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Can a flywheel add torque?

  1. Dec 16, 2015 #1
    Hello guys,
    I am working on designing a flywheel to couple to a generator however i have a small problem and i have low knowledge in flywheels

    I have a pinion rotating at w = 14.13 rad/sec and torque Tpinion = F*Rpinion = Mass of inertia *alpha pinion
    All is known I can find Tpinion euqal around 0.0006 N.m (Rp = 2cm , alpha pinion = 88 rad/s^2 , m = 700g) Now the question :
    After the pinion there is a gearbox of 1/20 coupled to a flywheel coupled to a generator

    My generator gives 10 w with minimum RPM of 2500
    My system gives me around 2700 RPM
    So i need a certain amount of Torque = 0.035 N.m to get this power output.
    Does the free wheel add torque ? and what the equation coupled with the gearbox be?

    thank you for your time and I appreciate every help
    Edit: I was considering the angular acceleration for the Flywheel to be alpha pinion *20 due to gearbox is it correct as well?
    Last edited: Dec 16, 2015
  2. jcsd
  3. Dec 16, 2015 #2
    This sounds like a small wind turbine connected to a 10W generator. The flywheel on the generator shaft is a good idea and will serve to smooth out speed variations. The flywheel adds no torque at constant speed. Yes, the angular acceleration of the flywheel would be 20 times that of the pinion.
  4. Dec 16, 2015 #3
    First of all thank you for your reply
    Second of all i need 0.035 N.m torque to get this 10 w however i only have 0.0006 N.m due to my small size and weight of the pinion. Can i increase this torque by adding a flywheel?
    ( the pinion size and weight cannot be changed )
    Last edited: Dec 16, 2015
  5. Dec 16, 2015 #4


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    Staff: Mentor

    As insightful said, the flywheel does not add torque. You need more input power to generate the output power you want.
  6. Dec 16, 2015 #5
    Now I'm confused about your setup. Could you provide a sketch? Your 0.035 N.m at 2700 rpm does give 10W at the generator, but would require 20 times that torque at 14.13 rad/s at the pinion input to the gearbox.
  7. Dec 17, 2015 #6
    Sorry for the confusion. This is a sketch.
    I am a bit as well confused due to many things.
    So as seen in the picture I have the pinion with the force F2 then gearbox with 1/20 then the flywheel then a generator.
    I derived two equations please correct me as I am not sure yet which one is correct.

    If I use equation 1 :

    F2.Rpinion = Jsystem * omega pinion

    with Jsystem = Jpinion + (smthing)*Jflywheel this means that the mass and radius of the flywheel can increase the torque.

    So can i increase my torque using this formula?
    Or equation 2
    Torque gearbox + Torque flywheel = 0.035 N.m (Power out = 10 W , RPM = 2700 so i need to supply to generator this torque of 0.035 N.m)
    Having Torque pinion = 0.5* mpinion * r^2 * alpha with m = 700 g and r = 2 cm and alpha =88.5 rad/s^2
    I can find the Torque done by the pinion which is this 0.012 N.m
    After gearbox , T = 0.012/20 = 0.0006 N.m
    I need 0.035 N.m so Torque flywheel = 0.034 N.m
    I know energy is conserved and i cannot add torque but the fact that moment of inertia can add torque i got a bit confused

    Sorry for the long post and thank you for your effort and time

    Attached Files:

  8. Dec 17, 2015 #7
    OK, this is the torque needed to accelerate your 700g, 2cm radius pinion disc at 88.5 rad/s2.
    I do not see how this is relevant at all to your operating a generator at a steady-state 2700 rpm.
    The flywheel simply cannot supply additional torque at steady-state.
    You need 20*0.035 = 0.70 N.m torque at the pinion at 2700/20 = 135 rpm.
    If you cannot provide this torque, your project (as I understand it) will not work.
  9. Dec 17, 2015 #8
    Mmm ok that i understand well bas the trick is if i need to writr the equation of motion to my system it will not be like equation 1 ?
    Equ 1 :
    F2*Rp = Moment of inertia system*alpha
    With MOI system = inertia of pinion +(smthing) inertia of flywheel
    Doesnt this mean if i add the flywheel it will help increase my torque?
  10. Dec 17, 2015 #9
    No. Adding the flywheel just adds to the moment of inertia of the system and so reduces alpha.
  11. Dec 17, 2015 #10
    Oh i see now
    Thank you a lot for your help and for your time
  12. Dec 20, 2015 #11
    I have one last question
    I understand i need now 0.7 N.m of torque
    However if equ of motion on pinion without flywheel is F*r = Jpinion* alpha
    If my torque should be 0.7 then using Jpinion= 0.5× mass pinion* r^2 i can find m = 40 kg. ( r = 0.02 m and alpha 88.5 rad/s^2)
    However my max pinion mass is 700 g so does adding a flywheel help me distribute this weight to get my desired torque?
    Because ill have F*r =0.7= Jsystem*alpha
    Where Jsys = Jpinion+ n^2 *Jflywheel ?
    I appreciate all your help and thank you for your time
  13. Dec 20, 2015 #12
    Why are you concerned with accelerating the pinion at 88.5 rad/s2?
  14. Dec 21, 2015 #13
    Suppose my design is giving me those values
    Can I get my torque to be 0.7 like i said in my previous post? (by adding a flywheel to distribute the weight)
  15. Dec 21, 2015 #14


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    Staff: Mentor

    s1h1ad2ow, what you are asking (or rather, how you are trying to go about it) does not make much sense. You should not be looking at a situation where the system is accelerating because your goal is to generate power under steady state conditions (not accelerating). Your approach is not helping you get where you want to go.
  16. Dec 21, 2015 #15
    That doesn't mean those values are relevant to your design.
    No. Your force on the pinion must be 0.7N.m/0.02m = 35N.
  17. Dec 21, 2015 #16
    I understand know I was using a wrong approach for this issue

    Thank you for your answers and your help guys
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