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In lugnet.technic, Thomas Avery writes:
> In lugnet.technic, Bob Kojima writes:
> > i will be showing this at a talent show on may 10th. i just finished it
> > yesterday. it was such a nice day outside and i don't really have a flat
> > surface long enough to set it up in my house, so i set it up outside. i
> > made this bridge from pictures of the real bridge and a little inspiration
> > from TJ Avery. the hawthorne bridge is about a mile away from my office so
> > i spent a lot of time just looking at it.
>
> I hope my bridge design spreadsheet worked okay for you. Your trusses look
> good! Nice work. Very nice!
>
> > you are right about synchronizing the motors. one of the motors runs
> > faster than the other. i made the string a little longer on one side. so
> > when the bridge starts to go up it is a little lopsided, but by the time it
> > reaches the top it evens out. not a very good solution but it works.
>
> I've hit a similar problem in planning out a crane. I've contemplated using
> two separate winches to lift the main hoist, but just like your drawbridge,
> one side can lift faster than the other. This depends on individual motor
> speed and the specific diameter on the winch drum (which varies as string is
> winched in or out).
>
> I'm not sure how to solve this effectively. The simplest way is to control
> each side independently, but that adds a lot of controls the user has to
> manage. You could use a RCX and two rotation sensors to keep the amount of
> string let out equal on both sides. But I've yet to try this.
>
> -TJ
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
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Message has 1 Reply:
 | | 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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Message is in Reply To:
| | Re: drawbridge
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| (...) I hope my bridge design spreadsheet worked okay for you. Your trusses look good! Nice work. Very nice! (...) I've hit a similar problem in planning out a crane. I've contemplated using two separate winches to lift the main hoist, but just like (...) (22 years ago, 29-Apr-03, to lugnet.technic, lugnet.trains)
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