Page Comparison

Operation Fundamentals

Planetary gears are a three part gearing system consisting of a sun gear, planet gears/carrier and outer ring gear. When operating, either the ring gear, planet gear carrier or sun gear is held fixed and one of the other two components serve as the driven and drive gear. When used as a motor reducer, typically the sun serves as the drive, the carrier serves as the driven output, and the ring gear remains stationary. 

Calculating gear ratios for planet gear are pretty simple. Once you have established which components are fixed, drive and driven you can figure out ratio by dividing # driven teeth / # drive teeth. To determine number of teeth you have, you can count can teeth on the sun and output rings, then # of planetary teeth in mesh are equal to # sun teeth + # ring teeth. 

Planetary gears are a lot better than standard spur gears, as they can reach ratios up to 10:1 in a single transmission stage. This is a much higher ratio than single stage spur gear options. I should mention this now, but limits on transmission ratios for standard and planetary gearing, or any gearing for that matter have to do with realistic sizing usage. Most applications of gearing operate under a space restriction, and transmission ratios typically are proportional to the difference in diameter of the two gears. Tooth pitch for driven and driving gears has to be the same to allow the teeth to mesh properly, so as you continue to scale things up ratios, gear operations become impossible without lowering pitch (which increases tooth spacing so high that the gears will be useless).

Backlash

Planetary gears can also suffer from backlash, with sources quoting 90-180 arcmin (1.5-3 degreees of play). That seems like a shit ton tho, and IDK if its that bad in real life. I think we would need to contact suppliers when determinig the true backlash of these gears (if we plan to buy it). I also don't know if that quoted backlash value is listed for input or output, as that would vary the resultant backlash. I've seen some high precision planetary gears that offer much lower backlash, so honestly I'm not 100% how to quanitfy expected backlash. Sources claim that backlash in planetary gears is lower than other gear types, but with no values provided. This is probably because for similar external gearing profiles, planetary gears can maintain the lowest tooth pitch due to their 3 component build, which helps reduce experienced backlash

Custom Manufacturability

If we want to use planet gears, some form of custom solution will likely be necessary. We would probably need wire edm to manufacture precise gears, and can probably test meshing and models with a 3d printer. Custom planetary gears would probably be limited to using standard spur tooth profiles, which is probably ok for our purposes. There are some good tutorials online on how to set up custom planetary gears.

OTS Options

There are some suppliers for planetary gearing sets, but typically we would need to integrate some sort of custom planet carrier to really use a planetary gearbox in our arm. As such, going custom for some of the parts would be a much better idea IMO. 

Joint Locations

In construction

Planetary gears would be useful in joints where high reduction stages are required. While they do have backlash, there are some techniques to mitigate that (like the video in the belt drive section which uses a second stage to reduce backlash and increase reduction ratio). Planetary gears can offer the highest single stage reduction out of all other gear types in a small profile and are super compatible with second stage reductions, as you can use coaxial shafts to easily set up a second stage. They might be pretty easy to use on the elbow