Motor Sizes for spinning a large object

[Asellith]

I am an Electrical engineer with enough mechanical engineering background to get me in trouble but not get very good answers so I thought I'd post my current problem here and see if anyone else could help me a bit.

The Problem: The lighting director at the college I work for has requested a custom design for his stage setups. He wants to build large panels out of wood in various sizes and shapes. Something like a large rectangle or a triangle. Then they will paint them different colors and shine lights on them to get different affects. Here comes my problem. He wants them to rotate by command from the light board. the elctronics side I can do. I actually just found a module that will take a pot feedback and a 36 vdc motor with 5a max output and make a giant DMX (the light board protocol) input and that will change the position of the panel to the desired angle.

So electronics are done but I need to size and build to motor and gear box to move this large panel. He estimates a max of 250 lb for the panel. His example was a 15 foot by like 5 or 6 foot rectangle with a wood frame and luan (thin plywood) skin.

It doesn't need to rotate very fast. It will need to get into position rather quickly and then stop but nothing to fast. A 180 degree rotation in a few seconds would be sufficient. My preliminary design was just a shaft with 2 bearings to support the weight and a gear on on end and a flange to attach the panel on the other. Then a simple chain drive to a DC motor.

They only want preliminary designs and cost estimates right now but I need to figure out how to size the motor and well its giving me a headache so I'm posting it here to see if someone with more experience can help me. Also any better ideas for moving this panel are welcome

Thanks

[Soeren]

Hi,

A drawing would help, as I'm not sure I get the right mental images.
If the axis of rotation is horizontal and you balance the weight to equilibrium in any position, it won't take all that much power to rotate it.

If you opt for 15RPM (2s/180°) a motor doing 1500RPM will get nearly 100 times as much torque, but you need to establish the initial torque needed to get it rotating and that would best be done on a test build up with a luggage type scale pulling the chain.

[Asellith]

I doodle this in sketchup really quick. i didn't draw the support members but I was thinking of holding the shaft with a few ball bearings connected to an A frame that would have legs off the bottom to put weights on to counter balance. I just didn't want to draw it all. So assume two points of contact to hold the entire rod up. I drew it at 4' long but the length could change if needed. It might be a little long right now. I can do the FBD for the entire 2d side to figure out length and support needed but its the rotational force I don't know how to figure out.

[nottoooily]

The torque will be the sum of the torques required to:
- overcome friction
- hold it in position if it's not balanced
- accelerate/decelerate it

I expect the first two would be negligible. The 3rd is easy -

torque = rotational inertia * angular acceleration

You can calculate rotational inertia from the shape and density of the board. Angular acceleration is radians/second^2.

That acceleration might be pretty high if you want it to get into position quickly without much overshoot, but I'm sure you understand all that control theory stuff.

the FBD for the entire 2d side to figure out length and support needed but its the rotational force I don't know how to figure out.

[antom]

I would use a regen. DC drive. A regen will ramp up & has regen braking to decel your load. You can usually power up to 1.5hp on a 115v input.
2 hp -5hp & up will need 230v single phase. Dart or KB make them
I used to sell a lot of DC drives. It doesn't seem like you would need more than 1hp. The beauty of a regen is that you can control them with
0-10v or 5-20ma.

[Asellith]

It needs to be DMX controlled but I might be able to find a 0-10v dmx controlled device for cheaper then the $200 it will cost me for the motor controller.

[Admin]

prepare to lower your head in shame:
http://www.google.com/search?&q=torque+required+to+rotate+a+mass

 

[Asellith]

I did google like crazy but my lovely filter at work blocks any buisness sites and all technical forums. They say they will open whatever I need but who has 2 days to wait for a site to open then just to see its not the right answer.

[Hertz32]

thx thats helped me a lot thx admin