Patent GB-519450-A (GB)
Title: Improvements relating to braking apparatus for railway and like vehicles
IPC: B60T8/32
Filed: 1938-09-23
Abstract: 519,450. Fluid-pressure brakes for rail vehicles. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd. Sept. 23, 1938, No. 27829. Convention date, Dec. 15,1937. [Class 103 (i)] To prevent wheel slip when braking a rail vehicle, the braking pressure is automatically limited in accordance with the condition of the rails as determined by the force of friction on a rail engaging shoe 95- which is pressed onto the rail when the brakes are applied by the admission of air from a reservoir 20 to a piston 88. The supply to reservoir 20 is cut off when braking is initiated, and one or more expansion reservoirs 21, 22 and 23 successively placed in communication therewith as the speed falls so that the pressure of the shoe 95 on the rail is decreased to compensate for the rise in coefficient of friction with the fall in speed. In the arrangement shown, the braking pressure is also directly limited in accordance with the speed, a speed responsive switch 13, comprising a contact-carrying rod 145 adapted to be displaced axially by centrifugal weights 150 driven by an axle of the vehicle, being arranged to control both the connection of the expansion reservoirs with reservoir 20 and a device 11 which controls the pressure in the brake cylinder 10. The device 11 is also under the control of a switch operated by the rail shoe 95, this switch comprising a contact arm 100 connected with the shoe and adapted to be displaced into engagement with contacts 112, 111 and 110 in succession as the frictional drag on the shoe increases. A chamber 42 in the device 11 connects the auxiliary reservoir 19 with the brake cylinder 10 and accommodates the brake cylinder supply and exhaust valves 33 and 30. These valves are controlled by a rocking beam 34 on a plunger 53 subject to the opposite actions of a spring 38 and a stack of diaphragms 48, 49, 50 and 51 of successively diminishing diameter. The spaces 56, 60, 61 and 63 between these diaphragms and an end plate 62 are adapted to be placed in communication with the train pipe 72 or the atmosphere by a series of valves 68, 69 and 70. Selective operation of these valves, in consequence of the differing diameters of the diaphragms, establishes pressures in chamber 42 and the brake cylinder at different predetermined ratios to the train pipe pressure. The valves are operated by magnets 65, 66 and 67 the circuits of which are controlled by the rail shoe and speed responsive switches. Assuming the vehicle to be travelling in excess of a predetermined high speed, e.g. 60 m.p.h., the circuits of magnets 65 and 66 will be closed at contacts 152 and 154 of the speed responsive switch, but not at their contacts 110 and 111 of the rail shoe switch, and the magnets will therefore be de-energized. The circuit of magnet 67 will be broken at both sets of contacts. To apply the brakes pressure is applied to the train pipe 72 and thence by way of valve 70 to diaphragm space 61 and directly to diaphragm space 63. The diaphragms 48, 49 and 50 are thus moved to the left and depress plunger 37 closing exhaust valve 30 and opening valve 33. Air is admitted to the brake cylinder 10 until the ratio of the pressure in the brake cylinder 10 and,chamber 42 to that in the train pipe 72 equals the ratio of the areas of diaphragms 50 and 48. Air is also admitted by valve 30 to the cylinders 115 and 123 of pressure-operated valves 119, 127 which thereupon isolate the reservoir 20 from the main reservoir 19 and apply it to the rail shoe cylinder 87. At this stage none of the reservoirs 21, 22, 23 communicate with reservoir 20 and cylinder 87, and the shoe is applied to the rail with a maximum force. The drag of the rail on the shoe displaces the contact arm 100 to a position determined by the frictional condition of the rail: If the rail condition is bad neither of contacts 110 or 111 will be engaged by arm 100, and the braking pressure will remain at the existing low value. If the rail friction is good and sufficient to cause both contacts 110 and 111 to be engaged, magnets 65 and 66 will be energized and pressure will be applied to the diaphragm chambers 56 and 60 to open valve 33 and close valve 30 until a higher and maximum pressure equal to that in the train pipe 72, is produced in the brake cylinder. An intermediate braking pressure is similarly produced if contacts 111 but not contacts 110, are engaged. Assuming good rail conditions, when the speed falls below 60 m.p.h., the speed responsive switch opens its contacts 152 and magnet 65 is de-energized so that valve 68 exhausts diaphragm space 56. The diaphragms consequently move to the right to open the exhaust valve 30 until a lower braking pressure is established. The opening of contacts 152 de-energizes magnet 130 whose valve 133 places expansion reservoir 21 in communication with reservoir 20 and cylinder 87. This reduces the pressure on the shoe 95 to compensate for the higher friction due to the lower speed. As the vehicle speed drops successively below two further predetermined speeds, the speed respon-, sive switch successively de-energizes magnet 69 and energizes magnet 70 to cause lowering of the braking pressure in two further stages. Expansion reservoirs 22 and 23 are also applied successively to the cylinder 87 to reduce the pressure on the shoe 95 to compensate for the still higher friction at the lower speeds. If, at any time, bad rail conditions are encountered, contact arm 100 will move anti-clockwise to lower the braking pressure if it is too high as determined by the speed responsive switch. The braking pressure will, however, be restored to the pressure determined by the speed responsive switch immediately good rail conditions are restored. The limiting braking pressure is thus always the maximum allowable by the rail conditions and speed. When the pressure in the pipe 72 is reduced to release the brakes, fluid escapes from the diaphragm spaces through check valves. 81, 82 and 83. Increase of pressure in the pipe 72 increases the braking pressure subject to the limit determined by the speed responsive and rail shoe switches. An alternative arrangement is referred to in which the fluid pressure controlled valves 30, 33 are replaced by an electrical type self-lapping valve device, the rail shoe and speed responsive switches being replaced by rheostats to vary its energization and consequently its delivered pressure.