Pitch Distance

[istambul]

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]

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

 

[rb1957]

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.

 

[GBor]

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]

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]

rb1957,

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

tg

 

[rb1957]

duh !

 

[graemew]

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]

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