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Vibration Monitoring and Analysis 167
Axial Of, on, around, or along an axis (straight line
about which an object rotates) or center of
rotation.
Bearing cap The protective structure that covers bearings.
Boundary condition Mathematically defined as a requirement to
be met by a solution to a set of differential
equations on a specified set of values of the
independent variables.
Displacement The change in distance or position of an
object relative to a reference point, usually
measured in mils.
Dynamics, operating Deals with the motion of a system under the
influence of forces, especially those that
originate outside the system under consider-
ation.
Fast Fourier Transform (FFT) A mathematical technique used to convert a
time-domain plot into its unique frequency
components.
Force That influence on a body that causes it to ac-
celerate. Quantitatively, it is a vector equal to
the body’s time rate of change of momentum.
Forcing function The cause of each discrete frequency
component in a machine-train’s vibration
signature.
Frequency Frequency, f, is defined as the number of rep-
etitions of a specific forcing function or
vibration component over a specific unit of
1
time. It is the inverse of the period, , of the
T
vibration and can be expressed in units of
cycles per second (cps) or Hertz (Hz). For
rotating machinery, the frequency is often
expressed in vibrations per minute (vpm).
Frequency, circular Another measure of frequency measured in
radians (w = 2p f).
Frequency, natural All components have one or more natural
frequencies that can be excited by an energy
source that coincides with, or is in proximity
to, that frequency. The result is a substantial
increase in the amplitude of the natural fre-
quency vibration component, which is
referred to as resonance. Higher levels of
