Now we are working on speed. Moving forward, we realize that using the bike hub as a model to gear up the Hand Drive is much too complicated. We came to the conclusion that although it would be nice, there is actually no reason for the Hand Drive to be able to switch gears. We are working on a design that allows the Hand Drive to be geared up all the time. While doing this we are learning a few things about planetary gears.
Planetary gears are pretty much the only way to gear up for circular motion. To open our minds to other possibilities though, we looked at different ways of gearing up for all motion and explored all sorts of pulleys and actuators. We didn’t find anything that would work for our design though, because the different ways of gearing up require a different kind of motion. In most cases this would involve the lever on each side of the Hand Drive being offset from the center, which would give us a whole different set of problems and complicate things substantially.
Even though we didn’t want to, we settled on planetary gears again, but this time we took a much deeper look. Planetary gears have three parts. There is the sun gear in the center, the planet gears that orbit the sun gear, and the annular gear that encloses the other gears. Although it would be nice if just adding these gears to the design would fix our dilemma, it is not that simple. Each of the three parts of the planetary gears has to have a different role. The most important part, which we didn’t realize before, is that one of the parts of the planetary gears has to be fixed.
In our case we have the sun gear fixed to the wheel, being driven by the annular gear that is fixed to the handle, and then the planets are fixed to the axle of the chair, and are not rotating. The hardest part about this design is fixing the planet to the axle. By fixing the planets to the axle, we have to extend the axle of the chair and add a nut to lock onto. With this new design, our Hand Drive is geared up with a one to five ratio which is pretty hefty, if we do say so ourselves.
We want our Hand Drive to be as thin as possible. The more the Hand Drive sticks out the more torque there is to deal with. It is also inconvenient to have something sticking out like that, and it’s ugly. We have come up with a very elegant solution that makes it both thinner, and prettier. Before we had a stacked gear that had to be thick so that it could absorb the force of the ratchet, but with this new design, we have made the stacked gear larger and hollowed it out. Now the planetary gears fit inside the stacked gear, thus taking out 20 mm of extra space that the planetary gears would require. We have the model and it looks beautiful!