Learning Forum

[Autonewbie]

Hi,

During the modal analysis, Part A will slide and go through Part B. May I know any linear element can be used to simulate the gap and stop by the Part B in linear Modal Analysis? Thank you!

[peteroznewman]

If you don't want to see Part A go through Part B in the Modal analysis, turn down the Result view scale factor.

Or you can just ignore this because it's not real.  The displacements in a Modal analysis are arbitrary because there is no load.

What is the frequency of the mode where Part A passes through Part B?  If it is zero or practically zero, that is because there is no stiffness connecting A to the structure, it must have a frictionless connection to the structure such as No Separation contact or a Joint.

A modal analysis characterizes the natural frequency of vibration and mode shapes.  Downstream analysis like Harmonic Response apply loads and use the data generated by the Modal analysis to compute a linear solution to the vibration problem with real displacement values.

Modal and Harmonic Response are linear analyses which means there cannot be any nonlinear elements or properties in the model. A gap element is nonlinear.  Frictional contact is nonlinear. When it exists in the model, it is converted to Bonded contact.

[Autonewbie]

Hi Peter, 

 

Thank you for your reply! The picture above was simplified. It is something as below, an assembly plastic parts with snap or catcher. The thing is some snap locations are not touching each other for assembly purpose, small gap like 0.1 mm. So, if model no separation at these small gaps seems adding more stiffness.

 

 

[Autonewbie]

I added one more picture to describe my model. Imagine two parts are assembled together using snap or catcher. There would be a small gap so that one part can snap in. So, in modal analysis, how to account for the part hitting each other during vibration?

 

[peteroznewman]

Show a cross-section through the snap for the two parts assembled. Where do they touch?  At the tip of the snap or at the base of the cantilever?  One relevant configuration of these two parts for Modal Analysis is to have No Separation on the flat faces where the two parts touch. That will let the two flat faces slide around on each other. Wherever the snap touches the other part, use a Bonded Contact. Then run the Modal analysis.

Why are you running a Modal analysis?

How will you use the results?

Parts can't hit each other in a Modal analysis because it is a linear analysis. Parts are either connected (with bonded contact or a joint) or they are not connected and free to pass through each other.

If you want to see parts hitting each other, then you have to run a Transient Structural analysis and you can have frictional contact where gaps can open and close.

[Autonewbie]

Hi Peter,

I was out of town. We use the modal result to check the first frequency. Typically, we will continue to run Random and Shock Analyses but for this case, we only run for Modal.

We want to check risk of noise based on the Modal analysis.

From on the result, we understand the risk is very low based on our experience. I was looking at those small gap that could contribute the risk of noise. 

[peteroznewman]

You can't evaluate risk of noise from a Modal analysis.

[Autonewbie]

it is looking at the first frequency based on our own design guideline. From the guideline, it gives us some quick idea about the level of risk of noise at different level of first frequency.