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Sizing a DC motor to move a weight and counterweight

  1. May 5, 2013 #1
    This might be a silly question for some but I was wondering how do I size a DC motor to move a weight and counter weight? Suggestions for a better design are also welcome.

    I want to build an enclosure with an automatic door (simple limit switches and an arduino to flip the motor on and off). But lifting the door seemed like it would take a large motor since the door is most likely going to be plywood and several feet x several feet. So I thought why not counterweight it to ease the load on the motor. But its been a long long time since I took physics so I don't really know how to find what size motor I will need. Or if there is anythign I'm forgetting to take into account here. So I thought I'd ask you guys.

    I'm attaching a picture of what I had in mind in case my explanation wasn't great.

    Thanks in advance.

    Attached Files:

  2. jcsd
  3. May 5, 2013 #2


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    Have a think about how quickly the door needs to move and accelerate. For example after time t it needs to be moving with some velocity v so it will need to have been given energy...

    = 0.5mv2.


    Power = energy/time

    So the power will be at least

    = 0.5mv2/t

    To that you need to add power to overcome friction. If friction force is f then the power required to overcome friction is f * v.

    There is more to it.. At some point you have to slow or even brake the motor to stop the mass of the door and counterweight crashing into the end stops. Simple limit switches and friction might not be enough to decelerate the door in the available distance.

    If you are actually planning to build this perhaps look into commercial garage door openers.
  4. May 6, 2013 #3

    Jano L.

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    I do not understand the picture, but counterweight is a great idea. The motor then needs to overcome only the force of friction. If the bearings and joints are well oiled so the friction is small enough, even a small torque motor should suffice. The power of the motor should not be that important, if you can wait a while for the opening. The important thing is to have enough torque (moment of force), which depends on the amount of friction and the radii of the sprocket wheels.

    I think best would be to ask someone who already has such setup or find the best motor by trying the ones you can get your hands on.
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