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Pitch Distance

Pitch Distance

Pitch Distance

(OP)
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.

 

RE: Pitch Distance

The net section stress (s = P/(w-d)t) is increased in the straps (w = 3.5d in your case).

RE: Pitch Distance

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.

RE: Pitch Distance

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.

RE: Pitch Distance

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.

RE: Pitch Distance

rb1957,

Don't you mean 4.8mm = 3/16"?

tg

RE: Pitch Distance

duh !

RE: Pitch Distance

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

RE: Pitch Distance

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.

Aerodesign
 

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