Turning giant turntables

[JCharlton]

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.

[pfreivald]

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

[colin340]

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"

[Andrew Schreiber]

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

[Retired Starman]

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.

[who716]

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

[Kevin Sevcik]

Originally Posted by Retired Starman

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".

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.

[RoboChair]

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.

[asid61]

Originally Posted by RoboChair

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.

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.

[GeeTwo]

Originally Posted by Retired Starman

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.

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.