×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Oil film stiffness and damping coefficients

Oil film stiffness and damping coefficients

Oil film stiffness and damping coefficients

(OP)
Hello,

I have a question about calculating oil film stiffness and damping coefficients. From literature, it says that

Kxx = dFx/dx
Kxy = dFx/dy
Kyx = dFy/dx
Kyy = dFy/dy
Cxx = dFx/dxdot
Cxy = dFx/dydot
Cyx = dFy/dxdot
Cyy = dFy/dydot

My question is that if you have a limit cycle (journal closed orbit), how can you find bearing coefficients from the orbit?

My external load applied on journal is the unbalance (omega is equal to the rotational speed of journal)

m*xdotdot + Fx = m*a*omega^2*cos(omega*t)
m*ydotdot + Fy = m*a*omega^2*sin(omega*t)+W

Where "Fx,Fy" are nonlinear forces, "W" is the gravity load, "t" is time, "m" is rotor mass, "a" is the distance from centre of rotor gravity to the centre of rotor geometry.

Thank you for your help.

 

RE: Oil film stiffness and damping coefficients

Great question.

As you know there are a lot of specialized programs that develop those coefficients based on study of the geometry, loading etc.

I can't recall ever hearing anyone mention estimating them from the filtered orbit, but it seems like a worthy goal.

As a starting point it seems logical to try to work with a matrix model.
M*q'' + C*q' + K*q = Q
where q are generalized coordinates and Q are generlized forces.  There is a lot of literature about modeling a machine in this form.

For a forced response, we don't need full modal analysis, just assume Q= Q0*exp(i*w*t) and the equation becomes
q*(-M*w^2 + i*C*w + K) = Q0

You know some or all of q depending on your model.  You know Q0 from your assumption of known unbalance force and weights acting on the system.  You know M.  You may know parts of K such as those associated with shaft.  There are unknown parts of C and K that you want to solve.  Hopefully there will be enough equations to solve the unknowns if things were set up well.

That is just some rambling thoughts fwiw.  A long way from solving anything.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Oil film stiffness and damping coefficients

(OP)
Thank you all for your reply.

electricpete, I am still not sure what you mean but thanks again.

unclesyd, I'll look into the thesis. Thank you very much.

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources