Axle Stability Brainstorm

We went to the National MakerFaire in DC and saw some really cool projects and products.

We didn’t actually have the gearbox working by the time we got to DC because the gears didn’t mesh. I was able to get that working yesterday when I figured out what gear modules actually were. There’s a formula that relates gear module to number of teeth and reference diameter.

Module = (Reference Diameter in mm) / (# of teeth)

I knew the number of teeth and the module of the gears I had so I was able to use the reference circles to make a gearbox that actually worked.

It looks like I screwed up the Faulhaber motor we were given by getting some adhesive into the internal shaft area. This is pretty unfortunate given that this motor was around $50. But it also forced me to find a new motor to work with. Hooking up a cheep motor we found in a box of stuff that Erica gave us to the gearbox actually showed promising results with the output current and voltage.

Spinning the input shaft of the gearbox by hand gave us around 0.7 volts and around 0.5 amps directly from the motor. This is pretty impressive for just hand speed. It looks like this could be a viable product after all.

The next issues I’m addressing is the stability of the entire device. Currently, there is no single shaft that extends through the entire device holding it all together. There are two separate stationary shafts that have a spinning component between them. This causes a discontinuous line of stability, as Myles* calls it, and could lead to failure at that discontinuous point.

(Above: The grey components are spinning while the red, blue, and yellow components are stationary. There’s no stationary component that runs continuously between the grey end caps.)

After some brainstorming with Myles, we decided that it was impossible to make a continuous shaft down the middle of the device while still having the input to the gearbox be in the center of the wheel. There would be interference with the spinning and stationary components that would be very complicated, if not impossible, to resolve.

So we came up with a solution to offset the input shaft of the motor and allow for a continuous support shaft through the entire device. It involves a bearing mounting inside a gear to allow for a stationary shaft to pass through a gear that is mounting rigidly to the spinning wheel of the EnGen. If this sounds pretty sketchy, it’s because it is, but I think there is some good reasoning behind it.

Having an extra set of gears would allow for another reduction and could produce even more power from the device. And pulling the gearbox input away from the center would allow for a continuous support to run through the device.

But this could also add an extra layer of complexity that we just don’t have the time for. The biggest gears we have in stock are too small to fit around the bearing we have and downsizing the bearings would reduce the shaft size, lessening any strength we’ve added to the device. We would have to find bigger gears and then bore out a hole for the bearing to be pressed into. This adds cost and labor to the manufacturing process.

So I’ve got a plan: I will continue along with my current prototype – the one with a discontinuous support system – until we get some sort of working model of it. Liani is still playing around with the electronics but pretty soon we’re going to need to test this baby out. If our first prototype doesn’t show signs of weakness at the discontinuous point in the support, we may not need this offset input idea anyway. Although, my guess is that there will be some obvious weakness and that this idea will have to be implemented eventually.

*Myles Cooper is a rising senior at Olin College who is working in the same lab space as Liani and I.

Starting a Linear Gearbox

Today I CADed a linear gearbox for the motor. I found nylon gears in the ME stockroom and was able to create a 15:1 gearbox in two stacked stages. These are so much better than the planetary gearbox I was trying to create from scratch.

Processes I tried for making my own gears:

  • 3D printing
    • Absolutely sucks for the small teeth profiles I needed. The profiles weren’t even there. They just sucked. Nothing meshed. Nothing fit. Ugly, awful gears that were only saved from being thrown in the trash by the wish to document the process.
  • CNC routing
    • Worked a whole lot better than 3D printing gears but still kind of sucked. The profiles were a lot nicer but it took forever to cut the gears out of 1/8″ plastic. They meshed and fit together very nicely but the time put into setting it all up was way too much.

And then I found nylon gears. So pretty.

I also found some dowel pins that pressed into the gears to get a good, solid axle.

These gears were bought in bulk by someone (no idea who the actual manufacturer is) and can be used in a mass production setting. I like these a lot better than anything that I was able to create myself.

The only issue I had with these gears was getting one of them to fit on to the motor that was provided for us. The hole in the last 10 tooth gear was too big to be pressed onto the motor shaft. It was down-right sloppy. I was hesitant about gluing the gear onto the motor because this is a $50 super fancy motor. But adhesive is what I ended up using.

I used Permatex medium-strength threadlocker (blue) to attach the gear to the motor shaft. The threadlock created a film around the motor shaft that helped to center the gear on the motor shaft. It didn’t take very long to cure and seems to be holding fairly well. I’m sure that if this ends up being the wrong gear to use, I can force it off the motor shaft without damaging the motor.

Currently, I’m designing a housing to hold the gears in place. I’m planning on 3D printing the housing but it could easily be injection molded if this were put into mass production. The entire gearbox will be around 2 inches long which is considerably smaller than the planetary gearbox monster I previously made.

The issue I’m having right now is trying to figure out what the pitch diameter is of the gears I found. I couldn’t find spec sheets on the gears so I have absolutely no idea what any of the designed dimensions are. I’ve contacted Nagy Hakim, who is the ME stockroom NINJA, about getting specs for the gears and I’m awaiting for a response. If he doesn’t get me that information in the next day or so, I’ll just do some trial and error designing to find the correct center-to-center distant for these gears.

Issue with Parametric modeling in Fusion 360

Issue encountered with Fusion:

I had already created a wheel housing for the 3rd version of the EnGen scooter when I created a linear gearbox to go inside. I created the gearbox in a separate Fusion file so that the part was clean and organized. When I inserted the gearbox into the wheel housing, I found it was too large for what I had previously designed.

This wasn’t supposed to be the end of the world – just make the housing larger and slip the gearbox in. But the size of the housing was determined by the very first sketch I made in this Fusion file.

Changing the height dimension from 50mm to 55mm suddenly broke a series of features in the timeline.

I had to roll back to the first error and start editing features and sketches until I was back where I started.