Glossary of Terms

Acceleration
The time rate of change of velocity. For harmonic motion, this is often expressed as g or a. Typical units for acceleration are feet per second per second (ft/s2) pk, meters per second per second (m/s2) pk, or more commonly g pk (= acceleration of earths gravity = 386.1 in/s2 = 32.17 ft/s2 = 9.81 m/s2). Acceleration measurements are generally made with an accelerometer and are typically used to evaluate high frequency vibration of a machine casing or bearing housing due to blade passing, gear mesh, cavitation, rolling element bearing defects, etc.Note: Acceleration, as well as velocity, is zero to peak measurement (Notation = pk)

Amplitude
The magnitude of periodic dynamic motion (vibration). Amplitude is typically expressed in terms of signal level, e.g., millivolts or milliamps, or the engineering units of the measured variable, e.g., mils, micrometers (for displacement), inches per second (for velocity), etc. The amplitude of a signal can be measured in terms of peak to peak, zero to peak, root mean square, or average.

Average Shaft Position
The static or average position of the shaft relative to a stationary component on the machine to which the displacement probe is mounted. The most common application is rotor axial thrust position relative to the thrust bearing. Another important application is shaft average radial position. These measurements are made using the dc (position) component of the proximity probe signal. Two proximity probes mounted in an XY configuration are required for this two-dimensional, radial position measurement.Correlation of shaft position measurements: stopped versus at speed, cold versus hot, no load versus normal load, is an important diagnostic tool.

Axial
In the same direction as the shaft centerline or the line around which the shaft rotates.

Balance Resonance Speed
A shaft rotative speed (or speed range) which is equal to a lateral natural frequency of the rotor system. Due to rotor unbalance, when the speed increases or decreases ranges, the observed vibration characteristics are (1) a peak in the 1X amplitude and (2) a more rapid change in the 1X vibration phase lag angle. The resonance speed is at the point which is 90 deg. phase lag from the angle of the heavy spot for that resonance mode. This may be slightly different than the rotative speed peak amplitude point. These changes may not happen at the same frequency because of nonlinearity, damping, and/or asymmetry in the system stiffness.

Bearing
Any low friction structure which supports the rotor and provides dynamic constraint in the radial (lateral) and/or axial directions. The two main categories are fluid film bearings and rolling element bearings. Type: journal bearing (radial), thrust bearing (axial).

Bow, Initial
A condition of plastic deformation of a shaft which results in a bent geometric shaft centerline. Often the centerline is bent in a single plane, due to gravity sag, thermal warpage, etc., however, the bow may be three dimensional (corkscrew). Shaft bow can be detected by measuring the shaft relative displacement with a proximity probe(s) at rotor slow roll speed. Note: Careful. A straight shaft becomes bowed after a balance resonance if it is unbalanced

Displacement
The change in distance or position of an object relative to a reference. Machinery vibration displacement is typically a peak to peak measurement of the observed vibrational motion or position, and is usually expressed in units of mils or micrometres. Proximity probes measure displacement directly. Signal integration is required to convert a velocity signal to displacement, but does not provide the initial displacement (distance from a reference) measurement.

EddyCurrent
Electric current which is generated in a conductive material when such material intercepts the electromagnetic field of a proximity probe

Electrical Runout
A noise component in the output signal of a proximity probe transducer system resulting from non-uniform electrical conductivity and magnetic permeability properties of the observed material. Also caused by local (spot) magnetic fields on the circumference of the shaft surface. A change in the Proximitor® output signal which does not result from a probe gap change (dynamic motion or change in average shaft position). The error repeats exactly with each shaft revolution.

Forced Vibration
The response vibration of a mechanical system due to a forcing function (exciting force). Typically, forced vibration has the same frequency as that of the exciting force.

Free Vibration
Vibration response of a mechanical system following an impulse-like initial perturbation (change of position, velocity or external force). Depending on the kind of perturbation, the mechanical system responds with free vibrations at one or more of its natural frequencies.

The repetition rate of a periodic vibration per unit of time. Vibration frequency is typically expressed in units of cycles per second (Hertz) or cycles per minute (to more easily relate to shaft rotative speed frequency). In fact, since many common machine malfunctions produce vibration which has a fixed relationship to shaft rotative speed, vibration frequency is often expressed as a function of shaft rotative speed. 1X is a vibration with a frequency equal to shaft rpm, 2X vibration is at twice shaft rpm, 0.5X vibration with a frequency equal to one-half shaft rpm, etc.

Frequency
Frequency of the vibration precession, f, of a rotor measured in Hz, rad/s (circular frequency) or cpm is often related to the rotative speed W measured in rpm or rad/s. For a synchronous precession:
 Frequency of vibration precession can also be independent of rotative speed, e.g., locked to a natural frequency of the rotor system.
Integrator
An electronic circuit that converts a velocity signal to a displacement signal or converts an acceleration signal to a velocity signal.

Keyphasor® Transducer
A transducer that produces a voltage pulse for each turn of the shaft, called the Keyphasor® signal. This signal is used primarily to measure shaft rotative speed and serves as a reference for measuring vibration phase lag angle. It is an essential element in measuring rotor slow roll bow or runout information.The Keyphasor® transducer is typically a proximity probe (recommended for permanent installations in which the probe observes a physical gap change event), an optical pickup (used for temporary installations in which the pickup observes a change in reflectivity event) or a magnetic pickup. Keyphasor® is a Bently Nevada registered trademark.
Mechanical Runout
A noise component in the output signal of a proximity probe transducer system; a probe gap change which does not result from either a shaft centerline position change or shaft dynamic motion. Common sources include out-of-round shafts, scratches, chain marks, dents, rust or other conductive buildup on the shaft, stencil marks, flat spots, and engravings.

Micrometer
A unit of length or displacement equal to 10-6 metres. One micrometre equals 0.0394 mil. (1mil = 25.4 micrometers.). Also called micron (obsolete).

Mil
A unit of length or displacement equal to 0.001 inch. One mil equals 25.4 micrometres.

Natural Frequency
The frequency of free vibration of a mechanical system at which a specific natural mode of the system elements assumes its maximum amplitude.

Optical Pickup
A noncontacting transducer which emits light from an internal infrared light emitting diode (LED), and detects the level of reflected light with a phototransistor. The most common application of this pickup is as a temporary Keyphasor® transducer, observing a once-per-turn change in shaft reflectivity (dark or light paint spot or small strip of highly reflective tape on the shaft).

Peak to Peak value
The difference between positive and negative extreme values of an electronic signal or dynamic motion

Phase Lag Angle
The timing relationship, in degrees, between two vibration signals, such as a Keyphasor® pulse and a vibration signal; also, the phase difference between two signals, such as the input force signal and output response signal. The "lag" corresponds to "minus" in mathematical formulations

Probe Gap
The physical distance between the face of a proximity probe tip and the observed surface. The distance can be expressed in terms of displacement (mils, micrometres), or in terms of voltage (millivolts). Standard polarity convention dictates that a decreasing gap results in an increasing (less negative) output signal.

Proximitor
A Bently Nevada signal conditioning device which sends a radio frequency signal to an eddy current proximity probe, demodulates the probe output, and provides output signals proportional to probe gap distances. Also called an oscillator-demodulator. Proximitor® is a Bently Nevada registered trademark.

Proximity probe
A noncontacting device which measures the displacement motion and position of an observed surface relative to the probe mounting location. Typically, proximity probes used for rotating machinery measurements operate on the eddy current principle, and measure shaft displacement motion and position relative to the machine bearing(s) or housing.

Radial
A direction on a machine which is perpendicular to the shaft centerline in the XY plane; usually refers to direction of shaft lateral or casing motion or measurement

Radial Vibration
Shaft dynamic motion or casing vibration which is measured in a direction perpendicular to the shaft axis, often called lateral vibration.

Relative Transducer
A proximity probe observing shaft motion referenced to the probe mounting, usually the bearing or bearing housing

Resonance
The condition in which the frequency of an external force coincides with a natural frequency of the system. A resonance typically is identified by an amplitude peak, accompanied by a maximum rate of change of phase lag angle.

Rolling Element Bearing
A bearing in which the low friction property derives from mechanical rolling (usually with fluid lubrication), using ball or roller elements between two constraining rings.

Signal Conditioner
A device placed between a signal source and a readout instrument to change the signal. Examples: attenuators, preamplifiers, filters, and signal converters (for changing one electrical quantity into another, such as voltage to current or analog to digital).

Slow Roll Speed
Low rotative speed of a rotor at which dynamic motion effects from forces such as unbalance are negligible. The speed at which shaft bow and runout can be measured. Typically, slow roll speed should be below 10% of the first balance resonance.

Transducer
A device for translating the magnitude of one quantity into another quantity. The second quantity often has units of measure different from the first and serves as the source of a useful signal. Vibration transducers convert mechanical motion into a proportional electronic signal (typically a voltage signal proportional to displacement).

Unbalance
Unequal radial mass distribution on a rotor system; a shaft condition where the mass centerline (principal axis of inertia) does not coincide with the geometric centerline. Eccentricity of local center of gravity (c.g.) of rotor from undisturbed axis of rotation. Also, product of rotor local mass times eccentricity of c.g. from shaft elastic axis. Often expressed in terms of eccentricity alone, e.g., in microinches of eccentricity. Also, the effective mass that causes rotor lateral synchronous vibration.

Velocity
The time rate of change of displacement. Typical units for velocity are inches/second or millimetres/second, zero to peak. Velocity measurements are used to evaluate machine housing and other structural response characteristics. Electronic integration of a velocity signal yields displacement, but not position

XY
Orthogonal (perpendicular) axes in a Cartesian coordinate system. Usually used to indicate orthogonal (mutually perpendicular) radial vibration transducers. Y represents the vertical axis, and X represents the horizontal axis.

Zero-to-Peak Value
One-half of the peak to peak value.