GB1601773A - Compressed-air brake for vehicles having a spring brake actuator - Google Patents

Description

(54) COMPRESSED-AIR BRAKE FOR VEHICLES HAVING A SPRING BRAKE ACTUATOR (71) We, WERKZEUGMASCHINENFA BRIK OERLIKoN-BUHRLE AG, a company organised and existing under the laws of Switzerland, of Birchstrasse 155, CH-8050 Zurich, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a compressed-air brake for vehicles, especially for rail vehicles, having a spring brake actuator in which a piston can be subjected to a release pressure and can be brought into a release position against the force of an accumulator spring by increasing the pressure in a main air pipe.

In compressed-air brakes of this type, the brake is to be in the fully released state when the air mains are, for example, under a pressure of 5kp/cm2, and a pressure of Skp/cm2 is also to prevail in the spring brake actuator. Full braking is to be attained when the pressure in the air mains is lowered to 3.5kp/cm2 and atmospheric pressure prevails in the spring brake actuator.

A large number of compressed-air brakes of this type are known, in which this requirement is met with the aid of pressure intensifiers of very diverse design.

The present invention provides a compressed-air brake device for a rail vehicle, which generates a braking force inversely proportional to the air pressure in a main air line the device having a spring brake actuator incorporating a piston which can be brought infinitely variably into a brakerelease position against the action of the actuator spring when pressure in the main line rises, and a control valve including a first piston rigidly joined to a second piston by a piston rod, and in which one side of the first piston can be subjected to the main line pressure and the other side thereof to a constant control pressure and one side of the second piston can be subjected to brake actuator release pressure and the other side to atmospheric pressure, and a spring biasing the pistons in the same direction as they are urged by the main line pressure so as to act against the brake actuator release pressure, whereby, when pressure reduces in the main air line, the control valve opens under the brake actuator release pressure to release the latter, the spring then reclosing the valve.

Various illustrative embodiments of the compressed-air brake according to the invention are described in detail in the following text by reference to the accompanying drawings in which: Figure 1 shows a first illustrative embodiment in a diagrammatic representation without a load-dependent pressure intensifier: Figure 2 shows a diagram in which the pressure conditions on braking can be seen; Figure 3 shows a second illustrative embodiment in a diagrammatic representation with a load-dependent pressure intensifier.

Referring to Figure 1, the spring-loaded brake of a rail vehicle has a control valve 10, to one side of which a main air pipe 11 is connected via two branch lines 12 and 13, and a spring brake actuator 15 as well as a control air container 14 are connected to the other side.

The spring brake actuator 15 contains a spring 16 which is supported on a piston 17.

This piston 17 divides the brake actuator 15 into two chambers 18 and 19. The chamber 18 is always connected to the atmosphere via an orifice 20 and the chamber 19 is connected via a brake line 21 to a chamber 22 of the control valve 10. If the chamber 19 of the spring brake actuator 15 is vented, the spring 16 of the spring brake actuator 15 is capable of displacing the piston 17 to the right in Figure 1, pressing the brake shoes, which are not shown, against the wheel which is to be braked. To release the brake, the chamber 19 of the spring brake actuator 15 must be filled with compressed air.

In addition to the said chamber 22 which is connected to the brake actuator the control valve 10 also contains a number of further chambers, namely a chamber 23 which is connected via branch line 13 to the air mains 11, a control chamber 24 which is connected via branch line 12 to the main air pipe 11, and a second control chamber 25 which is connected to the control container 14. Fi wally the control valve also possesses a chamber 26 which is always connected to the atmosphere.

The two control chambers 24 and 25 are separated from one another by a first piston 27 and the two chambers 22 and 26 are separated from one another by a second piston 28 and, finally, the two chambers 22 and 23 are separated from one another by a valve disc 29. The two pistons 27 and 28 are connected to one another by a valve tappet 30. The valve disc 29 is pressed by a spring 31 against a stationary valve seat 32 and against a movable valve seat 33 of the valve tappet 30. Together with the two valve seats 32 and 33. the valve disc 29 forms a double-seat valve. When the valve tappet 30 is in its uppermost position, compressed air flows from the main air pipe 11, the branch line 13 and the chamber 23 into chamber 22 and from there via brake line 21 into the brake actuator 15.When the valve 30 is in its lowest position, the compressed air flows from the brake actuator 15 via the brake line 21 and chamber 22 through a bore 34 of the valve disc 29 into the atmosphere. In the closed position, as shown, of the double-seat valve no compressed air flows in any direction.

A non-return valve 35 is connected to the two control chambers 24 and 25 of the control valve 10. This non-return valve 35 has two chambers 36 and 37 which are separated from one another by a diaphragm piston 38. This diaphragm piston 38 has a nozzle 39 through which the two chambers 36 and 37 can be connected to one another. If the diaphragm piston 38 is pressed downwards by excess pressure in the chamber 36, the nozzle 39 can be closed. For this purpose, the diaphragm piston 38 has a valve seat 40 which interacts with a stationary valve disc 41. A spring 42 tends to lift the diaphragm piston 38 with the valve seat 40 off the stationary valve disc 41.

Finally, at the lower end of the valve tappet 30 of the control valve 10, a spring 43 is fixed which is supported on a stationary adjustable screw 44. The bias of the spring 43 is adjustable by means of this screw 44. This spring 43 is stronger than the spring 31 and is thus capable of lifting the valve disc 29 off the valve seat 32 as long as this is permitted by the pressure acting on the two pistons 27 and 28.

The mode of action of the spring-loaded brake described is as follows: Before the vehicle equipped with the spring-loaded brake can move, the brake must be released. For this purpose, the main air pipe 11 is filled with compressed air, for example up to a pressure of 5 kp/cm2, as can be seen from Figure 2. Via the branch line 12, the chamber 24 of the control valve 10 is also filled up to a pressure of 5 atmospheres gauge, and the chamber 36, the control air container 14 and the control chamber 25 of the control valve 10 can also be filled with compressed air of 5 atmospheres gauge via the non-return valve 35, that is to say via the nozzle 39. Thus, identical pressures act from either side on the piston 27 of the control valve 10. The spring 43 acting on the valve tappet 30 of the control valve 10 is thus capable of lifting the valve disc 29 off the valve seat 32.Via the branch line 13, compressed air passes from the main air pipe 11 into the chamber 23 and, via the doubleseat valve 29, 32 which is open as mentioned, compressed air flows into the chamber 22.

The piston 28 is thus loaded on one side since atmospheric pressure always prevails in the chamber 26 located below the piston 28. As soon as the pressure in the chamber 22 has risen to 4 atmospheres gauge, the spring 43 is thus compressed and the valve disc 29 rests on the valve seat 32 and closes the doubleseat valve 29, 32; thus, the pressure in the chamber 22 cannot rise above 4 atmospheres gauge. The same pressure also prevails in the chamber 19 of the spring brake actuator 15 and is capable of compressing the spring 16 and thus releasing the brake. It can be seen from Figure 2 that a pressure of 4 atmospheres gauge prevails in the brake actuator 15.

If the vehicle is now to stop, the brake is put on. For this purpose, the pressure in the main air pipe 11 is lowered.

It is not necessary completely to vent the main air pipe 11 in order to obtain full braking. As can be seen from Figure 2, it is sufficient to lower the pressure in the main air pipe 11 to 3.5 atmospheres gauge in order to vent the chamber 19 of the spring brake actuator 15 completely and to enable the spring 16 of the spring brake actuator to press the brake shoes with the maximum force against the wheel which is to be braked.

This braking step proceeds as follows: As soon as the pressure in the main air pipe 11 is slightly lowered, the pressure in the first control chamber 24 and in the chamber 37 of the non-return valve 35 also falls. This has the consequence that the non-return valve 35 closes as a result of the valve seat 40 of the diaphragm piston 38 being pressed onto the stationary valve disc 41. A pressure of about 5 atmospheres gauge thus remains in the control air container 14 and hence also in the second control chamber 25. The pressures acting on the piston 27 are thus no longer in equilibrium and the piston 27 with the valve tappet 30 moves downwards. The movable valve seat 33 thus lifts off the valve disc 29 and compressed air can escape from the chamber 22 through the bore 34 into the atmosphere. As a result, the pressure acting on the piston 28 also decreases and the spring 43 is capable of moving the valve tappet 30 into the closing position as drawn. The pressure in the control chamber 24 determines when this closing position is reached.

If the control chamber 24 is fully vented, the chamber 22 will also be fully vented. As stated, it suffices to lower the pressure in the first control chamber 24 to 3.5 atmospheres gauge so that the pressure of 5 atmospheres gauge in the second control chamber 25 can overcome the force of the spring 43 on its own and the valve 29, 33 remains open. If the pressure in the first control chamber 24 is greater than 3.5 atmospheres gauge, the pressure in the second control chamber 25 is not sufficient to overcome the force of the spring 43, and a pressure in the chamber 22 on the piston 28 is still necessary in order to move the valve tappet 30 into the closing position as shown, against the force of the spring 31. The relationship between the pressure in the air mains 11 and the pressure in the brake line 21 can be seen from Figure 2.The line 45 represents the pressure in the main air pipe and the line 46 represents the pressure, depending thereon, in the line 21.

The illustrative embodiment according to Figure 3 differs from the illustrative embodiment represented in Figure 1 in that the brake system additionally has a load-dependent pressure intensifier, a braking accelerator and further control elements.

Corresponding parts are designated by the same reference numerals.

The spring brake actuator 15 is designed analogously; it contains a piston 17 which is loaded on one side by a spring 16 and on the other side by the air pressure of a chamber 19. The spring 16 is located in a chamber 18 which is vented through an orifice 20.

This spring brake actuator 15 is connected to the said load-dependent pressure intensifier 47, the parts of which are located in an area indicated by dots and dashes. The pressure intensifier 47 has a first double-seat valve 48 which is actively connected to two diaphragm pistons 53, 54. The first diaphragm piston 53 separates two chambers 49 and 50 from one another, the upper chamber 49 thereof being connected to the chamber 19 of the brake actuator 15 via a line 51. The lower chamber 50 is connected to the control valve 10, described further below, via a line 52. The second diaphragm piston 54 likewise separates two chambers 55 and 56 from one another, the upper chamber 55 thereof being always vented via an orifice 57. The lower chamber 56 is connected via a line 58 to a second double-seat valve 59. The second double-seat valve 59 is actively connected to a single piston 60.This piston 60 separates two chambers 61 and 62 from one another, the upper chamber 61 thereof being connected to the said line 58. The lower chamber 62 is always vented via orifice 63. The piston 60 is fixed to a valve tappet 64 which is supported on one end of a balance beam 65.

On the other end of the balance beam 65, a piston rod 66 is supported, which is fixed to a differential piston 67 which separates two chambers 68 and 69 from one another, the upper chamber 68 thereof being connected to a control air container 117 and the lower chamber 69 thereof being connected via the line 52 to the control valve 10. The balance beam 65 is supported on a displaceable sliding block 70 which is connected to a piston 72 via a rod 71. The piston 72 separates two chambers 73 and 74 from one another. The right-hand chamber 74 is connected to a pressure cell (not shown in the drawing) so that the pressure prevailing in the chamber depends on the load of the vehicle. This pressure is the larger, the more the vehicle is loaded.The left-hand chamber 73 is vented to atmosphere and contains a spring 75 which tends to displace the piston 72 to the right if the vehicle loading decreases. The position of the sliding block 70 thus depends on the vehicle loading. When the vehicle is fully loaded, the sliding block 70 is vertically below the piston rod 66 so that the balance beam 65 cannot pivot, as indicated by the reference numeral 70a.

The two double-seat valves 48 and 59 are connected via lines 77 and 78 to an auxiliary air container 76. The line 77 leads into a chamber 79 of the double-seat valve 48, which chamber is separated from the chamber 49 by a valve disc 80. This valve disc 80 rests on a valve seat 81 and co-operates with a valve tappet 82 to which the two pistons 53 and 54 are fixed. In the uppermost position of the valve tappet 82, compressed air can flow from the auxiliary air container 76 via line 77, chamber 79, chamber 49 and line 51 into the chamber 19 of the brake actuator. In the lowest position of the valve tappet 82, the compressed air flows from the chamber 19 via the line 51 and the chamber 49 through a bore 83 of the valve disc 80 into the open.

The line 78 leads into a chamber 84 of the double-seat valve 59, which chamber is separated from the chamber 61 by a valve disc 85. This valve disc 85 rests on a valve seat 86 and co-operates with a valve tappet 87 to which the piston 60 is fixed. In the uppermost position of the valve tappet 87, compressed air from the auxiliary air container 76 can reach the chamber 61 via line 78 and chamber 84. In the lowest position of the valve tappet, the chamber 61 is vented into the open via a bore 88 of the valve disc 85.

As in the first illustrative embodiment, the control valve 10 has a number of chambers, namely the chamber 22 which is connected via line 52 to the two chambers 50 and 69 of the pressure intensifier 47, the control chamber 24 which is connected to the main air pipe 11, the control chamber 25, which is connected to the control container 117, and finally the chamber 23 which is likewise connected to the main air pipe 11 via branch line 13. Additionally, the chamber 23 is connected to the auxiliary air container 76 via a first non-return valve 89. Finally, the chamber 26 is always connected to the atmosphere. The two control chambers 24 and 25 are separated from one another by the first piston 27, and the two chambers 22 and 26 are separated from one another by the second piston 28, and the two chambers 22 and 23 are separated from one another by the valve disc 29.The two pistons 27, 28 are connected to one another via the valve tappet 30. The valve disc 29 is pressed by the spring 31 against the stationary valve seat 32 and against the movable valve seat 33 of the valve tappet 30.

The valve tappet 30 carries a flange 90. A spring 91 which is supported at one end on this flange 90 and at the other end on a stationary plate 92, tends to lift the valve tappet and thus to lift the valve disc 29 off its stationary valve seat 32.

A braking accelerator device is located at the lower end of the valve tappet 30. This device comprises a carrier 93 which is fixed to the valve tappet 30 and to which a latch 94 is pivotably hinged. A spring 95 tends to hold the latch 94 in the working position shown.

Below this latch 94, there is a vent valve 96 which has a valve disc 97 which can be actuated by the latch 94 via the rod 97a. This valve disc 97 is rigidly joined to a piston 98 which separates two chambers 99 and 100 from one another. These two chambers 99 and 100 are connected to one another via a throttle 101. Two further chambers 102 and 103 are separated from one another by the valve disc 97, the upper chamber 102 thereof being connected to the said chamber 99 via a line 104 and the lower chamber 103 thereof being connected via branch line 12 to the main air pipe 11. Moreover, an accelerator chamber 105 as well as a vent nozzle 106 are connected to the chambers 99 and 102.

To actuate the latch 94, a piston 107 is provided which separates two chambers 108 and 109 from one another. On this piston 107, a piston rod is fixed which serves, on the one hand, to actuate the latch 94 and, on the other hand, to actuate a valve 110. The valve 110 serves to fill the control container 117 and has a valve disc 111 which is connected to the piston 107 via the said piston rod. The valve 110 also has a diaphragm piston 112, by means of which two chambers 113 and 114 are separated from one another, the chamber 113 thereof being connected via a branch line 115 and via chamber 24 to the main air pipe 11. The chamber 114 is connected to the control air container 117 via a line 116.The diaphragm piston 112 has a bore 118 and, at either end of the bore 118, in each case a valve seat 119 and 120, the lefthand valve seat 119 co-operating with the valve disc 111 and the right-hand valve seat 120 co-operating with a stationary valve disc 121. Transversely to the bore 118, there is yet a further bore 122 which remains open in any position of the diaphragm piston 112.

The mode of action of the illustrative embodiment, shown in Figure 3, of the spring brake actuator is as follows: Before the vehicle equipped with the spring-loaded brake can move, the brake must be released.

For this purpose, the main air pipe 11 are filled with compressed air, for example, as can be seen from Figure 2, up to a pressure of 5 kp/cm2. The chamber 103 of the vent valve 96 of the acclerator device is then filled via the branch line 12. The chambers 23 and 24 of the control valve 10 are also filled and the chamber 113 of the valve 110 is filled via branch line 115. Due to the bore 118 and the transverse bore 122 in the diaphragm piston 112, the control air container 117 can also be filled with compressed air. Likewise, the auxiliary air container 76 is filled with compressed air via the non-return valve 89.

Furthermore, the chamber 25, connected to the control air container 117, of the control valve 10 and the chamber 68 above the differential piston 67 of the pressure intensifier 47 are filled with compressed air. Thus, the same pressure prevails in the two control chambers 24 and 25 on either side of the piston 27. The spring 91 is therefore capable of lifting the valve tappet 30 and lifting the valve disc 29 off its stationary valve seat 32.

From the chamber 23, compressed air flows into the chamber 22 and via line 52 into the chamber 50 of the double-seat valve 48 as well as into the chamber 69 below the differential piston 67. As long as the same pressures act in the chambers 69 and 68, respectively below and above the differential piston, the larger area of the differential piston 67, however, being below, the piston 67 is lifted and the piston 60 can drop. Thus, compressed air flows from the auxiliary air container 76 via line 78 and chamber 84 of the double-seat valve 59 into the chamber 61 until identical forces act on the two ends of the balance beam 65. However, this equilibrium also depends on the position of the sliding block 70, that is to say on the loading of the vehicle, and a load-dependent pressure is thus generated in the chamber 61 and also in the chamber 56 of the double-seat valve 59, the load-dependent pressure being the smaller the more the vehicle is loaded.

Furthermore, compressed air from the chamber 22 reaches the chamber 50 of the doubleseat valve 48 via line 52, as a result of which the piston 53 is lifted and lifts the valve 80 off the valve seat 81 via the valve tappet 82.

Therefore, compressed air flows from the auxiliary air container 76 via line 77 and via chamber 79 into the chamber 49 and from the latter via line 51 into the chamber 19 of the brake actuator 15. The pressure in the chamber 22 acts on the piston 28 and will, at a certain value of, for example, 4 atmospheres gauge, compress the spring 91, as a result of which the valve tappet 30 reaches the closing position shown in Figure 3.

Corresponding to the bias of the spring 91, the pressure in the chamber 22 of the control valve 10 cannot become greater than, for example, 4 atmospheres gauge-as mentioned-and this pressure thus also prevails in the chamber 50 of the closure valve 48. As soon as a pressure of 4 atmospheres gauge has also been reached in the chamber 49, the same pressure prevails on either side of the piston 53 and the double-seat valve 48 reaches the closed position shown. Thus, this pressure also prevails in the chamber 19 of the brake actuator 15. This pressure is independent of the vehicle loading since, at a maximum pressure of, for example, 4 atmospheres gauge in the chamber 69 below the differential piston 67 and a maximum pressure of, for example, 5 atmospheres gauge in the chamber 68 above the differential piston, no forces are acting on the left-hand end of the balance beam 65.Likewise, no force is therefore allowed to act on the right-hand end of the balance beam 65, that is to say atmospheric pressure must prevail on either side of the piston 60 of the double-seat valve 59, since the chamber 62 is always vented.

Therefore, atmospheric pressure also prevails on either side of the piston 54 of the doubleseat valve 48.

It follows from this that, when the brake is released, the said 4 atmospheres gauge, that is to say the maximum release pressure corresponding to the bias of the spring 91, prevail in the chamber 19 of the brake actuator 15 independently of the vehicle loading.

For braking, the pressure in the main air pipe 11 must be lowered. Thus, the pressure in the chamber 24 below the piston 27 and in the chamber 113 of the valve 110 also falls.

This has the consequence that the valve 110 closes since the valve seat 119 strikes the valve disc. The control air container 117 and the chamber 25 of the control valve 10 as well as the chamber 68 of the pressure intensifier thus remain under the said pressure of about 5 atmospheres gauge. The pressures acting on the piston 27 are thus no longer in equilibrium and the piston 27 with the valve tappet 30 moves downwards. The movable valve seat 33 thus lifts off the valve disc 29 and compressed air can escape into the atmosphere from the chamber 22 through the bore 34. As a result, the pressure acting on the piston 28 also becomes smaller and the spring 91 will attempt to bring the valve tappet 30 into a closing position according to Figure 3. The pressure in the chamber 24, that is to say the main air pipe pressure, determines when this closing position is reached.

The vent valve 96 is opened by the downward movement of the valve tappet 30 via the latch 94. Thus, compressed air flows from the chamber 103 of the vent valve 96 into the chamber 102 and from there via line 104 into the chamber 99 and via throttle 101 into the chamber 100. Compressed air also flows from the chamber 102 into the chamber 109 and displaces the piston 107 to the left, and the latch 94 is thus also disengaged from the vent valve 96. Since, however, the chamber 99 has been filled more rapidly with compressed air because of the throttle 101, the vent valve 96 remains open until the same pressure prevails on either side of the piston 98. Furthermore, the accelerator chamber 105 is filled with compressed air, and this results in a fall of air pressure in the air mains. The air pressure in the chamber 22 will also fall corresponding to the air pressure drop in the chamber 24.The result of this is that the air pressure falls both in the chamber 50 of the double-seat valve 48 and in the chamber 69 of the pressure intensifier 47. Corresponding to the fall of air pressure in the chamber 69, the pressure in the chamber 61 rises, this rise depending on the vehicle loading, that is to say on the position of the sliding block 70. For example, when the vehicle is fully loaded, the pressure in the chamber 61 will always be negligibly small.

Furthermore, the pressure in the chamber 49 will also fall corresponding to the fall of the air pressure in the chamber 50 and the same pressure must always prevail on either side of the piston 53. Correspondingly, the pressure in the chamber 19 of the brake actuator 15 also becomes smaller and the spring 16 is capable of braking the vehicle.

If necessary, it is possible to connect the chamber 84 of the pressure intensifier 47 to the chamber 74 via a line 123, an electropneumatic valve 124 and a line 125. The same pressure as in the auxiliary air container 76 then prevails in the chamber 74, instead of a load-dependent pressure. The spring 75 is thus fully compressed and the sliding block 70 reaches the position 70a, that is to say vertically under the piston rod 66.

The braking force is then independent of the vehicle loading, that is to say braking always takes place as though the vehicle were fully loaded.

WHAT WE CLAIM IS: 1. A compressed-air brake device for a rail vehicle, which generates a braking force inversely proportional to the air pressure in a main air line, the device having a spring brake actuator incorporating a piston which can be brought infinitely variably into a brake-release position against the action of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. the latter via line 51 into the chamber 19 of the brake actuator 15. The pressure in the chamber 22 acts on the piston 28 and will, at a certain value of, for example, 4 atmospheres gauge, compress the spring 91, as a result of which the valve tappet 30 reaches the closing position shown in Figure 3. Corresponding to the bias of the spring 91, the pressure in the chamber 22 of the control valve 10 cannot become greater than, for example, 4 atmospheres gauge-as mentioned-and this pressure thus also prevails in the chamber 50 of the closure valve 48. As soon as a pressure of 4 atmospheres gauge has also been reached in the chamber 49, the same pressure prevails on either side of the piston 53 and the double-seat valve 48 reaches the closed position shown. Thus, this pressure also prevails in the chamber 19 of the brake actuator 15. This pressure is independent of the vehicle loading since, at a maximum pressure of, for example, 4 atmospheres gauge in the chamber 69 below the differential piston 67 and a maximum pressure of, for example, 5 atmospheres gauge in the chamber 68 above the differential piston, no forces are acting on the left-hand end of the balance beam 65.Likewise, no force is therefore allowed to act on the right-hand end of the balance beam 65, that is to say atmospheric pressure must prevail on either side of the piston 60 of the double-seat valve 59, since the chamber 62 is always vented. Therefore, atmospheric pressure also prevails on either side of the piston 54 of the doubleseat valve 48. It follows from this that, when the brake is released, the said 4 atmospheres gauge, that is to say the maximum release pressure corresponding to the bias of the spring 91, prevail in the chamber 19 of the brake actuator 15 independently of the vehicle loading. For braking, the pressure in the main air pipe 11 must be lowered. Thus, the pressure in the chamber 24 below the piston 27 and in the chamber 113 of the valve 110 also falls. This has the consequence that the valve 110 closes since the valve seat 119 strikes the valve disc. The control air container 117 and the chamber 25 of the control valve 10 as well as the chamber 68 of the pressure intensifier thus remain under the said pressure of about 5 atmospheres gauge. The pressures acting on the piston 27 are thus no longer in equilibrium and the piston 27 with the valve tappet 30 moves downwards. The movable valve seat 33 thus lifts off the valve disc 29 and compressed air can escape into the atmosphere from the chamber 22 through the bore 34. As a result, the pressure acting on the piston 28 also becomes smaller and the spring 91 will attempt to bring the valve tappet 30 into a closing position according to Figure 3. The pressure in the chamber 24, that is to say the main air pipe pressure, determines when this closing position is reached. The vent valve 96 is opened by the downward movement of the valve tappet 30 via the latch 94. Thus, compressed air flows from the chamber 103 of the vent valve 96 into the chamber 102 and from there via line 104 into the chamber 99 and via throttle 101 into the chamber 100. Compressed air also flows from the chamber 102 into the chamber 109 and displaces the piston 107 to the left, and the latch 94 is thus also disengaged from the vent valve 96. Since, however, the chamber 99 has been filled more rapidly with compressed air because of the throttle 101, the vent valve 96 remains open until the same pressure prevails on either side of the piston 98. Furthermore, the accelerator chamber 105 is filled with compressed air, and this results in a fall of air pressure in the air mains. The air pressure in the chamber 22 will also fall corresponding to the air pressure drop in the chamber 24.The result of this is that the air pressure falls both in the chamber 50 of the double-seat valve 48 and in the chamber 69 of the pressure intensifier 47. Corresponding to the fall of air pressure in the chamber 69, the pressure in the chamber 61 rises, this rise depending on the vehicle loading, that is to say on the position of the sliding block 70. For example, when the vehicle is fully loaded, the pressure in the chamber 61 will always be negligibly small. Furthermore, the pressure in the chamber 49 will also fall corresponding to the fall of the air pressure in the chamber 50 and the same pressure must always prevail on either side of the piston 53. Correspondingly, the pressure in the chamber 19 of the brake actuator 15 also becomes smaller and the spring 16 is capable of braking the vehicle. If necessary, it is possible to connect the chamber 84 of the pressure intensifier 47 to the chamber 74 via a line 123, an electropneumatic valve 124 and a line 125. The same pressure as in the auxiliary air container 76 then prevails in the chamber 74, instead of a load-dependent pressure. The spring 75 is thus fully compressed and the sliding block 70 reaches the position 70a, that is to say vertically under the piston rod 66. The braking force is then independent of the vehicle loading, that is to say braking always takes place as though the vehicle were fully loaded. WHAT WE CLAIM IS:

1. A compressed-air brake device for a rail vehicle, which generates a braking force inversely proportional to the air pressure in a main air line, the device having a spring brake actuator incorporating a piston which can be brought infinitely variably into a brake-release position against the action of

the actuator spring when pressure in the main line rises, and a control valve including a first piston rigidly joined to a second piston by a piston rod, and in which one side of the first piston can be subjected to the main line pressure and the other side thereof to a constant control pressure and one side of the second piston can be subjected to brake actuator release pressure and the other side to atmospheric pressure, and a spring biasing the pistons in the same direction as they are urged by the main line pressure so as to act against the brake actuator release pressure, whereby when pressure reduces in the main air line, the control valve opens under the brake actuator release pressure to release the latter, the spring then reclosing the valve.

2. A compressed-air brake device substantially as described herein with reference to Figure 1, or Figure 3 of the accompanying drawings.