Interference Diagrams: When Campbell Diagrams Aren't Enough
by: Eric Olson, Steve Hogg, Maki Onari, and William Marscher
May 2006

The intent of this article is to help readers understand more about Interference Diagrams, how they are used, and when an equipment user/buyer should specify the use of an Interference Diagram in assessing the potential for resonance problems in a centrifugal impeller or axial bladed disk. This article is of relevance to pumps, compressors, and turbines that are undergoing a design audit, or that have experienced unexpected fatigue failures of the blades, shrouds, hub O.D., or disk rim.

Interference Diagrams can be prepared using information from either numerical analysis or from modal testing. Sometimes the authors combine both the analysis and test techniques so that the effect of fit-up interferences and/ or casting tolerances can be cross-checked by the test, and the analytical model can be calibrated by these real-life effects before being used in downstream calculations, for example, of forced response stresses for plotting on fatigue Goodman Diagrams. The numerical results are produced using Finite Element Analysis (FEA) results, and are used to help identify potential fatigue and cracking problems in bladed disk elements such as turbine rotors and compressor impellers. One outcome of the typical FEA analysis is the identification of natural frequencies in resonance with machine running speeds multiplied by select excitation sources such as vane pass frequency, blade pass frequency, etc., producing so-called "excitation orders" EO (an EO of 5 is a frequency equal to 5 times running speed). This information alone, communicated on a Campbell diagram of frequency vs. running speed, is not enough to make design decisions. Typically, a Campbell diagram that includes all of a bladed disk's vibration modes indicates more potential problems than actually exist, as will be explained below. Interference Diagrams help the experienced engineer sort out which modes are most likely to actually cause problems...

This technical article appeared in two magazines.

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