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I am working on fabricating a large scale revolving disco ball for an event in Cincinnati. The scale is 10'diameter and the frame is flat bar steel. the exterior is cardboard, duct tape, and adhesive "mirror stickers." I am looking to have the ball rotate at 1 RPM. Total weight is 350 lbs. What type of motor should I research to purchase? The motor will be hard mounted at the top of the structure that is to be suspended.

Thank you advance.

Joe

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    $\begingroup$ Checking calendar... No appointments Cincinnati in the foreseeable future. Big sigh of relief. $\endgroup$
    – JRE
    Jun 6, 2017 at 13:41
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    $\begingroup$ 350lbs.... wow... seriously... sounds like an accident waiting to happen. Pretty sure you can make that a lot lighter... esp for a 1 time event. $\endgroup$
    – Trevor_G
    Jun 6, 2017 at 14:05
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    $\begingroup$ This is more of a mechanical engineering question than EE, and the EE part is a shopping question -- off-topic here anyway. The key issue is configuring a thrust bearing to support the 350 lb dead load. The motor and gearing will be sized primarily to handle the friction of the bearing. $\endgroup$
    – Dave Tweed
    Jun 6, 2017 at 14:09
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    $\begingroup$ A steel frame sounds like massive overkill (literally!). I would be thinking in terms of building a lightweight "buckyball" frame out of PVC tubing, wood lath or even strips of the cardboard. $\endgroup$
    – Dave Tweed
    Jun 6, 2017 at 14:19
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    $\begingroup$ ANd use aluminum foil instead of mirrors..... $\endgroup$
    – Trevor_G
    Jun 6, 2017 at 15:51

2 Answers 2

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There are two things the motor needs to be able to do. Firstly provide enough starting torque to overcome static friction in the bearings and accelerate the ball up to the required speed in a reasonable amount of time. Secondly to be able to provide enough torque to maintain the desired speed against the friction in the bearings for however long you want to run it for.

Bear in mind that not all motors are rated for continuous running, this really comes down to how well cooled they are.

Equally on of the big determining factors will be how good the bearings are and how well balanced the whole thing is. For a target speed of 1rpm you will need a fairly substantial reduction ratio gearbox anyway.

I suspect that the most convenient solution will be a DC motor with an integrated planetary gearbox as these are an off the shelf solution to high torque low speed applications. At a very rough guess I would say you are looking at something in the couple of hundred watt range.

You might also want to look as something like the hub and stub axle from a small trailer for the bearing as these are again a cheap and ready made solution.

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A worm / gear with the gear driving a shaft supported by a suitable ball race for the bearing may work. May need extra gearing though.

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  • $\begingroup$ I agree with this method. I would also look into replacing the flat barsteel with aluminum and wood. Correct me if I'm wrong, but If the outer case of the disco ball is made only of cardboard and stickers, there isn't much need for heavy steel in this case as most of the 350 lbs would be coming directly from the steel. $\endgroup$
    – Jason
    Sep 6, 2017 at 23:53

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