A Glossary of Terms for Sensors & Data SystemsA B C D E F G H I J K

L M N O P Q R S T U V W X Y ZAcceleration - The time rate of change of velocity.

Click the first letter of the ITEM you need.

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.

(Back to top)Angular Load Concentric - A load applied concentric with the Primary axis at the

point of application and at some angle with respect to the Primary axis.

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Balancing Resonance Speed(s) - A rotative speed that

corresponds to a natural resonance frequency.

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Balanced Condition - For rotating machinery, a condition

where the shaft geometric centreline coincides with the mass

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.

(Back to top)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.

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.

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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.

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Slow Roll Speed - Low rotative speed at which dynamic motion

effects from forces such as imbalance are negligible.

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Spectral Map - A three-dimensional plot of the vibration

amplitude spectrum versus another variable, usually time or rpm.

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Spectrum Analyser - An instrument which displays the

frequency spectrum of an input signal.

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Stiffness - The spring-like quality of mechanical and hydraulic

elements to elasticity deform under load.

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Strain - The physical deformation, deflection, or change in

length resulting from stress (force per unit area).

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Strain Gauge - A device with electrical resistance that is a

function of the applied measurand.For more details, clickhere

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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.

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Tracking Filter - A low-pass or band-pass filter which

automatically tracks the input signal versus the rpm. A tracking

filter is usually required for aliasing protection when data

sampling is controlled externally.

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Transducer - A device for translating the magnitude of one

quantity into another quantity.

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Transient Vibration - Temporarily sustained vibration of a

mechanical system. It may consist of forced or free vibration

or both. Typically this is associated with changes in machine

operating condition such as speed, load, etc.

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Transverse Sensitivity -See Cross-Axis Sensitivity.

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Trigger - Any event which can be used as a timing reference.

In a DSA, a trigger can be used to initiate a measurement.

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Uniform Window - In a DSA, a window function with uniform

weighting across the time record. This window does not protect

against leakage, and should be used only with transient signals

contained completely within the time record.

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Ultimate Overload Rating - The maximum load in percent

of rated capacity which can be applied without producing a

structural failure.

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Vector - A quantity which has both magnitude and direction (phase).

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Villari Effect - The change in magnetic properties of a ferromagnetic

material in response to the presence of stress in the ferromagnetic material.

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Waterfall Plot -See Spectral Map.

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Wiedemann Effect - The mechanical torsion that occurs when an

electric current is passed along or through a long thin ferromagnetic

material while it is subjected to an axial magnetic field.

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Zero Balance - The output signal rated excitation of a load cell

with no load applied, usually expressed in percent of rated output.

Home - Website - Search - Suppliers - Links - New Products - Catalogues - Magazines Problem Page - Applications - How they work - Tech Tips - Training - Events - Jobs -Register From Application Note: AN 243-1 - Hewlett-Packard Company, 1991Editing & additions by PCB Piezotronics, Depew, NY, 1998