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Help please with torque calculation

  1. Mar 30, 2015 #1
    Hello Guys,

    I have a 90mm=3.543" shaft going through 2 bearings that are the same shaft weighs 10lbs bearings weigh 25lbs each hub of bearing weigh 5lbs, bearing coefficient of friction is 0.15

    bearing OD: 190mm ID: 90mm

    ok so I thought Torque to rotate shaft is = Moment of Inertia x angular acceleration

    I want the shaft to be able to rotate at an angular speed of 15rpm so angular acc is 1.57 rad/s^2

    angular acc= 1.57 rad/s^2
    I= 1/2 M(r^2) --> 1/2 x 10lb x (1.77^2) = 15.7 lbm.in^2 (does this have to be converted to pound force???)

    T= 15.7 x 1.57

    is this right? Also how do I use coefficient of friction??
  2. jcsd
  3. Mar 30, 2015 #2
    If there is friction then a torque is needed to keep things moving at a constant rate, the applied torque just equals the torque needed to overcome friction. You are using the formula incorrectly?
  4. Mar 30, 2015 #3
    Without friction even minor torque can also rotate the shaft. So i don't suppose it will need such complicated calculations. Just force required will be equal to force required to overcome friction i suppose.
  5. Mar 30, 2015 #4


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    How do you figure this? If the shaft speed is a constant 15 rpm, then the angular acceleration is zero. Where did you get the 1.57 r/s^2 figure?

    I would strongly recommend getting everything into the same units system, either US Customary or SI, but not mixed as you have it.
  6. Mar 30, 2015 #5
    And i think for getting specific speed at specific acceleration friction should be considered first because without considering friction it will always be in acceleration even with a minor amount of force.
  7. Mar 30, 2015 #6


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    Kishan, I do not understand your statement,
    What are you saying, please?
  8. Mar 30, 2015 #7
    I want to say without considering friction and with application of minor amount of force body will always be in acceleration without considering magnitude of force except zero
  9. Mar 31, 2015 #8
    thanks guys for the replies

    ok so how I got angular acc is by converting rpm to rad/s which is 1.57 rad/s and I wanted it to reach that acc within 1second so acc=angular velocity/time basically thats the motor am going to be getting, one with 15rpm but torque is what am not sure of

    am having problems with units, whats the units of torque here when I use the following? I need it to be in ft lbs
    I= 1/2 M(r^2) --> 1/2 x 10lb x (1.77^2) = 15.7 lbm.in^2 (does this have to be converted to pound force???)

    T= 15.7 x 1.57

    also guys can I use this for starting toque??

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