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Pulley Driven System

  1. Aug 9, 2014 #1
    I would like to run 8 pulleys off of one motor. Each "pulley" would be 19" in diameter with 20lb of weight at 30RPM. I would like to know how to calculate the inch-lb that would be required of the motor. I read this on masterdrives.com

    Torque(inch-lb) = (.03908 x N x W x R^2)/t

    Where N = difference between final and initial rpm, W = weight of rim in lb, R = radius in ft, and t = time required to affect speed change in seconds.

    So if I wanted it to come up to seed in 5 seconds I calculate with one

    Torque = (0.03908 x 30 x 20 x 2.51)/5 = ~11.77 inch-lb of torque.

    If I were to run the motor at 90 rpm with a pulley a third of the diameter I would be using a third of the torque or ~4 inch-lb.

    Then since there's 8 do I need a motor capable of 90rpm and ~32 inch-lb of torque?
  2. jcsd
  3. Aug 9, 2014 #2


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    Science Advisor

    Why 8 pulleys?
    How do you intend to connect 8 pulleys to the one motor?
    why 5 seconds?
    Why size the motor power based on acceleration rather than steady state?
    Why 19'' pulleys?

    If you can explain what exactly you are trying to achieve we can give better advice. As it is it sounds like you've made a lot of arbitrary decisions and it'll be a poorly engineered solution.
  4. Aug 9, 2014 #3
    My apologies billy joule,

    I plan to connect the 8 pulleys using a belt like used on the belt driven ceiling fans. Each pulley will have a load of 20lb attached to it. The pulleys will not actually be 19" but the load will be 19" off center of rotation(weld bracket to pulley), the 8 pulleys will likely be 10" and the motor will be using a 6" pulley. It's for a decoration idea.

    The 5 seconds is arbitrary and I decided to size the motor for acceleration because I don't know how to size for steady state, or if my acceleration equation is even right.

    Thank you for helping me!
  5. Aug 9, 2014 #4


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    It sounds like you want unbalanced masses on rotating shafts:

    http://en.wikipedia.org/wiki/Vibrator_(mechanical [Broken])

    I wouldn't worry about motor selection until you're sure the thing won't destroy itself when you turn it on.

    Unbalanced mass demo:

    Now imagine those small weights are replaced with 8x20lb weights...catastrophic failure would likely follow..

    I suggest heading back to the drawing board.

    If you can tell us (preferably w/diagrams) exactly what you're trying to achieve we may be able to offer alternative solutions.
    Last edited by a moderator: May 6, 2017
  6. Aug 9, 2014 #5
    At different points on the ceiling will be 8 different pulleys and one motor/drive pulley. Each pulley will have a bracket welded to it in order to expand the diameter. From this bracket will be a few wires holding up some sort of halloween type object all pretty close to 20lb.

    At most they would rotate 30rpm, slower is fine. If I used a 3" drive motor pulley with the objects being 19" from center they'd turn at 1/6" the motor drive speed..if I were to start with say a 60rpm gear motor I don't think that at 10rpm it would get so out of control like in that video.

    I'd like to run all of them off of one motor:
    1. to save electricity
    2. to sync the movements..it gives more of an eerie feel.
    I thought it would make for a cool room in a haunted house.

    The belting is probably the most expensive part, $100 for 30'. I'd probably need more than that too, unless there is some other way to run them all together. I'm just trying to figure out how to size the motor at this point and looking on ebay to see if there is anything used that could run this. I've only seen equations on how to size inch-lb for acceleration/deceleration but not how to size for continuous operation. I've never taken a physics class, but reading about this stuff gives me much more respect for the intelligence of those during the industrial revolution.
  7. Aug 11, 2014 #6
    Bump, any input on if I'm using the correct calculations to size the motor or if there is a better equation? The less power use the better but I don't want to burn it out by under powering it either.
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