Sunday, September 26, 2010

Pneu Scooter: Δt1

I'm off the Singapore in about three weeks to visit the brand new Singapore University of Technology and Design. It should be an awesome trip - my third time in Asia, but my first trip longer than two weeks. Since it will be all cold and wintery in Boston by the time I get back, I sort-of want to finish Pneu Scooter before I go. Not that I'll have any time left to ride it, but it's just good to have some kind of deadline to force me to do it. (As a graduate student, it now takes me at least twice as long to do things I used to be able to do...)

So, here we go, time step #1:

I don't know why there is mixed fruit in this time step.

I really dislike adhesives. But they are a necessary evil and I am trying to get the sticky parts of the scooter build out of the way early. Gluing the stack of stator laminations together is one of those unavoidably messy processes involving three quick clamps, CA glue, some two-ton epoxy, and a sacrificial brush. The CA quickly wicks into the laminations, tacking them in a few places, then the epoxy makes a final coating on the outside surfaces.

You'll notice the two 1/16" plastic end laminations also glued to the stack. These are slightly oversized versions of the steel laminations that serve to protect the corners of the stack, preventing them from digging into the thin enamel coating on the magnet wire. This is in lieu of a complete epoxy coating on all the steel surfaces (or slot wedges). Hopefully the extra-large air gap will allow enough wiggle room for the epoxy coating and slight skew of the stack. If not, some creative lathe work may be in order.

This is the main wheel-motor shaft, made of 7075 (for the alloy cred). In the wheel-motor design, the shaft is stationary, so it's tapped on both ends for attachment to the rear deck forks by way of 1/4-20 machine screws. The two roll pins slide into a slot on the stator laminations, keying them in place and transmitting reaction torque to the shaft. Finally, the milled slot is one of many ideas I've stolen from Charles about how to build these things. Rather than drill cross holes and try to pass wires through the center of a hollow shaft, as in BWD, the three wires will simply lie side-by-side in the slot to pass through the bearings:

The slot faces upward, while most of the bearing load is borne by the bottom half under normal usage, so this doesn't significantly hurt the structural loop. So, please don't use this design for your inverting roller coaster with in-wheel motors (!!! That would be awesome!), but for a scooter it's fine. The pass-through wires are only 16AWG, but short and surrounded by aluminum, so they will definitely not be the I²R bottleneck.

Here's the stator-on-a-stick, next to the new rotor, also glued and drying now. The reason I don't have incremental pictures of building the rotor is because it literally took 10 minutes. It's made of three 1/4" plates of low-carbon steel, waterjet cut to the same time-saving shape as BWD's rotor laminations. The magnet indents remove all of the hassle of spacing and gluing (though it is still quite possible to crush your fingers). So, it went together so fast and my hands were so sticky that I couldn't take any pictures of the process.

Anyway, that's the current state of things. Next I will need to tackle one of many remaining large challenges: either machining the deck, winding the motor, machining the critical wheel adapter plate, or making the battery pack. Or, you know, all of those. Only the next time step will tell.

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