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Need help determining torque required

  1. Mar 25, 2013 #1
    Hi, all! I am looking for some assistance on designing a machine. I have to admit, I am just a Mechanical Designer/Draftsman, and need some help determining the right motor to use.

    Here is the setup:
    -I have a 60" diameter table. It is 0.25" thick. I am using general steel density to spec it's weight to about 185lbs. This weight is calculated with slots cut out of table, reducing overall weight.
    -On top of the table, there will be a load of 3,750lbs. This is a coil with an ID of 35", and an OD of 57".
    -All in all, there is 3,935lbs atop the table.
    -I have motors with a 1" shaft diameter.

    My question is, how much torque will be required to rotate the table top (with load) at a speed of around 1-2 RPM's?

    I am slightly confused on the formulas, so any help is greatly appreciated! Thanks!

    Also, see the sketch I did for descriptions in detail!

    Attached Files:

  2. jcsd
  3. Mar 26, 2013 #2

    Simon Bridge

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    You will need very little torque to maintain a constant rotation speed - just enough to balance the friction at that speed.
    You need more torque to accelerate it though - the amount depends on how quickly you want it to get up to speed.
  4. Mar 26, 2013 #3
    You'll need the equation for torque and angular acceleration: [itex]\tau = I\alpha[/itex], where I is the mass moment of inertia. Since the coil and the table have the same axis of rotation, you won't have to use the parallel axis theorem to get the combined moment of inertia, so you can just add them up: [itex]I = \frac{m(r_1^2+r_2^2)}{2}[/itex]. You'll have to decide how quickly the table and disk need to come up to speed at 1-2 RPM, and that will decide your alpha.

    Your motor should be rated to a certain max speed and max torque, so the torque needed to rotate the shaft of the motor will be factored in that. If you find that your motor isn't powerful enough to rotate the tabletop directly, consider using a gearing system. From a modified form of the power equation, [itex]\tau_1\omega_1 = \tau_2 \omega_2[/itex], which tells you that a motor with a low torque but high angular speed can produce a high torque at a low angular speed. The ratio of the angular velocities is determined by the gears you use.

    The last part to figure out is what the speed is that the motor will spin given the torque that it must generate. The max speed of the motor is determined at zero torque and the max torque is determined at zero speed. There is a somewhat linear relationship between these two, so plotting a torque vs. speed graph can help you ballpark what the motor will do.

    Hope that helps!

    EDIT: The textbook "System Dynamics" by Palm covers this kind of problem if you're interested.
    Last edited: Mar 26, 2013
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