train brakes - helpful info for train hoppers
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(47 votes)
Published: Jun 18, 2000 12:00 a.m.
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helpful guide to train brakes for fellow train hoppers
Early Train Brakes==================
The first train brakes were very simple. To stop the train, you blew a
certain pattern with the engine whistle and brakemen would jump from
car to car setting handbrakes. Brakemen, needless to say, had trouble
getting life insurance... :-)
The next generation of brakes added a compressor to the locomotive,
and a brake pipe running the length of the train, connected between
cars with gladhands, which were symmetrical ’non-gendered’ connectors
that were latched together by hand and would separate by themselves
if pulled on. The brake pipe was connected to an air cylinder on each
car, which pulled on the handbrake chain when the brake pipe waspressurized.
In other words, charge the brake pipe with air, and the brakes went
on. This worked MUCH better than brakemen, but it still took a long
time to pump all that air back to the cars. And, all it took was a
parted hose or other failure anywhere in the brake system to cause the
system to fail entirely.Westinghouse’s Invention========================
A guy named George Westinghouse didn’t like the direct air brake
either, and he invented a thing called a triple valve. This valve,
and a reservoir on the car, inverted the behavior of the direct air
brake: charging air into the pipe charged the system and released
the brakes, and draining air from the brake pipe applied the brakes.
He now had a much more responsive system that was fail-safe. It
worked, and became the basis of the modern air brake.
The triple valve attached directly to the brake pipe, then had a
connection to the reservoir, and to the brake cylinder. It was called
’triple valve’ not of the three connections, but of its three modes:
1. Charging. At rest, the Westinghouse brake system has no air in
it. So the air brakes in the train must first be charged. As air is
pumped down the brake pipe by the locomotive, the triple valve directs
it into the car’s reservoir, where it is held for use in applying the
brakes later. When the system is fully charged, brake pipe and all
the reservoirs in the train will be at a pressure designated by the
railroad, for this discussion let’s say 70 pounds.
2. Applying. When the engineer wants to apply the brakes, he sets
the brake handle such that air is removed from the brake pipe. When
the triple valve sees brake pipe pressure fall, it allows reservoir
air into the brake cylinder, and the brakes apply. It’s pretty
simple; if you reduce the brake pipe pressure 5 pounds to 65 pounds,
the triple valve transfers air into the cylinder until the reservoir
drops to 65 pounds. Due to the relative volumes of the reservoir and
(properly adjusted) cylinder, this will put 12.5 pounds of air intothe cylinder.
A 10 pound reduction will give 25 pounds of cylinder application (by
reducing the reservoir from 70 to 60). The maximum braking, then, is
a 20-pound reduction, which puts 50 pounds in the cylinder, and leaves
50 pounds in the reservoir. At this point, pressure in the reservoir
and cylinder are equalized, and that’s as hard as the system will
brake. Even if the brake pipe pressure is reduced further, nothing
more will happen.
3. Releasing. Once the brakes are applied, an *increase* in pressure
told the triple-valve to release the brakes. When it saw an increase,
it would vent the cylinder to atmosphere, and start to recharge thereservoir.
(In modern brakes there are two kinds of release actions:
- Direct Release, in which *any* increase in pipe pressure kicks the
brakes off fully. Freight brakes work this way.
- Graduated Release, in which a partial increase provides a partial,
proportional release. This is used in passenger trains, though the
valve becomes much more complicated. )
Westinghouse’s triple valve improved response times, because it didn’t
need to move all the air needed to apply the brakes. It only had to
move enough air to carry a _signal_ to the triple valve, telling it to
apply or release. But still, the signal took awhile to work its way
down the brake pipe. This would be improved later...Improvements: Emergency
=======================
An Emergency feature was an early addition to Westinghouse’s
technology. This added a second reservoir, and made the control valve
more complicated, but it added the means to make a harder application
of the brakes. And, with a propogation feature called ’Quick Action’,
it made them apply very quickly too.
’Emergency’ adds a fourth mode to the brake system. A _rapid_
decrease in brake pressure signals the valve to immediately throw the
’works’ into stopping the train. Including the full contents of a
second, larger reservoir, called the ’Emergency’ reservoir. (The
original reservoir is now called the ’Auxiliary’ reservoir.
Most freight cars use a ’duplex’ reservoir, which are two cast
halves separated by a steel plate. The steel plate is shaped like a
dome inside, which makes the emergency half of the reservoir larger.
A tab sticks out of this steel plate, one side labeled ’aux’ and the
other ’emerg’ so the sides can be identified.
In normal operation, the emergency-equipped control valve operates
just like the original triple valve, except, of course, that it also
charges the emergency reservoir. But part of the valve is designed
to detect a rapid drop in pressure, which trips the emergency mode.
The valve will then dump the entire contents of both reservoirs into
the cylinder, and when pressure equalizes, there will be nearly full
system pressure in the cylinder, 63 pounds or so on a 70-pound brake
pipe pressure. This is as hard as the brakes will go, and will often
lock up the axles at low speeds, skidding flats in the wheels. The
force of an emergency application can also damage lading or even
derail the train!
An emergency stop is now the default action almost anytime there’s a
brake failure. Any rupture in the brake pipe will cause an emergency
application, as will a defective brake valve pejoratively called a
’kicker’ or ’dynamiter’ (which puts the whole train in emergency.)
The reason a defective valve could disable a whole train, is that part
of the ’Emergency’ feature is another feature called ’Quick Action’.
Since an emergency application requires a rapid drop in brake pipe
pressure, there needs to be away to make sure the drop remains rapid,
even far back in the train. The locomotive alone can’t do that; by
the time the pressure drop got 100 cars back, it would not be sorapid.
So each valve repeats and _propogates_ the effect of the emergency
brake. That’s what Quick Action does. When the valve goes into
emergency, Quick Action vents the brake pipe itself, thus _causing_ a
rapid drop, amplifying the emergency action and making sure the next
valve goes into emergency too.
That also means that if a valve is defective, and produces what is
called an ’Undesirable Emergency Application’ (UDE), the entire train
will follow suit.
When conductors rode in cabooses, they knew of four reasons for their
train unexpectedly going into emergency: 1) An air hose has parted.
2) the train has broken in two. 3) One of the cars is a ’kicker’.
4) The head end is about to hit something.
Usually, they didn’t think too much about the cause before grabbing a
handhold; the slack action from an emergency brake could tear the
stove out of the wall. But as soon as they stopped, they’d radio to
the head end ’All stopped!’ Which gives the engineer some idea of how
many pieces the train is in.The AB Freight Control Valve
============================
The Westinghouse ’AB’ control valve is, essentially, the modern brake.
It has all the brake features discussed so far. While there have been
technological upgrades (’ABD’ to ’ABDW’ to ’ABDX’) the basic packaging
is standardized, and worth talking about.
The ’AB’ control valve consists of a pipe bracket, to which all piping
connections are made, and two control valve portions (the ’Service’
and ’Emergency’ portions) which bolt to the pipe bracket with three
bolts. Each of the three pieces weighs about 65 pounds, which is
(conveniently?) just light enough to be shipped by UPS.
The beauty of the system is its ease of maintenance. The two portions
(which are quite complex inside) simply bolt off; and you don’t
rebuild them, you just ship them off to someone who does it for you
for about $120. Add ten dollars worth of gaskets and filters, and
a field diagnostic, and you’ve done 16-year brake service on a
railroad car. The hard part is cutting out the stencil which says
’C.O.T.S. 1/4/94’ (Clean, Oil, Test and Stencil)
The pipe bracket does not just unbolt; there are pipes attached to it.
The five pipes are Brake Pipe, Cylinder, Auxiliary Reservoir,
Emergency Reservoir, and Retainer. The last one deserves someexplanation.
The retainer is a way of ’keeping’ some of the brake application even
after the brakes are released. When an AB brake releases cylinder
pressure, it vents it out the ’retainer’ port. On most cars, this
leads to a retaining valve located on the side of the car. The
retaining valve retains pressure in the cylinder when the control
valve tries to release it. It can be set for ’direct’, which lets the
air out directly, or ’retain 10 pounds’ which keeps the last 10 pounds
of pressure in the cylinder.
This is used to descend long grades: with the retainers turned up, the
cars will hold ten pounds of brakes even while the brakes are fully
released and recharging. More advanced retainers added two more
settings: retain 20 pounds, and slow release, which would release
fully but took about 90 seconds to do it.
On cars that didn’t use the feature, a screen was put over the
retaining valve port to keep wasps from building nests in thecontrol valve.
More Improvements: the ABD valve================================
The ABD valve added two features which made brake response faster, but
they worked mostly like AB valves, and were, of course, compatible.
However, they were completely different internally.
The old AB valve used technology of 100 years ago inside the valve:
small pistons moved brass slide valves, aligning or blocking ports to
make the valve function. They had to be lubricated with graphite, and
there were always problems with scoring and leakage. With the ABD
valve, rubber diaphragms (like in a car’s fuel pump) replaced the
pistons, and sliding shafts with rubber gaskets replaced the brass
slide valves. Although they did basically the same thing.
Two functional features were added to AB and newer valves: Quick
Service and Quick Release. Both worked like Quick Action,
manipulating brake pipe pressure to propogate the brake commands morequickly.
Quick Service propogates the effect of a service application. When it
sees an pressure drop of 1-1/2 pounds or so, it vents some more brake
pipe pressure to atmosphere, assuming a 5-pound application. This
means that the next valve sets brake more quickly, and so on.
Quick Release does the same thing, for releases. When it sees an
increase in pipe pressure, it adds some air from the emergency
reservoir into the brake pipe. This increases the pressure some more,
and all in all makes the train release much more quickly.
But... remember the trouble with ’Kickers’ and the Quick Action
feature, where one car could put an entire train into emergency?
The same thing can happen here, but now one car can *release* the
brakes on an entire train. This caused major problems until it was
learned that certain ways of handling the brake were causing it.
Picture, if you will, a train crew on a hill, trying to uncouple the
locomotives from a train. The engineer makes a heavy brake reduction,
which causes air in the pipe to move toward the locomotive.
Then, a brakeman closes the angle cock (the valve between cars that
closes off the brake pipe) on the first car, so they can uncouple.
Normal enough, but watch what happens in the brake pipe...
Air is moving through the brake pipe, and suddenly the air passage is
shut. But air has momentum, and it’s still moving toward the
locomotive. So it piles up at the closed angle cock, a little
pneumatic traffic jam. And as it does, the pressure at that spot in
the brake pipe, *increases*. Guess what the control valve in that car
does. It releases.
This wasn’t a problem with the AB valve; at most one or two cars at
the front of the train would release. But the Quick Release feature
changes all that, because it propogates the release back through the
train. That knocks the brakes off the second car, which knocks the
brakes off the third, and so on ...
By the time the crew realizes it, their entire train has released,
and it’s headed down the mountain.
Education has pretty much fixed the problem; wait until air has
stopped moving before closing the angle cock; or leave it open a bit
so air continues to drain.Braking down Hills==================
Diesel locomotives have two brake systems: an air system, which is
independent of the train brake; and dynamic brakes, which use the main
drive motors as generators, and discard the generated energy out large
resistor banks.
Ideally, braking down long grades is done with dynamic brakes. They
never overheat or wear out. Realistically, some help from air brakes
on the train is often needed, and that’s done either by normal use of
the brakes, or by stopping and turning up retainers.
I won’t talk about how air brakes perform over long periods of use,
but I believe they are sufficiently leak-resistant that they can make
it down the hill without needing to stop and recharge. However, if
the engineer repeatedly applies and releases the air brakes, he will
eventually drain the auxiliary reservoirs on the cars (but the
emergency reservoirs will be intact...) If an engineer foolishly does
this, he may need to use emergency to stop the train. Then he will
need to turn up enough retainers to hold the hill, and release the
brakes, and pump air into the system until it is recharged. While he
waits, the trainmaster will show up and cuss him out, of course...
There have been several rounds of evolution on locomotive brake stands
(which together with the control-valve-like Distributing Valve,
control the locomotive’s and the train’s brakes) The very early
varieties had three positions: Running (which charged the brake pipe
up to a set pressure), Apply (which discharged the brake pipe), and
Lap (which did nothing to the pipe). An engineer would move the
handle to ’Apply’ until he got the pressure he wanted, then he would
move the handle to ’Lap’. Unfortunatly, leakage would tend to reduce
the pipe pressure further, which would cause the cars to set the
brakes harder and harder. This caused problems on hills, as describedabove.
The most common contemporary model is the ’26’, which is a pressure
maintaining brake. Meaning, if you set the handle to 65 pounds, it
will automatically hold it at 65, even against leakage. This solves
the hill problem.
Before the invention of the pressure maintaining brake, engineers
would improvise one by adjusting the ’feed valve’, which is the
pressure regulator that feeds the brake system, and thus determines
maximum brake pipe pressure. That’s supposed to be set at whatever
brake pipe pressure the railroad runs as a practice (70, 80 or 90
pounds) and left there. To maintain pressure in their brake
pipes, engineers would dial the feed valve to the application they
wanted (say, 60 pounds) then let leakage bring it down to that. The
feed valve, thinking 60 pounds was the running pressure, would
dutifully hold the brake pipe there, against leakage. It was illegal
but it mostly worked.Runaways========
Seems like there’s a runaway in almost every train movie. But there’s
only about three ways a runaway can happen, given modern brakes...
1. The crew forgot to charge the system in the first place.
If you couple onto a string of cars that’s been stored for a week,
there’s no air in it. If you then haul it away without hooking up air
or doing an air test, you could lose your train simply because the
locomotive is the only brake on the train.
2. The ’Quick Release’ bug. Described above. If there’s anyone on
board capable of pulling an emergency cord, it’ll stop the train.
3. Going so fast down a hill that you gain kinetic energy faster than
your brakes can get rid of it. Like running down a steep hill.
4. The brake pipe gets ’bottled up’ behind the engine.
Either an angle cock vibrated shut (very unlikely) or the brake pipe
fell on the tracks and got bent and pinched shut in a way that
’bottled the air’ in most of the train, so there aren’t enough
responding brakes to stop it (also very unlikely, but it HAS
happened...) Again, if there’s anyone back there to pull the emergency
cord, they’ll stop the train. Or, if there’s enough brake pipe
leakage in the bottled portion, one of the valves will go into Quick
Service, and will cascade at least a minimum application...
These are all pretty bizarre, and they just don’t happen much.
And when they do, it’s in the paper :-) |
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 | May 03, 2006 12:33 am -
The electric trains over here do not use the air brakes untill its almost fully stopped. Even on the old electrics. Saves noise and air, plus the motors are really good at slowing athe train does, espically 8, 12 or 16 of them. :-) |
 | Sep 30, 2006 9:40 pm -
burn |
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