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Flywheel torque

  1. Aug 19, 2007 #1
    I am trying to understand how to calculate the torque on a flywheel. If I have a 90 pound flywheel that is 20" in diameter, how do I find the torque?

    Thanks for your help. :rofl:
  2. jcsd
  3. Aug 20, 2007 #2


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    What torque?
  4. Aug 20, 2007 #3


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    ...ie, torque comes from/causes acceleration/deceleration. So a flywheel at constant speed doesn't have any associated torque.

    We'll need to know more about what you want to do.
  5. Aug 20, 2007 #4


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    The angular moment of inertia for a solid disk is m x r^2 / 2 so in this case it's 90 lb (mass) x (10/12 ft)^2 / 2 = 62.5 lb ft^2

    1 lb mass = 1 slug / 32.174, so moment of inertia

    I = 1.94 slug ft^2

    Assuming no friction, torque would be associated with a rate of acceleration or deceleration.

    Assume that flywheel accelerates at a rate of 1000 rpm per second, rate of acceleration is:

    (1000 revolutions / minute x sec) x (minute / 60 sec) x ( 2 x pi x radians / revolution) = 104.72 radians / sec ^2 = alpha

    Conversion factors:

    1 slug = 1 lb sec^2 / ft

    Back to example:

    Torque = alpha(/radians) x I = (104.72 / sec^2) x 1.94 slug ft^2 = (104.72 / sec^2) x 1.94 (1 lb sec^2 / ft) ft^2 = 203.16 ft lb
    Last edited: Aug 20, 2007
  6. Sep 6, 2007 #5
    Torque is claclulated in absolute units, in this case foot pounds. You have to take into account the angular velocity, and the radius and mass of the fly-wheel. But here's the interesting thing if you have a fly-wheel that is two feet in diameter, and apply a force of 1lbwt. to turn the fly-wheel, the torque experienced at the axle is 1 foot pound. 1lb wt = about 1/32 of a foot pound, so you can see that the greater the diameter of the fly-wheel the larger the mechanical advantage. You can see why Archimedes was able to say give me a lever that is long enough and I will move the earth or words to that effect. django
    Last edited: Sep 6, 2007
  7. Aug 12, 2008 #6
    I need help as i am not good at physics, despite formula given by you gentlemen.
    I have made a flywheel for a bicycle. What is the torque for 6 kg steel Flywheel?

    The diameter is 3 1/2" ins by thickness 2 1/2" ins. The weight is 6 kg or 212 ounces ( 13.2 lb

    Flywheel rpm : 410 rpm ( Pedalling average at 60 rpm )

    Yo are a gentleman and a scholar thanks I rally appreciate your help.
  8. Aug 12, 2008 #7


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    Again, as per the previous posts, a flywheel requires no torque at a constant speed like you have mentioned. What exactly do you want to know when you ask for the torque? Are you wanting to know what the flywheel will provide or what it takes to get it to speed?
  9. Aug 12, 2008 #8
    Sorry for not stating clearly. I have a flywheel installed on a bike and it runs well. The system provide energy to the pedal when pedaling and it's balance out. Yes, what the flywheel provide? Thanks so much Sir!
  10. Aug 12, 2008 #9
    Re: Flywheel

    What is the angular velocity and moment of inertia for a flywheel weight of 6 kg ( 212 oz )with the diameter of 3 and 1/2 ins. and rpm is 479
    Last edited: Aug 12, 2008
  11. Aug 13, 2008 #10
    The crank power is 0.6 Hp ( 450 Watts ) and rotating speed of 100 rpm. A solid disc steel flywheel of 6 kg with a diameter of 3 1/2 ins.( rotates at 547 rpm if this is important ). Can this flywheel provide it's stored energy more than the crank's when release ?

    I don't mind anyone correcting me for not understanding flywheel principle, i don't have academic education coming from poor family, I self learn things that fascinates me. Thanks you!
    Last edited: Aug 13, 2008
  12. Aug 13, 2008 #11


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    Re: Flywheel

    Jeff went through a sample of all of those calculations in post #4. We like to try to get our users to learn instead of just giving answers - give the calculations a shot and see how you do.

    One thing though:
    Conservation of energy always applies. Assuming the flywheel is perfect, the energy you can get out of it would be exactly equal to the energy you put into it.
  13. Aug 14, 2008 #12
    Thank you Sir, and to all at this site. I am encourage.
  14. Sep 24, 2008 #13
    I managed to work out as you encourage following Jeffs example. Thanks so much I learn something when knowing its terminology.

    In designing the flywheel, the balancing of crank torque fluctuations produced by pedaling, I see that there is a transfer of energy to the low torque area in the pedal cycle. Thus the pedal is smooth and consistant. In all i wanted to know how it is that the extra weight added to the already heavy bicycle, when pedaling I don't feel the load at the pedal and the back wheel, instead it is light and easy throughout the pedal cycle. So I thought to workout the torque transfer from the flywheel. Perhaps in physics perspective I can see.
  15. Jan 10, 2009 #14
    Hi! edneo,
    Torque is calculated as follows: Torque = inertial mass x angular acceleration. It should be simple enough to substitute whatever figures apply in your case to the formula and to work out whatever you need to know.
  16. Jul 20, 2009 #15
    I have to design a flywheel to generate 30 ft lb torque. How do i go about it?
  17. Jul 20, 2009 #16


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    Welcome to PF.... first you'll need more information about the performance requirements of the flywheel. Hopefully what you read in this thread will help you understand why that is: torque is only one component of the performance of the flywheel. You also need to know how long you want it to be able to generate that torque and probably at what minimum rpm (giving you energy and power).
  18. Jul 21, 2009 #17
    we need the flywheel to run an alternator. the torque requirement of the alternator is 30 ft lb. and the RPM range will be 300~0. the alternator produces 71V and 2.1A at 300RPM and decreases uniformly with RPM. the flywheel should be albe to move the alternator for atleast 15 seconds.
    Thank you
  19. Jul 21, 2009 #18
    hello Russ,
    thanx for your post. can you suggest any book or link that i can refer and that will help me design the flywheel
  20. Jul 21, 2009 #19


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    Jeff did a walk-through of the calculation in post #4. You should be able to just plug your numbers in and get the answer. Could you give it a try....? We don't like just handing out answers here.
  21. Jul 22, 2009 #20
    hello russ,
    i used the calculation by jeff and came up with flywheel of mass 206lb and 16in diameter.
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