Stagecraft archive
http://stagecraft.theprices.net/stagecraft/ has had lots of tips about turn tables posted to it. You should consult it’s past subjects for a lot of help on it.
First, I am not sure that with only a double layer of plywood, with casters mounted 24" on center but only 12 of them, that you will have adequate support on the
platform. Depends upon how much both live and dead load are put on the plywood and what type of plywood it is such as say 7-layer verses normal 5-layer, you might and probably will need a rigid frame such as a steel one between them for more support.
Assuming that you were not planning on laminating the sheets permanently together, you would need drywall screws fastening them on approximately 4" centers around the edges and otherwise 6" centers all over the middle to achieve almost the so far desired 1.1/2" thick plywood strength and give you at best a 15# live load per
foot on the
platform. Way too low for an acting area with that kind of span between supports in my opinion and that's given the center pivot counts as a
leg.
Yes, platforms have single layers of plywood supported on 24 to 30"x48" spans, but it’s rigidly attached to a 2x4 frame, the plywood on the
turntable would not be attached to it’s supports and in my opinion would need more structure to it.
That said and to cut down on weight, given a 16ga box steel 1x1 welded frame, or 2x4 glued and screwed frame, the bottom plywood could be cut away into circles following the
caster supports without a problem. It’s purposes are in counter sway and support for the
platform at the casters. While the extra weight involved in doing a full sheet below the
platform might help in it’s stability, it will also add resistance due to it’s weight to being moved.
A pipe
base center is the best pivot
point to use for keeping the
platform aligned. Something such as a greased 1" Sch. 40 pipe welded to a 1/4 or 3/8" thick steel plate should suffice well, but in a pinch a plumbing
floor flange should work fine. It’s just not designed for the stress and might break if the
platform is jumped onto or given lateral force however. A 1.1/4" Sch. 40 pipe works really well in sleeving over the O.D. of the 1" pipe - better than most other sizes to each other. I use a similar pivot on the cantilevered
boom arm holding up my computer and dual monitors. Again, a welded attachment of the pipe to a thick
base will serve best. If possible, the 1" pipe in touching the top plate would be best terminated with some kind of ball bearing plate to
pick up and account for the weight of the
platform and prevent metal on metal friction you will get if otherwise this is how the center of the
platform is supported. This when dry might squeek and dig into the plate above. Short of mounting a ball bearing plate to the top of the 1" pipe, Perhaps it might be possible to drill an access hole from the top of the
platform into the space between pipes and pump it full of grease when it gets loud. This should help some. Otherwise the pivot
point would need to be non-load bearing to stay for the most part friction free and noise free. This would require more casters and a framework to support the larger spans. A further note would be for the welding seam on the cold rolled steel of the pipe. Unless smooth, it would need to be sanded.
On casters, it’s much easier and better to install the casters on the
deck, and provide a smooth high impact surface on the bottom of the
platform to
roll on. Lots of reasons for this such as should the surface the wheel have some give to it, the
caster can compensate without stressing the
caster in general, verses with the
caster mounted to the
platform, the entire
caster will be stressed. In general it's just easier to align the casters on the floor than to the
platform.
Masonite or other high impact/density materials noted in stagecraft work well for this bearing surface the casters
roll on. - Better than plywood that will compress with weight given the small surface area of the casters imposed on it. It will also be quieter because should the plywood have any flaws to it such as gaps in the plywood lamination, the casters will not break
thru into it and cause a noise each and every time it hit the pot hole - no matter how small.
You need to be really aligned with the non-swivel/dumb casters or you will get a lot of extra friction above the friction already imposed upon the
platform by the inner
edge of the
caster wanting to spin at a faster speed than it’s outer
edge. Swivel casters will also work well for this purpose and do not have to be as accurately aligned.
In the case of the dumb wheels, use a string and screw at the center of the
revolve and align all casters to the same
point by way of marking where each casters mounting holes will
line up. On this, the better the
caster in quality, and larger it’s diamater, the easier it will spin but the larger your span.
Solid rubber casters with just a
bolt thru them will not spin as easily or last as long as a nylon
caster with sealed bearing around it’s pin. Bearing casters will also be more quiet than normal ones with a metal pin as pivot
point under weight. Narrow casters while easier on the friction in contact with the
platform are usually not as heavy duty or easy in spinning under weight. You will have to balance the two or use some kind of
roller wheel such as at the end of a conveyer belt
system or a large ball bearing stuck in a frame. McMaster Carr has a really good amount of information about the various types of
caster construction and wheel types. If on a budget, it might be possible to
purchase better quality wheels and mount them on your own frame and pivot points.
The closer to center you get the more friction the casters will have to turning a tight corner/radius, but the more support and stability it will give your
platform. The more casters you have, the more friction will be involved with moving them, but the less stress or weight each one will be under. There is another balance to figure out what is best in.
Anyway, just some random thoughts on the subject. I have not built a
turntable for about 10 years now so will have forgotten much by now.