Bolt and nut versus bolt and plate nut

Is a plate nut with a bolt equivalent shear and strength to the same size bolt with a nut and washer? Wondering if I could substitute a nut for a plate nut or if there is more to it due to strength, so it is more dependent on the situation.

Some experimental results.

RVFan671 said:

Is a plate nut with a bolt equivalent shear and strength to the same size bolt with a nut and washer? Wondering if I could substitute a nut for a plate nut or if there is more to it due to strength, so it is more dependent on the situation.

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While I have no official reference, I wanted to know the strength of a nut plate by using torque for some important pieces that would be serious problem if they came loose. #6 screw was tested with a stainless all thread and stand off and a #6 regular aviation steel screw. The torque leveled (failed) at around 50 in-lb for 2 samples. Additional testing with 30 in-lb showed consistent for several samples showed the ability to achieve 30 and do it again. I used 15 in-lb for the assembly.

I think the max recommended is 12 in-lb. Hopefully this is helpful for your decision, although not specifically a shear test.

AN bolts

I'm particularly interested in AN 3,4,5 bolts rather than screws but thanks for the helpful information on your testing.

that question cannot be answered as a generality. it depends on the application.

i would say that in a application that is 100% shear, it would not be a problem as the shear strength of the fastener is important factor. but, if there is any tension load, then all bets are off.

bob burns
RV-4 N82RB

On my airplane, I used the platenuts for the extra holes that I need additional attachments such as on the firewall or mounting and it also means they aren’t structural. For the structural stuff, I stuck to the plan when it calls out for regular nuts.

Since you stated "depends on the situation" I'll throw in my 2 cents. I have no idea which option is stronger, and believe any difference that exists is minimal.
What is very important, however, is a reality that I've discovered since certifying my RV-7 15 years ago. Namely, that if you use a nut you will inevitably need to remove the screw/bolt years down the road and will regret not having used a plate nut. This is especially relevant when you can't physically put a tool on the screw/bolt at the same time as you put one on the nut - firewall applications, for example.
My recommendation is to always use a plate nut/nutplate. It takes more effort now, but you will probably reap the dividends later.

n82rb said:

that question cannot be answered as a generality. it depends on the application.

i would say that in a application that is 100% shear, it would not be a problem as the shear strength of the fastener is important factor. but, if there is any tension load, then all bets are off.

bob burns
RV-4 N82RB

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Yes, this is correct. In a shear application, the contact between the bolt and the parts is what matters. This is why close tolerance holes are important. The nut is just keeping the assembly together. You don't rely on friction at the assembly interfaces, nor at the nut.

In tension, although plate nuts are pretty hard steel, they have fairly small contact area with the mating parts, and also rather thin nut area surrounding the threads. Possible early failure modes are the plate nut extruding through the bolt hole, or badly deforming at the interface between the plate nut and the mating part, and the most likely one: the body of the plate nut where the threads are expands radially because of the wedge/ramp effect of the threads until the threads leapfrog over to the next thread. This often does not appear to damage the threads, and an observer may incorrectly conclude that the screw was just loose from turning.

Minor thread drift: This mechanism of threads leapfrogging is also observed in NAS679 nuts in tension. Same problem that the shell of the nut is rather thin surrounding the threads. Note especially that these NAS679 nuts are not a good choice for RV-8 landing gear bolts.

axial strength

The spec sheets for steel plate nuts K1000/MS21047 and K2000/MS21051 show the same axial strength as steel nylock nuts AN365/MS21044.

OTOH ref the next post by scsmith.
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johnbright said:

The spec sheets for steel plate nuts K1000/MS21047 and K2000/MS21051 show the same axial strength as steel nylock nuts AN365/MS21044.

Everyspec.com has spec sheets for free.
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Those specs are Minimum axial strength. Note that for the MS21044, the min required strength is 4580 for a 1/4-28 size, regardless of whether the nut is steel, aluminum, or CRES. The steel nuts are, in fact, quite a bit stronger than that. Plate nuts, perhaps not.