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Shanking fasteners
- Thread starter LuckyDuc
- Start date
luckyduc...
I think that Fieldteam’s explanation was for “thread-bottoming” or “nut-bottoming”… however thread-bottoming can lead to fastener-shanking (as described below).
Fastener Shanking.
Parts that fit poorly together, or are not clamped tightly together during assembly-fit-up, develop gaps at the mating-surfaces [faying surfaces]. Installing fasteners thru this stack-up [joint] results in exposed fastener shanks in each of the gaps. This is called "fastener shanking". Note: For driven-rivets this situation is made even worse, since the rivet-shank will swell-excessively [mushroom] in each of these gaps, resulting in a localized hydraulic-pressure force trying to separate the parts even more.
The remedy to avoid shanking fasteners is "simple": shim [metal or resin] all ill-fitting rigid parts... or "pull-down" thin sheet metal parts to full/tight contact during pre-fastening assembly.
Why is this so important to eliminate gaps [in-a-nutshell]?
The gaps allow localized-flexure during cyclic loading, which can cause pre-mature cracking or fretting of the hole or fastener. Also, these gaps are great moisture traps [CORROSION] and are almost impossible to seal for fluid-tightness, since sealant will fill these areas initially... but eventually break-down [crack disintegrate, disbond, etc] due to excessive flexure [LEAKS].
Also, In "significant gapping" situations, small-diameter fasteners [especially soft rivets] will see added pin-bending forces that may result in failures well-below design-load capacity (distortion/cracking, etc). These joints are highly likely to fail the fastener-shanks exposed in gaps due to fatigue, fretting, corrosion or over-load…. although heads/tails could pull-off”, too.
Regards, Wil Taylor
I think that Fieldteam’s explanation was for “thread-bottoming” or “nut-bottoming”… however thread-bottoming can lead to fastener-shanking (as described below).
Fastener Shanking.
Parts that fit poorly together, or are not clamped tightly together during assembly-fit-up, develop gaps at the mating-surfaces [faying surfaces]. Installing fasteners thru this stack-up [joint] results in exposed fastener shanks in each of the gaps. This is called "fastener shanking". Note: For driven-rivets this situation is made even worse, since the rivet-shank will swell-excessively [mushroom] in each of these gaps, resulting in a localized hydraulic-pressure force trying to separate the parts even more.
The remedy to avoid shanking fasteners is "simple": shim [metal or resin] all ill-fitting rigid parts... or "pull-down" thin sheet metal parts to full/tight contact during pre-fastening assembly.
Why is this so important to eliminate gaps [in-a-nutshell]?
The gaps allow localized-flexure during cyclic loading, which can cause pre-mature cracking or fretting of the hole or fastener. Also, these gaps are great moisture traps [CORROSION] and are almost impossible to seal for fluid-tightness, since sealant will fill these areas initially... but eventually break-down [crack disintegrate, disbond, etc] due to excessive flexure [LEAKS].
Also, In "significant gapping" situations, small-diameter fasteners [especially soft rivets] will see added pin-bending forces that may result in failures well-below design-load capacity (distortion/cracking, etc). These joints are highly likely to fail the fastener-shanks exposed in gaps due to fatigue, fretting, corrosion or over-load…. although heads/tails could pull-off”, too.
Regards, Wil Taylor
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