Bolt loosening in dynamic structure, occasional impact loads

[kaffy]

Good evening fellow engineers,

I am working on a rectangular frame (made up of steel c channels) which moves up and down and holds around 10000lb load. Previous engineers specified welds to keep the structural members in place.
I wanted to use bolted connections instead of welds but I was told that as frame is continuously moving and not a static structure, I shouldn't use bolts as bolts tend to loosen over time.
So my question is : Is self loosening of bolts really common?
I read somewhere that lock washers do not usually helps as axial tensions created because of preloading just flatten it out.
Are there any other ways to fix the issue and use bolts to attach the channels (picture attached for reference)?


Thank You
kaffy

 

[JoshPlumSE]

Interesting question. I don't think I've ever had to deal with it directly. Usually I just fully tension the bolts. Some quick thoughts (that hopefully can be improved upon by others).

I still think lock washers would help. I've got to imagine that there are multiple different types and some manufacturer should have figured out how to deal with this issues.
I've heard of people scoring or damaging the threads so that the bolt can't loosen.
I've heard of people doing a tack weld on the nut. Though I would think you'd have to be very careful about your materials to make sure the tack weld won't cause issues.

 

[Ingenuity]

Use preload in your anchor rods/bolts, and have min. 10x bolt diameter of free length. The object being that the magnitude of the preload always exceeds the dynamic uplift magnitude.

We had a situation recently where welded connections were previously used and the after 5 years the welded connections disintegrated due to fatigue.

 

[SlideRuleEra]

kaffy - Heavy duty chemical threadlockers are used on equipment in industrial plants. One of the available strengths is essentially permanent.

http://www.SlideRuleEra.net

 

[dik]

Lock washers are a bad idea if subject to vibrating loads... Use Loctite Red (aka Loctite 263)... For bolts in shear, I often use 'snug tight'... for bolts in tension, I always use Loctite Red or wreck the threads. I don't use jam nuts for bolts in tension.

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

-Dik

 

[kaffy]

Thank you very much guys

 

[JLNJ]

We've had welds break under cyclic loads. We've had pretensioned bolts break under cyclic loads. Sometimes without a clear cause or explanation. It's complicated and I'm not sure we really understand what's going on with dynamic loading.

 

[HTURKAK]

Yes..The use of split-ring lock washers is useless...



I will suggest ; choose a properly designed bolt that can develop preload to resist transverse slippage and use spherically seated washers with hardened nuts.. ( say , my opinion..)

 

[desertfox]

Hi kaffy

Yes bolts can work loose especially under dynamic loading and or vibration, however if you can determine the forces dynamically which the bolts are subjected to and set a preload accordingly then you shouldn’t have a problem, that said I would use norlock washers which are designed to prevent bolt loosening under dynamic loads.

https://www.nord-lock.com/en-gb/nor...MIitP32-7O8wIV0QN7Ch3PawoEEAAYASAAEgIfvvD_BwE

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

 

[Tmoose]

"I wanted to use bolted connections instead of welds."
Why?

I love threaded fasteners, maybe because there is often SO much to think about (after problems arise ).
10% avoiding fastener "design" quagmire ( ooooh, what if the threads strip!?) and 90% faying surfaces and the rest of the bolted system that must be right to prevent pre-load loss, which is behind pretty much all the problems I've seen at dozens of companies over the last 2 score plus years.

Realistically "Welding" can be mighty quick and eliminates issues with drilled holes location and quantity, fastener inventory, sketchy fastener quality (oh YEAH!), and fastener assembly issues a-plenty.
"Bob, have you seen Bob's torque wrench?"

https://www.youtube.com/watch?v=FU-tuY0Z7nQ

Cost analysis of existing welded joints vs proposed bolted joints would be interesting and probably necessary at some point to sell your design.It will be tricky to do right. Especially if the welding is already in-house and not just for assembling this frame.

Is this a new design?
Are the existing welded frames trouble free ?
Is this an existing design, assembled with established weld procedures, and a carefully developed weld pattern with very specific requirements for the weld returns (if not welded all around) ?

Of course the loads are not known, so it MAY be that requirements of keeping the frame together and aligned are easily satisfied with a few 1" long 1/4" 6013 fillet welds, or Home Depot 5/16" Grade blah hex head screws and 100% slip-proof when tightened to 10 lb-feets, and with NO regard to details uneven HR Mill scale faying surfaces or drilled hole de-burring. Maybe.

The picture is small and fuzzy, like our new BernaDoodle devil pup.
From what I can see there are 3 thru bolts or maybe 6 bolts individual bolts in each corner.
Long Thru bolts' preload will be limited to a uselessly low value limited by bending the flanges, unless there are nice unseen stiffeners in there.

Do the long (vertical) channels have tapered flanges or uniform thickness flanges? If tapered, fairly pricey beveled washers are required, adding to the parts count, and inviting assembly screw-ups with potentially big consequences, and even failure .

 

[dik]

Loctite Red or peening is easier...

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

-Dik

 

[kaffy]

Thank you guys. Really helpful information for a newbie like me

 

[drawoh]

kaffy,

Check out Bolt Science. There is all sorts of stuff there on bolts loosening.

The good people at Bolt Science argue that bolts loosen because the joint is moving. There is a mechanism present that rotates your bolt and/or nut. Are your corners highly strained and stressed? That will move your bolts, and break your welds. Possibly, you need to engineer your corner joints a bit.

--
JHG