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Torque Needed to Turn a Capstan

  1. May 22, 2015 #1

    I am to use a motor to turn a spool and lift a weight.
    The bit that's confusing me is the moments active in this situation.
    I would assume that the only torques to consider are the torque exerted by the motor and the torque caused by the weight of my object. I would think that the capstan equation applies in this situation and that the torque needed to pull up the weight would be RSpool*(Weight/e^(Coefficient of Friction*Theta)). However, I'm not sure if I'm missing something. I would like to raise the weight and keep it suspended with some kind of mechanical lock on the shaft. I looked at winches which, unless I'm mistaken, are the same thing I'm describing, but in McMaster the capacity of a winch is shown as less when the wire rope is fully wound. This goes against what I assumed which was that when fully wound it should be easier to hold a larger weight. Can anyone shed some light on my confusion or point out flaws in my thinking? My apologies if this was hard to follow I have included a rudimentary picture bellow. Let me know if any clarification is needed. Thank you.

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  2. jcsd
  3. May 22, 2015 #2


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    Winch capacity falls as the wire rope is wound in. That is because each layer of wire on the drum increases the radius of the drum and therefore the torque needed for the same rope tension.

    A capstan has a single layer of rope. Friction to the capstan drum being multiplied by e^theta. The rope creeps up the slightly tapered capstan drum as more rope is pulled onto and past the drum.
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