[N&W] Re: Train Air Brakes

[nw-mailing-list at nwhs.org]

 > First question-If a great drop in the trainline pressure will cause an
 > emergency application of brakes, what prevents this from happening when a
 > car is purposely cut loose from the train?  EG-humping?

Carmen would walk an inbound train to inspect it before humping, and while 
they did this would pull the release lever on each car to bleed the air out 
of the brake cylinders.  The air would have gone into emergency on the 
train when the engine was cut away.  There wouldn't be any air on cars 
until made up for an outbound train, or transfer cut, or something like that.

 > Second question-Am I correct in thinking that with no air pressure in an
 > individual car's brake system there will be no brakes unless the hand brake
 > is set?  EG-a single car sitting in a yard for an extended period of time
 > that has lost all pressure.  I am comparing this to  truck air brakes which
 > will engage if air pressure is below a certain level.

See above - a car would be sitting in the yard with air pressure on it.  It 
would have been put in emergency when it arrived and the engine cut off, 
and would have been "bled" by the car inspectors.  If it needs brakes, the 
hand brake must be  used.

EdKing
___________________________________________________________________
 >First question-If a great drop in the trainline pressure will cause an 
emergency
 >application of brakes, what prevents this from happening when a car is 
purposely
 >cut loose from the train?

The brakes will go in emergency

 >EG-humping?

Prior to humping cars, the brakes are bled off.

 >Second question-Am I correct in thinking that with no air pressure in an
 >individual car's brake system there will be no brakes unless the hand brake
 >is set?

That is correct.

Jimmy Lisle
___________________________________________________________________
When a car is cut off, as on a siding or in a switching operation, the
trainman (brakeman or conductor) closes the angle cock on the train to keep
the trainline pressure intact, and leaves the angle cock open on the car
that is being cut off so that the air pressure will be reduced to zero,
thereby applying the airbrakes on that car. Each car has an angle cock on
each end of the car for this purpose.

Incidentally, when cars are being humped, or otherwise switched without air,
the air has been bleed off these cars allowing them to roll free when cut
away from the rest of the cars.

You are right on your assumption that when there is no air in a car's system
there is no operative air brake on the car, therefore it must be secured by
applying the hand brake.

Hope this has helped you a bit.
Jeff Sanders
Salem, VA
___________________________________________________________________

Dave,

Thanks for your commendation. I am just happy to know that someone read
that lengthy article on freight car air brakes.

I know nothing about air brakes on semi's, but I surmised from the many
quadruple skid marks that lead gradually off to the highway shoulder that
the brakes apply automatically if an air hose bursts or becomes uncoupled,
as train brakes do.

Before addressing your questions, a quick review might help those who have
not read the article. When the brake pipe (trainline) is charged to, say,
70 psi at a particular car, the auxillary reservoir and the emergency
reservoir on that car will be charged to 70 psi and the brakes will be
released. If the brake pipe pressure is reduced at a rate below a certain
threshold rate, the control valve on our example car will send auxillary
reservoir air to the brake cylinder up to a 50 psi maximum. If the brake
pipe pressure is reduced at a rate above that threshold rate by any of
several means, e.g., the engineer's brake valve, some other manually
operated emergency brake valve on the locomotive or caboose (remember
those), a burst air hose, or an air hose uncoupling, the car's control valve
will send air from BOTH the auxillary and emergency reserviors to the brake
cylinder to develop brake cylinder pressure that is 20% higher than the
maximum that can be developed from a service application. Also, it is
important to know that there is a manually operated angle cock at each end
of each car and locomotive that closes or opens the brake pipe at that
location.. Naturally, on a train the angle cocks on the front of the
locomotive and the rear of the last car must be closed and all others must
be open in order to charge the brake system and have it operate properly.

The usual procedure when a train arrives at a yard where it will be
classified, e.g., hump yard, is for the engineer to apply the train brakes
by reducing the brake pipe pressure to 20 psi before closing the angle cock
at the rear of the locomotive, but not on the first car, and then pulling
away after the locomotive is uncoupled from the train. Although the
remaining air in the train's brake pipe will be exhaused suddenly, the
pressure is too low to cause the brakes to go into emergency. This has two
benefits, 1. The piston travel can be greater with an emergency application
due to elasticity of the brake rigging and displacement of the wheelsets in
a truck (particularly on an empty car) and the extra travel might exceed the
Federal limits, requiring the car to be shopped for brake attention, and 2.
It saves the air in the emergency reservoir (70 psi in our example car) so
that it does not have to be recharged when the car is placed in an outbound
train. Before the train is classified the brakes are released by someone
such as a yard brakeman who walks the train pulling on the brake cylinder
release rod handle that is on each side of each car. A short pull causes
the brake cylinder quick release valve to release the brakes on that car by
exhausing just the air in the brake cylinder while keeping the air in the
auxillary reservoir at 50 psi on our example car. Having the air in this
reservoir "bottled up" helps speed up the recharge when the car is placed
into an outbound train.

If a car is to be set out along line-of-road when it is necessary to drift
the car into a siding, for example, the angle cock on each end of the car
can be closed so that the air brakes will NOT apply when the car is
uncoupled and the air hoses part. The car can then roll free under the
control of a brakeman who applies the handbrake as necessary. Rules
generally specify that the air brakes should not be depended on to hold
standing cars -- the hand brakes must be set because air brakes can leak off
over time.

Unlike your description of truck air brakes, railroad air brakes do not
have set level of brake pipe pressure below which the brakes will apply. If
the brake pipe pressure leaks off faster than the auxilliary reservoir
pressure does, railroad brakes will apply, but if there is brake cylinder le
akage, the application itself can eventually leak off, thus the need to have
the handbrakes set.

Hope this helps some.

Gordon Hamilton

----- Original Message -----
From: "N&W Mailing List" <mailing-list at nwhs.org>
To: "N&W Mailing List" <mailing-list at nwhs.org>
Sent: Sunday, October 19, 2003 9:26 PM
Subject: Train Air Brakes

 > I'd like to commend Gordon Hamilton for his excellent article on train air
 > brakes in the latest edition of "The Arrow". I was definitely a
 > "preschooler" and now feel as if I've graduated to "lower elementary". I'd
 > recommend the article to every one. I do have some questions which will
 > either show my understanding or lack there of.
 >
 > First question-If a great drop in the trainline pressure will cause an
 > emergency application of brakes, what prevents this from happening when a
 > car is purposely cut loose from the train? EG-humping?
 >
 > Second question-Am I correct in thinking that with no air pressure in an
 > individual car's brake system there will be no brakes unless the hand brake
 > is set? EG-a single car sitting in a yard for an extended period of time
 > that has lost all pressure. I am comparing this to truck air brakes which
 > will engage if air pressure is below a certain level.
 >
 > Thanks in advance and please go easy on me. I've not yet graduated to the
 > "higher level of learning" when it comes to train air brakes;)
 >
 > Dave Moorehead
 > Milford, OH
 >
 >