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Thread: Help with Proper Torque Spec

  1. #1
    Associate Engineer
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    Help with Proper Torque Spec

    Hello,

    I am trying to determine a minimum torque requirement for sealing a flame proof enclosure that has 26 M14 bolts. Per the manufacturer, the couple of tightening of A2-70 M14 bolts of 117N/sq. mm.

    Using 7mm as the radius of the cross section, we get a required force of the bolt to be approximately 18 kN (117 * pi * 7^2). For the tightening, weíre using a driver (force straight down) as opposed to a wrench, so Iím not sure what the arm radius was, but letís say itís 200mm (~8 inches) ar, that gives us a torque of 3600Nm, or an ungodly number of inch pounds.

    Can you please let me know what Iím doing wrong? I would appreciate it. Thank you.

    Joe

  2. #2
    Lead Engineer
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    It is not clear to me what you mean by " Per the manufacturer, the couple of tightening of A2-70 M14 bolts of 117N/sq. mm."

  3. #3
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    Quote Originally Posted by JAlberts View Post
    It is not clear to me what you mean by " Per the manufacturer, the couple of tightening of A2-70 M14 bolts of 117N/sq. mm."
    I don't know what they mean by this either. The manufacturer is from Italy, so something may be getting lost in translation. Either way, the unit are is pressure/stress as opposed to force or torque.

  4. #4
    Administrator Kelly Bramble's Avatar
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    I think that what they want for
    A2-70 M14 bolts of 117N/sq. mm.
    is to calculate the applied axial tension force from the torque applied to the fastener.


    First, determine the cross sectional area of the M14 bolt - probably the minor diameter.

    Second, determine the axial force require to achieve 117 N/mm^2

    See:

    Estimated Fastener Bolt Clamp Force Calculator

    Cross section area = 3.14157 * r^2

    Force / area = 117 N/mm^2

    You would actually calculate the torque from the desired axial force.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
    Principle Engineer
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    117 N/mm2 is a stress level equivalent to 117 MPa or approximately 17,000 psi.



    This rather low stress for a bolt. This might be based on the unusual thermal requirements of a fireproof enclosure.


    The bolt stress is usually calculated at the threaded area where a combination of changing diameters and sharp corners create stress concentrations. Handbooks provide a value of the “stress area” for most common threads. This stress area is just a bit larger than the area formed by the root diameter of the threads.


    Bolt Load = stress value X stress area



    You did not specify a thread pitch. Based on a 2mm pitch, (M14x2), the stress area is approximately 114.1 mm2. Look up a value for your thread.


    117 x 114.1 = 13350 N.



    A simple estimate of the corresponding torque can be made by using a formula like:

    Bolt torque = .18 x major thread diameter x bolt load


    .18 x 14 x 13350 = 33642 N x mm or 33.6 Nxm or 24.8 ft lbs.


    Back to the part about this being an estimate for a moment. The .18 value in the formula is a friction factor in the threads. Not everyone uses the same value for friction. You may find values as low as .12 and up to .20. The value changes as the threads are burnished by repeated tightening. It is lowered by lubrication. Different materials have slightly different friction values.


    If you have adequate thread engagement, you could use the higher .20 friction factor as a worst case and raise the torque to 27.5 ft lbs as minimum value.



    This answer is based on the limited information that you supplied. Without a complete examination of the project, a definitive answer cannot be made in this forum.

  6. #6
    Technical Fellow jboggs's Avatar
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    For what its worth, I have seen the word "couple" used as a synonym for "moment".

  7. #7
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    This stress area is just a bit larger than the area formed by the root diameter of the threads.

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