Axial mode resonance		Radial mode resonance		Rocking mode resonance

Resonance frequency is determined by measuring the energetic coupling between the shaft and housing. There are 2 methods typically used:

(a) Power spectrum method. A source of white noise or swept sine wave with flat power spectrum over .5 to 20KHz (typically) is applied to the shaft of the assembly. The motion of the housing is monitored through a transducer (accelerometer, magnetic, LDV, or other) and fed into a dynamic signal analyzer. The FFT of this signal gives a power spectrum that will show a peak at resonant frequency(ies). A ping hammer can also be used to provide the stimulus to the assembly. Using a transfer mode where a second transducer measures the actual input motion of the shaft and the difference between this and output is analyzed can reduce errors from fixture response.

(b) Phase shift method. The shaft is excited with a variable frequency signal source and the phase-shift between this signal and the motion of the housing is monitored. The phase-shift is 180° at resonance.

It can be difficult to correlate resonance measurements for an assembly since the method used, fixturing and damping mass will affect results. For more information on this subject, contact Dynaroll engineers.

Radial resonance test (Courtesy Measurement Resarch, Inc.)