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Gravity in a Random Vibration Study

Question asked by Jay Liu on Mar 20, 2017

Hi guys, I am new to this simulation business and I am struggling to make sense of my result.


Basically I am shaking my 1500lb system with a profile from MIL-STD with a Grms of 2.2


I set up all the models, external loads, profiles, etc, and ran the simulation after many days of tinkering.


In my result, the stress seems to be very low, so I measured the reaction force on the mounting pads and (the following steps are all I can gather from internet, I am not sure if it is correct way to do)


1. find the curve of reaction force on all mounting pads vs frequency

2. integrated that over the whole range to the squared value of reaction force

3. sqaure root the result to get 1-segma force

4. times 3 to get the 3-segma force


The result is roughly 3000lb force for 3-segma. That seems so low given even the Grms is 2.2g. I also noticed nowhere in the setup am I allowed to define gravity. Come to think of it, how would the computer know which way is down?


Can anyone with more experience give me some pointers at high level because I am not sure if I am running the simulation correctly.