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Our customer is one of the leading providers of top-quality industrial fans. They are also engaged with installation of blower fans at site involving assembly of components like impeller, casing, shroud, motor, bearing and couplings. One critical aspect related to such installations relates to structural integrity of torque transmitting shaft connecting impeller to motor. The shaft is expected to bear not only the static loads corresponding to torque and impeller weight but also the dynamic loads due to impeller imbalance. The approval for installation required our customer to submit an analysis report validating the structural strength of shaft.

Zeus Numerix employed finite element method to analyze shaft behaviour under torsional and lateral loads. The shaft was modeled as one dimensional beam element whereas impeller was modeled as bulk mass element with an imbalance as per ISO 1940 Grade 2.5. Based on the dynamic FE analysis, torsional frequency, lateral frequency and critical speed were evaluated. Harmonic analysis was performed to get reaction forces on the bearing.

Left: Typical arrangement of blower fan shaft. Right: Reaction force peak at critical speed.

Customer was delivered with a report with torsional / lateral critical speed analysis, vibration analysis and bearing reaction forces. It included Campbell diagram for identifying critical speed. The ratio of a critical speed to operating speed was more than 1.5. The results indicated in the report obtained approval for the customer to go ahead with the installation.