## Using it!

I've decided I need a transmission for testing out the #FilaMecanum concept, so this is going to be it.

## Background

First I think I need to explain **how a normal planetary drivetrain** is constructed.

The power input turns the sun, and the sun drives the planets which rotate within a stationary ring gear. The planets are mounted to a carrier with bearings, and the carrier is where you take your output.

This arrangement gives you a gear reduction of:

To get further reduction, you can add more stages just like the above, all sharing the same ring gear. The stages multiply, so 3 stages of 3:1 would give you a 27:1 reduction. Problem here is now you need 9 planets, 9 bearings, 3 suns, 3 carriers, and 3 times the height too.

If you try to get more than about 3:1 reduction per stage, you end up needing fine teeth and a small sun gear. Small sun gears don't survive very long unless made of some heavy duty stuff and so 3D printing/laser cutting that doesn't work so well.

## How this works

The power input turns the sun, and the sun drives the planets which rotate within a stationary ring gear. So far the same.

However, instead of taking your output from the planets, we **allow the planets to just rotate within the ring, and cover them with a second ring gear**. The second ring gear is constructed to vary from the stationary ring gear **by exactly numplanets teeth**.

Now, on each revolution of the planets the two ring gears will advance with respect to each other *numplanets* teeth. We take our output as the second ring gear and get **massive** gear reduction from it.

This arrangement gives you a gear reduction of:

## The result:

- We can now use a much larger sun gear to drive the planets, and still get significant reduction. Perhaps even wrap the perimeter of a BLDC outrunner.
- We only need one bearing, which essentially supports the joint itself.
- The vast majority of the force applied to the joint is right at the outer edge of both ring gears and drives both in opposite directions at multiple locations.
- We can get away with using much cheaper materials to do it.

Picking a compatible gear ratio is critical, so here are the rules for the math required for the gearing. The simplified version for 3 planets is use a multiple of 6 for both the sun and ring gear teeth, and then an output ring gear tooth count of *ringteeth* - 3.

And here is a video.

Great project. I am using MPRT1.01.scad to compute gears. As we know, MPRT1.01.scad was designed for Inches. When MPRT1.01.scad computes gear parameters by parametric, It echoes wrong Diametral Pitch.

The source code is here:

...

input_pitch=ring_teeth/ring_gear_r/2; // compute Diametral Pitch for inches

...

echo(str("Input Pitch: " , input_pitch)); // echo the Diametral Pitch

When we use MPRT1.01.scad to compute parametric parameters, we should retrofit echo command by multiplying a 25.4

echo(str("Input Pitch: " , input_pitch* 25.4));

Because :

Inches= metricmeter/25.4

Diametral_Pitch = ring_teeth/ring_gear_r_inches/2 = ring_teeth/(ring_gear_r_meters/25.4)/2=(ring_teeth/ring_gear_r_inches/2)*25.4=(Diametral_Pitch echoed now)*25.4