pyotr filipivich wrote:

danmitch on Tue, 15 Jun 2010 14:12:22 -0400
typed in rec.crafts.metalworking the following:

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.


My understanding has been that the air pressure keeps the brakes
"open" (off), and that loss of air pressure applies the brakes. Sort
of a fail safe mode.


That's more-or-less what I said, but it's more complicated than you
imply. Essentially the air both APPLIES and RELEASES the brakes.

The locally STORED air in the main reservoir is what provides the force
needed to APPLY the brakes. A DROP in pressure in the train-line is what
triggers the brake application (via the triple-valve as previously
described). Once the pressure in the train-line is restored to normal,
the pressure in the brake cylinders is vented (again via the
triple-valve), and the brakes are released.

What's important to understand is the pressure in the train-line is used
to CONTROL the brakes, NOT to provide the force to apply them.

The brakes are NOT "spring loaded", and held "off" by the air in the
train line. It's only PREVIOUSLY stored air in the reservoir that can a
provide the force to apply the brakes (I suppose that could be thought
of as an air-spring). However, once that reservor air is used up, you're
in BIG trouble.

Once the main reservoir is depleted, as from too many brake applactions
in a short period of time, the brakes become inoperative regardless of
train-line pressure. Then you have a run-away train. NOT good!

It's the engineer's job to manage his available air supply to assure
adequate braking at all times.

Dan Mitchell
============