I'm trying to design a mechanism that will allow a part to rotate around an exterior axis using a mechanism on the interior of the structure

Question

I am trying to design a mechanism to allow a part to hinge around an axis which is external to the structure to which it is mounted (ie it is not practical to pin it at the theoretical axis). I have been trying various offset parallelogram linkage mechanisms but so far haven't been able to come up with a solution. Due to the parts being fairly small, there isn't room to to install a curved rail and slider which would allow the gold part to rotate about the desired axis so it needs to be done with linkages. If the path of the gold part deviates slightly from a true circle, this is not a problem as long as the start and end position are roughly the same as in the image. Any advice or suggestions on this would be greatly appreciated.

Thanks

Andy

Answer 1

I had the same problem, and while Rotation around an axis outside of the mechanism pointed me in the right direction, it didn't provide a straightforward solution.

There are other ways to solve this, but I found that a double parallel four-bar linkage is easy to design and manufacture.

The design process is:

1. Draw two lines through the wanted rotation center and place two points on each:

   1. The output bar (green) location. This should be positioned so that when rotated, it will remain in the desired area. Below I have positioned this at the top of the blue area, so it will move inwards when the mechanism opens.

   2. The location of two fixed mounting points (red).

   Note that the points can be placed anywhere along the lines, and the angle of the lines is free also. So you have great flexibility in the placement at this stage.

2. Add two link arms (yellow 1 and 2) from the fixed point that are parallel with the output bar.

3. Add two bars (yellow 3 and 4) that are parallel with the fixed mounting points and connect the link arms to the output bar.

4. Add a solid connection (dark green) between the output bar and the part to be moved. The connection can be any shape, as long as it is rigidly connected to the output bar.

How it works:

The link arm 1 acts as a model for the rotation of the output bar. Its rotation point is offset along the red line from the desired pivot point.

The parallelogram formed by 1, 2 and 3 offsets one point from the link arm to the output bar. The arm 3 always remains parallel with the red line, and the distance to the output bar is same as the distance of the fixed points from the desired pivot point.

The second parallelogram formed by 1, 3 and 4 then keeps the output bar parallel with arm 1.

Thus the output bar has same angle as arm 1, and is offset upwards by the same distance as the upper fixed pivot point is from the desired pivot point. The output bar, and anything rigidly connected to it, will rotate around the target pivot.