Mil-Spec searching - with results
Rick6a said:
I believe one is the minimum number of threads for those nuts made with an insert. In my experience, for all other type nuts, self locking and otherwise, the minimum number of exposed threads that had to protrude ranged from 2 to 2-1/2 threads. That was a long established policy at McDonnell-Douglas when I worked production there. I really don't know if that particular rule applies for our (experimental) purposes, but I've always felt comfortable following it as my own personal guideline.
Rick is correct.... but it depends on how you measure....
While the FAA in AC 43.13 allows for 1 thread to be exposed.... I did some hunting on the MIL-Specs, and reading the MS21042 specification (the typical all metal locknut sold by Aircraft Spruce), and found it referenced a NASM specification
NASM33588 with the nice long title "Nut, Self-Locking, Aircraft, Reliability and Maintainability Usage Requirements for"...
Anyway, this specification details the use of self-locking nuts in aircraft, and specifies...
2. Bolts, Studs or screws must extend thru the self-locking nut for a minimum length equivalent to two threaded pitches. This length includes the chamfer.
So it is tighter that the FAA spec. (and also includes the fibre insert nuts) but with the big difference that it includes the chamfered end of the bolt.
So off to the AN 3 specification....
....and the chamfer is specified at 0.015 to 0.047 inches on the end of the bolt. This is equal to 0.5 to 1.5 of a thread pitch.
So, measuring full threads, the MIL-Spec can be 0.5 to 1.5 full threads exposed, for a 10-32 bolt.
For an AN6 (3/8 bolt) the chamfer is 0.063 to 0.031 inches, which is about 0.75 to 1.5 full threads.... similar results...
I wonder if the FAA just went with a "thread" definition to get around the chamfer bit of the bolt.
It seems like
1 full thread is good as an
absolute minimum for the smaller size bolts we work with, but another 1/2 thread or so is better. The Mil-Spec is identical for both fibre and metallic lock nuts. Fibre ones are rated to 250F and metal to 450F.
Interesting hunting around the specifications..... good job it's slow at work since I'm retiring in 6 weeks....
gil in Tucson