Since single-row radial ball bearings have "small angular clearance", a shaft supported by them has a non-linear and non-symmetrical spring characteristic provided the bearing center lines of both bearing pedestals are not in alignment. In a small angular clearance, a shaft is freely supported, and the inclination angle of the deflection curve of the end of the shaft grows larger than a small angular clearance, then the shaft is not freely supported. When bearing center lines of both pedestals are not in alignment, the equilibrium position of a shaft is not located at the middle position of a small angular clearance, then the shaft has a non-symmetrical non-linearity. Therefore, sub-harmonic oscillation of order 1/2 can occur. In fact, the whirl of shaft with the mode of sub-harmonic oscillation of 1/2 appears at a rotating speed higher than the major critical speed.