Subject:
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Vertical lift spans (was: Re: drawbridge
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Newsgroups:
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lugnet.technic, lugnet.trains
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Date:
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Wed, 30 Apr 2003 01:59:31 GMT
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Viewed:
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197 times
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Not sure where to chime in, so I'll jump in here.
Regarding the lifting of vertical lift spans:
I've built many of these over the years and my latest version (yet to be
published on the internet) takes advantage of an engineering survey done on
one of the vertical lift bridges in Portland, Oregon - The Steel Bridge,
built in 1912 and still going strong today. It is the only double hung (ie
two decks that both go up and down) vertical lift span known in the world.
Key points relative to this discussion:
1) A counterweight is used in all vertical lift bridges. In every case I'm
aware of the counterweights are suspended on cables entirely different from
the cables that actually lift the span up.
2) Most of these bridges (of the ones I've seen) have a little house on top
of the lift span. This is where a single motor is housed that lifts the
entire span up and down. This motor is geared to two pulleys, generally
inside the house. The pulleys provide the lifting force to lift the bridge.
They act on cables attached to the top of each tower *and* at the bottom of
each tower. The specific rigging is a little complicated, but to try to put
it simply, cable is run through the pulleys from both sides at exactly the
same rate (because they are geared together) and the horizontal lift span
stays horizontal as a result.
I'll find some time in the next couple of weeks to photograph my bridge and
draw some pictures to show more clearly what I'm talking about.
Until then, let me say, the best way to build a vertical lift span is with a
single motor attached to the lift span. I base my opinion on the actual
workings of real vertical bridges all over the world and a LEGO model that
is true to this description.
Ben Fleskes
In lugnet.technic, Steven Lane writes:
> In lugnet.technic, Steven Lane writes:
>
> > Ok, here's my idea for a solution for a vertically lifting bridge.
> >
> > All you need is one winch connected to one lifting line. You also need one
> > control line(running off the same motor but geared down 2:1).
> >
> > The lifting line goes up the first tower down to the lift span, turns 90
> > degree's around a free running pulley, across the span, round another pulley
> > and up the other span where it goes over two more pulleys and then goes back
> > to the winch via a mirror image route back to a second winch drum on the
> > same winch axle.
> >
> > This is where you lot say it'll just pull from one side and jam. This is
> > true except that their is a control wire connected to the underside of the
> > span at the end nearest the winch. The control wire also goes to the lift
> > span across two pulleys and back to a second drum on the same control wire
> > winch axle.
> >
> > The bridge rises at speed X, the main winch pulls in the line at twice x and
> > the control wire is paid out also at speed x.
> >
> > Therefore as the 'controlled' end of the span cannot lift at a speed greater
> > than x the span has to stay level. And as the lines are pulling on the 4 top
> > corners the span should stay level in both the x and y plains.
> >
> > You would of course need a mechanism to tension the control wire correctly.
> >
> > Steve
>
> I've thought of a simpler way of describing my system.
>
> Basically you have a rope stretched between two towers. If you pull the rope
> the span goes up, release it and the span goes down. The wire is connected
> to the lift span at both ends via pulleys and the wire is pulled from just
> one end. Because this would proabaly cause the span to lift higher at the
> end closest to the winch a control wire also geared to the winch holds that
> end down. Because the control wire pulls down(paying out as the bridge
> lifts) opposite to the main wire pulling up, the system would remain level.
>
> Steve
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Message has 2 Replies:
 | | Re: Vertical lift spans (was: Re: drawbridge
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| (...) OK, this is similar, but first take a walk down to the local auto repair shop and see if you can look at a 'cats cradle' type auto hoist. Here is my description. It involves one string to lift one end of the span and another string, which is (...) (22 years ago, 30-Apr-03, to lugnet.technic, lugnet.trains)
| | | Re: Vertical lift spans (was: Re: drawbridge
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| Well put Ben. I have seen Ben's steel bridge (and helped re-rig the cables to the pulleys after a minor anomaly), and I can attest that it is truely one of the finest LEGO bridges in the AFOL community, or in the world for that matter. It is very (...) (22 years ago, 30-Apr-03, to lugnet.technic, lugnet.trains)
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Message is in Reply To:
| | Re: drawbridge
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| (...) I've thought of a simpler way of describing my system. Basically you have a rope stretched between two towers. If you pull the rope the span goes up, release it and the span goes down. The wire is connected to the lift span at both ends via (...) (22 years ago, 29-Apr-03, to lugnet.technic, lugnet.trains)
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