Bearing load

[Michael1968]

Hello experts,

I still had this question in Mechanical Acoustics/ Vibration engineering, but I think, it is better here in the bearing forum.

I'm doing a rotordynamic analysis (in ANSYS) of a shaft with journal bearings. There are time-independent loads (fixed in space) and unbalance. The journal bearings are simulated with matrix27 stiffness and damping elements with unsymmetric factors (C11, C12, C21, C22, D11, D12, D21, D22). I make a transient analysis with unbalance load. If I make a fsum in ANSYS of the matrix27 elements (both, stiffness and damping) I get periodic forces and I see, that a very large amount of the sum comes from damping ! I now want to check the pressure on the (stationary) bearing housing (outer part..). Which forces come to the bearing housing ? Do both come: stiffness and damping ? Which force would you take for calculation of a dynamic pressure on the stationary part ?
Thank you for your comments.

Michael

 

[sms]

I read your question in the other forum, and to be quite frank I am still not sure what you are asking. Would you be so kind as to rephrase it? For instance I am not sure what you mean by "Which forces come to the bearing housing?"

Are you asking which forces are transmitted to the bearing housing? Are you trying to design a bearing housing to take the load?

The support force in the bearing are related to both stiffness and damping, and in a fluid film bearing depending on speed relative to resonance, a significant portion of that will come from damping. I suggest that you get a copy of Thompson's Mechanical Vibrations for an overview...

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[Michael1968]

Hello sms,

thanks a lot for your reply. I try to rephrase it

We have designed a bearing and I try to show, that the bearing load (what is the "bearing load" --> this is the question) stays below a an allowed value. The static load I have and now I examine the dynamic loads. The postprocessing in ANSYS of the sum of the stiffness- and damping matrix gave a peridic force. I wondered and then I examined the force more exact and postprocessed damping- and stiffness force separatly and I saw: both are varying with 2x rpm and the level of damping is much more higher than that of stiffness.
To check the allowable bearing load, I simply wanted to sum up the static force on the bearing and a dynamic one. But which dynamic one ? Does damping force (because I think it is more tangential to the orbit and therefore to the bearing housing) also affect the bearing housing or does only the stiffness force affect the bearing housing? Certainly most conservative would be to take the maximum dynamic force of the sum of stiffness and damping...Another idea was, to take the only the radial components of the stiffness- and damping forces, because they bring pressure to the bearing housing...

Which dynamic forces would you take for the proove, that the existing loads are below the allowable loads ?


Michael

 

[sms]

Stiffness and damping should vary with the speed, that is normal. Dynamic forces are vectors just like any other vectors, so you need to add them up vectorially to get the total dynamic load.

Here is a paper that may help...

http://www.rmt-inc.com/assets/publications/stability-survey.pdf

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.