Our team is experimenting with designs for rotating joints. We’ve tried a few different designs for generic passive rotating joints (like the joints in a 6-bar linkage) with varying degrees of success. We are looking for one that have the following features:
Simple (few parts)
Low maintenance (doesn’t loosen over time)
Low play or slop
We’ve experimented with just bolts through two pieces of metal with a spacer, shafts through pillow blocks, hinges, and others. Our best solution is shown in this picture.
This is a long bolt through a pillow block attached to the first piece of metal, then a 1/8 inch plastic spacer, then through the second piece of metal with an attached pillow block, ending with a nylock nut. The nylock prevents the joint from loosening over time, and this approach works fairly well. We used this on all the rotating joints of our Sack Attack robot last year.
However, this type of joint suffers from extreme sensitivity to the tightness of the nylock. If it is too tight, the joint won’t rotate easily, if it is too loose, then the joint has too much play and becomes floppy. Our general approach is to tighten the nylock down all the way, then back it off until it feels free. But this is not easily reproducible - the “sweet spot” is very small.
We are interested in hearing what approaches teams use for a passive joint design like this. Does your team use bolts, shafts, shoulder bolts …? Our team is looking for a better design.
That’s the best small design. We have those on every robot we’ve built this year, from a Six-Bar to a Scissor Lift. Our defensive robot for Sack Attack last year had something like 50 of those, so the scissor could expand and compress as it drove. If you want them to spin a little better, even, take a drill and expand the hole in the Delrin Bearings. While they won’t grip an axle as tightly in the future, the screw will spin more freely.
The best design, though, is to use a turntable as a passive spinning point. Finding room to mount one is incredibly difficult, but they’re incredibly stable and have very low friction. Again, we used these last year on our defensive robot so the two bases could freely spin. It was fantastic. No tweaking, no worrying about tightness or Nylocks, no maintenance. By far the easiet thing I’ve ever been tasked to build.
We, too, use this type of joint very frequently. However, I believe they’re called Delrin Bearings, not pillow blocks.
You could swap the screw out for a short axle with a couple of collars, which is what we have on our scissor lift this year.
Also, putting one or two Delran washers between the bolt and the metal can, when the screw is lightly tightened, act as a sort of spring which helps keep the two arms pressed on the spacer. (at the cost of some friction)
We tried using shoulder screws before, but they are very small. I recall barely having enough space to connect two c-channels with a nylon spacer in between on the smooth part, and then the threaded part is too small to put a lock-nut on properly.
Our team is currently using shoulder screws. They’re really quite nice, no bearings required and they almost completely get rid of any unwanted play in the joints. Washers are needed to fill out the rest of the screw because they were designed to allow more than two plates rotating on one joint. You’ll need lots of washers, as 75% of them don’t have a large enough diameter to fit the screws. We’re using Keps nuts with great success, rather than nylock. Other than the washers issue, the biggest problem we’re running into is that they need to be tightened occasionally in order to remove all the play. Other than those two nuisances, shoulder screws are perfect for the job.
With that in mind, I would recommend them over a Saunders screw/Delrin bearing combo because shoulder screws are easier to work with. Teams probably have more screws and bearings on hand, which would make the latter cheaper. I would say, if you don’t mind making the $15 dip (screws + washers), go for it.