Hello,
I’m a new user trying to acclimate myself with PrePoMax by running some of the online cases (Thanks FEAnalyst!)
I’m specifically trying to replicate this YouTube version with different dimensions and in imperial units (inches, psi): https://www.youtube.com/watch?v=fT21I-fV1D0
For this design I’ve followed the boundary conditions correctly, and have “fixed” the .STEP import dimensions, ensuring it’s the correct dimensions and I’ve run hand calc’s to check stress.
Model Parameters:
Mat’l: Generic Steel, V=0.30, E=27,500,000psi, density = 0.283 lb/in^3
Outside Radius: 15.0 inch
Inside Radius: 5.0 inch
Thickness: 2.0 inch
Speed: 5,000 RPM (523.6 rad/s)
Expected Results (hand calcs):
Max Stress at I.D. : ~ 38,180 psi
Max Stress at O.D. : ~ 12,050 psi
Max Deflection at ID: ~ 0.0069 inch
As can be seen in the attached images, stress and deflection are off by orders of magnitude. This is a pretty fundamental case, so I’m wondering if it’s a bug with imperial units?
Additional Notes: I also tried refining mesh, and simulating a whole disk with no change. I’ve tried this in V1.4.1 as well as 2.0.0 with no change.
Thanks!
The first thing would be to check the consistency of the units used. Here follows the consistent set of units for your case, using steel as reference material:
It seems your density does not follow the consistent unit set. This will lead to wrong values of force, which in turn will make your results wrong. I am not so familiar with imperial units, but there is a common mistake in metric when you use millimeters, newtons, and seconds - one would expect the density to be kg/mm³, but to make consistent energy units we need to input the mass as tonnes (10³ kg). Maybe something similar is happening here?
Best regards,
Lucas.
1 Like
Hi Lucas,
Great observation thank you!! I tried lots of different unit conversions (mostly to metric, because of the .STEP importing issue in imperial), but not the one I should have!
This is sort of a nuance of the imperial systems that commercial FEA software must “fix” in the background (for nearly two decades I haven’t divided by g).
That seems to fix the problem, as PrePoMax is looking for conventional imperial published densities divided by gravity (e.g. .283 lb / 386.1).
These results now match Roark hand calculations very closely:
Updated Results:
Max I.D. Stress: 38,370 psi vs. Roark 38,180 psi
Max I.D. Stress: 12,010 psi vs. Roark 12,056 psi
Max Displacement at O.D.: 0.006597" vs. Roark 0.00658"
Max Displacement at I.D.: 0.006986" vs. Roark 0.00694"
Thanks again!
There’s even an interactive converter for consistent FEA units: Engineering Unit Converter
I moved this to general questions since there’s no PrePoMax bug.
It’s nice that it helped! Feel free to share your questions in this forum, and welcome to the PrePoMax community. 
