Restricted Bends

A project log for Antigravity Arm Floats

Wheelchair mounted orthoses to enable someone with very low muscle tone to move her arms without the encumbrances of gravity

mike-turveyMike Turvey 10/10/2016 at 06:180 Comments

One of the things I've played with a lot is restricting the angles that the joints in the plastic chain can take. For example, here is a piece of chain from the elbow:

Notice that in the lower left, there is a smaller circle, and then a partial larger circle around it. The larger circle acts as a "stop" to prevent the next link in the chain from turning in that direction. With the stops configured as above, the next piece of chain has a range of motion from either folded back to the right forming a "V" shape, all the way to extended and straight. But it cannot bend any further. A chain made out of links like the one above would readily bend in one direction, but would not flex in the other direction at all.

Here you can see the stops in action:

Here are a few more examples of trial pieces:

The top object is a fairly restricted chain that only allows about 40 degrees of turn in either direction. I initially thought that this would be a good approach for the shoulder joints. By restricting the angle this much, I was hoping that it would prevent any binding of the chain pieces because all curves in the chain would be fairly wide arches. It sounded good in theory, but in practice it was pretty bad. By restricting the movement of the chain, it significantly limited the positions that the chain could be in. And as a result, the rest of the arm floats often couldn't move into the positions that were most natural. It was a quick lesson, and instead of re printing all of the links, you can see in the middle that I took a small saw to the chain pieces and cut off much of the stop, enabling a much greater range of motion. When I redesigned this chain to use a larger bearing, I almost entirely eliminated the stop. While in some circumstances, the chain can somewhat bind, it turns out to be a very rare occurrence, and well worth the tradeoff of the increased freedom of movement.

The elbow, however is a different situation altogether. All of the chain in the elbow is limited to only bend in one direction. This works well to keep the Z rods away from her upper arm, and doesn't limit freedom of motion at all.