4.2. Defining Rotating Forces

 

4.2.1. Rotating Forces in Transient Analysis

In a transient analysis, the rotating forces are defined using table array parameters to specify the amplitude of the forces in each direction, at each time step. The analysis example provided in Example: Transient Response of a Startup shows how this is accomplished for mass unbalance.

The equations for the mass unbalance forces can be found in Mass Unbalance Transient Forces in the Theory Reference.

 

4.2.2. Rotating Forces in Harmonic Analysis

 

4.2.2.1. Nodal Force

Because complex notations are used in a harmonic analysis, a rotating load is defined with both a real component and an imaginary component. For example, to apply a rotating force F0 in the (YZ) plane, rotating in the counterclockwise direction (Y to Z), the force components are:

F0 = 1.e+6                  ! sample force component value
INODE = node(0.1,0,0)       ! sample node number
F,INODE,fy,F0               ! real fy component at node INODE
F,INODE,fz,,-F0             ! imaginary fz component at node INODE

If a phase needs to be included, the expressions of the forces components are as shown below:

Force Real (VALUE) Imaginary (VALUE2)
FYF0cosα-F0sinα
FZ-F0sinα -F0cosα

where:

F0 is the amplitude of the force. For unbalance, the amplitude is equal to the mass times the distance of the mass unbalance to the spin axis.

α is the phase of the force, needed only when several such forces, each with a different relative phase, are defined.

For more information, see Applying Loads and Obtaining the Solution in the Structural Analysis Guide.

If the rotating harmonic load is synchronous or asynchronous with the rotational velocity, use the SYNCHRO command. In this case, the amplitude of the force generated by unbalance represents the mass times the radius of the eccentric mass. The spin squared factor is introduced automatically. See Example: Unbalance Harmonic Analysis for more information about harmonic analysis with rotating forces.

Note:  When defining an unbalance force in an analysis that uses component-based rotation (CMOMEGA), make sure you apply the force at a node attached to elements belonging to the rotating component.

 

4.2.2.2. Distributed Forces Coming From Solid or Shell Model Unbalance

If a solid or shell model is not exactly axisymmetric, it induces unbalance forces that may need to be taken into account in your harmonic analysis. To determine and apply such forces, do the following:

  • Create nodes on the rotational velocity axis (center line) of the rotor model.

  • Couple the model to the center nodes that define a rigid region (CERIG command).

  • If the rotational velocity is along X, constrain UY, UZ, and ROTX degrees of freedom at the center nodes. Do not constrain the bearings locations.

  • Perform a static analysis with unit rotational velocity (OMEGX=1.0 on the OMEGA command).

  • In the post-processor (/POST1), retrieve and store the reaction forces at the center nodes (Entity=Node and Item1=RF on the *GET command).

  • You can perform your harmonic analysis after you have:

    • Removed the constraints at the center nodes.

    • Added your constraints at the bearings locations.

    • Applied the stored reaction forces at the center nodes. Ensure that you take the opposite sign and define complex rotating forces.