Combining Stresses 2

[USMechE6]

Hi All,

I need help calculating the combined stresses (max/critical stress) on the journal of a ball screw to determine FOS. I got the torsional stress and the axial stress (compression and tension), but I cannot remember how to combine them. I am trying to refresh on Mohr's circle from class over ten years ago, but admittedly it is throwing me for a loop. There is no bending stress or other shear stress, just pure axial and torsional, so it should be the same around the shaft circumference. Can anyone please help?

 

[FACS]

Research the Superposition method for combining stresses. Maybe that will refresh your memory.

 

[USMechE6]

@ FACS, I may be mistaken but I believe that is in reference to adding shear stresses (shear load + torsional) together and normal stresses (axial, bending) together, but not for combing shear and normal to determine the maximum stress within a part.

 

[goutam_freelance]

You have Sigma_x and Tau_xy. From Mohr's circle below you get principal stresses Sigma_1 and Sigma_2.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[desertfox]

Hi

Have a look at this site

https://academic.uprm.edu/pcaceres/Courses/MMII/IMoM-6B.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[USMechE6]

@ goutam_freelance: Thank you much. I also just found this calculator

https://mechanicalc.com/calculators/mohrs-circle/

that computes the same value.

 

[USMechE6]

Thanks, desertfox. I actually was reading through that, specifically the helicopter example but I did not follow how they got the 103 MPa. I need to review this further but for now can get this part drawn up and made

 

[rb1957]

I couldn't see "103 MPa" ... which example problem ? there seem to be lots directly applicable to your problem. You're right, that the outer surface is at the same stress level, but the combined stress, the principal stress is at an angle to the axial direction.


oh, I see it (on pg9) ...
maximum shear stress is (p1-p2)/2, the radius of mohr's circle; = (135--71)/2 = 103

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Hi USMechE6

The 103Mpa is the centre of the Mohr circle, or the radius arm if you like (171 + 105)/2


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[USMechE6]

Thank you all for the help.