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