Here’s an updated version of the gearbox-on-arm design, incorporating some feedback from this thread. It features a MAXSpline dead axle instead of round tube for better stability and a custom bearing plate design for better packaging. It also uses MAXSpline Shaft Collars to constrain the MAXSpline axially, which also function as sprocket spacers.
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You are correct, maybe I’m dreaming but I thought I saw somewhere that Rev was going to stock a bearing for this use case, but I can’t find it on their site, perhaps it was just reference to the same 35mm bearing I’ve seen.
This is purty!
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Do those sprockets kiss a bit there?
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All these designs are very cool.
The thing I’d be worried about with most of them though is the vertical spacing between the bearings. Especially with something like an arm attached to the carriage which will have a sizable weight far from the plane of the elevator, you’re putting a large torque on the elevator carriage. That torque is resolved by the radial forces on the front and back bearings, and a free body diagram will show that the forces are proportional to the vertical distance between the sets of bearings. If that distance is too small, the forces may be large enough to bend the cantilevered bolts, lock up the bearings, or dent the (presumably thin walled) tube the bearings are riding on. It’s one of those things in FRC that’s very hard to calculate but devastating if it goes wrong, so it’s usually left up to the designer’s best judgement based on what’s worked in the past. So by all means push the envelope, but if you’re thinking of using something like this in-season make sure it’ll work before you commit to it.
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The new REV extrusions are going to be more dent resistant than the usual tube!
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Geometry wise i agree . But it’s thinner than the vex equivalent, and from what i can tell the thicker tube only increase the wall thickness on the long side. would love to see someone do a comparison with both thickness of vex and rev.
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Some FEA analysis can be found on these threads (There’s a few more threads on the subject)
That’s all in the theoretical realm so I think it’d be fun to see some practical tests. There’s nothing like breaking things on purpose for science!
This is a really good point, and something I’d barely even considered to be honest. Looking at 254’s 2019 carriage that @Torrance linked above, the bearings are somewhat close together vertically but both sets are below the arm pivot by a significant amount. Is that something that would help reduce stress on the bearings as well?
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Not 254, nor have I heard the 254 explanation of this, so this is purely a guess, but:
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Moving the bearings below the arm should make the stress on the bearings from the arm greater, not less. (Longer torque arm caused by the arm because longer distance from arm CG to bearings.)
Incorrect guessing at 254's reasons
I’d hazard the guess that the bearings on 2019 254 were placed lower for their stinger climb. If you see this image:
and compare it to this image:
, you’ll notice that those blue boxes in the top image are the bearings. (The top image has the stinger, the bottom doesn’t.) It could then reasonably follow that the stinger/climb attaches in that general bearing area (see the hook). This would also make sense: The arm’s weight causes little stress relative to lifting the entire robot!
Edit: Torrance from 254 explained the correct reasons below.
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The Rev extrusions have extra meat and cross ties in the critical corner areas where bearings abuse the tubes…
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Fwiw
Once we switched to the r4a bearings we haven’t had any issues denting tubing. Ymmv.
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The bearings are such that their span is as long as possible, to support the loads, but short enough to allow for the travel/range-of-motion/stroke-length of the carriage on the elevator to enable all the different scoring positions. In this case, the arm pivot was placed high above the bearings because that is what worked better for the geometry. Needed a high pivot so the arm could be long but swing down to score on the level-1 discs. See pic of scoring geometries.
Note that the Stinger had teflon on a 6-8" span, and Suction Climber each had it’s own carriage with bearings on like an 8" span. So the loads from climbing were not carried by just the Elevator carriage’s bearings but rather by the combination of them with the stinger teflon or suction climber. It’s a pretty complicated free-body diagram.
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I am also not from 254
We can assume that the applied torque due to gravity or the motors is constant regardless of where the bearings are placed right? so in that case a longer moment arm between the arm’s pivot and the bearings will result in less force no? so moving these further away lessens the load on them, not increases it?
Thanks for the explanation! I forgot your carriage extended ‘above’ your first fixed stage. It’ll take me a bit to understand the bottom paragraph, haha, but it’s always a pleasure reading your explanations of this robot.
My understanding is this: The arm’s pivot only changes the torque a bit and it is not around the arm’s pivot that the torque acts on. It is around the bearings that the torque acts on because these are the only resisting torques. In a sense the whole carriage + arm body provides the torque; the bearings counter it. By moving the bearings further down, that torque arm increases. (The chain supporting the vertical load probably makes this a bit messy when the angle isn’t 0, but we can ignore that.)
I don’t want to drag away from the main purpose of this thread – I started moving the subject away so I feel bad continuing – though so if you want DM me to continue.
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Based on what I saw in FEA and general experience I think the little corner features in MAX tube will do a nice job of stiffening the bearing area used by most elevators.
No, I won’t model the contact stress of a carriage roller on different tubes. That stuff is real hard to get remotely close to correct.
@_cmb I appreciate your self reflection and design permutations. Nicely done.
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Since this thread is about the design of elevator carriages—and bearing placement is a critical consideration when designing elevator carriages—i think others could benefit from clarification of what we’re discussing. Plus, this way people with more expertise than either of us might be willing to weigh in on our discussion. Someone else please call me out if they think this is derailing the topic, im not sure what the norms are on CD for this kind of stuff.
I understand there are a few things going on here (free body diagram wise), but ill start with what I believe to be the most relevant.
I think about this in terms of a simple action-reaction torque pair around the pivot of the arm. The motor applies a torque to this joint (which can hold the arm in place, accelerate it up, etc.), and then the carriage sustains the reaction torque.
applied torque (from motor) in red, reaction In blue, pivot in green:
so then a contact force between some of the bearings and the elevator structure must create force that can counteract this. This force is the load on the bearings, and the torque it generates must be equal in magnitude to TR about P.
Sorry about the trash-tier image quality, but here in orange we see the potential bearing forces which could produce a torque equivalent in magnitude to TR (if the bearings were kept at the same spacing relative to each other but got moved closer or further from P.) As the bearings get further away, the load on them (force needed to counteract TR) gets smaller since we know that torque is the dot product of force and distance and in this case the sine term stays fairly constant.
Again, i understand there are other forces at play here, but like i said i think this is the main one we are concerned about when it comes to bearing spacings and placement.
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Sorry for the late reply.
Yeah I think I’ll need that haha.
So…: I hadn’t been thinking of this as when the arm is moving. I was thinking purely in the stationary case.
I’m pretty sure I understand what you are saying, and I guess what you are saying is true? Because you’re considering the rotation of the arm, I think that you are right in asserting that the further the bearings get moved, the less countering they most provide assuming an axis of rotation around the arm sprocket.
This prior disagreement stems because I was thinking stationary, where the axis of rotation is – or should – be around the bearings, as opposed the sprocket in the moving case. See below:
In writing out the physics I either made a hurried mistake or the torque of it sitting ‘at rest’ without the arm moving is always constant… which just hurts my head more.
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my case works when the arm is stationary also. Even if the arm is moving, a torque is still nessecery in order to stop the arm from falling. (maybe in this case some of that is don’t by a CF spring, but that doesn’t change my argument at all)
so in this case, if we are just ignoring the existence of the arm joint, the torque in both of these cases is equivalent. You can choose some numbers and throw this in cad and see for yourself, but if we assume the mass of the carriage is irrelevant (as your diagram shows), we see that as bearing set moves down the the sine term shrinks, and the radius grows proportionally. This means the torque produced by gravity acting on the arm doesn’t change.
We can also think about this in terms of line of action, which I’ve always found to be more intuitive since it avoids the math. If we draw the line of action of F (which I’ve done in blue) in both cases there is a point on this line where a radius (r in purple) meets it at a 90deg angle. In both cases at this point, the sine term is equivalent ( sin(90) = sin(90) ), and we can logically see that F is also the same (since that’s a given in this scinerio) so the torque is equal in both cases.
This kind of model is slightly more of a simplification than mine is. we can not use it to say anything about the height of the bearings relative to the arm. we can see from it that the bearings being further apart from each other does reduce the loads on them, but i think we both agree about that.
One other nitpick I have is that in your diagram I don’t think the rotational axis would be at that arbitrary center point between the bearings, it would probably be at the left side of the outer edge of the top right bearing on the carriage and this would act as our fulcrum. This is truly me being pedantic though, as this dosnt really effect anything as far as our discussion is concerned.
@Torrance do you think you could clarify this for us?
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I don’t think this is taking away from the purpose of the thread at all, I’m glad to be getting good feedback and provoking interesting and in-depth discussion about design. As an alum, I try to focus however I can on being a resource for other students - that’s why I write so much documentation for everything I design, and why I started this thread in the first place. And with the high possibility of a pick-and-place game this coming season, I think this kind of information could turn out to be really helpful for teams whose students have only ever experienced “shoot ball into goal, climb on bar”.
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