# Modal Analysis of a Rotating Shaft (mode-shape frequency issue) **URL:** \ **Category:** General Questions\ **Created:** [July 23, 2025, 6:08am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876 "2025-07-23T06:08:06Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![WIwat](https://avatars.discourse-cdn.com/v4/letter/w/22d042/32.png) [@WIwat](https://prepomax.discourse.group/u/WIwat)\ **Post date:** [July 23, 2025, 6:08am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/1 "2025-07-23T06:08:07Z")
Hi PMX team, I have a confusion about doing modal analysis of a rotating shaft, the results came out that the frequency at mode 2 is 0Hz which seems incorrect in my opinion. my project is in this link: > **[Marine Propulsion shaft FEM.pmx](https://drive.google.com/file/d/1Pm_S_fsqNBLsiggWYLUx9pGRG9DEUewh/view?usp=sharing)** > > Google Drive file. I still cannot figured out what part did I do wrong according to all the setups I made? ---
**Author:** ![Paul18fr](https://avatars.discourse-cdn.com/v4/letter/p/35a633/32.png) [@Paul18fr](https://prepomax.discourse.group/u/Paul18fr)\ **Post date:** [July 23, 2025, 6:24am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/2 "2025-07-23T06:24:44Z")
Hi, I cannot open your file; you seem using an old pmx release! Workaround: import the .inp file into the latest one ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/9/990f8e81f9232c2f938d657ca83a770309e16277.png) ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [July 23, 2025, 7:28am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/3 "2025-07-23T07:28:12Z")
I have a different problem with this file - I don’t see any geometry or mesh when I open it in v1.3.4. Something is off with units here. You seem to be using the SI(m) system. But Young’s modulus should be higher: 210000000000 Pa if it’s steel. Then the rotational speed is around 1047.2 rad/s - is that correct ? And finally - when I measure the model from exported .inp file, the length of the shaft is 1305 m. So it seems that you imported the geometry without scaling. You can do it in CAD software before exporting or in PrePoMax with Model → Part → Transform → Scale. Before you run the full 2-step analysis, try just the static or frequency step and see if the displacements, stresses or frequencies make sense. ---
**Author:** ![belov](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@belov](https://prepomax.discourse.group/u/belov)\ **Post date:** [July 23, 2025, 7:30am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/4 "2025-07-23T07:30:52Z")
Geometry in the form of a stl file [geometry.zip](https://prepomax.discourse.group/uploads/short-url/jxb6pMVsTeppRRfAVaETqjEP4EA.zip) (245.2 KB) ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/a/adf8789b78e4fc0ec983c7e00efc40bfec124dba.png) ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [July 23, 2025, 7:47am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/5 "2025-07-23T07:47:32Z")
Is this what you want to get (open it in PrePoMax v2.3.0) ? [Shaft fixed.pmx](https://prepomax.discourse.group/uploads/short-url/9PuVSowVen62Fs0oZTLG1b2fly.pmx) (2.4 MB) The shaft is 1.305 m long now. ---
**Author:** ![WIwat](https://avatars.discourse-cdn.com/v4/letter/w/22d042/32.png) [@WIwat](https://prepomax.discourse.group/u/WIwat)\ **Post date:** [July 23, 2025, 8:45am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/6 "2025-07-23T08:45:39Z")
Thank you so much, so the main problem is due to my geometry setup? Do I understand that right? ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [July 23, 2025, 8:50am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/7 "2025-07-23T08:50:29Z")
Yes, unscaled geometry, unit mismatch and Young’s modulus a few orders of magnitude too low. In the model I shared, this is corrected but you should still run these two steps separately (like I did with a frequency step) and see if the results make sense. Only then proceed to full multi-step analysis (preloaded eigenfrequency extraction). ---
**Author:** ![WIwat](https://avatars.discourse-cdn.com/v4/letter/w/22d042/32.png) [@WIwat](https://prepomax.discourse.group/u/WIwat)\ **Post date:** [July 23, 2025, 8:58am UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/8 "2025-07-23T08:58:01Z")
Oh, I see! Thank you very much for your help. I’ve already tried the edited file, and the results are exactly what I was hoping for. I apologize if I’ve asked a lot of questions — I’m a graduated high school student and will going to first-year undergraduate school soon, but I really want to practice using CalculiX and hope it will benefit me in the future. ---
**Author:** ![TKemeny86](https://avatars.discourse-cdn.com/v4/letter/t/58f4c7/32.png) [@TKemeny86](https://prepomax.discourse.group/u/TKemeny86)\ **Post date:** [July 23, 2025, 12:41pm UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/9 "2025-07-23T12:41:14Z")
If you’re using Salome for the 3d modeling, pay attention on the scale - in my case, each of my model is 1000× larger… You can use the query tools to check your actual dimensions in Prepomax ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [July 23, 2025, 12:49pm UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/10 "2025-07-23T12:49:52Z")
When I want to work in SI(m) system with model created in FreeCAD and exported to STEP, I have to scale it in FreeCAD first before exporting. As I’ve mentioned above, PrePoMax also has scaling. And, in addition to Query → Distance, it also has a convenient ruler at the bottom that allows you to estimate the size of the model. ---
**Author:** ![belov](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@belov](https://prepomax.discourse.group/u/belov)\ **Post date:** [July 23, 2025, 1:01pm UTC](https://prepomax.discourse.group/t/modal-analysis-of-a-rotating-shaft-mode-shape-frequency-issue/2876/11 "2025-07-23T13:01:43Z")
I change the unit prefix **.MILLI.** in the STEP file - **SI\_UNIT(.MILLI.,.METRE.)**