Rotors or shafts used in steam and gas turbines, turbo generators and compressors, or any large motor, have to work within set standards for runout to ensure efficient and safe operation.
Mechanical runout can be measured using a Dial Test Indicator (DTI) to establish the dimensional error as the shaft is rotated through 360º. Factors affecting this include the quality of the turning machine, the tooling being used, the grinding process, and material handling.
Electrical runout is the apparent displacement of a rotating shaft. This is measured with proximity probes (eddy current sensors). The unwanted shaft movement is caused by variations in the shaft materials electrical and magnetic properties. Factors affecting this will include the material composition, how the shaft is forged, heat treatment, grinding techniques, induced magnetism, stress, and material handling.
If excessive runout is not eliminated during the manufacturing process, the motor would have to be disassembled and the shaft reworked. This can be a time consuming and costly process and one the motor manufacturer would want to avoid.