Subject:
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Re: BBB wheels + availability?
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Newsgroups:
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lugnet.trains
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Date:
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Mon, 22 Dec 2003 06:06:56 GMT
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Viewed:
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1438 times
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> > But ,in my lego versions they
> > work better 90D apart. Is this what the counter balance on the wheels fixes
> > in real life ? Or is just the downward weight on the wheels?
>
> No the counterbalance is only to balance (as much as possible) the weight of
> the connecting rod and bearing on the wheel. You'll notice the counter weight
> is much larger on the driver that is connected to the piston, as that bearing
> needs to carry an extra con rod.
Think of it as a physics problem, since that is what it is. If the two
connecting rods (one each side) are at 180 or 0 to each other, then when one is
at a end, the other is at end as well. When you push on one or the other of
them, nothing will happen _if the force is straight on_. If the wheels are
quartered (either 90 or 120%, both are used(1)), then one rod being pushed on
_at all times_ will move the wheel, because the force can act in two vectors
rather than just one. (and since it is attached to the wheel, the wheel rotates
& the train moves forward).
Why two cylinders? Because a single cylinder is not usually self starting.
Under some conditions, a twin may not be self starting, but 99.95% of the time
it is. (there are a few tricks if a loco does stop at a dead spot- same as
there are some tricks that will usually start a single cylinder steam engine
without barring it over. If you have seen "Titanic", one of the tricks is shown
in it.)
Why more than 2 cylinders? Ballance. The peak forces are exerted when the
crank is at 90% to the cylinder (mid stroke). This has a rather bad aspect in
two ways
1: It encourages slipping
2: As the cranks aproach 0% (ends of stroke), there is a signifigant vertical
component to the forces. This can throw the loco right off the track (witness
the Southern (UK) Baltic tanks, SCL's 484's), and it also plays havoc on the
track (pounding it)
By dividing the power out over more than 2 cylinders, you reduce both effects.
It also allows aditional power in a restricted loading gauge (UK loading gauge),
because you can now get 3 cylinders of about the same bore where you had 2
outside only before.
I hope this explains why wheels are quartered, and why they may not be at 90
(but at 120 instead). I'd guess that you just didn't notice that the wheels are
quartered on the loco in the park- every rod loco that I know of is quartered.
James Powell
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Message is in Reply To:
 | | Re: BBB wheels + availability?
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| (...) That may be (as lar said) because it's a static display. (...) No the counterbalance is only to balance (as much as possible) the weight of the connecting rod and bearing on the wheel. You'll notice the counter weight is much larger on the (...) (21 years ago, 22-Dec-03, to lugnet.trains)
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