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Question about threads, stress concentration factors, and fatigue 1

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jgrady

Mechanical
Jan 30, 2015
34
Hi,

I have some questions about analysis of threaded parts.

1) When designing bolts in tension under fatigue loading, Shigley cites a stress concentration factor of 3.8
2) What about a threaded rod in bending in fatigue, do we use the same concentration factor of 3.8

Thanks in advance.

 
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Jgrady:
I don’t have a copy of Shigley’s book, so it would be interesting to see, for you to attach several pages of his text on either side of the 3.8 concentration factor that you cite, so that we more fully understood the context of that number. Certainly, there will be a stress concentration issue in the threads of nuts and bolts, but they are also well tested and accepted fastening systems. When you pretension them sufficiently so that they don’t have significant stress reversals during cyclic loadings, and they develop near their full cap’y., they perform quite well. Cyclic bending of a threaded rod is quite a different stress and fatigue problem, and while you know there will be stress concentrations at the roots of the threads, I’m not sure that Shigley’s number would apply to this problem. It would be far better to look at your design to try to eliminate the cyclic bending in the rod. Show us a sketch of your design so we can comment on how to minimize/eliminate the rod bending, that would be a significant improvement in the design.
 
dhengr:

threaded rod is part of an actuator used on a lever brake. Actuator rod is threaded and mates with a female threaded collar which sits in a ball and socket joint. Bending stress in the rod occurs when the rod is extended during braking and is imparted due to friction in between the ball and socket.
 
 http://files.engineering.com/getfile.aspx?folder=13a1f81b-017d-4bce-99e5-203a4ef008e0&file=actuator.JPG
That diagram appears to show a mechanism which would apply very very little bending moment to the threaded rod.
 
My machine design handbook says Kt is the same for tension or bending (for threads).
 
There's a difference in stress concentration factors from tension to bending, based on what I'm looking at. I've attached a chart in Shigley's 9th Ed (happened to be within arms reach) with some "test points" across the chart - it's a grooved round bar, probably the closest thing we can find to compare to a threaded rod. Based on a small-scale regression and the fact that I'm using the human eyeball, I'd say scaling that 3.8 down by about 1.1 to 3.5 would fit the pattern. In every example, the concentration factor for tension is higher than bending, which would mean using the factor of 3.8 given for tension is the conservative route.

Edit: read the scale factor inverted on first post.
 
 http://files.engineering.com/getfile.aspx?folder=dc790f4b-1099-4a9f-8020-7c588aeb1233&file=Kfactor.pdf
@jgKRI What about when the brake is engaged and there is friction between ball and socket due to deflection of the lever imparting a moment?
 
Imparting a moment into what?

The only way for those ball joints to apply any significant moment to the shaft is for them to seize or to bottom out in their range of rotational travel.
 
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