Counterbored rotors for natural frequency variation

Abstract

A motor having improved resistant to vibration has a shaft, a pair of bearings engaging the shaft, a set of pressed-on laminations stacked axially on the shaft between the pair of bearings, and a stator. Only a portion of the set of laminations actually engage the shaft, preferably because the central hole of some of the laminations is enlarged slightly by counterboring. Alternately, the diameter of the shaft or the length of splines on the shaft that engage the laminations may be adjusted so that not all of the laminations engage the shaft. A method of tuning a rotor to reduce unwanted noise and vibration from a motor includes the steps of selecting a first motor, measuring the natural frequency of the rotor system of the motor, selecting an increment to the natural frequency, and adjusting the construction of a second motor in accordance with the direction of the desired increment. If the desired increment is downward in frequency, the length of the region of engagement of the laminations to the shaft is reduced. If the desired increment is upward in frequency, the length of the region of engagement of the laminations to the shaft is increased. The adjustment of the length of the region of engagement is accomplished by a very slight counterboring of the central hole of some number of laminations, the actual number depending upon whether the first motor included such counterbored laminations and whether the increment is up or down. Alternately, the adjustment of the length of the region of engagement may be accomplished by adjusting the diameter of one or more portions of the shaft. Otherwise, the construction of the first and the second motors is the same, so that the motors have essentially the same size and weight, but different resonant frequencies.