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Torque required to tighten the cap for leak proof

  1. Oct 31, 2016 #1
    Hello Anyone,
    Could you help me in finding the torque req. for a cap to leak proof?
    My cap (polyproplene) which dia. was 32mm and its detail specs are,
    thread major dia.- 28.5mm, min. dia. - 26mm, pitch - 3mm, thread angle-45deg
    which has a EPDM rubber seal placed inside (outer dia 26.5mm & inner dia-18.5mm with thick 1.5mm)

    I want to know.. what should be the torque required to tighten the cap, so that It will be leak proof? or in other way what is the % of seal compression req. for leak proof? to handle the water pressure of ~600N in a plastic container.
     
  2. jcsd
  3. Oct 31, 2016 #2
    What approach should I consider? Shall I go with lead screw torque calc.? or any other
     
  4. Oct 31, 2016 #3

    Mech_Engineer

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    You should calculate how much you want to compress the interior rubber seal, this should give you an estimate of force, and from there you can estimate the torque needed to achieve the desired compression force on the seal.
     
  5. Oct 31, 2016 #4
    Thanks for your suggestion. As per the water pressure, I suppose to compress the seal by 40% based on this if I directly estimate the torque I'll get ~20Nm. If I use torque formula of lead screw concept, I'll get ~65Nm. So got confused. Guide me which way I need to go
     
  6. Nov 1, 2016 #5

    Mech_Engineer

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    The discrepancy in torque may have to do with friction, are you taking friction into account when calculating the required screw torque?
     
  7. Nov 1, 2016 #6
    Yeah. I consider frictional force of thread engagement too while doing torque and got the result of ~20Nm. Hope in lead screw torque Calc. approach also friction parameter will be considered right?
     
  8. Nov 1, 2016 #7

    Mech_Engineer

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    What equations are you using for each calculation?
     
  9. Nov 2, 2016 #8
    For thread engagement force :
    Frictional force, Ff = m g (sin α + μ cos α)

    For comp. of seal :
    E = Stress / Strain
    Stress = E*Strain
    Force/Area = E*Strain
    Force = E*Strain*Area

    Resultant force = Frictional force + Comp. force

    And the Compressive torque, Tc = K x Force x d (where k=Coeff. Friction of seal & d=PCD of seal)
    Here I got 20Nm

    2nd approach of lead screw torque formula:
    Torque = F*Dm/2*(L+u*PI*DM)/(PI*Dm-u*L)
    I got 60Nm.
     
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