Another Large Revolve on a Budget

scrogers

Member
Hey all,

First, big thanks to all those out there who help post on these forums for all things tech theatre. CB has been an invaluable resource to me for years, and now looking for a little guidance...

I'm looking to build a 24ft turntable/revolve and I have experience building a couple different versions of a turntable, but am intrigued by a design I've recently come across.

http://theaterdad.blogspot.com/2011/06/how-to-make-revolving-stage.html

Before I found the above link, I was essentially planning for this sort of design. Two layers of ply oriented 90 degrees from each other to form the disk over a bed of casters on the floor in concentric circles. I'd love some input on a few things from this "theaterdad" method.

1- The above method is designed around a 16ft revolve, but, think it would essentially "scale up" to 24ft?

2- Theaterdad used two layers of 1/2in ply, and was all about keeping it light. I too am concerned about weight, for reasons mentioned next. I was planning to go 5/8in since mine will be larger. That said, it seems like eveyone does 3/4, but I'd love the weight savings of 1/2 or 5/8th. Thoughts?

3- What I really liked... was his manual operation method. The "cog rope cup" as he describes, basically takes a knotted rope and loops it around the edge of the disk, and catches on several "cogs" or grooved 2x4's (see his description). I really like this idea because- a) it's relatively inexpensive, b) appears rather simple, and c) is operable from off-stage, BUT, would it scale up to a 24ft table? Rope suggestions? Any huge risks?

4- I'm planning for rings of casters at 3', 6', 8'8", and 11'4" from the center. My questions is, if I use the commonly used pivot method described all over CB of two pipes with flanges set within each other with grease, will the center of the disk have enough support? I have used a car hub and bearing assembly before as well as a slightly different take on the sleeved pivot, but the sleeve doesn't provide any support (vs say a car hub). No one has had issues with not enough central support? I'm just worried with a central ring of 3ft of center (so 6ft diameter) from center and 5/8in ply.

Basically, I really wanted to go full automation for this show, but after pricing our everything from rentals, to construction etc, motorized automation is just out of the cards for us this show. That said, again, this rope method looks slick... but will it just be impossible to move a 24ft table, let alone how about when I get a thousand pounds of scenery on it, and 20-30 people(?). The blog mentions how easy it was for "a 16 year old girl" to spin the empty table at ridiculous speeds, and still no issues for a strong person when loaded up... I just worry I do all this... and then cannot move it...

Again, thank you all in advance for any thoughts, but just trying to get some experiences or input on the above ideas before I'm deep into a design that's potentially problematic from the beginning.
 
psychlorama" e the time to touch all the details but yes you can build a manually powered 24' turntable. I did 40 years ago for a summer theatre and it worked all summer - maybe 40 performances. It was wire rope and a shop built (wood) hand winch.

The support - at center and elsewhere - is about span. Your span between supports across center is 6' but 3' or less further out. Rethink caster spacing. Add another ring and use double or triple ply and skip the framing. (Think about cutting middle layer of 3 enough smaller so it firms a groove for running line.) And I'd use fixed, not swivel casters, for sure.

And I have difficulty taking scenery advice from someone who uses the non-word "psychlorama" for a flat drop - just so many things wrong with that.
 
Bill,

Thanks for the reply!

Yeah... admittedly, I too was cringing a little to the cyc (and a few other) references...

About 15 years ago, I was involved in the construction (but not design) of a 16ft turntable similar to what you're describing, but we used 1/4in braided cable and a manual/hand winch (that we all called "the coffee grinder") to spin the revolve. It was a 3-layer sandwich design with the center layer/disk smaller to allow for a groove for the cable. The final product looked good in theory, but ultimately was incredibly heavy. The issue was we found the cable had to be under such high tension to make the table move reliably (and not slip) that we ran into all sorts of issues (cable stretch, splicing breaks, resplicing, super beefy tensioners.. etc). This also meant the pivot point needed to be quite sturdy.

Mostly, I liked the idea of these "cups/cogs" and a knotted rope, because it appears to allow for a relatively reliable (and slip free) method of spinning a table, without creating a central groove with a 3rd layer of ply (which adds cost and weight). The table is relying on the knots and cogs rather than tension around the disk. I worry that using rope or cable in the central grove method will land me back with the slipping issues(?).

Yes, thanks for the recommendation about casters. After posting last night, and doing some more reading and discussing... I think at least another circle of casters is needed, and trying to get the gaps down to more like 2ft rather than 3ft between each. I'd also plan for all non-swivel casters. I actually currently have a bunch of rather nice ones purchased for the project 15 years ago, I just need to get more. I also totally agree with what others have said, buy the nicest caster you possibly can. They TOTALLY make a difference.

I would plan to use two layers (not 1) of 5/8in ply with the seams at 90 degrees. I feel like that's a decent blend of weight savings and strength.

I do wish I could find someone else who's thought of, or used that rope knot cog method because again, it seems pretty solid in theory, but theories are just that.

Thanks again for any other thoughts!
 
I should clarify that in my reference the wire rope was attached and wound several times around revolve - so you could only go so many turns one way before having to reverse. It worked for the two (of four IIRC) shows in rep. Avoids slip problem. I think I had a tensioning device - like two pulleys with a tourniquet between them.
 
Research air casters. One source: www.aerogo.com
52 ton tank on an air caster revolve:
aircasterRevolve.jpg
 
While the revolving tank looks pretty epic, I think the air casters are a little out of our budget at the moment.

Putting the drive mechanism aside for a moment- I'll admit I'm having a hard time settling on a final table design. I had initially been thinking two layers of 5/8 ply cut into a circle, then simply laid on top of each other with the seams at 90 degrees, and screwed together. I have actually seen this method done (with 3/4 ply though) and had a recommendation of this since it's rather straightforward, light, and will breakdown back into 4x8 sheets (or less with the edges).

Recently, I've gotten another recommendation of making stretch skin platform pie wedges, which I understand would be stronger in terms of "flex," however, I think I'd need thicker ply, and lots more of it (not to mention 5/4) to make this work. I feel like this will ultimately add more weight (and cost) since it's now essentially the same design as two layers of ply, but with 5/4 framing in the middle (cut into wedges etc). Not to mention design complexity and I'm building a 24ft revolve, so it'll be a lot of pie wedges if I want to try and keep it out of single sheets of ply.

I have done considerable reading here and elsewhere and see the sandwich method (three layers, pie wedge, interlocking), the "Yale method," as well the stretch skin/framed method (mentioned above). I guess, if I get decent quality ply (BC or better) and simply lay two layers of 5/8 together with 80+ casters (on the floor facing up) every 2ft or less... am I setting myself up for issues?

Thanks again... just feeling a little stuck...
 
If by "strectch skin" you mean "stressed skin" I think that would be a lot of work for little gain unless you are planning on using this revolve a lot. Stressed skin panels are efficient for lengthening spans while minimizing thickness, but that does not seem to be the problem you are trying to solve.
 
I think your problems will occur not with the turntable empty but trying to turn it with the load you describe. That is why I brought up air casters, because the whole thing becomes frictionless when it is supported by the "air cushion".

The air casters I used over 40 years ago while in graduate school were the 8" AeroGo model, which was basically a vinyl or neoprene "doughnut" with an air inlet fitting. These were basically just "replacement" casters and not complete pallet assemblies. We used four of them on a large wagon that carried a set up and down a raked stage. There was a steel guide fin that rode in a slot in the rake, and a loop of rope was attached to each end of the fin and around floor-mounted pulleys at the upstage and downstage ends under the rake. One stage-hand sitting under the upstage end of the rake moved the wagon. He had to be careful because if he released the rope, the wagon would glide down the rake and crash into the orchestra pit!

As I recall (40 years ago) the air casters were quite inexpensive as they were just a vinyl/neoprene doughnut with a masonite landing pad /mounting block in the middle. The real trick was supplying enough air at low pressure and high volume, because it's the escaping air that "floats" the load. It was probably 20-25 PSI at maybe 10-15 CFM per caster. One of the shop staff had the bright idea to get an emission-control pump (remember this was the 1970's!) from a salvage yard and hook it up to a high horsepower electric motor (borrowed from the theatre's HVAC system) because under the hood it was driven off the car engine. It supplied the necessary low pressure/high volume air but was so noisy it had to be located in the basement under the stage with a large air hose coming up to the wagon.

The air casters typically lift a wagon about 3/4" off the deck and if the wagon framing is not rigid, there can be issues.
The smoother the floor, the less air loss as well. The only noise is the hiss of escaping air, and not the rumble of casters.

You will find it a fascinating experience should you decide to go this route. All of the air caster manufacturers have applications engineers who can offer advice for using their products and could suggest ways to supply the necessary air volume. Renting some kind of blower to supply the requisite pressure/volume is another possibility. I wouldn't be too quick to dismiss the air caster solution.

Here is another website: http://www.solvinginc.com/air_film_technology.htm[/QUOTE]
 
Haha... yes, thanks robartsd, I most certainly meant stressed skin (rather than stretched), platform wedges.

Yeah, I don't really care much about the thickness, but do care about the strength. I feel that two layers of sandwiched 5/8 BC ply sheets (seams at 90 degrees) will be plenty strong, but I acknowledge the risk (or more of a risk) of warping vs building the stressed skin wedges or platforms. That said, if the ply is badly warped to begin with, even a stressed skin platform won't help (the way I see it).

Thanks for this, I think I am leaning back toward two large 5/8 ply disks sandwiched... I know there are some assembly challenges as I'll basically have to lay it all out and then screw together on top of the caster bed... but, the construction and design feels more straightforward than some other options...
 
Whether double 5/8 is strong enough is a function of the span between casters.

Stressed skin does not require thick ply. Can do with as thin as 1/8" but easier to work with 1/4 or 3/8. Depending on core - 5/4 or other lumber or foam - it is more labor intensive but much lighter and can be thinner of similar spans.

Aircasters are great, if the noise is not a problem (farting us how some describe it) but must be higher cost.

A lot of analysis for structure and casters to find most efficient combination.

Keep in mind double 5/8 is half as strong as single 1 1/4 ply. I'd need to calculate but pretty sure 1/2 and 3/4 us stronger than double 5/8.
 
Bill

Can you clarify for me why a 1/2 +3/4 would be stiffer than two 5/8. I'm not understanding the physics. ( assuming they have a solid connection between the sheets )
 
Thanks again for the replies.

I spent a little more time looking at air casters. While I don't have exact figures, the basic casters do appear pretty simple, but I think the costs associated with the extra equipment etc needed to run/operate the caster still makes it a bit of a reach for us over a fixed caster. Additionally, I agree with Bill that at least in all the videos I've now watched with them, the hiss or "fart" produced by the caster ranges from a "slight noise" to quite a noticeable sound. I think it'd be an issue for some of my directors.

The stressed skin recommendation to me was 1/2in ply and 5/4 frame 2ft on center (~1'10" spans). I questioned that much of a span with 1/2in ply (since that leaves a nearly 2ft area of 1/2in ply unsupported), but then also read elsewhere the "gaps" could be filled with foam, or basically remove the interior 5/4 framing all together and use foam. That does seem strong (if foam filled) but I also question the weight. Does seem to make sense though if solid foam and thin skins. That said, I also agree with Bill that the stressed skins are more labor intensive, and I'm still not sure it's worth it for my project. I have no intention of ditching my ply/table when I'm done, but I think the individual ply discs will store better as 4x8 (or smaller) sheets than 8-9ft stressed skin pie wedges.

As for casters, I have 90 rather nice Bleutec (now Blickle) 4in rigid/fixed casters. Some are a older than others, but they are really pretty unbelievable how smooth and quiet they are. So, I've upped my casters from my initial design/post and am planning for rings at 1', 2', 4', 6', 8', 10', and 11' 4". This brings the spans as low as about a foot, up to about 2ft between casters in any direction.

I have only worked with 1 1/4 ply once (or it was something close to that, might have been 1 1/8 or something) as it's pretty hard to source (and expensive) in my experience, but it is pretty awesome stuff. Very rigid, but also crazy heavy. I'm not wedded to two layers of 5/8, especially if 3/4 and 1/2 are stronger, but from my calculations, they are basically the same weight, but unsure about "strength"(?).

Thanks again for the guidance.
 
Robartsd,

Thanks for the reply. Just wondering your (or others') recommendation? In my initial plan/post here, I was certainly low on casters (more like 60). That admittedly left some gaps well over 3ft (not including the center span which was huge). After talking with others (and recommendations here), it was suggested to be more like 80+ casters, so I increased the number of rings and upped it to 90. With the rings in the above mentioned positions (2 posts up), I think that leaves a maximum gap of 2ft-ish (or just over) with some even less.

I can get more... but just trying to get a sense if I'm "a few short" or like another 20-30 short... (for cost... sorta hoping closer to the first option than the second.)

Thanks!
 
Here's my thinking on estmating how many casters you need:

A 24 foot diameter circle is 452.4 sqare feet. 2 feet by 2 feet is 4 square feet (if more than 2 feet between rings is too much, more than 2 feet between casters in a ring is too much). 452.4 square feet divided by 4 square feet is 113 casters (I'd probably round that estimate up to 120 since you aren't laying them out on a grid). So it looks like 20-30 short is more like it. That is unless you're building a frame over the casters to support the deck - but my impression is that the deck will be two layers of plywood supported directly by the casters.
 
Here's my thinking on estmating how many casters you need:

A 24 foot diameter circle is 452.4 sqare feet. 2 feet by 2 feet is 4 square feet (if more than 2 feet between rings is too much, more than 2 feet between casters in a ring is too much). 452.4 square feet divided by 4 square feet is 113 casters (I'd probably round that estimate up to 120 since you aren't laying them out on a grid). So it looks like 20-30 short is more like it. That is unless you're building a frame over the casters to support the deck - but my impression is that the deck will be two layers of plywood supported directly by the casters.

Yeah, as you've probably seen from the above posts, I was leaning back toward two 5/8in ply disks with seams at 90 degrees, rather than stressed skin or framed wedges/platforms. So, the casters would directly support the two layers of ply.

While I think the actual point-to-point distance between casters for much of the table would be at or around 2ft, your math/logic is solid (and doesn't lie). To get all spans below 2ft, I need more like 110-120 casters or "points." I haven't had time to redraw out all my caster points in CAD from my initial (way under speced) design, so I need to sit down and do that... then I can feel more confident.

Makes sense though that with two layers of 5/8 (unframed), I really need to get all/ever spans below 2ft... Thanks... off to reevaluate some math...
 
if doing casters up , lay some out and lay two sheets of ply on them and (very carefully) walk and jump around on the ply and see if 24" or 28" or whatever will be good enough. You'd really like to cut one sheet in half so grain is opposite and you can see the effect of a seam. Nothing like empirical testing.
 
Yeah, a little real-world mockup is a great idea. Unfortunately, I don't stock much 5/8in ply, but I may still give it a shot.

I still haven't placed them out in CAD yet, but I have run some more math, and... yeah... well, to get the arc distances below 2ft between them on the circle will require quite a few more casters. Here's what I'm seeing:

Ring Rad - Circumference - # of Casters - Distance Apart

1’ - 6.28ft - 4 casters @ 1.57ft
2’ - 12.57ft - 7 casters @ 1.79ft
4’ - 25.13ft - 13 casters @ 1.93ft
6’ - 37.7ft - 19 casters @ 1.98ft
8’ - 50.27ft - 26 casters @ 1.93ft
10’ - 62.83ft - 32 casters @ 1.96ft
11’4” - 71.19ft - 36 casters @ 1.97ft
Total - 137 casters

That is quite a bit more than 90...

Unfortunately, between these caster increases and the need for good quality ply as my bottom "rolling" layer, costs are getting up there. I won't cheap-out on casters either since I know how much of a difference good ones make.

Going to need to come up with a rather cost effective manual drive system for this thing...
 
The first place I'd look to save money on this project would be reducing the size of the turntable - more than 25% of your casters are in the outermost ring - but I don't know how the decision that this needs to be 24 foot diameter was made.
 

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