Nyloc nut safety.

Aussie_Paul

A reforming stirrer!!!!!
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Since 1982 Gyro 5000+ mostly instructing, and approx. 200 fixed wing in the late 1960s.
The ASRA ops manual section 5.09 para 9b.
says, "All tightened bolts must have at leat 2 threads showing".

As the rules stand, the ops manual is the law for our gyros in Oz.

There was some discussion recently where it was reported that the current acceptable aviation practice was, "The bolt could be flush with the nut as long as the bolt thread comes through to the outer end of the nylon".

I have never seen any reference to this and was wondering if I have missed something along the way.

I am not asking which is the best practice only what the "Official" ruling is and why.
Any current LAMEs out there who can shed some light on this

Aussie Paul.:)
 
Last edited:
How does this stack up??

Aussie Paul.:)
 

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Paul, I think the concern revolves around the taper of the bolt at the very end of the threads. Obviously, if the last thread has a smaller diameter than the rest, the nylon won't grip it as tightly. "Two threads showing" is an easy rule to get you past the taper.
 
The FAA doesn't have regulations concerning things such as "threads showing", they instead refer us to an appropriate piece of "approved" data. One such piece of approved data is the "bible" of aircraft mechanics, the Advisory Circular AC 43.13-1B, Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair. Available for free download on-line at WWW.FAA.GOV

Here is an excerpt from the AC regarding Nylock nuts:

>> 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.<<<<
 
Ok, I am waiting for someone to tell us why the amount of thread showing iis important. I added a couple of words to my original post, "and why."

Thanks people.

Aussie Paul. :)
 
is there a better link to the advisory? tryin to find anything on the FAA website is nearly impossable. they should hire an 8 year old to fix their website..;)
 
Paul, I tried to answer the question: to make sure the nylon is past the tapered threads and fully engaged in the full-diameter ones.
 
Doug Riley said:
Paul, I tried to answer the question: to make sure the nylon is past the tapered threads and fully engaged in the full-diameter ones.

Doug, you did well, BUT (There is always a but with me!!) there is a critical reason why. Not so important with our over built gyros BUT very important in high payload airliners etc.

Aussie Paul.:)
 
When I was gainfully employed at McDonnell Aircraft, manufacturing Military Aircraft per Mil Spec...the spec was...a minimum of 1/32" protruding past the surface of the nut, OR, a minimum of 1 1/2 threads...either one was good.

I do not know if the Mil Spec has changed.


Cheers :)
 
Doug's correct..There can be some thread root taper at the very end of the bolt but the last thread is not 360 degrees.. The idea of a least one thread or a thread and a half is for the Nylock to engage a full 360 degrees.
 
Harry_S. said:
When I was gainfully employed at McDonnell Aircraft, manufacturing Military Aircraft per Mil Spec...the spec was...a minimum of 1/32" protruding past the surface of the nut, OR, a minimum of 1 1/2 threads...either one was good. WHY is the above standard required?

I do not know if the Mil Spec has changed.


Cheers :)

Aussie Paul.:)
 
Harry_S. said:
When I was gainfully employed at McDonnell Aircraft, manufacturing Military Aircraft per Mil Spec...the spec was...a minimum of 1/32" protruding past the surface of the nut, OR, a minimum of 1 1/2 threads...either one was good. WHY is the above standard required?

I do not know if the Mil Spec has changed.


Cheers :)

Aussie Paul.:)
 
On a bolted joint with a nut the thread exposure helps with the impact load capability of the joint. All bolted joints with standard nuts need between two and six threads beyond the nut... Blind holes should extend beyond the bolt by same. Blind hole using taper-lock studs have the last two threads... You don't care about that,,,,,you just want to know why...



The reason is to insure the nut has engaged fully formed threads for it's full height.
 
The bolt pattern is good though... Bolts are out of the elastic boundary of the gusset plate..
 

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.......And to decrease the possibility that the remaining unused threads are not used as a load carrying part of the bolt and that such section is a neat fit inside the hole.
 
Besides looking nice , the inspectors don't like to cut their fingers on cut off bolts so they Require you to use a bolt made to length ! ????? hehehehe Just jokeing !

Bob.......
 
This Oz guy got it correct in one short and to the point post on the Oz forum.

"Guys,
I was always taught to allow one and a half threads protruding, The Std Aircraft "hardware" bolt (grade five) has two built in length coverings..........the bolt lengths go up in 1/8 inch increments and there are two washer thicknesses, for example the 1/4 inch washer AN960-416 is 1.60mm thick and the AN960-416L (low profile) is 0.80mm, the whole aim is to make sure that the thread does not impinge into/onto in our case, cluster/head plates for example, that is the shank of the bolt takes the loading and not the thread which would be under shear.

Pete Barsden"


Another authoritive description written by Ron Alexander of Alexander SportAir Workshops.

Basics of Bolt Installation
Certain accepted practices prevail concerning the installation of hardware. A few of these regarding bolt installation follow:
1. In determining proper bolt length - no more than one thread should be hidden inside the bolt hole.
2. Whenever possible, bolts should be installed pointing aft and to the center of an airplane.
3. Use a torque wrench whenever possible and determine torque values based on the size of bolt.
4. Be sure bolt and nut threads are clean and dry.
5. Use smooth, even pulls when tightening.
6. Tighten the nut first - whenever possible.
7. A typical installation includes a bolt, one washer and a nut.
8. If the bolt is too long, a maximum of three washers may be used.
9. If more than three threads are protruding from the nut, the bolt may be too long and could be bottoming out on the shank.
10. Use undrilled bolts with fiber lock nuts. If you use a drilled bolt and fiber nut combination, be sure no burrs exist on the drilled hole that will cut the fiber.
11. If the bolt does not fit snugly consider the use of a close tolerance bolt.
12. Don't make a practice of cutting off a bolt that is too long to fit a hole. That can often weaken the bolt and allow corrosion in the area that is cut.


Aussie Paul.:)
 
;) Correct on the thread placement for bolts in shear.. but all bolted joints need between 2 and 6 threads beyond the nut. Any more than six threads doesn't add anything.
Shear load vs deformation curves are shown in the attached.

Eccentrically loaded and combination tension/shear joints behave slightly different.
 

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