# SMS Problem 57



## loudog (Sep 9, 2015)

Why can't you just calculate the max stress in the spring using 5 lbf and MERM equation 54.14: *Shear Stress Max = (8FDW)/(pi*d^2)*? SMS solves it by determining the Stress Amplitude, then the Mean Stress, then calculating Max as Mean plus Amplitude.

I understand the load is cycling, but don't fully understand how the max stress isn't the same whether it's cycling or not.

Please straighten me out.

Thanks,

LP


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## P-E (Sep 9, 2015)

The correct solution is to burn the SMS book. It's terrible. I did.


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## loudog (Sep 9, 2015)

I'm seriously considering it.

If there were more practice problems around...I'd burn it using pages from the Lindeburg Practice Exam to get things going.


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## denver1000 PE (Sep 9, 2015)

Yes all of the ppi books have round-about ways to solve some questions, and you certainly don't have to, or rather, should not follow the overly complicated way of arriving at an answer when you know a solution can be reached in a much simpler way. In this case just use the max stress.


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## denver1000 PE (Sep 9, 2015)

Another note in this particular problem and all the ppi solutions is the use of the Ks and Kw factor. The former is used for static loading and the latter for fatigue. The MERM practice book just uses Kw which is more conservative for a status problem and in this book they used both for an altetnating loaded spring which is incorrect to me.


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## P-E (Sep 9, 2015)

loudog said:


> I'm seriously considering it.
> 
> If there were more practice problems around...I'd burn it using pages from the Lindeburg Practice Exam to get things going.




I burned it in a camp fire up in the white mountains. So satisfying.

I felt that the TFS SMS required a bunch of reference books that I didn't have/didn't bring with me to the exam. Somehow, I passed it regardless, hmmm. Burn it.


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## denver1000 PE (Sep 10, 2015)

Oops, i meant static (not status) use Ks, and alternating (just spelled horribly!) use Kw (wahl factor or the other one discussed in Shigley's) -

:laugh:

MERM Practice book uses the Kw for both static and fatigue problems for some reason, and the SMS brought the fatigue load to alternating and mean for Kw and Ks, which wasn't necessary as Kw should be used for fatigure loading.

I would also throw my in the trash but there are a couple useful problems in the MD one.... trick is to use the solution as a guide of how not to solve the problem in some cases


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