Hand Drive Version Two


Kate Reed

We are taking the hand drive to a new level for independent studies. We are completely redesigning it. We realized with our last model that we couldn’t move backwards, and that was a problem.

We are now remaking the hand drive without the free wheel mechanism. The free wheel was what was preventing the wheelchair from going backwards. We are making our own ratchet mechanism, which is very similar to the free wheel, but it allows us to go backwards. Instead of having one ratchet like the free wheel, it has two ratchets each on a different plane. Depending which ratchet is engaged, it allows the wheelchair to go either forward or backwards. The two ratchets are connected by cables which allows the user to disengage one ratchet and engage the other through one swift and easy motion. 

Once we made our wooden prototypes in place it was time to start making 3D models. We started the modeling process with a clean slate. We made a new spider attachment that will fit more snug to the wheel, so hopefully it won’t pop off as easily. The gears for the ratchet are also on the spider attachment. We made the gears with a plate in between them so that the ratchets won’t slip out of place. Through testing the gears, we learned we don’t want the gear to curve as much, but want it to be more of a right angle. It really takes a minimal amount of distance for the gears to cover, for them to be functional. 

The lever piece houses the ratchet and the cabling. The lever is on the outer side of the ratchet. The lever piece consists of a disk with holes placed for the ratchet pieces to fit into. It also has pulleys for the cables, and a hole through the center where it screws into the spider piece. The level isn’t as complex of a piece compared to the spider, but it has to line up and fit perfectly with the spider, which is a frustrating little challenge for us to have.

Once we really started to chug along with our model, disaster struck. Fusion, the program we had been using for modeling, had decided it didn’t like our model anymore and therefore it wouldn’t work. We ended up having to remodel the entire project in different files. We lost a day and a half of work time in this process. 

When we remade the models in different files, and got the project back to the point it was before, we started moving forward again. One of our design challenges was figuring out the springs in the design. The springs push the ratchets into the gear, and are essential to the design. In our initial wooden prototypes we used coil springs. The unfortunate part about those springs is that they are too bulky. We want our project to be as slim and sleek as possible, so we started looking into other options. 

We ended up creating a spring from a piece of steel. It is a thin strip of steel that wants to stay straight. We found that if we use it in our design a little bent, it will want to stay straight- thus acting as a spring to get straight. We modeled and printed a prototype with this design. We found that the steel spring was pretty hefty, and too much spring. 

Once we realized the steel spring was too hefty, we started to come up with an alternative. We made a design with the regular coil springs as a back up in case the steel doesn’t work. I am personally routing for the steel springs, because it is a much more elegant design. 

When we had figured out the basic ratchet mechanism, we had to figure out how to make it work with cables. Only one of the ratchet pegs can be down at a time, and depending on which ratchet is down determines whether the user is wheeling forward or backwards. The cables are used to easily switch which ratchet is down, thus changing the direction of the wheelchair. 

The tricky part is figuring out how to use one cable to cause one ratchet to go down and one ratchet to go up. We were originally thinking the ratchets would each want to go a separate way naturally, because of the springs. When the cable was pulled, it would push one ratchet down and pull the other up. This wasn’t the most elegant solution, because the cables were getting tangled with the gear when the ratchets spun. 

Our new solution is to have the ratchet pegs be different. One ratchet peg attaches to the top of the cable and another ratchet peg attaches to the bottom. This way when the cable is pulled, it pulls one down by its back, and one up by its toes. We haven’t tested this solution yet, but it is looking good.

Throughout the project, we are finding more and more similarities to a bike. All of the cabling for our project is bike cabling, and we are going to use a bike break for the handles. It is interesting to compare a wheelchair to a bike and see the similarities.

The new springs came in. They are the same as the steel springs only less intense. We had to remodel our mechanism to take into account the new springiness and angles of the springs. We found that it was faster to just 3D print the pieces extruded from the base rather than print the whole base again and again. We used laser cut wood to replace the base in our prototyping.

We also redesigned the spider attachment for the wheelchair. We made it so it uses less material but still has the same structural integrity.

We added double sided tape to our gear to soften the sound of the ratchet. The lighter spring makes the system a little quieter, but the double tape seals the deal.

We realized that we had too many ratchet problems at once, so we decided to isolate each of the problems and deal with them one at a time. We started working on the individual pieces of the ratchet as opposed to the entire ratchet mechanism.

We were having a hard time getting the springs to work on the ratchets, and having them do what we wanted them to do. We realized that we needed longer springs, so that they would distribute the pressure, and have more springing power. We also realized that we needed to redesign the actual ratchet, so it would have space for the bicycle Knarp, which is the part that secures the end of the cables.

We made all these changes and it seems as if the right ratchet is set to go.

Part of the reason we had such a hard time making the ratchets is because the angle that the ratchet locks into the gear has to be very precise. If the angle is wrong, the ratchet has rotational force, and doesn’t actually lock into a stop. It is harder to place the ratchet that is in the upright position naturally, because we have to place the holes for the ratchet as if the ratchet is down. We are doing the guess and check method. 

We have been working on the placement of the ratchets and the springs and getting them just right. We went through and tried every single one of our prototypes again and really analyzed each one of them. It was good to see them all together. We decided that the sides should be as symmetrical as possible for the ratchets and it would be simplest to go back to the normal springs.

The steel springs are officially too unpredictable and hard to work with. We had the concept down and how the mechanism should work theoretically, but the steel springs were simply too unpredictable and were slowing us down, so we abandoned them. 

We decided to try out the traditional coil spring. This was just what we needed.