Train vacuum brakes work by creating a vacuum within a brake pipe system. When the vacuum is maintained, the brakes are off. Reducing the vacuum (by letting air in) applies the brakes.
Here's a more detailed breakdown:
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Creating the Vacuum: An ejector (usually steam-powered) or a vacuum pump, located on the locomotive, draws air out of the train pipe that runs the length of the train. This pipe is connected to brake cylinders on each carriage or wagon.
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Releasing the Brakes: With air removed from the train pipe, a vacuum is created. This vacuum is also extended to a reservoir (often called a "vacuum reservoir") on each carriage via a non-return valve. The vacuum in the train pipe allows atmospheric pressure to act on a piston in the brake cylinder, releasing the brakes. The brakes are kept off by being weight-biased or lightly sprung; the vacuum counteracts that bias.
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Applying the Brakes: To apply the brakes, the driver reduces the vacuum in the train pipe. This can be done by admitting air into the system from the locomotive.
- How it works: The higher pressure in the vacuum reservoir (because it is still close to full vacuum) in the auxiliary reservoir is now significantly different from the pressure in the brake pipe. Atmospheric pressure in the brake pipe pushes against a diaphragm or piston connected to the brake rigging. This applies the brakes. The degree of braking force is proportional to the amount of vacuum released in the train pipe; a full release of the vacuum gives maximum braking force.
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Brake Failure (Fail-Safe): A crucial feature of vacuum brakes is their fail-safe nature. If the train pipe is broken or becomes disconnected, air rushes into the system, automatically applying the brakes. This inherent safety mechanism is a key advantage of vacuum brake systems.
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Simple Diagrammatic Representation: The following illustrates the basic principles:
- Vacuum Created: Locomotive -> Ejector/Vacuum Pump -> Train Pipe (Vacuum) -> Brake Cylinder (Brakes Off)
- Brakes Applied: Locomotive -> Admission of Air into Train Pipe (Reduced Vacuum) -> Brake Cylinder (Brakes On due to atmospheric pressure)
In summary, train vacuum brakes rely on maintaining a vacuum for brake release and reducing it for brake application, providing a reliable and inherently safe braking system.
