According to the transfer mode of train kinetic energy during braking, the braking of different urban rail vehicles can be mainly divided into friction braking and electric braking.

First, friction braking.

Through the friction of the friction pair, the kinetic energy of the train is converted into heat energy, which escapes into the atmosphere, thus producing braking effect. The commonly used friction braking methods of urban rail vehicles mainly include brake shoe braking, disc braking and track electromagnetic braking.

(1) Brake shoe braking

Brake shoe braking, also known as tread braking, is the most common braking method. When braking, the brake shoe presses the wheel, and the wheel rubs against the brake shoe. Through the friction between the wheel and the brake shoe, the kinetic energy of the train is converted into heat energy and dissipated in the air.

In the friction pair of wheel and brake shoe, because the wheel mainly undertakes the driving function of the vehicle, other materials cannot be replaced casually. In order to improve the braking performance of brake shoes, only the material of brake shoes can be changed. At present, most urban rail vehicles use synthetic brake shoes. Synthetic brake shoes have poor thermal conductivity, so powder metallurgy brake shoes with good thermal conductivity and good friction performance are also used.

In brake shoe braking, when the braking power is high, the generated heat cannot escape to the atmosphere, but accumulates on the brake shoe and wheel tread, which increases its temperature and reduces the friction force. In severe cases, the brake shoe will melt and the wheel hub will relax. Therefore, the braking force is limited when braking the brake shoes.

(2) Disc brake)

Disc brakes can be divided into axle disc type and disc type. Generally, shaft disc type is adopted. When it is difficult to install the brake disc in the middle of the wheelset due to traction motor and other equipment, the wheel disc type can be used. When braking, the brake cylinder clamps the brake disc through the brake caliper, which causes friction between the brake disc and the brake disc, converts the kinetic energy of the train into heat energy, and escapes to the atmosphere through the brake disc and the brake disc.

(3) Track electromagnetic braking

Track electromagnetic braking is also called magnetic track braking. This is a traditional braking method. This braking method is to install a lifting cylinder between the front and rear wheels of the bogie. The top of the cylinder is equipped with two electromagnets, including electromagnet shoes and friction plates. The electromagnet is suspended at an appropriate height from the track surface. When braking, the electromagnet falls and is connected with the excitation power supply to generate electromagnetic attraction. The electromagnet is adsorbed on the rail, and the kinetic energy of the train is converted into heat energy through the friction between the wear plate and the rail, and escapes to the atmosphere. Rail electromagnetic braking can obtain large braking force, so it is often used as an auxiliary braking in emergency braking. This kind of braking is not limited by the wheel-rail adhesion factor, and it can effectively shorten the braking distance while ensuring the comfort of passengers. When the friction between the wear plate and the rail produces a lot of heat, the wear on the rail is also very serious. However, due to the short braking distance and simple and reliable structure, this device is widely used in trams and light rails.

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Second, electric braking.

From the point of view of energy, the essence of braking is to convert the kinetic energy of the train into other forms of energy. The ability of the braking system to transfer kinetic energy becomes the braking force. Generally, under a certain safe braking distance, the braking force of a train is a cubic function of its speed. The speed of modern rail transit vehicles is very high, and the quality of trains is also great. It is difficult to transmit the braking force only by mechanical means.

At present, the most widely used mechanical friction braking method is brake shoe braking. However, due to the limitation of braking power, the dust and heat generated by the wear of brake shoes and wheel treads will also cause serious pollution to the environment, especially in tunnels with poor ventilation conditions, which will have a serious impact on passengers and equipment. In addition, frequent use of friction braking will cause frequent replacement of brake shoes and increase the amount of wheel tread correction, which will not only lead to high maintenance costs, but also take a long time to maintain vehicles and reduce the frequency of vehicle use.

In order to reduce mechanical friction, pollution-free braking method should be adopted as far as possible. At present, the best method is electric braking. Electric braking can be divided into dynamic braking and electromagnetic eddy current braking according to different braking principles.

(1) dynamic braking

Because modern urban rail transit vehicles generally use electric traction emus and DC or AC motors as traction power, dynamic braking has become the development trend of urban rail transit vehicles. There are both moving cars and trailers in the EMU. Except that the trailer has no motor and can only be braked by friction, all motor cars can be braked dynamically and can also bear the braking force of some trailers.

(2)

Electromagnetic eddy current braking

In order to give full play to the advantages of track electromagnetic braking and avoid its disadvantages, electromagnetic eddy current braking is designed.

Electromagnetic eddy current braking is a battery braking method designed by using the Lorentz force generated by electromagnetic eddy current under magnetic field and the physical principle that the direction of Lorentz force is opposite to the direction of object motion. This braking method has the advantages of no friction, no noise, small volume and large braking force. At present, the electromagnetic eddy current braking methods for rail transit vehicles mainly include disc eddy current braking and track linear eddy current braking.

1, disc eddy current braking

Disc eddy current braking is to use the disc installed on the axle to cut the magnetic field lines to produce eddy current and Lorentz force. According to the mechanism of generating magnetic field, it can be divided into electromagnetic eddy current disc brake and permanent magnet eddy current disc brake.

The high-speed EMU of Shinkansen in Japan adopts the principle of electromagnetic eddy current braking.

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Permanent magnet eddy current disc brake uses permanent magnet to generate electromagnetic field, and the brake disc generates eddy current in the magnetic field to prevent the magnetic field from increasing, produce braking torque and produce braking effect.

The structure of disc eddy current brake is similar to that of mechanical disc brake, but there is no wear between brake disc and brake pad. Train braking is also limited by wheel-rail adhesion factor.