Hand Drive Version Six


Kate Reed

This version of the Hand Drive incorperates planetary gears, having a gearing up ratio of 1 to 1.6.


Kate Reed

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!

We always want to keep our big picture goal in mind of keeping the Hand Drive wheelchair accessible to all. With this particular model we would need to replace each wheelchair axle to give the gear something to hold onto, and that would make this design much more complicated and costly for the user. We came up with a redesign that just uses the natural axle of the wheel instead of replacing it. 

This new design has a fixed sun instead of the planets. The fixed sun is much easier because it is already in the center of the wheel, so we just have to give it a little divot so that it can lock onto the center of the axel. Now we have the planetary gears attached to the stacked gears and the annular gear attached to the spider, with a fixed sun gear. In technical terms, we are driving the planets now. The challenge of this design is that it would need a Lazy Susan bearing to hold it all together, because if not the entire system could just pull apart. The Lazy Susan would be attached on the annular gear as well as the stacked spider gear.

After making all the models for this newest version of the Hand Drive, we did some research and looked for a Lazy Susan. The Lazy Susan’s were not as accessible as we thought they would be. They only come in a few sizes, all of which are too large for our model, and they are square based and our Hand Drive is circle based, which would just look ugly if we incorporated this Hand Drive. We ended the day knowing we had to move on from this idea.

In trying to solve the dilemma of keeping the Hand Drive together, we needed to isolate various problems, the first being both the parts of the Hand Drive have to rotate separately with almost to no resistance in between. The second problem is that, in an ideal world, the user would crank the Hand Drive in a perfect straight line, but realistically the user would have some slop in their cranking and wiggle it a little. The wiggle that the user would naturally produce would create friction and potentially pull part of the Hand Drive right out if the connection was not strong enough. 

Our first solution was to try and create a series of tabs so the two sides of the Hand Drive could slide into one another. This wouldn’t work because both parts of the Hand Drive are circles, so there really is no way that we could slip the two pieces together. 

Our next solution was having one side have tabs sticking up and the other side having a track to receive the tabs, but this was a very messy solution, and wouldn’t work because it is relying on the flexibility of the plastic and essentially the weakness of the plastic to hold it in place, so it would likely break off quickly. This solution got us thinking in the right direction though. 

Our next solution involved loose bearings. In this solution, each of the two pieces of the Hand Drive would have a track for a bearing in a semicircle channel. The two pieces would fit together with just enough space for a bearing in between. Then you would slide the bearings in through the side, then put a screw in to lock it in place. This would both lock the two pieces in place, but also allow them to spin freely from one another. 

With the planetary gears it is crucial that everything stay aligned. We also added three bearings to the sun gear and a track on the stacked gear to insure there is no friction between all the layers. This design adds an extra 12 mm to the Hand Drive, which is not ideal, but should not disrupt too much. We think we can lose the height in some other areas of the Hand Drive, so we are staying right on track!

I went to test out the Hand Drive this week and it went really well. I took the Hand Drive to an assisted living home nearby called, the Village of Duxbury, in Duxbury, Massachusetts, and ooh boy, were they excited! I had about ten people test out the Hand Drive, and they were very impressed and enthusiastic. Almost every one of them said, “You should go on Shark Tank!”

I was impressed with how well the Hand Drive held its own. This was the first time we really just “let it go” for people to test. The testers were pretty brutal with it, but it didn’t show any wear and tear. They were banging it around, going in circles and dropping it all over the place, but the Hand Drive was strong and did not give in. 

It was interesting having people try it and hearing their instant feedback. One comment kept popping up over and over again- they kept saying how easy it was and how great their back felt. A lot of the older people had some back trouble in their past, and were hunched over, so they just kept saying how relaxed they felt. It was very inspirational for me because I felt we had made something that really could change people’s lives. I was just watching one face light up after another as people tried it. The testers really got it, and that was awesome. 

One observation in testing was that the height of the bar really depends on height of the person, and it is really a personal thing. The height of the bar also determines the posture of the user, for instance if the bar is shorter, the user has to lean forward more. A lot of the older people were small, and the Hand Drive did look a little big, but this is the beauty of it being a do-it-yourself project. The Hand Drive is 100% customizable. 

Another observation had to do with the brake lever and the age of our testers. Many of the older users couldn’t imagine a brake lever being anything other than a brake lever. On our Hand Drive, the brake lever reverses the direction. It was funny watching them reverse direction over and over again as they reached for the brake! When we had originally thought of the Hand Drive concept we hadn’t really thought of it helping older wheelchair users too, but now we can totally see why it makes perfect sense for this to be a considerable chunk of the market. With the Hand Drive using bigger muscle groups it’s less tiring, and many of the older people get fatigued easily. That hunched over look you often see in elderly people comes from leaning over a walker or wheeling a wheelchair in the traditional way.

Overall, it was an incredibly inspiring day - to get out in the world and have people try the Hand Drive. We’ve been working in a bubble for so many months, guessing and supposing what a user’s needs might be. Having live feedback, and such positive feedback, was both helpful and energizing!