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Simple Torque inertia question

  1. Mar 15, 2009 #1
    Hi I am designing a 3 axis robot but need help with one calculation.

    How do you calculate the torque required to rotate a supported vertical load.

    For instance the torque required to rotate a dead weight sitting on top of a thrust bearing. I am looking for some formula from mechanical design books but haven't found anything.

    Sorry about asking such a basic question.

  2. jcsd
  3. Mar 15, 2009 #2


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    Staff: Mentor

    Welcome to the PF, Tim. The equation you are looking for is:

    [tex]\tau = I \alpha[/tex]

    Where tau is the torque, alpha is the angular acceration in radians/s^2, and I is the Moment of Inertia:


  4. Mar 15, 2009 #3
    Thanks Berkeman,

    If I'm using a Stepper motor that has an input pulse of 100Hz (90 degrees a second in this setup) is my initial acceleration 1.57 rad per second^2?
  5. Mar 15, 2009 #4


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    Staff: Mentor

    If the motor swings 90 degrees in a second, that is an angular velocity omega:

    [tex]\omega = \frac{\pi}{2}[/tex] radians per second.

    The acceleration would be during the time it takes to go from omega = 0 to omega = PI/2 rad/s. The angular acceleration is the change in the angular velocity with respect to time.
  6. Mar 16, 2009 #5


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    Science Advisor

    Don't forget to include the frictional force from the thrust bearing.

    http://www.roymech.co.uk/Useful_Tables/Tribology/Plain_Bearing Friction.html

  7. Mar 29, 2009 #6
    Umm, don't forget to in include the weight either T = F.R . Which would be the "torque required to rotate a supported vertical load" as described in the first thread. Where F is the weight of the item and R is the horizontal radius of the load from the pivot position (without acceleration).
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