Hi, I just heard that we are getting an arbor press, I was wondering where we can buy gears that are interference fit on 1/2" hex shaft. In 2018 we had a wrist on our robot and it would lift and tilt, but we would slop a lot to the point where there was a solid 10 degree motion on the arm. we were able to wrap a round of Teflon tape and put the gear over it to fill the little space but that was still very “meh.” Thanks!
You’re not going to find a COTS solution for this. You’d need to custom broach your gears to match the size of your hex, most likely there’s a few vendors who sell 7075 hex, but it doesn’t come with tight tolerance so it’s hard to know what you’d get batch to batch.
2 Likes
Imma go complain in the 2020 Cots parts request thread then!
Why would you want interference fit hex shaft? The whole point of hex shaft is you don’t have to interference fit things, so assembly and maintenance are easy. If you want interference fit shouldn’t you just use round shaft?
4 Likes
If you want it tigheter to allow for less slop, 217 had a unique approach this season. On their arm, they had two sets of gears. They clocked the slop all the way one way, and all the way the other way, drilled and bolted the gears. This allowed for all the slop to be taken up on the hex.
4 Likes
So when are we all moving to splines? Hex doesn’t seem to be meeting all of our needs
3 Likes
But isn’t that comparing press fit hex to slip fit hex? I was talking about comparing press fit hex to press fit round. Either way there’s no backlash from the shaft interface but with round shaft you get the ability to use standard bearings + no weak points at the corners.
You can shim the hex shaft to reduce slop between the gear and shaft.
thats what we did, I might see if I can use the Machines at college to make some hex that is a tiny bit bigger so we can press fit on. So far I like 217’s method.
If you’re planning on press fitting things then I’d assume that you do’t have any plans to remove it from the shaft?
In that case just use loctite 680. It will 100% fill the gap and prevent the gear from moving at all.
And if you’re planning to move it/remove it, then set screws may be in order (for side-to-side movement, not rotary). There are a number of tricks that can be used to help make sure that it doesn’t move once screwed down. Press fits can be removed, but they’ll need the arbor press, which should make short work of any adhesive used (if it’s powerful enough).
Making your own hex is going to end up being an exercise in frustration. You HAVE to get all the sides correctly positioned, relative to each other and relative to the center–and you have to do it while changing the shape of the shaft. Fixturing is your friend for that, but when you have to change your alignment fixture every so often to handle the sudden appearance of flat sides where they weren’t expected by said fixture it’ll make life much more difficult. I could make a square shaft from a round one, and I’d have a reasonable chance of success. I could probably do a double-D shaft (two flats) no problem. But turning that double-D into a hex? Not easily.
Making hex shafts isn’t as hard as you’re making it out to be. One of these can be used to easily hold your workpiece. You just need a collet and a matching size of round stock to fit inside it. You can hold the hex block in vice the so it hangs out sideways. Easiest way is to lock your vertical axis to what it exactly needs to be, and just index your milling from the end of the shaft. Flip and repeat.
3 Likes
It can also be done pretty easily with a rotary index on the table of the mill. As long as you accurately locate the position of the first flat, they all should be right. We used this process to cut a round shaft into hex for our 2017 climber.
For mechanisms like arms, in my experience it is best try to avoid using shafts to transmit the torque. Instead, I usually advocate that the design should try to attach the gear directly to the arm. This is easier to do with sprockets (especially the plate sprockets that already have a large bolt pattern), but can be done with gears as well.
This is probably a bigger design consideration in FTC where the shaft diameters are really small and the torque through the gear hubs are generally only reacted with a small setscrew in a flat on the shaft. But we have also used this successfully in FRC to eliminate one big source of backlash in arm joints.
While the hex shaft is certainly rated to handle pretty high torques, I would expect to see a decent amount of wear in the hex bore of the gear and perhaps even in the hex flats on the shaft over the course of a season, especially if this mechanism was highly loaded and had a lot of backlash cycles.
We are not always able to follow this guideline. There are times where you just need to use a shaft to transmit the torque. But during our design reviews, I look for places where we can eliminate backlash by employing a technique like this - especially if the arm needs to be able to be controlled precisely.
3 Likes
Hm… attaching a gear to plate is pretty interesting, sadly we’d have no way other than clamp and drill, which might work, but the 775 mounting holes can be done so that we can adjust the holes.
You could always use bearing retaining compound. One of the loctite products. Don’t planning on moving the gear once it sets.
1 Like
You can consider a few solutions in addition to those already presented.
Learn how expanding arbors work and see if that technique could be adapted to your implementation.
Tapered pins can be used in several ways to remove backlash from torque-transmitting connection.
Shaft collars can be purchased, or made, with side flanges. They could be hex-broached to clamp down onto the shaft with the gear bolted to the flange.
The super old-school way to take out slop is peening. A careful application of manual peening after the gear is located would cinch it up quite nicely.
You could also glue them in place. Feeding epoxy into the small joint between the gear and shaft might be tricky, but I bet it could work nicely. Simply heat with a heat gun or torch to remove the connection later.
This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.