GB2093938A

GB2093938A - Quick take-up and release of brakes

Info

Publication number: GB2093938A
Application number: GB8205315A
Authority: GB
Original assignee: Davies and Metcalfe Ltd
Current assignee: Davies and Metcalfe Ltd
Priority date: 1981-02-27
Filing date: 1982-02-23
Publication date: 1982-09-08
Also published as: GB2093938B

Abstract

Air pressure in a brake pipe BP of a railway vehicle braking system varies between a maximum P1 corresponding to "run" condition with brakes fully released and a minimum pressure P2 corresponding to "full service" application. Between P1 and P2 is a predetermined brake pipe pressure P3 at which the brakes are at "initial application". As the brake pipe pressure drops from P3 to P2 the brakes are progressively increasingly applied. The brake pipe pressure is controlled by a known transmission valve 166 so that the brake pipe pressure equals the pressure in conduit 64. The pressure in conduit 64 is controlled by operation of a joystick to open or close brake application valve 22 or brake release valve 20. In control valve 74 chamber 70 is supplied with air at pressure P1 via conduit 34. Opening release valve 22 at "run" condition drops the pressure in chamber 78 below P1 to a predetermined value P4 between P1 and P3. This causes diaphragm 76 to move down lowering the spindle 82 to bring relatively large control reservoir 160 at pressure P1 into communication with relatively small minimum reduction reservoir 154 at atmospheric pressure. There is a sudden and automatic equalisation of the reservoir pressures by a drop to pressure P3. Since this pressure is on conduit 64 the brake pipe pressure rapidly drops to P3, the initial application pressure. If application valve 22 is held open the pressure in conduit 64 drops to minimum P2 by air escape through choke 52. The air pressure in conduit 64 is increased to release the brakes by supplying air pressure to the conduit through open release valve 20. When air pressure in chamber 78 rises to a predetermined value P5 between P3 and P4 the spindle 82 is lifted suddenly by the effort of springs 84 and 150 causing the chamber 78 to communicate with chamber 70 via conduit 110, poppet valve 102, 108 and bore 92. This causes a rapid rise in the pressure in chamber 78 and conduit 64 from P5 to P1 and a corresponding increase in brake pipe pressure. <IMAGE>

Description

SPECIFICATION Brake control apparatus This invention concerns apparatus for controlling variation in fluid pressure in a brake pipe of a rail vehicle braking system.

In such a system it is known to operate brake application and release mechanisms in response to change of fluid pressure caused by detecting variations in fluid pressure in the brake pipe.

Usually the fluid used is air.

The rail vehicle braking system is of the type in which: (i) the brakes are fully released at "run" condition in which fluid pressure in the brake pipe, which may extend the length of a railway train, is at a predetermined maximum pressure value P1; (ii) the brakes are fully applied at "full service" condition when the brake pipe pressure is at least at a predetermined minimum pressure value P2: (iii) between "run" and "full service" pressures there is a predetermined brake pipe pressure value P3 at which there is a minimum application of the brakes herein referred to as "initial application"; and (iv) dropping of the brake pipe pressure from P3 to P2 causes increasing brake application up to "full service".

According to the invention apparatus to control variation in fluid pressure in the brake pipe of a rail vehicle braking system of the type referred to comprises first valve means which in response to pressure of central fluid applied to the first valve means causes attainment of substantially the same pressure in the brake pipe, a joystick lever manually movable from a brake hold or lap position into respective brake application and brake release positions, movement of the joystick into the brake application position causing decrease in control fluid pressure when the latter is above the value P2 and movement of the joystick into brake release position causing increase in control fluid pressure when the latter is below the value P1, and second valve means being responsive to variation in control fluid pressure such that when the control fluid pressure drops to a predetermined value between values P1 and P3 and the joystick is in brake application or brake hold positions the second valve means operates to cause an automatic and rapid drop in control fluid pressure to the "initial application" value P3 and when the control fluid pressure rises to a predetermined value which is between P3 and P1 and the joystick is in brake release or brake hold positions the second valve means operates to cause an automatic and rapid rise in control fluid pressure to the "run" pressure value Pi.

The joystick can be of a kind which if not held manually in brake application or brake releasing positions is moved, preferably automatically, for example by resilient biasing, into the brake hold or lap positions which is the position that the joystick is in when in neither the brake application nor brake release positions.

Starting say with the brakes in "run" condition when the brake pipe pressure and the control fluid pressure have the value P1, movement of the joystick into brake application position and dwell there for at least an instant causes a drop in pressure in the control fluid to a predetermined pressure P4 which has a value between P1 and P3. The second valve means responds to a drop in the control fluid pressure it reaches the value P4 and causes an automatic and rapid drop in the control fluid pressure from P4 to the value P3, whether or not the joystick is in brake applying or brake hold positions. The rapid drop in control fluid pressure to P3 causes the brake pipe pressure to also drop rapidly to P3 and consequently the brakes go to "initial application".Should the joystick be maintained in or returned to the brake application position, the control fluid pressure drops progressively from P3 to P2 so the brake pipe pressure drops progressively for P3 to P2 until at brake pipe pressure P2 the brakes are at full service. If brake release is now required the joystick is moved to brake release position and the control fluid pressure increases from P2 causing a corresponding increase in brake pipe pressure towards P1 as long as the joystick is in the brake releasing position and so the brakes progressively release until fully off when the pressure is above P3.When the control pressure rises to a predetermined value between P3 and P1,for example P5, the second valve means responds to the control fluid pressure at P5 causing an automatic rapid increase in control fluid pressure from P5 to P1 and consequently the brake pipe pressure rises rapidly automatically from value P5 to P1 even if at this stage the joystick is in brake hold position. Preferably the pressure value P5 is lower than the value P4. Along a brake pipe are a number of distributors which respond to brake pipe pressure to cause appropriate brake application and release.The distributors are intended to latch, in readiness to operate, at a predetermined brake pipe pressure between values P1 and P3 when the pressure is falling and to unlatch out of operation at that predetermined pressure when the brake pipe pressure is rising, but stiffness or friction in the distributors can mean there is a failure to unlatch or latch when they are subject to a steady change in brake pipe pressure. When during a demand for braking, the distributors are subject to the rapid pressure drop from P1 to P3 the resultant shock makes them more likely to latch.Similarly they are more likely to unlatch out of operation when, during demand for brake release, the distributors are subjected to the shock of a rapid increase in brake pipe pressure from P5 to P1.The brake pipe pressure at which the distributors unlatch or latch can be of the order of P4.

The arrangement is preferably such that when the joystick is placed in the brake hold position directly after a demand for brake application so that brake pipe pressure is falling and is in the range of P3 to P2, the brake pipe pressure remains constant at the value that it had attained in that range at the time the joystick moved to hold position. Accordingly the degree of braking effort exerted remains substantially constant at a value corresponding to that of the maintained brake pipe pressure. A similar holding of the attained braking condition occurs when the joystick is placed in the brake hold position directly after a demand for brake release so that the brake pipe pressure is rising and is in the range of P2 to P5.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic view partly in section of apparatus formed in accordance with the invention to control variation in the brake pipe pressure of a rail vehicle braking system when the system is at "run" condition; Fig. 2 is a similar view of the apparatus in Fig.

1 at an early stage in brake application demand; Fig. 3 is a similar view to Fig. 2 but at a later stage in brake application demand from "initial application" to "full service" condition; Fig. 4 is a similar view to Fig. 3 but the apparatus at a stage of brake release from "full service"; Fig. 5 is a view similar to Fig. 4 but showing a final stage in brake release operation; Fig. 6 is a diagrammatic view of a modification of the control apparatus in Figs. 1 to 5, the apparatus in Fig. 6 being shown in the same stage of brake release as the apparatus in Fig. 5; and Fig. 7 is a block diagram of an electrical control used in the apparatus in Fig. 6.

Referring to Fig. 1 a joystick 2 is manually pivotable about a horizontal axis 4 in opposite directions A and R corresponding brake application and brake release respectively. when the joystick is pivoted in either direction A or R it pivots a carriage 6 about the axis 4. The carraige has a roller or cam 8 or 10 at each end to act, when the joystick pivots, on a cam follower or head 12 or 14 on push-rod 16 in a casing 18 of a respective valve 20 or 22.

The joystck 2 is mounted in a driver's cab of a railway locomotive or driving car of a multiple unit train. Resilient restoring means, for example spring (not shown) can automatically restore the joystick to the vertical position shown in Fig. 1 (and the carraige to the horizontal position) when manual effort pushing the stick in direction A or R is removed.

Springs (not shown) which may be the aforesaid restoring means urge each push rod 16 towards the carraige 6. On a rod 16 being pushed in the opposite direction by roller 8 or 10 the corresponding disc or poppet 24 is pushed off a valve seat 26 against action of spring 28 to open the valve 20 or 22 which closes when pushing effort on the rod is relieved.

Conduit 30 supplies air from any suitable source known per se at a pressure in excess of P1 to pressure regulating valve 32, for example the air pressure on the conduit may be 7 to 10 bar.

The valve 32 gives an air output at the predetermined pressure P1 to conduit 34 from which brartches conduit 36 leading to valve 20.

Pressure P1 can-be.of any desired value, for example 5 bar. Outlet conduit 38 from valve 20 branches through a T-junction into inlet conduit 40 to valve 22 and into conduit 42 connected to a pressure maintaining valve 44 connected to conduit 46 communicating with air pressure gauge 48. The valve 44 is to ensure air pressure in the conduit 46 does not fall below predetermined value P2. The pressure value P2 may be 3.5 bar.

An outlet conduit 50 from the valve 22 leads to a variable choke 52 comprising an annular seat 54 against which a plate 56 with a narrow central hole 58 is pressed by a calibrated spring 60. The force exerted by spring 60 holds the plate 56 against seat 54 when air pressure in conduit 50 is below P3 and thus air escapes from the conduit only through the small hole 58. When the air pressure in conduit 50 exceeds P3 the plate 56 is urged off the seat 54 so that the choke is fully open and air from the conduit escapes quickly around the edge of the plate.

Conduit 46 leads, through a check valve 62 from a conduit 64 which includes choke 66.

Conduit 46 is further connected with the conduit 64 via a conduit 68 by-passing choke 66 and check valve 62.

Conduit.34 leads to a chamber 70 in casing 72 of a control valve 74. Chamber 70 is separated by a flexible diaphragm 76 from a chamber 78 connected to the conduit 64. Diaphragm 76 is attached to a plate 80 affixed to stem 82 urged (upwardly in the figures) by a calibrated spring 84 exerting a predetermined force on the stem. The stem 82 which slidably passes through air tight seals 86 has open ends 88 and 90 opening into separate longitudinal bores 92 and 94 respectively. Bore 92 opens through stem wall passage 96 into the chamber 78. Near its other end bore 92 opens through stem wall passage 98 intQ cavity 100 leading through valve seat 102 into chamber 104 containing a spring 106 lightly urging towards seat 102 a poppet valve 108, which in Fig. 1 is closing the stem end 88.

Conduit 110 connects chambers 70 and 104.

Stem 82 extends through two chambers 112 and 114 and bore 116 between those chambers.

Lower end 90 of the stem passes with good clearance into bore 118 extending from chamber 114 to a chamber 120 through a valve seat 122.

The chamber 120 contains a poppet valve 124 lightly urged by spring 126 towards stem end 90 and (in Fig. 1) presses against the seat 122 to close bore 118.

An annular flange 128 affixed to the stem 82 restricts how far the stem may be lifted by abutting (as shown in Fig. 1) against an underside of web 130 between chambers 78 and 112.

Bore 94 opens into chamber 112 through stem wall passage 132 at the upper end of the bore.

The bore 11 6 has lower internal shoulders 134 serving as an abutment for a rubber piston head 136 capable of sliding, with an air tight fit, along the bore 116 and also along the stem 82 which can itself slide relatively through the head 136.

The head 136 is mounted on the lower end of a piston body 138 slidable in the bore 11 6. The piston body has a passage 140 through its side and an outer annular flange 142 against which presses a weak spring 144 engaging the underside of web 130. The upper end of piston body 116 has an internal radial flange 146 to engage the upper side of a ring 148 having a lower side engaged by the upper end of a calibrated spring 150. This spring exerts a predetermined force between the ring 148 and a radial inner flange on the lower end of the piston body 116.

Chamber 112 communicates with atmosphere through open conduit 152.

A minimum reduction reservoir 1 54 communicates with chamber 114 through conduit 156.

The chamber 120 is connected to conduit 64 by conduit 1 58.

A control reservoir 160 of larger volume than reservoir 154 is connected to conduit 64 by conduit 1 62.

The conduit 64 is connected to a conduit 164 leading to a transmission valve 1 66 which can be any suitable type known per se, for example of the diaphragm type. Valve 166 has an inlet conduit 1 68 supplying air to the valve from any suitable source, for example a main reservoir, at a pressure in excess of P1 for example the air pressure on conduit 168 can be about 10 bar. The valve is connected to the brake pipe BP by conduit 170, and the valve has an outlet conduit 172 leading to atmosphere. Air pressure in conduit 64 is applied through conduit 1 64 to the transmission valve 166 which operates to bring the pressure in brake pipe BP to the same value as that in conduit 1 64.

The joystick 2 is not only in the brake hold or lap position when the joystick is vertical but is also in the brake hold position when within an angle 0 in either direction to the vertical. This is because the joystick only causes valve 20 or 22 to open when the joystick makes an angle greater than 0 to the vertical.

In Fig. 1,the rail vehicle brakes are at "run" condition since the control apparatus is maintaining the pressure in brake pipe BP at P1.

In this case the air pressure at P1 in conduit 34 and chamber 70 is communicated through conduit 110, chamber 104, open valve 102 and 108, passage 98, bore 92 and passage 96 to the chamber 78. Because chamber 78 is at the same pressure P1 as chamber 70 conduit 64 and therefore conduit 164 are at pressure P1.

Accordingly the transmission valve ensures that the pressure in brake pipe BP is P1 and is maintained by virtue of the communication between conduit 34 and chamber 78. Control reservoir 160 connected to conduit 64 is also at pressure P1. Valve 122, 124 is closed. The minimum reduction reservoir 1 54 is at atmospheric pressure since that reservoir is connected to conduit 1 52 via conduit 156, chamber 114, open stem end 90, bore 94, passage 132 and chamber 112.

When the brake system is at "run" condition and there is a demand for brake application by movement of the joystick in the direction A (as shown in Fig. 2) which opens application valve 22 there is a flow of air from chamber 78 to atmosphere through conduit 68, 46, 42, 40, 50 and choke 52 which is fully open. Due to communication between chambers 70 and 78 being tortuous, there is a comparatively rapid drop in pressure in chamber 78 to a pressure value P4 between values P1 and P3. The pressure value P4 may be 4.85 bar. When the pressure differential across diaphragm 76 reaches (P1 P4) this is just enough to overcome the force of spring 84 and the stem 82 lowers (as in Fig. 2), sliding relatively to piston head 136. The length of stem 82 is greater than the distance between valve seats 102 and 122.Accordingly the descending stem has its lower end 90 closed by poppet 124 and simultaneously starts to open valve 122, 124 before the descending upper end 88 of the stem allows the valve 102, 108 to close. The closing of the stem's lower end 90 isolates the reservoir 1 54 from atmosphere via conduit 152. At the same time the opening of valve 122, 124 brings reservoir 154 into communication with the control reservoir 1 60 hitherto at pressure P 1. This communication establishes an equality of pressure between the reservoirs 1 54 and 1 60 at the predetermined value P3, the presence of choke 66 inhibiting the discharge of air from reservoir 160 into the chamber 78.The pressure drop in the reservoir 1 60 to P3 establishes that pressure in conduits 64 and 164 and consequently the brake pipe pressure drops to P3 and causes "initial application". The reservoirs 1 54 and 1 60 are of such relative sizes that once valve 122, 124 is open the pressure drop is automatically to P3.

This occurs rapidly and automatically once the pressure in chamber 78 drops to P4 which, if close to the value P1, can occur very quickly after the valve 22 is opened by even momentary displacement of the joystick 2 in the direction A from brake hold position. Upon the presssure in chamber 114 and reservoir 1 54 rising to at least a predetermined value P6 above atmospheric pressure, the upward thrust on piston head 136 overcomes the force of spring 144 and the piston 136, 138 rises relative to the stem 82 until it is stopped by the underside of the web 130.

Pressure value P6 is lower than value P2 and can for example be 0.7 bar. The spring 1 50 is considerably compressed between the raised piston head 136 and the ring 148 engaging the lowered stem flange 128.

When the pressure in conduit 64 falls below P3 and the valve 22 is still held open by the joystick 2, the air pressure in conduit 50 is also below P3. Thus the plate 56 in the choke 52 is urged against the seat 54. This situation is shown in Fig. 3, in which valve 22 is still held open by the joystick. Thus, the pressure in conduit 64 continues to fall by discharge to atmosphere via valve 22 and choke 52 but at a rate determined by the size of the hole 58. Accordingly, the air pressure in conduit 1 64 falls from P3 to P2 at that controlled rate causing the pressure of the brake pipe BP to fall from P3 to P2 at the same rate to progressively increase the braking up to "full service". Should the joystick 2 be allowed to return to brake hold state whilst the brake pipe pressure is falling from P3 to P2, both valves 20 and 22 will be closed.Thus, the pressure in conduit 64 will remain at the value it was when valve 22 closed and that pressure will also occur in the brake pipe. Therefore, the braking effort will continue at a constant degree commensurate with the brake pipe pressure.

When brake release is required and the pressure in the brake pipe is in range P3 to P2 when the joystick is moved in direction R to release position, the situation is as shown in Fig.

4. In Fig. 4 valve 22 is closed and valve 20 is opened allowing air at pressure P1 from conduit 34 to pass via conduit 36, 38,42, 46, 68 and check valve 62 to conduit 64 to increase the pressure therein and thus raise the brake pipe pressure to progressively cause brake release. At the same time the pressure rises in reservoir 160, in chamber 114 and reservoir 154, and in chamber 78. The effect of this is to continue to hold spring 1 50 compressed when the brake pipe pressure rises beyond P3.

When the air pressure in chamber 78 rises to a predetermined value P5 which is between P3 and P4, for example 4.7 bar the pressure differential across diaphragm 76 is no longer sufficient to overcome the effort exerted by compressed springs 84 and 1 50. This situation is shown in Fig. 5 in which at the attainment of the pressure P5 in chambers 78 and 112, the stem 82 is lifted slightly by the springs 84 and 150. Initially, the lifting of the stem causes its upper end 88 to contact poppet 108 lifting the latter off the valve seat 102. At the same time the lower end 90 of the stem is still closed for a moment by poppet 124, and valve 122, 124 remains open to maintain the pressure on piston head 136 and the spring 1 50 compressed.Because valve 102, 108 is open, the chamber 70 is in communication via conduit 110, passage 98 and bore 92 with the chamber 78 causing a rapid pressure rise therein to P1. This lifts the stem 82 further into the atttitude shown in Fig. 1, so that reservoir 160 and conduit 64 are isolated from reservoir 1 54 in which latter the pressure now drops quickly to atmospheric. In consequence the spring 1 50 expands and forces the piston 136, 138 to descend along the stem. The rapid and automatic pressure rise from P5 to P1 in chamber 78 is communicated via conduit 64, 164 to the valve 1 66 and consequently the brake pipe pressure also rises automatically and rapidly from P5 to P1.At this point the brake system is at "run" condition with the control in the attitude shown in Fig. 1. Therefore a momentary demand for braking by movement of the joystick 2 in direction A can result in an automatic and rapid attainment of "initial application".

In the modification in Fig. 6 an electropneumatic brake release valve 20a and eiectropneumatic brake application valve 22a are also provided and each is normally closed unless solenoid 202 in either is energised to open the valve. Outlet conduit 50a from brake application valve 22a branches into conduit 50 to the choke 52. Conduit 42a extends between pressure maintaining valve 44 and an electro-pneumatic changeover valve 204 having a solenoid 206 which when unenergised connects conduit 42a with conduit 208 and when energised connects conduit 42a with conduit 210 so that control is transferred to valves 20a, 22a. Conduit 208 is connected to conduits 38 and 40. Conduit 210 is connected to conduits 38a and 40a branching to valves 20a and 22a respectively.A pair of ganged switches 21 2a and 21 2b are actuated by the cam 8 when the joystick is moved to apply the brake.

In this event switch 212a opens and switch 212b closes. When the joystick is in brake hold or brake releasing attitudes the switch 21 2a is normally closed and switch 21 2b normally open. Another pair of ganged switches 21 4a and 21 4b is actuated by the cam 10 when the joystick is moved to release the brake. In this case normally closed switch 214a opens and normally open switch 21 4b closes. The valves 20a and 22a act as substitutes for valves 20 and 22 when an electrical control of brake application and release is desired in response to joystick movement.A suitable electrical circuit is shown in Fig. 7 in which proving switch 21 6 is automatically made in response to a demand for electrical control by the driver and an adequate supply of electrical power is found to exist. The driver using a control in his cab then makes switch 218 causing coil 206 of changeover valve 204 to be energised. Thus conduits 42a and 208 are blocked by the valve 204 which connects conduit 210 with conduit 42a. Operation of the joystick causes switch 21 2b or 21 4b to close or remain open resulting in the energising of one of the solenoids 202 so that valve 20a or 22b is open or closed and the brake system operates in the manner described above under control of valves 20 and 22.

The train may comprise at least first and second locomotives or driving cars (hereinafter called first and second control vehicles) each associated with a respective first and second brake pipe in which, to harmonise braking effort along the train, the air pressure should be the same and vary substantially identically. The term "first" and "second" control vehicle is merely intended to distinguish one vehicle from the other and imposes no limitation on their positions in the train. The first control vehicle has first control apparatus as described with reference to Figs. 6 and 7 to control pressure in the first brake pipe, and the second control vehicle has second control apparatus as described with reference to Figs. 6 and 7 to control pressure in the second brake pipe. Associated with the first control apparatus is transmitter means.For example, a radio transmitter, to telemeter signals from the first control apparatus to receiver means associated with the second control apparatus whereby the signals received cause the second control apparatus to automatically replicate substantially identically the operation of the first control apparatus. Accordingly, the second brake pipe pressure varies similarly with first brake pipe pressure.

For example, provided proving switch 216 on the second control apparatus is closed conductor 224 becomes energised when changeover valve 204 is energised by closure of switch 21 8. The signal in line 224 causes telemetering of an instruction to the receiving means of the second control apparatus to cause closure of its switch 21 8 and the energising of the solenoid 206 of its changeover valve 204. When the joystick in the first control vehicle is moved the electrical signals on line 220 or 222 correspond to whether or not application valve 22a or release valve 20a is open or closed.These signals corresponding to the condition of the valves 20a and 22a in the first control apparatus cause telemetering of instructions to the receiver means to cause the electro-pneumatic application and release valves in the second control apparatus to adopt the same attitude as the corresponding valves in the first control apparatus.

It is possible, when the joystick is in brake hold position, and it is intended to maintain a particular degree of braking effort, that the braking effort may vary due to air leakages at valves causing a change in the air pressure in conduit 64. Indeed, it is possible in a braking system provided with direct releasing distributors that if a smail increase in brake pipe pressure occurs whilst the distributors are in braking mode that an irreversible complete release of the train brakes could occur.

Figs. 6 and 7 illustrate electrical control, which may be based on a microprocessor, to maintain brake pipe pressure substantially constant at brake hold condition. This electrical control includes a pressure transducer 230 detecting the air pressure in conduit 64 by measuring that same pressure in control reservoir 1 60 and providing an output signal which is a function of the pressure in conduit 64 and therefore of the brake pipe pressure. This signal is supplied to a comparator 232 and to memory update 234. A pressure switch 236 closes when the air pressure in the minimum reduction reservoir 1 54 rises to a predetermined level above atmospheric pressure during reduction in the brake pipe pressure from P1 to P3 to cause "initial application".The switch 236 remains closed until the spindle 82 lifts during release operation to reduce the pressure in reservoir 1 54 to atmospheric when the pressure in brake pipe BP approaches P1 at the establishment of "run" condition.

With the system in the "run" condition the joystick 2 will be in the brake hold position with switches 212a, 214a closed and 212 and 214b open. The minimum reduction reservoir 1 54 is vented to atmosphere and hence pressure switch 236 is open. Accordingly, buffer 238 receives no input to produce an output acting as an inhibit signal. Thus, the inhibit signal is absent from the memory update 234 and output clamp circuit 240. Memory 242 is therefore storing the pressure value of the control reservoir 1 60 and the output clamp will be on, holding the output of comparator 232 at zero causing both switches S1 and S2 in a switch unit 244 to be open.

Movement of the joystick 2 to the brake application position will cause switch 21 2a to open and 21 2b to close. Switch 21 2b closing energises the application valve 22a which in turn causes at least a minimum reduction of the brake pipe pressure to P3 by the action of the control valve 74 connecting together the control and minimum reduction reservoirs 1 60 and 1 54.

Pressure switch 236 closes with the rising pressure in reservoir 1 54 but as switch 21 2a is open the inhibit signals will still be absent from the memory update 234 and output clamp 240.

The memory will therefore be updated to the current pressure of the control reservoir 1 60 and the output clamp 240 will ensure that the switches S1 and S2 remain open.

As soon as the control reservoir pressure has fallen to a value corresponding to the desired brake pipe pressure, the brake joystick is allowed to return to the "hold" position, opening switch 212b and closing switch 212a. Switch 212b opening de-energises the application valve 22a, preventing any further reduction in control reservoir pressure, and switch 21 2a closing causes the buffer to apply an inhibit to the memory update 234 and output clamp 240. The memory 242 therefore retains and uses as a reference the value of the control reservoir pressure existing at the time that the joystick was allowed to return to the "hold" position. This reference is compared in the comparator 232 with the actual control reservoir pressure as measured by the transducer 230.As the output clamp 240 is inhibited, any difference between the actual and reference control reservoir pressure will be corrected by the energisation of either the application or release valves 22a or 20a as appropriate, via output switches S1 or S2 in accord with output from the comparator. In this way any leakage of air into or out of the control reservoir 1 60 will be automatically compensated for, and the control reservoir pressure will be maintained at the value existing at the time the joystick 2 was returned to the "hold" position and thus the brake pipe pressure will be maintained at the control reservoir pressure.

Should the driver wish to further apply or release the brakes, the joystick is moved in the application or release position as appropriate.

Switch 21 2b or 21 4b closes energising the appropriate valve 22a or 20a directly causing the control reservoir pressure to decrease or increase as appropriate and switch 21 2a or 21 4a opens removing the inhibit from the memory update 234 and output clamp 240. The memory 242 is therefore automatically updated in line with any change in control reservoir pressure and the output from the comparator 232 is clamped ensuring that switches S1 and S2 remain open leaving control of the application and release valves 22a, 2Oa to be via switches 21 2b and 214b.

Claims

1. An apparatus to control variation in fluid pressure in the brake pipe of a rail vehicle braking system of the type referred to comprising first valve means which in response to pressure of control fluid applied to the first valve means causes attainment of substantially the same pressure in the brake pipe, a joystick lever manually movable from a brake hold or lap position into respective brake application and brake release positions, movement of the joystick into the brake application position causing decrease in control fluid pressure when the letter is above the value P2 and movement of the joystick into brake release position causing increase in control fluid pressure when the latter is below the value P1, and second valve means being responsive to variation in control fluid pressure such that when the control fluid pressure drops to a predetermined value between values P1 and P3 and the joystick is in brake application or brake hold position the second valve means operates to cause an automatic and rapid drop in control fluid pressure to the "initial application" value P3 and when the control fluid pressure rises to a predetermined value which is between P3 and P1 and the joystick is in brake release or brake hold positions the second valve means operates to cause an automatic and rapid rise in control fluid pressure to the "run" pressure value Pi.

2. Apparatus as claimed in claim 1, in which the drop in control fluid pressure to the value P3 is caused by establishing communication between first reservoir means containing control fluid at substantially the pressure value P1 and second reservoir means in which the pressure is lower than P3, such that substantial equalising of the pressure in the two reservoirs occurs at the pressure value P3.

3. Apparatus as claimed in claim 2, in which immediately before said communication the pressure in the second reservoir is atmospheric pressure.

4. Apparatus as claimed in claim 2 or claim 3, in which the second valve means comprises third and fourth valve means, and first and second chambers divided by a diaphragm, the first chamber is continuously maintained at the pressure value P1 ,the second chamber contains control fluid and is connected by a conduit to the first valve means, at "run" condition the third valve means being open to allow communication between the chambers and the fourth valve means being closed to isolate the reservoirs from each other, and upon a demand for braking at "run" condition a lowering of pressure in the second chamber to a value P4 between values P1 and P3 causes a differential pressure across the diaphragm sufficient to overcome opposing effort of first resilient means such that the diaphragm moves towards the second chamber and causes a member to move in a first direction, and said movement of the member being arranged to cause the fourth valve means to open and establish the communication between the reservoirs and the third valve means to close and isolate the second chamber from the first chamber.

5. Apparatus as claimed in claim 4, in which the fourth valve means opens before the third valve means closes.

6. Apparatus as claimed in claim 4 or claim 5, in which the second valve means includes means acting in response to an increase in pressure in the second reservoir means due to communication between the first and second reservoir means, to increase energy stored in second resilient means providing effort in opposition to movement of the diaphragm towards the second chamber.

7. Apparatus as claimed in claim 6, in which upon increasing the control fluid pressure to cause release of the brakes to an extent that the control fluid pressure in the second chamber increases to a predetermined value P5 between the values P1 and P3, force exerted by said first and second resilient means on the diaphragm moves said diaphragm causing movement of the member in a second direction to open the third valve means between the chambers.

8. Apparatus as claimed in claim 7, in which the pressure value P5 is lower than value P4.

9. Apparatus as claimed in claim 6 or claim 7, in which the movement of the member in the second direction causes closure of the fourth valve means and the second reservoir means to open to atmosphere.

10. Apparatus as claimed in any one preceding claim, in which movement of the joystick is arranged to operate at least two valve means (hereinafter referred to as brake application valve means and brake release valve means), wherein opening of the brake application valve means is arranged to cause reduction in control fluid pressure and opening of the brake releasing valve means is arranged to cause an increase in the control fluid pressure.

11. Apparatus as claimed in claim 10, in which joystick movement is transmitted mechanically from the joystick to the brake application and releasing valve means.

12. Apparatus as claimed in claim 10, in which an electrical control responsive to joystick movement is arranged to operate the brake application and releasing valve means.

13. Apparatus as claimed in claim 12, in which the brake application and releasing valve means have electrical valve actuating means.

14. Apparatus as claimed in claim 13, in which a second electrical control is provided for maintaining the control fluid-pressure substantially constant at a value it is desired to hold that pressure, said second electrical control comprising means to measure and memorise the desired pressure and to operate the electrical valve actuating means to restore the control fluid pressure to the desired value when an undesired variation of the control fluid pressure from the desired value is detected.

1 5. First and second rail vehicles respectively comprising a first apparatus as claimed in claim 13 or 14 and a second apparatus as claimed in claim 13 or 14, in which the first apparatus controls the pressure in a first said brake pipe and the second apparatus controls the pressure in a second said brake pipe, and telemetering means being provided linking the two apparatuses such that operations of one apparatus are automatically substantially replicated by the other apparatus.

1 6. Rail vehicles as claimed in claim 13, wherein the telemetering means comprises radio transmitting and receiving means whereby the telemetering signals between the two apparatus can be sent and received.

1 7. Apparatus to control variation in fluid pressure in the brake pipe of a rail vehicle braking system of the type referred to, substantially as hereinbefore described with reference to Figs. 1 to 5 or Figs. 1 to 7 of the accompanying drawings.

18. One or more rail vehicles each with apparatus as claimed in any one of claims 1 to 14 or in claim 17.