  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
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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.
Accelerometer - Transducer whose output is directly proportional
to acceleration. Most commonly use piezoelectric crystals to produce output.
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.
Alignment - A condition whereby the axes of machine components are either coincident, parallel, or perpendicular, according to design requirements.
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).
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.
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.
Anti-Friction Bearing - See Rolling Element Bearing.

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.
Asynchronous - Vibration components that are not related to rotating speed (also referred to as non synchronous).
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.)
Auto Spectrum (Power Spectrum) - DSA spectrum display whose magnitude represents the power at each frequency, and which has no phase.
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.
Axial - In the same direction as the shaft centreline.
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.
Axial Load - The load applied to the length of, or parallel to, the primary axis with which it shares a common axis.
Ambient Conditions - The conditions (humidity, pressure, temperature, etc.) of the medium surrounding the load cell.
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.
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.
Balancing Resonance Speed(s) - A rotative speed that corresponds to a natural resonance frequency.
Balanced Condition - For rotating machinery, a condition where the shaft geometric centreline coincides with the mass centreline.
Balancing - A procedure for adjusting the radial mass distribution of a rotor so that the mass centreline approaches the rotor geometric centreline.
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).
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.
Baseline Spectrum - A vibration spectrum taken when a machine is in good operating condition; used as a reference for monitoring and analysis.
Blade Passing Frequency - A potential vibration frequency on any bladed machine (turbine, axial compressor, fan, etc.).
It is represented by the number of blades times shaft-rotating frequency.
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.
Bode - Rectangular coordinate plot of lx component amplitude and phase (relative to a keyphasor) vs. running speed.
BPFO, BPFI - Common abbreviations for ball pass frequency of defects on outer and inner bearing races, respectively.
Bow - A shaft condition such that the geometric centreline of the shaft is not straight.
Brinneling (False) - Impressions made by bearing rolling elements on the bearing race; typically caused by external vibration when the shaft is stationary.
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.
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.
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.
Compensation - The utilization of supplementary devices, materials, or process to minimize known sources of error.
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.
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.
Cascade Plot - See Spectral Map.

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.
Centre Frequency - For a bandpass filter, the centre of the transmission band, measured in a linear scale.
Charge Amplifier - Amplifier used to convert accelerometer output impedance from high to low, making calibration much less dependent on cable capacitance.
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.
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.
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.
Critical Machinery - Machines which are critical to a major part of the plant process. These machines are usually unspared.
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.
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.
Critical Speed Map - A rectangular plot of system natural frequency (y-axis) versus bearing or support stiffness (x-axis).
Cross Axis Sensitivity - A measure of off-axis response of velocity and acceleration transducers.
Cross talk - The error output that results from mechanical coupling between the sensing axes on a multi-axis sensor.
Cycle - One complete sequence of values of a periodic quantity.
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.
Damping, Critical - The smallest amount of damping required to return the system to its equilibrium position without oscillation.
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.
Dead Volume - The volume inside the pressure port of a sensor, or transducer, at room temperature and barometric pressure.
Deflection - The change of length along the primary axis of the load cell involving no-load and rated-load conditions.
Diaphragm - The membrane part of a sensor that changes its value under pressure- induced displacement.
Drift - An unexpected change in output under constant load conditions.
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.
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.
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.
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.)
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.
Displacement - The change in distance or position of an object relative to a reference.
Displacement Transducer - A transducer whose output is proportional to the distance between it and the measured object (usually the shaft).
DSA - See Dynamic Signal Analyser.

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.
Dual Voting - Concept where two independent inputs are required before action (usually machine shutdown) is taken. Most often used with axial position measurements, where failure of a single transducer might lead to an unnecessary shutdown.
Dynamic 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.
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.
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.
Eccentricity Ratio - The vector difference between the bearing centreline and the average steady-state journal centreline.
Eddy Current - Electrical current which is generated (and dissipated) in a conductive material in the presence of an electromagnetic field.
Eccentric Load - A load which is applied parallel to, but not having a common axis with, the primary axis.
Electrical Excitation -The current or voltage that is applied to the input terminals of a transducer.
Error - The algebraic difference between the indicated and true value of the load being measured.
Electrical Runout - An error signal that occurs in eddy current displacement measurements when shaft surface conductivity varies.
Engineering Units - In a DSA, refers to units that are calibrated by the user (e.g., in/s, g's).
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.
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.
Flush Diaphragm - A sensing device that is located on the very end of a transducer with no pressure port.
Frequency Response - The range of frequencies over which the load cell output will follow the sinusoidally varying mechanical input within specified Limits.
Full Scale - The amount produced equivalent to the maximum load for a specific application or test.
Full Scale Output - The numerical distinction between the least output and the rated capacity.
Faraday Effect - The generation of a voltage by a coil when the coil is subjected to a changing magnetic field.
Fatique Life - The number of rated cycles that a sensor can be subjected to without changing its specification. Expressed as rated cycles.
Filter - Electronic circuitry designed to pass or reject a specific frequency band.
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.
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.

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).
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.
Free Vibration - Vibration of a mechanical system following an initial force - typically at one or more natural frequencies.
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.
Frequency Response Function - The amplitude and phase response characteristics of a system.
G - The value of acceleration produced by the force of gravity.
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.
Hanning Window - FFT window function that normally provides better frequency resolution than the flat top window, but with reduced amplitude accuracy.
Hysteresis - The greatest difference between load cell output readings for the
other by lessening the load from rated output.
Harmonic - Frequency component at a frequency that is an integer multiple of the fundamental frequency.
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.
Hertz (Hz) - The unit of frequency represented by cycles per second.
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).
High-Pass Filter - A filter with a transmission band starting at a lower cutoff frequency and extending to (theoretically) infinite frequency.
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.
Imbalance - Unequal radial weight distribution on a rotor system;
a shaft condition such that the mass and shaft geometric centre lines do not coincide.
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.
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.
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.

Impedance, Mechanical - The mechanical properties of a machine system (mass, stiffness, damping) that determine the response to periodic forcing functions.
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.
Influence Coefficients - Mathematical coefficients that describe the influence of system loading on system deflection.
Insulation Resistance - The resistance measured between the strain gauge circuit and the sensor metalwork. Expressed in megaohms at a stated voltage.
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.)
Journal - Specific portions of the shaft surface from which rotor applied loads are transmitted to bearing supports.
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.)
Lateral Location - The definition of various points along the shaft axis of rotation.
Lateral Vibration - See Radial Vibration.

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.
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.
Lines - Common term used to describe the filters of a DSA produced by the FFT (e.g., 400 line analyser).
Low-Pass Filter - A filter whose transmission band extends from dc to an upper cutoff frequency.
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.
Magnetic Flux Density (B) - The amount of magnetic flux that results from the applied magnetizing force. B is measured in newtons/ampere-meter.
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.
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
Magnetic Saturation - The upper limit of the abilIty of a ferromagnetic material to carry flux.
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.
Magnetostriction - The change of size and/or shape of a ferromagnetic material due to the application of a magnetic field.
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.
Micrometer (MICRON) - One millionth (.000001) of a meter.
(1 micron = 1 x E-6 meters @ 0.04 mils.)
MIL - One thousandth (0.001) of an inch. (1 mil = 25.4 microns)
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.
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.
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.
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.
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.
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.
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.
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.
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.
Octave - The interval between two frequencies with a ratio of 2 to 1.
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.

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.)
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.
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.)
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.
Peak Hold - In a DSA, a type of averaging that holds the peak signal level for each frequency component.
Piezoresistance - The change in resistance caused by an applied strain of the diaphragm.
Primary Axis - The geometric centerline (axis) along which the load cell is designed to be loaded.
Period - The time required for a complete oscillation or for a single cycle of events. The reciprocal of frequency.
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.
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.
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.
Power Spectrum - See Auto Spectrum.

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).
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.
Proximitor - See Oscillator/Demodulator.

Radial - Direction perpendicular to the shaft centreline.
Radial Position - The average location, relative to the radial bearing centreline, of the shaft dynamic motion.
Radial Vibration - Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centerline.
Rationalisation - The process of trimming the sensor output at the rated input to a target value.
Rated Capacity (Rated Load) - The maximum axial load the load cell is designed to measure within its specifications.
Rated Output - The algebraic difference between the outputs at no-load an at rated load.
Reference Standard - A force measuring device whose characteristics are precisely known in relation to a primary standard.
Resolution - The smallest change in mechanical input which produces a change in the output signal.
Real-Time Analyser - See Dynamic Signal Analyser.

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.
Rectangular Window - See Uniform Window.

Relative Motion - Vibration measured relative to a chosen reference. Displacement transducers generally measure shaft motion relative to the transducer mounting.
Repeatability - The ability of a transducer or readout instrument to reproduce readings when the same input is applied repeatedly.
Resolution - The smallest change in stimulus that will produce a detectable change in the instrument output.
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.
Rolling Element Bearing - Bearing whose low friction qualities derive from rolling elements (balls or rollers), with little lubrication.
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.
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.
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.
Runout Compensation - Electronic correction of a transducer output signal for the error resulting from slow roll runout.
Seismic - Refers to an inertially referenced measurement or a measurement relative to free space.
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.
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.
Sensitivity - The ratio of the change in output to the mechanical input.
Shear - Force that tends to divide an object along a plane parallel to the opposing stresses.
Shunt Calibration - Electrical simulation of load cell output by insertion of known shunt resistors between appropriate points within the circuitry.
Shunt-To- Load Correlation - The difference in output readings obtained through electrically simulated and actual applied loads.
Side Load - Any load acting 90 degrees to the primary axis at the point of axial load application.
Stabilisation Period - The time required to insure that any further change in the parameter being measured is tolerable.
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).
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.
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.
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.

Slow Roll Speed - Low rotative speed at which dynamic motion effects from forces such as imbalance are negligible.
Spectral Map - A three-dimensional plot of the vibration amplitude spectrum versus another variable, usually time or rpm.
Spectrum Analyser - An instrument which displays the frequency spectrum of an input signal.
Stiffness - The spring-like quality of mechanical and hydraulic elements to elasticity deform under load.
Strain - The physical deformation, deflection, or change in length resulting from stress (force per unit area).
Strain Gauge - A device with electrical resistance that is a function of the applied measurand. For more details, click here
Subharmonic - Sinusoidal quantity of a frequency that is an integral submultiple of a fundamental frequency.
Subsynchronous - Component(s) of a vibration signal which has a frequency less than shaft rotative frequency.
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.
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.
Temperature Range Safe - The extremes of temperature within which the load cell will operate within permanent adverse change to any of its performance characteristics.
Temperature Effect On Rated Output - The change in rated output due to a change in ambient temperature.
Temperature Range Compensated - The range of temperature over which the load cell is compensated to maintain rated output and zero balance within specific limits.
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.
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.
Temperature Range - Compensated - The temperature range over which the sensor will meet its stated specification.
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.
Torsional Vibration - Amplitude modulation of torque measured in degrees peak-to-peak referenced to the axis of shaft rotation.
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.
Transducer - A device for translating the magnitude of one quantity into another quantity.
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.
Transverse Sensitivity - See Cross-Axis Sensitivity.

Trigger - Any event which can be used as a timing reference. In a DSA, a trigger can be used to initiate a measurement.
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.
Ultimate Overload Rating - The maximum load in percent of rated capacity which can be applied without producing a structural failure.
Vector - A quantity which has both magnitude and direction (phase).
Villari Effect - The change in magnetic properties of a ferromagnetic material in response to the presence of stress in the ferromagnetic material.