Learning Forum

[omerugursimsek]

Hi,

I ' d like to perform a 3D rotor bearing system transient analysis subjected to unbalance load as a function of speed.

There is an example in Ansys Help named 'Transient Response of a Startup'. I can resemble my 3D FE model to this example. However

I have still questions that I can not handle :

-  First one is ,in my model there is two unbalance load which has different phase angle.I do not know how can apply these forces.

- Second  question is about definition of time-step. Near the resonance point I would like to use small time-step size while far beyond it ? want to use bigger time-step size.Hence, ? have to seperate the mission.Is it possible to restart process in transient analysis? or Can I do all those operations via single script.

I attached Ansys example here.Can anybody guide me how can I modify this script.

 

Thank you,

 

Ugur

[peteroznewman]

Ugur,

An image is always helpful to illustrate the problem.

When you say an "unbalance load", do you mean that there is a mass that is offset from the axis?
When you say "there is two unbalance load which has a different phase angle", do you mean that one mass is at a first angle and a second mass is at a second angle?

If this is the case, then the model should just have those two off-axis masses attached to the rotor at the correct radius and angle. That way, the "unbalance load is a function of speed" occurs naturally during the solution.

ANSYS can solve using multiple steps, and each step can have its own time-step, so you could define a 3-step solution where the ramp-up to just below resonance uses a large time-step, then the next time step to just above resonance uses a small time-step, and the third time step returns to a large time-step.

Regards,
Peter

[omerugursimsek]

Thank you for your comment Peter.

There is no more confusion about time-step arrangement now, it is clear.

Please simply imagine a system including a shaft, 2 bearings located both ends of the shaft and two disks on this system.

the unbalance mass is located at the top of the first disc and at the bottom for second disk if we think shaft axis is neutral axis.

So I called it phase angle which is 180 degree for this system.

I found two different ways modelling this problem :

- First way is : Use combin 214 elements which allow us modelling direct and cross-coupling stiffness of bearings.However if ? use this method the mesh will not

rotate and no unbalance load will be produced by masses I guess.Hence, I should calculate all unbalance loads before analysis and create table as example  I

sent.

- Second way is : I will use MPC184 joints element for modelling of bearings.In this case mesh will rotate and probably masses will produce unbalance loads.But, ? am not sure is there any joint element in which ? can model cross-coupling stiffness?.

if you have experience Peter , can you guide me how should ? progress.

Thank you,

Ugur