OT mechanism design

I know this off topic, but since i know this is a forum with very skilllful peope i will give it a try.. I want to build a mechanism with a virtual rotating point, imagine a
rectangular box that i want to rotate around its area center, but the link itself cannot ble placed in the area center. which type of mechanism do I use to do this?, any references to web sites or books is greatly appreciated. a similar mechanism is used for hinging the hood of automobiles, but there the point also translates.
Mads
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Multiple revolutions, or just a partial turn?
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Dale Dunn
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partial turn, approx 25 deg total.
Mads

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Do you want to turn about a true center, or approximately?
To turn around the true center, you might use a guide rail in the shape of circular arc. There are linear bearing systems that will work on a curved rail.
To turn about an approximate center using linkages, there are several possibilities I can imagine, and probably a lot more in the handbooks others have mentioned. Without knowing exactly what you're doing, I can only suggest guesses.
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"Mads" wrote:

have a look here : http://www.mfdabbs.pwp.blueyonder.co.uk/Maths_Pages/SketchPad_Files/Mechanical_Linkages/Mechanical_Linkages.html (sorry, the link is pretty long...)
there's a nice small software that's definitely useful to design such things : http://www.keypress.com/sketchpad /
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Philippe Guglielmetti - www.dynabits.com



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Do a google search on a "crank mechanism".
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Markus

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A converging (think trapezoid) 4-link mechanism will make an approximate center of rotation in space but has a limited arc. The point of rotation will resemble the letter E if it were written. Another method is to use a spherical joint similar to a computer monitor base.

skilllful
or
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The problem is that the point of rotation must not translate, spherical joint is difficult because of lack of space.
thanks for the suggestions, greatly appreciate it
Mads

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What sort of accuracy are you looking for?
Is the point of rotation in 1 or 2 planes?
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I have another solution but it will be difficult to describe. If you are interested, let me know and I'll email the model to you.
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I have received and tested your model, it was exactly what i was looking for, thanks.
Mads

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skilllful
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I've done something like this before. Build a circular guideway or track outside the part. Adjust it so the centerpoint of the track's radius is at the desired point of rotation. Attach the part with an arm to a carriage that follows the track. The motion will be circular about the centerpoint of the track's radius, but with no attachment to the centerpoint.
Art Woodbury
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I have used a double tracked cam follower systems in the past. Two cam followers mounted on a bar and being guided by the paths cut into a side frame. You can move points in space with almost no restrictions. There are some tricks to getting it to work really well though.
Jay

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