Elevator Design Help

[grem110]

Hi,
I'm on a second year team with only freshmen and sophomores (new school) so we're fairly inexperienced. We designed our elevator using wire rope and a worm (20:1, CIM). When we first ran it, we simply couldn't lift our grabber. Slapping a 7:1 planetary gearbox and reducing weight helped, but the worm started to skip and eventually ground down the worm gear into nothingness. We improved the rigidity of the assembly but didn't have time to test with a new worm gear (delays due to East coast storms). Our current plan is to fabricate a gearbox out of 4"x4" aluminum that's perfectly centered on all sides and hope that a new worm gear will do the trick. However, I'm still dubious that it will be capable of lifting our mechanism + 6 totes and we've majorly compromised our speed. I was stupid enough not to check Chief Delphi before designing, but now that I have, I'm trying to figure out where to go forward now that build season has stopped.

Would counterbalancing the elevator be a worthwhile endeavor? If so, do you recommend constant force springs (wouldn't they snap) or surgical tubing?

Would the elevator be effective without an anti-backdrive mechanism? I saw a Ri3D team that just used a 70:1 cim to drive their spool.

Any other thoughts? We're also pretty close to weight limit and are probably going to have to end up cheeseholing at competition regardless...

[Ether]

While you're waiting for responses, providing more detail would help.

How is the "worm" connected to the wire rope, and how is the wire rope connected to the load?

For example: the worm (part number ???) drives a ???-tooth gear which turns a ??? inch diameter winch drum which reels in the rope which goes over a pulley at the top and directly lifts the load. You get the idea. More detail. A series of properly-focused pictures taken from several angles would help greatly.

[asid61]

Wire rope? Does it wrap around the worm, or is it simply attached to a pulley?
If it's the latter, reducing the diameter of the pulley should act as a way to reduce weight and speed. We are using a 1.08"-ish pitch diameter timing belt pulley for ours.
IMO ditch the worm gear entirely and just make it a two-stage versaplanetary. We were running fine on a 28:1 reduction on an RS-775 (at least for a few totes) so a cim with a two-stage versaplanetary could suit you well.
Best of luck!

[Jared]

Can you provide some details on your worm gear? If you could fix the worm transmission, I think it would be easier than designing a new gearbox.

We're using a worm gear in our transmission, and we've experienced no problems after tons of abuse.

I would recommend using a hardened steel worm on a bronze worm wheel if that's not what you're already using.

Also, you should be using lots of grease on the worm gear, and your alignment needs to be pretty accurate. If there is slop, or if things are misaligned, you can really wear out the worm gear quickly.

If you can, could you post a picture of the worm transmission?

[dradel]

Why go for 6 totes. From what I have seen from most teams is it is going to take to long to make 6 stacks. And really a 6 stack is pointless without a can on top.

My thought is it is better to have a team make a 2 or three stack quickly and place on the scoring platform and perhaps have another team make another stack with a can at same time and combine them. Or perhaps a team that can top a stack with a can with a noodle.

There are many ways to go about it, efficiency is the key in my opinion.

[Joey Milia]

Posting a picture and more information on the worm gear set up would defiantly be useful. For the worm gear to work well you need to support it really well, or else you'll end up with alignment and wear issues already mentioned. I don't think this is a high shock load application so a cast iron worm gear will probably work if you can't get a brass one quickly.

It would help to increase the reduction in the worm gear stage, instead of before it, using something like a 40:1 or higher because you'll be putting less load on the worm gear teeth.

Counter balance will defiantly help. Appropriately strong constant force springs would be ideal but surgical tubing can work well too and will probably be easier to implement.

[grem110]

Sorry about the lack of detail.

The CIM Motor has a 7:1 Vex Planetary Gearbox attached to it facing the front. Hardened steel worm HLVH1 is attached to an 8mm output shaft by a spring pin which connects to a cast iron worm gear CG1042. There's a plate that mounts to the gearbox and also a supporting plate on the other side of the worm to improve rigidity. The worm gear is mounted via spring pin on a 1/4" shaft, which via a rigid set screw coupler connects to a Toughbox mini output shaft (am-2566). a 6" diameter drum supports 1/8" wire rope that goes up to a 3" diameter pulley on the top.

The pulley goes down and then loops around the grabber mechanism (2in pulley diameter) back up to the top, where it has a fixed stopping point.

We put a ton of Red N' Tacky Grease (white lithium grease is on order, we didn't have any) which became Black N' Sketchy when the worm gear ground down. One of the other things we need to improve is that spacing plate, but we know how to fix that issue.

Here are the best photos I have:



I don't have any good photos of the pulley arrangement, but here's a bad sketch of it.

[MaGiC_PiKaChU]

our team has a similar design but we use 2 mini CIMs on a 20:1 versa gearbox that runs a vex pro hex-shaft a chain to raise our elevator... we can lift up to 8 totes with a can on them and it's one of the fastest i've seen so far

[asid61]

The alignment of that worm gear is probably way off, with all those plates and screws.
If you replace the 6" drum with something like 1" and fix the worm gear you'll be fine. I still think a 2-stage versaplanetary will suit your needs, and it's going to be a lot simpler to implement than the worm box.
If you want to keep the 6" drum you will need to gear down a lot more. JVN design calculator will help you figure out exactly what you need in terms of motors and gearboxes.

[Jared]

We've used 16 pitch worm gears twice now, and we highly recommend the bronze worm wheel over the cast iron. The part number for the bronze is (from martin gear, not boston gear) wb1620. We had a few cast iron ones fail in 2013, but no bronze ones. We drove the bronze 1:1 with three CIM motors, so we saw, at most, 3x CIM stall torque. You have a 1:7 reduction on the CIM, so you could see up to 7x stall torque, so I would be a little worried with your setup, as I remember we were slightly above the max ratings at stall.

If you have room, you may want to try heavier duty worm gears.

[Knufire]

Worst case scenario, if you have the funds, you could purchase an AndyMark RAW Box.

[dradel]

I am assuming you are using the worm setup to prevent back driving.?
Perhaps get rid of the Cim and worm setup and go with a window motor ?

[Ether]

Originally Posted by dradel

Perhaps get rid of the Cim and worm setup and go with a window motor ?

You might want to compare the output power of a window motor to the power required to lift the load at the desired rate.

[Joey Milia]

I agree that your current issues are probably the result of poor alignment because it looks like none of your plates are directly connected to each other. If you can make the gearbox out of a single piece of 4x4 I think that would help a lot. I would still suggest a larger worm gear though. You should probably go with bronze as Jared said, I have no personal experience with cast iron worm gears so can't say anything for them.

(I just want to say thrust bearings please)

Originally Posted by Knufire

Worst case scenario, if you have the funds, you could purchase an AndyMark RAW Box.

This is a possibility if you also decrease the cable pulley diameter significantly.

[shewejff]

Worm gears rock, but my recommendation would be to go to a smaller drum. You are putting too much torque through the worm gearbox. A small drum and a large drum have the same force exerted on them from the wire (the weight of the totes), but with a smaller diameter you will have less torque on your gearbox which will result in less contact force between the gear and the worm, which should result in less wear. You'll just have to spin it faster which is no problem just by tweaking your Versaplanetary ratio.

We are using worm gears (steel H1407RH worm with bronze DB1400 gear, 10:1 gear ratio) with a CIM mounted to a 3:1 Versaplanetary with a 1.29" diameter sprocket as output. We aren't having any issues lifting the totes. We also used a very similar setup a couple years ago to lift our whole 100 lb robot to climb the tower. We had no noticeable wear at all through two competitions.

We made our gearbox out of a 2" X 3" X 1/8" rectangular aluminum tube. That works really well for the size of our gears, but the holes really need to be drilled perfectly. With our rudimentary workshop tools, it took us a couple tries to get it perfect.