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Building a turntable--help!

Discussion in 'Scenery, Props, and Rigging' started by dwsobel, Nov 2, 2003.

  1. dwsobel

    dwsobel Member

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    I am designing a turntable for an upcoming show, and I had some questions. It will be 8' in diameter, constructed out of 3/4" plywood. 2 8x4 sheets for the bottom, lapped with 2 8x4 sheets @ 90deg on top of that, glued and screwed.

    I have right now 12 4" rigid rubber casters that I plan of having tangential to the movement of the turntable, but I don't know whether to screw them to the turntable itself, or the screw them to the deck and have the turntable sit on top.

    I also am a little confused about how to center mount the turntable. I figured that 1 1/2" steel piping with a flange would work, but I don't know. Any thoughts on this?

    Also, Will noise be an issue with moving it? I was thinking of having 4 casters 2 feet from the center of the turntable, and then 8 casters on the edge (4 feet from center). Will rigid casters work? ( I thought they would because they would be tangential...)

    Any advice/suggestions?

    Thanks, David
     
  2. ship

    ship Senior Team Emeritus Premium Member

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    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.
     
  3. dwsobel

    dwsobel Member

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    Just finished it, 8' diameter made out of particle board (my carpenter assured me it would be solid as a rock, and weigh a ton :D ), with 4 swivel casters @ 1' from center, and 8 rigid casters @ 3' from center. Works great! Thanks for the helpful hints Ship! David
     
  4. wemeck

    wemeck Active Member

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    One thing you might want to be mindful of is the overall weight of the unit. I did the Music Man back in the early 90's and the set was three 16' revolves with the library, butcher shop, and house, etc. Well the over all weight of the units grooved the stage floor. We had two circle grooves in the floor. In retrospect we should have but masonite down to protect the stage floor some.
     

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