Turning giant turntables
 

So I've been thinking about making a turret for the offseason, to train the new one-year vetereans. We have around 3 or 4 working drivebases at this point and I think it's time to try something new that will teach them CAD and gearing and such.
I am planning it out and have run into a problem already: how do I turn a giant turntable? Originally I planned to use a large sprocket, but the Vex sprockets only go up to 60 tooth #35. McMaster is larger but more expensive and requires more machining.
How have teams turned large turrets in the past?

Re: Turning giant turntables
 

Try a Colson wheel on a smooth turret base. The Colson gets mounted to a motor/gearbox that gets you the speed you need.

Reference: Behind the Design, 2006: FRC121's turret power source.

Re: Turning giant turntables
 

One of the options I like is using whatever round thing you want in the diameter turret you want and mounting an inside out timing belt to it so the teeth point out. Use a small diameter belt on another sprocket or whatever on the output shaft of your gearbox. Mesh them together and boom! Super easy, Eric's method works as well.

Re: Turning giant turntables
 

So, are you looking for something this big, or smaller?

Re: Turning giant turntables
 

In 2012 we wrapped timing belt around the outside of a lazy susan and powered it with a pulley on a window motor. (what Akash mentioned above) This displays it pretty well. You could also check out 118's 2008 turret.

Re: Turning giant turntables
 

Does wrapping the belt around something cause a "bad mesh", so to speak?
I didn't know that AM product existed. Thank you for pointing it out!
Likely we might just cannibalize that for the turntable and gears, as the mounting needs to be flexible.

Re: Turning giant turntables
 

We used a timing belt mounted the "right way", so it wrapped around a small pulley. We cut the belt, and screwed one end to the turret, and attached the other end with a spring, so it could skip when it ran into the stop. (this turret only rotated about 180 degrees).

Re: Turning giant turntables
 

They have links on that page to buy parts separately.

Re: Turning giant turntables
 

I noticed that, but I would rather just buy the whole thing just in case we miss a crucial part.
Do you know how the table is vertically constrained? I can't see how it doesn't jump in the cad.

Re: Turning giant turntables
 

The only potential problem would be at the two ends. I think we got lucky and our ends matched perfectly. If they didn't we didn't have to worry about it because we didn't need to go 360 degrees anyway.

Re: Turning giant turntables
 

Well it may not be a turntable exactly, but if you are working on a project in the off-season to play this year's game a geneva mechanism would be really great.

You could power the lifts for all of the sections together, probably at least 3, and load 3 stacks of 6 from the hp and score them all together.

Really hope I see a bot use a system like this. Maybe I will cad one for fun later.

Re: Turning giant turntables
 

In 2006, 2012, and 2013 we manufactured a large (think 15") pulley that was bolted to a lazy susan similar to this.

Re: Turning giant turntables
 

As seen in this video, and one of my personal favorites.

Re: Turning giant turntables
 

Yep! :D Thinking back on it now we probably should have made it out of HDPE or some other light weight plastic rather than aluminum. I also feel the need to stress how important it is to get all of teeth spacing correct. We have an extra one of those pulleys laying around that has around 3 too few teeth. :(

Re: Turning giant turntables
 

We did a turret on our rookie robot with about 200 degrees of rotation. We used a lazy susan bearing and chain attached to the turret with screws. Turning was done with a window motor and about a 22 tooth sprocket, I believe. Not pretty, but it worked.

As we thought later about other ways to do this (including continuous rotation solutions), in addition to solutions similar to those I've seen here, using the teeth of an auto flywheel also came up.

The biggest problem with a continuous rotation turret is that if you want to put anything active on it, you need slip rings or similar. These aren't too expensive for signal level, but those with enough size to power some motors and more than two wires don't come cheap. Larger slip rings often have mercury.

Re: Turning giant turntables
 

We're using a turntable this year, it's powered by a 1/4" thick laser-cut ABS ring gear and pinion on a PG71. Cost all of $15 for laser time plus material. We used Inventor's design tools to get the tooth profiles, and added mounting holes as well.

Previous years we have tried reversed timing belts, flat urethane belts, friction drives but this seems the best.

Hasn't been to competition yet, but worked well during testing.

Re: Turning giant turntables
 

A terrible application for a terrible game, but here's our one turret in the past decade--it was dirt cheap and easy to manufacture (which are good attributes for an off-season project), and could be upscaled/downscaled in complexity/materials rather easily.

It's exactly the idea Akash mentioned.

https://www.youtube.com/watch?v=rfFGwIBrspM

Re: Turning giant turntables
 

rivet some #35 chain tight to the circumference of the turntable (http://www.mcmaster.com/#turntables/=w5dnug) then use a sprocket as a drive "gear"

Re: Turning giant turntables
 

27 has waterjet large gears in the past. http://www.chiefdelphi.com/media/photos/22666

Re: Turning giant turntables
 

One thing to watch is the construction of your turntable. If you are using one of those light-weight lazy Susan bearings, it will deform under the side stress of a belt or chain drive, and even a drive puck. The bearing loosens up and after a little use, it stops turning.

If you have the height to accommodate it, get a water pump from the auto parts store. It has a wonderful bearing, runs smooth, has a base with bolt holes and a disk where the fan clutch normally mounts. It can take all the side thrust you can give it. Should cost about $6.00 for one. Just ask the auto parts guys for the cheapest one they have or ask to "shop" in their shelves. With this set up, you can get under the large disk to mount a normal chain drive with sprockets in the off-the-shelf size.

Otherwise, I've had success with timing belts for driving turntables. But a water jet or laser cut giant sprocket sounds lovely with some 25 chain. By the way, you can get 25 chain in plastic if you need it. If you go with a smooth disk and belt, you might want to use a 10 turn precision pot or encoder on a drive puck to sense your disk movement rather than trying to measure the motion through encoders on the gear box. McMaster Carr sells multi-turn precision pots but they call them "variable output switches". Use that term in the search box to find a variety of them an reasonable prices.

Sounds like a useful project that could be applied to future games.

Re: Turning giant turntables
 

are you familiar with how a snow blower turns the snow chute we have done something similar to that in the past.

Re: Turning giant turntables
 

Why not Mouser? I suspect the cheapest 10-turn 10k at Mouser is as good as MC-Master's standard one for around half the price. And you can get 0.1% linear wire-wound pots for as much as Mc-Master's standard pot.

Re: Turning giant turntables
 

I have a Laser Cutter and a CNC Router at my work in Davis, CA, and I may be able to help you manufacture a solution such as a custom sprocket for #25 of #35 chain after the competition season ends. I also have a Sprocket Profile Generator for SolidWorks 2013 that you can punch in all of the variables such as Pitch, Roller Diameter, and Number of Teeth.

Re: Turning giant turntables
 

That would be the optimal solution for us, as I don't want to worry about maximum turns when moving it around.
Right now I might just use the AM one, but I will contact you postseason to see if it's any trouble to just make a custom one.
Thank you for the offer.

Re: Turning giant turntables
 

Reading this reminded me of our other turntable, on our air cannon. Since the turret isn't heavy (some sort pipes and the brackets to hold them), we weren't worried about deformation under the torque of rotation. However, because we throw t-shirts and footballs, we were worried about deformation due to lateral forces. After a rather embarrasing amount of mental fiddling with a lot of different things we could purchase to handle the lateral stresses, we finally pulled a toughbox 2 out of the old parts bins and mounted it so the shaft points up. We tie-wrapped a long shaft key at the bottom so we don't lose it as often. The turret is bolted to an AndyMark hub, and we hold the thing down with just a 1/4" thumb screw. This turned out to be really useful when we realized that the new tires made the air cannon wider than any of the doors on our build space - we popped the turret off and carried it out sideways. (We have since slimmed down the base.) Currently we're manually aiming the turret, but we have a mini-CIM and a potentiometer already mounted so we can convert the TB-mini into a servo.