Cyclic Loading of Fastener and Fatigue

[Braren]

Hi All,

I'm trying to determine the life of the threads in a particular application, specifically, I'm trying to come up with rationale to justify ability of my design to withstand cyclic loading over the course of the product life.

See image attached.
Knob Problem.png

Does anyone have any ideas how I could justify the verification of the design?
To me it's clear it will survive the application, very little load is applied, but the designer in charge is worried about galling.

Thanks,

[Hudson]

To you it is clear. To the designer it is not. We do not have enough information. The ones that count are those purchasing and/or using the product.

Take the tested sample to those who issued the specification and ask for their opinion if there are doubts.

Just my thoughts.

[JAlberts]

If this becomes a major issue then there is: ASTM G98 - 17 -Standard Test Method for Galling Resistance of Materials that can be utilized but it does not apply specifically to threaded assemblies.

[Braren]

Thanks for the response, I'm aware I didn't provide enough information for a complete answer. This is primarily a question of statistical confidence. I recall seeing a chart somewhere, sometime, in college, I think in one of my design and manufacturing courses in which we specifically looked at cycling of springs and of how the curve is exponential, with the spring degrading quickly if loading nears limit of spring, but approaches infinite as the load decreases. See image below.

Life of Spring.PNG

I was thinking that, since a fastener is essentially a spring, I could apply a similar rationale in my verification; if the load is light enough for the fastener specified, it should survive a number of cycles far above the number of cycles required (green region). I was expecting I could find some sort of study or table for standard fasteners or threaded holes specifically related to repeated cycling, but in my searching I couldn't find anything...

Also to clarify, it's not even so much that others doubt it will work, but rather, that I need more evidence to truly verify the design output meets the requirement input. My manager ended up arguing that since the requirement stemmed only from good engineering excellence, as opposed to "true product requirements" related to essential, risk-based, functionality, and we the requirement has no place. This is now getting more into systems engineering philosophy (I'm a systems engineer myself), I agree with him on us being able to drop the requirement, but I would be interested in hearing others' opinion on his rationale. I don't know if I quite agree with him that we shouldn't specify a requirement if it is not critical to product safety, if that's the case, how do you ensure the design team considers such things when developing the design? Perhaps if I included the requirement as a "should" instead of "shall" requirement?

[Braren]

Thanks for pointing me to the ASTM G98 standard, very useful, I'll need to add this to my list of standards!

[rhys_faulkner]

For the fatigue side, designing around the 'endurance limit' (A stress which will be far below the yield stress) is an option for you if you want to avoid plastic deformation.

However, reading your OP, it sounds like you may have more of a wear/tribology problem. In my experience with 'wear' (backed up by training), its actually very difficult to calculate because of the amount of variables involved, there's often no substitute for real life testing using 1:1 conditions.