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'Simple' air brakes are applied when pressure is applied to the brake line. Pressure in the line equals brakes going on. Similar for vacuum systems - vacuum present equals brakes on. This is a straight foward system and has been used for many years, but it is not fail-safe. In the event of a car in the train detaching from the train, that car will roll away and not have any brakes, and braking on the remainder of the train is often impaired. Full size railways use an automatic braking system where air pressure in the brake line keeps the brakes OFF, and in the event of a loss of pressure in the line, the brakes are applied. This is a fail-safe syetem, and often termed 'Westinghouse' brakes after the original manufacturer.
There are various methods of achieving automatic brake operation. The easiest way is to use spring applied brakes where springs apply the brakes fully ON as the default position, and use air pressure (or vacuum) to hold the brakes OFF. A break in the line means a drop in air pressure (or loss of vacuum) and the springs apply the brakes. While it is a low complexity system, it is very effective and works well.
Advantages: relatively simple to implement, can work easily with either air or vacuum.
Disadvantages: braking effort is pre-determined by the springs, must have some sort of brake release mechanism to permit shunting etc.
To achieve authentic automatic operation requires a relatively complex arrangment of plumbing, and a device called a triple valve to detect loss of pressure and permit the desired mode of automatic operation. This triple valve is a complex design and difficult to replicate in a fully functional manner in a scale model. However, a less complex automatic air brake system which more closely emulates the prototype operation can be made easily in the home workshop, and provide automatic braking. The diagram below best describes how it works, and apply the brakes with a drop in pressure in the brake line.
Take a few moments to study this diagram to understand operation.
This sytem requires the brake actuating cylinders to have double sided pistons and an air connection to each side. An air reservoir on each car is also required. This reservoir should be as large as possible, and a drain valve is provided on the reservoir to remove any condensation, or any remaining air for storage. A small 2-way valve to release the brakes for shunting is also required.
Note that the piston has only a single ended piston rod. This is intentional, and the effective piston area on the rod side of the piston will be a slightly lesser amount than the 'open' side area. This causes a small difference in the force on either side of the piston, and the piston tends to creep towards the OFF position when the air pressure is equal on either side (i.e. when brakes are not applied). [If it were the other way, the piston to would tend to creep ON and gradually apply the brakes. [Ref: Model Engineer 3981, 18 Nov 94 p632]]
The driver's control valve is a commercial plunger type air pressure regulator, modified to operate with a cam rather than the standard screw, and provides fully proportional control of the braking. The stop valves on the car ends can either be a separate valve, or use a self-sealing quick connect air connector.
[Note: A suitable 3-way valve for the release valve is used by the power boating community to switch from one fuel tank to a standby tank for outboard motors. They are readily available with 1/4" pipe fittings from most ship chandlers/marine supplers.]
[Updated: 15 Jan 2014]
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