Loctite on Structural Bolt?

[dozer]

I have here in my hand a structural steel spec that says in addition to pre-tensioning (structural) bolts, Loctite Threadlocker shall be applied. For the most part, these would be 3/4" A325 bolts. I've never heard of doing this before to structural bolts. It is for some modules that will be shipped by road and the theory is this will keep the bolts from vibrating loose. I would have thought the pretensioning would be sufficient to prevent that. Has anybody heard of doing this with structural bolts? If so, what type of Threadlocker would be used? I quick glance through the Loctite site doesn't suggest to me that it is intended to be used for this.

Bottom line is, should we push back on this requirement? Thanks.

 

[FoxSE14]

My guess is:

1. Pretension is intended as a design element and the EOR is relying on having the full pretension during the service life of the module.
2. Loctite as a measure to prevent any loss of pretension

If pushing back on the use of Loctite because you are unfamiliar, I would at a minimum, come back with an alternate suggestion to maintain full pretension during shipping.

 

[FoxSE14]

First move I would make is to contact Loctite directly and discuss the intended use with them.

 

[aggman]

We have never used loctight. I would be concerned that it wouldn't actually work right with the differences in the threads between structural and machine bolts. For vibration issues we have used anco lock nuts which are ok, but not great.

 

[BUGGAR]

You may be in violation of ASTM Specs. They're very specific about the threads and thread coatings. Bolts have been disqualified for being dropped in the dirt.

 

[FoxSE14]

Contacting AISC for guidance may be another avenue - they will likely be able to get some good ideas on approved ways to avoid vibration concerns.

 

[SlideRuleEra]

Caltrans, working with the Federal Highway Administration, performed tests of threadlocking compounds on various bolts, including A325. The results for the products, including a specific Loctite product, indicate it is ok. However, Caltrans authorizes use only on the type and size bolts tested.

Here is the report: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dozer]

I called Loctite. The guy I spoke to didn't even know what an A325 bolt was. I also looked at the Caltrans report. Did you notice these weren't fully tensioned bolts? They just ran a nut onto a bolt with thread locking compound and then after it cured checked how much torque it required to break it loose. I actually was surprised to see how high some of those torque values were. I still find it hard to believe that a 3/4" bolt with 28 kips of pretension is going back off after bouncing down a road for a few days but I guess if that's what they want we'll do it. I am going to send a note to AISC to see if they have any words of wisdom.

 

[dozer]

I found this Link on the AISC site which I feel supports my contention that a locking compound is not needed. Notice also that they don't mention thread locking compounds in cases where additional measures need to be taken.

 

[SlideRuleEra]

dozer - That AISC link is definitive. Questioning use of the theadlocking compound is surely justified, but if the requirement stands... doesn't look like a compound will cause problems.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[XR250]

They just ran a nut onto a bolt with thread locking compound and then after it cured checked how much torque it required to break it loose. I actually was surprised to see how high some of those torque values were.

Yea, Red Loctite is pretty tough stuff to remove. On cars and motorcycles, it usually requires heat.

 

[NS4U]

Another reason not to do it: If the bolt pretension is measured by torque based methods, the loctite will lubricate the threads. The amount of loctite applied to the bolts and the 'calibrated' bolt assembly will inevitably vary, so your ability to achieve consistent pretension will be tough.

If you use a strain based pretension in method, like turn of the nut then this concern probably isn't an issue.

 

[bigmig]

Be aware that loctite comes in multiple compounds. Some are for "hard" removal...some are permanent. If you have concerns about nuts moving, use a double nut. That should satisfy all the grey areas.