Turning giant turntables

[asid61]

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?

[EricH]

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.

[Akash Rastogi]

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.

[Electronica1]

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

[Mike Marandola]

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.

[asid61]

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.

[MrForbes]

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).

[Electronica1]

Originally Posted by asid61

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.

They have links on that page to buy parts separately.

[asid61]

Originally Posted by Electronica1

They have links on that page to buy parts separately.

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.

[Mike Marandola]

Originally Posted by asid61

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.

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.

[steelerborn]

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.

[Chad987]

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

[Mike Marandola]

Originally Posted by Chad987

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

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

[Chad987]

Originally Posted by Mike Marandola

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

Yep! 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.

[GeeTwo]

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.