Many years ago I saw a really cool apparatus invented by Steve Rea, a local physics teacher, for experimenting with concepts of rotational motion. It was a simple system of stacked pulleys of decreasing diameters. Metal rods with weights allowed the moment of inertia to be easily varied. The device was elegant in its simplicity and offered the opportunity for incredibly rich investigations and discussions.

As it turns out, Steve’s brother owns Arbor Scientific. Arbor adopted Steve’s design and they now sell it. Arbor’s Rotational Inertia Demonstrator works amazingly well and is very repeatable. I highly recommend it. At $160 cost really is quite reasonable for such a well built piece of lab equipment.

When I saw it for the first time I wondered aloud if I might be able to 3D print one. Steve told me I would have difficulty reproducing it because it was nearly impossible to find bearings that lacked grease. Grease is added to bearings to protect the metal bits from corrosion and increase the life of the bearing in high load/high speed operation. The viscosity of the grease in most bearings is too high for this sort of application. It would not allow for a good transfer of gravitational energy to rotational energy. Steve told me the hardest part of creating his prototype was finding suitable bearings. At the time I wasn’t interested enough to try to source acceptable bearings so I let the idea lay fallow.

Then, last year, the fidget spinner craze happened. Several months ago I was having a conversation about 3D printing with Andy Mann. He told me how his son designs and prints his own fidget spinners. Andy also related how his son is so into this he found a YouTube video showing him how to degrease his bearings to make them spin longer.

It took a day for the light bulb to turn on.  Two days after that I had a set of bearings from Amazon (I love Prime) and my prototype of a 3D printed fidget spinner. I started with this to make sure the degreasing worked and that I had dialed in the perfect size to hold the bearing.

With that done I knocked out a quick prototype, which failed utterly. Two versions later and I had it done. You can find my design on the Thingiverse and if you’re interested you can tweak it however you’d like in Tinkercad.

There are three different pulleys to provide different amounts of torque. With an extra set of hex nuts the washers can be positioned different distances from the center of rotation changing the moment of inertia. There are a lot of experiments that students can do to investigate rotational motion and the transfer of energy.

• • 24.5 mm
  • 49 mm
  • 73.5 mm

• • 2 each Skateboard Bearings, 608ZZ 8x22x7
  • 4 each 6″ long 1/4″ bolts
  • 12 each 1/4″ hex nuts
  • 40 each Fender washers, 1/4″ hole 1.25″ diameter (the number of fender washers can be varied)

As I mentioned above, the bearings need to be de-greased first. The grease protects the bearings from water and road grit, but it will keep the bearings from spinning freely. I used acetone since we had it in the chemistry supplies. I just dropped them in a small beaker for 20-30 minutes. I found I also had to take them out of the acetone and spin them a couple of times then drop them back in. A lot of the instructions on the net direct you to remove the metal shields. I’ve found you don’t need to do this. However, if you get “sealed” bearings you will need to remove the seals. You need two bearings, one in each end, for full support.

If you want to avoid using acetone do a little googeling for other ways to de-grease bearings. There’s lots of stuff related to fidget spinners kicking around right now.