What’s The principle Of Railway Train Braking?

When we drive a car and step on the brake shoes, the car slows down immediately. Even at a speed of 100 kilometers per hour, it only takes 30 to 40 meters to come to a complete stop during emergency braking. However, for trains, braking is far from being that simple. In China, when the load is 5,000 tons, the emergency braking distance for a train is 800 meters. Nowadays, with the increase in train weight and speed, the braking distance can extend to 1 to 2 kilometers. This article will discuss why the braking distance of trains is so long and the principle of train braking.

One of the important reasons for the high efficiency of railways is that trains run on steel tracks with steel wheels. Because steel is hard, the deformation when they come into contact is small; and because steel is strong, the contact area between the train wheels and the rails can be designed to be very small, about the size of a penny. These factors make the rolling resistance of the wheels smaller, thus achieving higher efficiency. However, while bringing these advantages, the steel wheel-rail system also brings a series of disadvantages, one of which is that it can only provide a relatively small braking force. This is the reason why the braking distance of trains is so long.

With the development of technology, the principle of train braking has become increasingly complex. However, on the simplest freight cars, a relatively simple and reliable system is still used. Let’s start with these simplest systems. The types of braking for railway locomotives and vehicles include tread braking, disc braking, and dynamic braking. On structurally simple freight cars, the most widely used is tread braking. When we drive a car and step on the brake pedal, the brake pump pressurizes the brake fluid, and the hydraulic pressure pushes the piston in the caliper to clamp the brake disc, thus achieving braking. The earliest brake system on trains was similar, except that it used air pressure instead of hydraulic pressure. A pressurized pipe (train line/brake line) connects the locomotive and all the cars. When the brake is applied, the air pressure increases, pushing the brake shoes to press against the wheel treads, thus achieving braking.

However, this braking technology has a fatal flaw: if the train is interrupted for some reason or there is a leak in the train line, the entire train will lose its braking force, which does not conform to the principle of fail-safe. To solve this problem, the automatic brake was invented. Unlike the forced braking in cars, the train brake uses pressure reduction braking, that is, a decrease in the pressure of the train line/brake line will cause the train to apply braking. How is this achieved?

On a train, there is a large “air tank” on the locomotive called the main reservoir, which is connected to the train line/brake line. The train line is connected to the braking system of each car. The braking system of each car consists of a three-way valve, an auxiliary reservoir, a brake cylinder, and a brake shoe, among other components. The train braking system has three positions in simple terms: brake position, release position, and lap position.

When the brake handle is in the release position, the train braking mechanism is in the position shown in the figure below. At this time, the air pressure in the train line is greater than or equal to the air pressure in the auxiliary reservoir. The three-way valve piston is on the far left, the brake cylinder is connected to the atmosphere, and the auxiliary reservoir is approximately at the same pressure as the train line.

When the brake handle is in the braking position, the main air reservoir is disconnected from the train pipe, and the pressure in the train pipe drops. This causes the pressure in the auxiliary air reservoir to be greater than that in the train pipe, pushing the piston of the three-way valve to the right. As a result, the auxiliary air reservoir is connected to the brake cylinder, and the brake cylinder is disconnected from the atmosphere until the pressures in both are equal. At this point, the pressure in the brake cylinder pushes the piston to move, causing the brake shoes to press against the wheel treads, as shown in the figure below.

However, this braking system is not without its drawbacks. Firstly, the train pipe serves both to transmit braking signals and to replenish air pressure. To transmit braking signals, the train pipe needs to be depressurized, and after the braking is relieved, the original air pressure needs to be restored. For normal braking, it takes 1 to 5 minutes to restore the air pressure; after an emergency brake, it can take up to 15 minutes to restore the air pressure. This means that the train cannot brake repeatedly in a short period, as it will not be able to apply the next brake before sufficient air pressure is restored. Therefore, braking a train is a technical task, unlike braking a car.

Furthermore, since the speed of air pressure transmission cannot exceed the speed of sound, on a heavy freight train that can be up to two kilometers long, the vehicles far from the locomotive receive the braking signal later. This time difference can have an impact on the train. Therefore, on heavy freight trains, electro-pneumatic combined braking is widely used. The train pipe only serves to replenish air pressure and does not transmit signals. The braking signal is transmitted by cables, thereby eliminating the time difference in braking between different vehicles.

Also, for the entire train to have braking force, the train pipe must be continuous without any interruption. On each vehicle, there are two valves: the angle cock and the brake cock. If the brake cock is closed, the braking of a single vehicle can be shut off, and such a vehicle is called a “closed car”. However, if the angle cock is closed, all the vehicles behind it will lose braking force, creating a safety hazard.

Moreover, from the description of the braking system principle above, it can be seen that if the braking force of the train is insufficient, the pressure reduction in the train pipe can be increased, thereby further reducing the pressure in the auxiliary air reservoir and further increasing the pressure in the brake cylinder. However, once the train is relieved, the brake cylinder is connected to the atmosphere, and the pressure in the brake cylinder will drop to atmospheric pressure in an extremely short time. That is to say, the train can only brake in stages but cannot be relieved in stages.

For freight trains, the weight difference between empty and fully loaded can be several times. If excessive braking force is applied to an empty car, it will cause the wheels to slide, damaging the vehicle and the track. However, if insufficient braking force is applied to a fully loaded car, it will cause the train to fail to stop in time. To address this issue, some freight cars are equipped with an empty/loaded device (E/L Device), which uses an additional pressure-reducing air reservoir to adjust the braking force applied to vehicles of different weights.

Supplier

Luoyang Fonyo Heavy Industries Co., Ltd, founded in 1998,is a manufacturer in railway casting parts. Our factory covers an area of 72,600㎡, with more than 300 employees, 32 technicians, including 5 senior engineers, 11 assistant engineers, and 16 technicians. Our production capacity is 30,000 tons per year. Currently, we mainly produce casting, machining, and assembly for locomotive, railcar, high-speed trains, mining equipment, wind power, etc.
We are the railway parts supply to CRRC(including more than 20 branch companies and subsidiaries of CRRC),Gemac Engineering Machinery, Sanygroup, Citic Heavy Industries, etc. Our products have been exported to Russia, the United States, Germany, Argentina, Japan, France, South Africa, Italy and other countries all over the world.
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