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Converting screw torque

  1. Dec 6, 2011 #1
    Hello MEs -

    I am an EE and have customer that insists in not following our qualified mounting instructions for a power module:

    Our instructions M6 8.8 screw torqued to 2.5 min to 5.0 Nm Max.

    Customer is using NAS 1802 torqued to 25 in lbs -->> Is this a suitable conversion?

    Of course they are breaking the modules - they have 10 installed, 8 with mounting hole cracks, and we make about 100,000 of these a year. I have customers in my territory using 2-5K per year - and never a problem of this type. We are a high end German co, that does not mess around - the modules are rigorously qualified.

    And of course - the customer is sure the modules are defective. They swear that the torque they are applying is correct - but they really have to be doing something to break the modules.

    Also - they are using a manual torque driver - the type that clicks when the torque is reached, our spec is for electronic driver with break away clutch - much more consistent
     
  2. jcsd
  3. Dec 6, 2011 #2

    Mech_Engineer

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    The two screws seems somewhat comparable. I'm assuming they're using a 1/4-28 screw out of this standard: http://www.av8design.com/Hardware/NAS1802.pdf.

    Diameter: .250" (6.35 mm) vs. 6mm
    Strength: ~160ksi (1100MPa) vs. 800MPa
    Thread pitch: .036" (.914 mm) vs. 1mm

    The screws are larger and stronger, but if they only torque to 25 in-lbf (2.83 N-m) they should be pretty close to your preload range. It could be they're over-torqued, or maybe they're using lubricant on the threads where you don't specify any. If the coefficient of friction in the joint is lower than expected from the calculation, a given torque will generate more preload force than expected, which might account for the problem too...
     
  4. Dec 6, 2011 #3

    Q_Goest

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    Hi Windadct,
    Torque is a measure of rotational force.
    T=Fr
    Where T= torque
    F= force
    r= moment arm or radial distance from the location of the torque to the force.

    So all you do to convert Nm to in lb is convert the units.
    1 meter = 39.37 inches
    1 newton = .2248 lb

    So 1 Nm = 8.850 inch lbs
    5 Nm = 44.25 inch lb

    So it looks like the 25 in lb (2.825 Nm) is not the issue.

    (conversions for torque can also be found here: http://www.onlineconversion.com/torque.htm)

    ***

    Edit: Couple more thoughts... for the torque you list, a stress analysis indicates the bolt stress is very low (on the order of 10,000 psi) but the area under the head of a bolt is roughly the same as the tensile area, maybe just slightly higher. So the compressive stress on your module, which I'm assuming is plastic, is roughly the same. That much compressive stress will break most unreinforced plastics. And as Mech Engineer mentions, adding a lubricant to the thread will increase that compressive stress substantially.

    Another thought that occurs to me is whether or not your standard torque assumes a self locking fastener is used. A self locking fastener, such as one with a nylon insert to prevent loosening, also creates a torque on the fastener that resists tightening. In other words, if you put 5 Nm of torque on a self locking fastener, it's like you're increasing the friction tremendously so the final load on the part is reduced. If your company assumes a self locking fastener is being used and they're not using one, the result will be a much increased force on the part which could be causing the breakage issue.
     
    Last edited: Dec 6, 2011
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