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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 20, Using Shaker to Determine Torsional Resonance

1. Mount shaker with its axis of rotation perpendicular to the axis of rotation of the rotor to be vibrated torsionally.

2. The shaker should be placed at the outer periphery of the rotor or rotor case in such a way as to generate reversible motion at the outer edge, tangent to the rotor or case outside diameter.

3. For most relatively small rotors, such as rotating pumps, the shaker cannot be mounted on the pump impeller since the shaker's weight will alter the torsional natural frequency too much. It is recommended that the shaker be mounted instead on the outer case. The resulting torsional vibration will, of course, be much greater on the case than that transferred to the rotor.

4. Place vibration pickup probe at outer periphery of the machine's rotor, tangent to its outside diameter.

5. Use the same principles for determining torsional resonant frequencies as for bending resonance tests. For example, phase will change as the shaker rotor's speed changes as the rotor goes through its torsional resonance. Phase will shift in the same manner, as if it were going through a bending resonance.

6. Often it is difficult to resonate a rotor torsionally without resonating the rotor in bending. This difficulty can be overcome in most instances by determining the trouble torsional vibration frequency such as rpm x number of vanes, blades, or gear teeth, etc. Quickly bring the shaker rpm up to that frequency. Adjust the shaker speed slightly lower and slightly upward. If the phase shifts with slight change in speed, the shaker is running at the suspect part's resonant speed.



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