How break discs work?
Why do trains have four wheels on one axle? What is the function of the other two?
In the video below, in addition to the two wheels that are rolling, there are two smaller wheels on the inside that are also spinning, but they are not in contact with the tracks. So what exactly is their function?
Actually, these two small wheels are not used for train movement, but for braking. It's called a brake disc system, and it's a common type of braking used on trains.
Train braking is more complex than cars, because it is fast and heavy. It is this disc braking method that is used more often nowadays. Early trains, because of their slow speeds, used break shoes to stop the train movement, the principle of which is similar to that of the automobile drum brakes. Through the cast iron material made of brake block, in the train braking, tightly holding the wheel, through friction to increase resistance to make the wheel stop rotating. The disadvantages of this way is very obvious.The braking process will produce a lot of heat which will result in wear and tear to the wheel, not only shortening the service life of the wheel, but also causing hidden danger on the traffic safety . So as the train speed is getting faster and faster, people invented this kind of disc brake.
The working principle of disc brakes is similar to that of automotive disc brakes, typically involving the installation of a brake disc on the axle. The entire braking system mainly consists of a hydraulic cylinder, brake calipers, and a brake disc. The brake disc is made of cast iron, with multiple transverse ribs between the upper and lower sections, primarily to enhance heat dissipation.
When braking is applied, the pressure in the hydraulic cylinder increases, pushing the piston outward and driving the calipers to clamp the brake disc, thereby generating frictional resistance. Since the brake disc is mounted on the same axle as the wheel, the clamping action of the calipers also reduces the wheel speed. This type of disc braking does not directly act on the wheel, thus avoiding wear on the wheel itself. Additionally, it provides smooth braking and can be installed in multiple sets as needed.
As faster high-speed trains are developed, disc brakes have become insufficient, so high-speed rail systems now use a hybrid approach combining electric braking and disc braking.