Nylock Nut Reuse 2

[JCasper]

If a nylock nut or nylon patch fastener is installed can it be backed off and then reinstalled? Is there decrease in resistance to backing off after being reinstalled?

 

[MintJulep]

MIL-DTL-45913 and -18240 are the relevant MIL standards.

They both require measurement of prevailing torque for 5 on-off cycles, with limits on the highest and lowest allowed torque for the first and fifth cycle.

 

[KENAT]

As Mint says, the Mil std's essentially allow for using them 5 times. Doesn't necessarily mean they aren't good after that though - it will depend on application.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[tbuelna]

JCasper-

The MIL standard requirements noted are for acceptance testing of lots, and it's possible that a certain percentage of each lot will not conform to these requirements. If you want to ensure that your locknut has the proper amount of prevailing torque when being re-used, you should measure the prevailing torque at each installation.

Nylock nuts can lose their locking ability if they have been installed for long periods or were subjected to elevated temperatures. Under these conditions the plastic locking element can experience creep and relaxation.

 

[racookpe1978]

Now, the next question is ... After 1, 2, 3 ... 10 years (10 months?) service, how can you tell how many times that particular joint has been loosened and tightened and re-tightened and re-tightened?

 

[MikeHalloran]

Thinking about that next question...

Okay, if you had a bolt that was hollow, at least through the head and a portion of the shank, you could install a microprocessor and some kind of strain sensor and a power source and a squib, so after X number of tension cycles, the squib would blow out a dye charge or an indicator pin to say 'end of life' for that bolt and any associated nuts.

Doing the same for the nut alone seems more difficult, but I can't say it's impossible.

The question after that becomes "How much would you pay for a wrenching cycles indicator"? The answer to that question is that nobody will want to pay a damn thing until the capability is mandated, after which volume will bring down the cost, and eventually, the price.

Current technology could do it, provided you're willing to deal with the paperwork of serializing and tracking individual nuts.

... which is probably possible with some of today's smartphone cameras, and a barcode type of marking that's suitable for locking nuts. Maybe a pin-stamped 2D barcode on one or more of the flats...





Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

The issue of tracking reuse is why on some airborne kit they get thrown out after single use.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[racookpe1978]

Hah! Marks-a-lot and the date written on the joint next to the nylock nut? 8<)

 

[MikeHalloran]

Given universal self-discipline and trust, sure, a handwritten Sharpie notation could work.
... but it's not 'verifiable' by some drone sitting at a desk, so it would never, er, fly, so to speak.
... and it actually would add weight to the aircraft.

Does the nuclear industry have an affordable way to deal with limited-reuse items?






Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Now, the next question is ... After 1, 2, 3 ... 10 years (10 months?) service, how can you tell how many times that particular joint has been loosened and tightened and re-tightened and re-tightened?

The point I made in my post was that by measuring the prevailing torque of the nut at each installation, the actual number of installation/removal cycles would be irrelevant. As long as the measured running torque of the locknut was within prescribed limits at each installation, then it would be suitable for re-use.

With most critical aerospace fastener installation procedures, it is standard practice to torque the fastener to a prescribed value above the measured prevailing (or running) torque of a self-locking locknut or insert.

 

[MikeHalloran]

With most critical aerospace fastener installation procedures, it is standard practice to torque the fastener to a prescribed value above the measured prevailing (or running) torque of a self-locking locknut or insert.

... which compensates for the torque wasted in turning the nut but not stretching the bolt.

... but does the typical procedure also include a requirement to measure and record the prevailing torque and to replace the nut if the value is below some threshold?



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

1997 US Navsea technical manual released for use by us civilians.

http://www.hnsa.org/doc/nstm/ch075.pdf

Some discussion of a nylok nut's prevailing torque, in regards to :
1 - it's use to establish a nut's suitability for use/re-use
2 - In section "075-4.5.1 TORQUE CONTROL. The first method of establishing preload is torque control."
"the final torque value that you can see on your torque wrench must equal the torque specified
for that particular application, plus the prevailing torque that you measured as you turn the nut on the bolt
in its unloaded state."

Also comments on the suitability and accuracy of various methods of establishing preload.
I don't think it says anywhere that torque control is forbidden.
There are many more red warnings, Figures and tables about using fasteners with tensile srength greater than 150,000 in wet or hot environments. Enough to make me rethink using them on trailer hitches and even undercar components in New England daily drivers.

 

[tbuelna]

... but does the typical procedure also include a requirement to measure and record the prevailing torque and to replace the nut if the value is below some threshold?

Great question. And the answer is yes.

Threaded fasteners for critical aerospace applications usually require two forms of locking function, one being the thread friction from installed pre-load, and the other usually being the thread friction from a locking element. QA procedures usually require that both the running torque and final torque are measured, and witnessed/recorded by an inspector.

With small diameter self-locking threaded fasteners (ie. .25 inch and smaller), it is important to ensure that the running torque of brand new locknuts or inserts falls within an upper and lower range. With small diameter all-metal locknuts or inserts it is very difficult to accurately control the running torque, and a percentage of brand new parts may greatly exceed the procurement standard limits. High running torque can result in excessive shear stress in the bolt/stud after installation. I've personally experienced situations where we had to trash around 10% of brand new 10-32 MS21043 all-metal locknuts because the running torque was too high. It was very tedious to check the running torque of dozens of locknuts for each assembly, but due to the relatively large number of brand new locknuts we rejected it was obviously worthwhile.

Regards,
Terry

 

[PersonalProfile]

Assuming there is no impact on the fastener itself (due to the induced tension during the initial tightening etc), ISO 2320 or the following may be of interest:

http://clok.uclan.ac.uk/6649/2/Eccles William Final e-Thesis (Master Copy).pdf

In an industrial environment, I propose a fastener that has loosening concerns, that have been attempted to be addressed via an "insert" type nut, a "pin" (i.e. it may prevent the nut becoming loose and falling off - hence the "pin", though I would not be relying on any preload / it acting as a fastener).
I can appreciate other industries may be different.

Regards,
Lyle