Brake (railway)
A brake (abbreviation: Br ) in railways and railways has similar operating principles as the commonly known types of brakes , but is systematically structured differently. The requirements for today's operation lead to different technical systems .
First developments
In the very earliest times, railroad brakes consisted of levers that acted on wooden brake blocks. Only the cars and the tender of the locomotive were braked . Later, as with road vehicles, crank drives and brake blocks were used to brake. Even back then, brakes were used not only on the locomotive , but also on several or all of the wagons of a train to brake manually in response to whistling signals from the locomotive. At that time, the cars with brakes usually had a high brakeman's cab at one end of the car.
In order to give passengers the opportunity to get the locomotive crew to stop the train, the communication cord was used to install a bell cord running through the entire train, which sounded a bell when the locomotive was operated.
The first really promising attempt at a continuous brake is the lever arm brake . The rotation of the axles is used to press the brake pads on. In order to release the brakes, the train driver has to use a winch to tension a rope that is guided over the roofs of the entire train. If this rope breaks, the train is automatically braked. This means that the lever brake still fulfills the basic requirements of a pull brake today.
The Görlitzer weight brake is a type of mechanical rope brake that is similar in its operating principle to the lever arm brake, but which varies greatly in structural details . Here weights are connected to the brake pads via levers. The braking force can be regulated by lifting or lowering the weights over the running rope. Another type of construction similar to the Heberlein brake is the Schmid continuous helical wheel brake .
From the mid-1870s, the suction air or vacuum brake was developed . It was used on almost all types of railways ( mountain railways , standard-gauge and narrow-gauge railways and trams). In this case, the spring-loaded brake is released by a negative pressure in a continuous suction line. The braking power could be regulated by reducing the vacuum in the brake cylinder. In the event of a train separation, the brakes in both parts of the train were activated automatically. The main drawbacks that prevented further use were the large suction cylinders in the car and the high steam consumption of the injector vacuum pump. In particular, narrow-gauge and mountain railways used the suction air brake for many decades because it was more suitable for long downhill stretches due to its general multiple solution than the initially single-release air brakes. The Rhaetian Railway and the Matterhorn-Gotthard Railway in Switzerland, for example, still use them today.
In order to avoid the disadvantage of the large brake cylinders in suction air brakes, steam locomotives and their tenders were often equipped with a steam brake in which a steam cylinder acted directly on the brake linkage. The disadvantage of the vapor barrier is its reduced braking effect due to cooling and condensation.
In the days of the steam locomotive, mountain railways in particular used the piston steam engine, which otherwise ran idle downhill, with a control and throttle valve moved in the opposite direction as a brake. One design is the counter-pressure brake according to Niklaus Riggenbach .
Braking systems
As a rule, the following braking systems are used in railway vehicles:
Friction brakes
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Pad brakes or disc brakes are mainly used as friction brakes . In special cases, drum brakes are also used, which can be designed as band or block brakes.
By the brake force at the wheel rim resulting friction generate the frictional force (braking force) at the wheel rim tangentially engages and the direction of rotation of the wheel is opposite. An equally large counterforce (adhesive force, static friction force ) is generated between the wheel and the rail , which inhibits the vehicle's movement.
The frictional force acting on the wheel from the brake pad must never be greater than the static friction force that can be transferred from the wheel to the rail, otherwise the wheels will slide and flat spots will arise . Because of the lower static friction compared to road vehicles, the braking distances are considerably longer . The coefficient of static friction between the wheel and the rail is greatest with clean, dry or completely wet rails. When it starts to rain, when there is fog, hoar frost, on level crossings due to road salt, but especially when leaves are falling or soiling with oil, the rails can become slippery and the coefficient of static friction can become very small. Anti-slip devices prevent slipping, possibly also helps sands .
Block brake
The block brake is the simplest and at the same time the oldest type of friction brake. It belongs to the group of friction-dependent brakes. The braking force is generated by brake pads that are pressed against the running surface of the wheels. As a rule, block brakes are designed as compressed air brakes .
Block brakes are cheap and light because only a few components are required. Braking cleans the running surface of the wheels, which improves the static friction between wheel and rail. Disadvantages are the high thermal load on the wheelset and the frictional wear of the wheel surface, as well as the high level of noise generated by block-braked cars due to the roughened wheel surface.
Disc brake
Another type of friction-dependent brakes is the disc brake . It is also usually designed as a compressed air brake. The brake lining press here on a specially provided a friction surface on the wheel or on the wheel set shaft mounted disc. Disc brakes mounted on the wheel disc are often used for driven axles, as there is sufficient space for the drive units in the center of the wheelset. Non-driven wheel sets usually have two to four shaft brake disks, often forcibly ventilated.
Disc brakes have a constant coefficient of friction over the entire speed range. Because of the better cooling, a higher braking power is possible, the wheel running surfaces are not worn by the brake and disc brakes generate less noise than block brakes. The disadvantage is the higher construction effort than with the block brake.
Mechanical brake
Most locomotives as well as passenger and most freight cars are equipped with a manually operated mechanical friction brake. This mechanical brake or handbrake acts directly (mechanically) on the vehicle's brake pads. It applies the braking force independently of the pneumatic brake and is therefore suitable for securing parked vehicles. A locking of the parked vehicle inactive allow only mechanical braking, since the holding force of the air brake is ensured due to unavoidable leakages, only up to 30 minutes after stopping.
There are two types of construction. The handbrake , which can be operated from the vehicle, serves on the one hand to secure vehicles against escaping and on the other hand to regulate the speed for certain maneuvering movements and to stop trains in the event of a faulty automatic brake. It is generally considered spindle brake executed and is operated by a brake platform or coaches from the vehicle interior, typically from an entry room. This braked weight is framed in white on freight wagons (white like the rest of the braking address, alternatively black on a white or light background). Hand brakes on tenders and tank locomotives are often designed as throw lever brakes .
The parking brake , which can be operated from the ground, is only suitable for securing against running away from parked cars. It can be designed as a handwheel or as a spring-loaded brake . This braked weight is framed in red on freight cars.
A direction-dependent ratchet brake is often installed on one-sided inclined rack railways . It only brakes when going downhill. When driving uphill, the applied ratchet brake is released by a ratchet mechanism and prevents the train from rolling backwards.
Rail brake
Rail brakes are usually designed as magnetic rail brakes . Brake shoes are lowered under the chassis and pressed onto the rail head by magnetic force . Because the frictional force is independent of the wheel-rail friction, they are suitable for shortening the braking distance when initiating rapid braking , especially on trams .
One of the first applications of the electric magnetic rail brake was in 1933 in the diesel-electric powered express rail car Fliegender Hamburger .
Another type of rail brake is the caliper brake , which acts on both sides of the rail head and which, in the case of funicular railways, bolts the carriage to the rails if the cable breaks.
Spin brake
The spin brake is not used to reduce the speed, but by applying the brakes lightly, it prevents the drive wheels from spinning in the event of poor adhesion . For this brake to work properly, the quick application of the brake pads and immediate release must be guaranteed. This is done with an electropneumatic valve that is operated by an automatic control unit or a push button switch.
Anti-skid device
In unfavorable cases, the braking force can exceed the static friction force that can be transferred from the wheel to the rail. There is a risk that the axles lock up when braking. This leads to longer braking distances and also means that the running surfaces of the wheels are damaged by flat spots . Anti- skid devices can prevent the bike from sliding.
The anti-skid device compares the speed of the vehicle axles with each other and against all-axle sliding with a virtual speed calculated from the applied brake pressure and the possible deceleration. As soon as the speed difference reaches a certain value, the brake cylinder is locked. If the speed of the wheel does not increase, it is vented. As soon as the axis has reached normal speed again, ventilation of the brake cylinder is interrupted and normal braking starts again. This process takes place in tenths of a second. If the friction conditions are very bad, the speed may not increase again at all.In this case, the control process is ended after 6 seconds and the full brake pressure is built up again, even if the axles are stationary. This prevents a vehicle from braking at all.
Electric and hydrodynamic brakes
Electric and hydrodynamic brakes are wear- free and support the air brakes in their effectiveness. The drive of the locomotives plays a constructive role here. In traction vehicles with electric traction motors , these can be used for electrodynamic brakes, while hydrodynamic brakes can be used in combustion drives with hydraulic power transmission. Recently, so-called retarders have also been found in railway vehicles - a variant of the hydrodynamic brake for non-drive axles. Of historical interest are the Riggenbach - back pressure brakes with steam locomotives .
Electrodynamic brake
In the electrodynamic brake, the traction motors of the traction vehicles act as generators . The electrical energy obtained in this way is fed back into the catenary network with the utility current brake , and with the resistance brake it is converted into thermal energy via resistors. It is possible to combine both operating modes. If the contact line network is capable of receiving, the braking current is fed back in this case. Otherwise it is switched to the braking resistors. The electrodynamic brake is used to regulate and reduce the speed with little wear, even on slopes, in certain cases up to a stop.
Electrodynamic brakes can be used on electric and diesel-electric powered locomotives . In diesel-hydraulically operated locomotives, there is no electric traction motor for generator operation for an electromotive brake.
Electrodynamic brakes are mostly used as an additional brake. A compressed air brake is usually the main braking system because, in contrast to the electric brake, it allows braking to a standstill and not just to almost zero. As long as the dynamic brake generates sufficient braking power, the compressed air brake is only pre-controlled, but not applied. Only brakes that are independent of the contact line or energy generation may be taken into account in the emergency brake calculation. This excludes all brakes that only feed back into the contact line or are excited by external current.
Hydrodynamic brake
Traction vehicles with hydraulic power transmission are ideal for installing a hydraulic brake . For this purpose, an additional hydraulic clutch is installed in the fluid transmission, the turbine wheel of which is firmly connected to the housing. The pump wheel is driven by the wheel sets (brake clutch, retarder). The braking force can be regulated by filling the clutch with oil. The clutch is consequently operated with 100% slip, which means that a large amount of heat has to be dissipated via the locomotive's cooling system. The efficiency of the cooling system therefore usually also limits the maximum output of the hydraulic brake.
Another possibility is the use of so-called turbo reversing gears, which use separate hydraulic circuits for each direction of travel. Here the circuits in the opposite direction can be used for braking. The disadvantage compared to the brake clutch is the increased fuel consumption, since the engine has to generate the braking force.
Eddy current brake
The linear eddy current brake is used on the ICE 3 operated by Deutsche Bahn . The advantages of this braking system are that the eddy current brakes are independent of the wheel-rail frictional connection and are therefore independent of the weather (so-called adhesion - independent brakes ) and that they apply their braking force to the rail head in a contactless and very precisely controllable manner , which enables wear-free operation. In connection with this, there is also the possibility of safely controlling the train on long descents, because - in contrast to block, disc or magnetic rail brakes - there is no risk of the brakes overheating. However, these braking systems cannot be completely dispensed with, since the braking force of the eddy current brake is speed-dependent and the vehicle cannot be brought to a stop in time with it alone. Other problems are the strong induction currents that heat up the rails and force the operator to monitor the temperature of the rails in order to prevent shifting of the track position due to excessive heating, as well as the strong magnetic fields that can disrupt the signals at the edge of the track.
With the rotating eddy current brake , the rail is used as an electromagnet and currents are induced in the wheels of the train, whose magnetic fields interact with those of the electromagnets and thus brake the vehicle. This brake is currently only used in test vehicles.
Gear brakes
In the case of cog railways, it is not possible to stop safely using only friction or magnetic rail brakes. The vehicles on these railways are therefore equipped with gear brakes. These are braked gears that mesh with the rack. Mechanical brakes are used as gear brakes. In traction vehicles, the gear brake is often designed as a drag or power current brake .
→ See sections Brakes and Self-excited inertia brake for converter vehicles in the article rack railway
Post brake
In Switzerland, rail tractors without an indirect compressed air brake are equipped with a secondary brake (N). They can bebrakedwhen towing . In an operating or emergency braking the Nachbremse does not respond. The secondary brake causes the air brake of a tractor connected to the main air line to respond when the pressure in the main air line is reduced by approx. 2.5 bar. When the main air line pressure is increased to the level of full braking, the secondary brake is released.
Because the indirectly acting compressed air brake is not effective when towing the Re 460 of the SBB , the locomotives were equipped with an after-brake, as was previously known on tractors.
For driving on rack and pinion sections, wagons with mixed gear / adhesion tracks can be equipped with an adhesion and a delayed acting gear brake. In the event of a moderate decrease in the vacuum (up to 25 cm Hg ) or the air pressure in the main air line, only the adhesion brake responds. That is enough to maintain the speed when going downhill. A greater reduction in the vacuum (from 24 cm Hg) or the pressure in the main air line causes the gear brake to also respond.
Today's operational needs
With the introduction of the Railway Building and Operating Regulations in 1967, it was prescribed that all vehicles - with the exception of small locomotives and cable cars - must be equipped with a continuous automatic brake.
Today trains with a maximum speed of more than 50 km / h must be equipped with a continuous and automatic brake. Continuous means that the brakes of all vehicles in a train can be operated centrally from one point. A brake is considered to be automatic if, when the brake line is disconnected, the train or both parts of the train automatically brake to a standstill. Other operational requirements are:
- Compatibility with the braking systems of other railways,
- Distribution of the braking force over the entire train,
- Brake force regulation according to train weight,
- high and continuously available braking power.
Compression and strain
A special feature of railway trains is that there must be adequate braking force at the end of a train in order to avoid compressing or pulling the train and to ensure that the train comes to a standstill in the event of a train separation.
A train is compressed when the rear pulling part is pushed unbraked against the front, already braked pulling part and thus compresses the train. Because of the breakthrough time, this occurs particularly on long trains with compressed air brakes, the brakes of which are controlled centrally from a driver's cab. The air pressure difference triggered by the driver's cab moves backwards through the train set at a finite speed, so that vehicles at the Zugspitze brake earlier than vehicles at the end of the train.
A strain in the train arises when the rear part of the train brakes earlier than the unbraked front part of the train or is still braking while the front part of the train no longer brakes. In this case, the forces occurring in the longitudinal direction of the train can be greater than the pulling and pushing devices can absorb. The result is a train separation , which poses a danger to the following trains.
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In order to master the problem of compression and strain in the train set in practice, the application and release time of the respective vehicle brake can be changed on most railway vehicles by means of switching devices , such as a brake position change. Depending on the selected braking position (G, P, P2, R, R + Mg), the vehicle brake applies or releases faster.
| Braking position | Characteristic | Mooring time | Release time |
|---|---|---|---|
| G | slow acting | 18 to 30 seconds | 45 to 60 seconds |
| P | fast acting | 3 to 5 seconds | 15 to 20 seconds |
The braking position to be set results from the timetable and the respective operational regulations of the railway company for operating the brakes. In rail freight transport , if the train weight (total weight of all vehicles behind the leading motor vehicle) is more than 800 t, the motor vehicle at the top will be or will be in braking position G and if the train weight is more than 1200 t, the following five will be added vehicles in the braking position G down.
Push locomotives
On routes with steep gradients , it may be necessary that particularly heavy trains (e.g. coal or ore trains) have to be pushed with additional locomotives. This is done with the help of so-called push locomotives, which bring additional drive power, but do not necessarily contribute to the braking effect. An additional braking effect of the compressed air brake depends on whether the push locomotive is coupled to the main air line . The electric brake of the pushing locomotive is used on the descent on mountain railways. It contributes wear-free to the braking performance of the train and the train remains stretched.
Safety brake
The safety brake is independent of the friction between wheel and rail. Rail brakes and gear brakes are among the safety brakes. Safety brakes are mandatory in Switzerland for driving on routes with an incline of more than 60 ‰ and for trams if the speed is not reduced appropriately.
Vehicles with a brake computer
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In modern locomotives and control cars, the control of the brake systems is integrated into the control technology . The controls in the driver's cab control a computer via the data bus and the vehicle control unit, and the computer controls the compressed air brake. In any case, rapid braking can be carried out directly by opening the main air line with an emergency cock or an emergency brake valve , bypassing the computer.
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 | 1 year after the last Br 1, 2 or 3 for freight wagons every 2 years |
Inspection for condition and proper functioning, if necessary repairs |
| Br 2 | 4 years after the last Br 2 or 3 for freight wagons alternating with Br 3 on the occasion of the general 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
- Sand spreader
- Track brake
- Brake test
- Emergency brake
- Whisper brake (composite brake block)
literature
- Federal Office of Transport: Driving Service Regulations (FDV) A2016 . Bern, 2016 . R 300.14
- Brakes . In: Deutsche Bundesbahn (Hrsg.): Railway teaching library of the Deutsche Bundesbahn . 4th edition. tape 122 . Josef Keller Verlag, Starnberg 1962.
- Friedrich Sauthoff : Braking customer for technical car service . Eisenbahn-Fachverlag, Heidelberg and Mainz 1978.
Web links
Individual evidence
- ↑ Bruno Lämmli: SBB CFF FFS Re 460 and BLS Re 465. Mechanical construction. Retrieved January 18, 2014 .
- ^ Ernst Kockelkorn: Effects of the new railway building and operating regulations (EBO) on railway operations . In: The Federal Railroad . tape 41 , no. 13/14 , 1967, ISSN 0007-5876 , pp. 445-452 .