Page 356 - Intro Predictive Maintenance

Establishing a Predictive Maintenance Program 347

will average $1,000. If each machine in your program requires 10 measurements, the
cost per machine will be about $10,000. Using displacement transducers for all plant
machinery will dramatically increase the initial cost of the program.

Displacement data are normally recorded in terms of mils or .001 inch, peak-to-peak.
This valve expresses the maximum deflection or displacement off the true centerline
of a machine’s shaft.

Velocity Transducers
Velocity transducers are electromechanical sensors designed to monitor casing or rel-
ative vibration. Unlike the displacement probe, velocity transducers measure the rate
of displacement, not actual movement. Velocity data are normally expressed in terms
of inches per second, peak (ips-peak) and are perhaps the best method of expressing
the energy created by machine vibration. Velocity transducers, like displacement
probes, have an effective frequency range of about 10 to 1,000Hz. They should not
be used to monitor frequencies below or above this range.

The major limitation of velocity transducers is their sensitivity to mechanical and
thermal damage. Normal plant use can cause a loss of calibration, and therefore a strict
recalibration program must be used to prevent distortion of data. Velocity transducers
should be recalibrated at least every six months. Even with periodic recalibration,
programs using velocity transducers are prone to bad or distorted data that results
from loss of calibration.

Accelerometers
Accelerometers use a piezoelectric crystal to convert mechanical energy into electri-
cal signals. Data acquired with this type of transducer are relative vibration, not actual
displacement, and are expressed in terms of g’s or inches per second. Acceleration is
perhaps the best method of determining the force created by machine vibration.
Accelerometers are susceptible to thermal damage. If sufficient heat is allowed to
radiate into the crystal, it can be damaged or destroyed; however, because the data
acquisition time using temporary mounting techniques is relatively short (less than 30
seconds), thermal damage is rare. Accelerometers do not require a recalibration
program to ensure accuracy.

The effective range of general-purpose accelerometers is from about 1 to 10,000Hz.
Ultrasonic accelerometers are available for frequencies up to 1MHz. Machine
data above 1,000Hz or 60,000rpm should be taken and analyzed in acceleration
or g’s.

Mounting Techniques
Predictive maintenance programs using vibration analysis must have accurate, repeat-
able data to determine the operating condition of plant machinery. In addition to the