Gearing Up, Round Two

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 09/02/2015 at 22:152 Comments

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!


zakqwy wrote 09/04/2015 at 13:53 point

Also, this picture is really awesome:


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zakqwy wrote 09/04/2015 at 13:51 point

I've thought about incorporating Lazy Susans into designs before, just because they always seem like a handy solution for handling axial loads; however, I inevitably come to the same conclusion you do--they're expensive, square, and only come in a few sizes. 

I'm looking forward to seeing images/models of the solution you came up with! I'm sure you've found a good network of suppliers, but I've had good luck getting various bearings from McMaster-Carr; not the cheapest, but they ship fast and sell single lot quantities, making them ideal for prototype development. 

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