Air brake (railway)

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Brake of a hopper truck
Brake pads on the wheels of a railway vehicle

The compressed air brake is mainly used in rail operations to brake trains . George Westinghouse developed them in the USA around 1869 especially for railroad operations .

The compressed air brake uses compressed air as an energy carrier and also to control the braking process. The actual braking effect is exerted by pressing brake pads either on the running surfaces of the wheels or on the brake discs . Energy storage devices are the main air reservoir of the locomotive and the air reservoir of the car. The compressed air from these energy stores acts on the brake cylinders on the wheel sets via control valves.


The air brakes on most European railways conform to an international standard and are compatible with each other . The main reasons for the introduction of the compressed air brake were the possibility of central and direct controllability by the driver as well as the uniform effect on all wagons of a train . The braking process could be triggered from any location on the train. For example, emergency brakes are installed on the train and must be operated by staff and passengers. Compressed air brakes were not very sensitive to slight air losses, since the working medium compressed air could be replenished as required.

Before the compressed air brake was available, the trains were braked by hand. The individual cars were manned by a brakeman who operated a handbrake. The brakes had to be applied or released in response to signals from the engine driver. On passenger trains, a communication cord was sometimes attached to the outside of the car , which was connected to the locomotive whistle. It served as a kind of emergency brake, in which the train crew or passengers could trigger a whistle signal for the brakes in the event of danger. The work of the brakemen was extremely stressful, as the seat on an open platform was unprotected for a long time. It was only later that the so-called brakeman's cabins offered a certain degree of protection against the elements. Because the brakemen had to be able to perceive noises from outside, the brakeman's cabins could not be thermally insulated.

The development of rail brakes was promoted by numerous accidents. After George Westinghouse had initially designed unsatisfactory brakes with chains and steam operation, he invented the direct-acting, non -automatic air brake in 1869 and the indirect-acting, automatic air brake in 1872 . On March 5, 1872, he received a US patent for his invention.

When introducing a continuous brake for the entire train that could be operated on the locomotive, other systems besides the compressed air brake were also considered. Transmission via negative pressure ( suction air brake ) or control with cable pull (e.g. lever brake ) was also used for this purpose. The cable brakes in particular had significant disadvantages compared to the pneumatic brakes because of the cable friction and were therefore limited to niches. The suction air brake became more widespread, partly also on main railways (e.g. from 1891 on the predecessor railways of today's Austrian Federal Railways , on railways under British influence or in Spain). It was the only one of the alternatives that could meet all the requirements placed on such a brake to the same extent as the compressed air brake. In the case of the suction air brake, the disadvantages of leaks compared to the air brake become noticeable, especially with longer trains. That is why it was only able to hold up to this day in areas with short trains, especially on narrow-gauge networks. The advantage of the indirectly acting suction air brake, on the other hand, was the fact that it could be released in stages from the start due to the two-chamber effect (multiple release). This made it particularly suitable for operation on long downhill stretches, which made it popular on mountain railways. In some cases it remained there until the turn of the millennium.

The passenger trains on main lines were largely equipped with compressed air brakes as early as the 19th century. The First World War delayed the development of a freight train brake. The international railway association founded in 1922 took on the development of a freight train brake for international traffic.

The compressed air brake was introduced for freight trains on a larger scale from the late 1920s.

Indirect air brake

Principle of the indirectly acting compressed air brake
Brake system of a freight wagon with brake slack adjuster, GP change and manual load change

  1   Brake clutch
  2 Clutch   cock
3 Emergency
  brake cable 4 Emergency
cock 5 Hand brake crank 6 Brake spindle
  7 Brake spindle nut
  8 Brake shaft
  9 Hand brake pull rod
10 Brake slack adjuster
11 Control rod
12 Horizontal
lever 13 Load rod
14 Mechanical load change
     with empty rod
15 Retraction spring 16 Brake

17 Fixed point
lever 18 Operating rod for mech. Load change
19 Brake pull rod
20 Vertical brake lever
21 Fixed point
22 Main air line
23 Brake pad
24 Brake triangle
25 Auxiliary air reservoir
26 Handle for release valve
27 Control valve
Control reservoir
29 Brake shut-off
30 Actuation handle for brake shut-off valve
31 GP changeover
device 32 Changeover device for load change

The force exerted by the piston of the brake cylinder 16 acts on the horizontal lever 12, from this on the combination of brake slack adjuster 10 - load rod 13 - brake pull rod 19 via the vertical brake lever 20 via the brake triangles 24 on the brake blocks 23 and thus on the running surfaces of the wheels.
With the handbrake crank 5, the force is transmitted via the brake spindle 6, brake spindle nut 7, brake shaft 8 and the handbrake pull rod 9 to the horizontal lever 12. From there, the force acts on the running surfaces of the wheels, as described above.

The indirectly acting , self-acting or automatic compressed air brake is the standard brake on railways. It is a continuous brake with which all connected vehicles of a train or a shunting movement are operated from the driver's cab of a traction vehicle or a control car . With this brake, the compressed air is supplied to the brake cylinder indirectly via the control valve from the auxiliary air reservoir, which is controlled by the main air line sdruck. The brake is also called automatic or automatic air brakes, because they are at a train separation , the automatic emergency brake causes the two pulling members.

Basic structure and mode of operation

In principle, the compressed air brake of a system of the pressure vessel , the brake cylinders and compressed air lines on each vehicle , which are connected to each other in the compilation of a train on interconnectors.

All vehicles in a train have a continuous, interconnected main air duct (HL). An air compressor in the locomotive supplies these via the driver's brake valve to air in Europe, the CIS and North Africa usually 5  bar pressure (the normal working pressure ).

Air brake according to Westinghouse

In addition to the main air reservoir line (HBL), the main air line also serves as an energy supplier and signal transmission path. Each car also has an auxiliary air reservoir that is constantly refilled from the main air line via a control valve, as well as compressed air-operated brake cylinders and brake pads on the wheels or disc brakes in the wheel or on the axle shaft. The principal control for the brake system is the driver's brake valve on the traction vehicle (z. B. a locomotive ) or the control car .

The brake is released (inactive) when all auxiliary air tanks are full and the main air line is at the normal operating pressure. If the pressure in the main air line is reduced, the control valves direct the compressed air from the auxiliary air tanks into the brake cylinders, which then press the brake pads against the wheels or brake disks via a brake rod or actuate the brake calipers of the disk brakes. The brake system is dimensioned in such a way that when the pressure in the main air line is reduced to approx.3.5 bar (full braking) and when the main air line is completely empty (0 bar for rapid, emergency or forced braking), a pressure of max. 3.8 bar is applied. After braking, the brake is released by refilling the main air line to the normal operating pressure of 5 bar. The control valves return to their original position, the auxiliary air tanks are filled, the air from the brake cylinders escapes into the open and the brake pads are released.

In order to trigger the pressure reduction in the main air line and thus the braking process, the driver 's brake valve on the motor vehicle or the locomotive is normally actuated by the driver . There is also a release option by actuating emergency brake valves , which are usually also available in passenger cars . If the main air line breaks in the event of a train separation while driving, this also leads to braking. In contrast to the metered braking by the driver's brake valve (service braking from the first braking level to full braking - VB), in the last two cases there is rapid braking .

When maneuvering , if the mass to be moved can only be braked with the locomotive brake and no excessive inclines have to be traveled, the car is driven to accelerate without the air brake acting by venting the brake cylinder and, in the case of older control valves, the auxiliary air reservoir. In the case of heavy shunting departments, it is necessary to use at least some of the existing vehicle brakes (so-called »air tips«). The continuous brake cannot be used in push- off and run-off operation .

Single brake

The single brake can be applied gradually, but only released all at once.

A brake designated as “single release” does not allow the braking effect to be gradually reduced. If there is only a slight increase in pressure in the main air line after a previous braking action, the control valves (of which each car has one) go into the release position, i.e. completely release the brake of the car in question.

If the driver's brake valve is not brought into the release or drive position for a long enough time after an (even slight) increase in pressure in the main air line, the auxiliary air tanks of each car are not refilled with compressed air.

If you want to brake again in this situation (e.g. because the engine driver misjudged himself), the pressure in the main air line must be reduced further than in the previous braking. If the braking force is adjusted several times without having to bring the driver's brake valve into the driving position for a long enough time and thus filling the braking system, the compressed air supply from the main air line, but also from the auxiliary air tanks connected to it, can be completely used up. There is then no longer any compressed air available for a braking effect. In technical terms , this is called "exhausting the brake" (exhausting the braking effect).

For this reason, repeated triggering and subsequent braking ("follow-up") is to be avoided, especially when entering stump tracks . If the brake has to be released on long downhill stretches, the journey must first be slowed down so much that there is enough time to refill the main air line and all auxiliary air tanks in the train via the release or travel position of the driver's brake valve. Operating the single-release brake required a lot of experience on long downhill stretches.

In normal operation in Central Europe, the single-release brake is only found in older traction vehicles today; it was almost completely replaced by the multi-release brakes of the types Knorr-Bremse with unitary action (KE) , Oerlikon (O) and Dako (Dk). An exception is the single-release Matrosov brake (M) of the Russian railroad freight cars, which can also be found on German tracks. As in the USA, this is adapted to the larger train lengths that are customary there, where multi-release brakes cannot be used without interference.

See also section Control valves of the earlier single-release air brake in the article Control valve (railroad)

Multi-release brake

The brakes used in Europe today, which can also be released in stages, are referred to as multi-release.

In order to avoid exhausting the brakes and to make it easier for the train drivers to regulate the braking effect, multi-release brakes were developed. The first types were two-chamber brakes based on the model of suction air brakes, in which both sides of the piston in the brake cylinder are pressurized. Examples of this are the Schleifer and Knorr two-chamber brakes (Kz). Their disadvantage is the high demand for compressed air, which at least makes them difficult to use on long trains. This led to the introduction of the Kunze-Knorr-Bremse developed by Bruno Kunze and Georg Knorr in Germany in 1918, which was introduced as a Kunze-Knorr freight train brake (Kkg) and later also as a passenger (Kkp) and express train brake (Kks) . This was further developed by Wilhelm Hildebrand and Georg Knorr. The Hildebrand-Knorr-Bremse (Hik) can also be braked and released in stages; when released, the control valve immediately fills the auxiliary air tank under each carriage again. In addition, the control valve and brake cylinder are separate, individually replaceable and significantly lighter.

While the Kunze-Knorr brake has an additional brake cylinder with two working chambers (two-chamber brake), the Hildebrand-Knorr brake (Hik) is a pure single-chamber brake. An important innovation of the Hildebrand-Knorr-Bremse compared to the Kunze-Knorr-Bremse is the introduction of the three-pressure principle. While with the Kunze-Knorr-Bremse only the pressure ratio between the main air line (HL) and the brake cylinder was used for the control, which can lead to a diminishing braking effect in the event of a leaky brake cylinder due to the constant emptying of the air reservoir, the Hildebrand-Knorr-Bremse also uses the Tank pressure included. If the brake cylinder leaks and the pressure in the main air line falls below the pressure in the reservoir, compressed air is fed directly from the main air line into the brake cylinder, thus preventing the braking force from being exhausted. To make use of this function, the self-regulating driver's brake valve was developed at the same time, which, however, was only used in large numbers around twenty years later and is now part of the standard equipment of the locomotives.

See also section Control valves of the multi-release air brake in the article Control valve (railroad)

Braking positions and switching devices

Brake equipment of a freight wagon, above in white lettering brake equipment, yellow above brake position change, red left brake shut-off valve, red right load change, yellow below release cable. The address Frein O-GP designates an Oerlikon brake with the braking positions G and P without automatic load braking
Brake address of a BR 146 locomotive

A distinction is made between the braking positions according to the braking effect they can produce and the response time. The braking positions G and P work without a power supply, which is why they are suitable for freight traffic. The R-brake requires anti-skid protection that is controlled mechanically in older vehicles and electronically in modern vehicles in order to prevent the wheels from locking; only this can mechanically apply significantly more than 100% braking weight because it increases the braking force above 55 km / h.

  • Braking position G = freight train, slowly responding brake with a brake cylinder filling time of 18–30 s and a release time of 45–60 s
  • Braking position P = passenger train, also called RIC brake, quick-acting brake with a brake cylinder filling time of 3–5 s and a release time of 15–20 s
  • Braking position R = express train (Rapid), high-performance brake with brake booster (passenger trains), but the same filling and release times as with the P brake
  • Braking position R + Mg = express train (Rapid) with magnetic rail brake (high-speed passenger trains)

The times indicated apply when filling up to 95% of the maximum brake cylinder pressure is reached, when releasing until the pressure in the brake cylinder falls below 0.3 or 0.4 bar (depending on the year of standardization of the brake). Switching between the braking positions is done manually. The required braking position must be set separately for each vehicle on the train using the switching device for compressed air brakes . The corresponding changeover levers are attached to the outside of the wagons and have a yellow ball handle (brake types freight train and passenger train) or loop handle (only passenger train brake types) to make them easy to distinguish. In the case of traction vehicles, the changeover levers are largely arranged within the vehicle.

In addition, there is also the load-dependent regulation of the braking force, which is intended to counter overbraking when the load is low and the braking force is too weak when the vehicles are loaded. Automatic load braking (abbreviated with A) is used for both freight and passenger coaches . It is available in different versions of mechanical transmission: by changing the brake linkage or pneumatic transmission (e.g. via a weighing valve ) with secondary springs made of steel. With secondary suspension by air springs, the air spring pressure is used as a load-dependent signal. Since the air suspension valve always regulates the same spring height, the pressure provides an evaluable load signal. There is linear or incremental gain. The manual load change can only be found on freight wagons . This has the positions "empty", "loaded" and sometimes also "partially loaded". The manual load change is set via a red crank handle on the solebar of the wagon.

The braking equipment of the vehicles is shown in abbreviated form on the long sides. The letters G, P and R are common internationally, but there are deviations such as M (French marchandises , Spanish mercancías ) for G or V ( voyageurs or viajeros ) for P.

High braking

The high braking is an extension of the compressed air brake for higher speeds. The braking force of a friction brake decreases at higher sliding speeds of the friction elements. To compensate for this, the speed-dependent braking was introduced. An axle bearing brake pressure regulator with a pressure intensifier regulates the current brake pressure. A pressure supply with more pressure is necessary for a higher brake pressure; but this is only possible with traction vehicles. In order to remain backwards compatible and fail-safe, the brake works just like a multi-release brake. Only the pressure booster is interposed between the brake cylinders and the control valve. For vehicles with electronic wheel slide protection, the high deceleration signal is generated by the wheel slide protection computer, the axle bearing brake pressure regulator is not required.

A distinction must be made between high-speed braking of locomotives and passenger coaches:

Traction vehicles

At speeds over 70 km / h, the pressure intensifier increases the pressure to a maximum of 5.5 bar (braking position P2) or 8 bar (braking position R). If the speed measured by the axle bearing brake pressure regulator falls below approx. 55 km / h (switching hysteresis), the pressure in the brake cylinder is automatically adjusted to the value of normal low braking. The compressed air for high braking is taken from the main air tanks.

Passenger coaches

It is true that the main air reservoir line (HBL), which supplies the train with 10 bar of compressed air, is generally also coupled in passenger trains ; in the event of a train separation, however, this line is opened so that no increased pressure is available. Even if older wagons without a main air reservoir line are placed in a wagon train, no HBL air supply of 10 bar is guaranteed. The high-speed braking in the passenger coaches is therefore fed with the normal 5 bar from the main air line. The higher braking is achieved with larger storage tanks (up to 200 liters per car) and usually two large-volume brake cylinders. With normal braking, the maximum brake cylinder pressure is around 1.7 bar, with high braking around 3.8 bar.

Anti-skid regulator

Due to the low coefficient of adhesion of steel on steel, train wheels can easily jam. The braking effect of a blocked wheel set is significantly lower, and in addition, due to the sliding friction on the rail, flat spots occur on the affected wheel set in a short time , which impairs the smoothness and, in severe cases, the safety. To minimize this damage, centrifugal governors were initially used as anti-skid protection . Two spring-loaded flyweights rotate with the axle and keep the anti-skid valve closed. If there is an abrupt change in speed, the weights are deflected and release the brake on the axis. If the axis accelerates again, the centrifugal weights close the valve again for continued braking.

Newer electronic wheel slide protection computers determine the axle speed using magnetic sensors and compare it with a virtual vehicle speed. If the axle slips, the brake pressure is first held and then gradually reduced until the axle rotates again. The required brake pressure is then built up again.

To increase the penetration rate , the control valve is usually equipped with a brake pipe emptying accelerator. It ensures that a certain amount of compressed air flows out of the main air line on the spot when braking.

Rapid brake accelerator

At high speeds, timing is very important. The speed at which a pressure wave travels in a pipe is relatively slow with a maximum of 290 m / s (the order of magnitude of the speed of sound ); the real pressure drop due to the expansion of the air is even slower. So that the trains brake more evenly, the reaction speed of the brake valves for long freight trains has been artificially slowed. This is neither necessary nor desirable for express trains with more uniform rolling stock and shorter train lengths. In order to accelerate the pressure drop and thus shorten the response time of the brakes in the train, valves are installed that register a rapid pressure drop in the main air line and further accelerate this pressure drop by opening additional outlets. Although this does not increase the breakdown speed, it does increase the speed of the pressure drop, which is necessary for the response of the rapid braking in the brake valve.

However, rapid braking accelerators also have disadvantages. Accelerators that are too sensitive can trigger during normal service brakes. Also, the train driver cannot close the rapid braking accelerator prematurely if he wants to cancel the rapid braking, for example if it was triggered by the Sifa or a braking curve exceeded by the LZB.

Brake address electro-pneumatic brake - control via information and control line (IS)

Electropneumatic brake (ep brake)

The indirect electro-pneumatic brake (better electro-pneumatic brake control with indirect effect) is a superimposition of the brake control via the compressed air line through the additional, but switchable, control of the brake valves by electronic signals. With the electro-pneumatic brake control, the disadvantage of the low breakdown speed of the air pressure brake is eliminated. In addition, it enables the train driver, in case of doubt, to bypass an emergency brake that has been pulled (so-called emergency brake bypass , NBÜ) in order to bring the train to a standstill at a convenient location.

With some types that do not conform to UIC , the electropneumatic brake is even operated without the effect of the main air line (so-called direct electropneumatic brake). The main air line is only switched on when the vehicle is being towed. All brake controls run via brake computers and electropneumatic converters. With this design, electrical active current brakes can work with pneumatic brakes at the same time and also act together when the axes slide.

Direct acting air brake

Direct-acting compressed air brake on a traction vehicle

With single-release brakes, there was a risk of exhausting the brakes and burning the train when driving on long and steep inclines, as well as the disadvantage of poor controllability. In order to remedy these grievances, the Compagnie Paris-Lyon-Méditerranée (PLM) equipped its passenger trains with a second air line and an additional, direct-acting air brake. This brake, together with the direct-acting Westinghouse brake, also known as the double Westinghouse brake , double brake or Henri brake, was also used in Switzerland and some other railways.

With the direct-acting compressed air brake, known in Switzerland as the regulating brake, the entire amount of air is only pressed into the brake cylinder via the additional driver's brake valve (regulating brake valve) on the traction vehicle. The compressed air is drawn from the main air tank via a pressure regulator. When releasing, the air escapes through the driver's brake valve. Small changes in pressure at the driver's brake valve can be used to continuously regulate the pressure in the brake cylinder when braking as well as when releasing. However, the direct-acting brake reacts extremely sluggishly in whole trains and requires a lot of practice from the engine driver, precise knowledge of the route and a far-sighted driving style.

The disadvantage of the double brake was the additional air couplings, which led to increased work when coupling and uncoupling and to higher maintenance costs. When multi-release brakes became more prevalent, direct-acting brakes were removed from cars from the mid-1950s. The direct brake acts as an additional brake ( marked by mZ in the brake address ) now only on the locomotive or the control car and, if necessary, on other locomotives in multiple units . Because the brake is only used for maneuvering, it is called in Switzerland Rangierbremse . In certain cases, the directly acting brake is controlled electro-pneumatically from the control car and in this case can also act on the traction vehicle.

Deadline work on brakes

As safety-relevant components, brakes on rail vehicles must be regularly checked and maintained. This work must be carried out by specially qualified personnel (“brake fitters”). For the area of ​​non-federally owned railways in Germany, the regulations of VDV -schrift 885 (maintenance guide for brakes and compressed air tanks for NE railways - IBD-NE) apply as recognized rules of technology . For the area of ​​the Deutsche Bahn AG there are regulations with similar content.

The IBD-NE currently provides for four types of brake revision (abbreviated representation):

Brake revision Rotation Scope of work
Br 0 if necessary Function and leak test. A Br 0 must be carried out after the brake system has been touched while working on the vehicle, for example by lifting the car body, machining wheelsets or replacing brake components.
Br 1 one year after the last Br 1, 2 or 3
for freight wagons every two years
Inspection for condition and proper functioning, if necessary repairs
Br 2 four years after the last Br 2 or 3
for freight wagons alternating with Br 3 on the occasion of the main inspection
Inspection for condition and proper functioning, including inspection of the compressed air tanks and partial dismantling of the brake system.
Br 3 during the general inspection of the vehicle Dismantle the brake linkage, refurbish or replace brake components, check safety valves, blow out lines, check pressure vessels.

See also


Web links

References and comments

  1. The passenger coaches for international traffic to Austria were equipped with an air brake in addition to the air brake. At the beginning of the Second World War, the railways in Austria switched to air brakes.
  2. ^ Heinz Russenberger: suction air or vacuum brake . In: Four-axle passenger coaches of the SBB from 1912–1929 (=  Loki special ). No. 31 . Lokpress, Zurich 2009, ISBN 978-3-9523386-2-9 , p. 10-11 .
  3. Daniel Jobstfinke, Matthias Gülker, Markus Hecht: Freight trains with ep brakes: higher speeds, less wear . In: ZEVrail, Glaser's Annalen . tape 143 , no. 4 , April 2019, ISSN  1618-8330 , ZDB -ID 2072587-5 , p. 124-129 .
  4. French for "brake".
  5. UIC Leaflet 540, 5th edition November 2006  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link /  
  6. Zugfunk Podcast: Episode 20 From 2:35:52