Gear pitch

I was looking at creating a custom planetary gearbox using off-the-shelf parts (no CNC) and I came across something interesting: There are no 20 pitch ring gears. It goes from 16 pitch to 24 pitch and skips the 20 pitch mark.
16 pitch gears are too large to get any appreciable ratios, so I was wondering what people thought about 24p 20* PA gears for drivetrain? The gears come in 2024 aluminum, 303 stainless steel, brass, or some kind of carbon steel (not sure which). Because of the design of planetaries, this means the whole gearbox would be made of steel/aluminum/brass 24p gears.

Also, I was wondering what pitch/module gears were used in the versaplanetary so I could approximate load ratings.

We did a project a while back with a planetary gearbox for the drivetrain. For a ring gear, we used the ring gear from a GEM planetary from AndyMark. It’s 20 DP if I remember correctly.

I’d go ahead and do the actual stress calculations for yourself (and you seem to be interested in doing that anyway), but I’d be inclined to believe 24 DP should be fine. Whether or not a specific gear will work for you depends on face width, the actual tooth profile, the material, and of course the application. You need to develop a good, conservative idea of the stresses that these gears will experience before asking questions about whether a specific gear is appropriate.

I’d guess that the gears for VersaPlanetaries are metric, and I’m fairly certain they have addendum modifications. You might have some luck contacting JVN or Paul Copioli on CD or via the VP site, but there’s always the possibility that they won’t release specifics on the technical details of the VersaPlanetary gears. If you just want a rough approximation of the pitch, count the number of teeth on a given pinion and measure the approximate pitch diameter with calipers or a ruler. Toothcount over approximate diameter would give you an approximation of the DP.

Thanks! I didn’t think of the GEM, and you are right on the 20 DP thing. They are a little large for my application (no 1.875" bolt circle) but I might be able to modify it.
40t ring, and the wide selection of CIM gears plus the 15t GEM gear give me a lot of options for gearing.
I was looking around and the Versaplanetary does have addendum mods to prevent over-meshing.

The addendum mods on the VP are to avoid undercut on the sun gears. The remaining mods are to make sure each ratio combination can work with the same ring gear and can be assembled.

I urge you to research the different ways one can screw up a planetary design. Things like assembly, vibration, and interference are all valid considerations that are equally important. Some combinations of valid tooth counts can’t actually be assembled. The information is easy to find.


Based on measurements from the CAD model of the VersaPlanetary 3:1 sun gear, it looks like they are 0.50 module.

The 9:1 gear stage is cool. 31*2+“9”=72