# Rotor dynamics and measurement

English

8.1 ACCEPTANCE REGION
Region formed by {the 1X or 2X vibration amplitudes and phase angles (in polar coordinates)} or {the rotor centerline (in Cartesian coordinates)} offering an important indicator of deterioration of the rotor.
8.2 AERODERIVATIVE
Aircraft jet {gas turbine} engine adopted and modified for industrial use.
8.3 ALIGNMENT
Process of positioning of machine components (bearings, rotor, casing, foundation, piping, etc.) with respect to each other for efficient operation.
8.4 ANALYZER, REAL TIME
Device displaying vibration quantities and characteristics in real time.
8.5 ANGLE, ATTITUDE
Angle between the resultant vector of all steady radial loads on a rotor and a line connecting the bearing and shaft centers.
8.6 ANTISWIRL
Injection of fluid, usually tangential, into the clearance between a rotor and a stationary part, in the direction opposing that, fluid being drugged in motion by the rotor rotation, in order to improve rotor stability.
8.7 APHT
Acronym for a plot of filtered vibration amplitude and phase lag angle versus time.
8.8 BALANCING (OF A ROTOR)
Reduction of the 1X lateral vibration of a rotor by adjusting its radial mass distribution so, that the mass centerline approaches the rotor geometric centerline.
8.9 BEARING, FLUID-FILM LUBRICATED
Bearing generating a rotor support by a fluid layer between the rotor and bearing surface.
8.10 BEARING, ROLLING ELEMENT
Bearing in which the low friction property derives from mechanical rolling (usually with fluid lubrication) of ball or roller elements between two constraining rings.
8.11 BOW
Plastic deformation of a shaft, which results in its bent geometric centerline.
8.12 CENTERLINE, (GEOMETRIC)
Line of all cross-section geometrical centers of a rotor.
8.13 CENTERLINE, MASS
Principal axis of inertia of a rotor.
8.14 CURRENT, EDDY
Electric current generated in a conductive material of a proximity transducer, when it intercepts electromagnetic field.
8.15 DATA, DIRECT [UNFILTERED, RAW, ALL PASS*, OVERALL*]
Data or a signal which represent the original transducer signal.
8.16 DATA, REFERENCE
Machine measurements at defined conditions used for comparison and correlation with measurements taken at a later time or at different operating conditions.
8.17 DATA, TREND
Periodic storage of static and dynamic data for observation of their changes as a function of time.
Direction of the rotor centerline or the line around which the rotor rotates. (Any direction starting at any point on the rotor centerline, perpendicularly to its tangent.)
8.19 DISK
Wheel, usually solid and axially slim, integral to or mounted on a rotating shaft. Note: In contrast to a shaft element, disk does not contribute to the rotor stiffness.
8.20 DROP, ROD
On a reciprocating machine, drop of a piston in a cylinder due to a piston rider ring deterioration.
8.21 ECCENTRICITY, BEARING
Radial displacement of a rotor journal centerline from the geometric center of a fluid-film lubricated bearing.
8.22 ECCENTRICITY RATIO
At steady state, dimensionless ratio of distance between the position of the rotor centerline and centerline of its bearing or seal, relative to their radial clearance.
8.23 EXPANSION, CASING
Measurement of changes in axial position of a machine casing relative to its foundation caused by temperature, during start-up and shutdown.
8.24 EXPANSION, DIFFERENTIAL [SPINDLE]
Measurement of axial position of a rotor relative to its casing at some distance from the thrust bearing caused by temperature, during start-up and shutdown.
8.25 FLUID AVERAGE CIRCUMFERENTIAL VELOCITY
Velocity at which the fluid radial damping force rotates in small clearances between a rotor and a stationary part of a rotating machine.
8.26 FLUID AVERAGE CIRCUMFERENTIAL VELOCITY RATIO [LAMBDA]
Non-dimensional ratio of the fluid average circumferential velocity to the rotor speed.
8.27 FORCE, ALFORD
Fluid-related tangential force due to clearance changes around the periphery of a turbine rotor, linearly dependent on rotor lateral displacements.
On any blade {vane} machine (turbine, axial compressor, fan, propeller, etc.), a possible vibration frequency equal to the number of blades {vaned stationary elements} times rotor speed.
8.29 FREQUENCY, GEAR MESH
Possible vibration frequency of any machine containing gears, equal to the number of gear teeth times rotor speed.
8.30 GAUGE, STRAIN
Transducer attached to a deformable solid, which reacts to changes in strain, typically through changes in deformation.
8.31 INFLUENCE VECTOR [COEFFICIENT*]
At balancing of a rigid rotor, the 1X vibration vector (amplitude and phase) due to any trial mass divided by the trial mass vector, at a particular rotor speed.
8.32 INFLUENCE VECTOR [COEFFICIENT*], DIRECT
Influence vector where the measured vibration vector and the unbalance force vector are at, or near, the same lateral plane along the rotor centerline.
8.33 INSTABILITY, FLUID-INDUCED
Limit cycle of self-excited lateral vibration of a rotor caused by interaction with fluid in its bearing or seal. Note: 1. It occurs after the system exceeds a threshold of instability when both direct dynamic stiffness and quadrature dynamic stiffness component of the rotor / fluid system equal zero. 2. It can occur in passive systems (like pumps) when any fluid (oil, process gas, etc.) enclosed in the rotor/ bearing {seal} {stator} clearance is dragged by rotating shaft into circumferential rotation or in active systems (like turbines) when the externally introduced axial fluid flow leads to a rotor driving torque and consequently creates a circumferential fluid flow. 3. Conventionally, fluid-induced instability is separated into two regimes called whirl and whip characterized by forward precession.
8.34 JOURNAL
Portion of a rotor inside the fluid-lubricated bearing.
1. Angular region around a rolling element bearing where, due to a radial force applied to the rotor, there is maximum compressive force between the shaft and outer race of the bearing. 2. Direction of steady-state load on any bearing (including fluid film).
8.36 LVDT
Acronym for Linear Variable Differential Transformer used for casing expansion or valve position measurements.
8.37 MAP, NATURAL FREQUENCY [CRITICAL SPEED*]
Cartesian plot of natural frequencies of a rotor system versus its bearing stiffness, rotor speed or any other parameter.
8.38 MASS, CALIBRATION [TRIAL]
In rotor balancing procedures, mass of known magnitude placed on the rotor at a known location under known operating conditions, in order to measure the resulting change in the rotor 1X vibration response.
8.39 MATERIAL, PIEZOELECTRIC
Any material, which provides a conversion between mechanical and electrical energy.
8.40 MODULATION, AMPLITUDE [DIRECT], [AM]
Variation in the amplitude of a carrier vibration signal.
8.41 MODULATION, FREQUENCY, [FM]
Variation in the frequency of a carrier vibration signal.
8.42 ORBIT [LISSAJOUS PLOT]
Path of a rotor centerline motion during precession [orbiting].
8.43 PLOT, BODE [UNBALANCE RESPONSE*]
Pair of plots in Cartesian coordinates displaying the nX (n=1,2,3...) vibration phase lag angle and amplitude versus frequency (e.g. rotor speed).
8.44 PLOT [DIAGRAM], CAMPBELL [CASCADE*, WATERFALL*]
1.Spectrum plot with frequency on the vertical axis, rotor speed on the horizontal axis and spectral amplitude indicated by the diameter of a circle (or square) at each point of the plot. 2. Plot of rotor lateral and torsional natural frequencies versus possible excitation frequencies.
Three-dimensional spectrum plot with rotor speed on the third (vertical or inclined) axis. Note: This term is sometimes reserved for a live display, continuously updated as new spectra are generated.
8.46 PLOT, FULL SPECTRUM
Enhanced spectrum plot produced by using the time base waveform from vertical and horizontal set of transducers to calculate the amplitudes of forward and backward (in relation to the rotor rotation) components for each frequency.
8.47 PLOT, NYQUIST
Graphical presentation in polar coordinates of the response of a system used to evaluate its stability. Note: This term should not be used to describe a similar polar presentation of machine vibration vector data.
8.48 PLOT, POLAR [NYQUIST*] (OF ROTOR VIBRATIONS)
Graphical presentation in polar coordinates of a locus of the rotor nX (n=1, 2, ...) filtered vibration vector as a function of rotor speed, usually during start-up and shutdown, or time (at steady state).
8.49 PLOT, AMPLITUDE {PHASE} {POWER} SPECTRUM
Cartesian plot representing amplitudes {phases} {squares of velocity amplitudes} of vibration components versus vibration frequency.
8.50 PLOT, TIMEBASE [WAVEFORM]
Cartesian plot of the instantaneous value of a signal as a function of time.
8.51 PLOT, TREND
Graphical presentation, in Cartesian or polar coordinates, of a measured variable versus time.
8.52 PLOT, WATERFALL [CAMPBELL*]
Plot similar to a cascade plot, except that on the vertical or inclined axis, instead of rotor speed, is usually time or other time-related variable.
8.53 POINT, NODAL [NODE]
Point of zero shaft deflection in a specific mode shape.
8.54 PRECESSION [ORBITING]
1. Motion of the rotor centerline around its static equilibrium position in the plane perpendicular to the spin axis, at any axial location. 2. Lateral vibrations of a rotor in two orthogonal directions in the plane perpendicular to the rotor centerline.
8.55 PRECESSION {WHIP} {WHIRL}, BACKWARD [REVERSE]
Precession{whip} {whirl} of a rotor in the opposite direction to its spin.
8.56 PRECESSION {WHIP} {WHIRL}, FORWARD
Precession {whip} {whirl} of a rotor in the same direction to its spin.
8.57 PRECESSION {WHIP} {WHIRL}, SUBSYNCHRONOUS {SUPERSYNCHRONOUS}
Non-synchronous precession {whip} {whirl} with frequency lower {higher} than the rotor speed.
8.58 PRECESSION , SYNCHRONOUS
Precession (most often caused by rotor unbalance) with frequency equal to the rotor speed.
8.59 PROBE
Any transducer or specifically an eddy current proximity transducer.
8.60 RACE, OUTER
For rolling element bearing, generally an annular component which is positioned between the rolling elements and the bearing housing.
8.61 ROLL-OFF
Rate of attenuation of an amplitude and corresponding rate of change in phase relative to frequencies above {below} a certain point.
8.62 ROTOR
Shaft possibly carrying disks and mounted in supporting bearings.
8.63 ROTOR, ANISOTROPICLY SUPPORTED
Rotor with bearings and supports, which have different stiffness and damping properties in different lateral directions.
8.64 ROTOR MODE (SHAPE), FORCED
1.Axial distribution of the filtered lateral vibration in certain direction at a specific rotor speed 2. For a symmetric rotor, a plain figure rotating with a particular frequency about the rotor centerline. Note: It is a combined characteristic of a rotor system and lateral forces distribution.
8.65 ROTOR MODE (SHAPE), FREE
Rotor relative deflections at each axial position in free vibrations at each natural frequency.
8.66 ROTOR, NONSYMMETRIC [ANISOTROPIC]
Rotor which has different distribution of mass and / or stiffness relative to different axes of the rotor.
8.67 ROTOR POSITION [ATTITUDE] ANGLE
Angle between an arbitrary reference line drawn through the center of a bearing and the line connecting the bearing and shaft centers, measured in the direction of rotor rotation.
8.68 ROTOR, SYMMETRIC [ISOTROPIC]
Rotor which has the same distribution of mass and / or stiffness relative to different central axes of cross section.
8.69 ROTOR VIBRATION REGION
Frequency range including the most influential frequency components due to rotor or rolling elements defects but excluding any significant frequency (nX components) due to bearing internal defects, typically n between 1/4 and 3.
8.70 RUNOUT COMPENSATION
Electronic correction of a transducer output signal for the error resulting from run-out.
8.71 RUN-OUT, ELECTRICAL
Noise component in the output signal of a proximity transducer system, repeating exactly with each rotor revolution, caused by non-uniform electrical conductivity and magnetic permeability properties of the observed material or by local magnetic fields on the circumference of the rotor surface.
8.72 RUN-OUT, MECHANICAL
1.Noise component in the output signal of a proximity transducer system caused by imperfections on the surface of a rotor. 2. Transducer gap change resulting from neither a rotor centerline position change nor rotor dynamic motion.
8.73 RUN-OUT, SLOW ROLL
Combination of electrical run-out and mechanical run-out measured at rotor speed for which dynamic effects associated with rotation are negligible.
8.74 SHAFT
Rotating element which provides mass, stiffness and damping.
8.75 SIGNAL-TO-NOISE RATIO
Ratio of the magnitude of a signal relative to the magnitude of the noise present in the signal.
8.76 SPALL [SPALLING]
Evidence of serious bearing degradation when a flock or chip of metal is removed from one of the bearing races or from a rolling element.
8.77 SPEED
Magnitude (absolute value) of a velocity vector.
8.78 SPEED, CRITICAL [RESONANT] {BALANCE RESONANCE*]
Rotor speed equal to any natural frequency of the lateral bending mode of a rotor excited by its unbalance.
8.79 SPEED, CRITICAL OF THE SECOND ORDER
Speed at which resonance of a rotor lateral vibrations with frequency 2X occurs due to a lateral constant force (e.g. gravity force acting on a horizontal rotor) if the rotor is laterally nonsymmetrical. Note: Corresponding 2X vibrations might be meaningful symptoms of a cracked rotor or other failures caused by rotor non-symmetric nonlinearities.
8.80 SPEED, ROTOR [ROTATIVE SPEED] , [FREQUENCY OF REVOLUTION] , [REVOLUTIONS]
Speed at which a rotor is rotating at a given instant, usually expressed in units of revolutions per minute (rpm) or per second (rps) or in hertz (Hz). Note: When compared to a frequency, a rotor speed is expressed only in rps or Hz.
8.81 SPEED, SLOW ROLL
Low rotor speed at which dynamic effects associated with rotation are negligible, typically below 10% of the first natural frequency.
8.82 SPIKE, PRIME
In the study of rolling element bearings, a frequency range which encompasses primary bearing fault frequencies and their harmonics.
8.83 SPIN
Rotation of a rotor about its centerline with a rotor speed.
8.84 SPOT, HEAVY
At single plane balancing, angular location of the unbalance vector (resultant of the mass unbalance distribution) of a rotor at its specific lateral plane.
8.85 SPOT, HIGH
At rigid rotor balancing, location on the shaft surface under a proximity transducer when the 1X filtered vibration signal produced by the transducer reaches its positive peak.
8.86 STABILITY (OF A ROTATING MACHINE)
State of a rotating machine when rotational motion of all rotating elements and the steady equilibrium of non-rotating parts are not accompanied by various modes of vibration with amplitudes exceeding prescribed acceptable levels.
8.87 STALL , ROTATING
Fluid whirl with flow of fluid detached from the end of a rotating machine blade or vane.
8.88 STIFFNESS, DIRECT DYNAMIC
Component of dynamic stiffness of a mechanical system co-linearly opposing to applied dynamic force.
Component of dynamic stiffness of a mechanical system opposing to applied force perpendicularly (90 degrees out of phase) and comprising possible gyroscopic effects of the system, hysteretic damping and fluid generated tangential forces.
8.90 THERMOCOUPLE
Temperature transducer comprised of two dissimilar metal wires which, when heated or cooled, produce a proportional change in electrical potential at the point of their join.
8.91 THRESHOLD OF INSTABILITY [STABILITY]
Value of a quantity or a parameter (most often rotor speed) at which self-excited vibrations begin.
8.92 TRANSDUCER [SENSOR], [PROBE], [PICKUP]
Device for translating the magnitude of one quantity into another quantity, serving as the source of a useful signal.
8.93 TRANSDUCER , DUAL
Transducer set consisting of a proximity transducer and a velocity transducer installed radially at the same point on a machine bearing housing.
8.94 TRANSDUCER, KEYPHASOR
Transducer producing a voltage pulse for each turn of rotor, used primarily for measuring of rotor speed and as a reference for measuring of vibration phase angle.
8.95 TRANSDUCER , PROXIMITY
Non-contacting transducer measuring the displacement motion and position of an observed surface relative to the transducer mounting location.
8.96 TRANSDUCER, RELATIVE
Transducer observing rotor motion referenced to the transducer mounting, usually the bearing or bearing housing.
8.97 TRANSDUCER, SEISMIC [INERTIAL]
Any vibration transducer which measures the absolute vibration of an object.
8.98 UNBALANCE
1. Unequal radial mass distribution on a rotor system. 2. Condition when the mass centerline of a rotor does not coincide with its geometric centerline. 3. Eccentricity of local centre of gravity of a rotor from its undisturbed axis of rotation. 4. Product of rotor local mass times eccentricity of centre of gravity from rotor elastic axis. 5. Effective mass that causes rotor synchronous lateral vibration.
Vibration of a rotor or casing in the direction perpendicular to the rotor centerline.
8.100 VIBRATION, RELATIVE
Vibration of a rotor measured with respect to a chosen reference, usually to a bearing or its housing.
8.101 WHIP [WHIPPING]
Self-excited vibrations of a rotor in a form of rotor precession with a frequency (constant or nearly constant relative to a variable rotor speed), usually close to one of natural frequencies of the system.
8.102 WHIP, DRY
Violent backward whip caused by full annular rub of a rotor in its mechanical circumferential seal {retainer bearing} with high dry friction whereby frequency is close to one of the frequencies of the corresponding coupled rotor / seal {retainer bearing} / stator system.
8.103 WHIP, FLUID
Fluid-induced self-excited forward whip with frequency near to a natural frequency of the rotor lateral bending mode.
8.104 WHIP [WHIRL*], HYSTERETIC
Forward whip caused by internal / structural damping of a hysteretic type in an elastic rotor.
8.105 WHIRL [WHIRLING]
Self-excited vibrations of a rotor in a form of rotor precession with a frequency nearly proportional relative to a variable rotor speed.
8.106 WHIRL , AERODYNAMIC
Sub-synchronous, usually backward fluid whirl of a propeller rotor at high air speeds due to destabilizing aerodynamic forces on the blades.
8.107 WHIRL , FLUID
Fluid-induced forward whirl with coefficient of proportionality close to the fluid average circumferential velocity ratio of fluid in a bearing{seal} {periphery} of a rotor, whereby whirl frequency corresponds to one of fluid-related natural frequencies of the rotor / fluid system.
8.108 WHIRL {WHIP}, OIL
Sub-synchronous fluid whirl {whip} with highly eccentric position during precession of a rotor in its oil lubricated bearings.
8.109 WHIRL {WHIP}, STEAM
Fluid whirl {whip} caused by steam flow in clearance between the blade ring and casing of a steam turbine.
8.110 1X {n X} COMPONENT
In a complex vibration signal, notation for the signal component that occurs at the rotor speed {n multiple of the rotor speed, n being an integer or fraction}.