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Shaft torque requirement (should be simple!)

  1. Dec 7, 2009 #1

    I apologize if this question appears simple; it may be but I cannot find the answer in any of my textbooks.

    I want to turn a shaft with a diameter, d, and length, L, which has a plate attached at the end, of diameter D.

    I made up a side view of what I'm trying to illustrate in MSPaint as attached (shaft holder.png)

    I need to be able to rotate this at the free end of the shaft at 100 RPM, and my plan is to attach a sprocket at that end, link it with a chain, and have another sprocket connected to a motor.

    My question is, how much torque should I need to rotate this given the following parameters?

    I'm not sure where to begin; I tried to analyze this as a problem with a circular shaft and a weight at the end, and also I've tried using the polar moment of inertia of the shaft but it's leading me nowhere!

    please help!

    Attached Files:

  2. jcsd
  3. Dec 7, 2009 #2


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    Staff: Mentor

    Sorry, but it is actually an impossible question. According to Newton's 1st law, constant speed motion requires no force.

    We need to know what force this device is acting against.
  4. Dec 7, 2009 #3
    Presumably the torque required to accelerate the shaft to 100 RPM

    All I know is that I selected a cheap motor capable of providing 100 RPM for this design and my professor yelled at me for not including 'Torque considerations'

    Am I misthinking this ?
  5. Dec 7, 2009 #4
    The torque needed to maintain an angular velocity is equial to the losses. You need to make sure that the motor has this capability at 100rpm.

    How you'd go about finding and quantifying the losses is another story.
  6. Dec 7, 2009 #5
    What do I need to do in order to figure this out ?
  7. Dec 7, 2009 #6
    There is no real way to even begin to estimate losses with the info posted. However they are likely to be small and insignificant.

    Do you need it to accelerate to top speed in a given time?
  8. Dec 7, 2009 #7
    Last edited: Dec 7, 2009
  9. Dec 7, 2009 #8
    The largest torque would come during acceleration. If you had some sort of time constraints then you could estimate these.

    But at steady state,(like someone already stated) you only need to overcome your losses.
    If you selected a motor that could run at 100rpm at no load, then it won't be able to do the same with losses. Maybe this is what your teacher was saying.
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