Can someone please explain the mechanism behind turrets like these? I don’t understand how the bearings are connected and how that allows things to move the way they are supposed to. How are the bearings connected to the plates?
I know team 148 did something like this in 2017, but I can’t get a good grasp of exactly how everything works.
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A section view might help:
The top plate is what the rest of your turret is mounted to, and the very bottom plate is a giant sprocket. These two plates are bolted together, with a stack of bearings on each bolt. By using a big-small-big bearing stack, you create a little C shape that rides on the edge of the middle plate, which is rigidly mounted to the robot. By having multiple of these bearing stacks in a circle, the entire turret is fully captured, riding on the edge of the big circle in the middle plate.
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How does the bearing stack work exactly? Does the middle smaller bearing not move and just the outside bearings move? (By move I mean if the inside of the bearing rotates or not?) Thanks!
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In a “perfect” world, all three bearings rotate as the turret turns. The thrust bearings take the moment load (and support the weight of the turret), while the center bearing keeps the turret running smoothly by rolling along the edge of the ring plate.
However, in reality, bearing stacks are imperfect and the turret ring plate might not contact every radial bearing at all times. For this reason those bearings tend to function more like “bumpers” and are there to make sure everything continues to run smoothly.
The thrust bearings act pretty much as you would expect, however the bottom one of course tends to spend more time supporting the turret due to gravity.
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Couple years ago we used the bearing stack for our turret. If we do a turret again, I think we would go with v groove bearings. 1 bearing instead of 3.
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I was about to bring up how a v-groove setup would require CNC machining operations that are available to fewer teams, but then realized that you could probably prepare a laser cut blank for this v-groove setup using a chamfer bit in a wood router. Anyone have any experience with that idea?
Rev sells V-groove bearings. They were on First choice for the longest time. Google also works well. They work best if you can chamfer the edge of the part that rides on the groove.
Turn Table bearings work well if you can find one that suits your needs. We used this in 2021 for the squishy yellow ball turret. It is not longer available, but similar ones are.
In 2020 2767 did a V-groove setup with a 2-D part plate, in 2022 we decided to use a COTS part because we didn’t wanna go through that all again (time wise it made sense to focus on other things)
An odd number of V-groove bearings were placed around (important incase the waterjet aluminum inner plate is out of round when tightened) (correction, comp bot did not do this). There was adjustment in placing these bearings. We used custom steel V-groove covers over radial bearings (i think it was stainless; non cots v bearings was for geometry reasons, so there was room at the root of the V for the plate so we didn’t have to chamfer). The plate slightly wore into the bearings, creating work hardened chamfers. You can see the setup here:
… and assembly, at least to some extent, in the build time-lapse
This was all a fair bit of hassle, but was very robust. In our first (and only) comp weekend of 2020 we had a ball get under the robot and we hit the turret on the color wheel. The turret popped off. No damage because of how things flexed. The video is on FUNs week 2 clips of the week. Here is the damage sustained:
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Or a lathe. I occasionally do something like that. It does work.
Adding to this, 3476 used V-groove bearings on their 2019 turret (which was 254 style, the whole elevator and end effector rotated) which, although it scares me a lot, clearly demonstrates the robustness of the setup if done correctly.
All that being said, big x-contact will always be the gold standard. Bit of a hot take, but I’d say that the only reason to do v-groove or bearing stacks is if you can’t possibly get a thin section big enough from China under the COTs price limit. Of all the things on the robot worth investing 600$ into to make it robust, a turret is probably the number two behind swerve.
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I generally agree so long as you are looking to pass through the center of the turret.
If not, (such as spinning an elevator around) any solution that decreases the moment loads by virtue of distance will work, and there are likely be lighter and cheaper ways of doing this than an x contact.
From a loads standpoint the difference is absolutely tiny between a “narrow”^ shaft with regular bearing surfaces spaced 8" apart axially and an 8" x-contact. Spacing along a narow shaft is cheaper and they have different packaging of course, but fundamentally they are doing the same job. The stiffness of the mechanism and the team’s manufacturing capabilities of course needs to be considered as well
^By comparison, i.e. a 1.5" OD thickwall piece of AL tubing vs the 8" bearing
I would challenge some of the assumptions about only doing X-contact bearings if you have a design spec for center loading through a turret.
It is really nice to be able to spec something for your application and just purchase it, and have a high degree of confidence in it working well. In a world where we build highly integrated robots with everything demanding some amount of bandwidth anywhere the engineering effort can be reduced matters. Using thin section bearings lets us shift the design effort away from the “means” (in this case, rotating some mechanism) and instead focus on the “end” (whatever sits on top of that rotation axis).
I know I can spec the appropriate thin-section bearing and it just “works”. We’ve done a lot of research on various bearing types and have gotten safety factors on stiffness from anywhere between 5x to 40x what our design targets are with x-contact and crossed roller bearings. In terms of loads we are at least 1.5x there, if not more. These bearings are also way less of a maintenance headache than a V-groove system. Price wise, they can be had quite reasonably from various online sellers, or eBay (our preferred choice).
I attached a few slides from our 2023 design notes that we drew up during our arm design process. The info there is still handy when working with any kind of DoF with high loads and stiffness reqs, turrets, arms, etc. And of course we’ve done a few Spartan Series talks on this as well - either @Travis_Schuh’s talk in 2017, or the talk Travis and myself did last year.
2023 Arm Design Notes - crossed roller.pdf (764.7 KB)
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