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Need help with torque to force conversion

  1. Jul 6, 2012 #1
    Scenario:
    Electric motor, 30 ftlb @ 300 rpm
    6000 lb load (13200 N) fixed to "this" machine
    Motor drives a shaft that drives tracks (like a tank)
    Will need to go up an incline (0 - 10°)
    Weight of machine is not a factor
    Assume frictionless system

    I can't figure out how to determine if my motor can drive that load on a flat surface, let alone an incline.
    I can design a gearbox to get the power needed but I need help to see if I can run it on a flat surface.
    Do I have enough info to see if this motor will move the machine?
     
  2. jcsd
  3. Jul 6, 2012 #2
    Weight of machine IS a factor. You must accelerate that mass up to your desired speed. You must also move that mass up the incline against gravity. That is done by torque being applied to the wheel shaft.

    If "frictionless" is assumed, then
    • a torque is applied to the wheel shaft
    • the wheel, in effect, acts as if it was a rack & pinion system
    • torque applied to the wheel will move the mass forward and/or up the incline

    The important bit of info you are missing is the amount of time required to accelerate all translational & rotational inertias up to speed.

    Do a free body diagram showing the forces & torques in X & Y & Theta.
    Use FBD to make force / torque balance equations.
    Resolve torque into all torque components needed to give "peak torque."
    Remember:
    • Torque = (rotary inertia J) * (rotational acceleration alpha)
    • Force = mass * (linear acceleration a)
    • alpha ~ delta(rotational speed) / delta(time)
    • a ~ delta(translational speed) / delta(time)
    • Peak torque includes every torque you can think of...motor & torque output must be sized for that.
     
  4. Jul 6, 2012 #3
    T,
    Thanks for the reply. Yes, the weight of the machine is a factor but it terms of my scenario, the weight of the machine is roughly 1% - 5% of the lifting load and I'm not really concerned about it yet. I will over design my system later on to include friction, machine weight, safety, etc. I just need help laying out the equations and making sure I've got the variables in the right place. I've been out of school for 6 years and I haven't done this stuff since then. Now, I'm in a job that requires some of this design stuff and I'm trying to "get back in the saddle".

    I'll see what I can put together based on your comments. It may not look correct, but maybe you can help with that.
     
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