Measuring Shaft Runout on Electric Motors

Shaft Runout Tolerances & Standards

What is meant by mechanical runout? It is the measure of a shaft's deviation from an absolute uniform radius as the circumference of the shaft is traversed. Mechanical runout is frequently the result of machining processes such as lobing, tool chatter, and/or improper feed rate and speed of the cutting tools; dents from handling; patches of rust; bowed rotor; and defective or worn bearings in the supports of the machine or lathe.

A proximity probe will measure both mechanical and electrical runout in which case the whole measurement is known as TIR or Total Indicated Runout. In order to bring TIR within acceptable tolerances, mechanical runout must be mitigated prior to addressing electrical runout. One must make a precise measurement of the physical profile of the shaft in order to ascertain mechanical runout. Mechanical runout should be measured with an electronic dial indicator with digital readout or a LVDT (Linear Variable Differential Transformer) for accuracy. Either of these tools is capable of resolving increments as small or smaller than 0.1mil, rendering the more commonly owned mechanical dial indicator impractical.

It is common practice when performing machinery diagnostics is to subtract a known runout signal from the overall vibration waveform to obtain a "runout-free" waveform. Known as compensation, it is a way of dealing with both mechanical and electrical runout. Compensation can be valuable as the runout signal can generally be validated and updated as needed as part of the diagnostic process but is not recommended as part of a permanent monitoring system since runout signals can change over time and skew true results. These changes are most often due to surface scratches incurred during operation or maintenance, and/or changes in the amount and distribution of shaft magnetism. Therefore, compensation could be used for diagnostics only and not for ongoing machinery protection.

Standards for new and refurbished motors are set by the American Petroleum Institute (API) and are frequently cited. Generally, API specifications require that the shaft be supported in v-blocks; the probe be perpendicular to one face of the v-block; and that runout be measured in terms of peak-to-peak probe output. For example, API Standard 687 (Repair of Special Purpose Rotors) provides a very detailed description of how to measure runout. Standard 612 (Petroleum, Petrochemical and Natural Gas Industries - Steam Turbines - Special-purpose Applications) deals with TIR requirements of mechanical drive steam turbines, requiring the TIR to be 0.25 mil pp or 25% of allowable vibration, whichever is greater. API 617 (Axial and Centrifugal Compressors and Expander-compressors for Petroleum, Chemical and Gas Industry Services) has identical runout requirements dealing specifically with process centrifugal and axial compressors as well as turbo-expanders while API Standard 670 (Machinery Protection Systems) deals with the subject of using compensation in permanent monitoring systems.