B

Brambo

Well Known Member
I just finished attaching the seat ribs to the center section aft bulkhead and after torquing the AN3-10A bolts, there is only 1 or two threads showing. It seems like I've read somewhere about how many threads should show but can't find it. What is the minimum number? If this isn't enough I guess you just order some longer bolts. Anyone else have a problem in this area?

Bill Rambo
RV-7a
 
It's OK...

Brambo said:
I just finished attaching the seat ribs to the center section aft bulkhead and after torquing the AN3-10A bolts, there is only 1 or two threads showing. It seems like I've read somewhere about how many threads should show but can't find it. What is the minimum number? If this isn't enough I guess you just order some longer bolts. Anyone else have a problem in this area?

Bill Rambo
RV-7a
Click to expand...

One thread is the minimum.... sounds like you are OK...

Don't forget, if you are a little short on the bolt length, changing from a thick (AN960) to a thin (AN960L) washer will give you another thread showing on an AN3 bolt...

It's in the "bible" AC 43.13 ... on-line here from the FAA

http://www.airweb.faa.gov/Regulator...99C827DB9BAAC81B86256B4500596C4E?OpenDocument

Hardware is covered in Chapter 7.

The relevant section is here...

f. Fiber or nylon locknuts are constructed
with an unthreaded fiber or nylon
locking insert held securely in place. The fiber
or nylon insert provides the locking action because
it has a smaller diameter than the nut.
Fiber or nylon self-locking nuts are not installed
in areas where temperatures exceed
250 ?F. After the nut has been tightened, make
sure the bolt or stud has at least one thread
showing past the nut. DO NOT reuse a fiber
or nylon locknut, if the nut cannot meet the
minimum prevailing torque values. (See table
7-2.)


gil in Tucson
 
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Brambo said:
..........there is only 1 or two threads showing. It seems like I've read somewhere about how many threads should show but can't find it. What is the minimum number?......
Click to expand...
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.
 
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.
Click to expand...


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.... :D

gil in Tucson
 
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I may be missing something but if the bolt is the proper length then the amount of threads should be taken care of automatically. I've always made sure that I get the correct grip length for the application (no threads in the hole) and after the nut is put on it always works fine with plenty of threads exposed. Am I missing something?
 
Plans

Todd said:
I may be missing something but if the bolt is the proper length then the amount of threads should be taken care of automatically. I've always made sure that I get the correct grip length for the application (no threads in the hole) and after the nut is put on it always works fine with plenty of threads exposed. Am I missing something?
Click to expand...

Yes... the plans are not always correct on calling out the bolt length. How many washers, and the type (thick or thin) also makes a difference...and the thickness of multiple players of primer..... :D

gil in Tucson
 
az_gila said:
Yes... the plans are not always correct on calling out the bolt length. How many washers, and the type (thick or thin) also makes a difference...and the thickness of multiple players of primer..... :D

gil in Tucson
Click to expand...

Agreed, but it seems like you'd have to be pretty far off on your bolt size, or have 1/4 of primer to leave only 1 thread exposed when the nut is tightened. :confused:
 
The plans

Todd said:
Agreed, but it seems like you'd have to be pretty far off on your bolt size, or have 1/4 of primer to leave only 1 thread exposed when the nut is tightened. :confused:
Click to expand...

Todd ... I think it's mainly an incorrect call out on the plans.

I know this is the source of most "defects" I find during a TC inspection.... even more so on glass planes siince the glass thickness is less controlled.... :)

gil in Tucson
 
Thanks for all the replies. Yes, I think it will be good, but I thought I'd better ask now rather than find out during the inspection that it's not acceptable. And yes, the plans SHOULD be correct, but I'm sure I'm not the only one here that's found problems with the plans. I've found that if something doen't look quite right, better to look into it further than to ASSUME that the plans are always correct.

Bill Rambo
RV-7a
 
No plastic nuts

Hi Bill,
Did you also note Gil's quote of 43.13 where it mentions no plastic nuts are/should be used in areas where temps exceed 250F?

The reason all locknuts in the firewall forward area should be the all-steel variety.
Regards,
 
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