[PRCo] Re: PCC__Handbrakes

[Boris Cefer]

My feeling is that the lower valve is a clack-valve, which allows to fill
the small air reservoir with a compressed air under normal running
conditions (brake released, brake piping under full pressure), but does not
allow the air to flow from the reservoir into the brake cylinder. If the
pressure disappears from the car piping (it is called for spring-applied
braking), the air remains in the reservoir for an eventual emergency
release.
If there is any magnet valve on the truck, I don't know, but I don't think
so.

Boris

----- Original Message -----
From: "Fred Schneider" <fschnei at supernet.com>
To: <pittsburgh-railways at dementia.org>
Sent: Sunday, May 01, 2005 7:48 PM
Subject: [PRCo] Re: PCC__Handbrakes


> Ed:
>
> I had a hectographed page from the engineering office in Homewood (for
those
> who think it matters, it is not on company letterhead) that lists all of
the
> numerous variants in brake schemes among the Pittsburgh PCC cars.  I said
a
> page ... a whole page of different schemes and retrofits.  I have no idea
where
> it is today but you might look in all my left over files from the PCC
books
> that we moved to the library.
>
> In general all-electric cars had spring applied, electrically released
drums,
> and the shoes could either be internal expanding or external contracting.
In
> other words, any way it could be built, it was.  I have no objection to
what
> anyone posted here about air drums ... again, they could be internal or
> external to the drum, and air applied / spring released or vice versa.
> Whatever a company wanted to buy.  If you can find that sheet, you might
have
> proof that Pittsburgh did it both ways.
>
> What I'm most curious about is Boris's picture showing the air cylinder
next to
> what looks like an air reservoir.   It appears that the air reservoir
being
> right next to the cylinder was an attempt to either give a bast apply or a
fast
> release by not having  100 feet of pipe to fill with air.  One of the
problems
> with some air-PCCs (the only one with which I have experience) is the slow
> application time after the lockout relay commands a brake application.  It
> takes time to charge all that pipe.  Therefore you tend to get a momentary
> brake release when the dynamics fade and it takes a second or two for the
air
> to build up.  This may have something to do with my observing track brakes
> being used on air cars as they came to a stop on a safety island in East
> Liberty.  Perhaps the motorman was pushing the pedal to the floor to avoid
> running off the end of the island, and getting track brakes.  My
experience is
> with Baltimore 7407.  I'm waiting to see if PRC 1138 behaves the same way.
>
> For those of you totally oblivious to what I'm trying to say, with any air
> brake system, the brakes will not apply (or release) until the pipes are
filled
> with compressed air or all the air is released.  Some of the newer Gomaco
cars
> are using relay valves to connect an adjacent air tank to an air cylinder,
and
> using very small diameter pipes to connect the relay valves to the
motorman's
> brake valves.  This appears to cut the application and release times down
to a
> fraction of a second.   This was not a problem with all-electric PCC cars
> because ... think back to high school physics ... electricity travels at
the
> speed of light, or 186,000 miles per second.  If you all-electric car was
> equivalent to a snake wrapped twelve times around the earth, then you
would
> have a similar delay.
>
> So Boris ... you want to post a drawing to show us just how this works.  I
can
> see two valves.  One probably a magnet valve (the lower one) and the other
> valve hand controlled from a pull / push rod.
>
> fws