Page 140 - Intro Predictive Maintenance

Vibration Monitoring and Analysis 131

[Note: Most vibration-monitoring programs rely on data acquired from machine
housing or bearing caps.] In most cases, peak velocity values are used with vibration
data between 0 and 1,000Hz. These data are acquired with microprocessor-based,
frequency-domain systems.

Acceleration. Acceleration is defined as the time rate of change of velocity (i.e.,
2
dX
¨
second derivative of displacement, 2 or X) and is expressed in units of inches per
dt
2
second squared (in/sec ). Vibration frequencies above 1,000Hz should always be
expressed as acceleration.
Acceleration is commonly expressed in terms of the gravitational constant, g, which
2
is 32.17ft/sec . In vibration-analysis applications, acceleration is typically expressed
in terms of g-RMS or g-PK. These are the best measures of the force generated by a
machine, a group of components, or one of its components.

Measurement Classifications. There are at least three classifications of amplitude
measurements used in vibration analysis: broadband, narrowband, and component.

Broadband or overall. The total energy of all vibration components generated by a
machine is reflected by broadband, or overall, amplitude measurements. The normal
convention for expressing the frequency range of broadband energy is a filtered range
between 10 to 10,000Hz, or 600 to 600,000cpm. Because most vibration-severity
charts are based on this filtered broadband, caution should be exercised to ensure that
collected data are consistent with the charts.

Narrowband. Narrowband amplitude measurements refer to those that result from
monitoring the energy generated by a user-selected group of vibration frequencies.
Generally, this amplitude represents the energy generated by a filtered band of vibra-
tion components, failure mode, or forcing functions. For example, the total energy
generated by flow instability can be captured using a filtered narrowband around the
vane or blade-passing frequency.

Component. The energy generated by a unique machine component, motion, or other
forcing function can yield its own amplitude measurement. For example, the energy
generated by the rotational speed of a shaft, gear set meshing, or similar machine com-
ponents produces discrete vibration components whose amplitude can be measured.

Common Elements of Curves. All vibration amplitude curves, which can represent
displacement, velocity, or acceleration, have common elements that can be used to
describe the function. These common elements are peak-to-peak, zero-to-peak, and
root-mean-square, each of which are illustrated in Figure 7–11.

Peak-to-peak. As illustrated in Figure 7–11, the peak-to-peak amplitude (2A, where
A is the zero-to-peak) reflects the total amplitude generated by a machine, a group of
components, or one of its components. This depends on whether the data gathered are