Summary

If you built my Hypocycloid Gear Reduecer (Thing 79231), it is not unlikely that a few questions occurred to you. This Thing will encourage you to explore these questions and learn some interesting things about hypocycloid gear reducers at the same time.

Instructions

Begin by printing one each of the parts. I am assuming you have already build the example (25:1) included in Thing 79231. These parts are designed to work with it. Note that the gear reduction of Thing 79231 is really 24:1, not 25:1. I discovered this by carefully counting the input revolutions required to get one revolution of the output. Then I studied the behavior to understand what's going on. The lower cam on the rotor contributes 4:1; the upper adds 6:1. Sorry for the confusion!

Assemble the parts. You will need:
4 - Bearing 6700ZZ 10x15x4 Shielded Ball Bearing from vxb.com
1 - Bearing 5x10 Shielded 5x10x4 Miniature Ball Bearing from vxb.com
1 â€“ M5x25mm (or the length for your application) hex head bolt. This is the output shaft.
Put two 10mm bearings into the pockets on each rotor.
Put one 5mm bearing into the pocket on the output block.
Put the M5 bolt into the bolt pocket on the output block.

For the purpose of these experiments, let's identify the original rotor as a â€œ4x5 lobe rotorâ€. That is, 4 lobe cam on the bottom, 5 lobe on the top. The new rotors you will make are a â€œ4x4 lobe rotorâ€ and a â€œ5x5 lobe rotorâ€. Call the original output block a â€œ6 roller blockâ€ and the new one, a â€œ5 roller blockâ€.

The first question that might have occurred to you is, â€œHow can I get less reduction?â€ Well, obviously, you could use fewer cam lobes. But you might be concerned about having so few lobes (2 or 3) on a cam. Will it work smoothly? ( Great question! And one I haven't answered yet - stay tuned!) So another approach might be to use just one cam instead of two. But then how to get an output shaft to turn when the cam moves eccentrically? Time for the first experiment.

Dissemble your Hypocycloid Gear Reducer and remove the 4x5 lobe rotor. Put the 5x5 lobe rotor on in its place. Before you put the rest of the reducer back together, turn the motor so the input shaft rotates, moving the rotor eccentrically. Note the motion of the rotor: It's not rotating, only translating in an eccentric pattern! I found this amazing. Now put the 6 roller output block on and assemble the reducer. Make the motor turn (electrically or by hand) and note the output shaft is turning 1/6th turn for each turn of the input (6:1 reduction). So the rotor is only translating, yet the output block is turning. Even more amazing!

Let's understand this a little better. Try the 4x5 rotor with the 5 roller output block. Orient the 5 lobe rotor to engage the lower case (which has 5 rollers). Assemble the reducer (using the 5 roller output block) and measure the revolutions of the input required to produce one output revolution. Although the rotor does not rotate, the 4 lobe rotor drives the 5 roller output shaft to produce a 5:1 reduction.

â€œWhat if I turn the rotor over?â€ I hear you say. Try it! Put the 4x5 rotor in with the 5 roller output block again. This time, orient the 4 lobe rotor to engage the lower case (which has 5 rollers). Assemble the reducer and measure the revolutions of the input required to produce one output revolution. Post your results and let's see if we all agree.

Now for another obvious question. â€œHow about if I just want a little less reduction. Can I use a 4 lobe rotor on both sides?â€ I'm glad you asked, 'cause that was my first question. Install the 4x4 lobe rotor and the 5 roller output block and assemble the reducer. Now there are 5 rollers and a 4 lobe rotor on both levels. I expected a 20:1 reduction. What happens? And, no, you didn't break anything.

These experiments should teach you some useful things about the Hypocycloid Gear Reducer and help you design one that meets your needs. Have fun!

Hypocycloid How To Part 2 - Some Experiments