Preferred method for preventing bolt loosening 1

[jk2017]

Been doing some research on bolt loosening for a job I'm on and while it seems like there are tons of options, I'm curious to know what your preferred method is. Doing some searching in this forum has yielded varying opinions, with some of the most popular including defacing the threads above the top nut, loctite (blue and red), jamb nuts, and tack welding the nut down. I've also seen plenty of suggestions to use spring washers, nyloc nuts, and products like Nordlock. These all seems to have their pluses and minuses. For me, while tack welding seems popular, I'd be pretty worried about cracking, and I've seen varying opinions on spring washers. Seems to me that jamb nuts are a strong option.

 

[phamENG]

As you said, they each have their pluses and minuses. The pro's and con's mean more or less depending on the nature of the job, loading, etc.

Can you give more information on the size of the fasteners involved, grades/materials, loading magnitude and direction, fatigue considerations, type of joint (bearing, slip critical), prying action?, etc. A sketch of the connection would also be helpful.

For instance, I'd use a nyloc nut or spring washer on my kid's bike, but not on a Gen-Set anchorage.

 

[jk2017]

phamENG - thanks for the response. Question was more about general practice, but to be a bit more specific about my case (without divulging too many details for legal reasons): it's a 1/4"-20 t-bolt, sees dynamic loading (think rattling due to wind), service life of the supported element is about 25-30 years. Pryout is my main concern here. Basically I have a cfs u-shaped rail with a t-bolt connecting the rail to a cfs angle that supports the element subjected to wind.

 

[GC_Hopi]

Some of these methods are common based on the industry. IMO, traditional structural engineering uses defacing the threads above the top nut, loctite (blue and red), jamb nuts, and tack welding the nut down. Mechanical engineering uses spring washers, nyloc nuts, and products like Nordlock. Aerospace uses safety wire, rivets(for more reasons that bolt loosening).

Maybe go with loctite and a jam nut in your case if you have concerns. 1/4" is a litte light to be welding or fouling threads.

 

[rb1957]

Assuming no one will look at this until it breaks and someone has to fix it, I'd prefer the most secure mechanical solution ... either tack welding the nut or using an anti-rotation washer (with a tab). If it had an annual? maintenance inspection then you could go with lock-nuts (with a nylon insert).

another day in paradise, or is paradise one day closer ?

 

[dik]

From my notes:

BOLTS
-HS BOLT SPECIFICATION TO ASTM F3125, GRADE A325 HAS BEEN USED. THREADS CAN BE INCLUDED IN THE SHEAR PLANE. INSTALLATION SHALL BE ‘SNUG TIGHT’.
-ALL ‘SLIDING’ CONNECTIONS AND HS THROUGH BOLTS FOR HSS TO BE FINGER TIGHT. TORQUE TO 2.3N-M (20IN-LBS) (DO NOT CRUSH HSS WALL) AND THREADS SECURED BY PEENING OR LOCTITE 263 (RED) THREAD LOCKER.
INSTALLATION OF ‘JAM’ NUTS:
THE INNER JAM NUT SHALL HAVE A THICKNESS OF NOT MORE THAN 75% OF THE OUTER NUT;
THE INNER NUT SHALL BE INSTALLED FIRST AND SNUGGED TO 50% OF THE TORQUE VALUE;
THE OUTER JAM NUT SHOULD BE INSTALLED SNUG TO THE INNER JAM NUT;
THE INNER NUT SHOULD BE RESTRAINED TO KEEP FROM ROTATING AND THE OUTER JAM NUT SHOULD BE FULLY TORQUED TO IT; AND
WRECK THE THREADS BELOW THE NUT.
and I don't often use jam nuts... I prefer Loctite 263 (Red).


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ATSE]

In order of reliability, for A325 structural, coarse thread (similar to Gr 5 for strength):
1. Cut washer + pretensioned
2. Nylock + pretensioned
3. Cut washer + jam nut
4. Anco (aggressive, one time use)

 

[dik]

by split washer, I hope you are not referring to lock washer (helical spring washers)... they should not be relied on for securing nuts.

https://www.boltscience.com/pages/helicalspringwashers.htm

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

So dik, are you saying that helical spring washers are totally useless? Or do they have some useful purpose other than acting as a locknut?

BA

 

[drawoh]

BAretired,

The lock washer serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time, it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lock washer of this type is useless for locking.

Mechanical practise is to specify the highest bolt torque possible, use a torque wrench, and apply thread-locker if possible. I do not know how this translates to structural design.

--
JHG

 

[phamENG]

Here's a broad view from AISC. For bolted connections in building structures using high strength bolts per the RCSC, you don't need to do anything but pre-tension it. Mechanical applications with higher amplitude or constant vibrations warrant chemical and mechanical locking mechanisms, but it's rare in a structural application.

 

[dik]

BART:
Just be careful when you use them...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dauwerda]

As phamENG pointed out in his latest post, AISC and the RCSC use pre-tensioning as an effective means of preventing nut loosening. The caveat that AISC gives to this is for anchors - This is because anchors will lose their pretension over time due to concrete creep, and with less or no pretension, there is less friction between threads and nuts can come loose. A double nut can be a good solution for this as the two nuts tightened against each other will not lose their pretension (similar to a typical steel only connection) and will effectively maintain the required thread friction to prevent loosening (that's not to say the anchor itself won't lose its pretension however).

Double nuts (and other types of locking nuts) are a terrible solution for typical structural bolts as A325 bolts have a specific (short) thread length which provides a very small window for proper bolt length. The bolt needs to be short enough the nut doesn't bottom out on the shoulder before it clamps the connection together and long enough that it provides enough thread for the nut. This is a small window for a single nut, start adding other devices (lock washers, MF nuts, etc) and it's near impossible to get right. A standard A325 bolt does not have long enough threads to accommodate 2 standard nuts (which I have seen called out many times, especially when seeing through-bolts used on HSS). This thread length issue is not a problem for anchors as they can have any thread length specified. (The newest ASTM standard does allow for structural bolts to have longer threads specified, but these are still special order with long lead times)

There have been a few AISC steelwise articles addressing the use of helical spring washers in structural joints - they all agree, don't do it.

Note that lock washers are not specifically
addressed in either the RCSC (or AISC) specifications,
and these products are not used in structural
bolting. RCSC Specification Section 3.1
states that “compressible materials shall not be
placed within the grip of the bolt.” The Commentary
further explains that “the presence of
gaskets, insulation or any compressible materials
other than the specified coatings within the grip
would preclude the development and/or retention
of the installed pretensions in the bolts, when
required.” Lock washers could be considered a
compressible material, and the installation methods
in the RCSC Specification are not formulated
or developed to accommodate lock washers.

Back to the OP however, it is clearly not a question about structural bolts. The 1/4" bolt described, I would guess, is not high strength anyway so could not even be effectively pre-tensioned to prevent loosening. I would also be a fan of loctite red in this situation.

 

[Agent666]

Also, don't tack weld high strength bolts, just mentioning because there have been a few suggestions for this but no mention of the material you're using to know if this is an appropriate suggestion. .

https://engineervsheep.com

 

[jk2017]

The 1/4" bolt described, I would guess, is not high strength anyway so could not even be effectively pre-tensioned to prevent loosening. I would also be a fan of loctite red in this situation

Thanks for the reply, and this is correct, they're not high-strength bolts. Anything against loctite blue? I ask because the supported elements will likely have to be replaced in 25-30 years and I've heard loctite red can be a beast. Access to these elements requires a crane as well, so any advanced removal techniques would be very costly. Thankfully, in my case, we have plenty of thread available for another nut.

 

[dik]

@dauwerda: almost my exact sentiments.
@jk2017: that's why I nearly always loctite red... blue is OK, just not as good. I often use it for sliding connections and I really don't want them to loosen. They've changed the designation from red to 236 and blue to 243 (my favourite calibre). I don't know when Henkel took over the product...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[3DDave]

In a static loading condition any loosening of the fastener will be because the joint is compromised. Either embedment, differential thermal cycling, prying loads, et al. None of the schemes to lock the rotation of the fastener or nut will make up for these effects. At best, they will keep the broken parts from escaping; at worst they will conceal impending failure.

To an extent, thread locking compounds will offer some resistance to embedment by filling the tiny gaps in the threads, but I've seen the ends of the bolts with the nuts still securely attached after the bolt fatigued and broke just below the nut.

 

[desertfox]

Nordlock washers would get my vote, we used them on steel rolling mills which are subject to impact loading and vibration and to my knowledge we never have a problem.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[3DDave]

desertfox - to be fair, by the time people are willing to pony up for nordlocks, they've already spent a lot of time eliminating all other sources of problems. Still, no one seems to get fired for choosing nordlock.

 

[dik]

@3D and fox: Nordlock and Belleville, both great. Pricey but good... I've gotten away from safety wiring and castellated.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik