# How to create gear ratios for Arm rotation

Hello!

We have a few mechanisms on our bot this year: a rotating arm, a AM climber in a box on the arm, and an intake. All of these are powered by a NEO brushless.

We figured the following gear ratios would be good from the JVN calculator.
arm rotation: ~120:1 (we need a 360:1 total reduction, but we have an essentially 3:1 sprocket).
AM climber telescope: ~140:1 (We’re not sure if this is right. It’ll extend ~1m and be ~30kg, so we assumed it would be (1 * 30 * 9.8)/3.75=80 N/m, and we doubled it to maximize power).

We’re not sure how to actually create these gear reductions, however. We were planning on using a VersaPlanatery, but we just saw that they don’t really support over 45:1 for a NEO.

We’re pretty confused now, because we’re not really sure how teams are able to create such high gear ratios.

Does stacking versaplanaterys fix this issue? (i.e. will using 2 versaplanaterys with a 45:1 and a 7:1 work to make a 315:1 without exceeding the VP load ratings)?

We wanted to stick to using COTS solutions like the VersaPlanatery’s, since we don’t really have the tools and manufacturing methods to create our own gearboxes. Does anyone have any solutions in mind?

“30kg”

Is that meant to be 3?

This obviously matters very much in calculations.

Probably not, but I’d agree thats HEAVY. For other 'muricans, thats ~66lbs, which is over half the allowed weight of the entire bot.

@DespicableMonkey, can you clarify what exactly was measured to get this 30kg figure?

The Climber in a Box springs are only 3.5lb, so you could never raise the arm if it weights 7# (3.1kg).

Also, back to your original question - no, stacking multiple gearboxes does not change the maximum load rating.

Some of AM’s HD Sport gearboxes go to 170 ft-lb (230N-m).

Generally with a custom or COTS gearbox with a final chain reduction.

No it doesn’t. You really, really shouldn’t use a VersaPlanetary gearbox for a long heavy arm such as the one you have described (I’m assuming you meant 3kg
not 30kg, so around 7lbs).

Your best bet at this point given that you shouldn’t use a VersaPlanetary and can’t make custom gearboxes is to use another COTS gearbox instead. My go to recommendation is the REV MaxPlanetary but that’s out of stock, so I’ll instead recommend the Andymark CIMSport which are also built like a tank. Other non planetary COTS gearboxes can work as well but will have more packaging constraints.

I’ve posted a lot recently about some best practices for these sorts of high torque arms so I’ll go ahead and link to one of those posts:

Not sure how exactly you have your telescoping arm setup, but 140:1 seems to be a pretty high-gear reduction. Do you have any idea what your spool diameter is, load (30kg does not seem right unless you are using it for a buddy climb maybe?), and motors? I like to use Reca.lc Arm for arms and Reca.lc Linear Mechanism for elevators+telescoping tubes, as it is online and easier to share with others.

For reductions, do not stack versaplanetarys as others have said. AndyMark CIM Sport is a great solution that has gear ratios up to 100:1. I would also reccomend the REV MAXPlanetary but these are out of stock at the moment.

For arms you would usually want some type of chain reduction at the end as chain takes sudden forces better (ie a robot hitting your arm) than the teeth of gears do.

You could also achieve “custom COTS” gearboxes using the MAXTube pattern/Ratio Plates but that is bulkier.

(The Idea would be to have different-sized gears/sprockets for more gear reductions along MAXTube. Photo shows a 1:1 after the MAXPlanetary reduction)