Design Feedback?
 

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.

Re: Design Feedback?
 

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?

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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 :confused:

Re: Design Feedback?
 

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!

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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!

Re: Design Feedback?
 

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!

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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!

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

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.

Re: Design Feedback?
 

The prinicple Art was getting at is over-constraining a system. A good example is a 3 legged vs. 4 legged table. A three legged table does not rock. It may not be level, but it does not rock, even on an uneven surface. A four legged table is very tricky to not have it rock. This is because 3 points make a plane. 4 points overconstrain. With a stiff driveshaft, 2 bearings will constrain the shaft in all planes except for possibly concentric sliding (you need a collar or a lip of a clip). If you try to do 3 bearings, and they are not perfectly aligned, the 3rd bearing may fight with the other two and cause stress in the bearings which will cause them to fail early.

As far as critique goes, what does the "spool" of your winch look like (from a size perspective). I know that 20:1 for a 2" Diameter spool was just about right for a fast lifter that would meet your desired rating. What DIA are you using for your spool?

I too am a bit worried about the cantileer of the spool driving element as shown. How is it supported? Also, will this be another overconstrain on that shaft?

Re: Design Feedback?
 

Seeing I am electrical and controls...

How 'bout an encoder mount on the output shaft?

Also at that gear ratio, the lift will backdrive very easily, so you may wish to look at ways to add a brake and a brake release mechanism.

Re: Design Feedback?
 

This is slightly off topic, but I'm curious; has anybody had an actual problem with over constraining a hex shaft in an FRC application?

The versaplanetary gearboxes have two bearings on the output shaft, so anybody who supports the vp output shaft with an additional bearing is locating the shaft in three places. We ran setups with four bearings on the same shaft both this year and last year with no problems, including a bearing that was pressed into a welded sleeve that couldn't have been extremely accurate.

I'd guess we can get away with this because of the fit between hex shafts and bearings and the clearance holes for standoff bolts. A #10 is a slip fit in a .1875" hole, so it has almost .01" of slop in the recommended .196" clearance.

I'd also assume that the hex bearings which tend to come in a little oversize are also pretty forgiving.

Re: Design Feedback?
 

We had some minor problems with inserting the hex shafts into bearings in the new VEX clamping gearboxes and the clamping bearing blocks. The hex shaft would go into the hex bearing on one side just fine but would not go into the bearing on the other side. We had to loosen the clamping screws before the hex shaft would go into the second hex bearing. I suspect that the axes of the two bearings were not perfectly in line and/or parallel. Once the hex shaft is inserted into both bearings, the clamping screws could be tightened and the shaft would spin freely. I guess the plastic around the bearings deform slightly to allow them to align to the hex shaft.

Re: Design Feedback?
 

Absolutely. In 2012, we had an intake / ball conveyor powered by a gearbox with two bearings capturing the extended output shaft. We then tried to slide this extended output shaft through another bearing, the conveyor roller, and finally a fourth bearing. As you might expect, the system was heavily binding.

In general, we have found that once you hit the third bearing, in a third part, you get reduced efficiency, and four is a death sentence. The farther apart the bearings are, the less of an issue this is (I.e. if you have a 24" long shaft, you can probably do two bearings per side).

If you need to use more than two bearings per shaft, it can be done, but you need to make an extremely deliberate effort to ensure they are all aligned. In our drivetrain, our gearbox has two bearings, and then the output shaft goes through a third bearing in our tube. We use counterbored standoffs to hold the two gearbox plates captive relative to each other, so those two holes are as aligned as possible. We also counterbore the output bearing so it sticks out of the gearbox. This lets us tuck the bearing into the hole in our drive tube - this hole is cut through both sides of the tube and thus the third bearing is mounted through it as well. When we go to this much effort, we don't run into problems.

The reason this is okay is because two of the three bearings are held concentric with the same feature in the same part. It's very hard for two things in the exact same hole to be misaligned. It's much, much easier for two parts to be misaligned, so it is much more of a problem.

This is actually why I would say you can't get away with it. The slop leads to the misalignment we're talking about. Because both bearings are tucked into the same part in the VP, I think those bearings are as close to perfectly aligned as possible and for alignment purposes count as one of your two bearings.

Honestly, every hex bearing I have ever used has had a far, far tighter fit than any of the gears I put on those shafts. It's annoying.

Re: Design Feedback?
 

If were gonna use 1/8" then its gonna be up to our sponsor to laser cut it for us. I think that 1/8" is sufficient if there are frequent standoffs for support. Anyways, each gearbox (2 in total) will be lifting a max of ~30 pounds (6 stack weight / 2). Illl go ahaid and experiment with flanges too. Thanks!

Re: Design Feedback?
 

I understand, I think ill opt for a 2 plate design with frequent standoffs. As for the spool, the gear ratio is designed for a 2" diameter spool. It will be mounted almost right next to the plate of the gearbox, so correct me if I'm wrong, I don't think that that should be much of a concern. I have also started redesigning with a ~8.5:1 ratio; the math tells me its more that enough to lift 60+ lbs at ~56 in/sec with one RC, that's calculated with the fork assembly factored in too, and ~36 in/sec with a full 6 stack, and this is with two winch assemblies of course. All the tips I received are coming in really helpful, so I'm great full for that.

Re: Design Feedback?
 

Friction is a thing. I calculated for our lift a total load of 120lbs, for 6 totes + RC, and that was too low. In the end I should have calculated with around 160lbs, because we were using a forklift style lifter. I think using a C-shaped chassis would change how much friction you have to account for.

Re: Design Feedback?
 

Yeah we were planning for that since the start of the season, but we never got to it for the lift. Even if its included, I don't think we will have the time to make use of it for automated actions, but If our programmer is confident, then ill go ahaid and add it on. Speaking about back drive, I'm still trying to find something like a break, ratchet, etc. Worm gear is kind of debatable for us since our original lift design used one, but slight miss alignments and improper lubrication caused huge amounts of friction. If anyone can give me some ideas that would be awesome.

Re: Design Feedback?
 

Disc brakes! :D We used one this year and it went perfectly. Just stick a piston on the part that would normally connect to the bike handle and you're good to go. I believe we used an Avid BB5 or Avid BB7.
Mounting it is a little tricky, but the mounting specs for disc brakes are actually listed online.

Re: Design Feedback?
 

Friction isn't a problem for us, we use a c-channel for the racing on the lift along with rollers; quite smooth, but I will have to double check for that.

Re: Design Feedback?
 

I think the friction he is talking about isn't just the simple contact friction between the carriage and the channel, but the friction caused by misalignment between the two channels. If your chassis / structure isn't rigid enough, load on the elevator will cause some degree of flex between the two channels, which leads to more friction force. It's not something to be written off, certainly. Good design can help keep a chassis rigid and elevator beams roughly coplanar.

Re: Design Feedback?
 

Adding an encoder and PID controlling the elevator position is arguably the best way to do this. Trades mechanical complexity for controls complexity, but saves weight and allows students to learn basics of controls engineering.

Keep in mind that you'll be stalling the gearbox motors, so keep your current draw well below 40 amps. I wouldnt worry about CIM motors overheating, but breakers like to blow if you keep them hot for a while.

Re: Design Feedback?
 

Our structure was quite rigid. Most of the friction came form the massive forces on the bearings due to a forklift-like design (so the lever arm on teh carriage was huge) and the motors themselves. 2 RS-775s going through a 21:1 reduction to a 1" diameter pully led to a lot of friction. With no motors it was okay, but once the motors went on it was very hard to lift.

Re: Design Feedback?
 

Good thing you have access to a pro miller at SLEHS! And the part will only cost you $300 per hour that I machine it. Good deal, act fast... :D