NCEES Practice Exam MD&M #108 question

[katiejune]

On the new NCEES Practice Exam for MD&M, the question #108 is using g as 386.4in/s^2. Which gives me an answer of Critical Speed = 3954 revolutions per second, not per minute as the answer is stating. And this is not corrected in the errata so I am wondering if I am in a brain fog missing something?

Thanks for any help

 

[JHW 3d]

Can you post the question?

 

[jonjjj]

Katie, I am working/researching the conversion, but believe the 215 in the numerator accounts for the conversion from rad/sec to rpm, the coefficient of the beam deflection using whatever simplification for the loading (uniform load).  So long as the units under the root are the same of the length in the denominator for that equation (e.g. you converted everything to inches under the root to match the length in inches), you will get rpm as your answer.

 

[jonjjj]

I gave up on finding the loading condition for the provided equation in problem 108.  Hopefully an exam problem that provides an equation specifies the units when there is a coefficient (e.g 215).  I am having my own challenges with 510 and 533.  Were you able to solve those two problems as is?  I have another post that details my challenge...

 

[J_MEC]

I did look into this myself and came to the same conclusion on the assumption that the 215 number includes a conversion factor to rev/min.

I found a reference from a manufacturer of ball screws that gives the critical speed of the ball screws with a modification factor of Cs (see 1st dropbox link below). I did some quick comparisons of the equation given in the practice exam, the equation given in Shigley's for critical speeds, and the equation given in the catalog and it seems reasonable to assume that the 215 includes the conversion factor to rev/min (see 2nd dropbox link below)

I agree with jonjjj, hopefully on the exam when a coefficient is included in a given equation they will specify the units of that coefficient. This question would have been much clearer if the equation was stated with as the 215 as a variable C with units of (rev x sec)/(rad x min) (see 3rd dropbox link below).

Unfortunately everything is too large for me to attach directly to the message, but you can view everything from dropbox links below.

https://www.dropbox.com/s/z2w4nkpznl3g9g0/Critical Speed Formula.pdf?dl=0

https://www.dropbox.com/s/pvhw5ubvhibu4tv/Comparison of Equations.jpg?dl=0

https://www.dropbox.com/s/756220bzlj1nm8x/Question 108 Proble Statement Improvement.jpg?dl=0

 

[katiejune]

Thank you so much for helping me understand this and for validating my confusion as well. I am not to the 500s yet but when I am, I will get back to the post on that one. Thank you again and good luck to you all on the test.