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Calculating torque required to operate a flywheel

  1. Jan 12, 2016 #1
    So basically, i'm making a vehicle that should regenerate energy from braking. the car will start at rest at a ramp between 30 to 50 degrees slope. a flywheel will be connected to the front wheels of the vehicle

    I want to determine what kind of a flywheel will be sufficient to store the potential energy and that is not too big to make the vehicle heavy!

    then I want to calculate the torque required to start the flywheel spinning when the vehicle is free to move?
    ( how do I determine the torque required to start the flywheel spinning when the vehicle is at rest?)

    What's the difference if the flywheel is connected to the vehicle before it gains speed vs. the same flywheel is connected after the vehicle gains some speed?

    Lets assume the vehicle is 5 kg and is placed on a 1 m long ramp
    Last edited: Jan 12, 2016
  2. jcsd
  3. Jan 12, 2016 #2


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    There is no minimum torque required to accelerate a flywheel. Any non zero torque will cause an angular accelration found by:
    T = Iα
    α = https://en.wikipedia.org/wiki/Angular_acceleration
    I = https://en.wikipedia.org/wiki/Moment_of_inertia
    T = https://en.wikipedia.org/wiki/Torque

    however, in real systems there will be static friction that must be overcome - that is what you must find. You'll need to know a lot more about your vehicle eg wheel diameters, tyre friction coefficient (to find if you'll roll or slide) flywheel bearing friction, transmission (eg pulleys or gears) friction etc etc
    For some small vehicle that you'll be building yourself these things are hard to predict (eg the losses in cheap plastic gears is strongly dependant on alignment - what is the tolerance of your manufacturing process? etc etc).
    I assume this is for some university design challenge? I think your best bet is to build in a lot of adjustability and tune performance via experiment. Design so the flywheel mass, vehicle mass, wheel diameter etc are adjustable.
  4. Jan 12, 2016 #3
    Thank you very much for the information. Do you know if there is a way to calculate the maximum energy a flywheel can store? Or should it be specified from the manufacturer?
  5. Jan 12, 2016 #4


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    The maximum energy is limited by the max RPM the flywheel can spin at before it self destructs.

    I doubt it'll even be a consideration in your case, your RPM & energy stored will be likely be very low and so far from the safe limit for any reasonable flywheel design.
    I also doubt you'll need to buy from a manufacturer, using a stack of washers or turning a simple disc from scrap steel will likely meet your needs. Your money is probably better spent on bearings or something (I assume you've been given a strict budget).
  6. Jan 12, 2016 #5
    I see,
    How do you think I should determine what flywheel size to make? In the vehicle requirements, it should be light as possible, guess it would be between 3-5 kg.
  7. Jan 12, 2016 #6


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    This is a small project and the flywheel isn't going to be absorbing much energy or spinning anywhere close to fast enough to tear itself apart. I'd approach this from a standpoint of more operational concerns: calculate how much energy you want it to store and then play with some numbers for rpm and moment of inertia to get an idea of how big it needs to be and how fast it will need to spin. Then you should think about how you can make it spin in order to absorb the most energy. You'll need it to be able to spin-up from a stop, covering a vast range of RPMs even as the vehicle moves slowly down the ramp. That's not necessarily a straightforward thing to do.
  8. Jan 12, 2016 #7
    Thanks i'll do that.
    Is there a difference in terms of distance traveled if I have the flywheel engaged at the top of the ramp vs having the flywheel disengaged and engaged after the vehicle gains some speed? in terms of energy loss will any method perform better than the other?
  9. Jan 13, 2016 #8


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    Engaging and disengaging will have some energy losses (how much depends on your clutch mechanism). So it's best to do it as few times as possible.

    With that said, it depends on the best flywheel for your needs, maybe the best overall design won't start from a standstill (or you thought it would but the static friction is greater than expected) and it needs to build some speed before the flywheel is engaged.
  10. Jan 20, 2016 #9
    If the flywheel is engaged at the top of the ramp, it's pretty much useless.. you'll only be slowing down the acceleration of the car. As the car accelerates down the ramp, the flywheel speed follows, and unless you can select from many gears, it won't be able to provide ANY braking when you want it to.. it'll already be at speed.

    For this project to make sense to me, I'd think you'd want the car to roll down the ramp and hit level ground, then regeneratively brake as much as possible, stop (with friction probably), and use the flywheel energy to get going again.
    For a 5kg car, I thought of the perfect place to look fora flywheel, but it might be hard to find nowadays.. An old 8 Track player has about a 4" flywheel and a good set of bushings.. it'll spin for a while.

    The biggest problem in efficiency is any energy left in your flywheel at the end of the run is wasted energy, so if you gear the flywheel so it turns fast compared to the vehicle speed, your acceleration will be slow, it will be able to store a lot of energy though... but once you re-engage it after a stop, you won't get going very fast because of it.
    If you gear it so it turns slowly, you'll accelerate fine, but it won't be able to store any significant amount of energy.. after you stop, it'll only give you a little nudge to get going again.
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