Amplitude modulation phenomena of vibrations in belt-driven centrifugal fans

This paper investigates amplitude modulation (AM) of a relatively long-cycle vibration when excited by a double belt in a belt-driven centrifugal fan. Since the inner belt is stretched more than the outer belt due to the longitudinal difference in bending displacement of the overhung shaft by the pulley, tension and rotational speed of the inner belt are greater than for the outer belt. Two types of V-Belt excitation occur as a result of the small difference in rotating speeds of the belts, and cause amplitude modulation of the vibration during fan operation. On the other hand, the previous investigation confirmed that V-Belt excitation due to belt joins incorporates some higher harmonics. Superposition of amplitude modulation due to each higher harmonic was observed, and it was therefore concluded that measured data were consistent with calculated data based on the results of the previous investigation. Furthermore, it was shown that there is an amplitude modulation of the vibration in the tested fan, consisting of the amplitude modulation due to first, second, fourth and sixth order components corresponding to the rotational speed of the belt.