×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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?

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?

"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.

RE: Why do bolts work loose?

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

RE: Why do bolts work loose?

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.

RE: Why do bolts work loose?

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.

RE: Why do bolts work loose?

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).


=====================================
(2B)+(2B)' ?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources

3D Scanning in the Automotive Industry
With over 100 years of production history, the automotive industry has been at the forefront of manufacturing technology since its inception. Whether the transformative technology of the day was the assembly line, the integration of robotics into the manufacturing process, or the switch from steel to aluminum frame chasses, the automotive industry has consistently implemented advanced technology into its manufacturing and production workflow to improve manufacturing and product performance. Today, the same is true. Download Now
Green light on lidar: Developing low cost systems for autonomous vehicles
Lidar has been around for quite some time, but to date, it’s been custom—and expensive. Right now, there isn’t a clear-cut solution that’s suitable for all applications, such as lidar in autonomous vehicles. As they explore options, optical and mechanical engineers are forced to make choices and tradeoffs during the design process. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close