      Glossary
•  Absolute Pressure
• The pressure measured relative to a perfect vacuum.
•  Acceleration

• The rate of change of velocity often depicted as "g's" or in "mm/s2" in the metric system or "in/sec2" in the English system. Acceleration is not constant but will vary through the vibration cycle, reaching maximum levels as velocity reaches its minimum. This is typically when a particular mass has decelerated to a stop and is about to begin accelerating again.
•  Accelerometer

• A transducer whose electrical output responds directly to acceleration. Accelerometers typically cover a much wider frequency range, along them to pick up signals not present with other types of transducers. Due to the frequency range, accelerometers are ideal for most types of rotating equipment, making them the most used transducer for vibration measurements.
•  Accuracy

• The ratio of the error to the output or to the full scale output, as specified, expressed in percent.
•  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 minimized by filtering the signal to a bandwidth less than: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 where components within a drivetrain are parallel or perpendicular, according to design requirements. A tester such as the Fluke 810 Vibration Tester* can diagnose misalignment conditions where these components are no longer aligned according to design requirements, causing excessive bearing wear and power consumption in the machine.
•  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 magnification of force, displacement, or acceleration from a known reference point.
•  Anti-Aliasing Filter

• Most commonly a low-pass filter designed to filter out frequencies higher than:the sample rate in order to minimize aliasing.
•  Anti-Friction Bearing

• Most commonly a low-pass filter designed to filter out frequencies higher than:the sample rate in order to minimize aliasing.
•  Asynchronous

• Vibration components that are not related to rotating speed (also referred to as nonsynchronous).
•  Atmospheric Pressure

• The pressure caused by the weight of the earths atmosphere; varies with geographic location, altitude, and weather.

• Attachment pads (bronze or stainless steel) can be placed at appropriate measuring locations on machines using an industrial adhesive. The triaxial accelerometer is attached to these pads for measurement collection. The pad may include an alignment notch to ensure the consistent orientation of the accelerometer to the three vibration axes (Radial, Tangential, and Axial). The pad ensures a good transfer of vibration data to the transducer by providing a strong and consistent mounting location.

• The angle between the direction of steady-state preload through the bearing centerline, and a line drawn between the shaft centerline 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.
•  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

• One of the three vibration axes (Radial, Tangential and Axial), the axial plane is parallel to the centerline of a shaft or turning axis of a rotating part.
•  Balanced Condition

• For rotating machinery, a condition where the shaft geometric centerline coincides with the mass centerline.
•  Balancing  (mechanical)
• Adjusting the distribution of mass in a rotating element, to reduce vibratory forces generated by rotation.
•  Balancing Resonance  Speed(s)
• A rotative speed that corresponds to a natural resonance frequency.
•  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.
•  Barometric Pressure
• See Atmospheric Pressure.
•  Baseline Spectrum

• A vibration spectrum taken when a machine is in good operating condition; used as a reference for monitoring and analysis.
•  Best Straight Line

• The line parallel to, and centered between, two parallel straight lines enclosing all calibration data points.

• 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 1x component amplitude and phase (relative to a keyphasor) vs. running speed.
•  Bow

• A shaft condition such that the geometric centerline of the shaft is not straight.
•  BPFO, BPFI

• Common abbreviations for ball pass frequency of defects on outer and inner bearing races, respectively.
•  Bridge Resistance
• See Input Impendance and Output Impedance.
•  Bridge

• A Wheatstone bridge configuration utilizing four resistive elements.
•  Brinneling (False)

• Impressions made by bearing rolling elements on the bearing race; typically caused by external vibration when the shaft is stationary.
•  Calibration

• The comparison of transducer voltage outputs against the outputs of a reference standard.
•  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 resonance. 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.
• See Spectral Map.
•  Cavitations

• 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 vaporization occurs. The result is mixed flow which may produce vibration.
•  Center Frequency

• For a bandpass filter, the center of the transmission band, measured in a linear scale.
•  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.
•  Common Mode  Pressure
• See Line Pressure.

•  Condition Monitoring  (CM)

• The measurement, recording and analysis of machinery parameters (such as acceleration) to determine machine health. Current condition is compared to when the machine was new. Also known as machinery health monitoring.
•  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.
•  Constant Bandwidth  Filter

• A band-pass filter whose bandwidth is independent of center frequency. The filters simulated digitally by the FFT in a DSA are constant bandwidth.
•  Consta nt Current

• Electric current independent of either voltage or resistances and fixed at a specific value. A constant current ower supply varies its output voltage, up to its maximum compliance voltage, to maintain the fixed current into the load.
• Constant Percentage  Bandwidth

• A band-pass filter whose bandwidth is a constant percentage of center frequency. 1/3 octave filters, including those synthesized 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.
•  Critical Speed Map

• A rectangular plot of system natural frequency (y-axis) versus bearing or support stiffness (x-axis).
•  Critical Speeds

• Vibration components that are not related to rotating speed (also referred to as nonsynchronous).
•  Cross Axis Sensitivity

• A measure of off-axis response of velocity and acceleration transducers. Cycle:One complete sequence of values of a periodic quantity.
•  Damping, Critical

• The smallest amount of damping required to return the system to its equilibrium position without oscillation.
•  Damping

• The dissipation of energy. Two types of damping are: Coulomb (friction) Damping and Hysteritic (inherent) Damping.

• The volume inside the pressure port of a transducer at room temperature and barometric pressure.
•  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.
•  Deflection

• The change in length along the primary axis or the distance a diaphragm moves at the center between no-load and rated-load conditions.
•  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.
•  Diaphragm

• The sensing membrane which is deformed when pressure is applied.
•  Differential Pressure
• The difference in pressure between two measurement points.
•  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.)
•  Digital Filter

• A filter which acts on the data after it has been sampled and digitized. Often used in DSAs to provide antialiasing protection before internal re-sampling.
•  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  Transducer
• A transducer whose output is proportional to the distance between it and the measured object (usually the shaft).
•  Displacement

• The change in distance or position of an object relative to a reference.
•  DSA

• Vibration components that are not related to rotating speed (also referred to as nonsynchronous).
•  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 Pressure
• See Impact Pressure.
•  Dynamic Signal  Analyzer (DSA)

• Vibration analyzer 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 Ratio

• The vector difference between the bearing centerline and the average steady-state journal centerline.
•  Eccentricity,  Mechanical

• The variation of the outer diameter of a shaft surface when referenced to the true geometric centerline of the shaft. Out-of-roundness.
•  Eddy Current

• Electrical current which is generated (and dissipated) in a conductive material in the presence of an electromagnetic field.
•  Electrical Runout

• An error signal that occurs in eddy current displacement measurements when shaft surface conductivity varies.
•  Engineering Units

• In a DSA, refers to units that are calibrated by the user (e.g., in/s, gs).
•  Excitation, Electrical

• The voltage or current applied to the input terminals of the transducer.
•  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.
•  Failure Mechanism
• The mechanical or physical parts that results in failure.
•  Failure

• The event, or inoperable state, in which any item or part of an item does not, or would not, perform as specified.
•  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, DFT, where the number of samples is constrained to a power of 2 for speed.
•  Filter

• Electronic circuitry designed to pass or reject a specific frequency band.
•  Finite Element  Modeling

• A computer aided design technique for predicting the dynamic behavior of a mechanical system prior to construction. Modeling 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).
•  Flush diaphragm

• Sensing element is located on the very tip of the transducer (No pressure port).
•  Forced Vibration

• The vibration of a machine caused by some mechanical excitation. If the excitation is periodic and continuous, the response motion eventually becomes steady-state.
•  Free Vibration

• Vibration of a mechanical system following an initial force - typically at one or more natural frequencies.
•  Frequency Domain

• Since vibration exists within the time domain, a vibration signal is represented as a time wave form if viewed on an oscilloscope. If plotted, the time waveform would represent a plot of amplitude vs. time. If the waveform were transformed to the frequency domain, the result would be a spectrum representing a plot of amplitude vs. frequency.
•  Frequency Response

• The range of frequencies over which the transducer voltage output will follow a sinusoidally varying mechanical input within specified limits.
•  Frequency

• The number of oscillation in a given time period. Usually expressed in terms of cycles or vibrations per second (Hertz).
•  Full Scale Output

• The algebraic difference between the output with zero input and output with full scale input (range) applied.
•  Full Scale

• The maximum measurand that a transducer is designed to measure within its specification.
•  G
• The value of acceleration produced by the force of gravity.
•  Gage Pressure

• The pressure above (or below) atmospheric. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual atmospheric pressure value.
•  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.
•  Harmonic Distortion

• In the output signal of a device, distortion caused by the presence of frequencies not present in the input signal.
•  Harmonic

• A sinusoidal quantity having a frequency that is an integral multiple (2, 3, etc.) of a fundamental (´1) 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 dead band, or that portion of a systems response where a change in input does not produce a change in output.
•  Imbalance

• A condition on rotating equipment where the center of mass does not lie on the center of rotation. Imbalance can severely reduce bearing life as well as cause undue machine vibration.
•  Impact Pressure

• The pressure in a moving fluid which is exerted parallel to the direction of flow, caused by the inertial effects of the mass of the fluid. Also called DYNAMIC PRESSURE or VELOCITY PRESSURE.
•  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.
•  Impedance

• 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.
•  Independent Linearity

• Maximum deviation from the linear regression line (least squares fit) for all measured points, expressed as percent of full scale output.
•  Influence Coefficients

• Mathematical coefficients that describe the influence of system loading on system deflection.
•  Input Impedance

• The resistance measured across the excitation terminals of a transducer at room temperature.
•  Insulation (Isolation)  Resistance

• The DC resistance, expressed in ohms, measured between any electrical connector pin or lead wire and the transducer body or case. Normally measured at 50 Vdc.
•  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.)
•  Isolation

• A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates steady-state vibration.
•  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
•  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.
•  Line Pressure

• The maximum pressure in the pressure vessel or pipe for differential pressure measurement. Also called COMMON MODE PRESSURE.
•  Linear Averaging
• See Time Averaging.
•  Linearity

• The maximum deviation of the calibration curve from a specified straight line expressed as a percent of full scale output and only measured on increasing measurand.
•  Lines

• Common term used to describe the filters of a DSA produced by the FFT (e.g., 400 line analyzer).
•  Low-Pass Filter

• A filter whose transmission band extends from dc to an upper cutoff frequency.
•  Machinery Health  Monitoring
• See Condition monitoring (CM).

•  Mean-Time-Between- Failure (MTBF)

• A measurement of reliability for repairable items: The mean number of life units during which all parts of the item perform within their specified limits, during a particular measurement interval under stated conditions.
•  Mean-Time-To- Failure (MTF)

• A basic measure of reliability for non-repairable items: The total number of life units of an item divided by the total number of failures within that population, during a particular measurement interval under stated conditions.
•  Measurand

• The physical quantity, property, or condition which is measured. (e.g.: pressure, load, weight, acceleration).
•  Mechanical Runout

• An error in measuring the position of the shaft centerline with a displacement probe that is caused by out-of-roundness and surface imperfections.
•  Medium (MEDIA)

• The fluid(s) in contact with the diaphragm, the pressure of which is being measured.
•  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.
•  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 modulating signal. Frequency modulation produces a component at the carrier frequency, with adjacent components (sidebands) at frequencies around the carrier frequency related to the modulating signal. The carrier and sidebands are described by Bessel functions.
•  Natural Frequency

• The frequency at which a system will oscillate if moved from its free position and allowed to vibrate without external forces.
•  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.
•  Noise

• Any component of a transducer output signal that does not represent the variable intended to be measured. 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.
•  Nonlinearity
• Used interchangeably with linearity.
•  Nonrepeatability
• Used interchangeably with repeatability.
•  Octave
• The interval between two frequencies with a ratio of 2 to 1.
•  Oil Whirl/Whip

• An unstable free vibration whereby a fluid-film bearing has insufficient unit loading. Under this condition, the shaft centerline 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.
•  Orders

• In rotating machines, orders are multiples or harmonics of the running speed (or associated reference component).
•  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 Impedance

• The resistance as measured on the output terminals of a transducer at standard temperature, with no measurand applied, and with the excitation terminals open-circuited.
•  Output

• The electrical signal measured at the output terminals which is produced by an applied input to a transducer.
•  Overrange

• The maximum pressure or load which may be applied to the transducer without causing a permanent change in the performance specifications.
•  Partial Pressure

• The pressure which would be exerted by one constituent of a mixture of gases, if it alone were to occupy the same volume as the mixture. See also TOTAL PRESSURE.
•  PASCAL
• Pressure of one Newton (force) per square meter.
•  Peak Hold

• In a DSA, a type of averaging that holds the peak signal level for each frequency component.
•  Period

• The time required for a complete oscillation or for a single cycle of events. The reciprocal of frequency.
•  Phase Shift
• The phase angle between the output and the applied signal.
•  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.

• 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).

• The height of a liquid column at the base of which a given pressure would be developed due to gravity acting on the fluid mass.
•  Proximitor

• See Oscillator/Demodulator. Radial:Direction perpendicular to the shaft centerline.
•  PSI
• Pounds per square inch.
•  PSIA
• Pounds per square inch absolute.

• The height of a liquid column at the base of which a given pressure would be developed due to gravity acting on the fluid mass.
•  PSID
• Pounds per square inch differential.
•  PSIG
• Pounds per square inch gage.

• The average location, relative to the radial bearing centerline, of the shaft dynamic motion.

• Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centerline.

• One of the three vibration axes (Radial, Tangential and Axial), the radial plane represents the direction from the transducer to the center of the shaft on rotating equipment. For typical horizontal machines, Radial equals the vertical axis. For Horizontal machines Radial refers the Horizontal axis to which the accelerometer is attached.
•  Range

• The measurand values, over which a transducer is intended to measure, specified by their upper and lower limits.
•  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.
•  Real-Time Analyzer
• See Dynamic Signal Analyzer. Real-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.
•  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.
•  Reflected  Overpressure

• The total pressure that results at the interface when a shock wave traveling in a medium encounters a discontinuity such as a rigid surface or another shock wave.
•  Repeatability

• The maximum deviation from the mean of corresponding data points taken under identical conditions. The maximum difference in output for identically-repeated stimuli when there is no change in other test conditions.
•  Replication
• Testing that reproduces a specified desired history.
•  Resolution

• The smallest change in stimulus that will produce a detectable change in the instrument output.
•  Resonance

• A condition occurring when the forcing or operating frequency of a system coincides with the natural frequency.
•  Response Time

• The time required for the output of a transducer to increase from zero to some specified percentage of its final value when excited by a step change in measurand.
•  Rise Time

• The time required for the output of a transducer to rise from 10% to 90% of its final value as a result of a step change of measurand.
•  Rolling Element  Bearing
• Bearing whose low friction qualities derive from rolling elements (balls or rollers), with little lubrication.
•  Root cause analysis
• Determining what actually caused a failure.
•  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.
•  Running Speed

• The speed, usually expressed in revolutions per minute (rpm), at which a rotating machine runs. It may also be expressed in Hz by dividing rpm by 60.
•  Runout Compensation

• Electronic correction of a transducer output signal for the error resulting from slow roll runout.
•  S.I. System
• The international (metric) system of units.
•  Sealed (or Sealed  Gage) Pressure

• Velocity is the rate of change in position, measured in distance per unit of time. When measuring vibration signals, velocity also represents the rate of change in displacement and is expressed in inches (in) or millimeters (mm) per second.
•  Seismic

• Refers to an inertially referenced measurement or a measurement relative to free space.
•  Sensing Element

• The part of the transducer which reacts directly in response to the measurand.
•  Sensitivity

• The ratio between electrical signal (output) and mechanical quantity (input).
•  Shock

• A transient event defined by a sudden change in motion, force, or velocity.
•  Shunt Calibration

• The change in electrical output caused by placing a fixed resistor between the appropriate transducer terminals. Used in the field for quick calibration.
•  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.
•  Solid-state sensor

• Velocity is the rate of change in position, measured in distance per unit of time. When measuring vibration signals, velocity also represents the rate of change in displacement and is expressed in inches (in) or millimeters (mm) per second.
•  SPAN

• The algebraic difference between the limits of the range from zero to full scale.
•  Spectral Map

• A three-dimensional plot of the vibration amplitude spectrum versus another variable, usually time or rpm.
•  Spectrum Analyzer

• An instrument which displays the frequency spectrum of an input signal.
•  Spring Rate

• The value defined by the ratio of force (load) to deflection. Usually expressed as pounds per inch. Also referred to as stiffness.
•  Specification
• The group of error limits within which each device will operate.
•  Stagnation Pressure

• The sum of the static pressure and the impact pressure. It can be measured at a point where the velocity of the fluid is zero.
•  Standard Pressure

• Pressure of one normal (standard) atmosphere, defined (in the United States and some other countries) as 101.325 kPa (14.696 psia).
•  Static Pressure

• The pressure of a fluid, exerted normal to the direction along which the fluid flows.
•  Stiffness

• The spring-like quality of mechanical and hydraulic elements to elasticity deform under load.
•  Strain gage

• A measuring element for converting mechanical strain into an electrical signal.
•  Strain

• The physical deformation, deflection, or change in length resulting from stress (force per unit area).
•  Structural Damping

• Damping which reduces the vibration of resonating surfaces that radiate noise. Damping is accomplished by affixing a material directly to the vibrating surface. This material converts the mechanical vibration to a minimal amount of heat energy.
•  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.
•  Supply Voltage
• See EXCITATION.
•  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.
•  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.
•  Tangential

• One of the three vibration axes (Radial, Tangential and Axial), the tangential plane is positioned 90 degrees to the Radial plane, running tangent to the drive shaft. For typical horizontal machines, tangential equals the horizontal axis. For typical vertical machines tangential equals the second horizontal axis perpendicular to the mounting of the accelerometer.
•  Temperature  Coefficient

• The percentage change in the sensitivity of a transducer as a result of a change in the operating temperature of the transducer (expressed as percent per degree [%/oF]).
•  Temperature  Compensated

• The range of temperature over which a transducer can operate up to full scale and still meet all specifications.
•  Temperature  Compensation

• The utilization of supplementary devices, materials, or components with the transducer to minimize sources of error caused by changing temperature.
•  Termperature,  Operating

• The range of temperature over which a transducer may be safely operated up to full scale without causing failure; but specifications may not be met.
•  Thermal Zero Shift

• The change in zero balance due to a change in ambient temperature. Usually expressed as the maximum percentage change of FSO over the compensated temperature range.
•  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.
•  Torsional Vibration

• Amplitude modulation of torque measured in degrees peak-to-peak referenced to the axis of shaft rotation.
•  Total Pressure

• The sum of the pressures (partial pressures) which each gas (in a mixture of gases) would exert were it to occupy the containing vessel alone.
•  Transducer
• A device for translating the magnitude of one quantity into another quantity.
•  Transducer

• A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take a physical phenomenon (pressure, temperature, humidity, flow, etc.) and convert it to an electrical signal.
•  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.
•  Transmissibility

• The ratio of dynamic output to dynamic input. This can also be expressed as a percentage of isolation.
•  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.
•  Unbalance

• Unequal mass distribution on a rotor. The mass centerline does not coincide with the rotation or geometric centerline. Also known as imbalance.
•  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.
•  VACUUM

• Pressure measured below atmospheric pressure and with reference to atmospheric pressure (Negative gage pressure).
•  Vector
• Quantity which has both magnitude and direction (phase).
•  Velocity Pressure
• See Impact Pressure.
•  Velocity

• Velocity is the rate of change in position, measured in distance per unit of time. When measuring vibration signals, velocity also represents the rate of change in displacement and is expressed in inches (in) or millimeters (mm) per second.
•  Vibration

• An oscillation of force, displacement or acceleration in a mechanical system about some reference point. Expressed in terms of frequency and amplitude.
•  Waterfall Plot
• See Spectral Map.