Why split pins are the safest option for securing bolted connections

Discussion in 'Workshop Tips and Secrets / Tools' started by Daniel Tyrkiel, Jun 14, 2010.

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  1. Jun 14, 2010 #1

    Daniel Tyrkiel

    Daniel Tyrkiel

    Daniel Tyrkiel

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    Hello, I don't post much these days, but I thought this may be useful.

    A salesman from nord lock came in to the place I work at the other day to peddle their special washers and showed us a vibration test using various nuts (plain, nyloc, ovalised, and their own of course) and the graph attached is the result. On the vertical is the clamp load of the joint, and on the horizontal is time (of the vibration test).

    The results for standard nuts and nylocs and other wonderful but standard fittings got me the chills. Basically they will lose all of the clamping load and the joint will fail at some point.

    The only thing that would hold was their thing and split pins. He also showed that a double nut but tightened using the 'top' nut only holds for many times longer than others but will lose the load eventually anyway.

    I'm not affiliated with this company, but this being a critical element in aircraft that are to carry us safely and give us pleasure, I thought I'd post it here.

    regards

    Daniel
     

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  2. Jun 14, 2010 #2

    Hot Wings

    Hot Wings

    Hot Wings

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    Interesting. But do you have a key to go with the graph?

    I found a video for part of the graph. I notice they didn't demonstrate a tabbed or cotter keyed fastener.

    Nord-Lock - NORD-LOCK Video

    This gives me reason to rethink the use of nylocks in some situations.
     
  3. Jun 14, 2010 #3

    Daniel Tyrkiel

    Daniel Tyrkiel

    Daniel Tyrkiel

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    What I remember is that the red ones are plain nuts, green blue and purple are nyloc, ovalised nut and spring washer, and lastly the very flat ones are nord locks.
     
  4. Jun 15, 2010 #4

    wsimpso1

    wsimpso1

    wsimpso1

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    Don't go and get your panties in a wad over this. I have been involved in this stuff. Engines and transmissions with well designed joints don't require substantially less torque to disassemble than to assemble, even after huge duty.

    That particular rig is designed to simulate a particularly bad joint, and yes, if your joint will slip in use, it is a badly designed joint, and will loosen the fasteners.

    In particular, if a single bolt is holding a joint and the joint sees torsional vibration that will slip the faces, you do need to cotter pin or safety wire the joint. Or use a set of Nord washers... Me, I am sticking with 80 years of successful AN hardware experience and joints designed to not slip. See Shigley for the basics of good joint design...

    Billski
     
  5. Jun 15, 2010 #5

    Daniel Tyrkiel

    Daniel Tyrkiel

    Daniel Tyrkiel

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  6. Jun 15, 2010 #6

    Hot Wings

    Hot Wings

    Hot Wings

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    As should all of us. But we also encounter joints during maintenance that aren't always designed to such ridged standards. What got my attention was the insidious nature of the failure mode.

    While they are two different creatures, in a past life as a truck mechanic it was part of the inspection process to physically grab each nut and bolt to check for looseness. On occasion I'd find broken or missing bolts that I KNOW I'd just checked a few hours of use earlier. In hind sight it appears that there was just enough residual clamping force that the bolt appeared to be tight when in fact it had already failed. I now believe that I have seen the same sequence of events on aircraft, both certified and home-built.

    I don't miss working on trucks:uzi:
     
  7. Jun 16, 2010 #7

    Daniel Tyrkiel

    Daniel Tyrkiel

    Daniel Tyrkiel

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    I'm glad that this has opened a discussion, as this is quite important. It's obvious that Nord Lock are trying to run a business and sell a product, but so do nut and bolt manufacturers. Anyways, hope this helps. To be honest, I won't be building anytime soon, because of m.o.n.e.y. or lack of it ;)
     
  8. Jun 16, 2010 #8

    Dana

    Dana

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    Like Billski, I'll stick with good ol' AN hardware for airframe connections. But there are a few places... I first saw washers like the Nord-Locks some 25 years ago and thought they'd be great for holding on model airplane propllers, which are notorious for coming loose (unsurprising, with a single nut holding the prop on a single cylinder engine).

    -Dana

    Beware of strange faces and dark dingy places,
    Be careful while bending the law.....
     

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