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Stiffening Effect of Motor Core Webs for Torsional Rotordynamics

[+] Author Affiliations
Chris D. Kulhanek, Stephen M. James, Justin R. Hollingsworth

Southwest Research Institute, San Antonio, TX

Paper No. GT2012-69967, pp. 721-729; 9 pages
  • ASME Turbo Expo 2012: Turbine Technical Conference and Exposition
  • Volume 7: Structures and Dynamics, Parts A and B
  • Copenhagen, Denmark, June 11–15, 2012
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-4473-1
  • Copyright © 2012 by ASME


Longitudinal webs or spider bars are often placed mid-span of a motor shaft and are primarily used to support the windings or rotor laminations while allowing sufficient space for cooling air flow. When subject to a torque, the radial webs experience a loading configuration that includes bending and torsion while the base shaft experiences pure torsion. A webbed cross-section has a higher torsional stiffness as compared to the torsional stiffness of just the circular portion of the shaft section. This influences the torsional critical speeds and can become important for torsional systems that operate with minimal separation margins from resonance frequencies. This work presents various approaches to calculate the stiffening effect. The approaches include empirical and analytical methods described by Nestorides and API 684. An additional method uses a solid model of the motor core and a commercial Finite Element Analysis (FEA) solver to predict steady-state deflection under a torsional load. This in turn allows for a torsional stiffness calculation. Motor core configurations with various shaft diameters, number of spider bars, and spider bar geometries are considered. Good agreement is shown between the FEA results and the Griffith and Taylor method described by Nestorides. The other methods considered, including the calculation method described in API 684, show generally poor agreement with the FEA torsional stiffness results for the webbed shaft geometries studied.

Copyright © 2012 by ASME



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