Force needed for rotating platform

[IntimeTD]

Hey everyone,

I'm designing a set for Lost in Yonkers, it's a 16'x12' free-rotating platform made up of 6 stock 4'x8' platforms coffin locked together. Each smaller platform will have 6 ball casters (part 5674K57 over at McMaster) to take the weight of the whole set, which I calculate to be around 1300 lbs. My question is basically: will it rotate? If we have 4 stage hands (your average college student) pushing it, do you think we could do a 180 degree turn in a reasonable amount of time?

I've attached a ground plan for reference. It has two rooms, a sofa, bunkbed, side table, and two dining room chairs.

Thank you so much in advance!

 

[derekleffew]

Welcome, IntimeTD. Promoting your, most excellent, post to the Question of the Day forum. Since this area is primarily for students only, there is a "waiting period" for professionals, so if you need a professional opinion in less than one week, let us know. (But I don't think you will.)

 

[gbdesign]

What sort of floor will the platform be rolling on? By 'free rotating' I assume the platform can move in any direction, not just rotate around a center pivot point? Hence the use of the ball casters?

 

[IntimeTD]

@derek: Thanks for the welcome and the Q of the day! Very excited about Control Booth, and luckily I have plenty of time until the build starts.

@gbdesign: The stage floor is tempered masonite, and reasonably level (just had it redone). and yes, it has no central anchor point, since we might have it come downstage for the bedroom scenes.

Thanks!

edit: was reviewing some of the discussion between Van, MPowers, etc in other threads on revolves, and would appreciate if someone could confirm that the ball casters (now part 5674K1, others were out of stock) facing downwards will do the job. Any other tips or tricks always appreciated. Thanks!

 

[mstaylor]

I am not allowed to answer the question but "reasonable amount of time" needs to be qualified. Are you asking it in the sense of will the bearings make it possible. The other part is since there is no center point will the casters create other issues of time involved to turn.

 

[shiben]

I believe it will rotate, if you have 4 larger guys doing it (one on each corner, i guess). My main concern would be how straight it would revolve. Not totally sure on the whole physics of it, would need to know a bunch of things and set up a couple diagrams... However, I just feel (gut thing here) that 9 casters might work better?

 

[IntimeTD]

Sorry, to clarify I guess I'm asking two questions. 1) will the casters I've chosen adequately execute the turn? (I chose ball casters to avoid the torque problems swivel casters have) and 2) given the weight of the set, would stage hands be able to make the turn in around 30 seconds if they were pushing on handles attached near the corners (probably vertical 2x4 secured with brackets so they get more leverage). Obviously they'd have to apply fairly even forces since there's no pivot point.

Shiben, there will actually be a total of 36 casters on the set. I know it sounds excessive, but because the turntable actually consists of 6 4x8 platforms, it needs one on each corner and one in the middle of the longer edge to avoid snapping the frame (has happened to us in the past). I want the coffin locks holding them together to take as little of the normal force as possible and just hold them together for the rotation.

I tried crunching some numbers on the rotational forces needed, but didn't get anything conclusive, so I guess I'm looking to see if anyone has managed to do something of similar scale, or is better at physics then I am

Thanks!

 

[gbdesign]

I would be a little concerned about the small footprint of the ball casters. Even on tempered maso, they might create little dimples when they sit for long periods of time. Another concern is the weight. 1300 pounds being pushed by 4 guys equals 325 pounds per guy. Not easy to get moving from a standstill, and even harder to get to stop if you build up any momentum. Plus you probably have to land the platform on its spike marks every time.

 

[Footer]

Just a few notes:
This set was turned by 2 over 40 women. It easily weight 5,000-6,000 lbs.

And they did it pretty quick....

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In order to find how much force something is going to take, one must consider the efficiency of the casters...

 

[FACTplayers]

It all depends on the type of wheels you use. The structure is very light and will easily move as long as your wheels are rated for the weight. Rolling friction is very low and rolling mass is also much lower than the actual mass of the structure. This explains why someone can push a car (in neutral) or load a wheel barrel up and move the load around.

 

[standup]

I would be a little concerned about the small footprint of the ball casters. Even on tempered maso, they might create little dimples when they sit for long periods of time. Another concern is the weight. 1300 pounds being pushed by 4 guys equals 325 pounds per guy. Not easy to get moving from a standstill, and even harder to get to stop if you build up any momentum. Plus you probably have to land the platform on its spike marks every time.

I can push my dad's pickup truck if it's in neutral. That thing weighs about 4-5X what his estimated weight is. You "should" have plenty of manpower to turn it. I'd agree that the biggest concern would be the wheels go flat. That would make it harder to turn, and i don't know how to avoid that from happening.

 

[drummerboi316]

what about using zero throw casters?
http://www.rosebrand.com/product165...4000&tid=1&info=Theatrical+Casters+%26+Brakes


EDIT: I'm sorry, I have no idea how to link.

 

[MPowers]

Just to clarify Nick's post. Four sources for the product he is describing. In each case it is the tri wheel caster or turtle.

Casters from Rose Brand

Casters & Triway Swivels / Home - Mutual Hardware

Theater Casters | Access Casters Caster Wheels

http://www.bmisupply.com/bmicat/bmicat11/BMI.2011.Hardware.pdf

 

[IntimeTD]

Nick those are some sweet looking casters, unfortunately I have $500 for the whole set, so I need to be rather frugal. But it's good to know it can be done. Looks like my next step is to get my hands on some samples and just play around for a bit. Thanks for all the feedback everyone! I'll post some pictures or video if anyone's interested (so long as it actually works of course )

 

[65535]

One or last Main Stage show we had a 16' by 27' platform rotate behind the curtain, didn't take too long except for the fact that we had a grand total of a little over a foot clearance at a diagonal between the curtain and the back wall.

Shouldn't be too bad, good casters are you friend.

 

[Van]

I think the main theme that you will see from this thread is that "Good Casters are a must". What makes a 'Good Caster' ? Several different factors, depending on application. Let me first state that Steel ball casters, such as those depicted by the OP, are an extremely bad choice. Why are they a bad choice ? Well let's looks at it this way; you are supporting a weight of 10 tons. you have 5 casters distributed evenly around the base of your weight. Each caster is carrying 2 tons, right ? ok, Now how big is the point of contact between the caster and the floor? on a "regular" rubber or urethane or phenolic wheeled caster you might have as much 3/4sq.in.< let's call it 1sqin for ease of mathmatics, shall we?> Ok so that's a point load of 2 2 tons per square inch. That's not so bad as long as you are on a strong, durable, non-flexible surface. Let's say you're on 1/4" Tempered Masonite. 2 tons over a 1sq.in. area might be ok but if you concentrate that same weight into a <theoretical> single point, like would exist at the contact point of a steel ball and a hard surface... you might as well be putting the weight on steel spikes and trying to shove it sideways, The balls are going to dig right into the deck. On a softer surface, like Plywood, the effect would be even more severe.
Shoot, got to go do some work, I'll write some more later, let's see some more thinking about the steel ball caster issues

 

[mstaylor]

I think the main theme that you will see from this thread is that "Good Casters are a must". What makes a 'Good Caster' ? Several different factors, depending on application. Let me first state that Steel ball casters, such as those depicted by the OP, are an extremely bad choice. Why are they a bad choice ? Well let's looks at it this way; you are supporting a weight of 10 tons. you have 5 casters distributed evenly around the base of your weight. Each caster is carrying 2 tons, right ? ok, Now how big is the point of contact between the caster and the floor? on a "regular" rubber or urethane or phenolic wheeled caster you might have as much 3/4sq.in.< let's call it 1sqin for ease of mathmatics, shall we?> Ok so that's a point load of 2 2 tons per square inch. That's not so bad as long as you are on a strong, durable, non-flexible surface. Let's say you're on 1/4" Tempered Masonite. 2 tons over a 1sq.in. area might be ok but if you concentrate that same weight into a <theoretical> single point, like would exist at the contact point of a steel ball and a hard surface... you might as well be putting the weight on steel spikes and trying to shove it sideways, The balls are going to dig right into the deck. On a softer surface, like Plywood, the effect would be even more severe.
Shoot, got to go do some work, I'll write some more later, let's see some more thinking about the steel ball caster issues

I am confused by your response. The OP has the set as 1300 lbs and 36 casters. That comes out to 36 lbs per caster. The other thing to consider is where the platforms join the casters double or quadruple. There are only four casters that will actually point load solo.

 

[stoccoj]

What your looking to do is definitely possible, though success is heavily dependent on your choice of casters.

I'm not familiar with the ball casters you've specified, but my two concerns would be the concentration of the weight in a small area and the amount of sound created by a metal caster.

We used StageMaster casters for our last production on a 30' wide x 16' deep x 20' high castle and 4 high school students were able to move it without problem. That being said, those particular casters aren't cheep and are probably overkill for your set. If you could find similar casters at a lower price point, that is what I would use. The swiveling casters on a swiveling platform solve the problem of them binding if they are facing the wrong direction.

The other thing I learned during the production was to give your stage hands plenty of time to practice rotating the platform. It takes a while for them to get a rhythm down and consistently hit spike positions. Also, make sure you use the same 4 stage hands for every rehearsal and performance. If you replace one, it throws the whole thing off. I still have nightmares about them catching a curtain with the castle at the final dress rehearsal.

Good Luck!

 

[IntimeTD]

Thanks for some truly fantastic responses everyone. stoccoj, I passed on the tip about stagehands to the relevant people.

In general the reason I chose ball caster was to avoid the problems of swivel casters without the cost of the tri-casters, which are just out of our range. I'm hoping the numbers work out, since each caster is taking an average of half its load capacity. But at this point the casters have actually been ordered, so lord help us if they don't. If I'm just totally off in my thinking though, there should still be time to return them and get new ones.

 

[Footer]

Thanks for some truly fantastic responses everyone. stoccoj, I passed on the tip about stagehands to the relevant people.

In general the reason I chose ball caster was to avoid the problems of swivel casters without the cost of the tri-casters, which are just out of our range. I'm hoping the numbers work out, since each caster is taking an average of half its load capacity. But at this point the casters have actually been ordered, so lord help us if they don't. If I'm just totally off in my thinking though, there should still be time to return them and get new ones.

There is one other massive problem with the ball casters beside the infinite point thing. Because the bearing that makes them move is the ball itself, they can quickly foul up. So, your going to have to keep your deck spotless. Sawdust will grind them to a halt faster then you can imagine. Paint flecks can also get inside an also cause a huge amount of resistance. One fouled up caster and your pretty much done. So, mop often, do no wood working onstage, and keep your eyes out for anything that can get in. It does not take much.