Page 357 - Intro Predictive Maintenance

348 An Introduction to Predictive Maintenance

transducer, three factors will affect data quality: measurement point, orientation, and
compressive load.

Key measurement point locations and orientation to the machine’s shaft were selected
as part of the database setup to provide the best possible detection of incipient
machine-train problems. Deviation from the exact point or orientation will affect the
accuracy of acquired data. Therefore, it is important that every measurement through-
out the life of the program be acquired at exactly the same point and orientation. In
addition, the compressive load or downward force applied to the transducer should be
the same for each measurement. For accuracy of data, a direct mechanical link to the
machine’s casing or bearing cap is necessary. Slight deviations in this load will induce
errors in the amplitude of vibration and may create false frequency components that
have nothing to do with the machine.

The best method of ensuring that these three factors are the same each time is to hard-
mount vibration transducers to the selected measurement points. This technique will
guarantee accuracy and repeatability of acquired data, but it will also increase the
initial cost of the program. The average cost of installing a general-purpose accelerom-
eter will be about $300 per measurement point or $3,000 for a typical machine-train.

To eliminate the capital cost associated with permanently mounting transducers, a
well-designed quick-disconnect mounting can be used. This mounting technique
permanently mounts a quick-disconnect stud, with an average cost of less than $5, at
each measurement point location. A mating sleeve, built into a general-purpose accel-
erometer, is then used to acquire accurate, repeatable data. A well-designed quick-
disconnect mounting technique provides the same accuracy and repeatability as
the permanent mounting technique but at a much lower cost.
The third mounting technique that can be used is a magnetic mount. For general-
purpose use, below 1,000Hz, a transducer can be used in conjunction with a
magnetic base. Even though the transducer/magnet assembly will have a resonant
frequency that may provide some distortion to acquired data, this technique can be
used with marginal success. Because the magnet can be placed anywhere on the
machine, it will not guarantee that the exact location and orientation is maintained on
each measurement.

The final method used by some plants to acquire vibration data is handheld transduc-
ers. This approach is not recommended if any other method can be used. Handheld
transducers will not provide the accuracy and repeatability required to gain maximum
benefit from a predictive maintenance program. If this technique must be used,
extreme care should be exercised to ensure that the exact point, orientation, and
compressive load is used for every measurement point.

15.6 GETTING STARTED

The steps we have defined provide guidelines for establishing a predictive mainte-
nance database. The only steps remaining to get the program started are to establish