Capacitive and Eddy-Current Measurement Solutions for Condition Monitoring

For condition monitoring, shaft runout is a common measurement. Eddy-current and capacitive sensors offer useful non-contact measurement solutions with distinct advantages and disadvantages. While there are techniques for refining a shaft runout measurement to just one or a few of these components, the purpose of this article is to measure total runout with all of its contributing factors (except sensor errors).

The techniques described here are intended to reduce or eliminate the sensor’s contribution to the final result. When properly applied, non-contact eddy-current and capacitive sensor measurements of shaft runout will produce results with negligible sensor errors.

Radial shaft runout is defined as a measurement of radial displacement of the shaft surface as the shaft turns. For a round shaft, contributing factors to radial runout include shaft straightness, drive/shaft alignment, bearing stiffness, and increasing runout as the bearings wear. Radial shaft runout is generally used to indicate wear in the drive bearings. Figures 1 and 2 show radial runout.

Figure 1. Runout is the displacement of the surface of a rotating object. Non-round shafts will have significant runout by definition.

Figure 2. Radial runout is perpendicular to the axis of rotation.

Axial Shaft Runout

Axial shaft runout is determining the axial displacement of the shaft as it rotates. This measurement is taken at the shaft center on the rotary axis. Off-center measurements are known as “face runout” in which the flatness and squareness of the surface become contributing factors to the measurement. Axial shaft runout is primarily used for condition monitoring of the thrust bearing. Figure 3 shows an axial runout measurement.

Figure 3. Axial runout is measured at the center of rotation to prevent shaft end flatness/squareness errors from affecting the measurement.

Shaft Shape

It has been observed that non-round shapes have considerable runout. A hexagonal or oval shaft that is perfectly rotating will have considerable runout as the indicator responds to radial displacements of the shaft surface due to the shaft shape. In this article, the focus is that the shaft being measured is round.

Shaft Straightness

Radial runout is affected by shaft straightness. If the shaft is bent, runout measurements will be based on the location of the measurement along the length of the shaft and the location and severity of the bend. If a shaft is fixed at both ends (e.g. between the drive and a gear box) the maximum runout will tend to be near the center. Figure 4 shows how shaft straightness impacts runout measurement.

Figure 4. Shaft straightness affects runout measurement.

Synchronous and Asynchronous Shaft Runout Components

Certain runout components such as shaft out-of-roundness or a tilt in the drive will repeat at certain angular locations of the rotation, which are synchronous error motions. Other shaft runout components such as bearing frequencies are cyclical but do not repeat at the same angular locations and are called asynchronous error motions.

Total Shaft Runout

The only value of concern in condition monitoring is a single value indicating total shaft runout. This is normally a peak or average of multiple TIR readings over a period of time and multiple rotations.

As other components and bearings wear, the total shaft runout will increase. In condition monitoring, a threshold value is set above which the system is shutdown and repair or rebuild is commenced.

Runout Measurements with Noncontact Sensors

Measuring shaft runout while in operation requires a non-contact sensor. The types of sensors best suited to this measurement are capacitive displacement sensors and eddy-current displacement sensors

Capacitive or Eddy Current

Capacitive displacement sensors offer high precision; they work equally well with all conductive materials; they work well with small diameter shafts. But they require a clean environment.

Eddy-current displacement sensors work in wet, dirty environments and can be mounted further away from the shaft, but they must be calibrated to a specific material, don’t work as well with smaller shafts (< 8 X Probe Diameter), and are more “noisy” when used with magnetic steel shafts because of “electrical runout”.

Mounting the Probe

These non-contact sensors include a probe connected via a cable to electronics that drive the probe and provide an output voltage proportional to the changes in distance between probe and the shaft. The probe is mounted at a distance from the shaft approximately at the center of its measurement range.

Deriving Total Shaft Runout

The shaft runout measurements from the non-contact sensor track the instantaneous displacements in real-time as the shaft rotates. This output must be conditioned to derive a single “total runout” measurement.

The runout value can be a type of average value or a peak value. The specific method for creating a total runout value will depend on the application. In this type of condition monitoring system, the units of measurement are not critical; whatever the units, the establishing of baseline and threshold values is the critical piece of the measurement. Figures 5 and 6 show how the runout can be measured.

Figure 5. “Total Runout” can be measured with TIR (peak-to-peak) captures of the runout signal.

Figure 6. Changing “total runout” can be measured with Tracking TIR option of the MM190 Module.

Average Values

The output values can be averaged over time by using some type of AC voltmeter. These are available as discrete instruments or may be available in support software for a data acquisition system. It is important to consider the meter’s ability to measure at the rotational frequency of the shaft.

Peak Values

Peaks of output values can be captured and the system can report the difference between the maximum and minimum peaks. This is a TIR (total indicator reading) measurement. Systems that capture these peaks have to be periodically reset to keep the value current should it decrease.

Unique Considerations for Eddy-Current (Inductive) Measurements of Shaft Runout

Eddy-current sensors are calibrated for a unique material. To maintain precision, the sensors must be used with that specific material. Eddy-Current sensors are normally calibrated to a flat target. Shaft diameter should be 8-10 times larger than the eddy-current probe diameter to provide a sufficiently flat target for accurate measurements.

Electrical Runout

Eddy-current sensors read “electrical runout” errors from magnetic steel materials; capacitive sensors do not. Magnetic materials have a property called electrical runout. Small localized differences in magnetic properties within the material affect the interaction with eddy-current sensor magnetic fields. The electrical runout is usually less than 75 µm (0.003 inches), which is often only a fraction of the measurement range of the eddy-current shaft runout sensor.

Conclusion

The measurement of shaft runout is a useful and common measurement, especially for condition monitoring. Baseline runout numbers and thresholds for operator intervention are set using a single sensor and a method to derive a single, total runout value. Capacitive and eddy-current sensors both provide excellent solutions depending on the shaft runout measurement specifics and the environmental condition of the application.

This information has been sourced, reviewed, and adapted from materials provided by Lion Precision.

For more information on this source, please visit Lion Precision.

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