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Pitch Distance
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Source：Internet Author：Unknow Pubdate：2010-03-13
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istambul (Aerospace)
21 Dec 09 23:13
Hello Everybody,
I would like to have the clarification on the effect of pitch distance in stress analysis. I have fitting which needs get connected through fasteners of 4.80 mm dia. The normal pitch kept is 4.5d-5.0d. In this case, due to space constraint, the pitch is 3.5d. The fasteners are shear loaded. How does it affect the stress results? Thanks in advance.
SWComposites (Aerospace)
21 Dec 09 23:58
The net section stress (s = P/(w-d)t) is increased in the straps (w = 3.5d in your case).
rb1957 (Aerospace)
22 Dec 09 6:21
closely pitched fasteners have an increased Kt ... 3.5d doesn't sound too bad. 4.8mm (3/32") dia rivets(?) sound pretty small for structural apps.
字串7
GBor (Mechanical)
22 Dec 09 9:44
If you are working on a commercial aircraft and Part 26 is involved, you will have to do a DTA, and the foreshortened pitch will likely result in reduced cycles to failure. Also, with such a short pitch, detection of a crack may not be possible without some significant expense due to the method that would be required to detect such a small crack.
737eng (Aerospace)
22 Dec 09 11:18
If it is a "fitting" and the fitting has an offset load the actual shear stress could go up per fastener due to the eccentric load acting on the closer spaced fasteners.
trainguy (Structural)
23 Dec 09 18:57
rb1957,
Don't you mean 4.8mm = 3/16"?
tg
rb1957 (Aerospace)
31 Dec 09 14:12
字串8
duh !
graemew (Aeronautics)
4 Jan 10 1:04
G'day Istambul
As 737eng says, changing the pitching of fasteners in a fitting can vary the load distribution between the fasteners, as well as the reactions to applied loads which may have a component parallel to, but displaced away from the mating surfaces.
Without knowing what the fitting looks like, I recommend that you consider performing a bolt group analysis (perhaps even a 3D one if necessary) using unit applied loads (in appropriate directions) to determine comparative fastener loads between the nominal and the varied configurations.
Regards
Graeme
Aerodesign (Structural)
5 Jan 10 5:09
Istambul,
Biggest influence is that more fasteners = more weight = less efficient. Unless you have an inter-rivet buckling issue (comp loading) or bearing-bypass 'interaction' concerns the best-practice is to revert to 5d or thereabouts. 字串1
Aerodesign
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