Brake Question

[Murphy Siding]

I thought I understood that a rail car's brakes were operated by brake air pressure, or lack of brake air pressure: The air pressure would pump up the brake lines, which would open the brake shoes on the wheels. Draining some of the air pressure would cause the brake shoes to start closing on the wheel, causing it to slow the train. I've read where a brake line *break* will cause the train to "dump the air", causing the train to go into emergency braking. So, my question is this: At every grain elevator around here, there is a little machine on the tracks to move individual cars (a trackmobile?). At several wholesale lumberyards, they have a front end loader, with some sort of *coupler* to move cars a short distance. How do they move the cars? Wouldn't the brakes be set, with no line pressure to open the brake shoes and unlock the wheels?

[tree68]

IIRC, if you dump ALL the air, the brakes release, which is why the engineer can't make repeated applications without giving the system time to recharge.

I may be wrong, but the same situation you cite exists at a hump yard...

On the topic of moving the cars - I've seen places that have a winch system to move the cars. It's evident from the large pulleys you'll see alongside the tracks.

[GN-Rick]

When freight cars are set out-either at industries or while
dwelling at yards-the brakeman or switchman will pull a
lever under the side of the cars and thereby release the
air from the main reservoir. With this emptied, there is no
air left to set the brakes. This is why cars roll freely while
they are being switched. This is also why a train preparing
to depart has to pump up air-not just to fill the train line, but
to also fill the reservoirs so that the brakes function.
Hope this helps.

[Anonymous]

Every freight car has a "bleed rod" whose operation (pull or push) will exhaust the air from the cylinders and brake reservoirs. This releases the brakes. Without any source for air, they can't be applied again without use of the hand brake.

About a year ago, TRAINS had a story on mountain railroading where the operation of air brakes was pretty well covered.

Old Timer

[Jackflash]

When you pull this small handle you wrote of , you are releasing the air
from the brake cylinder of the car, with no pressure in the cylinder the
piston will retract allowing the brakes to release on that one car, thats how
a forklift or towmoter can move a car around, now to make it stay put
you must put a hand brake on the car.

[Murphy Siding]

OK- I'm confused. Does the air pressure keep the brakes open or shut? As in- if all the air leaks out due to a leak or something, does that set the brakes, and hold the car fast, or does that release the brakes allowing the car to roll freely?

[tree68]

http://www.railroad.net/articles/railfanning/airbrakes/index.php

All sorts of details on how brakes work. I think Al Krug also has a great explanation, but I couldn't find it right off. It can be a confusing topic to get your head wrapped around - sort of a "less is more" type of thing.

Briefly - pressure in reservoir actually applies brakes, as determined by a reduction in the brake pipe pressure, via the "triple valve." When pressure goes down in the brake line, pressure goes up in the brake cylinder (with the pressure coming from the reservoir). So it's positive pressure in the cylinders pushing the shoes against the wheels.

No air in reservoir means no pressure to push shoes onto wheels.

IIRC, early air brake systems did work on the principle of more trainline pressure, more brakes. Of course that meant that in the case of a break-in-two, there were no brakes...

[jchnhtfd]

OK, Murph... railway air brakes can be confusing. Don't feel bad! Starting from the brake shoes... the brakes are applied by air pressure inside the brake cylinders. No air pressure, no application. Now. The air pressure to operate the brakes comes from either the main air reservoir or the auxiliary reservoir on the car itself. If you open a valve (we'll get to that in a minute!) between the reservoir(s) and the brake cylinders, the brakes will apply. The valve can be opened part way, to allow the brakes to apply partially.

Now applying the brakes lowers the pressure in the reservoir(s)... we'll get to that too!

The valve in question is a clever doodad. It is closed if the air pressure in the train line (a long pipe basically running the whole length of the train) is greater than the air pressure in the reservoir(s), and opens if the air pressure is less. So if you reduce the air pressure in the train line, the valve will open and the brakes on the cars will apply. There are several ways to reduce the pressure in the train line. The engineer can do it with his brake valve in the engine. The EOT device (in the old days, it was the Conductor!) can do it with a valve at the back end of the train. However, if something happens to the train and that long pipe is broken (say the hoses between two cars comes apart -- like a broken coupler or a derailment!) that will do it too. Which is one of the real benefits of the system -- something goes wrong, and the brakes apply.

Now back to the reservoir(s). If the pressure in the train line is greater than the pressure in the reservoirs, a valve allows air to flow from the train line into the reservoirs, thus restoring the pressure in them for the next brake application.

Before someone jumps on me, the above is somewhat simplified...

If you lose the air pressure in the reservoirs, then you have to apply the hand brake to keep the car from moving. Any time a car or cut is left standing, the rules say you have to apply enough handbrakes to keep the whole thing from moving, as it is assumed (usually correctly!) that in a while there will be enough leakage, even if the air brakes were initially applied, to release the brakes. It's happened...

[oltmannd]

What's really confusing (and what make air brakes such a neat invention) is that the air is used for both power and as a control signal.

The air is used to fill a reservior on each car to provide the muscle for braking.

When the engineer wants the brakes to apply, he "tells" the brake valve to let some air from the reservoir into the brake cylinder.

He "tells" the brake valve what to do by venting some of the air in the train line.

When the pressure in the train line is lower than that in the reservoir on the car, the brake valve will let air pass from the reservoir to the brake cylinder, applying the brakes.

When the pressure in the train line is greater than that in the reservior, the brake valve vents the brake cylinder, releasing the brakes, and refills the reservior.

In actual practice, the things are a bit more complicated as brakings systems have matured from when Geo. W invented them, but that is the basics.

[chad thomas]

Here is Al Krugs excellent rundown on rr air brakes:

http://www.alkrug.vcn.com/rrfacts/brakes.htm

[Murphy Siding]

Thanks to all. I've read and re-read both links provided. I now have a general understanding of railroad air brakes. I never would have thought that it was that complicated![:O]. That's a lot to know, just to get your train to stop.

Thanks

[coborn35]

Also about the switching, sometimes they will drain the pressure, (when using a trackobile) have the brakeman ride either the end, or point, (engineers side of course) and when they need to stop, they put the brakes on the trackmobile on, and then radio to the brakeman to put on the handbrake of the car. Sometimes this an be tedious and works only in short cuts.

[Anonymous]

The pressure in the line DOESN"T force the brakes off the wheels. Without any pressure in the system in any part the brakes are released. The only way the brakes are applied is when the brake cylinder is pressurized.

When the train goes in emergency the air from the emergency reservoir (half of the air tank under the car) is vented into the brake cylinder, putting on the brakes. If the seals are worn or the pipes or valve leak, the air will eventually leak out of the brake cylinder and the brakes will release by themselves. That's why railroad rules say that air brakes cannot be counted on to hold a cut of detached cars, the crew has to apply handbrakes.

There is a valve that bleeds all the air out of the system which then releases the brakes so the car will roll free when being switched.

Dave H.