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Why do bolts work loose?
2

Why do bolts work loose?

Why do bolts work loose?

(OP)
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

RE: Why do bolts work loose?

Just another punishment from the Original Sin. You know, Adam and Eve timeframe.

RE: Why do bolts work loose?

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.

RE: Why do bolts work loose?

racookpe1978,
I haven't come across a better explanation. Materials are perverse.

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

RE: Why do bolts work loose?

"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.

RE: Why do bolts work loose?

"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 .

RE: Why do bolts work loose?

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."

RE: Why do bolts work loose?

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.

RE: Why do bolts work loose?

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

Ted

RE: Why do bolts work loose?

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.

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