Design Feedback?

[SerpentEagle]

I've recently been working on a custom winch gearbox for off season. Basically, it's a ~12.4:1 ratio, powered by one mini CIM. The output shaft is attached to the spool which reels in and lifts the fork mechanism (~10 lbs) + a six stack w/ RC (~60 lbs). I'm looking for some advice on design; the faceplates are 1/8" thick, two per each side with 1/4" standoffs. Here's a link to the solidworks files on GrabCad: https://grabcad.com/library/gearbox-updated-1


Any and all advice appreciated, thanks.

[Joe G.]

Good start!

What was the rationale for the split plate structure? As drawn, you don't really gain any practical strength from the outer plates, since they do not constrain the shafts in any way, or reinforce the parts of the inner plates which will see stress. Tolerance stackup between the inner and outer plate may also cause you issues with gear mesh at the miniCIM pinion, depending on how it is done. The only thing I can see that the outer plates are doing for you is standing off the miniCIM so that it doesn't interfere with the 1st stage shaft's bearing, but this could be achieved through either a thicker plate or a standoff plate slipped in front of the miniCIM.

There's a lot of material up top that isn't really serving any purpose. This can be narrowed up a lot to save some weight. A cool trick which could come in handy here is to run the motor mount bolts all the way through both plates, with spacers serving as your upper standoffs.

It will probably be fine as is, but it's generally considered good practice with this kind of lightening to put a band which runs all the way around the outer edge of the miniCIM, so that it has a solid surface to react against to prevent bowing.

You're missing a few fillets in your lightening, which will cause problems in manufacturing if this is made with a mill, and will generate stress concentrations where you least want them no matter what.

How do you plan to retain the output shaft?

[SerpentEagle]

Originally Posted by Joe G.

Good start!

What was the rationale for the split plate structure? As drawn, you don't really gain any practical strength from the outer plates, since they do not constrain the shafts in any way, or reinforce the parts of the inner plates which will see stress. Tolerance stackup between the inner and outer plate may also cause you issues with gear mesh at the miniCIM pinion, depending on how it is done. The only thing I can see that the outer plates are doing for you is standing off the miniCIM so that it doesn't interfere with the 1st stage shaft's bearing, but this could be achieved through either a thicker plate or a standoff plate slipped in front of the miniCIM.

There's a lot of material up top that isn't really serving any purpose. This can be narrowed up a lot to save some weight. A cool trick which could come in handy here is to run the motor mount bolts all the way through both plates, with spacers serving as your upper standoffs.

It will probably be fine as is, but it's generally considered good practice with this kind of lightening to put a band which runs all the way around the outer edge of the miniCIM, so that it has a solid surface to react against to prevent bowing.

You're missing a few fillets in your lightening, which will cause problems in manufacturing if this is made with a mill, and will generate stress concentrations where you least want them no matter what.

How do you plan to retain the output shaft?

Thanks for all the advice. The reason I have two 1/8" plates is because we currently only have access to a laser cutter and a mill, and the laser cutter cant cut more than 1/8" thick aluminum. Originally, the plates were 1/4" thick, but since we don't have access to a CNC mill, I had to resort to 1/8". I do plan to add bearings to the outer plates. Also, correct me of I'm wrong, but wouldn't adding more standoffs at critical points between the inner and outer plates increase rigidity? I'm aware of the faulty fillets and will fix. I'll also add a ring to support the CIM, thanks for bringing it up. Didn't think that gear meshing would be a problem, but I can see it now. Would mounting the cim to the inner plate instead be a solution?
And regarding the extra material at the top, I've left it there for now since I don't have a clue how its going to be mounted.

Again, much thanks for all the advice, this is my first year so I'm not too experienced. When I get the time I'll go ahaid and update the model.

[Chris is me]

I haven't had time to really look at the cad, but quick question, do you have access to a manual brake? If you could add a few flanges to a 1/8" gearbox plate, you can make it much stiffer. If you don't need to mount anything to the flanges for structural reasons, the bends don't even have to be particularly precise.

[SerpentEagle]

Originally Posted by Chris is me

I haven't had time to really look at the cad, but quick question, do you have access to a manual brake? If you could add a few flanges to a 1/8" gearbox plate, you can make it much stiffer. If you don't need to mount anything to the flanges for structural reasons, the bends don't even have to be particularly precise.

Yeah we do, but correct me if I'm wrong, flanges on the edges would only prevent the plates from twisting and bending, but the segments holding the bearing seats would still be able to flex

[pmangels17]

If you have access to a manual Mill, you might be better off just doing the whole thing out of quarter inch plate, and doing the lightening by hand in rectangular patterns or round holes. Other than that it looks great!

[artdutra04]

Originally Posted by SerpentEagle

...I do plan to add bearings to the outer plates...

Don't do this. This will mean that the shafts would be quadruple supported by four different bearings in four different plates. This is not desirable, as any slight differences in the location of the holes in each of the gearbox plates will cause binding in the bearings.

The only reason why singular 1/8" 6061 aluminum plates without any flanges might have issues for FRC gearboxes is because of flexing in the gearbox plates. Thicker plates or flanges make the plates stiffer. If you have a small gearbox, one 1/8" 6061 plate on each side of the gearbox without any flanges will most likely work just fine, as they won't be able to flex that much.

Alternately, if you have a larger gearbox you can most likely get by with only one 1/8" 6061 plate per side without flanges as long as you have frequent standoffs between the two plates, or if the gearbox plates are tied well into other structure on your robot in a way that prevents the gearbox plates from flexing.

The flexing of a gearbox is not desirable because it could cause bearings to pop out of their holes or shafts to bind.

I would avoid making gearbox plates on a manual mill, unless you have either a highly-experienced machinist or a moderately-experienced machinist and DROs. Otherwise, it's way too easy to put the bearing holes a few thousandths off which could cause excessive binding of the gears.

[Jared]

My biggest concern would be the bearing in the .125" thick plate with a cantilevered spool at the end. If you're moving that six stack up and down quickly, there's going to be a lot of forces trying to bend the plate or make the hole into an oval.

Art Dutra brought up a valid concern with putting too many bearings on the same shaft and 'overlocating' it, but I think you could get away with a third bearing on the plate closest to the output side if you used a previously cut plate as a fixture for the next plate so they would be lined up. You're also using hex shafts and hex bearings, which tend to have sloppy fits anyway.

[SerpentEagle]

Originally Posted by pmangels17

If you have access to a manual Mill, you might be better off just doing the whole thing out of quarter inch plate, and doing the lightening by hand in rectangular patterns or round holes. Other than that it looks great!

Thanks. I don't think manually milling it would be a good idea, as any off-measurement would cause problems. Hopefully I find a close by fabricator with a cnc mill. Thanks for the help!

[SerpentEagle]

Originally Posted by artdutra04

Don't do this. This will mean that the shafts would be quadruple supported by four different bearings in four different plates. This is not desirable, as any slight differences in the location of the holes in each of the gearbox plates will cause binding in the bearings.

The only reason why singular 1/8" 6061 aluminum plates without any flanges might have issues for FRC gearboxes is because of flexing in the gearbox plates. Thicker plates or flanges make the plates stiffer. If you have a small gearbox, one 1/8" 6061 plate on each side of the gearbox without any flanges will most likely work just fine, as they won't be able to flex that much.

Alternately, if you have a larger gearbox you can most likely get by with only one 1/8" 6061 plate per side without flanges as long as you have frequent standoffs between the two plates, or if the gearbox plates are tied well into other structure on your robot in a way that prevents the gearbox plates from flexing.

The flexing of a gearbox is not desirable because it could cause bearings to pop out of their holes or shafts to bind.

I would avoid making gearbox plates on a manual mill, unless you have either a highly-experienced machinist or a moderately-experienced machinist and DROs. Otherwise, it's way too easy to put the bearing holes a few thousandths off which could cause excessive binding of the gears.

What if instead of bearings, I just leave a slot with a bushing slightly larger than the shaft on the outer plate? Or would there be too much friction when load is taken? Ill go ahaid and create some more standoffs in
the critical areas. Thanks a lot for the help!

[Joe G.]

Originally Posted by SerpentEagle

Yeah we do, but correct me if I'm wrong, flanges on the edges would only prevent the plates from twisting and bending, but the segments holding the bearing seats would still be able to flex

Go for the flanges. Unstiffened, any bends of the plates large enough to cause issues would have to go across the entire plate. You probably won't have any issues with distortions which only exist in the center of the plate, which are the only kind which will be allowed to form with proper flanges in place. If you're still concerned, ease off on the lightening a little bit, it's probably overkill for 1/8" plate anyways.

My team has done similar gearboxes in sheet metal with flange reinforcement in .090" thick metal with no issue, and I've heard of lighter.

[SerpentEagle]

Originally Posted by Jared

My biggest concern would be the bearing in the .125" thick plate with a cantilevered spool at the end. If you're moving that six stack up and down quickly, there's going to be a lot of forces trying to bend the plate or make the hole into an oval.

Art Dutra brought up a valid concern with putting too many bearings on the same shaft and 'overlocating' it, but I think you could get away with a third bearing on the plate closest to the output side if you used a previously cut plate as a fixture for the next plate so they would be lined up. You're also using hex shafts and hex bearings, which tend to have sloppy fits anyway.

Yeah that's true. I'll go ahaid and experiment with 1/8" thick sheet metal and bearings at our next meeting, and make a little prototype with just the shaft and bearings so that I can apply a load and see how it holds up. Thanks!

[philso]

Originally Posted by artdutra04

I would avoid making gearbox plates on a manual mill, unless you have either a highly-experienced machinist or a moderately-experienced machinist and DROs. Otherwise, it's way too easy to put the bearing holes a few thousandths off which could cause excessive binding of the gears.

What about clamping and bolting the pairs of the plates together then using the mill to drill the holes for the standoffs and bearings?

Start by clamping the pair of plates together then using the mill to drill the holes for the standoffs. Bolt the pair of plates together using the standoff holes. Machine the holes for the bearings so that the holes in the two plates match. You may have to make a block to support the work piece that has recesses for the bolts used to hold the pairs of plates together.

[cadandcookies]

Originally Posted by philso

What about clamping and bolting the pairs of the plates together then using the mill to drill the holes for the standoffs and bearings?

Start by clamping the pair of plates together then using the mill to drill the holes for the standoffs. Bolt the pair of plates together using the standoff holes. Machine the holes for the bearings so that the holes in the two plates match. You may have to make a block to support the work piece that has recesses for the bolts used to hold the pairs of plates together.

Would you try to do the gear spacing this way? Seems to me that would still be difficult to do manually, though I have limited machining experience.

[asid61]

Originally Posted by cadandcookies

Would you try to do the gear spacing this way? Seems to me that would still be difficult to do manually, though I have limited machining experience.

I usually clamp all my plates together and mill them at once; it's far easier to do all the plates at once than one at a time. If the plates are all the same, I just clamp them all together with some kant-twists.
From the design I'm seeing here, you could try and make all the plates identical and do it on a manual mill. However, lightening patterns on a mill are rather tricky to pull off, and that may be a deal breaker for you if you're dead set on making this thing light (in which case I'm wondering how you plan on lightening that gear).
Putting bearings a few thousandths off on a mill is really hard if you're using a DRO, and still pretty hard if you're not. The mill takes the guesswork and eyeballing out of it in my experience, as the dials are all graduated and easy to read. If you're using a bad mill, that's one thing, but it's hard to screw up on a Bridgeport.
Flanges are magic. We supported the entire superstructure and our intake last year with a 1/16" thick sheet metal gearbox which also powered our intake. It held up until a head-on collision with our intake bent it up.

If this is your first year doing this, you're really good. Keep it up!

EDIT: You may want to opt for a 2-plate design if only 2 plates will have bearings anyway. JMO.