I recently started my YouTube channel, Finally Functional. Currently, I’m uploading videos about a project I’m working on in my spare time, which I call the VR shoe.
The VR shoe is a motorized shoe that is intended to be used when playing virtual reality games. The shoe is meant to keep you in the same place as you walk, like a treadmill. Unlike a treadmill, the VR shoes are omni-directional, meaning that you can walk in any direction, or even strafe side to side. The VR shoes will allow you to put your VR headset on, start up your game, and walk around in any direction in the virtual world while staying in the same spot in the real world.
In this article I’ll talk about some improvements I made to the wheel hub drive from the last article and some other designs I have tried since then.
The planetary gear I was using used 5 planets. You can see in Figure 1 that the planets are so close together that they actually touch each other. This was causing extra friction so I tried 4 planets instead, shown in Figure 2. This seemed to work better.
Figure 1: Helical planetary gear with 5 planetsFigure 2: Helical planetary gear with 4 planets and sleeve bearings
In Figure 2 you can also see that I inserted sleeve bearings. I hoped that the sleeve bearings would help decrease the friction caused by the planets spinning around the bolts that attach them to the Y carriage.
I ended up redoing the rear motor mount, shown in Figure 3. Also shown are the 3D printed bearings I used that go around the motor.
Figure 3: New rear motor attachment
To test the assembly’s strength I tied a rope around it and attached the other end to the ball transfer skateboards I made, as shown in Figure 4.
Figure 4: High load test
I stepped on the skateboard and turned the motor on to see if it could pull me (I weigh about 160 pounds). The results were disappointing. It was not able to pull me when I stood on the skateboard with the 3D printed ball transfers. It was able to pull me when I stood on the skateboard that had 5 ball transfers I bought from McMaster, but it took a lot of current to do so (I believe around 30 amps).
The planetary gear seemed to not be causing much friction once I went down to 4 planets and put the sleeve bearings in. I thought that maybe the bolts that go through the planets were being pulled to one side when under a heavy load, causing the planets to tilt a little. But after taking a video of the planets under load it looked like they were not being tilted.
I believe the problem is mainly with the 3D printed bearings shown in Figure 3 and discussed in part 1. I don’t think they can handle the load I’m trying to put them under. Since there is only one ring of BBs, and the motor and hub spin at different rates, the BBs probably do more sliding than rolling, causing friction. I may have to redesign the bearings to have 2 rings of BBs, one for spinning with the motor and one for spinning with the hub. The plastic parts may also flex under high load, causing more friction.
Other Motor Drive Designs
I created a few more designs that I hoped would be able to handle heavy loads. These designs focused on minimizing friction. The first is shown in Figure 5.
Figure 5: Small wheel design
This design drives a smaller wheel. I wanted to see if the motor could provide the torque needed to rotate the large gear I slid around it. I cut out the wood pieces on my CNC. The wheel was printed out of TPU and attached to the gear via 1/8″ steel rods, as shown in Figure 6 and 7. The tire tread was copied from here.
Figure 6: Wheel, gear, and 1/8″ steel rods unassembled Figure 7: Wheel, gear, and 1/8″ steel rods assembled
I performed the same kind of load test as before, shown in Figure 8.
Figure 8: Small wheel motor drive being set up for testing
Disappointingly, the this motor drive was not able to pull me either, even when I used the skateboard that used the ball transfers I bought from McMaster.
The last design I tried is shown in Figure 9.
Figure 9: Gear train motor drive
This design used a simple gear train to get a gearing ratio of 4:1 between the motor and the longboard wheel. The wheel was attached to the gear using 1/8″ steel rods like the last design, as shown in Figures 10 and 11.
Figure 10: Wheel, gear, 1/8″ steel rods, and other parts unassembled
Figure 11: Wheel and gear assembled using 1/8″ steel rods
This design was tested like the others, as shown in Figure 12.
Figure 12: Testing the gear train design
This design actually worked! It was able to pull me when I put all my weight on both of the skateboards shown in Figure 12. The current it took to pull me is shown in Figures 13 and 14.
Figure 13: Current draw when the gear train design pulled my weight while I stood on the skateboard using my 3D printed ball transfers Figure 13: Current draw when the gear train design pulled my weight while I stood on the skateboard using the ball transfers I bought from McMaster
You can see that the ball transfers from McMaster used less current. It takes more force to start pulling a load from a dead stop than to keep it going, so I believe that’s why the current is highest in the beginning (and the startup boost may be too high).
Conclusion
I’m going to move forward with the gear train design. I’ll improve the design in several ways. For example, I’ll try to redesign it so that the middle gear is supported on both sides and I’ll probably make the gears thicker.
Figure 2: 3 bearings going around the motor and attachment for the rear shaft
First I designed bearings that go around the motor. The motor is an outrunner, so the outside spins when the motor is powered on. 3 of them are pushed onto the motor, as shown above. I also 3D printed 2 spacers to keep the bearings from moving. Also shown below is the attachment I made that screws into the rear of the motor. A 5/16″ square rod is inserted into the attachment.
The other end of the square shaft is inserted into the sun gear of a planetary gear, shown above. The gear was modified from this design by emmett on Thingiverse. With his design, you normally print the ring, planets, and sun gears all at once. The parts will be slightly fused together, so you need to break them apart by forcing the sun gear to turn. Instead, I broke the ring gear into 2 pieces so that I could print each part separately and then assemble them.
Figure 5: Y carriage
Shown above is the Y carriage that attaches to the planetary gear. 5 bolts are going through it. The bolts are what will attach the Y carriage to the planet gears.
Figure 6: Y carriage and motor
The motor and bearings go inside of the Y carriage (or hub I guess you’d call it?), as shown below.
Figure 7: Spacers for the bolts that attach the Y carriage to the planet gears
I printed some spacers that go on the bolts, shown above.
Figure 8: Y carriage attached to planetary gear via 5/16″ boltsFigure 9: Nylon nuts are used to secure Y carriage to planetary gear
Shown above the Y carriage is attached to the planetary gear. I used nylon nuts to secure it. The nuts are screwed down just enough to prevent the bolts from wiggling around as the gear spins, but not enough to cause extra friction.
Figure 10: Motor inserted into Y carriage hub
Shown above, the motor and bearings shown in Figure 1 are inserted into the Y carriage hub.
Figure 11: Cap that screws into the other end of the Y carriage hub
Next, I made a cap that fits over the other end of the Y carriage hub to keep the bearings from coming out.
Figure 12: Mount that front of the motor attaches to
Shown above, I mounted a piece to the front of motor.
Figure 13: Cap that fits over the motor shaft
Finally, I made a cap that fits over the shaft of the motor.
Figure 14: First test of the hub motor
Shown above is my first test of the wheel hub drive. You can see when I zoom in on the planetary gear that the sun gear is spinning faster (4 times faster) than the planet gears and the Y carriage. This is the gearing that I wanted to achieve.
Figure 15: Tire that fits around the Y carriage hub
Since I didn’t need it for testing, I didn’t print the tire that will fit around the Y carriage hub yet, shown above in black.
Improvements
There are several improvements that I’d like to make to the wheel hub drive. First, the planetary gear was printed with too small of a tolerance. The ring, planet, and sun gears fit together too tightly making them hard to turn. The gears are generated using an OpenSCAD file and the tolerance is one of the parameters. Increasing the tolerance should solve this issue.
Second, I’m going to replace the spacers shown in Figure 7 with metal washers instead. I believe washers will have less friction and better handle the heat generated by the turning.
Third, I believe I only need 1 bearing, not 3 as shown in Figure 2. I would only leave 1 bearing on the end to support Y carriage hub. With only one bearing, there will be less friction. However, I’m worried that if I take the middle bearing out, the Y carriage hub will strain under the weight I plan to put it under. To help, I’m going to try to redesign the Y carriage hub so that I can reinforce it with at least 8 1/4″ steel rods.
Fourth, and finally, I’m going to see if I can make the bearings shown in Figure 2 without using glue. Even though the glue seemed to work very well, I’m worried that it will degrade over time. I tried to redesign them to use snap-fit joints instead, as shown below.
Figure 16: Redesign of bearing to use tabs
However, they didn’t seem to work too well. I may try to make the snap-fit joints work, but right now I’m thinking that I might be able to use zip ties instead.
I will make these improvements and then post the results in part 2. If I’m happy with the results, I will upload the 3D model files to Thingiverse. Stay tuned!
