Threaded Rod (not bolt) Loosening

[271828]

Greetings. I'm working on a project that's under construction. It has mild steel threaded rods hanging vertically from steel beams in the roof. At the beam connections, there are double nuts below and above the beam flange. The rods are about 20 ft long and hang under only their own weight. Later, unistrut will be connected to them for duct and pipe supports.

The concern is that vibratory compaction below will cause vibrations at the roof that might cause the connections to loosen. I measured the lateral acceleration at the rod connections and it's roughly sinusoidal with an amplitude of 0.25-0.3g. Vertical (axial) vibration is about 0.1g.

I dug up a lot of papers on bolt loosening, but I haven't found anything that tells me if 0.25-0.3g lateral or 0.1g vertical is enough to be concerned about.

Any advice or references would be greatly appreciated!

 

[GregLocock]

Typical 4 cylinder engines reach 20g rms at 6000 rpm. They use (well designed) bolted joints.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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

 

[Tmoose]

Do the roof beams have tapered flanges ?

https://www.researchgate.net/public...n-Bolt-shank-gets-bent-on-tightening-from.png

.3g lateral at the roof beam?
What frequency?

 

[271828]

Greg: Understood. These are just mild steel threaded rods that will be used to support ducts. They're nowhere near as robust as the bolts in an engine. I don't think we can count on any pretension.

Tmoose: No, they're standard wide-flange beams with constant thickness flanges. The rod goes through the flange like in your link, but the rod is vertical and not bent. There is a washer and double nut below the flange, and another washer and double-nut above the flange.

Thanks Greg and Tmoose.

 

[3DDave]

It's not g-loads that cause the problems - it's rocking back and forth that acts to lever the nuts and cause some of the original elastic deformation to be overcome by plastic deformation.

The local DoT kept having a large traffic light array come apart multiple times until someone finally added a couple of diagonal wires to stop the side-to-side motion.

However, the chances these rods will simply unscrew themselves is also low. Not being tight and unwinding many turns are usually far apart.

 

[TugboatEng]

When double nutting, a thin nut is typically used to draw up tension and then a thick nut is used to jam. Two nuts of the same size don't produce appreciable locking. This works for flange studs but I don't know if it would work for a hanger.

If the temperature are low, nylock nuts actual work quite well for this. A nylock could be used as the backup in the double nut arrangement.

Threaded shaft collars that can clamp onto the threads work extremely well.

Shaft Collars, for 1/2"-13 Thread Size, Black-Oxide 1215 Carbon Steel

https://www.mcmaster.com/6438K21

 

[TugboatEng]

Also, if you're expecting a lot of vibration, treat your washer as a bearing or isolation. Consider using rubber or composite bearings to isolate the rod and washer from the beam.

 

[271828]

Thanks 3DDave and TugboatEng.

Clarification of the question:

The nuts on the rod apply dubious pretension on the connection. There might not be any pretension. The vibration perpendicular to the rod is 0.25-0.3g. Is that enough to be concerned that the nuts might back off? Is this enough to warrant recommending that loctite application or some other action be taken?

There is a tremendous number of these rods, so if I tell them to apply loctite, change the two equal nuts to unequal nuts, nyloc, etc., then I really need to have a good reason.

Thanks to everybody for the help so far.

 

[TugboatEng]

You can apply Loctite 290 after installation.

 

[271828]

Thanks TugboatEng. That should mitigate the push-back if I decide to recommend it.