a fan is modified by replacing its rigid rotor shaft by a flexible shaft so the normal operating speed range is above the first transverse critical speed . during run - up , the assembly of the fan impeller 12 and the drive shaft 11 is accelerated rapidly through the first critical speed at which the peak transverse vibrations could be encountered , and during run - down the shaft 11 is braked to minimize the same exposure to transverse vibrations . the use of a more flexible fan drive shaft of smaller diameter , allows a higher fan operating speed to be used without risk of detrimental shaft aerodynamic chokage effects , and the utilization of the existing fan foundation .