re-using lock nuts, good, bad?

[James Brown]

hear's the poop,

The prevailing torque locknut retains its locking ability even when the preload or tightening torque has been lost.

A review of research papers tends to conclude that of the two; the elastic lock nut is more re-usable than the all-metal lock nut. Two problems with the all-metal design:
The all-metal locking mechanism rubs the threads and removes the protective plating and damages the threads. This makes the bolt more prone to corrosion and galling damage. Replacement with a new nut does not restore the damaged bolt threads.
The wearing away of metal tends to loosen the grip of the all-metal lock nut.


The elastic lock nut is more friendly to the threads, locks out moisture and prevents corrosion; and the nylon deforms rather than gouges. Except in areas of high-temperature, the elastic lock nut is generally preferred.

Recommendations for use per FAA AC43.13-1B Acceptable Methods and Practices:
Elastic lock nuts are not to be installed in areas exceeding 250 degrees F.
Do not reuse elastic lock nuts if the nut cannot meet the minimum prevailing torque values shown in the chart
Do not use self-locking nuts on parts subject to rotation
Do not use self-locking nuts where the loose nut, bolt, or washer may fall or be drawn into the engine air intake scoop.
Do not use self-locking nuts to attach access panels, doors, or any parts that are routinely disassembled before or after each flight.

Several aircraft accidents (Canadian TSB Report A97O0055) and aircraft control problems (C-130 aileron) caused by lock nuts coming loose have challenged the idea that lock nuts can be reused.

Air Force T.O. 1-1A-8 now states: "New self-locking nuts shall be used each time components are installed in critical areas throughout the aerospace vehicle" This seems a more practical policy given the low cost of a lock-nut.

The idea of inspecting a lock-nut for minimum prevailing torque sounds good on paper, but as illustrated in the Canadian accident, it may not be a good practice to use in the field. In this accident the engine manufacturer's maintenance manual (ROTAX), stated: "self-locking nuts must be replaced with new items after removal in the event the friction torque has diminished."

The usual method a mechanic would use to check for "diminished friction torque" is to see if the nut would unscrew by hand. If it does, then it is no good and is replaced. If it cannot be unscrewed by hand then it is OK. THIS METHOD DOESN'T WORK!

Probably what the engineer who wrote this had in mind is that the mechanic would test several new nuts, obtain a statistical average break-away-torque, and compare this with the old nut. If the break-away-torque fell within one standard deviation of the mean, then the nut was still good and could be re-used.

When the Transportation Safety Board did their own tests on the M8 lock nut PN 942-035 used on the exhaust, they found that friction torque diminishes each time the M8 locknut is installed and removed. In this accident the loose lock nuts created a situation that led to an accident.

[JJ 911SC]

James

It's late, I'm tired, I'm on my 3rd scotch and I'm trying to catchup on 17 pages of new Post...

So, you basically lost me after hear's the poop,

So now that I'm subscribe, I'll check it out tomorrow morning to see if I can come back with a better Reply Post

After all this typing (mainly yours), I'm poop (start to look like a Caca post). I'm going for a 4th one

Bonne nuit

J.J.

[James Brown]

LOL you go JJ!! its just mech. porn, not mutch to it. But have a go when sober.

[JJ 911SC]

Originally Posted by James Brown

LOL you go JJ!! its just mech. porn, not mutch to it. But have a go when sober.

Hi James

As promise...

I've been working in/for the Navy for the last 35 years. My previous job was the Configuration Manager for the Victoria Class Submarines.

We follow very thigh specifications (NES & Def Stand) when it comes to re-use of Fastener and the likes, especially on a First Level/QCA Systems (system that a failure would be catastrophic to the Sub or the crew) no room for error at depth.

We have rules for the re-use of stud, since a simple one with QCA paper work could be in the $1,000 a piece and takes a while to procure.

Since I'm at home (nothing to do with the scotch I'm converting my garage to an heated one), I will not bet money, but I'm quite sure that we forbid to re-use of locknut. I'll check tomorrow at work and let you know.

Cheers (trigger for a drink )

J.J.

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[RWebb]

Originally Posted by JJ 911SC

...

We follow very thigh specifications ...

very common in all Navies