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A magnetic track brake (Mg brake) is a brake for rail vehicles. It consists of brake magnets, pole shoes, a suspension, a power transmission and, in the case of mainline railroads, a track rod. When current flows through the magnet coil, the magnet is attracted to the rail, which presses the pole shoes against the rail, thereby decelerating the vehicle. [1]
While brakes such as disc brakes or shoe brakes depend on the frictional connection between wheel and rail, the magnetic track brake acts directly on the rail. Therefore, its brake effect is not limited by wheel-rail contact. Thus, environmental factors such as wetness or contamination of the rail have less influence on the brake force. [2]
Magnetic track brakes are used on rail vehicles in addition to the primary, wheel-effective brake systems. As an additional brake system, they help to ensure that the prescribed brake distances of rail vehicles can be complied with.
Since magnetic track brakes always act unregulated and at their maximum brake force, they are only used as safety and emergency brakes. They can be used at speeds of up to 280 km/h (170 mph). With the usage of special friction materials they can be used up to speeds of 350 km/h (220 mph).
Due to their track-cleaning effect, magnetic track brakes increase the coefficient of adhesion between the following wheels and the rail during the brake process. This additionally leads to an improvement of the wheel-effective brake systems. [3]
Magnetic track brakes are distincted between rigid and articulated magnets. [4]
On April 5, 1900, the patent (AT11554) for the first electromagnetic brake for rail vehicles was registered by the Westinghouse Air Brake Company London. Three years later, the electromagnetic track brake was introduced in Germany by the Westinghouse Company.
The Mg brake was characterized by the fact that the electromagnets were magnetized to different degrees by the exciter coils, which made the brake force dependent on the strength of the brake current. Even the winding numbers of the exciter coils were different in order to be able to regulate the brake force. Thus, the track brake was also equipped with several shoes in order to be able to adapt to possible unevenness of the rails.
In 1905, the first tests were carried out by the Rhine Railway Company. These were track magnets with an attractive force of around 4 kN, which lowered automatically onto the rails when the current was switched on, pressing onto the brake shoes and on the wheels of the cars via a lever rigging. At that time, it had not yet been recognized that track brakes should work independently of the friction between the rail and the wheel.
In 1908, Mr. Jores took over the Westinghouse representation for track brakes in Germany and played a major role in their continuation. After World War I, Jores led the production of his own track brakes after the patent rights had expired. The track brakes were based on drawings taken from Westinghouse. They were manufactured until 1929 without any major changes. The main feature of the track brake at that time were the rail shoes, which were made of a special rolled section.
In 1920, the Magnetic Brake Company, headed by Mr. M. Müller, entered the market with track brakes. Müller attempted to improve the track brake with new designs. For example, he replaced the profiled shoe with a pole shoe made of commercially available flat iron. Until then, track brakes had only been used for streetcars and thus for speeds of up to 40 km/h (25 mph).
At the beginning of 1930, the German Imperial Railways initiated a high-speed rail project that envisaged speeds of up to 160 km/h (99 mph) and was to be of great significance for the track brake.
In 1931, Jores´ company was bought by Knorr-Bremse AG, and the technical director Müller from the Magnetic Brake Company was convinced to join the company. For the first time, the track brake for fast-moving vehicles was developed within the Knorr-Bremse company. In cooperation with the German Imperial Railways, the first tests were carried out with the "Flying Hamburgian". For braking, special brake pads with linings made of synthetic friction materials were used, which acted on brake drums and were attached to the wheel spiders. There was also an electromagnetic track brake available, which however was only to be used as an additional emergency brake.
It became apparent that the pole shoe commonly used up to then was no longer able to cope with the demands of the high speed and the associated high level of heating. Hence the pole shoes were first slit, then divided and made from individual segments. This increased brake performance by 20%. The coil was now fixed to the core and then inserted into the box from the end face together with the core. The coil box was tightly screwed between the core and the webs of the magnet coil, making loosening impossible. The further development of the track brake now appeared to have been completed for the time being.
The coefficient of friction between the rail shoe and the rail is dependent on the speed, i.e. with increasing speed, the coefficient of friction decreases. As the project "speed up to 350 km/h" became official, it appeared as if the track brake could no longer be of use for this purpose.
It was not until passenger train speeds exceeded 140 km/h (87 mph) and a friction-independent brake system became necessary that the plans for the track brake were brought out again and the design improved. To improve the contact surfaces with the rail, articulated magnets were developed and patented. [5]
The main component of the magnetic track brake is the brake magnet. Following the principle of an electromagnet, it consists of a coil wound around an iron core, which is enclosed by horseshoe-shaped magnets.
Direct current is passed through this magnet coil, generating a magnetic field. This causes an attractive force between the brake magnet with the pole shoes attached to it and the rail. The pole shoes are pressed onto the rail, and the resulting friction converts the kinetic energy of the movement into heat (dissipation) until the kinetic energy is consumed or the brake is deactivated. [6]
Magnetic track brakes must also work safely in the event of a contact line failure. The braking system must therefore be designed in such a way that, in the event of a power failure, a supply from the vehicle's batteries is guaranteed at all times.
Rigid magnets contain a single steel core running the entire length of the magnet body, with pole shoes located on the underside as wear parts. [7]
Rigid magnets are typically used for streetcars, where they are usually suspended in a low position.
The suspension is responsible for holding the switched-off magnet above the rail. In the event of braking, the magnet automatically attracts itself to the rails against the effect of the suspension springs. After switching off, the springs of the suspension pull the magnet back into the readiness position. [8]
The drivers are responsible for the transmission of the brake force from the magnet to the bogie. It takes place via tie bars or driver towers.
Tie bars are attached to the front and rear ends of the brake magnet respectively. They are the preferred and most effective way of transmitting brake force.
If there is not enough space in front of or behind the brake magnet to mount the drivers, they are mounted on top of the magnet. These are referred to as driver towers. This type of driver should only be used in exceptional cases. [9]
The pole shoes are located on the underside of the brake magnet. Between the two pole shoes, a non-magnetic strip ensures that a magnetic short circuit does not occur. [10]
The friction material of the rail shoes can be made of different materials, each of which determines the service life and brake performance of the rail shoes. [11]
Articulated magnets have magnetic cores that are divided into two end pieces and several intermediate links separated by partitions. While the end pieces are tightly screwed together with the coil body, the intermediate elements can move freely in the openings of the coil case. Thus, they can adapt themselves better to unevenness of the rails during the brake process. [12]
The track rods are used to keep the brake magnets at a distance. They also ensure their parallelism and stability. Together with the two brake magnets, the track rods form the so-called brake frame. Track rods must be individually adapted for each vehicle model. [13]
The actuating cylinders are located on top of the brake square. They are responsible for lowering the brake frame onto the rails and raising it again. [14]
Built-in springs hold the brake frame in the high position when the brakes are not applied. When the brakes are applied, the brake frame is pneumatically lowered onto the rails against the force of the springs. The compressed air supply required for this is provided by a separate compressed air reservoir. This ensures that the brake system is still working even if the vehicles brake pipe fails. When the brakes are released, the springs in the actuating cylinders lift the brake frame back into the high position. [15]
In the deactivated state, the magnets are de-energized and the brake square is brought into the high position. In this case, the centering device ensures that the brake square is centered and fixed in its position. While braking, the brake magnets are activated and center themselves on the rails by the magnetic force. [16]
Also with articulated magnets, drivers ensure that the brake force is transmitted from the brake magnets to the vehicle. They are located in all four corners on the inside of the brake frame. [17]
If required, a buffer switch can be mounted on the brake frame. It signals when the brake frame leaves its high position and thus provides information on the status of the track brake. [18]
The pole shoes in magnetic track brakes can be made of different materials. These differ primarily in their magnetic properties, brake force coefficient, and wear. [19]
Steel is the standard friction material for track brakes. The wear of steel pole shoes is low, but they form weldings which have to be knocked off regularly.
Pole shoes made of sinter offer increased brake performance and do not form weldings, but their wear is higher. Sinter is used in cases where brake force is critical. It is currently used, for example, by Vy in Norway.
Pole shoes made of cast iron are only used in mainline. They have reduced brake force and increased wear, but do not form weldings. In France, cast iron is the standard friction material used for magnetic track brakes.
Magnetic track brakes are installed in almost all rail vehicles. Only high-speed trains use eddy current brakes instead of magnetic track brakes for technical reasons.
Rigid magnets are usually suspended in low suspension and are used on streetcars. In special cases, the use of track rods is possible.
Articulated magnets are usually suspended in high position and are used in mainline railroads. However, they can also be used in low suspension, for example in subways.
Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed "rheostatic" if the generated electrical power is dissipated as heat in brake grid resistors, and "regenerative" if the power is returned to the supply line. Dynamic braking reduces wear on friction-based braking components, and regeneration lowers net energy consumption. Dynamic braking may also be used on railcars with multiple units, light rail vehicles, electric trams, trolleybuses, and electric and hybrid electric automobiles.
In electromagnetism, an eddy current is a loop of electric current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnetic field. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material. When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid.
Electrodynamic suspension (EDS) is a form of magnetic levitation in which there are conductors which are exposed to time-varying magnetic fields. This induces eddy currents in the conductors that creates a repulsive magnetic field which holds the two objects apart.
A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational features are more complex because of the need to control multiple linked carriages and to be effective on vehicles left without a prime mover. Clasp brakes are one type of brakes historically used on trains.
Automatic Warning System (AWS) is a railway safety system invented and predominantly used in the United Kingdom. It provides a train driver with an audible indication of whether the next signal they are approaching is clear or at caution. Depending on the upcoming signal state, the AWS will either produce a 'horn' sound, or a 'bell' sound. If the train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS. However if the driver correctly acknowledges the warning indication by pressing an acknowledgement button, then a visual 'sunflower' is displayed to the driver, as a reminder of the warning.
An eddy current brake, also known as an induction brake, Faraday brake, electric brake or electric retarder, is a device used to slow or stop a moving object by generating eddy currents and thus dissipating its kinetic energy as heat. Unlike friction brakes, where the drag force that stops the moving object is provided by friction between two surfaces pressed together, the drag force in an eddy current brake is an electromagnetic force between a magnet and a nearby conductive object in relative motion, due to eddy currents induced in the conductor through electromagnetic induction.
The SCMaglev is a magnetic levitation (maglev) railway system developed by Central Japan Railway Company and the Railway Technical Research Institute.
Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are composed of steel backing plates with friction material bound to the surface that faces the disc brake rotors.
Electromagnetic brakes or EM brakes are used to slow or stop vehicles using electromagnetic force to apply mechanical resistance (friction). They were originally called electro-mechanical brakes but over the years the name changed to "electromagnetic brakes", referring to their actuation method which is generally unrelated to modern electro-mechanical brakes. Since becoming popular in the mid-20th century, especially in trains and trams, the variety of applications and brake designs has increased dramatically, but the basic operation remains the same.
The Westinghouse Brake & Signal Company Ltd was a British manufacturer of railroad signs. Founded by George Westinghouse, it was registered as "Westinghouse Brake Company" in 1881. The company reorganised in 1920, associating with Evans O'Donnell, and Saxby and Farmer which merged to form the "Westinghouse Brake & Saxby Signal Company". The 'Saxby' would be dropped from their title in 1935.
Wheel slide protection and wheel slip protection are railway terms used to describe automatic systems used to detect and prevent wheel-slide during braking or wheel-slip during acceleration. This is analogous to ABS and traction control systems used on motor vehicles. It is particularly important in slippery rail conditions.
Knorr-Bremse AG is a German manufacturer of braking systems for rail and commercial vehicles that has operated since 1905. Other products in Group's portfolio include intelligent door systems, control components, air conditioning systems for rail vehicles, torsional vibration dampers, and transmission control systems for commercial vehicles.
A wheel cylinder is a component of a hydraulic drum brake system. It is located in each wheel and is usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto the shoes so as to bring them into contact with the drum and stop the vehicle with friction. The wheel cylinders are usually connected to the shoes with small bird-beak shaped rods. Wheel cylinders were first invented by Bendix in 1958.
Theodor Georg Knorr was an engineer and entrepreneur on the field of railroad technology and founder of the company Knorr-Bremse. He is particularly remembered for his role in the development of the compressed air brake.
The Kunze-Knorr brake is an automatic compressed-air brake for goods, passenger and express trains. It was the first graduated brake for goods trains in Europe. When it was introduced after the First World War, goods train brakes switched from hand operation to compressed-air in various European countries. The Deutsche Reichsbahn alone put the cost of equipping German goods wagons with Kunze-Knorr brakes between 1918 and 1927 at 478.4 million Reichsmarks. The operating cost savings from faster goods services and having fewer brakemen was assessed by the Reichsbahn at almost 96.3 million Reichsmark annually.
Kiepe Electric GmbH is a German manufacturer of electrical traction equipment for trams, trolleybuses other road and rail transport vehicles, as well as air-conditioning and heating systems, and conveyor device components. Founded in 1906, it was known as Kiepe Elektrik GmbH until 2003, when it was renamed Vossloh Kiepe, following its acquisition by Vossloh AG. Vossloh sold the company to Knorr-Bremse in January 2017, and in May 2017 Knorr renamed it Kiepe Electric GmbH.
Electromagnetic clutches and brakes operate electrically, but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches or brakes. Over the years, EM became known as electromagnetic versus electro mechanical, referring more about their actuation method versus physical operation. Since the clutches started becoming popular over 60 years ago, the variety of applications and brake and clutch designs has increased dramatically, but the basic operation remains the same.
The intermittent inductive automatic train stop is a train protection system used in North American mainline railroad and rapid transit systems. It makes use of magnetic reluctance to trigger a passing train to take some sort of action. The system was developed in the 1920s by the General Railway Signal Company as an improvement on existing mechanical train stop systems and saw limited adoption before being overtaken by more advanced cab signaling and automatic train control systems. The system remains in use after having been introduced in the 1920s.
Electromagnetically induced acoustic noise, electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.
Electromagnetism is one of the fundamental forces of nature. Early on, electricity and magnetism were studied separately and regarded as separate phenomena. Hans Christian Ørsted discovered that the two were related – electric currents give rise to magnetism. Michael Faraday discovered the converse, that magnetism could induce electric currents, and James Clerk Maxwell put the whole thing together in a unified theory of electromagnetism. Maxwell's equations further indicated that electromagnetic waves existed, and the experiments of Heinrich Hertz confirmed this, making radio possible. Maxwell also postulated, correctly, that light was a form of electromagnetic wave, thus making all of optics a branch of electromagnetism. Radio waves differ from light only in that the wavelength of the former is much longer than the latter. Albert Einstein showed that the magnetic field arises through the relativistic motion of the electric field and thus magnetism is merely a side effect of electricity. The modern theoretical treatment of electromagnetism is as a quantum field in quantum electrodynamics.
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