hi，i have a question that when i do dynamic analysis,what shold i set to the stiffness if the model includes the hangers or other snubber.shold i input the stiffness of hangers,or the stiffness of model shold be input ?


thank you for your help !

jian zhang.

Your question is unclear to me.
The dynamic analysis should include the stiffness of all supports/restraints.

hi，Dave
it may be that i make a mistake to do dynamic analysis.when build the model ,i take the support as rigid and do not build the model of support,but there two hangers in the model,what i had thought was that the stiffness of hangers was needed to be include.how i do to make it ?

hoping for your reply soon!thank you very mauch!

jian zhang

Correct me if I am wrong but why would you do a dynamic analysis on a line that you have Springs? Springs are used to absorb thermal growths, not vibrations. Vibrations are best absorbed with Fabreeka pads. (you can do a search on the net with "Fabreeka" to know more!!) In order to provide a good response to your problem, please state what dynamic problem you are trying to resolve.

Assuming you let CAESAR II design the springs, the stiffness will be automatically included in the dynamic model.

hi,shamim Huq,
i want to konw how the vibration of main engine do influnce on the exhuast pipe. The exhuast has two hangers in the model and the other supports are considered to be rigid(i do not be sure if it is correct actually).If i am going to study the inlunce of the vibration to the exhuast pipe,what shuold i do ?

hope for your help

hi,Dave,
I got what you said,but if i want to konw the influence of the main engine's vibration to the pipe. what should i do ?

hope for your reply soon!

If you have springs in the system they should be in the model as well - statics or dynamics. If you only wish to add stiffness, then a restraint will do the job - simply add the stiffness in the stiffness field (a blank stiffness indicates rigid). If you wish to better model a hanger support, the spring preload (theoretical cold load)should be included. To do this, define a hanger rather than a restraint. The bottom of the hanger input window allows you to set the stiffness AND preload.
Without good load information (to define the cyclic mechanical vibration of the engine on your exhaust) the most you should expect out of the program is the possibility of exhaust resonance with the engine. Do this by simply calculating the natural frequencies of the exhaust. If the exhaust frequencies are close to the cyclic speed of you engine, then you can expect trouble. How close is close? Some would say within 20%. Others may also be interested in multiples of the running speed.

hi，Dave！
it's very nice of you.As to the excitation,i don't know something about vibration of the main engine when exhuasting.How exhuast happens,and how it does to the exhuast gas pipe?

Like I said, you can't very well analyze your system response without good load information. That's why I suggest calculating natural frequencies - no load, no displacement, no stress, just system natural frequencies.
The engine may produce harmonic loads which may be troublesome if they happen to have forcing frequencies close to these system natural frequencies. The source of these harmonic loads could be mechanical vibration or pressure pulsation.