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How to calculate force

  1. Aug 10, 2010 #1
    Hi All,
    Since from childhood we are reading a formula Torque,T = F x r where f is the force and r isthe radius , I want to just know in this formula how to calculate force.

    My Problem:-
    I got Barrel Drum whoes O.D is 762mm or 30 inches and whoes ID is 742.3 or 29.5 inches . And the Barrel Drum weight after rolling the Hyd Hose is about 1700kg and there is one shaft going through the centre of the drum and the shafts sits on two bearings on each sides.
    I want to calculate the Torque required to turn this Drum should I use the formula T= Fx r or should I use another formula T= I x Alpha where I is moment of Inertia and alpha is Angular acceleration . in the formula T = F x r I dont know the value of F, how to calcu;ate F(force ) .

    Can anybody help me to suggest which formula is the best in this situation.
    With Regards
  2. jcsd
  3. Aug 10, 2010 #2
    Why do you need to know the torque?
  4. Aug 10, 2010 #3


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    Staff: Mentor

    The torque is dependent on the angular acceleration and friction, so it isn't an easy thing to just calculate.
  5. Aug 11, 2010 #4
    Hi Zhermes I need to design a Shaft which will act as axle to the drum thats why I need to know the Torque transmitted by the Shaft

    Hi Russ I have calculated the Angular accelearation by considering the RPM of the Drum and which are initial RPM =0 final RPM = 20 then I calculated Angular velocy 2xpixN/60 then I assume the time taken for the drum to reach from 0-20 rpm would be 3 sec and then Angular accelartion comes ou to be Angular Velocity/delta T = 0.7rad/sec^2 . I have neglegted the froction values in the bearings.

    So which formula is the best T=F x r or T= I x Alpha. If first formula is better then how to calculate Force F?

    Thanks for both of you for replying
  6. Aug 12, 2010 #5


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    Staff: Mentor

    You need to combine the two equations to calculate force from that information.
  7. Aug 13, 2010 #6
    You are way overthinking this, you dont need the 'force' as this is all rotation, should should only be using torques. T = Fr should only ever be described in terms of T.

    You have the acceleration torque, which is T= I alpha. This is by FAR the largest torque the shaft will experience.

    You will also have some braking torque due to component friction (ie the bearings). It's impossiblt to predict this with the information you'd given, but it's likely to be miniscule in comparison to the acceleration torque.

    However, unless you are designing the shaft with a factor of safety of 1x failure stress (which you should never do) it's irrelevent. As the biggest stresses will be during acceleration, and if you design to a FOS of 2, you know you are well in with the stresses and max torsion.

    To put it in maths terms.

    Total T = I * alpha + Brake T

    Where I*alpha >>>>> Brake T. So much so we can ignore the shaft friction altogether. So the equation we need to design to is:

    T = I*alpha.
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