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  1. #1
    Co-Pilot jetlag's Avatar
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    Oct 2015
    Charente Maritime
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    Replacing screws/bolts and nuts

    I have my prop off at the moment and will be remounting in the next few days. The machine is 10 years old and I assume the nuts n bolts are the same age and looking at the nuts they have been on and off a few times. So I've decided to replace them for the hub and also for the mounting, presently they are 8.8 steel but I thought for a little extra I could replace with either the 10.9 or even 12.9 giving a much greater resistance to rupture. I would still torque to the norms as prescribed for the 8.8 simply having a much higher strength in reserve for the eventual weakening of them over time. Is this OK .... or have I missed something.
    Today is the tomorrow you worried about yesterday.


  2. #2
    Co-Pilot Martin Watson's Avatar
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    Apr 2012
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    Metals do not weaken over time. Cyclic stresses induce fatigue cracks that may result in failure of a component over time, however. Resistance to fatigue is complex and, crucially, stronger is not always better.
    The design and materials specified will have been a compromise between various factors that you don't know about. Trust the original designer and do a like-for-like replacement.
    BMAA 5370
    Fixed wing instruction, examinations and revalidations in Norfolk.

  3. The Following 2 Users Say Thank You to Martin Watson For This Useful Post:

    Arielarts (09-03-18), jetlag (08-03-18)

  4. #3
    Co-Pilot Arielarts's Avatar
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    Feb 2011
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    Do not use 10.9 or stronger. 8.8 or S or R if 5/16" is the magic balance of strength verses ductility. Put simply, the stronger bolts are more likely to break when subject to any bending as they are more brittle. Elsewhere there's a discussion about the use of non-Rotax rubbers on airfilter boxes, with ney-sayers pointing out (incorrectly) that this invalidates the permit, yet prop bolts are far more dangerous to play around with...
    And the length of unthreaded shank is critical too.

  5. The Following User Says Thank You to Arielarts For This Useful Post:

    jetlag (09-03-18)

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