Vacuum brakes are applied by creating a vacuum. It’s controlled by the driver with braking equipment in the drivers’ cab through a brake valve. The brake valve is connected to a brake pipe which run throughout the whole train.
There is a special equipment called Exhauster. Its function is removing atmospheric pressure from the air in the brake pipe to create a vacuum. When the brake pipe is at its full vacuum, brakes are fully released & when the brake pipe pressure is equal to the atmospheric pressure, brakes are fully applied. Now let’s take part by part & examine its function.
Connected to the brake valve of the driver. The exhauster should turn on at high speed to release brakes & should maintain at slow speed to maintain the vacuum.
This is the part which touches & presses the wheels when applying brakes.
This is included to provide a source of vacuum always to apply or release brakes because the vacuum brake system is operated on the pressure difference between two sides of the piston in brake cylinder.
This is the most important part in the vacuum brake system. It carries vacuum & transmits pressure variations which is needed to control brakes. Flexible hoses are used to couple this pipe between wagons & there is a dummy coupler on each wagon to fix this hose when not connected.
Duty of this part is maintaining the vacuum in the vacuum reservoir at required level. When applying brake the connection of reservoir to the brake pipe closes & it opens when releasing the brakes. This function is carried out by the ball valve.
In this system of braking when the driver applies the brakes, a valve opens which is situated between brake cylinder & intake manifold. Therefore existed vacuum vanishes & atmospheric pressure act on the brake piston. This force is powerful than the force from the other side. Therefore the piston moves forward with the rod attached to it.
When driver releases the brakes, brake valve closes. Then the valve between brake cylinder & intake manifold opens. The rod & the piston doesn’t move anywhere because there is no pressure difference. Then the brake shoes & rod return to their original state with the help of the spring connected to them. Now let’s see what happens inside the brake cylinder.
When brakes are released
This diagram shows the internal parts of the brake cylinder when brakes are fully released. That means the brake cylinder, vacuum reservoir & ball valve are in the release position. Note the location of the piston. It’s in the bottom of the cylinder & the brake cylinder is directly connected with the vacuum reservoir because the cylinder is open from the top.
There is a vacuum in the brake pipe, underneath the piston & vacuum reservoir. This vacuum allows the ball valve to expose a connection between brake pipe & vacuum reservoir. Therefore the piston moves to the bottom & brake shoes release from the wheels.
When brakes are applied
When brakes are applied the vacuum inside the brake pipe get reduced by taking atmospheric air inside. When air pressure increase the ball valve moves up & it closes the connection between brake pipe & vacuum reservoir. Therefore the vacuum in the vacuum reservoir remains unchanged.
Now there is a pressure difference between the sides of the piston due to the entering of atmospheric air into the pipe. A force act upwards on the piston & the piston moves up. Therefore brake shoes touch with the wheels & it allows the train to slow down.
The advantage of vacuum brake system are the simplicity in design, ability to release partially (Air Brakes can’t do this without the help of additional equipment). There are some disadvantages too in this system. Larger cylinders & pistons are needed to apply the necessary force on wheels. Existing vacuum in the brake pipe could cause particles to be sucked in. This system is less efficient than the air brake system. It takes long time than air brakes to apply & release brakes normally. If you want to speed up this function you’ll have to use additional equipment.