A Glossary of Terms for Sensors & Data Systems

Click the first letter of the ITEM you need.
Acceleration - The time rate of change of velocity.
Typical units are ft/s2, meters/s2, and G's
(1G = 32.17 ft/s2 = 9.81 m/s2).
Acceleration measurements are usually made with accelerometers.
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Accelerometer - Transducer whose output is directly proportional
to acceleration. Most commonly use piezoelectric crystals to
produce output.
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Aliasing - A phenomenon which can occur whenever a signal
is not sampled at greater than twice the maximum bandwidth
of the signal. Causes high frequency signals to appear at low
frequencies. Aliasing is minimised by filtering the signal to a
bandwidth less than ½ the sample rate. When the signal starts
at 0 Hz (baseband signals), bandwidth can be exchanged to
maximum frequency in the definition above.
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Alignment - A condition whereby the axes of machine
components are either coincident, parallel, or perpendicular,
according to design requirements.
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Amplification Factor (Synchronous) - A measure of the
susceptibility of a rotor to vibration amplitude when rotational
speed is equal to the rotor natural frequency (implies a flexible
rotor). For imbalance type excitation, synchronous amplification
factor is calculated by dividing the amplitude value at the
resonant peak by the amplitude value at a speed well above
resonance (as determined from a plot of synchronous response
vs. rpm).
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Amplitude - The magnitude of dynamic motion or vibration.
Amplitude is expressed in terms of peak-to-peak, zero-to-peak,
or rms. For pure sine waves only, these are related as follows:
rms = 0.707 times zero-to-peak; peak-to-peak = 2 times zero-to-peak.
DSAs generally read rms for spectral components, and peak for
time domain components.
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Anti-Aliasing Filter - Most commonly a low-pass filter designed
to filter out frequencies higher than 1/2 the sample rate in order to
minimise aliasing.
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Anti-Friction Bearing - See Rolling Element Bearing.
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Asymmetrical Support - Rotor support system that does not
provide uniform restraint in all radial directions. This is typical for
most heavy industrial machinery where stiffness in one plane
may be substantially different than stiffness in the perpendicular
plane. Occurs in bearings by design, or from preloads such as
gravity or misalignment.
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Asynchronous - Vibration components that are not related to
rotating speed (also referred to as non synchronous).
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Attitude Angle (Steady-State) - The angle between the
direction of steady-state preload through the bearing centreline,
and a line drawn between the shaft centreline and the bearing
centerline. (Applies to fluid-film bearings.)
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Auto Spectrum (Power Spectrum) - DSA spectrum display
whose magnitude represents the power at each frequency, and
which has no phase.
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Averaging - In a DSA, digitally averaging several measurements
to improve accuracy or to reduce the level of asynchronous
components. Refer to definitions of rms, time, and peak-hold
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Axial - In the same direction as the shaft centreline.
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Axial Position - The average position, or change in position,
of a rotor in the axial direction with respect to some fixed reference
position. Ideally the reference is a known position within the thrust
bearing axial clearance or float zone, and the measurement is made
with a displacement transducer observing the thrust collar.
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Axial Load - The load applied to the length of, or parallel to, the primary axis with
which it shares a common axis.
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Ambient Conditions - The conditions (humidity, pressure, temperature, etc.) of the medium
surrounding the load cell.
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Angular Load Eccentric - A load applied eccentric with the primary axis at the
point of application and at some angle with respect to the primary axis.
(Back to top)Angular Load Concentric - A load applied concentric with the Primary axis at the
point of application and at some angle with respect to the Primary axis.
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Balancing Resonance Speed(s) - A rotative speed that
corresponds to a natural resonance frequency.
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Balanced Condition - For rotating machinery, a condition
where the shaft geometric centreline coincides with the mass
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Balancing - A procedure for adjusting the radial mass distribution
of a rotor so that the mass centreline approaches the rotor
geometric centreline.
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Band-Pass Filter - A filter with a single transmission band
extending from lower to upper cutoff frequencies. The width of
the band is normally determined by the separation of frequencies
at which amplitude is attenuated by 3 dB (a factor 0.707).
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Bandwidth - The distance between frequency limits at which
a band-pass filter attenuates the signal by 3 dB. In a DSA, the
measurement bandwidth is equal to [(frequency span)/(number of filters)
x (window factor)]. Window factors are: 1 for uniform, 1.5 for Hanning,
and 3.4 for flat top (P301) and 3.6 for flat top (P401). See flat top for
more information.
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Baseline Spectrum - A vibration spectrum taken when a machine
is in good operating condition; used as a reference for monitoring
and analysis.
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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
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Block Size - The number of samples used in a DSA to compute
the Fast Fourier Transform. Also the number of samples in a
DSA time display. Most DSAs use a block size of 1024.
Smaller block size reduces frequency resolution.
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Bode - Rectangular coordinate plot of lx component amplitude
and phase (relative to a keyphasor) vs. running speed.
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BPFO, BPFI - Common abbreviations for ball pass frequency
of defects on outer and inner bearing races, respectively.
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Bow - A shaft condition such that the geometric centreline
of the shaft is not straight.
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Brinneling (False) - Impressions made by bearing rolling
elements on the bearing race; typically caused by external
vibration when the shaft is stationary.
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Bridge Resistance - The nominal electrical resistance of the
sensor circuit measured at the excitation connections of the
sensor with zero measurand applied and the output
connections open circuit.
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Calibration - A test during which known values of the measured
variable are applied to the transducer or readout instrument, and
output readings varied or adjusted.
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Combined Error - (Non linearity and Hysteresis) The maximum
deviation from the straight line drawn between original no-load
and rated load outputs expressed as percentage of the rated output
and measured on both increasing and decreasing loads.
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Compensation - The utilization of supplementary devices, materials,
or process to minimize known sources of error.
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Creep Recovery - The change in no-load output occurring with time
after removal of A load which had been applied for a specific period
of time. Usually measured over a specific time period immediately
following removal of rated load and expressed as a percent of rated
output over a specific period of time.
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Campbell Diagram - A mathematically constructed diagram
used to check for coincidence of vibration sources
(i.e. 1 x imbalance, 2 x misalignment) with rotor natural resonances.
The form of the diagram is like a spectral map (frequency versus rpm),
but the amplitude is represented by a rectangular plot, the larger the
amplitude the larger the rectangle. Also known as an interference
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Cascade Plot - See Spectral Map.
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Cavitation - A condition which can occur in liquid-handling
machinery (e.g. centrifugal pumps) where a system pressure
decrease in the suction line and pump inlet lowers fluid pressure
and vaporisation occurs. The result is mixed flow which may
produce vibration.
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Centre Frequency - For a bandpass filter, the centre of the
transmission band, measured in a linear scale.
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Charge Amplifier - Amplifier used to convert accelerometer
output impedance from high to low, making calibration much
less dependent on cable capacitance.
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Coherence - Measures how much of the output signal is
dependent on the input signal in a linear and time-invariant way.
It is an effective means of determining the similarity of vibration
at two locations, giving insight into the possibility of cause and
effect relationships.
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Constant Bandwidth Filter - A band-pass filter whose
bandwidth is independent of centre frequency. The filters
simulated digitally by the FFT in a DSA are constant bandwidth.
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Constant Percentage Bandwidth - A band-pass filter whose
bandwidth is a constant percentage of centre frequency.
1/3 octave filters, including those synthesised in DSAs, are
constant percentage bandwidth.
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Critical Machinery - Machines which are critical to a major
part of the plant process. These machines are usually unspared.
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Creep - The change in sensor output that occurs with time when
a constant measurand is applied with environmental and other
variable remaining constant. Expressed as a percentage of
applied measurand over a specified time.
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Critical Speeds - In general, any rotating speed which is
associated with high vibration amplitude. Often, the rotor
speeds which correspond to natural frequencies of the system.
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Critical Speed Map - A rectangular plot of system natural
frequency (y-axis) versus bearing or support stiffness (x-axis).
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Cross Axis Sensitivity - A measure of off-axis response of
velocity and acceleration transducers.
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Cross talk - The error output that results from mechanical
coupling between the sensing axes on a multi-axis sensor.
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Cycle - One complete sequence of values of a periodic quantity.
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Damping - The quality of a mechanical system that restrains
the amplitude of motion with each successive cycle. Damping
of shaft motion is provided by oil in bearings, seals, etc.
The damping process converts mechanical energy to other
forms, usually heat.
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Damping, Critical - The smallest amount of damping required
to return the system to its equilibrium position without oscillation.
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Decibels (dB) - A logarithmic representation of amplitude ratio,
defined as 10 times the base ten logarithm of the ratio of the
measured power to a reference. dBV readings, for example,
are referenced to 1 volt rms. dB amplitude scales are required
to display the full dynamic range of a DSA. dB values for power
or voltage measurements yields the same result.
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Dead Volume - The volume inside the pressure port of a sensor,
or transducer, at room temperature and barometric pressure.
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Deflection - The change of length along the primary axis of the
load cell involving no-load and rated-load conditions.
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Diaphragm - The membrane part of a sensor that changes its
value under pressure- induced displacement.
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Drift - An unexpected change in output under constant load conditions.
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Driveline Shaft - A steel tube with a u-joint at each end that
transfers torque from the output of the transfer case to the axle.
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Degrees of Freedom - A phrase used in mechanical vibration
to describe the complexity of the system. The number of degrees
of freedom is the number of independent variables describing the
state of a vibrating system.
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Digital Filter - A filter which acts on the data after it has been
sampled and digitised. Often used in DSAs to provide antialiasing
protection before internal re-sampling.
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Differentiation - Representation in terms of time rate of change.
For example, differentiating velocity yields acceleration. In a DSA,
differentiation is performed by multiplication by jw in the frequency
domain, where w is frequency multiplied by 2p. (Differentiation can
also be used to convert displacement to velocity.)
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Discrete Fourier Transform - A procedure for calculating
discrete frequency components (filters or lines) from sampled time
data. Since the frequency domain result is complex (i.e., real and
imaginary components), the number of frequency points is equal
to half the number of time samples (for a real FFT). When using
zoom analysis, the FFT uses complex time data and then the
number of frequency lines is equal to the number of time samples.
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Displacement - The change in distance or position of an object
relative to a reference.
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Displacement Transducer - A transducer whose output is
proportional to the distance between it and the measured object
(usually the shaft).
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DSA - See Dynamic Signal Analyser.
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Dual Probe - A transducer set consisting of displacement and
velocity transducers. Combines measurement of shaft motion
relative to the displacement transducer with velocity of the
displacement transducer to produce absolute motion of the shaft.
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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.
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Dynamic Motion - Vibratory motion of a rotor system caused by
mechanisms that are active only when the rotor is turning at
speeds above slow roll speed.
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Dynamic Signal Analyser (DSA) - Vibration analyser that uses
digital signal processing and the Fast Fourier Transform to display
vibration frequency components. DSAs also display the time
domain and phase spectrum, and can usually be interfaced to
a computer.
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Eccentricity, Mechanical - The variation of the outer diameter
of a shaft surface when referenced to the true geometric centreline
of the shaft. Out-of-roundness.
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Eccentricity Ratio - The vector difference between the bearing
centreline and the average steady-state journal centreline.
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Eddy Current - Electrical current which is generated (and dissipated)
in a conductive material in the presence of an electromagnetic field.
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Eccentric Load - A load which is applied parallel to, but not having
a common axis with, the primary axis.
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Electrical Excitation -The current or voltage that is applied to
the input terminals of a transducer.
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Error - The algebraic difference between the indicated and true
value of the load being measured.
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Electrical Runout - An error signal that occurs in eddy current
displacement measurements when shaft surface conductivity varies.
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Engineering Units - In a DSA, refers to units that are calibrated by
the user (e.g., in/s, g's).
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External Sampling - In a DSA, refers to control of data sampling
by a multiplied tachometer signal. Provides a stationary display of
rpm-related peaks with changing speed.
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Fast Fourier Transform (FFT) - A computer (or microprocessor)
procedure for calculating discrete frequency components from
sampled time data. A special case of the Discrete Fourier Transform,
DFI, where the number of samples is constrained to a power of 2
for speed.
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Flush Diaphragm - A sensing device that is located on the very
end of a transducer with no pressure port.
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Frequency Response - The range of frequencies over which
the load cell output will follow the sinusoidally varying mechanical
input within specified Limits.
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Full Scale - The amount produced equivalent to the maximum
load for a specific application or test.
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Full Scale Output - The numerical distinction between the
least output and the rated capacity.
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Faraday Effect - The generation of a voltage by a coil when the
coil is subjected to a changing magnetic field.
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Fatique Life - The number of rated cycles that a sensor can be
subjected to without changing its specification. Expressed as
rated cycles.
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Filter - Electronic circuitry designed to pass or reject a specific
frequency band.
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Finite Element Modelling - A computer aided design technique
for predicting the dynamic behaviour of a mechanical system prior
to construction. Modelling can be used, for example, to predict the
natural frequencies of a flexible rotor.
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Flat Top Filter - FFT window function which provides the best
amplitude accuracy for measuring discrete frequency components.
Note: there are several different flat top windows. The HP proprietary
P401 is the "best" flat top window, P301 is the most common.
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Fluid-Film Bearing - A bearing which supports the shaft on a
thin film of oil. The fluid-film layer may be generated by journal
rotation (hydrodynamic bearing), or by externally applied pressure
(hydrostatic bearing).
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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.
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Free Vibration - Vibration of a mechanical system following an
initial force - typically at one or more natural frequencies.
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Frequency - The repetition rate of a periodic event, usually
expressed in cycles per second (Hz), revolutions per minute (rpm),
or multiples of a rotational speed (orders). Compare to orders
that are commonly referred to as 1x for rotational speed, 2x for
twice rotational speed, etc.
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Frequency Response Function - The amplitude and phase
response characteristics of a system.
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G - The value of acceleration produced by the force of gravity.
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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.
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Hanning Window - FFT window function that normally
provides better frequency resolution than the flat top window,
but with reduced amplitude accuracy.
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Hysteresis - The greatest difference between load cell output readings for the
same applied load. One reading is obtained by escalating the load from zero, the
other by lessening the load from rated output.
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Harmonic - Frequency component at a frequency that is an
integer multiple of the fundamental frequency.
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Heavy Spot - The angular location of the imbalance vector
at a specific lateral location on a shaft. The heavy spot typically
does not change with rotational speed.
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Hertz (Hz) - The unit of frequency represented by cycles per second.
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High Spot - The angular location on the shaft directly under the
vibration transducer at the point of closest proximity. The high
spot can move with changes in shaft dynamics (e.g., from
changes in speed).
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High-Pass Filter - A filter with a transmission band starting at
a lower cutoff frequency and extending to (theoretically) infinite
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Hysteresis - Non-uniqueness in the relationship between two
variables as a parameter increases or decreases. Also called
deadband, or that portion of a system's response where a
change in input does not produce a change in output.
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Imbalance - Unequal radial weight distribution on a rotor system;
a shaft condition such that the mass and shaft geometric centre
lines do not coincide.
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Input Impedance - The resistance measured across the
excitation terminals of a transducer at room temperature at
the point where there is no load applied and the output terminals
are open-circuited.
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Insulation Resistance - The dc resistance measured between
the load cell circuit and the load cell structure. Normally measured
at fifty volts and under standard test conditions.
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Impact Test - Response test where the broad frequency range
produced by an impact is used as the stimulus. Sometimes
referred to as a bump test.
See impulse response for more information.
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Impedance, Mechanical - The mechanical properties of a
machine system (mass, stiffness, damping) that determine the
response to periodic forcing functions.
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Impulse Response - The response of a system to an impulse
as input signal. The output then produces the impulse response
that is the time domain equivalent to the Frequency Response
Function, FRF.
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Influence Coefficients - Mathematical coefficients that describe
the influence of system loading on system deflection.
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Insulation Resistance - The resistance measured between the
strain gauge circuit and the sensor metalwork. Expressed in
megaohms at a stated voltage.
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Integration - A process producing a result that, when differentiated,
yields the original quantity. Integration of acceleration, for example,
yields velocity. Integration is performed in a DSA by dividing the
frequency lines by jw, where w is frequency multiplied by 2p.
(Integration is also used to convert velocity to displacement.)
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Journal - Specific portions of the shaft surface from which
rotor applied loads are transmitted to bearing supports.
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Keyphasor - A signal used in rotating machinery measurements,
generated by a transducer observing a once-per-revolution event.
The keyphasor signal is used in phase measurements for analysis
and balancing. (Keyphasor is a Bently Nevada trade name.)
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Lateral Location - The definition of various points along the
shaft axis of rotation.
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Lateral Vibration - See Radial Vibration.
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Leakage - In DSAs, a result of finite time record length that results
in smearing of frequency components. Its effects are greatly
reduced by the use of weighted time functions such as Flat top
or Hanning windows.
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Linearity - The response characteristics of a linear system
remain constant with input level and/or excitation signal type.
That is, if the response to input a is k.a, and the response to
input b is k.b, then the response of a linear system to input
(a + b) will be (k.a + k.b), independent of the function k.
An example of a nonlinear system is one whose response
is limited by mechanical stop, such as occurs when a bearing
mount is loose.
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Lines - Common term used to describe the filters of a DSA
produced by the FFT (e.g., 400 line analyser).
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Low-Pass Filter - A filter whose transmission band extends
from dc to an upper cutoff frequency.
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Magnetlc Field Intensity (H) - The force that drives the
generation of rnagnetic flux in a material. it is also called
magnetizing force and can be produced by the application
of an electric current. H is measured in amperes/meter.
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Magnetic Flux Density (B) - The amount of magnetic flux
that results from the applied magnetizing force. B is measured
in newtons/ampere-meter.
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Magnetic hysteresIs - When a ferromagnetic material is
placed in an alternating magnetic field, the flux density (B) lags
behind the magnetizing force (H) that causes it. The area under
the hysteresis loop is the hysteresis loss per cycle, and is high
for permanent magnets and low for high-permeability, low-loss
magnetic materials.
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Magnetic Permeability (µ) - This indicates the ability of a
material to support magnetic lines of flux. The µ of a material
is the product of the relative permeability of that material and
the permeability of free space. The relative permeability of most
nonferrous materials is near unity. In free space, magnetic flux
density is related to magnetic field intensity by the formula:
B = µ(0)H
1O-7 henry/m. In other materials, the magnetic flux density at
a point is related to the magnetic intensity at the same point by
B = µH
µ = µ(0) µ(r) and µ(r) is the relative permeability
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Magnetic Saturation - The upper limit of the abilIty of a
ferromagnetic material to carry flux.
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Magnetization Curve - Shows the amount of magnetising
force required to saturate a ferromagnetic material. It is normally
shown as a graph with B as the ordinate and H as the abscissa,
and is known as the B-H curve.
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Magnetostriction - The change of size and/or shape of a
ferromagnetic material due to the application of a magnetic field.
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Mechanical Runout - An error in measuring the position
of the shaft centreline with a displacement probe that is caused
by out-of-roundness and surface imperfections.
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Micrometer (MICRON) - One millionth (.000001) of a meter.
(1 micron = 1 x E-6 meters @ 0.04 mils.)
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MIL - One thousandth (0.001) of an inch. (1 mil = 25.4 microns)
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Modal Analysis - The process of breaking complex vibration
into its component modes of vibration, very much like frequency
domain analysis breaks vibration down to component frequencies.
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Mode Shape - The resultant deflected shape of a rotor at a
specific rotational speed to an applied forcing function. A three
-dimensional presentation of rotor lateral deflection along the
shaft axis.
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Modulation, Amplitude (AM) - The process where the
amplitude of a signal is varied as a function of the instantaneous
value of a another signal. The first signal is called the carrier,
and the second signal is called the modulating signal.
Amplitude modulation always produces a component at the
carrier frequency, with components (sidebands) at the frequency
of the carrier frequency plus minus the modulating signal.
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Modulation, Frequency (FM) - The process where the
frequency of the carrier is determined by the amplitude of the
related to the modulating signal. The carrier and sidebands
are described by Bessel functions.
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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.
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Nonlinearity - The maximum deviation of the calibration
curve from a straight line drawn between the no-load and
rated outputs; expressed as a percentage of the rated output
and measured on increasing load only.
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Nodal Point - A point of minimum shaft deflection in a specific
mode shape. May readily change location along the shaft axis
due to changes in residual imbalance or other forcing function,
or change in restraint such as increased bearing clearance.
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Nyquist Criterion - Requirement that a sampled system needs
to be sampled at a frequency greater than twice the bandwidth
of the signal to be sampled.
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Nyquist Plot - A plot of real versus imaginary spectral
components that is often used in servo analysis. Should not be
confused with a polar plot of amplitude and phase of 1x vibration.
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Octave - The interval between two frequencies with a ratio
of 2 to 1.
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Output - The signal (voltage, current, pressure, etc.) produced
by the load cell. Where the output is directly proportional to
excitation, the signal must be expressed in terms of volts per volt,
per ampere, etc., of excitation.

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Oil Whirl/Whip - An unstable free vibration whereby a fluid-film
bearing has insufficient unit loading. Under this condition, the
shaft centreline dynamic motion is usually circular in the direction
of rotation. Oil whirl occurs at the oil flow velocity within the bearing,
usually 40 to 49% of shaft speed. Oil whip occurs when the whirl
frequency coincides with (and becomes locked to) a shaft resonant
frequency. (Oil whirl and whip can occur in any case where fluid is
between two cylindrical surfaces.)
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Orbit - The path of the shaft centerline motion during rotation.
The orbit is observed with an oscilloscope connected to x and
y-axis displacement transducers. Some dual-channel DSAs also
have the ability to display orbits.
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Oscillator-Demodulator - A signal conditioning device that
sends a radio frequency signal to an eddy-current displacement
probe, demodulates the probe output, and provides output
signals proportional to both the average and dynamic gap distances.
(Also referred to as Proximitor, a Bently Nevada trade name.)
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Output Symmetry - The difference in rated output of a sensor
between its negative and positive operation. Expressed as a
percentage of rated output.
Overload - Safe - The maximum input in excess of the rated
input that can be applied along the principal axis without
producing a permanent change in the sensor specification.
Expressed as a percentage of the rated measurand.
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Peak Hold - In a DSA, a type of averaging that holds the peak
signal level for each frequency component.
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Piezoresistance - The change in resistance caused by an
applied strain of the diaphragm.
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Primary Axis - The geometric centerline (axis) along which
the load cell is designed to be loaded.
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Period - The time required for a complete oscillation or for a
single cycle of events. The reciprocal of frequency.
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Phase - A measurement of the timing relationship between
two signals, or between a specific vibration event and a keyphasor
pulse. Phase is often measured as a function of frequency.
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Piezoelectric - Any material which provides a conversion
between mechanical and electrical energy. For a piezoelectric
crystal, if mechanical stresses are applied on two opposite faces,
electrical charges appear on some other pair of faces.
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Polar Plot - Polar coordinate representation of the locus of
the 1x vector at a specific lateral shaft location with the shaft
rotational speed as a parameter.
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Power Spectrum - See Auto Spectrum.
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Preload, Bearing - The dimensionless quantity that is typically
expressed as a number from zero to one where a preload of zero
indicates no bearing load upon the shaft, and one indicates the
maximum preload (i.e., line contact between shaft and bearing).
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Preload, External - Any of several mechanisms that can
externally load a bearing. This includes "soft" preloads such
as process fluids or gravitational forces as well as "hard"
preloads from gear contact forces, misalignment, rubs, etc.
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Proximitor - See Oscillator/Demodulator.
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Radial - Direction perpendicular to the shaft centreline.
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Radial Position - The average location, relative to the radial
bearing centreline, of the shaft dynamic motion.
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Radial Vibration - Shaft dynamic motion or casing vibration
which is in a direction perpendicular to the shaft centerline.
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Rationalisation - The process of trimming the sensor output
at the rated input to a target value.
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Rated Capacity (Rated Load) - The maximum axial load
the load cell is designed to measure within its specifications.
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Rated Output - The algebraic difference between the outputs
at no-load an at rated load.
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Reference Standard - A force measuring device whose
characteristics are precisely known in relation to a primary
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Repeatability - The maximum difference between load
cell output readings for repeated loadings under identical
loading and environmental conditions.
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Resolution - The smallest change in mechanical input
which produces a change in the output signal.
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Real-Time Analyser - See Dynamic Signal Analyser.
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Real-Time Rate - For a DSA, the broadest frequency span
at which data is sampled continuously. Real-time rate is mostly
dependent on FFT processing speed. If the definition of
realtime rate is "not miss any data", the real-time rate will be
window dependent. The real-time rate will decrease when using
any other window than uniform.
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Rectangular Window - See Uniform Window.
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Relative Motion - Vibration measured relative to a chosen
reference. Displacement transducers generally measure shaft
motion relative to the transducer mounting.
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Repeatability - The ability of a transducer or readout instrument
to reproduce readings when the same input is applied repeatedly.
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Resolution - The smallest change in stimulus that will produce
a detectable change in the instrument output.
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Resonance - The condition of vibration amplitude and phase
change response caused by a corresponding system sensitivity
to a particular forcing frequency. A resonance is typically identified
by a substantial amplitude increase, and related phase shift.
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Rolling Element Bearing - Bearing whose low friction qualities
derive from rolling elements (balls or rollers), with little lubrication.
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Root Mean Square (rms) - Square root of the arithmetical
average of a set of squared instantaneous values. DSAs perform
rms averaging digitally on successive vibration spectra, frequency
line by frequency line.
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Rotor, Flexible - A rotor which operates close enough to, or
beyond its first bending critical speed for dynamic effects to
influence rotor deformations. Rotors which cannot be classified
as rigid rotors are considered to be flexible rotors.
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Rotor, Rigid - A rotor which operates substantially below its
first bending critical speed. A rigid rotor can be brought into,
and will remain in, a state of satisfactory balance at all operating
speeds when balanced on any two arbitrarily selected correction
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Runout Compensation - Electronic correction of a transducer
output signal for the error resulting from slow roll runout.
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Seismic - Refers to an inertially referenced measurement or
a measurement relative to free space.
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Seismic Transducer - A transducer that is mounted on the
case or housing of a machine and measures casing vibration
relative to free space. Accelerometers and velocity transducers
are seismic.
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Safe Overrange - The maximum pressure or load which may
be applied to the transducer, load cell or sensor without causing
permanent damage or a change in the performance specifications.
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Sensitivity - The ratio of the change in output to the mechanical
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Shear - Force that tends to divide an object along a plane parallel
to the opposing stresses.
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Shunt Calibration - Electrical simulation of load cell output by
insertion of known shunt resistors between appropriate points
within the circuitry.
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Shunt-To- Load Correlation - The difference in output readings
obtained through electrically simulated and actual applied loads.
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Side Load - Any load acting 90 degrees to the primary axis at
the point of axial load application.
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Stabilisation Period - The time required to insure that any
further change in the parameter being measured is tolerable.
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Standard Test Conditions - The environmental conditions
under which measurements should be made when measurements
under any other condition may result in disagreement between
various observers at different times and places. These conditions
are as follows: Temperature 23 degrees +or- 2 degrees C
(73.4 degrees +or- 3.6 degrees F).
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Strain Measurement - The ratio of the change of the length
of a structure when force is applied to it to the dimension of the
original length.
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Signal Conditioner - A device placed between a signal source
and a readout instrument to change the signal and/or bandwidth.
Examples: attenuators, preamplifiers, charge amplifiers, filters.
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Signature - Term usually applied to the vibration frequency
spectrum which is distinctive and special to a machine or
component, system or subsystem at a specific point in time,
under specific machine operating conditions, etc. Used for
historical comparison of mechanical condition over the
operating life of the machine.

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Slow Roll Speed - Low rotative speed at which dynamic motion
effects from forces such as imbalance are negligible.
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Spectral Map - A three-dimensional plot of the vibration
amplitude spectrum versus another variable, usually time or rpm.
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Spectrum Analyser - An instrument which displays the
frequency spectrum of an input signal.
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Stiffness - The spring-like quality of mechanical and hydraulic
elements to elasticity deform under load.
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Strain - The physical deformation, deflection, or change in
length resulting from stress (force per unit area).
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Strain Gauge - A device with electrical resistance that is a
function of the applied measurand. For more details, click
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Subharmonic - Sinusoidal quantity of a frequency that is an
integral submultiple of a fundamental frequency.
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Subsynchronous - Component(s) of a vibration signal which
has a frequency less than shaft rotative frequency.
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Synchronous Sampling - In a DSA, it refers to the control of
the effective sampling rate of data; which includes the processes
of external sampling and computed resampling used in order
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Temperature Effects - (On rated output) The change of
rated output for a specified temerature change at steady state
temerature conditions. Expressed as a percentage of the
output per ºC. (On zero output) The change of zero output
for a specified temerature change at steady state temperature
conditions. Expressed as a percentage of the rated output per ºC.
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Temperature Range Safe - The extremes of temperature
within which the load cell will operate within permanent adverse
change to any of its performance characteristics.
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Temperature Effect On Rated Output - The change in
rated output due to a change in ambient temperature.
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Temperature Range Compensated - The range of
temperature over which the load cell is compensated to maintain
rated output and zero balance within specific limits.
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Terminal Resistance Corner To Corner - The resistance
of the load cell circuit measured at specific adjacent bridge
terminals at standard temperature, with no load applied, and
with the excitation and output terminals open-circuited.
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Terminal Resistance Input - The resistance of the load cell
circuit measured at the excitation terminals at standard
temperature, with no load applied and with the output terminals
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Temperature Range - Compensated - The temperature
range over which the sensor will meet its stated specification.
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Time Record - In a DSA, the sampled time data converted to
the frequency domain by the FFT. Most DSAs use a time record
of 1024 samples.
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Torsional Vibration - Amplitude modulation of torque measured
in degrees peak-to-peak referenced to the axis of shaft rotation.
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Tracking Filter - A low-pass or band-pass filter which
automatically tracks the input signal versus the rpm. A tracking
filter is usually required for aliasing protection when data
sampling is controlled externally.
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Transducer - A device for translating the magnitude of one
quantity into another quantity.
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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 such as speed, load, etc.
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Transverse Sensitivity - See Cross-Axis Sensitivity.
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Trigger - Any event which can be used as a timing reference.
In a DSA, a trigger can be used to initiate a measurement.
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Uniform Window - In a DSA, a window function with uniform
weighting across the time record. This window does not protect
against leakage, and should be used only with transient signals
contained completely within the time record.
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Ultimate Overload Rating - The maximum load in percent
of rated capacity which can be applied without producing a
structural failure.
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Vector - A quantity which has both magnitude and direction (phase).
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Villari Effect - The change in magnetic properties of a ferromagnetic
material in response to the presence of stress in the ferromagnetic material.
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Waterfall Plot - See Spectral Map.
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Wiedemann Effect - The mechanical torsion that occurs when an
electric current is passed along or through a long thin ferromagnetic
material while it is subjected to an axial magnetic field.
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Zero Balance - The output signal rated excitation of a load cell
with no load applied, usually expressed in percent of rated output.

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From Application Note: AN 243-1 - Hewlett-Packard Company, 1991 Editing & additions by PCB Piezotronics, Depew, NY, 1998