Turntables are a PITA! if you do some searching on here you will find a TON of articles and post from folks talking about the advantages of motorized or not.
Some of the things to consider beyond all the questions and issues above:
If you want to drive the
turntable one of the best ways is to run a
roller chain around the perimeter. If you want to operate the
turntable manually you will need a
hand crank capstan of some kind. I suck at math but I know the formulas; you are going to need something that gives you enough mechanical advantage that you can easily manually operate the crank. <small drive sprocket> You are going to need something that drives the chain fast enough to move the circumference of an 18' diameter disc at walking speed, or 3-5 MPH <or 5-9 Kph, if you are
@RonHebbard > (that's going to take a BIG sprocket)
One of the biggest issues with moving a
turntable is that initial moment of inertia. A mass at rest wants to stay at rest. There are a ton of formulas that can tell you exactly how much force it's going to take to get that
turntable to start moving then how much force it will take to maintain it at a given velocity. Most of them calculate a best case scenario and therefore you can add 25-50% to the force necessary due to friction. Suffice to say it take a big force to get 700 pound of plywood to start to rotate.
One good design I saw just had a two wheels sandwiching the outside
edge of the
turntable. there was an adjustment that would pinch the wheels together tighter to get better
grip. Try as they might they could not get the wheel to stop screeching when they started it up. this is because a drive motor in a situation like this requires a soft-start. Remember that
Electric motors are Instant Torque; instantly throwing 3HP at a wheel will make it skid on anything. <Another discussion to be had: AC or DC motor?
PWM drive?
etc,
etc,
etc.>
***
Safety ***
Say you've done all the calcs and you are ready to assemble your
system. You've decided to drive it with a motor. Your
voltage supply is correct. Your motor size is sufficient. You have a soft start motor controller or you have a ramp-able
PWM programmed to provide the right start up torque. What about
safety? As stated there are formulas for this but 700 pounds moving at 3-5 MPH <5-9Kph if you are Ron> has a butt-ton of kinetic energy. Now imagine you are an actor stumbling around in the dark and he manages to
wedge himself between a set piece on the
revolve and a
flat on the
stage. "Put the Candle Back!" Yeah you need brakes/braking
system/ dead man stop. If it's an
electric set up you should have at least Two dead man buttons in addition to the one on your operating
console. If you are manually operated you need to devise a dead-man
system in which all operators can communicate and stop the disc at a moments notice. Either way you HAVE to instruct the Actors and Stagehands of the danger involved and the situational awareness that MUST be maintained while working on on around a
revolve.
I Operated a small manually operated
revolve for a show back when I was a kid. Had whole ninja costume complete with Balaclava I had a pole that locked into a
socket on the floor so I could push around the back side and the audience wouldn't notice me. It was small Only 10' in diameter and didn't have much on it and I was young strong and stupid. Yep, had an actor step between to opening for the
revolve and the
edge of the wall opening. Damn near killed him and me stopping that thing but I did it.
I don't know what kind of budget you have or where you are located but you might want to check out "Creative Connors" or one of the other moving scenery companies. They can supply you with the motors and control systems, Hell, they could probably rent you a whole
revolve system, but I don't know what their pricing is like. I'm sure there are other folks in different regions that offer similar services. I'm hoping of of the other members chimes in.