On Fri, 20 Aug 2010, David Cantrell wrote:
On Thu, Aug 19, 2010 at 09:48:11AM +0000, d wrote:
Surely it has some sort of handbrake? How else could it be parked safely
for long periods when the air will have all leaked out?
Uh, aren't railway brakes held *off* by vacuum,
Once upon a time, but, UIVMM, not for several decades.
as opposed to being held *on* by air?
They're held both on and off by air. Pressure from a reservoir pushes them
on, and pressure from a brake pipe pushes them off. Whichever has the most
pressure wins.
The problem with the vacuum system is that you can never have more than
one atmosphere of pressure pushing the brake on, because that's all you
can ever muster to keep it off. Whereas with air, you can have as much
pressure as you like in the reservoir, as long as you can summon up the
same amount of pressure in the brake pipe to keep the brake off.
That's fail-safe - lose power or burst a pipe and the vacuum goes away,
and the brakes go on.
This is still the case with the current system - if the brake pipe comes
unstuck or the power to the compressor fails, its pressure drops, and the
pressure in the reservoir will overcome it, and apply the brakes.
The weakness, of course, is the reservoir. If it isn't filled (eg a train
has been parked for ages), or it runs out (eg a train has parked
recently), or it leaks or is vented by mistake, you've lost your ability
to apply the brake. I don't know how this is dealt with - i would guess by
making the reservoir quite large and very reliable (and it is, after all,
just a big tank with a pipe coming out of it), although this doesn't
address the cold start problem.
Of course, this doesn't help if you've deliberately disabled the braking
system, but I would expect the operating procedures to only permit that
if there is a hand-operable braking system, even if it's just screwing a
shoe down onto the wheels and really ****ing up the wheels.
Exactly. The sort of thing that in nuclear power engineering is called a
scram - a last-ditch, absolutely foolproof, not necessarily recoverable,
way of stopping a runaway.
tom
--
i know how they do it i think they just put on a song then the mario thing
plays it i think im not sure though im still looking on it -- darkcat102