Vibration Terminology Dictionary
Acceleration
- The time rate of change of velocity. Typical units are m/s2,
and g (1 g = 9.81 m/s2). Acceleration measurements are usually made
with accelerometers.
Accelerometer
- A sensor whose electrical output is proportional to acceleration.
Piezoelectric crystals are the most common type.
Aliasing
- A phenomenon which can occur whenever a signal is not sampled at
greater than twice the maximum frequency component, causing high
frequency signals to appear at low frequencies. Aliasing is avoided
by filtering out signals greater than half the sample rate.
Angle domain
- Analysis of a Non-stationary signal using recursive filtering,,
i.e. to produce a signal suitable for the usual analysis procedures.
Anti-aliasing filter
- A low-pass filter designed to filter out frequencies higher than
1/2 the sample rate in order to prevent aliasing.
Asynchronous
- Vibration components that are not related to rotating speed (also
referred to as non-synchronous).
Average - A measure of the effective energy in a vibrating part based on the arithmetic average of a set of rectified (i.e. positive) instantaneous values.
Averaging
- The summing of
vibration signals triggered at the same point of the cycle in a
rotational mechanism, such that random noise is minimised to leave
the dominant mechanical vibration.
B:
Bandpass Filter
- A filter with
a single transmission band extending from lower to upper cut-off
frequencies. The width of the band is determined by the separation of
frequencies at which amplitude is attenuated by 3 dB (0.707).
Bandwidth
- A finite range of frequencies within the total signal, which are those
used for further analysis. Often the spacing between frequencies at
which a bandpass filter attenuates the signal by 3 dB, or the half-power
points.
- Narrow bandwidth
- A constant bandwidth on a linear frequency scale being significantly
smaller than one-third octave.
- Octave Bandwidth
- Filters set in frequency according to IEC Recommendation 225 based on
a reference centre frequency of 1 kHz.
- One-Third Octave bandwidth
- Filters obtained by dividing each octave band into three geometrically
equal sub-sections.
Baseline
spectrum - A
vibration spectrum taken when a machine is in good operating condition;
used as a reference for monitoring and analysis.
Blade passing frequency - A potential vibration frequency on any bladed machine (turbine, axial compressor, fan, etc.). It is represented by the number of blades times shaft rotating frequency.
Bode plot
- The portrayal of the motion of a shaft against a fixed point, showing
the rectangular co-ordinate plot of IX component amplitude and phase
versus running speed.
Buffer
1) An isolating circuit used to avoid distortion of the input signal by
the driven circuit. Often employed in data transmission when driving
through long cables.
2) A temporary
software storage area where data resides between time of transfer from
external media and time of program initiated I/O operation.
C:
Campbell diagram
- A
mathematically constructed diagram used to check for coincidence of
vibration sources with rotor natural resonances, e.g. 1XRPM imbalance,
2XRPM misalignment, etc. The form of the diagram is a rectangular plot
of resonant frequency vs excitation frequency. (see also Interference
diagram.)
Cascade plot
- See Waterfall analysis.
Cavitation
- A condition which can occur in liquid-handling machinery (e.g.
hydraulic pumps), where the inlet pressure is inadequate and local
vaporisation of the fluid occurs under vacuum conditions. Tiny air
bubbles are released and as they collapse on the metal surfaces
extremely high pressures are generated with a characteristic vibration.
Cepstrum
-The Fourier transform of the logarithm of the mean square density, i.e.
simply speaking, the spectrum analysis of a spectrum analysis.
Charge amplifier
- An amplifier whose output voltage is proportional to the input charge
from a piezoelectric transducer (voltage output unaffected by саble
length from the transducer). (See also Voltage amplifier.)
Coherence
- The ratio of coherent output power between channels in a dual channel
DSA (Dynamic Signal Analyser). An effective means of determining the
similarity of vibrations at two locations giving insight into the
possibility of cause and effect relationships.
Condition
monitoring -
Determination of the health or condition of a machine or system by
analysis of signals generated by the machine. Measurements may be taken
either periodically or continuously and can cover a very wide range of
features including vibration.
Crest factor
- Relationship
between peak value and rms value (Peak divided by rms).
Critical speeds
- In general, any rotating speed which is associated with vibration
amplitude. Often the critical speeds are the rotor speeds which
correspond to natural frequencies of the rotor/disk system, including
the stiffness / damping of the oil film and pedestal/foundation.
D:
Damping - The quality of a mechanical system (natural or added) that maintains or reduces the amplitude of motion with each successive cycle. The damping process converts mechanical energy to other forms, usually heat. Damping of shaft motion may be provided by oil in bearings, seals, etc.
- Critical damping - The minimum viscous damping that will allow a displaced system to return to its original position without oscillation.
- Damping ratio
- The ratio of the actual damping in a system to the critical damping,
at a resonant frequency.
- Database - A
group of SETs, subSETs, and POINTs arranged in a hierachy that define a
user's facilities (i.e., buildings, areas, machines, data gathering
locations).
Decibels (dB) - A logarithmic
representation of amplitude ratio, defined as 20 times the base ten
logarithm of the measured amplitude to a reference.
Defect bearing frequency - Frequency
generated as a result of a defect in a bearing.
Degrees of freedom - (Mechanical system) The minimum number
of independent co-ordinates required to continuously define the position
of all parts of the system at any instant of time.
Differentiation -
Representation in terms of time rate of change. For example
differentiating velocity yields acceleration and differentiating
displacement yields velocity. In a DSA (dynamic signal analysers)
differentiation is performed by:
Amplitude * 2 * PI * Frequency
Digital-to-analogue conversion (D-A)
- The process of producing a continuous analogue signal from discrete
quantised levels. The result is a continuous waveform designed to match
as closely as possible a previously sampled signal or a synthesised
result.
Displacement - The change in
distance or position of an object relative to a reference.
Displacement sensor - A transducer
whose output is proportional to the distance between it and the measured
object (usually the shaft).
Download - Transferring information
to a data collector from a host computer.
Dual voting - Concept where two
independent inputs are required before action (usually machine shutdown)
is taken. Most often used with axial position measurements, where
failure of a single transducer might lead to an unnecessary shutdown.
Dynamic range - For spectrum
measurements, the difference, in dB, between the overload level and the
minimum detectable signal level (above the noise) within a measurement
system.
Dynamic Signal Analyser (DSA) -
Vibration analyser that uses digital signal processing and the Fast
Fourier Transform to display vibration frequency components. DSAs may
also display the time domain and phase spectrum.
E:
Eddy current - Electrical current
which is generated (and dissipated) in a conductive material in the
presence of an electromagnetic field.
Electrical runout - An error signal
that occurs in eddy current displacement measurements when shaft surface
conductivity varies.
Enveloping - The screening of random
noise from a vibration signal before any transformation analysis to
enhance pure repetitive elements of a signal.
Excitation force - An external force
applied to a system causing it to respond.
Exponential Averaging - Generates
a continuous running average where the most recently sampled spectra
have more influence on the average than older ones. This provides a
convenient form to examine changing data with the benefit of some
averaging to smooth the spectra
F:
Filter - Electronic circuitry
designed to pass or reject a specific frequency band.
Forced vibration - The oscillation
of a system under the action of a forcing function. Typically forced
vibration occurs at the frequency of the exciting force.
Fourier transform - A mathematical
operation for decomposing a time domain function into its frequency
domain components (amplitude and phase). The process is reversible, and
the signal can be reconstructed from its Fourier components.
Fast Fourier Transform (FFT) - The
transformation of the vibration signal from the time domain into
discrete frequency components.
Free vibration Vibration Of a mechanical system following an initial
temporary force, i.e. the vibration which occurs when a machine is
allowed to vibrate in the absence of any additional motivating forces
-typically at one or more natural frequencies. (Transient vibration.)
Frequency - The repetition rate of a
periodic event, usually expressed in cycles per second (Hz), cycles per
minute (cpm), revolutions per minute (rev/min), or multiples of
rotational speed (orders). Orders are commonly referred to as 1XRPM for
rotational speed, 2XRPM for twice rotational speed, etc.
Frequency component - The amplitude,
frequency and phase characteristics of a dynamic signal. Frequency
domain - The analysis of vibration amplitude against frequency. An FFT
graph.
Frequency range - The frequency
range (bandwidth) over which a measurement is considered valid; (i.e.
within the manufacturer's specifications).
Frequency response - The amplitude
and phase response characteristics of a system.
Fundamental frequency - The lowest
frequency periodic component present in a complex spectrum. At least one
complete period of a signal must be present for it to qualify as the
'fundamental'.
G:
Gear mesh frequency
- A potential vibration frequency on any machine that
contains gears; equal to the number of teeth multiplied by the
rotational frequency of the gear in a fixed train of gears.
H:
Harmonic - A multiple
of the Fundamental frequency.
Sub-harmonic - A sub-multiple of the
Fundamental frequency.
Hertz (Hz) - The unit of frequency
represented by cycles per second.
High-pass filter - A filter with a
transmission band starting at a lower cut-off frequency and extending to
(theoretically) infinite frequency.
Impedance - Ratio of applied force to
resulting velocity during simple harmonic excitation.
- Driving point impedance -
Mechanical impedance where the force and velocity are measured at the same
point.
- Transfer impedance -
Mechanical impedance where the force and velocity are measured at different
points.
Integrating circuit - An electrical
frequency filter which converts a vibratory acceleration signal into one in
which the amplitude is proportional to velocity of displacemen
Integration - A process involving which
converts a vibration acceleration signal to velocity, or a velocity signal to
displacement. Calculation is:
Amplitude / 2 * PI * Frequency
Interference diagram - see Campbell diagram.
J:
Jerk - A vector quantity specifying rate
of change of acceleration with respect to time.
Joint time-frequency analysis - The
simultaneous analysis of a vibration signal in both the time domain and the
frequency domain.
Keyphasor phase reference - A signal used
in rotating machinery measurements, generated by a sensor observing a
once-per-revolution event. The keyphasor signal is used in phase measurements
for analysis and balancing and although widely used is techncially a trademark
of the G.E (Bently Nevada)
Kurtosis (K) - The fourth moment of a
probability distribution. Kurtosis was sometimes used for analysis of
rolling element bearings prior to the widespread adoption of Enveloping.
L:
Linear range - The portion on the curve of a
sensor's output voltage versus parameter where the slope linearity does not vary
significantly from the nominal slope.
Linear Averaging
- The process of adding together a sequence of spectra measurements
and then dividing the total by the number of samples. The result is a true
arithmetic average on a sample by sample basis. Averaging smoothes out random
noise components in a spectrum
Lissajou figure - Time based 2-dimensional
waveform from two transducers with a phase difference of 90°.
Low pass filter - A filter whose
transmission band extends from dc to an upper cut-off frequency.
M:
Mechanical admittance
- see Mobility
Mechanical impedance - see Impedance
Mechanical runout - An error in measuring
the position if the shaft centerline a displacement probe that is caused by
out-of-roundness and surface imperfections.
Memory length (period) - The size of
storage, usually referring to the input memory section of an FFT processing
system. Typically expressed in units of time for a specified sampling rate.
Mobility - The inverse of Mechanical
impedance. (Mechanical admittance).
Modulation, Amplitude (AM) - The process
whereby the amplitude of a signal is varied as a function of the instantaneous
value of another signal. The first signal is called the carrier, and the second
signal is called the modulating signal. Amplitude modulation produces a
component at the carrier frequency, with adjacent components (sidebands) at the
frequency of the modulating signal.
Modulation, Frequency (FM) - The process
whereby the frequency of the carrier is determined by the amplitude of the
modulating signal. Frequency modulation produces a component at the carrier
frequency, with adjacent components (sidebands) at the frequency of the
modulating signal.
Multiplexer - A hardware device that allows
multiple channels to be digitised by a single ADC (Analogue to Digital
Converter).
N:
Natural frequency - The frequency of free
vibration of a system. The frequency at which an undamped system with a single
degree of freedom will oscillate upon momentary displacement from its rest
position.
Noise - Any component of a transducer output
signal that does not represent the variable designed to be measured.
Non-stationary random vibrations -
Vibrations whose statistical properties vary with time within time intervals
considered essential for their proper description.
Orbit
- The path of the shaft centreline
motion during rotation. The orbit may be detected by X and Y axis displacement
transducers, and observed with an oscilloscope.
Order - A multiple of some reference
frequency. An FFT spectrum plot displayed in orders will have multiples of
running speed along the horizontal axis. Orders are commonly referred to as IX
for running speed, 2X for twice running speed, and so on.
Order analysis - The ability to study the
amplitude changes of specific signals that are related to the rotation of the
device under test.
P:
Peak to Peak value - The maximum
excursion between the upper and lower limits of a vibrating motion, Generally
the difference between positive and negative extreme values of an electrical
signal or dynamic motion.
Peak value - The higher of the positive and
negative extreme values.
Period - The time required for a complete
oscillation or for a single cycle of events. The reciprocal of frequency.
Periodic vibration - An oscillatory motion
whose amplitude pattern repeats after fixed increments of time.
Phase - Angular position between one
vibrating part at a given instant, and a reference point or other vibrating part
at the same time. Phase of a periodic signal at a given instant of time is
the fraction of cycle of a periodic waveform, measured from an arbitrary
reference.
Phase angle - Time displacement between two
currents or two voltages (or their mechanical analogues) or between a current
and a voltage measured in electrical degrees, where an electrical degree is
1/360 part of a complete cycle of the frequency at which the measurement is
made.
Piezoelectric - Any material which provides
a conversion between mechanical and electrical energy.
Piezoelectric crystal - If mechanical
stresses are applied on two opposite faces, electrical charges appear on some
other pair of faces.
Polar plot - Polar co-ordinate
representation of the locus of the 1XRPM, 2XRPM, 3XRPM, .... vector at a
specific lateral shaft location with the shaft rotational speed as a parameter.
See Bode plot.
Power spectral density - The energy content
of a vibratory signal within a given bandwidth.
Process Domain
Averaging - Averaging, where a series of individual spectra are added
together and the sum is divided by the number of spectra.
Each time a process is executed, the individual values of the output can vary to
some degree. For example, the values in each line of a spectral output can show
some variation in amplitude, although tending towards a mean value. Averaging
will provide a better estimate of this mean value.
Process domain averaging will provide a
statistically more accurate estimate of the signal. It will not improve the
signal-to-noise ratio.
Q:
Quefrency - The independent variable of a
Cepstrum, i.e. the scale of the magnitude of harmonics on the Frequency scale of
a spectrum.
R:
Random vibration -
A vibration whose
instantaneous amplitude is not specified at any instant of time, i.e. the
vibration is unpredictable.
Receptance - Displacement per unit force,
particularly sinusoidal force. The inverse of dynamic stiffness (Mechanical
admittance.)
Relative motion transducers - Transducers
producing signals proportional to the relative motion between a reference point
and the point of interest. Displacement transducers generally measure shaft
motion relative to the transducer mounting.
Resonance - A condition of vibration where
the excitation frequency coincides with a natural frequency of the system. Thus
a small change in excitation frequency causes a decrease in system response.
Root mean square (rms) - A measure of the
effective energy in a vibrating part based on the square root of the arithmetic
average of a set of squared instantaneous values.
S:
Seismic transducers - Transducers producing
signals proportional the absolute motion in space (compare with Relative motion
transducers)
Shock - Rapid transient transmission of
mechanical energy.
Shock Pulse Method (SPM) - A method of
monitoring (usually bearingss) by means of the detection of pulses generated by
defect impacts
Side-bands - Additional frequencies either
side of a major frequency peak generated by amplitude and frequency modulation.
Signal conditioning - The transformation of
a transducer signal into a form which is suitable for analysis, metering or
feeding into an alarm system.
Signal-to-noise ratio - A measure of signal
quality. Typically, the ratio of voltage or power of a desired signal to the
undesired noise component measured in corresponding units.
Signature - A vibration frequency spectrum
which is distinctive to a particular machine or component, system or subsystem
at a specific point in time, under specific machine operating conditions. Used
for historical comparison of mechanical condition over the operating life of
the machine.
Spike Energy method (SE) - A method of
monitoring which combines SPM, the rate of occurrence of pulses and the
broad-band rms acceleration over the fre-quency range 5 kHz to 45 kHz. (Units in
gSE.)
Stationary random vibrations -
Vibrations those whose statistical characteristics do not change with time
within time intervals considered essential for their proper description.
Sub-harmonic - Sinusoidal quantity of a
frequency that is an integral sub-multiple multiple of a fundamental frequency.
Sub-synchronous - Component of a vibration
signal which has a frequency less than shaft rotational frequency.
T:
Time Averaging - In a Dynamic Signal
Analyzer, the averaging of time records using triggering at the same point of
the waveform to reduce a synchronous components and improve the signal-to-noise
ratio.
Time domain averaging means that data is averaged as it is acquired by the
instrument, before processing commences. It should only be used, when the signal
is repetitive, and a consistent trigger point is available.
Time averaging is particularly useful in the analysis of signals from faulty. It
cannot be used with success on rolling contact bearings, because there is not a
purely rolling action, and there is a small amount of slippage of the roller on
the track.
Time domain - A dynamic amplitude versus
time graph.
Transient vibration - Temporarily sustained
vibration of a mechanical system. It may consist of forced or free vibration, or
both. Typically this is associated with changes in machine operating condition,
e.g. speed, load etc.
Transmissibility - Ratio of the amplitude
response of a system in steady state vibration to the excitation amplitude.
Ultrasonics
- The frequency band above
that normally associated with human hearing, i.e. generally greater than 20 kHz.
Upper limit sometimes considered as 1 MHz.
Upload - Transferring collected data from
the Data Collector to the host computer.
Vane passing frequency
- A potential
vibration frequency on vaned impeller compressors, pumps, and other machines
with vaned rotating elements. It is represented by the number of vanes (on an
impeller or stage) times shaft rotational frequency.
Vector - A quantity which has both magnitude and direction (phase).
Velocity - The time rate of change of
displacement. Velocity leads displacement by 90 degrees in time. Typical SI
units for velocity are m/s, mm/s. Velocity measurements are usually obtained
with an accelerometer and integrated to velocity, or a mechanically activated
velocity transducer, and are used to evaluate machine housing and other
structural response characteristics. Electronic integration of a velocity signal
yields displacement.
Vibration - A periodic or random motion
about an equilibrium position.
Vibration isolator - A resilient support
that reduces transmissibility.
VibroNurse - The web's
number 1 vibration analysis and condition monitoring resource site.
Voltage preamplifier - A preamplifier which
produces an output voltage proportional to input voltage from a piezoelectric
accelerometer.
W:
Waterfall analysis
- The
three-dimensional portrayal, on a single graph or plot, of the change in
vibration amplitude against another variable, e.g. time or revs/ min of the
system or component, over a series of trials.
Waveform - A presentation or display of the
instantaneous amplitude of a signal as a function of time. A vibration waveform
can be observed on an oscilloscope in the timebase mode.
Wavelength - The distance along a periodic
wavefront between points of comparable amplitude with a phase difference of one
period.
Z:
Zero-to-peak value
- The larger of the
positive and negative extreme values of an electronic signal or dynamic motion
form the zero axis.
Zoom - Feature to magnify portions of a
selected spectrum plot for more detailed examination.