How do air brakes on a train work?

Question

How do air brakes on a train work?

Answer 1

Trains actually have several types of brakes. Hydraulic, dynamic, & air. The air brakes work like the brakes on big diesel trucks. It's an air assisted brake pod. The air is stored in tanks & acts as boost when you apply the lever. You can create several hundred pounds of air pressure by opening the valve. Duplicating that kind of force manually could prove very diffacult.

Answer 2

The airbrakes on a train work off resevioirs that are charged up by a compressor on the locomotive/locomotives - Air pressure in the reseviors on the RR cars is distributed by the brake pipe through the universal valve.  As long as the system is charged the brakes are applied when the cylinders are actuated and through the bake rigging push the brake shoes against the wheels.

Figure it takes about 3 hours of more for the pressure to bleed off and the brakes to release unless the hand brakes are tied down.

Never expect air brakes to hold a train.

When the engineer does a reduction on either the engine brake or the train brake, the loss of air pressure sets the brakes on the train.  Think of it as a kind of signaling device.

on Freights in the US you want a 90 lb brake pipe pressure and on passengers you want 110 pounds.

A good set would show 20 pounds on the train.

Answer 3

The air is compressed into a cyclinder which pushes against a spring.  If the air is removed or a line gets cut, a big engages the brakes.  That is why the cars can sit on the track and not roll away.

Answer 4

There are two types of air brakes on a train: independent (locomotive) brake, and automatic (train) brake.

Independent brakes work with straight compressed air.  A reservoir of compressed air on each locomotive is kept, and on the engineer's command, the compressed air is sent to the brake cylinder, which pushes a piston out, pushing brake shoes against the wheels, and stopping them.

Automatic brakes have a little more involved, but are a simple concept in principle.  The locomotives and cars will all be hooked up to the "train line", which is a line of compressed air running the length of the train.

The train line is usually around 90 psi.  Compressed air from this line charges two air reservoirs in every car: the service, and the emergency.  When the engineer wishes to apply the brakes, he uses a handle on his/her control stand to reduce the pressure in the train line.

When the pressure is reduced in the train line, valves in each car begin to shift, and allow air from the service reservoir into the brake cylinder.  The compressed air now flowing into the brake cylinder pushes out a piston, which, through a system of rigging, pushes a brake pad against the wheels.

When the engineer releases the brakes, he allows the pressure to return to the normal amount (90 psi).  The same valves that applied the brakes now move again, allowing air to release from the cylinder, the piston comes back in, and the pads release from the wheels.  The rise in compressed air also recharges the service reservoir, so it can be used again.

When the air brake system detects a sudden loss of air pressure, it will deploy the "emergency" braking.  The system will take all of the compressed air from both the service and emergency reservoirs, and send it directly to the brake cylinder.  This will provide a sudden and hard pushing of the brake pads on the wheels, and is the fastest (but often dangerous) way to stop the train.

Once the emergency brakes have been deployed, everything must be recharged - the train line must be returned to 90 psi, and the service and emergency reservoirs need their compressed air recharged.  If the emergency is applied, it can take some time to recover the air brakes, depending on how big a train you have.

Hope this helps.

Answer 5

The air brakes in their present form evolved over a period of time.

As late as the late 1880s, cars did not have air brakes, nor did locomotives.  The only way to slow or stop a train was by means of operation of a hand brake, by the brakeman.  The engineer would blow the whistle to signal the brakemen to run on the roof top from car to car, tightening "staff" brakes as they went, often aided by a "brake club", sort of an ax handle, always made from Hickory.

Of course this was extremely dangerous, inconsistent and above all a slow process to control speed or stop.  It meant that, due to lack of braking ability, if the engineer could see you, it was already too late to stop for you.  It is the same way today but for different reasons.  Stay off the right of way and stay alive.

The next development was a simple device on the engine, where the engineer would open a valve to let steam into a piston that applied a cast iron brake shoe to the drive wheels.  This was the "steam jam brake",  And everyone has used the phrase or heard it said, "I had to 'jam' on the brakes to keep from hitting the moron!"

The engine brake, aka "independent" or "jam" (a term still in use today) is essentially the same concept.  The engineer puts compressed air into the brake cylinder that puts a composition shoe against the wheel.  This brake can be operated independently from the train brakes, which is both useful and necessary, hence the term, "independent brake."

Then along came George Westinghouse, and everything changed.  There were many train collisions in his time with appalling loss of life in many instances.  He developed the system still used today, from the Westinghouse Air Brake Company, "WABCO".

The answer above by Steven B about the train's air brakes working similarly to a big rig is absoluely right.  I am not familiar with an hydraulic brake, but dynamic brake is another question, requiring a different answer.  In these types of air brake systems, however, the brakes are applied by letting air OUT of the system.

The basic parts are the brake pipe, brake valve, air reservoirs, brake cylinder, brake rigging and retainer, in that order.  The system is charged with air, 90 psi for US freight trains.  To apply the brakes, the engineer lets air out of the brake pipe, which causes the brake valve on each car to admit air into the brake cylinder from the air reservoir.

It takes TIME to evacuate the air from the brake pipe to apply the brakes.  The air flows through the engineer's "automatic brake valve" to atmosphere.  This is the "service rate" of application, where air flows (propagates) at (rounded off) 500 feet per second.  If I am the engineer of a train that is 8,000 feet long, of reasonable tonnage, traveling at 60 MPH and I see you on the track a mile in the distance, you've got a big problem.

Our 8,000 foot train takes a full 16 seconds before the brake application on the rear car begins.  The train has travelled more than a quarter mile, and still doing 60 MPH.  Now more air is needed, still only accessable at 16 seconds a whack.  Then, "Splut!"  You're fertilizer.  It's not 1885 anymore, but you're still just as dead.  

Make everybody's day and stay off the right of way.  When crossing the tracks at any time, look both ways before crossing.  This is even more important if you cross the same crossing two or three times a day as part of a normal routine.  You will become complacent, then you will become dead.

Thanks for asking.  I hope everyone learned something.