K1000 nut plate locking feature wear

[jpolenek]

Is there a specification or objective measure for determining the effectiveness of the locking feature on K1000 nut plates so we know when they should be replaced?

Joe

[Sam Buchanan]

Is there a specification or objective measure for determining the effectiveness of the locking feature on K1000 nut plates so we know when they should be replaced?

Joe

As long as you cannot remove a screw from the plate nut with your fingers it is still functioning as designed. I've removed screws from plate nuts in my 20-year-old RV-6 dozens of time during inspections/maintenance and all are still in good condition.

[Marc Zeitlin]

Is there a specification or objective measure for determining the effectiveness of the locking feature on K1000 nut plates so we know when they should be replaced?Although AC43.13-1B, Chapter 7-64 doesn't specifically speak to the issue of prevailing torque for metal lock-nuts or metal locking nutplates, it does talk about not re-using fiber or nylon locknuts if they cannot meet the prevailing torque values in table 7-2.

So I'd argue that if a metal locknut or metal locking nutplate cannot meet these same values that are used for fiber/nylon nuts (and whether you can turn the bolt by hand at these prevailing torque values is very dependent upon the strength of your fingers - mine are not what they used to be) then they should be replaced.

But a large problem is that the table has no values for bolts smaller than 7/16 diameter - so we have no guidance for #6, #8 or AN-3 through AN-6 nuts in AC43.13-1B.

However, this document from Bell Helicopter:

https://www.fire.lacounty.gov/wp-content/uploads/2017/02/412-Standard-Practices-Manual-Chapter-02-Torque.pdf

shows what they use for minimum "Tare" torque values, and they do have AN-3 and up nuts. Their values for 7/16 and larger are higher than AC43.13-1B, so they're being more conservative, it seems.

Lastly, we get to this:

http://tinelok.com/wp-content/uploads/2013/09/IFI-100-Prevailing-Torque-Locknuts.pdf

from a MFG of locking nuts, and they have a table for nuts down to #4. Their values are lower than Bell's are and bracket those in AC43.13-1B, depending upon how many removals.

Now, for the widely used #8 and AN-3 nutplates, you're looking at somewhere around 1 - 2 in-lb, so for most folks, it may very well be the case that "hey, I can't unscrew it with my fingers" is an adequate measure, and in any case, I don't have a torque wrench that can measure 1 or 2 in-lb - that's a tiny torque wrench :-).

That's all certainly more than _I_ wanted to know...

[Sam Buchanan]

As I was sayin'.......... ;)

But that was an intriguing stroll through documentation and one I had not seen previously.

[jpolenek]

Marc,
From your experience, what kind of lifespan can we expect from nut plates used on something like aileron hinges (e.g. Long-EZ/Cozy), given the removals and re-installations required in the building process and afterwards?

Joe

[vaflier]

If you have concerns and the nut plate cannott easily be replaced you could always add a drop of low strength Locktite to the fastener for insurance.

[Marc Zeitlin]

From your experience, what kind of lifespan can we expect from nut plates used on something like aileron hinges (e.g. Long-EZ/Cozy), given the removals and re-installations required in the building process and afterwards?As Sam stated, dozens of times is not at all unreasonable. Cowls, wheelpants, and the like come apart way more often than primary structure and/or control system components, so that's where nutplates would be more susceptible to losing prevailing torque. I can't say, in checking many hundreds of nutplates that are on airplanes up to 40 years old that I've run across more than a few that have lost their locking capability. Mostly on cowls, due to the fact that the cowls come off 10 - 20 times during the Phase I period, and then a few times/year after that. So on a 30 - 40 year old EZ, each nutplate may have been used up to 100 times. And still, only a few lose prevailing torque.

On EZ's, it's rarely a big deal to replace one in the unlikely event you need to, unlike on an RV, where there are at least 10 gazillion (technical term) nutplates and screws, many in places you can't get to.

My $0.02.

[cwilliamrose]

Then there are people like me who, after more than a few experiences with stainless screws galling in a K1000 nutplate, take a tap to any nutplate used with stainless screws to remove the friction. In the case of screws holding non-structural sheet metal panels I'd rather replace a few lost screws a year than deal with a single screw galled in place. Nutplates used in other areas are a different story, for those being used as described above I'd say the less friction there is the better I like it.

[Sam Buchanan]

A little dab of BoeLube or beeswax (or a toilet floor wax seal) on stainless screws will prevent galling and retain the friction lock of nutplates.

[cwilliamrose]

I agree but it's simpler to just live without the friction. Since I'm looking for the same functionality as a sheet metal screw the machine screws are a good improvement. They don't get loose as easily and the treads are much less likely to strip in service. Light friction would be better but there's no choice there.

[Paulie123]

Then there are people like me who, after more than a few experiences with stainless screws galling in a K1000 nutplate, take a tap to any nutplate used with stainless screws to remove the friction. In the case of screws holding non-structural sheet metal panels I'd rather replace a few lost screws a year than deal with a single screw galled in place. Nutplates used in other areas are a different story, for those being used as described above I'd say the less friction there is the better I like it.
That defeats the purpose of the self locking feature. Do all you builders have limp wrists?

[cwilliamrose]

Say what??

The self-locking feature is not a requirement and in fact does not exist in the sheet metal screws the machine screws are often replacing.