Before starting today's topic, let's introduce the brake shoe. Brake pads are called brake shoes in railway terms. The brake shoes are located on the tread of the wheel. When braking, the two brake shoes in the front and rear of the wheel clamp the wheel to achieve the purpose of parking. The train needs to be braked in the process of running, and the contact with the wheel tread produces friction, which converts the kinetic energy of the train into heat energy and dissipates it into the atmosphere to achieve the part that decelerates or stops the train. The braking part that directly rubs the wheels to stop the train is the brake shoe. Shoe-shaped brake pads made of cast iron or other materials hold the wheel tread tightly when braking, and stop the wheel from turning through friction. Brake shoes can be divided into cast iron brake shoes and synthetic brake shoes according to their materials.
Synthetic brake shoe. Also known as non-metal brake shoe, it is made of asbestos and other fillers mixed with resin or rubber as a binder and then hot pressed. Synthetic brake shoes are also reinforced with steel backing. If the brake shoe is pressed into a piece for disc braking, it is called a brake pad. Synthetic brake shoes were first used on London Underground vehicles in 1907. Since the 1950s, applications have become increasingly common. Synthetic brake shoes are lightweight, wear-resistant and virtually spark-free when braking. The friction coefficient between it and the steel wheel changes little with the increase of speed, and is basically the same as the change of the braking adhesion coefficient between the wheel and rail, so that the adhesion can be better used to improve the braking performance and shorten the parking braking distance. Synthetic brake shoes are divided into high friction coefficient and low friction coefficient. The friction coefficient of the high-friction-coefficient synthetic brake shoe is about twice that of the cast-iron brake shoe. Smaller diameter brake cylinders and auxiliary air cylinders can be used, thereby reducing the weight of the basic brake device and saving compressed air. It has many advantages. . The low friction coefficient synthetic brake shoe can directly replace the cast iron brake shoe, which is suitable for retrofitting old cars. The disadvantage of synthetic brake shoes is that the thermal conductivity is poor, and the heat generated by friction increases the temperature of the wheel tread, and even causes local high temperature on the tread to cause thermal cracking. In recent years, in order to avoid pollution to the environment, synthetic brake shoes without asbestos, lead and other harmful substances have been used more and more. So how exactly does it work?
Braking principle of train brake shoe: In the process of train braking, the braking device converts huge kinetic energy into heat energy and dissipates it in the atmosphere. The quality of this braking effect mainly depends on the ability to dissipate frictional heat energy. When using this braking method, the friction area of the brake shoe is small, and most of the thermal load is borne by the wheel. The higher the train speed, the greater the thermal load on the wheels when braking. If cast iron brake shoes are used, the temperature can melt the brake shoes; even if more advanced synthetic brake shoes are used, the temperature will be as high as 400~450℃. When the temperature of the wheel tread increases to a certain extent, the tread will be worn, cracked or peeled off, which not only affects the service life but also affects the driving safety. It can be seen that the traditional tread pad brake cannot meet the needs of high-speed trains.