Moving Forward

A project log for Hand Drive

A wheelchair attachment that allows any wheelchair to be powered in a rowing motion. It is 3D printable, open source, and available to all.

Kate ReedKate Reed 08/14/2015 at 18:250 Comments

Moving Forward

The Hand Drive went to the White House. Whoa. Although meeting the president was super-duper crazy cool, we are not done with this project yet. The Hand Drive broke when President Obama tried it at the White House.The reason it broke was because the brake was on when the president tried to use the hand drive, and this caused way too much force to be on just one ratchet piece, so it snapped off. Believe it or not, we were expecting this to happen, just not when the president tried it. This has been a problem that we have been struggling with in our design. As soon as we got back from the White House, we started a redesign.

When we sat down for our redesign, we came up with the idea of a planetary ratchet system, which means the force could be distributed throughout three ratchets as opposed to one. This works by having the ratchets on the inside of the gear rather than the reverse. When we figured out the mechanics of it, it worked out so that we could make both the top and bottom ratchet have the same rotation point. We thought about having them be on two different planes, but our epiphany came with the idea of combining the two ratchets into one. This way we still end up having the stacked gear as we did before so it can still go both forward and backward, but the entire unit is much more compact.

We had one main design change in figuring out how to make the ratchet mechanism. In our previous design, squeezing the bike handle would disengage one ratchet and engage the other. This was a little weird because instead of having a spring to push the ratchet down giving it resistance, the ratchet was relying on human error of the person squeezing the bike handle to have some give in their grip. Although this worked, it was not ideal. Nonetheless, we know more now and have come up with a better solution.

With the inner ratchets, we came up with a solution that involves a centerpiece in the center of the gear and the ratchets. It has three springs sticking out the sides that push outwards. The ratchets are designed with a curved back. When you twist the center piece, it pushes the springs to one side of the ratchets or the other, thus engaging either the top or bottom ratchets. It is a very elegant solution. We like elegant.