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Mechanical Braking Systems for Electric Trains: Requirement and Types of Mechanical Brakes
For stopping the electric train and holding it stationary, in addition to the electric brakes, the mechanical brakes are also required. The mechanical braking is required because the electric braking is unable to hold the train at standstill.
In modern electric locomotives, the regenerative braking is employed down to a speed of 16 kmph, then rheostatic braking down to about 6.5 kmph and finally mechanical braking to a standstill.
The mechanical braking is usually applied by brake block shoes pressed with force against the tread of the wheels and the required braking force is caused by the movement of the piston which is transmitted to the brake block shoes through a system of levers.
Types of Mechanical Brakes
The mechanical brakes are classified into two types viz. −
- Compressed air brakes
- Vacuum brakes
The compressed air brakes are widely used in electric locomotives whereas the vacuum brakes are used in steam locomotives. The compressed air brakes are little advantageous to the vacuum brakes as compressed air can conveniently be stored up and released for quick action while in case of vacuum brakes the necessary vacuum is to be created by the pump.
Compressed Air Brakes
A typical schematic of the compressed air brake is shown in Figure-1. The compressed air brake consists of a reservoir of compressed air, a brake cylinder, compressed air pipe, a valve, springs, piston and a piston rod.
The piston is connected to the brake shoe through piston rod and levers. The springs are provided to keep the brakes in the off position. While applying the brakes, the compressed air is allowed to enter into the brake cylinder through the compressed air pipe and valve, which presses the piston against the force of springs. The force with which the brakes are applied depends upon the quantity of the compressed air allowed to enter into the brake cylinder.
The flow of the compressed air into the brake cylinder is controlled by a valve placed at the inlet of the brake cylinder. The brakes are released by exhausting the air from the cylinder. When the brakes are released, the piston along with the brakes return to its original position under the influence of spring force on exhausting the air.
In the compressed air brakes, the compressed air used is at about 5.5 kg/cm2 pressure. The compressed air brakes are widely used in the electric locomotives.
The construction of the vacuum brakes is shown in Figure-2. The vacuum brake consists of a vertical cylinder with a piston inside and the piston is provided with a rolling rubber ring. The piston is connected to the piston rod which operates the braking arrangement through a system of levers.
In the vacuum brakes, the vacuum is always maintained on the top and the under-side of the piston so that in normal conditions the piston rests at the bottom of the cylinder. While applying brakes, the air at atmospheric pressure is gradually admitted beneath the piston through the air pipe and value, the piston moves and applies the brakes.
In case of vacuum brakes, the force with which the brakes are applied depends upon the rapidity with which the vacuum is destroyed. The brakes are released by recreating the vacuum. The vacuum is recreated by drawing air from lower side of the piston through the air pipe. The piston falls to bottom of the cylinder on account of its own weight. The vacuum brakes are extensively used in steam locomotives.
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