Nut Plate Fastener Loads

[lildaveyboy]

Hello. I have a general question about how to account for fastener/pin loads applied to nut plates. Is it correct to assume that the large hole nut plate fasteners take all of the pin load and that the small hole nut plate rivets do not take any of the pin load? I realize it is conservative in an analysis to assume that the large and small hole fasteners each take all of the pin load but I am wondering if the small hole rivets truly take any of the pin load or a certain percentage? Does anyone know how this situation is typically analzyed? Thank you.

 

[dwallace1971]

The small, or satellite holes are for holding the nutplate in place and are not meant to carry any of the pin loads.

 

[rb1957]

but isn't the hole in the parent material a clearance hole ?

so the only connection between the screw and the rest of the world is the nutplate, and it's fasteners ?

 

[dwallace1971]

Only if one lives in a world without friction.

 

[YoungTurk]

In my experience, neither the effects of friction or any structural contribution from the nutplate rivets are considered for static strength.

Flabel lays out the friction issue as clearly and plainly as I've seen:

"In analyzing a typical mechanically fastened connection, it is generally accepted practice by industry professionals to assume that the frictional resistance between the joint fasteners and the plates of the connection are minimal at ultimate load. Generally speaking, if the frictional effects between the plates and fasteners of a joint have been overcome, the joint may be analyzed for its "ultimate strength capacity." "

This is conservative but reasonable because of the uncertainly of preload, surface coating and finish, and possible joint slip due to shocks and vibration.

As to load capacity of the rivets, consider that the relatively weak rivets on the nutplate may fail under ultimate load (if the hole size is excessive), but the joint will retain the strength of the primary fastener where it bears upon the mating part.

I generally only consider the nuplate rivets when concerned with fatigue aspects of a design.

 

[dwallace1971]

When evaluationg for ultimate load, correct on every point. However, when evaluating for operational loads, friction and preload in a joint should be considered. Other than fatigue, the only consideration I make for satellite holes is if they are capable of reacting out the specified torque. For reasons of fatigue, it would be nice to get rid of those little buggers altogether.

 

[lildaveyboy]

My question was actually related to fatigue. We consider pin loads in our fatigue analysis so I was wondering if you have a fastener/pin load on the nut plate joint do you consider any of that load going to the NP small hole rivets or do you just assume the large hole bolt carries all of the pin load. Thanks.

 

[YoungTurk]

No response...

Take the pin load out at only at the bolt / pin. Should be conservative that way.