help with Vex versaplanetary gear ratios

I am new to gearboxes and gear ratios. I need some help understanding the Vex versaplanetary gear kit information. For example, if I look at the Versaplanetary gear kit ratio identification guide for the VP 5:1 gear kit (217-2819) I find the following info. There is a 18 tooth sun gear driving four 27 tooth planet gears. The planet gears fit into a 72 tooth ring gear. Thus, the gear ratio would be (72/27) * (27/18) = 2.666 * 1.5 = 4.0 but this is listed as a 5:1 gear kit. I see that the planet gears are on a plate with a 28 tooth input and the sun gear is attached to a 28 tooth output gear. Does this matter?

Following the same approach, I calculate a gear ratio of 3 for the VP 4:1 gear kit and a gear ratio of 9 for the VP 10:1 gear kit. Each gear ratio seems to be one less than the stated value.

What am I missing? Do I have something inverted?

Any guidance for a novice would be greatly appreciated.
I am definitely :confused:

Dan Dominik

Planetary calculations aren’t that simple. Read for here for a better explanation.

But for the 5:1 stage it’s works out to 18 / (72+18) = 0.2 or 1/5 or 5:1

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Allen, thanks for the quick reply. The link reference you provided was very helpful. I should have done a better search of the web for planetary gear ratio calcs but just tired from a long week of robotics and bag day. Thanks again.

You did the right thing by asking. Never stop learning!