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Planetary sizing

Planetary sizing

Planetary sizing


I am sizing planetary gear set for an automotive transmission using KISSsys. But when I use its rough sizing feature I get large number of teeth on sun gear as compared to planet gears. My previous experience is always sun gear teeth are lower than planet gears. So wanted to understand if anybody has better approach for deciding no of teeth of planetary gear set.


RE: Planetary sizing

What ratio are you trying to get, and which elements are being driven and which ones are being locked? That's what it's going to depend on.

I don't think I've ever seen an automotive planetary gear set in which the sun gear did not have more teeth than each of the planet gears.

Usually there is at least one concentric shaft going up the middle of that planetary set, often two or three, so having a decent amount of diameter to the sun gear helps to make room for that!


RE: Planetary sizing

Thnaks Brian.
Yes, my previous experiece has been with planetary design of tractor transmissions as a final rediction ratio. Where we had sun gear teeth cut on the sunshaft and we always had configurations where sun teeth were lesser than planet. But as you say it is not the case most of the time in automotive transmissions.
I have a reduction ratio to be achived in tha range of 9 to 9.5. I am using two planetary in line with each set gving reduction of around 3.0. I am locking ring gear in both sets.Input is given to sun gear. Power is taken out from forst planetary carrier and is input to secong sunshaft.

RE: Planetary sizing

Assuming you are using a licensed and therefore LEGAL version of KISSsys, then I would highly recommend you contact them directly. KISSsoft has an office in the USA.
They offer tutorials and help in learning how to operate the software.

RE: Planetary sizing

I am calculating ratio using (Sun no of teeth + Ring no of teeth)/Sun teeth. With this formula I get reduction ratio of 3 but if I use the formula, Reduction ratio = (2 x ring_diameter/sun_diameter) -1 then I get higher than 3.

RE: Planetary sizing

That's because the first formula you are using is the right one, and the second formula is wrong.

RE: Planetary sizing

A simple epicyclic stage with the ~3:1 ratio and sun input/carrier output/fixed ring configuration described should allow a large number of planet gears to be used (maybe six?). Using the largest number of planets practical provides high torque capacity from minimum volume. So I assume that's one reason why your software application rough sizing function gave you this result.

RE: Planetary sizing

That's a good suggestion. Currently I am using 3 number of planets, let me try with more than 3 number of planets.
Thanks you.

RE: Planetary sizing

There are some tooth-count restrictions to watch out for, though. The planets can only fit in places where the teeth will line up properly. If you want three or four or five planets and you want them to be evenly spaced, the tooth-count has to work out in a way that will allow them to mesh.

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