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Motor/Gearbox Calculation

  1. Sep 1, 2013 #1

    I have an engineering project where I need to be a able to lift a 1kg load with a balsa wood crane, but have it fail at 1.5kg so as to be not over engineered.

    Various components need to fail (eg truss, which is easy) but I am trying to work out how to get the electro-mechanical component to fail.

    One of my ideas was to utilise the friction in the gearbox to hold the weight of the 1kg load when the motor is not running, but when there is a 1.5kg load and the motor stops running, the friction in the gears will not be strong enough and the gears will slip into reverse and drop the load.

    I have gauged that this is possible, but am getting a bit confused with Holding/Stall torque and if I should be calculating this for the motor or gearbox or both.

    Any guidance?

    Cheers! Sean.
  2. jcsd
  3. Sep 1, 2013 #2


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    A gearbox is a mechanical torque transformer.
    If you use a DC motor then the torque on the stalled output shaft will be proportional to the motor current.
  4. Sep 1, 2013 #3
    Sorry could you explain further. Essentially (I Think) I want the friction in the gearbox to resist a certain load when the motor is not running. I am trying to work out how to calculate this?
  5. Sep 1, 2013 #4


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    There is no way to accurately predict the friction through a normal gearbox. It is an exponential problem highly dependent on temperature and lubrication. Only with a worm drive could friction lock the gearbox when without motor power.

    If you limit the stall current to a DC motor it will fail to lift 1.5kg, but it will still be able to lift 1.0kg. That can be very accurately controlled.

    If you can get a small wiper motor out of an old 12V vehicle. They often have a 12V motor with a worm driving an external crank. Consider removing the crank and replacing it with a drum.
  6. Sep 1, 2013 #5
    the only thing you can take Is a power screw. you can find the equation for all
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