Hand Drive Version Four


Kate Reed

Once we had the center ratchet design  working, it was time to face the facts. Although the planetary ratchets allowed for the Hand Drive to stay in one piece, it still didn’t fix a big picture concept that we wanted to address. The Hand Drive is slow, and we feel we can make it more useful to the user if we address this. To make it faster we have to use planetary gears. In trying to learn about planetary gears and how they work, we met with a bike mechanic, Jason, from Broadway Bike. We realized that what we wanted to do was very similar to how a bike works. With Jason, we took apart a bike hub, and he showed us step by step what each part does. 

We learned that with one set of planetary gears you can have three different speeds depending on what gear you’re driving - “driving” meaning which gear is being spun. You can drive the planetary gears, which is high gear and the fastest gear. If you drive both the planetary gears and the annular gear (the ring gear) together that is considered neutral, meaning it’s a 1 to 1 ratio between pedaling and the wheel. Driving just the annular gear is the slowest gear combination and is considered gearing down.

The bike hub works with a clutch that moves up and down pulled by a cable. The cable is under tension so that when you click to switch gears it pulls the clutch at just the right tension to engage either the planets, the planets and annular, or just the annular gear. 

Knowing all this caused us to completely rethink our entire design, again. Learning about the concept of a clutch really changed our mindset, and we came up with two different variations of the Hand Drive involving the planetary gears.

The first was the same as our current planetary ratchet design but with a clutch instead of the detent, as well as planetary gears embedded into the base. The idea was that when the clutch is down, it engages the bottom ratchet and locks into the planetary gears, but when the clutch is up, it unlocks from the planetary gears and switches the ratchets into reverse. In this design the Hand Drive was only geared in the forward position.

The second design involved an extremely similar mechanism to the bike hub. It had a set of planetary gears in the middle and a ratchet mechanism on either side. One ratchet mechanism was always engaged with the planetary gears and the other would engage when the clutch was pulled. The ratchets would be in opposite directions. 

In the end neither of these solutions worked. The first didn’t work because the planetary gears weren’t actually doing anything, because the gears were not connected right. The second solution didn’t work because the ratchets ended up locking each other in place so the wheelchair wasn't able to move.

Through long debate and lots of headaches we came up with a solution that would work. It is a combination of our two previous solutions. This solution involves a set of planetary ratchets with a ratchet on both the top and the bottom, with the ratchets facing opposite directions. The ratchets and planetary gears move as a unit either up or down when the clutch is pulled. The entire unit of ratchets and gears is inside an outer shell. The outer shell has the gear that the ratchets latch onto. When the clutch is pulled it pulls the entire set of gears and ratchets into the reverse direction, and when it is released it has a spring to pull it back into the forward position. 

So far we have modeled the planetary gears, the gear cage and the ratchets. We are thinking that we are going to end up milling a lot of the parts out of aluminum, but reuse as many of the bike parts as we can. We are starting to feel the time pressure of the end of school and are working hard!

So far we have been through five major prototypes and dozens of sub-prototypes. It's been a long process but we finally feel like we are getting somewhere significant. We now have three final stages of the Hand Drive, one with a single double ratchet, one with three double sided ratchets, and one with planetary gears incorporated into it. Although we have gotten each design to a high level of functioning, we are still working and still trying to improve our designs.

In the big picture, one of our goals is to work towards a design where the planetary ratchets can be 3D printed instead of milled. We never want to lose sight of our goal to make the Hand Drive affordable and accessible for all. Wheelchair user statistics inform us that wheelchair users are very unlikely to have jobs and, partly as a consequence, are substantially more likely than the remainder of the population to live in poverty. At all ages, income levels for mobility device users tend to be low. If we can pull this Hand Drive off we can significantly improve the quality of life for many- at a price point they can handle, and with the personal satisfaction of building their Hand Drive themselves with their own specifications.