Nut plates overly tight

Hi OME,

 

For a start, there are no assembly instructions.  This is a scratch build and the plans don't go into the detail of inspection ports etc.

 

In this particular instance I'm using 8 nutplates to attach a 0.040" instrument panel to the standard 0.025" panel (the majority of which has been cut away, leaving a frame for the front panel to attach to).  This will enable me to remove the panel easily.

 

As a picture tells a thousand words, the following 2 show how far the screw goes in before becoming too tight to turn.

 

[ATTACH]43018[/ATTACH] [ATTACH]43019[/ATTACH]

 

Everything looks normal.....are you sure you aren't just a bit limp wristed? 

To me the "brand" of anything doesn't mean much these days. Few actually manufacture themselves.

The best you can ask for is a robust quality control system.

The chinese have taken great pleasure in buying brands and replacing top quality products with cheap and nasty replacements (obviously maintaining the quality product pricing). Or they lower the price to put pressure on competitors.....with the intention of driving them out of business.

After a few of years the public work it out......sometimes....

Just because these part have a "number" it doesn't mean jack .....

If airliner parts can be faked, a few nut plates are nothing....

https://www.nytimes.com/1995/05/25/us/counterfeit-airliner-parts-are-said-to-be-often-used.html

https://en.m.wikipedia.org/wiki/Unapproved_aircraft_part

Boeing has stated that mechanical parts, electronic parts and materials have been counterfeited. Physical parts include bolts, nuts, and rivets. Electronic parts include resistors, capacitors, and integrated circuits. Materials include composite chemicals, steel, and titanium.[8] In 1996 unapproved parts were found to originate from sources such as counterfeiters, thieves, "strip and dip" operations which hide defects with metal plating, and from production overruns.[7]

Marty, get hold of a thread pitch gauge and check the nut against the screw. Despite the full traceability, there's always the chance of a mix-up. Traceability merely finds the person responsible.

 

The short distance the screw is going in, before it binds, indicates a serious thread mismatch.

 

Yes, you really need a pitch gauge and verniers (digital will give imperial or metric readout)

First 100% confirm pitch and dimensions of the bolt/screw and go from there....

Often the actual OD of a thread is slightly smaller than the nominal size....

It has to be smaller than the nominal size.  The flats UNF or round with BS do that, Aren't we talking of 8 gauge 32 here? Nev

Your friendly local LAME should be able to supply the correct nut plate and screw and rivets, we never have issues. 

Use your verners to measure inside of both ends as some of these are slightly narrower dia at inside end to ensure not working loose in operation under vibration etc.

There are metric nut plates in europe that are like a nylock and easier to put in and out but like all nylock methods only tight for a number of uses.

I used camlocks for half my panel access panels, works great.

Your getting closer with the build; well done.  Cheers

Yes I'd prefer the dzus type fastener generally  if it's coming off much and I'd give stainless a big miss in a general sense, as unless it's lubed it may easily seize . It's a bugger for galling. Also when press fitted.  Nev

Dzus fasteners are great for access panels, but Marty is simply fixing a panel that he "might" want to remove once or twice in the life of the aircraft.

I'm still betting on the screws he is using being too long for the nutplate. 

Marty,

If you are scratch building, how did you determine that you needed and AN525 as opposed to another type of screw?

I'm still betting on the screws he is using being too long for the nutplate. 

 

The photo in his above post clearly shows they are not too long.

Dzus are ok or bigger stuff, but for panels Turnlock fasteners are the go. Nothing wrong with nutplates and screws either if access isn't required every day,  it sure beats having to get a spanner in a blind confined space.

They probably just need some lube and run on and off a couple of times.

I meant to type 8 instead of 6. A no 8 will screw into a 10 easily and feel correct until it gets a bit of torque applied, when nit will just pull through. It would jiggle a bit when inserted showing that it is too small.

I can see 5 threads not into the nut before it has bottomed out.

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That's 5/32" plus the grip length of 2/32.   7/32 = 0.71875" Marty says he's fastening two sheets with a total thickness of 0.065". He only needs the grip length of the AN525 to fill the hole space in the two sheets. Then he only needs a threaded length which is enough to go to the bottom of the nutplate. 10R7's are obviously too long since the threaded length is 3/8". A 10R6 has a grip length of 1/32" and a threaded length of 11/32. To use a 10R6 would mean that one thread of the threaded length would be in the hole through the back sheet. That leaves 10/32 (5/16) of thread to go through the nutplate.

Until Marty can tell us the length of the threaded section in the nutplate (measured in 1000th's of an inch, none of your poofy French stuff) we can't determine the correct length for anything that he wants to screw into the nutplate successfully.

I can see 5 threads not into the nut before it has bottomed out.

 

/monthly_2020_01/20200117_103235.thumb.jpg.5003b9018896ddb1ab5748fe624c6176.jpg

 

That's 5/32" plus the grip length of 2/32.   7/32 = 0.71875" Marty says he's fastening two sheets with a total thickness of 0.065". He only needs the grip length of the AN525 to fill the hole space in the two sheets. Then he only needs a threaded length which is enough to go to the bottom of the nutplate. 10R7's are obviously too long since the threaded length is 3/8". A 10R6 has a grip length of 1/32" and a threaded length of 11/32. To use a 10R6 would mean that one thread of the threaded length would be in the hole through the back sheet. That leaves 10/32 (5/16) of thread to go through the nutplate.

 

Until Marty can tell us the length of the threaded section in the nutplate (measured in 1000th's of an inch, none of your poofy French stuff) we can't determine the correct length for anything that he wants to screw into the nutplate successfully.

 

Unless he specifically ordered the nutplates with a closed bottom, (and the one pictured isn't) any length screw can be used until it runs out of thread.

The screw in the pic is not shanked out and should screw right through.

BTW, grip lengths are not measured in thousandths of an inch, but the far more masculine 1/16 or 1/8 

This thread is starting to sound like the "How many forum posters does it take to change a lightbulb" thread.

I don't know what brand they are but I have never had such an issue with any of the nutplates I've used. The screws tighten up due to the thread being slightly tapered but they go right through without excessive force and keep going till whatever is being clamped is tight. By the current number of posts it probably takes about 20 posters to put a screw into a nutplate.

 

BTW, grip lengths are not measured in thousandths of an inch, but the far more masculine 1/16 or 1/8 

 

Or even 30tooths. I just used the decimals for easier computation.

This thread is starting to sound like the "How many forum posters does it take to change a lightbulb" thread.

 

I refuse to be LED into that enquiry.

Here's the data on the two types of fasteners:

Hi all,

Thanks to all of you for your advice.

I'm just going to go with the theory that I'm a wimpy wuss-bucket and not worry too much about it.  The 8 plates I'm using for the panel are working now that the screws have been driven in and out several times, for other uses I'll attempt lubing them first and if that doesn't work I'll use the same persuasive technique I used with these ones.

OME, I'm not sure where I got the screw size, it may have been mentioned in the plans for another area (probably the rear fuse hatch).  In any case, as M61 said, the nut plates are open ended so the length of the screw shouldn't matter - when the panel is tightened up the screws protrude a good 5mm beyond the end of the nut plate.

Thanks again!

Cheers, Marty

I refuse to be LED into that enquiry.

 

Did you mean "LEAD" or "LED", as in Light Emitting Diode?

Did you mean "LEAD" or "LED", as in Light Emitting Diode?

 

I just can't resist being punny.

when the panel is tightened up the screws protrude a good 5mm beyond the end of the nut plate.

 

A good 5 mm is 3/16", or so. That means that you have nearly as much thread gone through the nut and out the other side as is inside the nut doing the work. The nuts might be open-ended, but they are also oval so that the nut gets a good grip on the screw. By running the screw as far as you have done, you are rounding the oval and reducing the holding ability of the nut. You are giving the screw the chance to loosen with vibration, although in this location you will notice it and can access it easily for re-tightening.

If a 10R7 has an overall length of 7/16", and you have about 3/16" protruding from the bottom of the nutplate, then I suggest you buy some AN526-1032R4, or even R5 screws to use with those nutplates.  Length of R4 is 1/4" and R5 is 5/16"

AND PLEASE STOP USING THE METRIC SYSTEM WHEN MEASURING AN, MS, or NAS HARDWARE. ?

By running the screw as far as you have done, you are rounding the oval and reducing the holding ability of the nut.

 

That's what they are designed to do. That is how they lock.

Generally it is accepted that a screw is "in safety" when at LEAST 2 threads protrude through the nut.

If I was supervising/ or inspecting a job where the tradie had screws that didn't go all the way through, he would be replacing all the short ones with the correct length fastener before anything got signed.

I agree that it is desirable to use one system or the other. I currently work on metric aircraft, and as far as rivets go, most of them are just standard rivet sizes but labelled in metric, but many are still imperial part numbers. it's damn annoying having a hybrid system.

All the screws though are metric.

 Depending on the material the thread is into you need varying  length of engaged thread  operating. Unless it's real crap 2x dia is enough from the strength aspect  as a minimum, but self locking and wear come into it as well if it's removed often. Worn or loose threads have a potentially large strength deficit and  you run the risk of thread stripping before the design tension is achieved. Long thread depth also is loading the first few threads unavoidably as the screw stud etc is stretching making the thread more coarse in the most stressed areas. Properly designed screws/ studs have reduced shank diameters to put the stress away from the top  threads and spread it over a larger length. Have the right thing in the right place.. Simple covers like this one are not loaded much  The main thing is the fasteners stay there..Nev

If you've got an instrument panel vibrating that much, that the screws are going to fall out, I'd have to opine that;

A. You've got some serious engine/prop imbalance issues - and

B. You'd be having trouble reading the instruments.

I would imagine you'd be quite well served with regular screws and nuts retaining the instrument panel, just tightened to correct torques.

Lubing the screws would only assist in loosening them, IMO.

OT .put your hand on any part of a piston engined plane in flight and you will "feel" the rumbles. Nearly everything flexes too. A LOT of instrument panels are rubber mounted even in the old days makes the instruments last longer and makes an IVSI need a little motor to vibrate it. Nev

Needed to be spring mounted in the gyro days.