Condition Monitoring Systems



A direction perpendicular to a shaft’s centerline.

Radial Position

The average location, relative to the radial bearing centerline, of the shaft dynamic motion.

Radial Vibration

Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centerline.

Radio frequency

The frequency range between ultrasonic and infrared. AM broadcast frequencies range from 540 to 1,800 kHz, while FM broadcasts from 88 to 108 MHz.

Radio frequency interference (RFI)

Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring the instrument signal.

Random Noise

A random Signal Generator output exhibiting uniform spectral density over Fspan, Gaussian amplitude distribution and a crest factor of 5.


Broad-band random mixed with random narrowband tones.

Random vibration

(See Probabilistic vibration.) One whose instantaneous magnitudes cannot be predicted. Adjective “Gaussian” applies if they follow the Gaussian distribution. May be broad-band, covering a wide, continuous frequency range, or narrow band, covering a relatively narrow frequency range. No periodic or deterministic components.


A statement of the upper and lower limits over which an instrument works satisfactorily. / The region between the limits within which a quantity is measured, received or transmitted. It is expressed by stating the lower and upper range values in engineering units.

Rate gyro

A kind of gyroscope that measures rotational velocity (degrees or radians per second) around a fixed axis.


A sound exemplified by shaking a steel can full of steel nuts and bolts.

Real or Normal Modes

In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass through equilibrium at the same time.

Real Part

A plot of the real part of the frequency response function versus frequency. For a single degree of freedom, the magnitude is zero at the damped natural frequency.

Real-Time Analyzer

See Dynamic Signal Analyzer.

Real-time closed loop control

Real-time closed loop control resembles iterative closed loop control but continuously modifies drive signals throughout the test.

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.

Rectangular Window

See Uniform Window.


The existence of more than one means for accomplishing a given function. Each means of accomplishing the function need not necessarily be identical. The two basic types of redundancy are active and standby.

Active Redundancy

Redundancy in which all redundant items operate simultaneously.

Standby Redundancy

Redundancy in which some or all of the redundant items are not operating continuously but are activated only upon failure of the primary item performing the function(s).

Relative Motion

Vibration measured relative to a chosen reference. Displacement transducers generally measure shaft motion relative to the transducer mounting.


The likelihood or probability that an equipment will “do its job” for a specified length of time (say 1000 hours) under specified circumstances (such as cycling in a specified manner over certain temperature limits, experiencing a particular vibration spectrum, etc.). Reliability defined in this way can be determined experimentally. Take 1000 units. Operate them under specified conditions. At the end of 1000 hours, how many are still operating correctly?

MIL-STD-721C offers (1) The duration or probability of failure-free performance under stated conditions. If you want to be complete, add (2) The probability that an item can perform its intended function for a specified interval under stated conditions. (For non-redundant items this is equivalent to definition (1). For redundant items this is equivalent to definition of mission reliability.)

Atomica (formerly says “The extent to which an experiment, test, or measuring procedure yields the same results on repeated trials”. This definition is found in a number of dictionaries.

A student dictionary just says “dependable” when defining “reliable”.

The probability that a device will perform its objective adequately for the period of time specified, under the operating conditions specified.

Reliability engineering

Reliability engineering is the doing of those things which insure that an item will perform its mission successfully. The discipline of reliability engineering consists of two fundamental aspects:

(1) paying attention to detail

(2) handling uncertainties

Remote terminal unit (RTU)

Industrial control and data collection device similar to a PLC but designed for remote data collection, transfer and communication via wire-based or radio telemetry links to DCS or computer systems.


(1) The maximum deviation from the mean of corresponding data points taken under identical conditions. (2) The maximum difference in output for identically-repeated stimuli (no change in other test conditions). Do not confuse with accuracy. / The ability of a transducer or readout instrument to reproduce readings when the same input is applied repeatedly. / The maximum difference between output readings when the same input is applied consecutively. This is the closeness of the agreement among consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction. It is usually measured as non repeatability and expressed as in percent of span.

Repetitive shock machine

A platform to which products (to be tested or screened) are attached. Often this platform forms the bottom surface of a thermal test chamber. Pneumatic vibrators are attached to the bottom of the platform, causing it to vibrate, usually simultaneously in several axes.


Testing that reproduces a specified desired motion history (time domain) or waveform.

Residual Terms

Terms added to a curve fit algorithm to take into account the effects of modes outside the range being fitted. These terms consist of a mass term on the low frequency end and a stiffness term on the high.


The smallest change in stimulus that will produce a detectable change in the instrument output. / The smallest change in input which produces a detectable change in the output. This is the smallest increment of change that can be detected by a measurement system. Resolution can be expressed in bits, in proportions, in percent of actual reading or in percent of full scale. For example, a 12-bit system has a resolution of one part in 4,096 or 0.0244% of full scale. / The smallest change in input that will produce a detectable change in an instrument’s output. Differs from precision in that human capabilities are involved.

Resolution Bias Error

See Picket Fence Effect.


Forced vibration of a true SDoF system causes resonance when the forcing frequency equals the natural frequency, when any forcing frequency change decreases system response. (See also critical frequency) Therefore resonance represents maximum sprung mass response, if forcing frequency is varied while input force is held constant. More complex systems have many resonances. / 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. / A condition of oscillation caused by a small amplitude of periodic input has a frequency approaching one of the natural frequencies of the driven system.


The vibratory motion or force that results from some mechanical input.

Response signal

The signal from a “response sensor” measuring the mechanical response of a mechanical system to an input vibration or shock.

Response Spectrum

The frequency response function, also called the response spectrum, is a characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, and this is most easily done with a dual-channel FFT Analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems.

Response time

An output expressed as a function of time, resulting from the application of a specified input under specified operating conditions.

Response Window

A decaying exponential Window of adjustable Time Constant used on the motion signal in an impact Modal test.


Continued oscillation after an external force or excitation is removed, as after a guitar string is plucked.

Rise time

The time required for the output of a transducer to rise from 10% to 90% of its final value, as it responds to a step change in the measurand.

RMS or Root-Mean-Square value

The square root of the time-averaged squares of a series of measurements. Refer to a textbook on electrical engineering. In the exclusive case of a sine wave, the RMS value, is 0.707 the peak value.

RMS value

See Root Mean Square.

RMS Responding

A measurement equal to the RMS value of the input signal for all waveforms within the specified frequency range and crest factor limit.


Rotation about the axis of linear motion.

Rolling Element (anti-friction) Bearing

Bearing whose low friction qualities derive from rolling elements (balls or rollers), with little lubrication. A shaft rotates on rollers or balls.

Root cause analysis

Determining what actually caused a failure, as opposed to what appears to have been the cause. Usually requires the services of a specialized failure analysis lab.


The roots of the characteristic equation are complex and have a real and imaginary part. The real part describes the damping (decay rate) of the system and the imaginary part describes the oscillations or damped natural frequency of the system.

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. / The square root of the average of the squares (root-mean-square) of the instantaneous values. It is the square root of the arithmetical mean of the squares.

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.

RPM Spectral Map

A spectral map of vibration spectra versus rpm.

Runout Compensation

Electronic correction of a transducer output signal for the error resulting from slow roll runout.