Page 156 - An Introduction To Predictive Maintenance

146 An Introduction to Predictive Maintenance

A 1
for . This means that there are two solutions for this example, which are of the form
A 2
A 1 sin(wt) and A 2 sin(wt). As with many such problems, the final answer is the super-
position of the two solutions with the final amplitudes and frequencies determined by
the boundary conditions.

Many Degrees of Freedom
When the number of degrees of freedom becomes greater than two, no critical new
parameters enter into the problem. The dynamics of all machines can be understood
by following the rules and guidelines established in the one- and two-degree(s)-of-
freedom equations. There are as many natural frequencies and modes of motion as
there are degrees of freedom.

7.6 VIBRATION DATA TYPES AND FORMATS
There are several options regarding the types of vibration data that can be gathered
for machine-trains and systems and the formats in which the data can be collected.
Selection of type and format depends on the specific application. There are two major
data-type classifications: time-domain and frequency-domain. Each of these can be
further divided into steady-state and dynamic data formats. In turn, each of these two
formats can be further divided into single-channel and multichannel.

7.6.1 Data Types
Vibration profiles can be acquired and displayed in one of two data types: time-domain
or frequency-domain.

Time-Domain
Most of the early vibration analysis was carried out using analog equipment, which
necessitated the use of time-domain data, because it was difficult to convert time-
domain data to frequency-domain data. Therefore, frequency-domain capability was
not available until microprocessor-based analyzers incorporated a straightforward
method (i.e., Fast Fourier Transform, FFT) of transforming the time-domain spectrum
into its frequency components.

Actual time-domain vibration signatures are commonly referred to as time traces or
time plots (see Figure 7–17). Theoretical vibration data are generally referred to as
waveforms (see Figure 7–18).

Time-domain data are presented with amplitude as the vertical axis and elapsed
time as the horizontal axis. Time-domain profiles are the sum of all vibration com-
ponents (i.e., frequencies, impacts, and other transients) that are present in the
machine-train and its installed system. Time traces include all frequency components,