Shaft dynamics in the rotors of large centrifugal fans

Abstract

1. A realistic way of improving the viability of the rotor in a centrifugal fan is to reduce the amount of metal, particularly in the shaft, whose mass exceeds half the mass of the rotor. The critical speed of the rotor should not thereby be reduced. 2. Hollow and other types of light two-bearing shaft have natural frequencies 15–20% lower for the same diameters in relation to solid shafts (for the usual masses of the fans). 3. A light single-bearing shaft under the same conditions has a natural frequency virtually equal to that of a solid shaft. 4. If the outside diameter of a hollow shaft is 6–8% larger than that of a solid one and the wall thickness is 8% of the outside diameter, the natural frequency of the hollow shaft should be not lower than that of the solid one. 5. The mass of the rotor as a whole with a single-bearing hollow shaft is reduced by 35–40%, while that of a rotor with a two-bearing hollow shaft having the same natural frequency as a solid one is reduced by up to 25%, which substantially improves the reliability and working life of the bearings. 6. One obtains reliable and viable hollow shafts with welded bearing rings if the technological requirements are met. 7. In calculating the critical speed of a two-bearing shaft in a centrifugal fan, there is only a very small correction from the gyroscopic effect, which may be taken as a certain safety margin only for rotors where the fan proper is off center. This correction is 9–11% for a single-bearing shaft.