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

 

have a look here :
http://www.mfdabbs.pwp.blueyonder.c.../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/

 

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.

 

partial turn, approx 25 deg total.

Mads

 

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

 

What sort of accuracy are you looking for?

Is the point of rotation in 1 or 2 planes?

 

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

 

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.

 

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

 

I have received and tested your model, it was exactly what i was looking
for, thanks.

Mads