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Thread: NASA fastener design manual

  1. #11

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    It's helpful to distinguish between "locking" and "retention" when discussing fasteners; these terms tend to be used interchangeably when in fact, they're entirely separate functions.

    When speaking of a "locking" feature, this is a mechanism intended to prevent a fastener from loosening or otherwise disrupting the installed preload (otherwise known as "torque"). A "retention"" feature does not prevent a fastener from loosening. Its purpose is to retain or capture the fastener (once loose) from separating from its mating part and causing further damage. Cotter pins, safety wire, [lock washers] and related mechanisms will NOT prevent a fastener from loosening.

    As noted, a good quality lock washer will, once relaxed (fastener broken free from its installed preload), resist further loosening of the fastener (acting as a retention mechanism). Fully compressed with the correct fastener preload, though, it acts as a simple flat washer with no locking attributes.

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  3. #12
    Andy from Workshopshed Workshopshed's Avatar
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    This video shows the differences.



    NASA fastener design manual-graph.png

    Which also seems to be referenced in this article.

    https://engineerdog.com/2015/01/11/1...out-fasteners/

    So looking at the graph they are better than a plain washer but not much. I've mostly seen them used when the clamped material is softer hence you'd not want to tighten the bolt too much and also they would work better in that case.
    Andy from Workshopshed
    "Making and repairing things in a shed at the bottom of the garden"
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  5. #13
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    Agreed Jon, the moment the nut is loosened, the raised ends of the washer dig in and resist further rotation. Quality in manufacture make some split washers more effective than others. One thing to note is that split washers are not generally used in the aviation industry. Here reliance is made on lock nuts of various designs, usually clamping the threads of the bolt through deformation of the nut or a nylon type insert. Inserts of this type are sometimes inserted into screw threads as well. The best know, and most disliked by aircraft mechanics, is the locking wire system where nuts and or bolts are secures in a tightening direction by stainless steel wire. Moving off the point, there is a right and a wrong way to fit a plain washer. Washers are stamped out and have a 'flat' side and a 'rounded' side. The rounded side should normally be under the nut and the flat side, which might have a bit of a feather on the edges, should be sited towards the casing secured.

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  7. #14
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    That video and the related ones are truly excellent, a picture is worth..... Lets not forget simple staking as well, at least if done well fastener is reusable.
    On the subject of insert type nuts there are 'Hex - Seal' brand nuts that while they have some anti-vibration traits their primary function is to seal against the ingress on contamination or fluid along the fastener.
    This is a fascinating subject.
    Best anti-seize next?

  8. #15
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    Nuts with inserts are manufactured by Simmonds Nuts, earlier versions had a fibre insert as a locking device. Later versions employed nylon and were known as Nylock nuts.

  9. #16

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    https://www.google.com/url?sa=t&sour...UzNNMybMPPhK-Q

    I googled "what is the intended purpose of a split washer" and this was among the first links returned. As mentioned previously, the term "lock" is apparently open to interpretation, especially by those folks at NASA. It seems these weren't meant to "lock" something, and are incorrectly named in the public eye of most hardware stores.

  10. #17

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    Over-torquing has well known results, crushing, stripped threads and broken studs/bolts being the most obvious.

    What is less understood is the results of under torquing.

    In a Douglas Aircraft publication I had eons ago there was a large section on torquing and correct torque procedures with lots of emphasis on determining the running torque on a friction lock-nut (nyloc or metalok) and adding that to the required torque to determine the correct torque setting for the wrench. Using only the lubricant specified (Xxxx or none), surface cleanliness and other factors were also discussed

    Most important to me was the long paragraph discussed actual tests to determine the results of under-torquing and discussed why an under-torqued joint could fail (bolt shear or component material failure) in as little as 4,000 stress cycles whereas the identical test piece correctly torqued lasted close to 200,000 cycles before failure.
    Last edited by MiTasol; 08-06-2017 at 10:57 PM.

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  12. #18
    NortonDommi's Avatar
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    Here's an ad for the 'Hex-seal seel nuts'. I worked a while in the waste industry and the leachate that developed in the back of packers did not always fall out with the rubbish. Corrosive and very bad if it was in a pocket when welding a tear in a bin. This stuff was a great penetrant and would migrate huge distances and there was always the lazy guy who could not be bothered to put any sort of sealant or lube on fasteners. Apart from correct torquing procedure I am of the opinion that prevention of a problem is far easier than dealing with a problem,(Murphy's Law applies to weather, location, time etc),and hidden corrosion is a problem.
    I was told the first day of my apprenticeship:
    " Do the very best job you can and think of the next guy because that 'next guy' might just be you". I have never forgotten that and just wish it had been drummed into a few I have met early on.

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  14. #19
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    Hex-seal Seel-nut.

    Attachment 19098

    I forgot to add this.

  15. #20
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    Torque Charts.

    I had to go and look at some torque charts which was interesting. I'm in the habit of always lubricating threads with something except when there is a specific callout not to, plated threads are an example as the plating can be the lubricant. Zinc and Cadmium 1 are the most common. These platings are usually associated with lower grade tensile fasteners but have some valuable attributes such as not being great contributors to Hydrogen embrittlement, both are good lubricants under pressure and both provide cathodic protection. The tribological interactions as a fastener is tightened and the effect of surface treatments is something that any Aeronautical Engineers on this site would I hope be able to explain in a bit more depth than I have knowledge of.
    Attached a couple of charts.Attachment 19111Attachment 19112Attachment 19113

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