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Combining Stresses
2

Combining Stresses

Combining Stresses

(OP)
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?

RE: Combining Stresses

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

RE: Combining Stresses

(OP)
@ 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.

RE: Combining Stresses

(OP)
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

RE: Combining Stresses

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 ?

RE: Combining Stresses

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

RE: Combining Stresses

(OP)
Thank you all for the help.

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