The Automatic And Independant Air Brake
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Shown on the left is the standard "Westinghouse" Air Brake System. This is a fully fail safe system which provides passenger and freight safety the world over. The following article does not provide all of the system details as we could write a book on this if we felt so inclined. The purpose here is to provide an insight into the basic control and operation of the system and the reasons behind its design. |
This is of course not the only brake system in use around the world, but it is the most common. The other systems include the basic air brake sometimes used on tram cars, the vacuum brake (also quite a comon system ) and several other minor brake systems.
However as the automatic air brake is the most comon, that is the one we will look at here. We are interested in the brake from the point of view of the train driver, so lets begin with the drivers controls and how they work.
On the right is the standard brake control which will be familiar to almost every train driver, steam or diesel, anywhere in the world. This particular example is a Westinghouse L-8-PA Brake Valve from one of the American railroads.
You will notice that it has two main control handles. 1) Automatic Brake System, 2) Independant Brake System.
The automatic brake controls the air in the train pipe. That is the pipe which runs the entire length of the train connected between vehicles by flexible hoses.
The independant brake controls only the locomotive brakes and is as the name implies independant of the train brakes.
It is important to understand at this stage why there are separate systems. Let me give you an example to illustrate. Lets say we are standing in a station and all the coaches are bunched together at the couplings. If we just pulled away the couplings would receive a hard snap as each one in turn down the length of the train took up the slack .
Not only would this stress the couplings but the passengers would recieve a very hard jolt as each coach pulled away. It is therefore quite normal to release the locomotive (independant) brake and then release the coach brakes gradually as the slack is taken up. This ensures that none of the couplings become overstressed and the passengers dont end up with their lunch in their laps. We can also use this system of braking to prevent coaches from bunching up on down hill gradients, when a small amount of braking force applied to the coaching stock will certainly help.
There are very many situations and conditions where brakes are used and it would be well beyond this article to list them all but as you can now see brakes have other uses than just stopping trains.
The Brake Controls
The brake controls have very specific method of operation In this article it is only possible to outline these and give a rough description of their function as follows:
The brake control positions are listed in the following table and should give a good indication of how the brake operates.
Release:
This position releases all brakes and charges the air reservoirs as quickly as the compressor can supply the air.
Running:
This position also releases the brakes and charges the reservoirs but at a slower rate. The reservoirs are charged to a pre set pressure depending on the train weight.
First Service:
This Position is used to apply brake increase action gradually
Lap:
In this position the brake pipe pressure is maintained . keeping the braking force at whatever level it has been set at by service or first service.
Service:
Applies brake force more rapidly than first service.
Emergency:
Used only in dire emergency situations this will apply full brake force and usually lock the wheels.
Links To Great Railway Web Sites ( including the one already mentioned where you have the chance to drive trains)
Basic Stuff Introduction Plus How To Stop A Train
How Diesel Railway Engines Work (This Page Still Under Construction)
