Symmetry on rotating part 1

[dweier]

I am attempting to analyze a rotating part using a centrifugal force. The part is complicated and I am trying to analyze a pie section of the part. I am having difficulties with boundary conditions. If I place a 0 translation on the symetric faces the ends of the parts distort out further from the axis than the center. The entire part should distort the same amount from the axis. What boundary conditions need to be applied to properly simulate the part in a pie section?

 

[cbrn]

Hi,
bad news: you can not do cylindrical-symmetric analyses with CosmosWorks.
In other words, in CW symmetry is enforced only by setting to 0 the appropriate displacement, which is not completely correct for cylindrical-symmetry. One thing you can do however is to use a 90° sector, so that sector-cyclic symmetries do are on perpendicular planes, so that you can set to 0 the corresponding "out-of-plane" displacement for the sector's endfaces.
Please note that in order to do this, the part AND the loads MUST have a cyclic period of 90°, otherwise you will have to shift to 180° or, in the worst case, to the full 360° model.

Regards

 

[dweier]

I suspected that the cylindrical-symmetric analysis was not possible.
I do not quite understand the 90 degree sectoring. I created a 90 degree sector restrained it to the planes for 0 displacement, but the ends of the 90 degree sector are still considered free and displace more than the center of the part (@45 degrees) I need to maintain the hoop stresses in the part.

 

[cbrn]

Hi,
you're right, 90° sector is still dangerous with CW. The two endfaces of the sector will behave in the same way only if the load is also rigorously 90°-symmetric, and it's true that you may have discrepancies in the displacements along the sector if the load is not rigorously constant with theta. Be careful: the minimal unsymmetry in the load will invalidate this fictitious cylindrical-symmetry. This is also true for 180°-symm, however.
But the 90°-method is not wrong "per se": I had two or three cases where this approach was perfectly acceptable (for example, when the part is loaded by internal / external hydrostatic pressure, which is constant with theta and can vary, if needed, only along the axis of the sector).

Regards

 

[ERT]

Hi all,

I disagree. CW can accurately analyse cylindrical-symmetric models in pie sectors, if it is restrained properly. The key is to find the smallest sector angle that represents a repeating pattern of geometry, restraints and loading.

The key is to use the "Symmetry Restraint" on all the cut faces. This restraint is exactly the same as the "On Flat Face" restraint with normal displacement set to zero. This is important to get past the orthogonal sense of symmetry.

For example, in the theoretical case of a wheel with three equally spaced spokes, subject only to centrifugal loading, this model can be analysed with sector angles of 180°, 120°, and even 60°.

Regards,

Ed

http://www.akeng.com

 

[cbrn]

Hi,

sorry; ERI, you're right in what you describe. But I suspect CosmosWorks to behave exactly as you write: "it is the same as 'on flat face' with normal displacement set to zero". This is correct ONLY for loading patterns that are not dependent on theta (the angular coordinate). And, correctly, you make the example of the wheel under centrifugal force.
Otherwise, a deformation displacement of the sector's delimitation faces will exist, and you MUST NOT force it to be zero, ONLY you must ensure that, vectorially, u(lower edge) = u(upper edge) in the cylindrical coordinate system.

Regards