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Re: NT Class again

Aus loco discussion mailing list

Peter, I can't help you much here, except to guess at what might have
happened.

The control system air reservoir is usually fed from the main reservoir
through a protection valve that will not feed until main pressure is above a
value like 75 lbf/in2.  Likely the DH class has an air-operated reverser (I
think that most Voith hydraulic transmissions have an electro-pneumatic
arrangement.)  So even if the driver reset the direction, the reverser
itself might not do anything if the control air pressure was low.  The
reverser can be the heaviest load for the air control system, and so likely
the first to suffer from non-function from low pressure.   I think that the
DH might also have an air throttle, so with low control air pressure, it
might not open very far, but nevertheless enough to move the loco.  And the
throttles often operate at reduced pressure (e.g. 60 lbf/in2 max for the
standard Westinghouse system, and even lower for the EE system) so would be
less affected by low control reservoir pressure.

As far as I know, most locos have interlocks that prevent the application of
power (or cut off an existing application of power) if there is insufficient
air pressure in the main reservoir to properly operate the brakes.  So you'd
think that this would supervene.  But maybe there's a situation when you can
have enough main reservoir pressure to release brakes, but still be a bit
low in terms of control air pressure.

Maybe someone familiar with the Walkers DH locos can confirm/refute/modify
the above guess?

Steve P.


----- Original Message -----
From: Peter Donaghy <PeterDo@als.com.au>
To: 'Aus loco discussion mailing list' <Ausloco@listbot.com>
Sent: March 23, 2000 07:00 PM
Subject: RE: NT Class again


> Aus loco discussion mailing list
>
> What arrangement exists on the ex-QR DH class units?  When I was in
> Alice Springs recently, the two operators rearranging pass cars and
> locos at the Ghan Preservation group invited me onto DH14 with them.
> The loco had just been re-started, and when the brakes were released,
> rolled the opposite direction to that which the driver tried to move.
> He re-braked, and said the compressor hadn't pumped the air up
> adequately.  After another minute or so, everything was OK.  Is this a
> throttle matter, or reverser?
>
> Peter
>
>
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