Questions about VEX clamping gearbox + VersaDrop

Technical Technical Discussion
#1

We are toying around with several drivetrain ideas in offseason, and had a couple questions about using VersaDrops with 4" mecanum wheels and VEX clamping gearboxes as shown on the octocanum tab of the VersaDrop product page.

The Toughbox Micros we used this year with 6" AM mecanums had a LOT of “slop.” The wheels could easily turn a good half inch without actually turning the motor. We dissected the gearbox and it was a combination of the hub on the mecanum wearing down and the first gear on the output shaft causing this. Would using VEX clamping gearboxes with VersaDrop result in this same “slop” after a fair amount of use? We need the gearing to be tight enough to work well with a Talon SRX and encoder for velocity closed-loop control.

That brings me to my next question… has anyone mounted an encoder to either the VersaDrop or the clamping gearbox? I was wondering if a CIMcoder would fit on the CIM output shaft as well as the gear and the clamping gearbox’s motor mount. The CIMcoder page says it takes 0.31" off the output shaft, if that helps…

Lastly, JVN’s spreadsheet tells me I could get an adjusted 13.4 ft/s with a 14:70 gear and 16:18 sprocket reduction with the 4" mecanums. This year’s robot felt slow to us, with an 8.54:1 ratio on the 6" AM mecanums. How accurate is the spreadsheet’s estimate? We’re aiming for 12+ ft/s, so this would be perfect if the estimate comes close.

Thanks in advance

#2

A bit of backlash shouldn’t affect **velocity **control too badly, unless you’re doing some fancy maneuvering with direction reversal; it’s much more likely to affect an autonomous routine like driving up to a side peg this year. That said, a chain or belt reduction is likely to introduce more backlash than a gear setup.

Do I understand correctly, that the backlash was due to wear of the hub and output gear where each interfaced with the output shaft? The only time I’ve seen serious wear on a 1/2" AM hub was when we were swinging a hammer with one in 2014, so I have to wonder what you were doing.

Was that output gear steel or aluminum? The output gear is the last one you should downgrade to aluminum, as it carries the most torque.

#3

First off, I would urge you to go with no larger than 4" wheels and gearing for 15fps-ish free speed (or about 12-13 adjusted) for the purposes of saving weight and remaining maneuverable.
With that out of the way…
If you want to use an SRX+encoder, it will be much easier to use a VP mag encoder instead of other options due to the integration with the SRX. If you use a breakout, I would recommend Munchskull’s Cimcoder over the AM one; many teams including 1072 this year have had issues with it. The CUI encoder by contrast is incredibly robust- I have never broken one despite seeing horrible, abusive use cases for them. Plus, it’s only $20 + 3D printing compared to ~$40 for the AM version.

#4

Is what you call backlash what we have been calling “slop”?

And yeah, not sure why it wore down. The output gear is steel. Just seems like too much backlash. Here’s a video of the said shaft/gear: https://streamable.com/j4hm1

It doesn’t look very dramatic but in combination with the worn down hub the 6" mecanum moves a fair amount before actually turning the motor.

Bad camera angle is because I didn’t have anyone to hold the phone :smiley:

#5

I like that encoder design a lot. Thanks for linking.

#6

I worked a bit with a team (6493) this year that used a setup like what you’re describing. The CIMcoder and pinion gear do fit on the shaft and have enough room (barely). The pinion gear doesn’t fully engage the complete width of the larger gear, but it should be enough for this kind of drive (honestly I think this is more of a flaw in the design of the clamping gearbox than anything else). I don’t think you would have any problem doing this if you wanted to go this route.

That said, that same team that attempted to do that ended up switching out the CIMcoders for GreyHill Encoders coupled to one of the wheel hex shafts on each module because they couldn’t figure out how to get any usable data from the CIMcoders.

#7

How did you couple the Grayhills with 1/2" hex? I’ve only ever seen them coupled to 1/4" cylindrical before…

#8

Backlash is a gear’s ability to rotate slightly without it’s teeth moving the gears it is meshed with. It’s where the teeth haven’t yet contacted the meshed teeth so some rotation is allowed. I doubt it really wore down, that is just how these gears work. This will always be the case with most COTS stuff that you buy for FRC.

It also looks like the shaft is spinning slightly inside the gear though (slop). Do you know if it was always like that? We used Micro’s a couple years ago and I don’t remember seeing that exact problem, but I know we had a similar amount of backlash. If that is standard, you might want to look into purchasing different gears that don’t allow so much slop.

Lastly, switching to a smaller wheel size will reduce the effect of this problem. As the circumference of the wheel decreases, the linear distance that the wheel can turn without engaging the gear train is also reduced. Just another reason why small wheels are generally awesome.

#9

I’m not sure if it was always like that… but I think that slip inside the gear combined with the backlash from the worn down AM 1/2" wheel hub is what caused the majority of the play. I don’t think the gears are necessarily responsible.

I’ve only been hearing good things about small wheels… are there any disadvantages?

#10

You can drill a 1/4" hole in the center of the hex shaft, press fit the Grayhill into the hole, mount the Grayhill to something kind of flexible, and you should be good. Use Thunderhex, which comes with a hole perfectly centered in the hex, if you don’t want to try centering the hole for drilling yourself.

#11

It’s certainly possible that a lot of wear has made the issue more noticeable, but there will always be backlash with gearboxes like this, it’s just the way of the geometry :confused:

I’ve only been hearing good things about small wheels… are there any disadvantages?

The biggest thing, in my opinion, is if there is some sort of rough or slopped terrain to get over then a larger wheel can often get over it easier. It’s possible to get away with small wheels in those situations though, but there are other factors like ground clearance that you have to deal with. Looking at Stronghold most teams used large wheels for good reason.
There is also less gear reduction required for the same speed with large wheels, but that isn’t a big concern in my opinion.

#12

Unfortunately I wasn’t around when the team actually implemented that change, I was just aware of it. The options they had been looking at were either 3D Printing a 1/2"Hex to 1/4"Round coupler or drilling a 1/4" hole in the shaft (as others have suggested), not sure which they ended up using.

#13

I noticed that the octocanum example on VEX’s site uses the clamping gearboxes and the VersaDrops. In the VersaDrop assembly manual, however, they directly attach the CIM to the VersaDrop. If we used the 12:66 ratio shown in the manual with a 16:18 sprocket down to the actuating mecanum, would that work equally as well?

JVN’s spreadsheet would suggest so…

#14

Backlash is the clearance between the gears and too little increases friction and wear.

Slop is what you see in the interface between the shaft and gear or hub. Preferably you would want a tight slip fit or a light press fit in those situations to reduce wear. However with parts made to a price a little slop is preferable to maintain a slip fit across the potential components that may mate together.

#15

They don’t handle rough terrain as well (covered), they wear faster, and they move more slowly for a given axle speed. I think that’s it. Well, you can’t drive them dead axle with big sprockets, but you don’t need such big sprockets, do you?

#16

Are you sure about that? Maybe I’m misinterpreting what you’re saying.

Let’s say you have two robots, one with 4" wheels and one with 8" wheels, and you want them both to go 12 ft/s. The 4" wheel robot would need a 7.7:1 reduction and the 8" robot would need a 15.6:1 reduction. That extra reduction means a heavier gearbox, plus the wheels themselves weigh more.

#17

Heh, whoops that’s not what I meant to say at all. I meant to say that with a certain reduction, you can get higher speeds with large wheels. So a young team could just slap larger wheels to increase speed on their robot and not mess with their gearbox at all. But, that because changing ratios in COTS gearboxes is so simple now, I don’t think that’s too big of a concern.

Thanks for pointing that out!