Page 139 - Intro Predictive Maintenance

130 An Introduction to Predictive Maintenance

quency would be indicated as 4X, or four times the running speed. In addition, because
some malfunctions tend to occur at specific frequencies, it helps to segregate certain
classes of malfunctions from others.

Note, however, that the frequency/malfunction relationship is not mutually exclusive,
and a specific mechanical problem cannot definitely be attributed to a unique fre-
quency. Although frequency is a very important piece of information with regard to
isolating machinery malfunctions, it is only one part of the total picture. It is neces-
sary to evaluate all data before arriving at a conclusion.

Amplitude
Amplitude refers to the maximum value of a motion or vibration. This value can be
represented in terms of displacement (mils), velocity (inches per second), or acceler-
ation (inches per second squared), each of which is discussed in more detail in the
Maximum Vibration Measurement section that follows.

Amplitude can be measured as the sum of all the forces causing vibrations within a
piece of machinery (broadband), as discrete measurements for the individual forces
(component), or for individual user-selected forces (narrowband). Broadband, com-
ponent, and narrowband are discussed in the Measurement Classifications section that
follows. Also discussed in this section are the common curve elements: peak-to-peak,
zero-to-peak, and root-mean-square.

Maximum Vibration Measurement. The maximum value of a vibration, or amplitude,
is expressed as displacement, velocity, or acceleration. Most of the microprocessor-
based, frequency-domain vibration systems will convert the acquired data to the desired
form. Because industrial vibration-severity standards are typically expressed in one of
these terms, it is necessary to have a clear understanding of their relationship.

Displacement. Displacement is the actual change in distance or position of an object
relative to a reference point and is usually expressed in units of mils, 0.001 inch. For
example, displacement is the actual radial or axial movement of the shaft in relation
to the normal centerline, usually using the machine housing as the stationary refer-
ence. Vibration data, such as shaft displacement measurements acquired using a prox-
imity probe or displacement transducer, should always be expressed in terms of mils,
peak-to-peak.

Velocity. Velocity is defined as the time rate of change of displacement (i.e., the first
dX .
derivative, or X) and is usually expressed as inches per second (ips). In simple
dt
terms, velocity is a description of how fast a vibration component is moving rather
than how far, which is described by displacement.

Used in conjunction with zero-to-peak (PK) terms, velocity is the best representation
of the true energy generated by a machine when relative or bearing cap-data are used.