Page 174 - An Introduction To Predictive Maintenance

164 An Introduction to Predictive Maintenance

Table 7–2 Vibration-Severity Standards
(Inches/Second-Peak)
Machine Classes
Condition I II III IV
Good Operating Condition 0.028 0.042 0.100 0.156
Alert Limit 0.010 0.156 0.255 0.396
Alarm Limit 0.156 0.396 0.396 0.622
Absolute Fault Limit 0.260 0.400 0.620 1.000
* Applicable to a machine with running speed between 600 to 12,000rpm.
Narrowband setting: 0.3¥ to 3.0¥ running speed.
Machine Class Descriptions:
Class I Small machine-trains or individual components integrally connected with the complete machine
in its normal operating condition (i.e., drivers up to 20 horsepower).
Class II Medium-sized machines (i.e., 20- to 100-horsepower drivers and 400-horsepower drivers on
special foundations.
Class III Large prime movers (i.e., drivers greater than 100 horsepower) mounted on heavy, rigid
foundations.
Class IV Large prime movers (i.e., drivers greater than 100 horsepower) mounted on relatively soft, light-
weight structures.
Source: Derived by Integrated Systems, Inc. from ISO Standard #2372.

with true running speeds between 600 and 12,000rpm. The values from the table
include all vibration energy between a lower limit of 0.3¥ true running speed and an
upper limit of 3.0¥. For example, an 1,800-rpm machine would have a filtered nar-
rowband between 540 (1,800 ¥ 0.3) and 5,400rpm (1,800 ¥ 3.0). A 3,600-rpm machine
would have a filtered narrowband between 1,080 (3,600 ¥ 0.3) and 10,800rpm (3,600
¥ 3.0).

7.8.3 Signature Analysis
The phrase “full Fast Fourier Transform (FFT) signature” is usually applied to the
vibration spectrum that uniquely identifies a machine, component, system, or subsys-
tem at a specific operating condition and time. It provides specific data on every fre-
quency component within the overall frequency range of a machine-train. The typical
frequency range can be from 0.1 to 30,000Hz.

In microprocessor systems, the FFT signature is formed by breaking down the total
frequency spectrum into unique components, or peaks. Each line or peak represents
a specific frequency component that, in turn, represents one or more mechanical com-
ponents within the machine-train. Typical microprocessor-based predictive mainte-
nance systems can provide signature resolutions of at least 400 lines, and many
provide 12,800 lines or more.

Full-signature spectra are an important analysis tool, but they require a tremendous
amount of microprocessor memory. It is impractical to collect full, high-resolution