What are some light/effective/elegant/long-term solutions for pivot points? For example, the pivots or points of rotation on teams’ cube intakes this year (especially those with a left/right wheeled intake with some sort of spring/pneumatic system to grip the cube) – any recommendations for how to execute those well? Thanks in advance for the help!
T100 did our pivots with typical 1/2 hex x 1-1/8 flange bearings on hex shaft, because the pivots were both pivots and shafts for timing belt pulleys driving the intake wheels.
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We originally used hex shafts in standard 1.125" OD bearings, since we were planning to use them to drive the intake wheels as well. We ended up putting motors on the arms of the intake, so we switched over to 1/2" bolts inside bronze bushings, since the bushings seemed to support the sides a bit better (they had less play compared to the ball bearings). Plus, it was only a ~1/4 rotation movement, so the bearings weren’t really necessary.
Bearings or bushings for high load, speed and/or duty cycle.
You can get away with bolts as pivots for lighter duty things or for systems that adjust only a few degrees (like a 2013 shooter deck being asjusted by pneumatic cylinders 3*)
3928 likes to use Oil-Embedded Flanged Sleeve Bearings. Just press them in and put your pivot on a round shaft or shoulder bolt.
We used 1/2" round for our intake pivot side to side this year, one being just a 1/2" hole and the other being where the bearings are, then 5/16" shoulder bolts for the rotation point at the start of the match to drop the intake.
This. Compared to a bearing, less space, less cost and easier installation. Worked flawlessly.
We’ve gotten away with using bolts for pivots for a whole bunch of things…
one trick is to use a long bolt, with two nuts, one nut on either side of the part that the pivot is attached to. Then tighten the nuts against each other, and the part that pivots can be on the bolt, retained loosely by the head of the bolt. hard to describe, and even harder for students to figure out which things to tighten…but it works pretty well, and don’t cost much, or take much machining. Just drill a few holes.
That is none of those.
We’ve done this - a lot lighter and simpler and more robust than idle hex shafts in hex bearings! For lighter duty pivots, including our off-season robot’s arm to raise a cube about 20" and to 45 degrees so we can launch it into a switch, we use hinges - sometimes riveted, sometimes bolted depending on the forces. For 2017 STEAMworks, we had *spring *hinges on two doors and a popup gear intake ramp.
I guess you have a different conception of these principles, than I do.
That’s ok.
I’ll give you that it isn’t elegant, but it is certainly light, effective, and capable of lasting more than a season.
Solutions like these honestly work acceptably if working under a deadline or at a competition, but should never be designed into the robot from the start if you try to follow best practices. Standard bolts are soft and make for terrible shafts/bearing surfaces, the threads of them make for even worse. Higher contact pressures at the crests of the threads will quickly wear away the bearing material, or vice-versa, especially if just drilled through alu. frame members. Even for stationary loads, avoid any shear forces on the threads as excess stress risers in the root will lead to decreased fatigue life/increased risk of failure. Ideally, only the shank (unthreaded portion) of the bolt is loaded in double shear (supported on both ends) and the threads/lock-nut simply retains and tensions the fastener.
More to the OP’s question: For rotating joints, Shoulder Bolts work well as they are made from harder steel and ground for precise bearing surfaces and fits They also can greatly simplify an assembly over steel shaft and collars. The stationary portion of your mechanism should support the shank at both ends. The bolt does not rotate in this arrangement- all sliding interactions should happen between the ground shank of the bolt and the inner surface of the bearing. Press fit the bearing into the rotating portion of your mechanism. Oil-Embedded Flanged Sleeve Bushings work well for low speed mechanisms or incomplete rotations. Needle-Roller Bearings work well for higher speeds or when low-friction is required of the joint. Really only use them on hardened steel shafts, they will chew through aluminum pretty easily, but you can get shaft liners for larger IDs. Keep in mind any axial forces in the joint as well, as needle roller bearings will not support these. I like Nylon Thrust Washers for lighter loads as you can also stack or custom turn them to remove axial play if need be. There are other types of thrust bearings as well though, and put one on each side against the stationary faces to keep the joint from seizing when you tighten the lock-nut. Good surface finish helps. Also pay attention to any bending moments applied to the joint. Use two bearings on both sides of the rotating mechanism, and try to minimize the space in between the bearing and the inner faces of the joint, to keep the shear force on the bolt closer to ideal shear.
This is engineering “best practice” for single DOF joints with high strength, and low backlash requirements. For race teams, a common use case is the suspension rocker arms. They redirect the motion of the push/pull arms into the shocks/dampers and experience extremely high and repeated load, so fatigue is a large factor. The joints must have no play as that will translate into suspension compliance, giving the driver a poor and unpredictable feel for the car. They also must be low friction as to minimize the joints damping effect. The actual dampers should handle all damping, as that can be tuned when setting up the car, and the telemetry will more closely match the predicted model. Stiff joints will also wear quicker, bad all around, so from a maintainability standpoint these components can be replaced instead of the whole frame.
I hope this helps you OP, and I can answer some questions in regards to FRC design/manufacturability as we employed this method on the wrist joint of our intake this year, and several other places.
(full disclosure: after a revision to the carriage before the second competition, I neglected to order a new shoulder bolt for the rear pivot of the pneumatic piston that lifted the intake. We used a bolt through to Detroit. The threads bore directly on the bronze bushing we had, and the load was applied FAR from ideal shear. As expected, it quickly wore into the bushing, despite us actuating the piston a few times a match. I can’t say I’m proud, but that was far from the example of “worst engineering practice”. Your robot won’t fall apart from a joint like that. Race cars would though)
I thought we were designing robots, for a competition season. I don’t use this design on my race car!
My idea of “elegant” includes figuring out what the requirements are, and designing to meet those requirements with the least cost and effort. Simplicity is good.
I guess next year’s design will include the requirement to get into deep space, so we’ll have to step up our game. Or maybe it will still just be a high school robotics competition? 
edit: oops! I forgot…I actually did use this design on my race car, to hold the hood prop. It works great.
Simplicity is a great thing, I completely agree. On critical and non-critical mechanisms, I never want to have to worry about them failing or replacing parts on them or having to fiddle with bolt tension or ten shaft collars spaced out across the shaft- I want them to just work! Well designed joints are simple in this regard. Often lots of thought goes towards simplifying the manufacturing, and less so the reliability or maintainability.
The second half to my post was addressed at the OP who asked for examples, I know that you know how to build race cars 
. Its just an example use case that others can add to their repertoire, and hopefully I explained the design requirements and intent behind it well enough in my post that they would know when to use methods like this… For me, FSAE was a natural step from FRC. Judges there will tear cars apart in design reviews if they see stuff like this. So I want my students to put that much thought into even “simple” systems like a high-school robotics competition. And it won’t be next year, but eventually they *will *be required to get to deep space!
Disgusting. I bet it’s in single shear as well, you monster. That’s going to break. GTB!
I like using 1/2" x 5/8" flanged bronze bushings with 1/2" Vex tube axle. The black anodize makes the surface hardness nigh enough to resist wear. Dead axles also work great for large arms and such.
On our intake this year, we went with regular flanged thunderhex bearings just for standardization purposes. I believe the original design called for a 1/2" tube axle and bushings, but we were running low on time and had the thunderhex in stock.
For smaller pivots I like putting R188 bearings or bushings on 1/4"-20 screws. For pneumatic cylinders I love just using plain screws for pivots. 3/4" bore cylinders will take 10-32 on the back and 1/4"-20 screws on the rod end/clevis that attaches to the front. 1-1/6" bore cylinders can take a 1/4"-20 on the back, but need a 5/16"-18 on the rod end (we put a spacer over a 1/4"-20 on ours). Never had an issue with using screws as pivots for these smaller cylinders. We used the screw-pivot method from above on our cylinders to rotate the intake up and down.
Thunderhex pivots can work great for a live/dead axle, where an arm or intake pivots on the axle, but the axle is also used to drive wheels somewhere down the line. They can transfer torque but still use round bearings with low play.
(I think my response got eaten up by an auto moderator? Forgive me, complete noob to posting on CD… haven’t browsed forums in years…)
Simplicity is a great thing, I completely agree. On critical and non-critical mechanisms, I never want to have to worry about them failing, or replacing parts on them, or having to fiddle with bolt tension or ten shaft collars spaced out across the shaft-- I want them to just work! Well designed joints are simple in this regard. Often lots of thought goes towards simplifying the manufacturing, and less so the reliability or maintainability.
The second half to my post was addressed at the OP who asked for examples, I know that you know how to build race cars . Its just an example use case that others can add to their repertoire, and hopefully I explained the design requirements and intent behind it well enough in my post that they would know when to use methods like this… For me, FSAE was a natural step from FRC. Judges there will tear cars apart in design reviews if they see stuff like this. So I want my students to put that much thought into even “simple” systems like a high-school robotics competition. Wont be next year, but eventually they will be sending robots to deep space.
Added advantage: see the thread regarding backlash. May not be so much of a stretch afterall.
Maybe I was a little brash. Your race car hood prop is a perfect use case for your pivot design. It’s a non essential item that will see relatively few cycles and is actuated by hand so wear isn’t a concern. Slop in the joint isn’t a concern as it will always be pre-loaded against gravity. It experiences almost no load when in motion so a proper bearing surface is unnecessary.
The fact of the matter is a majority of the pivots in FRC don’t follow these same design criteria. That’s not to say your pivot won’t work but I think we can at least try to do better, “Shoot for the moon. Even if you miss, you’ll land among the stars." or whatever.
Here’s my design criteria for a pivot in FRC like one that would be used on an arm:
Low Friction - A little bit of friction can be useful but we still need our pivot to actually pivot.
Robust - It needs to be able to stand up to impact and handle unexpected loading. It shouldn’t need replacing within the FRC season.
Easy to maintain - An issue with a lot of pivots I’ve seen is the performance of the pivot depends on assembly. Over tighten a bolt and it locks up, under tighten it and it’s sloppy. This may not seem like a big deal but when your in the finals and making a quick fix it’s easy for someone to make a mistake. A big part of design is considering your end user.
Rigidity/Slop - Don’t make things harder for your drivers than they have to be.
I strongly recommend reading Charles Guan’s (Battlebot builder and FRC alum) “How to Build your Everything Really Really Fast”](http://www.instructables.com/id/How-to-Build-your-Everything-Really-Really-Fast/). It’s full of a ton of good info on making pivots.
For our main arm pivot on 5406’s 2018 robot we tapped both ends of the 5/8" igus shaft that comes in the kop, turning it into a solid dead axle between two plates. We then machined a bushing out of Delrin that fit snugly into versablocks in place of the normal flanged bearing. If we didn’t have lathe access this could of easily been 3D printed. You can find CADhere. Since the bushings were placed as far as part as possible it made for a very durable, rigid setup. It also eliminated the need for any spacers.
This is the key - if it needs replacing during an FRC season, that replacement needs to be built into the schedule. A well-built bolt joint (as Forbes described) can go many hundreds or a few thousands of right angles before wear becomes a real issue. If you add bronze bushings, take it up another factor of ten or a hundred. Bolts and bushings certainly seem much more capable at handling FRC shock than bearings.
This situation is pretty much the opposite of one I heard a couple of decades ago. Prior to Desert Shield and Desert Storm, DoD had procured a great number of hardened GPS (Global Positioning System) radios to use for navigation and targeting. The MILSPEC (military specification) GPS devices were capable of falling 100 ft (30m) onto a flat rock without breaking - but they had an effective battery life of only a few hours - and the batteries were rechargeable and not readily replaceable. Commercial devices ran for dozens of hours on AA alkaline primary batteries, which were cheap and easily replaceable. Admittedly, they’d likely be KOd by falling 30 ft (10m) onto a flat rock or other hard surface. I’ve heard several stories of lives saved and missions accomplished due to “personal” GPS devices coming through when MILSPEC failed. In the rare case that ENS Schuckatelli’s GPS fell 30 ft onto a flat rock, AGC Wojohowicz’s carried through. The bottom line is - design to deal with what you believe are your most likely failure scenarios, but plan backups to get you through the season.
Our claw pivot points this year were just a simple bolt interface (see a photo here). 1/4" bolt going through a 1/4" hole in each of the plates. The entire grabber arms were about 2" tall themselves, so the slop in the bolt interface didn’t affect the function of the grabber noticeably. It survived three events and cycled a lot of cubes. The quick cheap simple pivot bolts work for some mechanisms like this.
If you are transmitting any torque through the pivot point, such as a gear or sprocket that is bolted coaxially to the pivot, then you should opt to use bushings or bearings (though watch out, bearings take up space and get heavy). You don’t want any slop in those kinds of applications. For lots of passive pivot joints where slop isn’t as important, simple bolted connections can function just fine, but they don’t look as refined or professionally designed as if you use bushings for every pivot joint. We don’t tend to design past a 1 season lifespan.
EDIT:
For fun, here’s an example of a terrible way to build a pivot joint that is still functional for FRC. Our pneumatic catapult from 2014 pivoted on a 1/2" hole in an aluminum bracket, that rode on a 1/2" aluminum shaft. No bushings. It survived 100’s of shots over the season and was very repeatable. I think it might be kind of triggering though, so you may want to avert your eyes.