Source-Controlled Vibration Reference Glossary

A component whose frequency equals the instantaneous rotational reference frequency.

f = 1·fᵣ

Scope / caution: It is common in rotating machinery but is not a unique or primary proof of unbalance.

Kilder: S4, S12

A component whose frequency is twice the instantaneous rotational reference frequency.

f = 2·fᵣ

Scope / caution: It can arise from several mechanisms and is not a unique indicator of misalignment or looseness.

Kilder: S4, S12

The time derivative of velocity and the second time derivative of displacement when all quantities use the same coordinate and reference frame.

a(t) = dv/dt = d²x/dt²

Scope / caution: A measured acceleration value also depends on the sensor, mounting, filter, bandwidth and amplitude descriptor.

Kilder: S10, S12

A transducer system whose primary measurand is acceleration.

Scope / caution: Technology,sensitivity,frequency response,dynamic range,mounting,cross-axis response and environmental limits are model-specific; no universal 0.5Hz–20kHz range is asserted.

Kilder: S3

The geometric state of machine axes or components relative to specified position and angular tolerances.

Scope / caution: A vibration pattern alone does not prove alignment or misalignment; verify geometry and the applicable coupling/machine procedure.

Kilder: S3, S5

A magnitude used to describe a signal or motion. The quantity, amplitude descriptor, reference, frequency band and processing must be stated.

Scope / caution: RMS, positive peak, negative peak, maximum absolute peak and peak-to-peak are not interchangeable.

Kilder: S1, S10

An older or informal label for a rolling-element bearing, such as a ball or roller bearing, contrasted with a plain or fluid-film bearing.

Scope / caution: Use “rolling-element bearing” when the bearing type matters; theoretical bearing frequencies do not by themselves prove a defect.

Kilder: S9

A direction parallel to a defined shaft or machine axis.

Scope / caution: An axial component can have many causes; axis, point, sensor orientation and operating state are required.

Kilder: S3

A process for assessing and, when required, correcting rotor mass distribution so that residual unbalance meets an applicable specification.

Scope / caution: Correction-plane count and tolerance depend on rotor behaviour, configuration, service speed and the applicable part of ISO 21940 or another specification.

Kilder: S7, S8

The interval between stated lower and upper frequency limits of a measurement, filter or analysis process.

Scope / caution: Measurement bandwidth, filter bandwidth, resolution bandwidth and DFT-bin spacing are different quantities.

Kilder: S3, S4

A documented reference measurement or state used for later comparison under sufficiently comparable operating and measurement conditions.

Scope / caution: A baseline is not automatically a limit; preserve point, direction, load, speed, temperature, instrument and processing records.

Kilder: S3, S5

A commonly used label for a theoretical rolling-bearing kinematic frequency associated with a race, rolling element or cage.

Scope / caution: Geometry, which ring rotates, contact angle and rolling assumptions matter. Slip and load can shift observed components; a spectral line is not standalone defect proof.

Kilder: S9

For Z equally spaced blades rotating at fᵣ past a fixed reference,the ideal passage frequency.

BPF = Z fᵣ = Z·RPM/60

Scope / caution: State blade count,shaft reference and geometry. A high component is not by itself proof of obstruction,resonance or blade damage.

Kilder: S12

For a stationary outer ring and the stated pure-rolling geometry, the theoretical rate at which rolling elements pass a point on the inner race.

BPFI = (n/2) fᵣ [1 + (d/D) cos α]

Scope / caution: n = rolling-element count; d = rolling-element diameter; D = pitch diameter; α = contact angle; fᵣ = shaft rotational frequency. Other ring motions require another model.

Kilder: S9

For a stationary outer ring and the stated pure-rolling geometry, the theoretical rate at which rolling elements pass a point on the outer race.

BPFO = (n/2) fᵣ [1 − (d/D) cos α]

Scope / caution: The line location is an analysis overlay, not proof of an outer-race defect. Confirm geometry, speed reference, sidebands, harmonics and other evidence.

Kilder: S9

Signal content distributed over a stated frequency interval rather than represented only by isolated narrow components.

Scope / caution: Always state the band, filter, weighting, quantity and descriptor; “broadband” alone is not a diagnosis.

Kilder: S3, S4

For the stated rolling-bearing geometry, the theoretical spin frequency of a rolling element about its own axis.

BSF = (D/2d) fᵣ {1 − [(d/D) cos α]²}

Scope / caution: SKF distinguishes ball-defect frequency from BSF: a localized rolling-element mark can produce contacts at twice BSF. Slip can shift observed components.

Kilder: S9

A plot of measured or calculated frequency against rotational speed, often including order lines, modal branches or response amplitude.

Scope / caution: A crossing can identify a condition for further study; it does not alone prove resonance or acceptability.

Kilder: S4, S5

Formation and collapse or transport of vapour cavities when local liquid pressure falls sufficiently low relative to vapour pressure.

Scope / caution: Noise, vibration and process changes can accompany cavitation, but broadband vibration is not unique to cavitation.

Kilder: S5

Acquisition and interpretation of information about an item or machine over time for assessment of condition and change.

Scope / caution: Vibration can be one input among process,temperature,lubricant and other evidence. Exact normative wording requires the licensed vocabulary.

Kilder: S2, S3

A mechanical element or assembly connecting shafts to transmit torque while providing the designed accommodation or constraint.

Scope / caution: Coupling type,installation,load and alignment requirements are product-specific.

Kilder: S5

A frequency expression counting cycles per minute.

CPM = 60 f[Hz]

Scope / caution: RPM equals CPM only for a one-cycle-per-revolution (1×) component.

Kilder: S12

For a stated record and preprocessing, the maximum absolute magnitude divided by the RMS magnitude.

CF = max |x(t)| / xRMS

Scope / caution: A zero-mean sine has crest factor √2. No universal value such as 3.5 diagnoses bearing impacts; bandwidth, record length, clipping and transients strongly affect it.

Kilder: S10, S12

A rotational speed at which a rotor-system response associated with a mode can become resonant under a particular excitation order and operating condition.

Scope / caution: It is not necessarily a single permanent number or always tied to 1× excitation.

Kilder: S5

Mechanisms that dissipate or redistribute vibration energy and influence response amplitude, phase and decay.

ζ = c / (2√(km))

Scope / caution: The formula is only for a linear single-degree-of-freedom model with viscous damping coefficient c, stiffness k and mass m.

Kilder: S12

Signal processing that estimates information carried by modulation of another signal.

Scope / caution: Rectification,envelope detection and analytic-signal methods are not interchangeable without stated filters,bandwidth and scaling.

Kilder: S4

Change of position relative to a stated reference. In machinery work it can be absolute motion, relative shaft-to-bearing motion, or one coordinate of an orbit.

Example sine: x(t) = X sin(ωt + φ)

Scope / caution: State axis, reference, amplitude descriptor and whether the value is scalar, waveform or two-channel orbit data.

Kilder: S10, S12

Separation between specified geometric, rotation or mass-centre axes or centres.

Scope / caution: Geometric eccentricity, mass eccentricity, electrical runout and indicated runout are different. In an ideal point-unbalance model U = m e only when U and m use consistent definitions.

Kilder: S7

Demodulation of amplitude variation from a selected carrier band, often after band-pass filtering and rectification or analytic-signal processing.

Scope / caution: Report sensor, carrier band, filter, demodulation method, sampling and speed. It does not automatically prove a rolling-bearing defect.

Kilder: S4, S5, S9

A family of algorithms that efficiently computes a discrete Fourier transform.

Scope / caution: The displayed spectrum also depends on sampling, record length, window, scaling, averaging and leakage; FFT output is not an infinitely resolved continuous spectrum.

Kilder: S4, S12

Rate of repetition of a periodic phenomenon, expressed in hertz when measured in cycles per second.

f = 1/T

Scope / caution: The period relation applies to a periodic component with period T.

Kilder: S1, S12

For the stated bearing model, the theoretical rotational frequency of the cage or rolling-element train.

FTF = (1/2) fᵣ [1 − (d/D) cos α]

Scope / caution: Do not replace the geometry with a universal fraction of shaft speed; ring motion and slip affect the value.

Kilder: S9

An acceleration magnitude expressed as a ratio to the conventional standard acceleration gₙ.

1 gₙ = 9.80665 m/s² exactly

Scope / caution: This is a reference conversion, not local gravitational acceleration and not a vibration-severity category.

Kilder: S11

For a gear with Z teeth rotating at fᵣ, the ideal tooth-engagement repetition frequency.

GMF = Z fᵣ = Z·RPM/60

Scope / caution: Observed components and sidebands depend on geometry,load,speed variation and measurement path; they are not standalone damage proof.

Kilder: S12

A component whose frequency is an integer multiple of a stated fundamental frequency.

Scope / caution: Harmonics can arise from waveform shape, nonlinearity, modulation and many mechanisms; they are not a unique fault signature.

Kilder: S4

A vendor- or procedure-specific label for measuring or processing a selected high-frequency band.

Scope / caution: There is no universal 5–60kHz HFD band. Report sensor,band,filter,detector,units and reference procedure.

Kilder: S4, S5

The SI derived unit for frequency, equal to reciprocal seconds.

1 Hz = 1 s⁻¹ = 60 CPM

Scope / caution: State the physical phenomenon whose frequency is being reported.

Kilder: S11, S12

A common informal synonym for unbalance. ISO rotor-balancing work uses the term “unbalance”; exact normative wording requires the licensed vocabulary.

Scope / caution: This glossary uses “unbalance” for the mass-distribution quantity.

Kilder: S7

A dynamic test using a measured impact input and measured response to estimate frequency-response or modal information.

Scope / caution: An unmeasured “bump test” is not equivalent to a calibrated modal test; windowing,coherence,force spectrum and boundary conditions matter.

Kilder: S5

A legacy multi-part family for evaluating machine vibration measured on non-rotating parts. Parts have different life-cycle states and many have corresponding ISO 20816 replacements.

Scope / caution: Never cite “ISO 10816” without the exact part,edition,machine scope,quantity,point,band and support condition.

Kilder: S6

A multi-part family titled “Measurement and evaluation of machine vibration”. ISO 20816-1:2016 is the published general-guidelines edition and is marked for revision.

Scope / caution: The family permits different well-defined quantities and methods. It does not supply one universal broadband-velocity table for every machine.

Kilder: S6

The rotor-balancing series. ISO 21940-1:2019 is the current introduction; ISO 21940-11:2016 plus Amendment 1:2022 addresses procedures and tolerances for rotors with rigid behaviour.

Scope / caution: Do not transfer quality grades or tolerances to a rotor without the applicable part,rotor behaviour and service-speed context. ISO 19499:2007 is withdrawn.

Kilder: S7, S8

Time derivative of acceleration and third time derivative of displacement when the derivatives share one coordinate and reference.

j(t) = da/dt = d³x/dt³

Scope / caution: Numerical differentiation amplifies high-frequency noise; report filtering and bandwidth.

Kilder: S12

A plain bearing in which a journal is supported by a sliding interface,commonly with a lubricating film.

Scope / caution: Bearing type,load,clearance,lubricant and dynamic coefficients determine stability; not every journal bearing exhibits oil whirl or oil whip.

Kilder: S5

A once-per-revolution or otherwise defined shaft-reference signal used for speed,phase or order tracking.

Scope / caution: Pulse geometry,latency,polarity,missing pulses and reference angle must be documented. A phase reference does not itself locate a heavy spot.

Kilder: S3, S4

A normalized fourth central moment of a distribution or sampled record. Pearson kurtosis for an ideal Gaussian distribution is 3; excess kurtosis is Pearson kurtosis minus 3.

Scope / caution: State convention,record length,band and preprocessing. No universal threshold such as 4 diagnoses a particular defect.

Kilder: S12

Natural logarithm of the ratio of same-direction amplitudes separated by one or more cycles in a decaying response.

δ = (1/n) ln(xₖ/xₖ₊ₙ);  δ = 2πζ/√(1−ζ²)

Scope / caution: The ζ relation assumes an underdamped linear single-degree-of-freedom system with viscous damping.

Kilder: S12

Loss or variation of an intended mechanical constraint,fit,preload or support condition.

Scope / caution: Harmonics,subharmonics or waveform clipping are possible observations,not standalone proof; inspect the joint and operating condition.

Kilder: S5

A geometric condition in which intended coupled or supported axes do not meet the applicable offset,angular or positional requirement.

Scope / caution: Axial,radial,1× and 2× components are non-unique observations. Confirm by geometry and machine-specific evidence.

Kilder: S5

Identification or calculation of modal properties such as natural frequencies,damping and mode shapes for a stated system and boundary condition.

Scope / caution: Experimental and analytical modal models require validation and uncertainty records.

Kilder: S5

A modal frequency of free vibration for a specified system and boundary condition.

Undamped SDOF: fₙ = (1/2π) √(k/m)

Scope / caution: Real machines have multiple modes and frequency-dependent,boundary-dependent properties; the formula is not a general machine model.

Kilder: S12

A frequency ratio of 2:1. Fractional-octave bands require stated centre-frequency and band-edge conventions.

f₂/f₁ = 2 for one octave

Scope / caution: An octave is a ratio,not an absolute bandwidth or diagnostic criterion.

Kilder: S12

A fluid-induced rotor instability in which a subsynchronous response can become associated with a rotor mode over a speed range.

Scope / caution: No universal “above 2× critical speed” rule is published here; verify the rotor-bearing model and operating evidence.

Kilder: S5

A fluid-induced subsynchronous precessional response associated with a fluid-film bearing system.

Scope / caution: Frequency ratio and onset are machine-dependent; do not use a universal 0.42–0.48× band as a diagnosis.

Kilder: S5

The locus of shaft or rotor position in a transverse plane,constructed from two simultaneous displacement channels and their geometry.

Scope / caution: State whether motion is relative or absolute,probe angles,polarity,filtering,runout compensation and phase reference. Shape alone is not a unique fault diagnosis.

Kilder: S3

A display of one transverse displacement channel against another to show a shaft or rotor locus.

Scope / caution: See Orbit. Probe geometry,relative/absolute reference,filter,runout compensation and direction convention are essential.

Kilder: S3

Frequency normalized by a rotational reference. Order k means f = k fᵣ at that operating instant.

order k = f/fᵣ

Scope / caution: The order number is dimensionless; its frequency in hertz changes with rotational speed.

Kilder: S4, S12

An aggregate signal magnitude over a stated measurement chain and frequency band.

Scope / caution: State quantity,descriptor,band,filter,point,direction and operating state. It is not universally velocity RMS or 10–1000 Hz.

Kilder: S3, S6

A stated extreme of a record:positive peak,negative peak or maximum absolute peak must be distinguished.

Zero-mean sine: Xpeak = √2 XRMS

Scope / caution: The √2 relation is not valid for arbitrary waveform peaks.

Kilder: S10, S12

Difference between the maximum and minimum values in a stated record.

Xpk-pk = xmax − xmin

Scope / caution: It equals twice the absolute peak only for a waveform symmetric about the stated reference.

Kilder: S10, S12

Angular or time relationship between components at a stated frequency and a stated reference.

Scope / caution: Reference location,polarity,rotation convention,filter and time synchronization are required. Phase does not directly equal a rotor heavy-spot angle.

Kilder: S4, S10

A non-contact displacement transducer system,often eddy-current,used to measure probe-to-target gap and dynamic relative motion.

Scope / caution: Sensitivity,linear range,target material,cable/driver combination and installation are system-specific. No universal API 670 sensitivity is asserted.

Kilder: S3

A direction perpendicular to a defined shaft or rotation axis.

Scope / caution: Horizontal and vertical are installation coordinates,not universal fault labels; record probe angle and positive direction.

Kilder: S3

A condition in which forcing near a modal frequency produces a response strongly influenced by that mode.

Scope / caution: Amplification and phase depend on damping,forcing,nonlinearity and boundary conditions; frequency coincidence alone is not a complete assessment.

Kilder: S12

Square root of the mean of squared values over a stated record or interval.

xRMS = √[(1/(t₂−t₁)) ∫ₜ₁ᵗ₂ x²(t) dt]

Scope / caution: For a zero-mean sine XRMS = Xpeak/√2. RMS is not by itself “energy content” or a universal severity measure.

Kilder: S10, S12

The rotating assembly or component considered in a machine model or balancing procedure.

Scope / caution: Define what is included in the rotor and its service/boundary conditions.

Kilder: S7

Rotational rate expressed as revolutions per minute.

1× rotational frequency fᵣ[Hz] = RPM/60

Scope / caution: Only a one-cycle-per-revolution component equals RPM in CPM; other orders and event counts differ.

Kilder: S12

Apparent variation associated with target geometry,shaft motion,target electrical properties or the measurement system as the shaft turns.

Scope / caution: Mechanical and electrical runout require a defined measurement and compensation procedure. Do not blindly subtract a TIR value from dynamic vibration.

Kilder: S3

A rotating member used to transmit torque or support rotating components.

Scope / caution: Straightness,surface,material,loading,boundary conditions and cracks require their own measurements; vibration alone does not define shaft condition.

Kilder: S7

A component adjacent to a carrier or principal component,with spacing associated with modulation or another periodic process.

Scope / caution: State carrier,spacing,orders and operating condition. Sidebands are not unique proof of gear,bearing or electrical damage.

Kilder: S4

A representation of signal magnitude,phase or power-related quantity versus frequency.

Scope / caution: State transform,scaling,units,window,record length,averaging,bandwidth and frequency resolution.

Kilder: S4

A component at a rational fraction of a stated fundamental or reference frequency.

Scope / caution: A subharmonic is a frequency relation,not a unique diagnosis of looseness,whirl,rub or belt behaviour.

Kilder: S4

Phase-coherent with a stated rotational reference. Some practices use “synchronous” for integer orders; others reserve it for 1×,so the convention must be stated.

Scope / caution: Do not assume every synchronous component has the same cause.

Kilder: S4

Recorded signal value versus time.

Scope / caution: It is the output of a measurement chain,not automatically the true mechanical motion. State sampling,units,filtering,range and sensor orientation.

Kilder: S4

Oscillatory variation of angular position,angular velocity or torque about a shaft axis.

Scope / caution: It requires a suitable torsional measurement or model. Ordinary transverse accelerometers and radial proximity probes do not directly measure torsional motion,although coupled responses can occur.

Kilder: S6

A ratio of response to input under a precisely defined excitation and output convention.

Base-displacement SDOF: |X/Y| = √[(1+(2ζr)²)/((1−r²)²+(2ζr)²)], r=f/fₙ

Scope / caution: The previous page omitted the numerator. Force,base-motion,relative-motion and acceleration transmissibilities use different definitions.

Kilder: S12

A time-ordered series of comparable measurements used to examine change.

Scope / caution: Comparable point,direction,sensor,band,load,speed and processing are required. A trend change is evidence for review,not an automatic failure forecast.

Kilder: S3, S5

A rotor mass-distribution condition for which the principal inertia axis does not coincide with the rotation axis as required by the model.

Ideal unbalance-force magnitude: F = Uω² = m e ω²

Scope / caution: The force identity is an ideal rotating-unbalance model. A measured 1× component is not unique proof or a complete balance tolerance.

Kilder: S7, S12

For Z equally spaced vanes rotating at fᵣ past a fixed reference,the ideal passage frequency.

VPF = Z fᵣ = Z·RPM/60

Scope / caution: Unequal spacing,multiple rows,slip or several rotors can require another event model; amplitude is not a standalone condition verdict.

Kilder: S12

Time derivative of displacement in the same coordinate and reference frame.

v(t) = dx/dt

Scope / caution: Velocity RMS is used in some applicable machine-evaluation procedures,but is not the universal primary parameter for every ISO part or machine.

Kilder: S10, S12

Variation of a mechanical quantity about a reference value,commonly described using displacement,velocity or acceleration versus time or frequency.

Scope / caution: Quantity,reference,axis,descriptor,band and operating condition must accompany a useful record.

Kilder: S1

An evaluation outcome obtained by applying stated criteria to a defined machine,measurement quantity,location,direction,band and operating condition.

Scope / caution: No universal A=new,B=acceptable,C=alert,D=danger mapping or numeric boundary is reproduced. Use the exact applicable current standard part and licensed text.

Kilder: S6

A sequence of spectra displayed against time,speed or another ordering variable.

Scope / caution: Axes,scales,normalization,window,record spacing and operating history are required for interpretation.

Kilder: S4

Spatial period of a propagating wave.

λ = cₚ/f

Scope / caution: cₚ is phase speed for the relevant wave mode and medium; the relation is not a machinery-vibration severity rule.

Kilder: S12

Precessional or orbital motion of a rotor or shaft centre. Forward and backward directions are defined relative to shaft rotation and the coordinate convention.

Scope / caution: State whether the orbit is relative or absolute and how it was filtered and compensated.

Kilder: S5

A weighting applied to a finite record before a transform to control spectral-leakage behaviour.

Scope / caution: Use the correct name Hann (not “Hanning”). Window choice trades amplitude,noise bandwidth and frequency separation; there is no universal general-purpose choice.

Kilder: S4

Excursion from a stated zero or mean reference to one stated extreme.

Scope / caution: Specify polarity or maximum absolute convention. It is not automatically half peak-to-peak for an asymmetric or offset waveform.

Kilder: S10