CSI RBM University 2001 – ODS & Modal
Figure 25 Second bending mode of the Pump1 structure showing the drive shaft vibrating in-phase with the support pipe at 29.2 Hz.
Since the natural frequency at 29 Hertz is at the upper end of the 2x operating speed range, the structural engineer recommended increasing the machine stiffness. Increasing stiffness results in a higher natural frequency. To accomplish the change, several potential solutions were evaluated using the FEA software including:
Shortening the length of the pump tube Increasing the wall thickness of the tube A different motor Improving the attachment to the floor Bracing the tube to the building wall
The only option that was feasible was to reduce the length of the pump tube. The customer agreed that changing the tube length was an acceptable solution. Tube length reductions of 48, 62 and 72 inches were evaluated. All three models produced acceptable results. Table 5 shows the change at each natural frequency based on pipe length reductions. The stiffness of the support pipe (Tp) was considered by treating the support pipe thickness as a design variable. It was found that this parameter has little effect on the modes near 29 Hz.
© Copyright 2001, Computational Systems Incorporated. All rights reserved.
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