Page 355 - Intro Predictive Maintenance

346 An Introduction to Predictive Maintenance

above 1,000Hz) should have an absolute fault of 3.0 inches per second, g’s-Peak
(acceleration).

Rolling-element bearings, based on factor recommendations, have an absolute fault
limit of 0.01ips-Peak. Sleeve or fluid-film bearings should be watched closely. If
the fractional components that identify oil whip or whirl are present at any level, the
bearing is subject to damage and the problem should be corrected. Nonmechanical
equipment and systems will normally have an absolute fault limit that specifies the
maximum recommended level for continued operation. Equipment or systems vendors
can usually provide this information.

15.5.5 Selecting Transducers
The type of transducers and data acquisition techniques that you will use for the
program is the final critical factor that can determine the success or failure of your
program. Their accuracy, proper application, and mounting will determine whether
valid data will be collected.

The optimum predictive maintenance program developed in earlier chapters is pre-
dicated on vibration analysis as the principle technique for the program. It is also
the most sensitive to problems created by using the wrong transducer or mounting
technique.

Three basic types of vibration transducers can be used to monitor the mechanical con-
dition of plant machinery: displacement probe, velocity transducer, and accelerome-
ters. Each has specific applications and limitations within the plant.

Displacement Probes
Displacement, or eddy-current, probes are designed to measure the actual movement
(i.e., displacement) of a machine’s shaft relative to the probe. Therefore, the dis-
placement probe must be rigidly mounted to a stationary structure to gain accurate,
repeatable data.

Permanently mounted displacement probes will provide the most accurate data on
machines with a low—relative to the casing and support structure—rotor weight. Tur-
bines, large process compressors, and other plant equipment should have displace-
ment transducers permanently mounted at key measurement locations to acquire data
for the program.

The useful frequency range for displacement probes is from 10 to 1,000Hz or 600 to
60,000rpm. Frequency components below or above this range will be distorted and
therefore 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,