Motors
I started with four surplus Kiddie Car gearmotors that I purchased (thanks to Curt) from Surplus Center (1-800-488-3407 item number 5-1144 @ US$6.99 each). I only needed 2, but I bought 4 in case I needed the parts. While testing, I did break one (more on that later), so I was glad that I got extras.
I have not tested them yet, but I have been told that the motors turn at 172 RPM @ 12 volt with 110:1 geardown. At this point, I do not know the torque, but they are strong.
These motors include a plastic gearbox with a large hub on the drive gear. Inside there is a metal pinion gear pressed onto the motor axle and three plastic gears.
The drive hub had a funny kind of connector that was not well suited to direct connection, but offeres a pretty good surface for connection if you only had the molded piece required.
I first tried using a two part epoxy called PC-7 to make a molded piece to fit the hub. To keep the epoxy from sticking to the hub, I sprayed it with WD-40. I showed the result of this at the last meeting. It works and makes a pretty good connection, but I did not like the way it looked.
I called the Manufacture of PC-7 and asked what I should use as a release agent, and was told to try silicon spray. I did this, but I guess I waited to long to remove it. It was rock solid and in trying to remove it from the hub I cracked the hub. (Good thing I ordered extra motors, eh?)
I happened to stop by TAP Plastics and noticed that they sell something called Quick-Cast. This stuff is a fast curing 2 part polyurethane casting resin. This stuff is great! It is a two part liquid that once mixed, gets hot and cures to a hard plastic in about 10 minutes. Once cured, it tools very well. It costs $15.00 which gives you 1 16oz part A and 1 16oz part B can. Each casting I made uses about 1oz of each (A&B), so the castings cost about $1 each.
I also got really lucky and out of the 8 molds that they had, I found a $2 plastic molding cup (Castin' Craft MC3 - Reusable) that is the exact size and shape needed for the gear hub.
I poured 1oz of each part into two paper measuring cups, then poured them together into a mixing container and stirred them for 10 seconds. I poured the result into the mold and placed the gearhub into the mold.
Within about 3-5 minutes it hardend into a rubbery state. While still rubbery, I pulled it from the mold and from the gearHub. Then I waited for it to finished hardening. 5 minutes later I had a perfect match for the hub.
I then drilled out the center hole using the Hub as a guide and drilled 2 bolt holes.
While this one was removable, I decided that it fit so well and looked so well that I would fill the mold a little deeper and so the liquid would flow thru the 4 slots into the inside of the hub. That way when it hardened, it would be locked to the gearHub. This one was even better than the removable one so I made another to match it.
The Quick-Cast worked so well, I think that I will be using it for other plastic parts as neded. Of course, the final test will be to see how durable it is. This could be a problem as it is so easy to tool.