Load transfer through anchor nuts / nut plates 2

[yorp15]

I have a piece of non-critical equipment that's mounted underneath a bracket using floating anchor nuts, and I need to reverse-engineer some allowable material removal from the bracket. However I'm having trouble visualizing load transfer through the anchor nuts.

For the vertical load case the bolts are in tension, so I don't have a problem with visualising that.
But for Fwd, Aft and Sideward load cases, how does load transfer into the bracket? I'm pretty sure that I'm missing something here... I'm assuming negligible bearing on the bracket at the bolt holes, because the bolts won't necessarily be in contact with the bracket due to the floating anchor nuts. So the anchor nut itself has to take the shear load and transfer it via the little rivets that aren't supposed to be taking any load? Or should I be assuming that these load cases aren't important (or else the joint would've been designed differently)?

 

[SWComposites]

The load is either transfered thru friction, or the equipment moves enough to bottom out the fasteners in the holes.

 

[Sparweb]

For floating anchor nuts especially, there is freedom to slide until the screw can contact the side of its hole.
For fixed anchor nuts, this is less true, however, the holes that *should* be drilled for non-floating anchor nuts should not have as much clearance, either, so the same condition should be the result.
You might also want to compare the total shear strength of the two little rivets (say... 200Lb * 2 = 400 Lb) with the shear strength of the screw (say... 2000Lb or more) and you can see that the joint does not fail EVEN IF the little rivets were to fail. But in reality, load transfers through the stiffest (steel, solid) members of the joint and the little rivets (aluminum) are more flexible.
The rivets are just there to resist torque when the screw it tightened, nothing more. There are plenty of ways to screw-up an anchor nut rivet installation in such a way that the rivets do get loaded structurally and/or cause fatigue/cracking problems, but your brackets aren't likely one of these ways.

STF

 

[yorp15]

Thanks SWComposites and SparWeb. That does make sense. I suppose in the event that one hole becomes oversized, the bolt may never reach the edge of the hole and so load would just transfer through all the other bolts/holes (until the dodgy bolt ends up with a bending load). One of the holes I'm looking at is going to end up slightly oversized but I don't really want to stuff around with it much if I can avoid it. From memory the oversize is only double the tolerance on the bolt diameter, so finding a sleeve seems like more effort than it's worth.

As you can tell, I'm new to this type of work so there's a lot of knowledge that I need to absorb.

 

[WKTaylor]

yorp15

Sparweb's description is very accurate: nutplates are simply a nut constrained on installation... then the characteristics of the nut, with a properly torqued bolt or screw, take-over.

Just guessing: You are probably installing full/mostly threaded screws in this non-structural installation, where shank-in-bearing [for close-tolerance shear] is non-existent. In this case, torque-tension generates clamp-up friction between parts to accommodate shear.

CAUTION.
Nutplates installed on primary structure mandate a LOT of other rules for airworthiness... but for Your example [a non-structural case]... the usage is simple/clear.

NOTE.
Various terms for the same item, drive me to drink! The term 'nutplate' is traditional/current USA slang... as opposed to the technical [specification] term 'plate nut' or 'nut, plate'... or the European [OK, non-USA specification] term 'anchor nut'.

FYI, only.
SAE ARP1928 TORQUE RECOMMENDATIONS FOR ATTACHING ELECTRICAL WIRING DEVICES TO TERMINAL BOARDS OR BLOCKS,STUDS, POSTS, ETC
NAS1737 INSTALLATION TORQUE LIMITS FOR 160-KSI THREADED FASTENER SYSTEMS SUBJECT TO PERIODIC REMOVAL



Regards, Wil Taylor

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