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 Threaded fasteners - torque tightening.
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Bruce Sutherland

United Kingdom
817 Posts

Posted - 17/03/2017 :  22:04:39  Show Profile
To many, fastening parts together with a bolt/screw/stud and a nut would seem to be simple and straight-forward. This is far from the truth and much engineering investigation has taken place to understand the nut-and-bolt jointing process.
Factors from varying qualities/tensile strengths of steel, thread surface finish (e.g. plain or zinc plating, etc), and lubrication all vary the correct torque tightness of the joint.

I am a retired automotive product design and development engineer and, whilst not an expert in fastener technology, recent forum queries about big-end torque tightening prompted a little investigation into the ‘science’ behind threaded fasteners.

A draft of my investigations can be found from the “Home” page in “Document downloads – Technical” “Miscellaneous – Analysis of threaded fasteners”
http://www.triple-mregister.org/uploads/retro/Threaded%20Fasteners%20Analysis.pdf

Other forum contributors with an engineering background are invited to correct any errors and/or provide further clarification “builds”.


Bruce. (PB0564)

Nick Feakes

USA
1930 Posts

Posted - 20/03/2017 :  11:49:01  Show Profile
Bruce
Thank you for an excellent exposition on a topic I have never thought much about, I certainly learned quite a bit about screw threads. Since no-one has made any comments I assume the document rises from being a draft to "final" status?
Nick
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NeilS

United Kingdom
125 Posts

Posted - 20/03/2017 :  12:55:59  Show Profile
A well written document that is factually correct as far as i can see. Thank you Bruce

Neil


Neil
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Simon Johnston

United Kingdom
2166 Posts

Posted - 20/03/2017 :  13:45:22  Show Profile
Bruce,

Can you offer any guidance on the use of bolts that are fully threaded and those that are only partially threaded, the latter often referred to (perhaps incorrectly ) as 'set screws' ?

Simon J
J3437
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Oz34

United Kingdom
1328 Posts

Posted - 20/03/2017 :  14:27:32  Show Profile
Is it not the fully threaded ones which are known as set screws Simon?

Dave
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Barrie G

United Kingdom
19 Posts

Posted - 20/03/2017 :  14:49:09  Show Profile
Hello All,

My father, who was a motor mechanic, always referred to "set screws"
when dealing with pointed bolts which had heads for spanners....not 4 candles!!

Barrie
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Simon Johnston

United Kingdom
2166 Posts

Posted - 20/03/2017 :  14:50:40  Show Profile
You could well be right, Dave!

Simon J
J3437
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Peter Green

United Kingdom
1446 Posts

Posted - 20/03/2017 :  16:49:22  Show Profile
A Google search finds this:-

A bolt is not fully threaded, whereas a set screw is. Bolts are generally not available in the shortest lengths for each thread size, as there is insufficient room to form a useful length of thread and to still have an unthreaded section below the head.

Peter
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Bruce Sutherland

United Kingdom
817 Posts

Posted - 20/03/2017 :  16:55:04  Show Profile
Simon, Dave, and others,
From my apprenticeship days I’ve always understood the differences between a bolt and a set screw to as follows:




A point to emphasise, particularly for highly stressed fasteners (e.g. big-end bolts), is the importance of the small radius under the bolt head.
A ’sharp’ corner here tends to promote the formation of micro cracks, which grow leading to bolt failure. The cyclic loading in big-end bolts leads to fatigue.

In materials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading.
Thus radii help to reduce sudden/sharp corner transitions in metal parts and aid the reduction of localised stress raisers and improve fatigue life.

Bruce. (PB0564)

Nick, the picture above is a bit large and I didn't know how to reduce it; can you? BS.

Edited by - Bruce Sutherland on 20/03/2017 17:16:29
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Bruce Sutherland

United Kingdom
817 Posts

Posted - 20/03/2017 :  17:08:33  Show Profile
quote:
Originally posted by Nick Feakes

Bruce
Thank you for an excellent exposition on a topic I have never thought much about, I certainly learned quite a bit about screw threads. Since no-one has made any comments I assume the document rises from being a draft to "final" status?
Nick



My thanks to those who have found this of interest.

Nick, OK, let's change from draft to "final", but I'll send you a further copy with minor editorial changes.

In the event that others find errors or suggest improvements then we can always update it.

Bruce. (PB0564)
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Simon Johnston

United Kingdom
2166 Posts

Posted - 20/03/2017 :  20:07:00  Show Profile
Bruce,

Thank you for clarifying which is a bolt and which a set screw. I had written the query hurriedly on my phone with no time to check which was which. But my query was really whether bolts and set screws are interchangeable in use. For example, when fastening the clutch cover to the flywheel, or the propshaft to the flange on the differential, does it matter whether bolts or set screws are used?

Simon J
J3437
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Peter Green

United Kingdom
1446 Posts

Posted - 20/03/2017 :  23:14:43  Show Profile
Simon,

I always use bolts wherever possible. In the two instances you mention I would use bolts

Peter
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Simon Johnston

United Kingdom
2166 Posts

Posted - 21/03/2017 :  09:14:18  Show Profile
Thank you , Peter. That would have been my understanding as well, but I was wondering if there was any technical engineering basis for this.

Simon J
J3437
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Bruce Sutherland

United Kingdom
817 Posts

Posted - 21/03/2017 :  11:30:46  Show Profile
Simon,
Referring to my earlier comment about the desirability for a small radius under the head of the fastener, it is easier to achieve this with the plain shank on a bolt than with the run-out of the thread with a set screw, hence the preference for a bolt where the application allows.


Bruce. (PB0564)
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Nick Feakes

USA
1930 Posts

Posted - 21/03/2017 :  11:57:00  Show Profile
Bruce
I have uploaded the latest version of your document, it can be found here: "Document downloads - Technical - Miscellaneous"

Nick

Webmaster
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Bruce Sutherland

United Kingdom
817 Posts

Posted - 21/03/2017 :  12:33:47  Show Profile
Nick, thank you.
quote:
Originally posted by Nick Feakes

Bruce
I have uploaded the latest version of your document, it can be found here: "Document downloads - Technical - Miscellaneous"

Nick

Webmaster



Bruce. (PB0564)
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