From: bobmacd@well.sf.ca.us (Robert MacDowell)
Subject: Re: More air brake questions
Organization: Whole Earth 'Lectronic Link
Date: Tue, 24 Nov 1992 04:06:37 GMT



burke@doright.Tymnet.COM (John Burke) sez in <2853@tymix.Tymnet.COM>:
>I have some more questions about air brakes.
>
>* What happens if the engineer keeps brakes applied for a long time,
>for example, on long downhills?  And if he releases partially and
>then re-applies, and does this several times?

Re: Freight.
With modern equipment, it's just fine to hold them applied for a long
time on downhills.  Modern pressure- maintaining brakes, like the 26,
will hold the air at that pressure.

The old manual-lap brakes, like the 6, would not compensate for brake
pipe leakage, and so they would set up slowly as air leaked out of the
pipe.  The knee-jerk solution was to turn down the feed valves to
whatever pressure you wanted.  The feed valve would pressure-maintain.

There's no such thing as releasing partially.  If you release *at
all*, each car's brake valve goes into *full* release, and the newer
valves (ABD and above) will propogate the release to other valves.
(ask Montana Rail Link what happens when your people don't understand
this concept...)

Once released, you must apply again, except that the brake pipe and
car reservoirs are partially depleted so you must apply harder to get
the same amount of braking.  Do it too many times, and all that's left
is emergency...  you have to use it.  Then you call the dispatcher to
tell him you're in emergency and to stop traffic on the adjacent
mainline in case any of your cars derailed and fouled it, then you
walk the train to see that they didn't, including the ones on the high
trestle with no walkways.  Then you sit for a half hour and hold the
train with engine brakes or retainers while you pump your air back up
to where it's safe to descend the hill again.  Meanwhile the
trainmaster shows up.

So to your question of
>* What happens if the engineer ... releases partially and then
>re-applies, and does this several times?
the answer is, he gets to meet the trainmaster.  :-)

>* Details of operating a 26L brake: Now, what I've seen the NYC subway
>motorman do normally is: throw the handle rather far right, then move it
>back immediately some distance left; but after that, they use smaller
>changes in position of the handle to re-adjust braking.  Application
>appears to always involve a right-then-left motion, but for less
>distance to get a lighter increase; while release appears to be done with
>just small movements farther left.  Does this sound like the same
>operation as on mainline locomotives with a 26L brake?

The NYCTA probably does that because they think it speeds up
application.  Whether it actually does or not depends on the valve I
suppose.  I would think Amtrak would dial up the brake they want and
leave it there.

In freight service, *any* release is a full release, so doing this
is an extremely bad idea.

>* Details of the old 24L brake control:
>        Release, Running, First Service, Lap, Service, Emergency
>
>Question is: do the Running and Service positions have a range so
>that, for example, if you move the handle farther right in Service
>the pressure drops faster?

They don't *intentionally* have a range. But they were just one piece
of brass rotating on another, and lining up holes. So I suppose if you
half-aligned a hole it would apply/release slower.
But there'd be no benefit to doing that.  Application *rates* are very
important, and in any case are faster than the cars' brakes will
respond.  For instance, a slow pipe reduction is service application.
A quick pipe reduction, even for milliseconds, is an emergency app and
once one valve sees it, it will "dynamite" and set off adjacent
valves, which will propogate the action through the train.  Likewise
release.  Any pressure increase is a signal for full release and ABD
or newer valves will propogate that through the train.

>* What would typical trainline pressure reductions be for 1) a very
>minimal application,

I'm not sure, but...  I think as little as 1.5 pounds reduction will
put most newer freight car valves into "initial quick service", which
will be like a 5-pound reduction.  The newer valves will also
propogate it by drawing a little air from the brake pipe.

> and 2) maximum in normal service?

The answer to your question is a reduction of 2/7 of the running
pressure.  For instance if your running pressure is 70 pounds, making
a 20-pound reduction (to 50 pounds) gives you all you're going to get.
Drawing down further than 2/7 does nothing beneficial.
BTW, *any* sudden drop in pressure will kick off emergency, regardless
of pipe pressure.

>* Gauges: which readings can the engineer/motorman see on air gauges
>in typical control setups?

Mainline engineers can see (on two duplex gauges):

 - Main reservoir
 - Brake pipe
 - Equalizing reservoir (either you know what it is or you don't :-)
 - Cylinders (on this locomotive unit)

Most transit installations seem to have "Brake Pipe" and "Cylinder" on
one gauge. Sometimes they have a separate gauge for "Main Reservoir".

>* Other related info which anybody has for me?

There's an excellent trilogy of books called "Chalk Talks" by C.M.
Drennan. Obviously, being almost 40 years old, they don't mention the
new ABDX-R brake that Westinghouse just announced :-) but they cover
the basics well, in an understandable way.

 - ABC's of Air Brakes, 1954.
 - Locomotive Air Brakes, 1955.
 - Car Air Brakes, 1955.

They're published by Simmons-Boardman Books, The Railway Educational
Bureau, 1809 Capitol Ave, Omaha, NE, 68102.  About $20 each. I got
mine from Milepost One in Old Sacramento, CA. for $40 the three, but
that may have been a TRAIN'88 convention price.

--Bob MacDowell
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