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Force required to turn shaft

  1. Jun 14, 2009 #1
    Hello fellow members. I have a question and was wondering if someone could help me out. How could one figure out how much force is required to turn the shaft of say a large (9" diameter) DC motor directly from the shaft, and also with a lever of any length? Is there some sort of machine that has to turn it to measure it, and is this information available anywhere on the net?

    I am only interested in the numbers, so if someone has the information on say the amount of force required to turn a 9" DC motor I'll accept that as well.
  2. jcsd
  3. Jun 14, 2009 #2
    A shaft is turned by torque, not by force.

    How much torque is required depends upon what the resisting torques are. Is the shaft trying to turn a large mass, is it stuck in a hole with a lot of resisting friction, or what is the reason the shaft does not turn freely?

    I have no idea what you mean when you speak of turning a shaft using a motor but involving a lever. This requires more explanation.
  4. Jun 14, 2009 #3
    Yeah I realized my mistake in asking for the force required, and apologize if it is unclear.

    I want to know how much torque is required to turn the shaft (rotor) of a large stationary motor with and without using a lever. Does this need to be measured, or is there data out there where I can find this out?

    The motor is being used as a generator.
    Last edited: Jun 14, 2009
  5. Jun 15, 2009 #4


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    You say it is a 9" DC motor you wish to use as a generator, you need to supply a lot more information in order to produce your answer. It sounds like you might be describing a motor used to power an electric vehicle.

    Any motor or generator is considered an electrical energy transfer device, and will transfer only what is supplied or demanded of it.
    You can measure the torque @ no load, by winding a string around the shaft, then hook a fish scale to the string and pull steady and as the shaft turns the scale will show the effort in ounces or pounds (use the shaft radius).

    There are a lot of sites that will offer some sort of calculators, you have to know what loads are going to be required.

    Last edited: Jun 15, 2009
  6. Jun 15, 2009 #5
    As RonL indicated, the motor used as a generator will serve as an electromechanical energy converter. If there is no electrical energy taken out, then the only torque required to turn the shaft is that required to overcome friction. If there is electrical power being taken out, then in addition to the friction torque, the input torque must provide the power for the electrical power being taken out.

    Much depends on the type of motor you have. For example, if you have an isolated three phase induction motor, simply turning the shaft will not produce any current because there is no magnetic field in the machine. It is a different story with a permanent magnet machine.

    It sounds like you are seeking a simple answer to a question that is much more complex than you realize.
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