This is how I made the drive system for version 2 of my BB-8. It's commonly called the hamster because it works like a hamster running in a hamster ball to make the hamster ball rotate.
As previously stated, one of the goals for version 2 of my BB-8 was to make a more powerful drive system so that I could have more weight in the bottom of the ball. That was done by using drill motors from cordless drills and drill batteries. That also gave me the needed weight. To make turning work well, it's also a tank drive, or differential drive. That means that to turn, the two motors rotate in opposite directions.
In the photo below is the drive system in the bottom half of the ball for testing while I was finallizing the circuit. Note that the rollers are attached to the two ends of the drive plate below but weren't attached yet at the time the above photos were taken. (They're the small brass colored objects at the uppermost end and lowermost end, just touching the inside of the ball.) There's more about the rollers at the bottom of this page.
The following video is part 1 in a series on making this BB-8 droid and though it includes putting together the drive system as shown on this page.
The drive plate
The drive plate is a piece of 1/4" thick acrylic (or possibly Lexan, a polycarbonate plastic) that I cut out using my scroll saw.
The wheel and gear assemblies
Three gears were needed to go from each motor shaft to the wheels since the motors were on the sides and the wheels in the middle as you can see from the photos at the top of this page. For each motor/wheel the same sized small gear is at the motor shaft and the wheel and then a bigger gear is between the two. I designed these gears in Blender (3D modelling software) and 3D printed them. The STL files are available below. Note that for some unknown reason I had to drill out the center holes to 1/4" as they came out smaller.
bb8_v2_small_gear_for_motors_and_wheels.stl - 55KB, STL file format
bb8_v2_bigger_gear_for_in_between.stl - 74KB, STL file format
The first mechanical components I made were the wheel assemblies with their gears and bearings. You can see the bearings in the photo below. They were bought from a local bearing supply store such that they were small and had a 1/4" inner diameter so that I could use 1/4" bolts and threaded rods as the axels. I also needed somewhere to house the bearings, holding them in place. For that I cut the small blocks of hardwood to make bearing housings, also shown below. I used a spade drill bit to make the big hole.
As shown below, I wanted two bearings per axle. Basically the recipe was a nut, a washer, a small lock washer, a bearing, a nut to act as an internal spacer, another bearing, another small lock washer, another washer, and another nut. The two outer nuts sandwiched together tightly on the ends. The bearings, lock washers and center nut were fit inside the hole while the washers were larger than the hole and acted as endcaps to keep the other parts in the hole. But, even though the whole thing was compressed tightly together, the washers did not press hard against the wood. Instead the washers touch the wood lightly and the whole thing rotates easly while the wood stays in place.
Next, as shown below, the gear was tightened to the axle with washers and nuts on either side.The same could not easily be done with rollerblade wheels though. If the rollerblade wheel was tightened to the axle, the inner part of the wheel would stay with the axle but the outer tire of the wheel would spin freely on its bearing. To fix the tire part firmly to the axle and have it turn only when the axle turned, I made what I called wheel locks using some aluminum bar that I'd cut and drilled into the shapes shown below. A screw was then put through one of the holes in the wheel lock and screwed into a part of the tire that conveniently already had a hole. Once fully assembled, this would fix the tire relative to the lock and the axle.
As shown below, then the axle was put through the other hole in the lock and then nuts tightened against this and the wheel on both sides. The tire part of the wheel now rotated only with the axle.
Also shown below, two more assemblies were done in very similiar fashion except with a larger diameter gear and no wheel. This was all installed on the drive plate.
Note that a lot of time and patience was taken to make sure the bearings and parts in the housing all turned as easily as possible, mostly making sure that the washers didn't press on the bearing housing, restricting the rotation. The power from the motors had to be translated from the motor, through the gears and bearings, to the wheel. Any losses at any step along the way reduces the final torque at the wheel, and I wanted all the torque I could get.
Mounting the drill motors
I needed some drill motors and this page explains how to get drill motors from cordless drill. I also needed a way to mount the drill motors that was strong but that did not take up a lot of room on the drive plate since I still had other things to keep room for. I settled on making some metal brackets using 1/8" thick aluminum bar from Home Depot. As shown below, I put the bar in my vice and made a 90 degree bend in it. After cutting it and drilling a hole, I also put a Dremel cutting tool on my power drill and cut a slot in the elbow of the bracket.
As shown below, the slot in each bracket was for putting the bracket on a hose clamp for attaching to the motor. For each motor, three bolts were then added to the drive plate and the motor was lowered onto these. After making sure the gear on the motor shaft meshed and transfered power efficiently to the wheel, the bolts were tightened.
I later found that using just nuts wasn't enough to keep the motors from moving under the stress and causing the gears to slip. As shown below, I went back and added washers and lock washers.
Attaching the drill batteries
The drill batteries are for powering the drill motors of course, but they're also mounted as low as possible to act as heavy weights for contributing to the stability of the BB-8 droid. As shown below, to attach them to the drive plate I started by cutting up some acrylic pieces and sewing velcro strips to the pieces. I then used the velcro to strap the acrylic peices to the tops of the batteries.
I next drilled holes in the drive plates for bolting the batteries on. I then drilled matching holes in the battery's plates.
To make sure there wasn't unneccessary room taken up by the heads of the bolts, as shown below, I countersunk the bolt heads in the acrylic. I then added more nuts to the bolts so I could adjust the spacing between the drive plates and these battery plates if needed. Finally, I pushed those bolts up through the holes I'd made in the drive plate and used nuts to fix them in place.
Rollers for the drive plate
As shown in first the photo below, two rollers were added to the ends of the drive plate. The rollers were taken from the bar shown below that had a bunch of rollers mounted in it. I found this bar in a local hardware store and have no idea what its original purpose was. I removed two rollers from this bar.
If you can't find a bar of rollers like this then you can make your own rollers from a brass tube from a hobby store and marbles as I show how to for my BB-8 version 1.
Notice that the rollers each have a narrow stem. I drilled holes in two pieces of dowel for this stem to fit into and hot glued them into the dowels. Ideally the dowel should have a diameter that is the same or larger than the larger diameter part of the rollers.
To mount the roller on the plate, two slots were cut out of the plate at either end by melting the plate using an old soldering iron. Then a hose clamp was pushed through the slots and tightened around the roller, holding the roller firmly in place. By loosening the hose clamp it's easy to move the roller in and out to adjust its distance to the ball.