The Automatic Air Brake
"Automatic" meant that brakes would apply automatically on all cars in the train in case of an undesired train parting. This system is also controlled by the engineer with a valve handle in the cab.
A compressor on the locomotive fills the main reservoir with high pressure air (125-140 psi).
The brake pipe connects the locomotive with the rest of the train, with each car in the consist equipped with an auxiliary reservoir which is charged by the brake pipe.
The train brakes are released in this condition where the brake pipe pressure is high. This pressure is typically 70 to 90 psi for freight trains (depending on terrain) and 110 psi for passenger trains.
The train brakes are applied when the engineer moves the valve handle to the service position, which causes a reduction in brake pipe pressure. This reduction is detected by a valve on each car which causes air in the aux. reservoir to be directed to the brake cylinder on that car. This causes the piston on that cylinder to extend and push against the brake rigging which then pushes the brake shoes against each wheel on that car. Result: brake horsepower!
When an emergency occurs so that a very quick stop is necessary the engineer slams the handle all the way to the right to the big hole position. This causes a very rapid decrease in brake pipe pressure on each and every car in the consist. The vessel which contains the aux. reservoir on each car also contains a (slightly larger) section for air to be used only in these emergency situations. This air is suddenly dumped into the brake cylinder, resulting in a much higher brake shoe pressure. This stops the train relatively quickly, sometimes causing flat spots on the wheels from the sliding of locked wheels. An interesting side note here is that the "regular" aux. air is not dumped during a "big hole" application.
For those interested in a more thorough treatment of this subject please see this link.
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