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A Glossary
of Terms for Sensors & Data Systems
A B C D E F G H I
J K L M N O P Q R S T U V W X Y Z
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 averaging.
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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.
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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 centreline.
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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
frequency.
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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 diagram.
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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 frequency.
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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
where:
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
where:
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.
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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 standard.
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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
planes.
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Runout Compensation - Electronic correction of a transducer
output signal for the error resulting from slow roll runout.
(Back
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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 input.
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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.
(Back to top)
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.
(Back
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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).
(Back
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Strain Gauge - A device with electrical resistance
that is a function of the applied measurand. For more details,
click here
(Back
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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 tracking.
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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 open-circuited.
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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.
(Back
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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 rotection 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.
(Back
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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).
(Back
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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.
(Back
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Waterfall
Plot
- See
Spectral Map.
(Back
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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.
(Back
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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.
From Application
Note: AN 243-1 - Hewlett-Packard Company, 1991
Editing & additions by PCB Piezotronics, Depew, NY, 1998
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