# Does this sound right for a turntable motor?



## ajb (Dec 11, 2009)

I'm pretty sure this doesn't break da rules, but I suppose if the thread is gone and my account banned tomorrow I'll know why .

The turntable in question will be 8-10' diameter, will have a total weight (including scenery) of about 600lbs, and should be capable of a 180° turn in about five seconds. According to my calculations, I need 2.68HP, so ultimately a 3HP motor to accommodate drive losses. Does that sound about right based on others experiences?

If you're interested in the nitty gritty (read: nerdy) details, the turntable and supported scenery have a total mass moment of inertia of 4671 slug-ft² and the motor should be able to accelerate all of it to 6rpm in 2.5s.

For the final drive I'm leaning toward roller chain wrapped around the circumference of the table.


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## Van (Dec 11, 2009)

Your math seems sound and your question is, IMHO well within our TOS. Your drive solution is one that tends to work rather well even under heavy loads. My only concern is that moving that much mass that quickly demands the installation of some serious safety measures. Obviously a deadman switchis a must and I would seriously look at some form of friction braking systemintegrated into the kill switch.


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## jwl868 (Dec 11, 2009)

While I can't comment on the motor, I think you need to take a closer look at that weight. It seems low. (I see there is some uncertainty in the diameter and the weight difference between 10' diameter and 8' diameter is significant.) Plus, if an actor or stage hand (or two) are on it, you've got another 200 - 300 pounds. (Not to mention a change in the set design.)

The initial estimate of 2.7 hp is very close to 3 hp, so the extra weight will put you over 3 hp.)

Joe


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## Footer (Dec 11, 2009)

Are you going to gear the motor down at all? Do you have a controller to operate the motor? Does the motor have a reducer on it currently? 

I have moved large revolves with 1hp motors with a very large reducer. In my experience, there is not much onstage that can not be moved with a 3hp motor. You might be able to get away with less motor.


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## ajb (Dec 11, 2009)

Thanks, everyone. This is still very much under negotiation, so I'll take all this under consideration when making final plans and selections. 

The motor will definitely be reduced. I should be able to get ~12-25:1 between the final sprocket and the actual turntable so I'll need to get the motor down to 80-150 rpm. The final sprocket will be my fudge point to make all of the ratios match. I'm still in the process of spec'ing everything out, including a VFD to drive it, so we're not committed or limited to anything yet.

For the idea of the brake, I think it's a good one, but not sure how feasible it is. 6-0RPM in .5s for an emergency stop is an acceleration of 2.5 rad/s² which would require a stopping torque of 14545 ft-lbs or around 3600lbs at the edge of the revolve. That's quite a lot! I think what will be required in lieu of that is a thorough preventative approach. 


> In my experience, there is not much onstage that can not be moved with a 3hp motor.



True, but it's a question of how fast you can move it and how long it takes to get up to speed. If this were an intermission change I could get away with 1HP and 16 seconds to turn 180--unfortunately I have to deal with a vista changes that need to be a wee bit snappier than that, I think. Still harassing the director to nail down that requirement.

All that said, I just found a supplier for 5HP 117RPM gearmotors at an attractive price, so we'll see what they-who-hold-the-pursestrings say when I show them the numbers. The extra capacity would be nice to have <where's the Tim Allen smiley when you need it. . .>.


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## Footer (Dec 11, 2009)

ajb said:


> True, but it's a question of how fast you can move it and how long it takes to get up to speed. If this were an intermission change I could get away with 1HP and 16 seconds to turn 180--unfortunately I have to deal with a vista changes that need to be a wee bit snappier than that, I think. Still harassing the director to nail down that requirement.



With the getting it moving thing more power will help, but I don't like to move things that fast onstage due to safety concerns. Be sure to put in at least two dead man switches and have hard limits on everything.


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## coldnorth57 (Dec 11, 2009)

For all the HP you are talking about for moving this turntable you have glossed over the fact you have to stop it and that takes even more power that getting it to move in the first place. You need some very strong braking system. Getting it moving is easy stopping well that is another matter think it thourght how to stop it.


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## scenerymaker (Dec 11, 2009)

Two more things to be concerned about. The chain drive on the rim of the table is liable to be problematic. I built a 25' table with 65 people standing on it for our Easter pagent. I thought (wrongly) that laminated OSB, with the rough side out would be grippy enough. The chain just slid on the rim .  Then I covered it with spray urethane bed liner that is supposed to keep things from sliding around in your pickup bed. Strike 2.  Now we are down to 4 hours before the doors open. I covered the rim with industrial carpet and the chain bit into that and everything worked.  Nobody in the audience ever noticed the 10 lbs of sweat I lost on that. Bottom line here is be certain about the friction between your table and chain. 

The other thing you need to do is calculate the tensile force you will be applying in your chain and think about how you will hold your motor rigid on the stage.


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## ajb (Dec 13, 2009)

coldnorth57 said:


> For all the HP you are talking about for moving this turntable you have glossed over the fact you have to stop it and that takes even more power that getting it to move in the first place. You need some very strong braking system. Getting it moving is easy st opping well that is another matter think it thourght how to stop it.



Actually it takes a bit less power to stop--rate of acceleration being equal--than to start because now friction is working in your favor. If you need to stop in, say, 0.5s versus starting in 5s then yes, it takes ~10x as much power to stop. For normal, gentle stops I am equipping the the VFDs with braking resistors which provides for up to 125% braking torque. For secondary/emergency brakes I have several mechanical options in mind that I'm still evaluating. Just because I haven't mentioned it in the thread doesn't mean I haven't thought about it, but I appreciate the reminders nonetheless .


scenerymaker said:


> I covered the rim with industrial carpet and the chain bit into that and everything worked.  Nobody in the audience ever noticed the 10 lbs of sweat I lost on that. Bottom line here is be certain about the friction between your table and chain.



I was going to start with some Buna-N rubber I've got lying around, but carpet's a great idea! The ultimate option seems to be timing belt, if you size it to match the pitch of the drive chain and securely attach it to the revolve it can provide excellent grab.


> The other thing you need to do is calculate the tensile force you will be applying in your chain and think about how you will hold your motor rigid on the stage.



Done and done. Of course the big disadvantage to this drive style is the price of the chain required to withstand those forces.


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## kicknargel (Dec 13, 2009)

Somewhere in Yale Tech Briefs is a good system for building a tensioning frame to cable-drive a revolve.


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## DuckJordan (Dec 17, 2009)

As for the breaking system since i have no clue about rotating sets with a motor. what might work is an air bag system with 1/4" ply with a ruber strip underneath the platform with many diffrent points of slow down. the airbag system would be similar to the air ride caster system we have in our system (I'm not sure if it has a standard name) but basicly when you turn a valve they inflate pushing about 25 sq. ft. pounds of pressure down. I know it works great to move about 300 lbs of equipment on and off. it may also work in a static situation where you have the airbags inflate for the breaking system... just my two cents may or may not work.


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