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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 ˝ the sample rate. When the signal starts at 0 Hz (baseband signals), bandwidth can be exchanged to maximum frequency in the definition above.
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-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.
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.)
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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 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.
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.
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).
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.
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).
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.
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.
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.)
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 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.
Linear Averaging - See Time Averaging.
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 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: µ(0) = permeability of free space,having the value 4 pie cubed X 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
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.
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 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 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.
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.
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.
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.
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.
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.
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.
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.
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.
Strain Gauge - A device with electrical resistance that is a function of the applied measurand. For more details, click here
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 şC. (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 şC.
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.
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.
Unbalance - See 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.
Waterfall Plot - See Spectral Map.
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.