Cascading gearboxes - VP into MP

Like most teams we’ve been hit by the MaxPlanetary shortage. Luckily we ordered a couple sets before the season started, but not knowing which ratios we’d want we got a mix. Now we find ourselves needing two relatively high reduction gearboxes to manage arm and wrist motion, and not enough stages to cover both.
Our plan is to use a straight MaxPlanetary 5 x 5 x 4 for the arm motion (with a NEO drive, and a chain reduction afterwards). Should be relatively problem free for the gearbox at least.
But for the wrist motion, we’re gonna try something weird and I would like to hear the community’s opinion.
The plan is to use a NEO driving a single stage Versaplanetary (5x stage) with a CIM output shaft, and connecting that into a MaxPlanetary 4 x 3 x 3.
We had quite a few VP gearboxes around from previous robots, including several CIM output shaft kits. It all fits together even if it looks a little weird.

I’m not worried about breaking the Max, but I do wonder about the CIM output shaft. VEX doesn’t give load ratings for a single stage. The math says with a NEO at 80A stall, the VP output shaft would have 8.1 Nm of torque and the listed breaking strain is 29 Nm.

If it works, it seems like it might be a solution for some other desperate teams in need of high reductions, who have MaxPlanetaries (and Versaplanetaries, and CIM output shaft kits) but not the right reduction stages.

This could work but makes for a long stackup that you would want to support so its not all hanging so far off of the max planetary mounting.

Imo if you need another 5:1 stage, I would personally opt for using chain/sprocket or gears to complete the final reduction.

Vex Does not recommend VP with CIM Shaft and a NEO. (sorry I looked to quick this is a double reduction listed in the load guide. I will leave it in the case it helps someone else.)

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VersaPlanetary Gearbox - VEX Robotics

Edit see later post for further review

But that’s for 2 stages, and we’re only using 1 5:1 stage. The smallest reduction you can get with 2 stages is 9:1.

Good point on wanting to support the stackup. I’ll talk to the team to design a support to hold the NEO end.
As for chain & gears - chain is difficult due to packaging. With gears it will require multiple stages to get 5:1 reduction but we’ll look at that.

Edit:

I was corrected. I looked at this too quickly and thought it was a single stage. This is for a two stage. Well It is clear you should not use a 2 stage. Might be okay with a single stage and this arrangement.

It says the CIM is okay at a 9:1. So at stall torque that would be 2.41 Nm * 9 = 21.69 Nm

The Neo at stall torque and a 5:1. 3.36 N*m * 5 = 16.8 Nm

So if it can handle the CIM at 9:1 it would likely be able to handle the Neo at 5:1.

Just checked and I’m wrong - you can get a 5:1 reduction with a single stage of gears now (thanks REV) Might have to look at that much harder. Still, I think this will work if we add support for the NEO end.

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It may be able to handle it but also aluminum does not make me feel especially confident with reversing loads at that % of strength. Maybe it never actually see that torque but do have to consider what would happen if the mechanism is back driven, it may experience more stress than intended

Is there a reason you don’t want to go with the versaplanetary outright for the wrist or use a CIM Sport from AndyMark? Sounds like you have a good idea of what final gear ratio you desire so the flexibility of the MaxPlanetary might not be needed and the CIM Sports are pretty bullet proof.

Alternatively, we are using REV’s 2 Motor Gearbox - Through Bore with a 6:1 ratio and feeding that into the MaxPlanetary for a shoulder joint. There are mounting holes to do this and it just requires a custom adapter (we are just taking 1/2 hex to 8mm Round adapters and 8mm key stock from McMaster to make them). This could free up a reduction from the MaxPlanetary cartridges you already have to get the right combo of ratios.


:thinking:

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This also exists for reasons.

We’ve had bad experiences with VPs at high reduction. Even operating within VEX’s published limits we’ve broken a hex output coupler, and had a planet pin come loose on a 10:1 stage, locking the gearbox. The 5:1 and lower VP stages I will admit are more robust. But when I saw the MAXplanetary launch I said “let’s buy some of these” to the team since they seemed far more robust than VPs in general and the hex shaft coupling is really nice.
A CIM sport is interesting, and maybe worth considering, as is the solution with the 2 Motor through bore gearbox.
This idea was “free” in the sense that we had the VP stages and CIM output shaft just sitting around. Basically we have 2 each of 5x, 4x and 3x stages for the MAX, but many old VP gearboxes from old robot intakes / conveyors where they weren’t subject to nasty loads.

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WHOA! is that a custom Ultraplanetary to MaxPlanetary adapter? That’s cool!

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