Vacuum brake
The vacuum brake is used in railway operation for braking of trains operating brake on locomotives and cars. In contrast to the predominantly used today compressed air brake, the vacuum brake does not work by overpressure, but by a negative pressure relative to the prevailing atmospheric pressure. The brake was named after the aspirator used to generate the negative pressure. The name is common in Switzerland vacuum brake was also used earlier in Germany.
Principle of operation
To use the vacuum brake as a continuous train brake, all vehicles of the train with a suction air ( Switzerland: main vacuum line ) are connected. The mechanical part of the vacuum brake is similar to that of the air brake. After the operation, no difference -automatic and automatic brakes. Both types correspond in structure to the direct-acting or indirect -acting pneumatic brake.
Not self-acting vacuum brake
In the non- self-acting vacuum brake for braking must be made a negative pressure in the brake cylinder and the suction air. To release the brake, to open a valve that connects the conduit to the atmospheric air pressure. At full pressure balancing the brake pads are removed from the tires again.
The non- self-acting vacuum brake was responsible in 1889 for the railway accident at Armagh, which claimed 80 lives. The non-automatic vacuum brake is not approved as a service brake for railways.
Automatically acting vacuum brake
The self-acting suction air or vacuum brake, and automatic vacuum brake ( aV ) called works, similar to a pneumatic brake with dual-chamber master cylinder such as the Kunze -Knorr freight or passenger Kunze -Knorr brake. The mechanical part of Saugluftbremsanlage with brake rigging, brake pads, etc. largely corresponds to the applied pneumatic brake designs.
Evacuation or release
The vacuum pump on the train drawn ( evacuated ) from via a check valve from the entire system, the air, by which a negative pressure is built up. When steam locomotives of this is generated with an ejector, which is combined with the driver's brake valve. Via the main air line, the sub-chambers of the brake cylinder and via check valves the upper chambers and the special containers to be evacuated. When horizontally mounted brake cylinder, a return spring or when vertically mounted brake cylinder holds the weight of the piston this in the release position. The brake is ready for operation or solved if between the atmospheric pressure and the system a pressure difference of 52 cm Hg is created and exists between the lower and upper chambers of the connected brake cylinder pressure equalization. On the traction vehicle is a control valve limits the pressure difference between the atmospheric pressure and the main air pipe on 52 cm Hg A check valve prevents stoppage of the vacuum pump, the uncontrolled ingress of outside air.
Brakes
With the driver's brake valve, the vacuum is reduced in the brake pipe and the lower chambers of the brake cylinder by carefully admitting outside air. The negative pressure of 52 cm Hg in the upper chambers, and the special tanks is maintained thanks to the automatically acting sealing by the built-in check valves. As a result of the pressure difference in the two chambers, the force of the return spring and the weight of the piston is overcome and the latter moves to the braking position.
At a quick or emergency braking, the suction air is connected directly with the outside air. So the vacuum is destroyed in no time and achieve the maximum braking effect. If a train separation, the negative pressure at the separation point also builds from now on.
Advantages and disadvantages compared to the air brake
- The vacuum brake can be adjusted well and resolve particular stages. It is inexhaustible. These benefits are no longer fell into the weight than the mehrlösige air brake was introduced.
- Because of its simple structure, the vacuum brake is robust and easy to maintain. It can no malfunctions caused by freezing condensation occurs in the air line.
- While the air brake cylinder pressures of several bar allows - usually up to 3.8 bar - the difference in pressure at the vacuum brake from the outset by the atmospheric air pressure (about 1 bar) is limited; actually be achieved only around 0.7 bar. To a similar braking force ( equal time pressure differential piston area ) to reach the brake cylinders must have approximately two and a half times the diameter.
- The vacuum brake reacts slower.
- The penetration speed of the vacuum brake is typically 15 m / s, at an emergency application, about 200 m / s Thus it is lower than that of the compressed air brake.
- A mechanical slide protection can not be achieved in the vacuum brake.
- For steam operation is the high steam consumption of the ejector disadvantage.
Vacuum -controlled pneumatic brake
The vacuum- controlled pneumatic brake has the advantage that it can be equipped with a Gleitschutzregler compared to the pure vacuum brake. For tractive rolling the combination with a jumper and a centrifugal brake. However, the vehicle needs compressed air, which is supplied either from an air compressor, or from the feed line.
The vacuum controlled air brake is in addition to the pure vacuum brake equipped with a vacuum control valve. This has the same function as the control valve of the pneumatic brake.
See also section vacuum control valve control valve in the article (railway)
Types and distribution
About the names of the manufacturers of the brake equipment, the Hardy vacuum brake and vacuum brake Körting be distinguished.
Both designs are still on lines of Saxon narrow gauge railways (eg Döllnitz web Lößnitzgrundbahn, Weißeritztalbahn, Preßnitztalbahn ) and experience in the museum railway Bruchhausen- Vilsen in action. See also: Narrow gauge railways in Saxony
In Austria, the designs and Hardy Körting were from 1891 until the beginning of World War II with the previous tracks of today's Austrian Federal Railways Saugluftbremsen used. Coaches for international traffic were also fitted to the vacuum brake with pneumatic brake. In narrow gauge railways the vacuum brake is still widely used, eg in the Mariazell Railway or the Waldviertel narrow gauge railways.
In Switzerland, vacuum and compressed air brake for narrow gauge railways were around 1900 or less equally distributed. On the occasion of modernization of the rolling stock changed since then most tracks on compressed air brake. Apart from Museumsbahnen you meet saugluftgebremste trains in to the Rhaetian Railway, the Matterhorn- Gotthard -Bahn, the Montreux-Bernese Oberland Railway and the Transports Publics Fribourgeois.
In the United Kingdom (including Ireland) and in the British colonies, the automatic vacuum brake was standard. Gradually it was replaced by the compressed air brake, in the United Kingdom from the 1970s. In the Indian Railways and in South Africa, the vacuum brake is still used.