View Full Version : Working out epicyclic gear ratios.

10-31-2009, 08:44 PM
I've been checking out this site and I'm struggling to understand the formulas, could someone run me through it please?


I have an epicyclic gearbox that I want to figure the ratio for. It looks like exapmple 1 on that page. On my gearbox, S is the input shaft, L is the output and A is fixed.

Input gear S has 9 teeth
Planet gears P have 17 teeth
Annular gear A has 45 teeth.

From turning the input and watching the output I can see my ratio is about 6:1

I have another which is much lower geared and has the following values:


What formula should I use?

John Stevenson
10-31-2009, 09:32 PM
If you are using the setup in fig 1 then ignore the planets, they are only idlers.

The formulae is Ring / Sun +1 so 45/9 +1 =6:1

Your other ratios will be 64 / 8 = 8 +1 = 9:1


10-31-2009, 10:47 PM
Thanks John. I don't understand why they couldn't put that formula on the page I linked to :(

This gearbox is stacked with two 6:1 ratios and a 9:1 so I could have 6:1, 36:1 54:1 or 324:1.

John Stevenson
11-01-2009, 07:39 AM
Yes I read the page and then reached for another book, even after reading what the formulae was that page still made no sense to me.

I think the phrase "Educated beyond the ability to understand" comes to mind :rolleyes: