What are some good design options for securing a dead axle in a highish-load pivot application? The model below shows a 3/4" rod bolted into supports as a dead axle and an arm pivoting around that axle using Oilite sleeve bearings or similar. For simplicity, nothing is shown to set the arm position between the supports.
Under load, with sleeve friction, the dead axle wants to become a partially live axle, rotating relative to the supports on the threads of the bolts attaching the axle to the supports. Tightening the axle bolts provides limited friction to stop the axle from rotating. Over time, rotation of the dead axle and bolts would cause wear and enlargement of the mounting holes in the supports if the bolts rotate with the axle or the bolts will come loose from the axle.
Some potential options to manage the situation:
Increase the thickness of the support tube wall with another plate inside the tube, Loctite those bolts in the axle, and just live with the wear in the axle mounting holes - hope you can remove the bolts without damaging the axle too much if you need to swap out the arm
Embrace the partially-dead, partially-live axle concept and mount the axle to a plate-reinforced tube wall with a little sleeve bearing and shoulder bolt to control the wear
Mill some flats on the ends of the dead axle and add a slotted plate on the inner surface of the support tubes so the axle cannot rotate - milling flats adds complexity and a slotted plate would limit the direction in which the dead axle could be removed
One of these options might be acceptable, but I’m sure there are some better ways to install a dead axle so it doesn’t rotate, but it also maintains that primary dead axle feature of being very easy to remove for quick replacement of the pivoting assembly. What are your favorite designs?
This sounds like exactly the type of application that lubricated brass bushings are perfect for - just some graphite or something on the surfaces that contact.
Bearings aren’t great for high loads but are excellent for high speeds. For FRC usually bearings work fine for everything, there’s nothing that’s such a high load that bearings cause major issues.
I’ve also heard Delrin makes pretty decent bushings. The trick is to get something that doesn’t have much friction with the surfaces it contacts (lubrication helps a lot) and that will wear down faster than the other parts because bushings are cheap.
Personally, I would fix the dead axle in place and prevent it from moving in relation to the mounting and use a bearing or bushing, depending on the application. To fix the axle, welding is probably the easiest thing to do depending on material and access to welding equipment. Another really simple solution would be some aircraft safety wire to prevent rotation - it’s what aircraft do to keep bolts from unscrewing themselves. Just drill a few small holes and thread the wire through, twist it together, and bob’s your uncle. However just chucking some bushings on everything the shaft contacts would probably be plenty for FRC and would allow the axle to stay partially live.
We did not have any issues with wear as you described; but this generally what I would recommend:
a. Double check the axle fit on the sleeve bearings as well as their alignment. I would be surprised that there is that much reaction torque from the sleeve bearings and may be an indicator of excessive binding.
b. While we did implement it in the same manner you did for maintenance reasons, we generally prefer to put a clearance hole for the axle itself in the inside wall of the vertical supports and bolt into the end of the axle through the outer wall. This gives you a much larger bearing surface if the axle starts rotating which should resist wear better, but it harder to disassemble. You can see an example of that on the arm crossbar just below the main axle. We also did opt to add some plates to effectively increase the tube wall thickness near the axle mount point.
Another option in line with this may be to drill a 3/4" hole through both walls of the vertical supports, and add some plates on the end to capture the angle. This way you’re fully supporting the OD of the axle and you can remove the plates to push the axle out without having to splay the tubes apart.
c. I have in the past used airline safety wire for applications like this in FRC; I do think that’s a great solution.
Usually people use a star nut. If you want it to last in a high load application though you gotta make sure that that force is distributed somewhat. One way to do that is to have the star nut be farther away from the fulcrum. A design like this where you press the tube thru two plates is what many teams do. Some also make a metal plate for the bolt so that they can really torque it down and distribute the force evenly on the polycarb.
The much simpler solution is to pass the axle all the way through the support tube walls and secure it with cross bolts through both the support tubes and the axle. We did that this year and it proved very secure. Here’s a couple of pictures of our axle and it’s supports:
The support in the second picture has an auxiliary plate on the outside to help support the end of the axle. That’s not needed on the other side, since we had the axle extend out to the camera boom instead (and pass through that as well.)
Our team had a very similar dead axle situation, we originally had bolts on a dead axle exactly like the picture. The bolts sheared, and we ended up enlarging the holes and just putting the shaft completely through both tubes, with e-clips on the ends to stop it from falling out.
If you don’t want to make grooves, you could still do tapped ends with a washer-just make sure the washer is flush with the shaft so that the shaft stays supported by both parts of the tube
This is the perfect thing the REV max spline was made for. Use the thriftybot 35mmx47mm bearings for the arm or shaft to rotate on and using the spline plates to lock it on the arm or support depending which way you want to go
Keep in mind that the bolts into either end of a dead axle do not have to mate with the center-of-rotation of that dead axle. You can purposefully install these bolts eccentrically (or even multiple bolts) in order to resist rotation. Do be aware of how this changes the loading on said bolts (putting a shear load on the bolt from arm rotation, in addition to the shear load caused by the weight of the axle and arm).
How about using mounted flange bearings instead of the bolt in end to hold shaft in place? UCFL204-12. I am using 2 of these on my welding cart to share the weight of tool box and 2 Size T welding tanks mounted on platform directly center of axis of bearing rotation. So ~500LB load on axle shared between 2 bearings
A fender washer and friction washer to remove the moment caused by static friction/tolerances in the bushing.
Ensure there is minimal friction or high spots on you bushings and axle, bushings run tight tolerances and a generic shaft that is out of round wil really fight you. You will probably want to run a bushing material softer than your axle material and ideally wear it in with the arm disconnected and the motor running.
Larger design critique,
I would turn the dead Axle into a tube vs rod for the weight saving and use tube nuts
do what @MisterGrapfruit diagramed to support the rod to offload the axial load from the shaft.
3d print a plug for the end of the tube with a cross hole and cross drill the upright if it still spins.
I would also look into counterbalancing the arm with a gas springs attached to the sprocket and the other end to solid frame. Chain won’t stretch so much, less motor power needed, all around good practice.
One thing to note is that if you decide to index the dead axel through the 2x1 you may consider indexing it through both sides and have a gusset on the outside as the final mounting point. This is to make assembly/disassembly easier because you can slide out the axel without needing further disassembly of the frame which could be a chore. See 7461’s 2023 arm for an example of this. Always design for repair!
Zombie axles can be useful, especially when you can easily plop an encoder on there.
A good number of the 330 arms worked like that, sometimes with bushings but often just whatever bearing block was handy. Using the same joint style regardless of live or dead axle is more flexible, though may require an additional crossmember between the supports.
Excellent point regarding extra cross members. Although you could pass load through a live axle in this way in a pinch, it would likely require a bit more hardware and/or additional machining operations.
I would actually recommend against oil impregnated bronze bushings in most FRC applications. Oilite type bushings are essentially bronze powder and oil pressed into the shape of a bushing. When you run a shaft in them they warm up and the oil flows out to the bearing surface. They are only low friction if you are adding additional lubricant or they warm up from use. I’ve never seen people lube them in FRC, and they don’t have time to self lubricate properly in a match. In sporadic use they can actually wear pretty quickly, especially when they are running on aluminum and get aluminum oxide in the bearing surface. I have seen bushings wear completely through in FRC. They are also very weak, so you’re only really getting strength where they are pressed into a plate or block. To everyone using a 1” long bushing pressed into a 1/4” plate, 3/4” of that bushing is serving very little purpose. Bronze bushings are great for continuous use applications (fast or slow) or in well lubricated environments, such an oil filled gearbox or small cheap motor.
For low speed applications in FRC I would highly recommend using plastic bushings, UHMW or other bearing plastics will tend to be extremely wear resistant themselves and very kind to aluminum shafts. As an added bonus, you can cheaply buy UHMW blocks or plate and drill your own holes to combine the bearing, mount and gussets into a single piece. Adjusting friction is as simple as adjusting the tolerances.
Side note from a pedantic mechanical engineer: A bearing is anything that enables other things to slide past each other. A bushing is any (usually round) tube that fills the space between a shaft like object and a larger hole. An Oilite or UHMW bushing is both a bushing and a bearing. An interference fit tube allowing a 1/2” shaft to be press fit into a 3/4” hole is a bushing even though nothing can move after it’s pressed in. Bearing refers to a use, bushing refers to a shape.
So I guess my question is, are they good enough for FRC? Because I can buy pretty much any size I need from McMaster for a few bucks without needing to machine them myself.
I like this design a lot. Alternatively, one side can just have one wall of the 2x1 pass the whole tube through, with a small hole on the other side. I would definitely not just rely on the shear strength of the bolt or tube nut to secure the tube - instead, run the round tube through at least one wall of the 2x1 so that the wall of the 2x1 supports it and takes the bulk of the load.
We also had something very similar for our elbow joint this year: a 17mm steel rod as a dead-axle with bushings pressed into the second arm tube rotating around the axle. To secure the axle to the first arm tubes on either side, we actually threaded the steel axle to M16 and used locknuts to hold it in place. We were debating drilling and tapping like you did, but having a large thread that we could torque down made the whole first arm structure a lot stronger. Since the bottom axle of the first arm (aka the shoulder joint) was live-axle with the first arm, the whole first arm became a solid rectangle that was very hard to deform.
Sure, in most cases FRC bots don’t live long enough to wear out a bronze bushing, so it really doesn’t matter in most cases. I prefer plastic because I’ve found them to be even more durable, less likely to get sloppy from small amounts of wear, they feel smoother to me. Also you can easily buy plastic bushings that are a 1:1 for most any bronze bushing from McMaster. Custom manufacturing just lets you combine the bushing with a plate or two the we would still have to make in house, so it can reduce overall part count without increasing custom part count.
@Skyehawk we actually managed to wear through several bronze bushings in our intake in 2022 even though they were on ground shoulder screws, granted I would switch that design over to ball bearings not plastic bushings as it was high enough speed that if probably would have melted UHMW.
