About natural frequencies

[lucaengineer]

This is the first time I use the frequency analysis module of Catia. I have performed a normal modes analysis to determine the natural frequencies of the support you see in the next picture

This support will sustain a mass of 600 kg and this mass cannot vibrate, to place this support with its mass I will use precise rails. After the modal analysis the frequencies I obtained were:

5.85242 Hz
22.7846 Hz
38.5786 Hz
52.584 Hz

What frequencies should I compare these with? How can I obtained them?

 

[IRstuff]

Well, what is your requirement? How would you know that you're done?

TTFN

FAQ731-376

 

[DLite30]

You need to determine at what frequency your exictation forces are going to be, and ensure that you have about a 20% difference between your modal and excitation frequencies.

Examples would be: attached motors (what rpm do they run at?), vibration from the floor or building columns.

I would also consider, if you haven't already, the pendulum effect frequency of the mass and the torsional stiffness of the bracket, to make sure they don't coincide.

 

[Tmoose]

Are those ground symbols indicating infinite stiffness? I'm not sure I'd consider a 100 mm thick beam, cantilevered several meters with a 600 kg load somewhere as infinitely stiff.

 

[kpt4321]

Take care with how you account for the mass of your "probe," or whatever that mechanism that you are supporting actually is. At the very least, you need to have the mass of that accounted for at the end of the support structure. However, given the aspect ratio of the device, I would suggest that that will not give you satisfactory results, and that you should be simulating it in conjunction with the support structure.

For what it's worth, and to add to Tmoose's concern, a 4in thick, 40in wide, and 160" long beam (roughly the dimensions of your part, as per the drawing), made of steel, will deflect about 1/4 of an inch simply supported if the end load is 1300 lbm. With a simple point mass assumption (IE that all of the mass resides at the tip of the cantilever), the natural frequency of that beam in the vertical direction is in the ballpark of 6Hz. In other words, it is far from infinitely stiff.

 

[Twoballcane]

"What frequencies should I compare these with? How can I obtained them?"

You have to tell us what frequencies "you" are worried about. It's your part. Does your mass vibrate or is there a dynamic load (shock) that you are concerned with? The Fn is a good place to start, but we also have to know the response.

Tobalcane
"If you avoid failure, you also avoid success."