Vibration or improper pretension?

[sotir]

Hello all,

I've been combing these forums and other sources concerning pre-tensioning, the use of lock nuts and vibration in structural applications. The consensus seems to be that if a bolt is properly pretensioned then it should not back off no matter what the vibration is in the system. That being said, I have a situation where a large conveyor system seems to have connection bolts backing off and clamp plates falling to the ground. The conveyor was installed around 3 years ago. The plant has gone through and retightened some of the bolts in one confined area. Upon observing the area of conveyor where the retighten/falling clamp plates are, there seems to be more vibration then in the rest of the system. So, my question is twofold....How can I tell if it's a vibration issue or an improper pretension issue? And how do I prevent this from occurring again? The bolts are 3/4" diam. A325 and they used ANCO PN LOC nuts.

 

[rb1957]

for my money, I use pre-tension to contorll the tension loads on the bolt (to limit the joint gapping), and I'd use anti-rotation features to prevent the bolt from backing off. if lock-nuts don't do it for you, what specialised hardware is appropriate for a high vibration environment ? you could add anti-rotation brackets, that would be insalled after tightening the nut, capturing it in place.

 

[sotir]

Thank you rb1957,

I'm not 100% sure what an "Anti-rotation bracket" nor how it's used.

I've been everywhere looking for a sure fire way to stop the nut from backing off.

Since they already used lock-nuts, I could try another type of lock-nut, but I don't know that I'll have any success with those either. Is any one better then another?

Tack welding the nut and bolt were also an idea, but I wasn't sure if heating the components would relax the pretension in the connection.

How can anyone be sure the installation was correct in the first place? Were the bolts originally torqued enough? Is there a way of telling this while the bolts are in place?

 

[msquared48]

If there are enough threads above the bolt head, why not screw up the threads? Is it ever necessary to remove the nuts?

Initially I thought of tack welding too, but was concerned about the vibration and repeat loading causing cracks in the welds. I have seen that happen.

Have you thought about modeling the conveyor on the computer and subjecting it to cyclical loading to see if you can duplicate the vibration seen, then adding mass and/or structural members as needed to dampen, or change the modal frequencies to a range that does not produce the vibrations currently seen? Might be too difficult a solution for the problem.

Mike McCann
McCann Engineering

 

[desertfox]

Hi sotir

When the bolts were retightened what were the tightened too?
Also are there no manuals you can refer too for maintaince? and finally have you contact the supplier of the conveyor?

regards

desertfox

 

[sotir]

Mike,

I have thought about damaging the threads as well, and that may be the way I go, but from some of the other threads I've read here, there were people who disagreed with that method. It may end up being my only choice.

I have also thought about reinforcing the whole conveyor in this area to reduce the vibration. As of yet, the owner does not want to go down that route, so I'm trying to see if there is a more economical solution. If worst comes to worst, then that may be what is done. As I said, this problem is not occurring on all parts of the conveyor, just in this area of high vibration.



desertfox,

Unfortunately, I do not know what the bolts were re-tightened to, we were not hired by the owner at that time, maintenance went through did the work. I also wanted to see the bolts that had fallen out, but they had just thrown them away before I had a chance to see if there was any deformation or stripping or anything with the bolt itself. I'm not sure if there are any maintenance manuals for this type or work but I will look into it. Yes I'm in contact with the supplier; they assume it's not their problem.



So, without stepping on too many toes I'm trying to figure out why these lock nuts have backed out and a reasonable solution for ending the back out.

 

[dik]

Can you use loctite? some types require removal by torch...

Dik

 

[sotir]

Dik, Yes there is a possibility that I can use loctite, but the owner will ask what is the advantage of loctite compared to the anco lock nut used before. I don't have that kind of data to say that one lock nut is better then another, and all the testing I've seen out there is used to sell one product over another. I'm just not sure what has already happened will not happen again. Have you used loctite in a high vibration area?

 

[DarthSoilsGuy]

my understanding of pre-tension is that it is hard to measure while you're working and impossible to measure afterwards. you could torque-test, but the test is very flawed and especially so with old bolts. turn-of-nut method is the best direct correlation for pre-tension, but it doesn't work well unless all the bolts are equally snug (like changing a tire) before pre-tensioning.

is replacing the bolts and observing/documenting the new installation out of the question? i would think that if it fails again it would have to be a bad connection design which you could address better 3 years from now.

i wonder if a gap/gaps between faying surfaces was closed in a bad way.

My preference for backing down nuts is another nut after the 1st one is tight, if you've got thread to spare.

 

[HgTX]

Properly tensioned bolts won't back off. The same force that's holding the connection together is holding the nut and the head of the bolt to the plate. I'm with DarthSoilsGuy--replace the bolts and make sure they do it right.

Hg

Eng-Tips policies: faq731-376

 

[desertfox]

Hi sotir

If you're in contact with the conveyor supplier you can ask them what the bolts should be tightened too or indeed ask whether you need to check bolt torques periodically.
Once you know what the bolts should be torqued too you have a point to start from.

regards

desertfox

 

[sotir]

DarthSoilsGuy and HgTX,

I am also under the same impression that a fully pre-tensioned nut and bolt is the way to go. But I'm sure you know as well as I do, in the field, no ones work is flawed. The "make sure they do it right" is the tough part. On a conveyor system, with 1000 bolts and nuts....any many installers, how is that achieved? I've never used the Direct Tension Indicators, but I'm willing to call them out if that will help. How reliable are they? Would you advise using a locktite type of system as well, for insurance? Or would it be a waste of money?

 

[EddyC]

I recently worked on a project where a bolted connection failed repeatedly. After further investigation, it was found that the torque wrench wasn't calibrated and that the installer was looking at the metric scale on the dial when using the wrench (He was tightening to 80 n-m instead of 80 ft-lb).

 

[swearingen]

DTI's are the way to go in my opinion. It's the only method of the four allowed by the Code of Standard Practice that does not depend on friction (which varies widely), but depends on the actual force in the bolt itself. They're essentially fool-proof as well - no 80Nm vs. 80ftlb problems.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[MarcbSE]

I'm with swearingen and would go with the DTI's.

 

[DarthSoilsGuy]

i was imagining you observing/documenting the installation, but that was before you said 1000 bolts/nuts.

i haven't had the opportunity to be involved with structural steel inspection of DTIs yet, just the twist-offs.

here's a neat little web-site i've gone to a bunch of times.

http://www.boltscience.com/

 

[dik]

I've used loctite for 'mild' vibration problems, sounds like you're on the upper edge... You can talk to the manufacturer and see what they can recommend. Any nuts I've had to remove are a real pain! With some forms of loctite you have to use a torch to heat the material to several hundred degrees to remove/loosen the coating (glue? <G>)

Dik

 

[sotir]

Well, thank you all for your input.

Let me see if we can all agree on a few points....

1. There is no way to tell if the bolts were pretensioned correctly in the first place.
2. Proper Pretension is all that is required to overcome vibration.
3. Bolt replacement will be necessary, and use of DTI's or twist-offs for new bolts is a good idea for being able to sample installation processes.
4. Lock nuts, whether chemical or physical, are not necessary if the correct pretension is achieved in the beginning.
5. If all else fails, redesign the conveyor system to minimize the vibration.

Once again, thank you all for your input. If you have anything new to add, of if I missed something that should be looked into, please feel free to let me know.

Regards,

Sotir