Modal Analysis: Problems with constraining & external force

[calvin.schraderB3A4M]

Hello,

I hope this is the right place to ask for help regarding the modal analysis of my assebly.

I'm designing a tower that is attached to a bearing and hold upright by a spring. (See Screenshot 1, spring-end and bearing-mount are fixed.)

I have two problems:

1) I cannot figure out how to constrain the pin going through tower and mount properly. I used the normal undirected mate constrain between pin and tower. Also defined it as a pin in the modal analysis. Contacts are set to on, but during modal analysis the parts are split up into components and the tower seems to ignore the pin. (See Screenshot 2) I can avoid that by adding manual spring contacts, but they have a big impact on the Modal Frequencies depending on the spring contact strength. How can I constrain pin, tower and mount so they are regarded as a normal bearing, without adding springs in the modal analysis?

2) Gravity does not seem to affect the eigenfrequencies, even with added point mass on top of the tower. According to my analytical formulas it should have an effect. Even applying a vertical point force on the tower top does not have an impact on the modal frequencies. (Setup see Screenshot 3) Does Inventor ignore external forces when it comes to mode analysis or am I doing something wrong?

Hope somebody here has some ideas to help me find my mistakes.

Thanks,

Calvin

[JDMather]

Attach assembly here.

[calvin.schraderB3A4M]

Here the assembly for Problem 1).

Thanks,

Calvin

[KKizildemir · Accepted solution]

Hi @calvin.schraderB3A4M ,

I think you're trying to solve this mechanism with not right solution environment. This mechanism is an essential dynamics problem. So I think you should seek a solution in Inventor Dynamic Simuation environment.

I've done it for you! Please first watch the video and see how the oscilating frequency changes when I change the stiffness of the spring.

Then, here are the details which makes difference:

1. I created a virtual spring instead of your spring model. I just suppressed the model. Please see the screenshots below
2. I defined an 10 N external force as a step input for 0,1 second. Please see the screenshot below:
3. I defined a trace at the top of the rod. So I was able to draw oscilation graph by using it. Please see related screenshot below

   

You can export all the data of each solution step to an excel file. So, you can calculate the vibration frequency by using simple Excel formulas. I've showed how to export the datas in the video.

Don't hesitate to ask about any questions that I've missed to explain about software or engineering side.

Regards,

[calvin.schraderB3A4M]

Hi @KKizildemir ,

thats exactly what I was looking for. Didn't know I had to do it with a dynamic simulation. Thanks for the detailed explanation about the simulation.

Calvin

[KKizildemir]

You're welcome @calvin.schraderB3A4M !