Page 162 - Intro Predictive Maintenance

Vibration Monitoring and Analysis 153

Figure 7–21 Displacement probe and signal conditioning system.

Three basic types of vibration transducers can be used for monitoring the mechanical
condition of plant machinery: displacement probes, velocity transducers, and
accelerometers. Each has limitations and specific applications for which its use is
appropriate.

Displacement Probes
Displacement, or eddy-current, probes are designed to measure the actual movement,
or displacement, of a machine’s shaft relative to the probe. Data are normally recorded
as peak-to-peak in mils, or thousandths of an inch. This value represents the maximum
deflection or displacement from the true centerline of a machine’s shaft. Such a device
must be rigidly mounted to a stationary structure to obtain accurate, repeatable data.
See Figure 7–21 for an illustration of a displacement probe and signal conditioning
system.

Permanently mounted displacement probes provide the most accurate data on
machines with a rotor weight that is low relative to the casing and support structure.
Turbines, large compressors, and other types of plant equipment should have dis-
placement transducers permanently mounted at key measurement locations.

The useful frequency range for displacement probes is from 10 to 1,000Hz, or 600 to
60,000rpm. Frequency components above or below this range are distorted and, there-
fore, unreliable for determining machine condition.

The major limitation with displacement or proximity probes is cost. The typical cost
for installing a single probe, including a power supply, signal conditioning, and so on,