www.ansys.comANSYS Advantage • Volume II, Issue 4, 200836

TURBOMACHINERY

What’s Shakin’?The combination of 3-D structural dynamics, ANSYS Workbenchand classical rotordynamics modeling techniques helps solverotating machinery vibration problems.By Josh Lorenz, Senior Principal Mechanical Engineer, Kato Engineering Inc., Minnesota, U.S.A.

In the electric power generationindustry, the ability to design and pro-duce reliable and long-lasting rotatingmachines is, in part, dependent on theability to control machinery vibration.Due to the dynamics of rotatingmachinery parts, mechanical vibrationcan’t be eliminated completely, butdesigning machines that meet industrystandards for acceptable vibration levels is a crucial part of the productdesign process. Kato Engineering hasbeen in the business of designing suchmachines since the 1920s.

Kato Engineering uses 3-D struc-tural modeling tools from ANSYS toincrease their ability to simulate andpredict machine vibration. Classicrotordynamics modeling techniquesinvolving spreadsheet-style programswith axisymmetric beam elementsolutions have been around for manyyears and are very useful tools. However, there are times when the

complex 3-D structural details of arotating machine cannot be cast

into the form of an axisymmetricbeam element model with

sufficient accuracy. Forcritical applications andnew designs in whichvibration prediction isof the utmost impor-tance, simulation using

software from ANSYSallows Kato Engineering to

bridge the gap between rotor-dynamics spreadsheet modeling

techniques and real-world vibrationbehavior.

Recently, Kato engineers inheriteda relatively large common shaftmotor–generator design with unfavor-able vibration performance. Their taskwas to modify the design, making itmore reliable and easier to produce.This included improving the vibrationcharacteristics of the machine.

In order to tackle this type ofproblem using structural softwaretools from ANSYS, the engineers atKato performed frequency sweepharmonic response analyses using a mixed-element modeling tech-nique. This method combines the efficiency of beam element models— for the rotating portion of themachine — with more structurallycomplex 3-D shell and solid elements that represent the sur-rounding stationary structures. Thesesurrounding structures include themachine frame, mounting structure,foundation and other components.

The simulation results providedstructural vibration displacements andphase angles at each node of the

model for all of the frequencies considered. Using the time–historypost-processor, the team at Kato tookdata from nodes in the model wheresensors would be installed during real-world vibration tests of the equipmentand examined the predicted magni-tude of the vibration response versusfrequency. Later, the engineering teamcompared the simulation results withmachine vibration Bode plots obtainedduring vibration testing of the produc-tion equipment.

An important portion of the rotor-dynamic analyses focused on thebearing connections between rotatingand stationary structures. Through the use of ANSYS Parametric Design Language (APDL) and MATRIX27 elements in software from ANSYS, theengineering team allowed for the inclu-sion of complex bearing stiffness anddamping phenomena in the simulationmodel. They were able to include thebearing stiffness and damping charac-teristics, as a function of rotationalspeed as well as all of the cross-coupled stiffness and damping termsthat are important in the simulation ofthe bearing behavior. While thesetypes of bearing behaviors have longbeen a part of spreadsheet-style rotor-dynamics programs, the Kato teamwas now able to efficiently incorporatethese behaviors into 3-D full-modelvibration simulations using the flexibleAPDL structure and data input–outputoptions. Speed-dependent bearingstiffness and damping coefficients —calculated using dedicated bearingperformance software — were curve-fit and programmed into the structuralmodel using an APDL routine.

A portion of the finite element mesh for theshaft of a motor–generator

Contour plot of vibration displacement for the whole assembly at a particular forcing frequency

ANSYS Advantage • Volume II, Issue 4, 2008

TURBOMACHINERY

www.ansys.com 37

The ANSYS Workbench framework has been a verypowerful tool for Kato Engineering with regard to simu-lation model construction time. Using the ANSYSWorkbench environment, the team was able to make thetransition more quickly from CAD geometry to complexfinite element meshes. Where 3-D structural model creation for a machine previously may have taken a week,it was now taking only a day or two with the enhancedgeometry and meshing capabilities of ANSYS Workbench.

For this particular application, the Kato engineering teamwas able to predict the steady-state vibration response ofthe redesigned machine to a reasonable degree of accuracyusing structural simulation tools. While the modeling tech-nique relied on the placement of rotor imbalance forces,which were somewhat nebulous in reality, the team was ableto get good correlation between simulation and test resultsusing reasonable assumptions for expected magnitudes andlocations of rotor imbalance. By iterating with the simulationmodel during the design phase, Kato engineers were able todetermine that, if they control the imbalance of the rotor to acertain degree and at certain locations, they can expect tomeet targets in terms of machine vibrations. The improvedsteady-state vibration performance of the redesignedmachine provided a significant boost for customer confidence in Kato Engineering’s capability to produce a reliable product. ■ Comparison of simulation and test data at one of the vibration

sensor locations, indicating trends

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