modal analysis and snubbers

Posted by: vermaccio

modal analysis and snubbers - 01/25/21 11:09 AM

my system has a modal vibration mode 3.1Hz. So i have to chasge system to have a mode higher than 4Hz.
I can't insert a guide because the vibration mode is in a location where i have an huge displacement (a branch at the end of a veery long hot main header).

i thought to use a snubber.

Do i insert snubber only in dynamic input or have i to model both in static input both in dynamic input?
Posted by: Richard Ay

Re: modal analysis and snubbers - 01/25/21 11:05 PM

View the animation of the 3.1hz mode of vibration. See if there are any locations of significant displacement where you could add a restraint. Any restraint at a (displacement) high point will kill the mode.
Posted by: vermaccio

Re: modal analysis and snubbers - 01/26/21 01:40 AM

i can't insert a restraint where is needed. the only solution maybe is a snubber. so the question remain open: i have to insert the snubber only in dynamic input or also (only) in static input? what does change inserting it or not in static input?
Posted by: Michael_Fletcher

Re: modal analysis and snubbers - 01/27/21 03:50 PM

Have you considered adding a loop to avoid a snubber?

Have you given any thought as to how you might specify such a snubber?
Posted by: Richard Ay

Re: modal analysis and snubbers - 01/27/21 04:17 PM

Michael makes some good points to consider.

However, to answer your question, you can define the snubber either in the model itself, or in the dynamic input - BUT NOT BOTH. If you put the snubber in the model, it will only be considered in a static analysis if the load case is OCC and/or you check the "activate snubbers" checkbox in the Static Load Case Editor.

Snubbers will always be considered in a dynamic analysis.
Posted by: vermaccio

Re: modal analysis and snubbers - 01/28/21 05:24 AM

thank you. better inserting in static imput or i can forget they are there smile

note: i solved the problem changing pipe routing (client didn't want but at the end they unserstood it was cheaper and better than using a snubber), but i had that question smile the answer will be ususeful if in future i will use a snubber smile