of fluid, each pump contains two unit volumes, leaving only the aforesaid minimum quantity within the brake. In the illustrated condition of the system, the supply line 78 to the inlet ports 80 of the pumps 61 and 62 is connected through the changeover valve 77 to the device 76, the line 79 to the inputs 81 being, at this stage, closed off. When an air signal derived from the brake-actuating air pressure is applied to the device 76, this pressure is applied along the lines 76A, 76B and by-passes 76C to the "NOT" valves 74, 75 at respective locations c, urging the "NOT" valves into the positions shown so that the lines 76A, 76B connected to the "NOT" valves at locations p, q communicate through these valves with ports d, enabling a small pressure to be applied at locations e and f respectively at the ends of the changeover valve 77. The piston areas of the valve 77 experiencing the pressure from the device 76 are different, with the larger area on the left hand side, so that the pressure urges the changeover valve 77 rightwards to connect the device 76 to the left hand inlet port y, thereby permitting pressure to be supplied through the valve 77 into the line 79 at the outlet port g, and thence to the pistons 64 at the right hand ends of the pumps 61 A, 62A. The respective connections of the device 76 to port y and of the port g to line 79 are represented in broken lines in the drawing to allow for the valve 77 being shown only in its initial position in which these connections are not made.

The aforesaid pressure on the right hand pistons 64 moves them inwardly through the chambers 65A against the opposition of springs 65, so as to expel fluid from the chambers 65A through the one way valves 67A and outlet ports 67 for supply to the brakes B 1 and B2, in order to cool the brakes during actuation. Simultaneously, appropriate ones of the lines 70 transfer fluid from the ports 69 associated with the chambers 65A to opposite ends of the "OR" gate 72, the fluid passing through the one-way valves 72A, 72B and entering the line 73 to be applied to the left hand end of the "NOT" gate 75 at location b. This urges the "NOT" gate to the right and closes off communication at d with the connector device 76. This condition remains, until the moving pistons 64 reach the inner ends of the chambers 65A, whereupon the pressure signal to the "OR" gates diminishes to zero, or substantially so, permitting the "NOT" gate 75 to move once again to the left to apply a heavy pulse of pressure to the changeover valve 77 at location f. This has the effect of pushing the changeover valve to the left, until this valve assumes the illustrated position in which the device 76 is connected, via locations y and h of the valve, to line 78 which supplies the full air pressure from the device 76 to the pistons at the left hand sides of the pumps 61 A, 62A, causing them to move inwardly and expel fluid into the brake. Simultaneously with this, the removal of pressure from the right hand chambers 65A of the pumps enables the springs 65 to return the right hand pistons 64 to the outer ends of the chambers 65A, thereby drawing hot oil from the brakes through non-return valves 68A for cooling within the cylinders. As previously, the back pressure from the cylinders 64A applied through the "OR" valve 71 to the "NOT" valve 74 cuts off the supply to the left hand end of the changeover valve until such time as the back pressure decays towards zero, whereupon the "NOT" valve 74 changes over, in the manner previously described for valve 75, allowing a pressure pulse to be applied to the left hand side of the changeover valve 77.

It can be seen that, during brake application, cooling oil is continuously supplied to the brake for cooling, whilst hot oil is simultaneously withdrawn for cooling within the pump bodies. This arrangement enables excellent cooling of the brake to be achieved over prolonged brake applications, since the cooling cycle continues for as long as the brakes are actuated. When the actuating air pressure is removed, all of the pistons 64 return to the outer ends of the piston bodies, thereby withdrawing fluid from the brake, leaving a minimum amount of oil in the brake for normal operational purposes. The system described senses the inner ends of the piston strokes by responding to the decay in back pressure from the chambers, as described. It would, however, be possible to sense the piston movement in other ways, using suitable sensors and electronic circuitry for example. Although the described system provides pumping of fluid through the brake at a rate proportional to the braking effort, it would be possible for the system to respond to other pressures and/or signals by using appropriate sensors and circuitry. Although the pumps 61 A, 62A are illustrated as composite devices each containing two pistons, it would be possible to place each cylinder in an individual cylinder body, with suitable modifications to the valve arrangements.

A further alternative embodiment of the braking system of the invention is illustrated in Figures 5 to 8. From the diagrammatic representation in Figure 5, it will be seen that the system includes a pump module 100 connected by respective pairs of liquid supply and return lines 100A and 100B to a pair of brakes contained within brake housings 101 , 102. The input line 100A is connected to a liquid input 100C of the brake disposed at a higher level than a liquid outlet 100D to which the return line 100B is connected. Each supply and return line contains a non¬ return valve 103 arranged and operating in the manner to be described, although Figure 5 illustrates a simplification of a practical system, as will become evident hereafter. The system contains a main valve module 104 connected by respective air supply lines 105, 106 to the pump module 100. The valve module 104 controls the flow of pressurised air from a reservoir 107, which may conveniently be the main air actuation supply reservoir on a vehicle to which the brakes are fitted, the valve module controlling operation of the system in response to an air signal derived from the actuating pressure applied to one of the brakes 101 , 102 and applied to the valve module along line 108.

One practical form of the pump module is partially illustrated in Figure 6. Only one half, 100', of the module is illustrated in detail, the other, 100" being a mirror image of the first about the centre line CL of the module and containing identical components. The half-module 100' contains pistons 110, 1 1 1 each urged towards a position adjacent the centre of the module by a respective return spring 112. When each piston 110 and 1 1 1 is in its rest position, as exemplified by the piston 1 10, the chamber 113 behind the piston is filled with cooling liquid which may conveniently be derived from the engine or gear box of the vehicle incorporating the braking system. Air pressure derived from the main brake air actuating system is supplied when required, through individual air conduits 105, 106 between respective pairs of pistons each containing a corresponding piston from each half module. Each piston from each pair is associated with a different one of the brakes 101 , 102. It can be seen from Figure 7 that output and return conduits, designated respectively 114 and 1 15 are provided for the introduction to and removal from the chambers of cooling liquid. Non-return valves 103, are provided to control the flow of fluid between the chambers 113 and conduits 1 14 and 115 which lead respectively to the supply and return lines 100A and 100B. The valves 1 16 are arranged to permit expulsion of fluid from the chambers to the brakes along the conduit 1 14, outlet 1 14A, and supply line 100A but to prevent return of fluid along this conduit. Similarly, the valves 117 are arranged to permit return of heated fluid from the return line 100B into the chambers via an inlet 115A, but to prevent fluid flow in the reverse direction. Figure 8 illustrates in more detail the control valve module 104 of Figure 5 in relation to a diagrammatic representation of the pump module 100. It will be understood that the lines 100A, 100B and valves 103 are connected to the right-hand end of the module 100 in a manner identical to those shown at the left-hand end thereof, as can be seen in Figure 5. A first part of the valve module 104 consists of a slide valve 120 to which is connected the vehicle air actuation reservoir 107. A spool 121 of the valve 120 is subjected to an air pressure signal S taken from one of the brake actuators, so that, upon brake actuation, this signal is applied to the spool 121 to move the latter into an operative position (as shown) in which the air supply 107 is connected through the slide valve 120 to a control valve 122 forming a second part of the valve module 104 and thence to an air inlet conduit 105 connected to the pump module. At this time, an alternative inlet line 106 is connected by the valve 122 to an exhaust path E1. When the air pressure signal S is removed, the spool 121 returns to a position in which the reservoir 107 is disconnected from the control valve 122 and conduit 105 is connected to an exhaust path E of the slide valve which can be seen to operate as an on/off switch between the air supply and control valve 122.

When pressurised air is supplied to the valve 122 in its position as illustrated, it passes through the air inlet conduit 105 to a space between the pistons 1 1 1 , causing these pistons to separate against the action of the return springs 1 12 in order to expel cooling liquid along lines 100A and associated valves 103 into the brakes 101 , 102 via upper inlets 100C (Figure 5). Liquid flow between chambers 1 13 of each half module along lines 100A, 100B is blocked at this time by pairs of non-return valves 103. For simplicity, these valves are represented as single items in Figure 5. When the pressure between the pistons reaches a predetermined value, which arises when the pistons have reached the ends of their respective strokes, a pilot air pressure from the inlet conduit 105, which is applied to a spool of the valve 122 via a branch path 123, causes this spool to switch from its illustrated position to its opposed position in which air pressure is removed from the inlet conduit 105 and applied via a path E3 of the valve to the inlet conduit 106, the inlet conduit 105 then being connected to an exhaust path E2 of the valve. Fluid is expelled into the brakes by the pistons 1 10, whilst at the same time, the pistons 1 1 1 are returned to their innermost positions by the action of the springs 1 12, which has the effect of withdrawing heated fluid from the outlets 100D of the brakes for cooling by heat dissipation from the body of the module 100, which, as can be seen from Figure 6 is provided with cooling fins 140 to enhance the heat dissipation. The return lines 100B may be provided with a one-way valve and throttle arrangement (not shown) such as that shown at 14, 15 in Figure 1 for example, in order to control the withdrawal movement of the pistons under the action of the springs 1 12. A branch 124 between the inlet line 106 and the spool of the valve 122 is provided so that the valve can be reversed when the pressure between the pistons 1 10 reaches the aforesaid predetermined value. Pressure is then once again applied at inlet 105 whilst the pistons 1 10 are returned by the springs 1 12 to withdraw heated liquid from the brakes for cooling and the cycle continues for as long as a signal S is applied to the spool 121 of the slide valve 120. Upon removal of this signal, the spool reverts to its position in which pressure flow from the reservoir 107 is interrupted and the cycle is switched off. It will be understood that the full operative cycle of simultaneous liquid charging and withdrawal only commences, when the system is first used, after an initial liquid expulsion stroke of at least one piston.

Claims

1. A braking system comprising a liquid-cooled brake having a housing containing at least one rotary braking element coupled to a rotary member to be braked, and a relatively fixed surface for engagement by the braking element to effect braking thereof, the system including actuator means operable to bring the braking element and fixed surface into braking engagement, and fluid flow means for supplying cooling liquid to the brake during actuation thereof, the fluid flow means serving both to charge the brake with cooling liquid during brake actuation and to withdraw liquid from the brake for cooling.

2. A braking system according to Claim 1 , wherein the cooling liquid is contained in a chamber and fluid pressure is applied to the liquid for urging it into cooling engagement with the brake.

3. A braking system according to Claim 2, wherein the fluid pressure is applied to the cooling liquid by way of an intervening element.

4. A braking system according to Claim 3, wherein the intervening element is a piston of which one side partially defines the chamber containing the cooling liquid, and to the other side of which the fluid pressure is applied, the piston being subject to a force which opposes the pressure and acts to retract the piston upon reduction of the pressure, thereby to withdraw cooling liquid from the brake.

5. A braking system according to any one of the preceding claims, wherein the fluid flow means is arranged to operate so as to supply cooling liquid to the brake whilst simultaneously withdrawing liquid from the brake for cooling.

6. A braking system according to Claim 5, wherein the fluid flow means is a double-acting device having a pair of pistons arranged to perform alternating movements in respective cylinders, such that one piston moves to charge the brake with liquid while the other moves to withdraw liquid from the brake for cooling, and vice versa.

7. A braking system according any one of the preceding claims, wherein the fluid flow means is arranged to act in response to brake actuation energy input.

8. A braking system according to Claim 7, wherein the fluid flow means acts in proportional response to the brake actuation energy input.

9. A braking system according to Claim 6, wherein pistons of the double-acting device are arranged in opposed relationship, being urged apart by force-generating return means and subject to the fluid pressure applied in opposition to the return means, application of the fluid pressure to the respective pistons being effected alternately so that as one piston is pressurised to expel fluid to the brake, the other is returned by the return means to withdraw liquid from the brake for cooling.

10. A braking system according to Claim 9, wherein valve means controlling the flow of cooling liquid to and from the brake is disposed between the pistons.

1 1 . A braking system according to Claim 9 or Claim 10, wherein the application of fluid pressure alternately to the pistons is effected in response to pressure signals derived from the liquid pressure in the fluid flow means at end of travel positions of the respective pistons reached under the influence of said fluid pressure.

12. A braking system according to Claim 1 1 , wherein the application of fluid pressure to the pistons alternately is effected by a changeover valve controlled by a pair of NOT gates responding respectively to said pressure signals, the NOT gates cooperating with respective OR gates to cause fluid pressure to be switched from one piston at its end of travel to the other piston at the beginning of its travel.

13. A braking system according to Claim 12, wherein the changeover valve acts in response to differential pressures applied thereto.

14. A braking system according to Claim 9, wherein opposed pistons of the double-acting device are urged by the return means towards each other and fluid under pressure is applied between the pistons.

15. A braking system according to Claim 14, wherein control of the double-acting device is effected by a two-position valve which, in each of its two positions respectively, permits the supply of pressure fluid exclusively to one of two pairs of opposed pistons of the module, the valve being moved between the two positions in response to one pair of pistons reaching end of travel positions, the application of pressure fluid to the two-position valve being switched on by a further valve responsive to a signal derived from a brake of the system upon brake actuation.

16. A braking system comprising a liquid-cooled brake having a housing containing at least one rotary braking element coupled to a rotary member to be braked, and a relatively fixed surface for engagement by the braking element to effect braking thereof, the system including actuator means operable to bring the braking element and fixed surface into braking engagement, and fluid flow means serving to supply cooling liquid to the brake during brake actuation, the cooiing liquid supply being provided in proportion to the actuating force applied to the brake.

17. A braking system according to Claim 16, wherein the cooling liquid is contained in a chamber and fluid pressure is applied to the liquid for urging it into cooling engagement with the brake.

18. A braking system according to Claim 1 7, wherein the fluid pressure is applied to the cooling liquid by way of an intervening element.

19. A braking system according to Claim 18, wherein the intervening element is a piston of which one side partially defines the chamber containing the cooling liquid, and to the other side of which the fluid pressure is applied, the piston being subject to a force which opposes the pressure and acts to retract the piston upon reduction of the pressure, thereby to withdraw cooling liquid from the brake.

20. A braking system according to Claim 19, wherein the force opposing the pressure is a spring acting against said one side of the piston.

21. A braking system according to Claim 19 or Claim 20, wherein the piston is provided with one or more axial through passages controlled by a one-way device and serving to permit the bleeding, when the piston is retracted, of air entrapped in liquid withdrawn from the brake.

22. A braking system according to any one of the preceding claims, wherein fluid pressure derived from the brake actuating fluid is applied to the cooling liquid in the chamber by way of one-way valve means and leaves the chamber by way of throttling means which permits a gradual decay of said pressure.

23. A braking system according to any one of the preceding claims, wherein a housing of the fluid flow means is provided with an extended external surface to promote cooling of the housing.

24. A braking system according to Claim 23, wherein the extended surface is in the form of projecting fins.