Distorted Thread Nut:
This type of nut has been slightly crushed at the top. (notice the marks on the side of the nut). When the round bolt reaches the oval portion of the nut it springs the nut back round. This spring action grips the bolt and adds friction that prevents loosening.
Spring Beam Nut:
This type of nut has thin slots cut down through the top few threads with the resulting fingers bent slightly inward. At installation, the bolt springs the fingers out and the fingers grip the bolt with a prevailing torque.
(Engineers call them "prevailing torque nut")
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:
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.
- 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.
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.
In 1969, Cessna issued Service Letter SE69-28 after finding that NAS679 nuts, size 10-32 were cracking due to "heat treatment embrittlement". SE69-28 indicated that Cessna had discontinued their use and called for their replacement in certain critical applications.
NAS679 nuts, other than size 10-32 were used extensively by Cessna until 1980 when they were replaced by MS21042 nuts. While the company amended its parts catalogues for the affected aircraft to reflect the change, Cessna did not specifically indicate that the NAS679 nuts had been superceded and there was no service information requiring replacement of NAS679 nuts, other than those called out in SE69-28.
There has been other reports of these nuts cracking in the Cessna Wing Carry Through Spar Assemblies. For this reason, were allowed and applicable, MS21042 may be a better choice than NAS679
Service Difficulty Reports (SDR) published stated that the 16 nuts were MS21042L6, although when investigated by the Transportation Safety Board of Canada it was found that the cracked nuts were actually NAS679. Reference AAC6-54 dated 2/95
Read about nylock nut torques
NAS 679 Low Height Lock Nut