Ignoramus3037 wrote:

On 2010-06-15, danmitch wrote:

Ignoramus9619 wrote:


Dan, awesome post, thanks a lot for educating me.

You are quite welcome.

Train air brakes are a very complicated affair. snip


Dan, write something else about trains. I love reading this stuff. snip

"OK", here's some more.

Depending on how you look at it, the air both applies, and releases the
brakes. It's all controlled by ONE air supply, the "train line", pumped
up by the locomotive's air compressor. The compressed air in the train
line both charges the local reservoirs on each car, AND controls the
brake application and release.

The important safety features of the Westinghouse brake system are that
1) each car has it's own reservoir, and THAT air is used to apply that
car's brakes, independently of any other car. and 2) that a DROP in air
pressure in the train line applies the brakes. Hence, in the event of a
separation of the train line (a break-in-two of the train for example),
the pressure loss applies the brakes.

All this is actuated by a complicated pressure sensitive valve on each
car. This is called a "triple valve". Depending on the pressure changes
in the train line, this valve routes air either from the train line to
the car's reservoir (charging), or routes air from the reservoir to the
brake cylinders on that car applying the brakes, or vents air from the
brake cylinders, releasing the brakes.

The degree of brake application is also controlled by the pressure drop
in the train line ... the greater the pressure drop, the more strongly
the brakes are applied.

The train line pressure is controlled from the locomotive by selectively
venting pressure from the train line. a series of vents ("holes") can be
selected to obtain various rates of pressure loss. An "emergency"
(maximum) brake application is obtained by selecting the largest vent
.... hence the railroad slang term for an emergency brake application is
the "Big Hole". This is also what results from a break in the train line.

Such emergency applications are frowned upon unless really needed, since
they often result in equipment damage, derailments, or wrecking the
entire train. At best, one can expect flat-spotted wheels from locking
and sliding.

Sometimes an individual wheelset (axle) will improperly lock in even an
ordinary brake applcion, resulting in a flat-spotted wheel. Hence the
loud "Clunk-Clunk-Clunk" noise one sometimes hears in a passing train.
This intermittent "hammering" is hard on both the car and the track
(aside from making an awful noise).

Also: Each brake application depletes the car's air reservoir a bit.
Normally this is soon recharged by the loco's air compressor once the
brakes are released. However, one has to be careful to not apply the
train brakes too hard and too frequently, without allowing adequate time
for the loco's air compressor time to properly recharge the cars' air
reservoirs. Insufficient air in the reservoirs can lead to inadequate
braking being available when needed, and a train run-away. Again, proper
brake usage requires both experience with the equiupment, and a prior
knowledge of what lies ahead on the route. A train may need to "rest"
briefly to recharge the reservoirs before beginning a long descent.

The above problem can be reduced by using another appliance on each car
called a "retainer" valve. Actuating this device (manually on each car)
retains at least some air in the reservoir. It is used mainly when
descending long grades, where the brakes need to be "dragging" but not
fully applied. Thus some "air" is held in reserve. "Setting retainers"
must be done manually by crew "walking the train", and done BEFORE
descending a long "Often only some subset of cars in the train are so
configured.

Also, when a car is parked, with the train line open, the brakes are
applied ... but there is always some leakage, and the car's reservoir
will bleed down, eventually releasing the brakes. To prevent this
situation, each car has a mechanically applied "parking brake" actuated
by either a large wheel or lever mechanism with some form of ratchet to
lock it. This mechanically pulls on the end of the brake cylinder's
piston rod, extending it, and applying the brakes the same as if the air
cylinder had done so.

The above is much the same system as used in olden days to mechanically
apply the brakes before air-brakes were invented. The crew had to walk
the length of the moving train, usually along the roof, setting or
releasing the brakes car by car as needed. It was a VERY dangerous
business, especially in bad weather. Also, imagine the time delay
involved in getting most of a train's brakes applied! This made even
early air brakes a wonder of responsiveness.

And "brake inspections" are needed periodically when running any train.
Normally they are inspected BEFORE the train is allowed to leave a yard,
and after any really hard brake applcation. Crew must "walk the train",
on each side, and inspect each car's brakes to be sure they are either
applied or released as may be required. "Stuck" (dragging) brakes are
rather common.

The brake shoes (cast iron or composition) wear, and need to be replaced
perodically, or as may be needed. The wheels also wear (due to both
braking and track contact), and need to be periodically re-machined to
the proper contour (turned on a large lathe), or replaced, as needed.

Dan Mitchell
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