Why do bolts work loose? 2

[ZaraSailor]

I'm keen to understand how dynamic loads cause bolts to unwind. Is it more to do with shock and vibration than dynamic loading?
Feel free to leave your opinion on locking nuts, Nord-Lock washers and nylon locking patches etc below too.

Thanks very much

 

[XL83NL]

Check boltscience.com, especially the video in this article.

http://www.boltscience.com/pages/vibloose.htm

Great website, great articles, useful knowledge.

 

[CWB1]

I'd recommend reading Bickford's "Intro to the Design & Behavior of Bolted Joints."

Whenever anything fails its usually a good idea to check the simple things first. In the case of a bolted joint I would check fastener installation procedures vs what the technician is actually doing, then run a quick bolted joint analysis to confirm the procedure (namely the torque) itself is correct before considering fatigue and other factors. In many cases its the easy stuff that gets missed.

 

[desertfox]

Hi

This site gives some information about bolt loosening

http://www.smartbolts.com/insights/loose-bolts-causes-ways-prevent/

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

 

[Tmoose]

"I'm keen to understand how dynamic loads cause bolts to unwind."

Are some of your bolts "unwinding?"

Before the parts become available to inspect, I'm inclined to put a couple of items on my list.
- insufficient installation torque allowing micromotion and resulting embedment and cascading loss of preload.
- embedment due to badly designed or executed joint. Rough, damaged or non-flat faying surfaces, stuff like that. Including split lockwashers tearing up the bolt and nut and their counterfaces.

 

[chicopee]

"I'm keen to understand how dynamic loads cause bolts to unwind." I believe that I may have an answer from an episode that took place in a manufacturing company. The maintenance crews had over the years assembled and disassembled the same bolts over a period of several years. There was a preload requirement each time the same bolts and nuts were assembled back on the power press stretching the bolt to the point when either necking took place and the bolts broke or the nuts loosened due to too much slack and tore the bolts apart jettisoning the whole assembly. This incident nearly killed the operator of the power press .

 

Tmoose,
I think what you're getting at is that a bolted joint should be considered as a system.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Tmoose]

Hi IM,

Pleas permit me to rephrase that.

If some of your bolts are unwinding, even before the parts become available to inspect, or I place an order for NordLocks, I'm inclined to put a couple of items on my list of likely suspects to investigate as possibly causing the "unwinding."
- insufficient installation torque allowing micromotion and resulting embedment and cascading loss of preload.
- embedment due to a badly designed or executed joint. Rough, damaged or non-flat faying surfaces, stuff like that. Including split "lock"washers tearing up the bolt and nut and their counterfaces.

 

[hydtools]

Unwinding is an irreversible process. I have never seen bolted assemblies wind or rewind.

Ted

 

[Tmoose]

Hi hydtools,

"I have never seen bolted assemblies wind or rewind."

Isn't there an old engineer's joke about that?
The punch line is "how would you know?"

Actually the cheesy plastic cover on my snowblower unwinds it's wing bolts when I tighten them as hard as I dare.
They seem to loosen a turn or so ( completely loose ) and then stop. I keep forgetting to watch what they are doing whilst in the throes of de-snow-ification.

 

[URL unfurl="true"]https://pieng.com/threaded-fastener-loosening/[/url]

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Garret Henry]

In my opinion bolts can loose due to the following reasons-
Under-tightening
Vibration
Embedding
Gasket creep
Differential Thermal Expansion
Shock
However, you can overcome these issues or minimize them to some extent by using these-
Washers
Mechanical devices
Prevailing torque nuts
Adhesives

 

[ZaneC]

I agree with the post above. Torque specifications must be adhered to. Over-tightening can strip the bolt and cause issues.
I'd use a locking compound on the bolt thread and Nordlock washers and/or locking nuts.

 

[gwalkerb]

Maximizing bolt stretch can also help with loosening bolts, especially in vibrating service. For example, it's recommended that any rotating equipment has at least a 7:1 bolt stretch ratio for hold-downs, and similarly, reciprocating equipment should have at least 12:1.

 

[electricpete]

This has not much to do with answering the op's question, but it's timely for me so I'll share my recent experience.

My garbage disposal started leaking water last weekend (photo attached). Nothing has been done to the disposal recently, and it didn't used to leak (any leak is obvious because the water collects in a container which is stored underneath).

There is a metal ring that screws upwards on the plastic for tightening. It has three 120-degrees apart cylinders to insert a screwdriver for tightening (indicating to me it should be more than hand tight). When I found it, it was loose enough to easily tighten by hand.

Why did it start leaking out of the blue? I think the threaded ring loosened under the influence of vibration when the disposal was running (sort of an inadvertant homemade Junker test).


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(2B)+(2B)' ?