glossary for vib terms

8/3/2019 Glossary for Vib Terms

1/22

ERI News

your reliability newsletter

Newsletter Archives

Vibration & Shock

Glossary

Technical Presentations

Useful Links

Search this site

Search

Free Newsletter

To subscribe to ERI

News - your reliability

newsletter, just fill in

the form above and click

on Sign up now! You will

receive an e-mail

message to confirm your

subscription immediately.

Just click on the link

provided on the message

and you'll be all set.

Visit our newsletter

archives page to take a

look at our past issues!

Company

First Name

Last Name

E-mail

Sign up now!

RESOURCES

Vibration & Shock Glossary

We offer engineers, technicians and others an extensive glossary of

terms relating to vibration and shock testing, measurements, analysis

and calibration.

Select the first letter of the word from the list below to jump to

appropriate section of the glossary. If the term you are looking for

starts with a digit or symbol, choose the '#' link.

ABCDEFGHI JKLMNOPQRSTUVWXYZ#

- A -

Absolute. A term applied to calibration (e.g. of an accelerometer)

based upon the primary standards of mass, length and time. (See also

c o m p a r i s o n calibration .)

Absolute Accuracy. A measure of the uncertainty of an instrument

reading compared to that of a primary standard traceable to NJST.

Absolute vibration. Vibration of an object relative to a fixed point in

space. Seismic sensors (accelerometers and velocity pickups) measure

absolute vibration. Contrasts with relative vibration.

Absorber. A device capable of reducing, of attenuating, of "soaking

up" vibration, usually converting it to heat.

Accelerated Life Testing. An activity during development of a newproduct. Prototypes are subjected to stress levels (including vibration,

usually random) that are much higher than those anticipated in the

field. The purpose is to identify failure-prone, marginally-strong

elements by causing them to fail. Those elements are strengthened

and tests are continued at higher levels. Sometimes called Test,

Analyze & Fix (TAAF) testing.

Accelerated Stress Testing. A post-production activity on a

sampling (100% at first) of units. The intent is to precipitate hidden or

latent failures caused by poor workmanship or by substitution of

weaker parts and to prevent flawed units from reaching the next

higher level of assembly or the customer. Intensity is typically half

that achieved in accelerated life testing.

Acceleration. Acceleration is rate of change of velocity with time

(denoted as dv/dt or d2x/dt2), usually along a specified axis, usuallyexpressed in g or gravitational units. It may refer to angular motion.

Accelerometer . A sensor or transducer or pickup for converting

acceleration to an electrical signal. Two common types are

piezoresistive and piezoelectric.

Accessibility. A measure of the related ease of access to various

portions of an item for operation or maintenance.

Accuracy. The capability of an instrument to indicate the true value.

Do not confuse with inaccuracy (sum of hysteresis + non-linearity +

drift + temperature effect, etc.) nor with repeatability .

A/ D Converter. A device that changes an analog signal such as

voltage or current into a digital signal (consists of discrete data

Page 1 of 22Vibration and shock test glossary

9/20/2011http://www.equipment-reliability.com/glossary.html


2/22

values).

Aggravated Test. A test in which one or more conditions are set at a

higher stress more level than the test item will encounter in the field,

in order to reduce test time or assure a margin of safety.

Aliasing. A spectrum analysis problem resulting from sampling data attoo low a sampling frequency. I t causes high-frequency signals to

appear in a spectrum at low frequencies. In a sampled data system,

the analog input must be sampled at a rate at least twice the

bandwidth of the signal to avoid loss of data (Nyquist Theorem).

Alignment. A desired machinery condition, in which the axes of

components of a machine are adjusted so as to be colinear, parallel or

perpendicular, and thus creating least vibration.

Ambient environment. The conditions (e.g. temperature and

humidity) characterizing the air or other medium that surrounds

materiel.

Amplitude. The magnitude of variation (in a changing quantity) from

its zero value. Always modify it with an adjective such as peak, RMS,

average , etc. May refer to displacement, velocity, acceleration,

voltage, current, force or pressure.

Angular Frequency. (Also known as circular or torsional frequency.)

is the torsional vibration frequency in radians per second. Or

multiply by 2 and express in hertz (Hz) or (obsolete) cycles per

second (cps).

Angular rate sensor. A sensor that measures rotational velocity

(degrees or radians per second) around its sensitive axis.

Anti-aliasing filter. A low pass filter designed to stop frequencies

higher than some fraction of the sample rate, in order to minimize

aliasing.

ASD . Auto spectral density. The measure of acceleration power per Hz

of analysis bandwidth. Also known as Power spectral density, PSD. The

square root of the area under the ASD curve is defined as the g RMS of

acceleration.

Attenuate. Reduce.

Autoranging. The capability of an instrument to switch among ranges

automatically. The ranges usually are in decade steps.

Auto spectrum (power spectrum). A spectral display of the power

(voltage squared) at each frequency. Phase is ignored.

Availability . A measure of the degree to which an item is in an

operable and committable state at the start of a mission, when the

mission is called for at an unknown (random) time.

Average. Refer to a textbook on electrical engineering. In the

exclusive case of a pure sine wave, the average value is zero.

Average Responding. A measurement proportional to the average of

the absolute values of all input waveforms within a specified frequencyrange.

Averaging. Summing and suitably dividing several like measurements

to improve accuracy or to lessen the effect of any asynchronous

components.

A-weighting Emphasis given by filtering to sound measurements with

the goal of compensating for the non-flat frequency response of

human hearing, in order to get numbers approximating human

response. See Equal Loudness Curves.

Axial. Along the centerline of a shaft.

Axial (thrust) position. Change in a rotors position or displacement




3/22

along its axis, relative to some nearby fixed point.

back

- B -Balancing. (mechanical) Adjusting the distribution of mass in a

rotating element, to reduce vibratory forces generated by rotation.

Bandwidth. The frequency range (usually stated in hertz or Hz) within

which a measuring system can accurately measure a quantity.

Barrier . Blocks airborne sound from entering passenger compartment

via an aperture. Example, a piece of sticky elastomer that is pressed

on so as to cover a hole.

Bias. Refers to a more or less persistent tendency for measurements,

as a group, to be too large or too small.

Bounce Test: A shaking of unrestrained (loose) cargo. The cargo is

repeatedly thrown a short distance into the air and then falls onto the

vibrating platform.

Broadband. Vibration (or other) signals which are unfiltered. Signals

at all frequencies contribute to the measured value.

Baseline spectrum. A vibration spectrum taken when a machine is in

good working condition (new or just overhauled), used as reference for

future monitoring or analysis.

Blade-passing frequency. A potential vibration frequency on any

bladed machine (turbine, fan, etc.), the number of blades x shaft

speed.

Bode plot. The magnitude of vibration at 1x shaft speed, also its

phase relative to shaft position, both plotted against running speed.

Bow . An undesirable shaft condition (rotating machinery) in which the

shaft centerline is not straight.

Bump Test: A horizontal striking of an impacting surface againstcargo, or cargo motion arrested by impacting a stationary object.

Burn-in. Continuously powering a product, often at constant elevated

temperature, in order to accelerate the aging process. Much less

effective, in my opinion, than power-on ramping of temperature +

random vibration.

Buzz A sound exemplified by loose power transformer laminations

(dominated by 120 Hz where the power frequency is 60 Hz).

back

- C -

Calibration . (as applied to vibration sensors) An orderly procedure for

determining sensitivity as a function of frequency, temperature,altitude, etc.

Campbell diagram. A mathematically-constructed diagram used to

check for coincidences of vibration sources (1x, 2x, etc. shaft speed)

with rotor natural frequencies, resulting in rotor resonances. It plots

frequency vs RPM, with plot size growing with increasing amplitude.

Sometimes called an interference diagram.

Cavitation. A localized low-pressure vaporization condition (cavities or

bubbles) within a liquid, as around a propeller or within a pipe or in a

pump's suction line. Bubble collapse creates unwanted sound and/or

vibration. May be destructive.




4/22

Channel. A sensor (or pickup or transducer) with its associated signal

conditioner and monitor (for observing the signal) and recorder (for

storing the signal).

Characterization is aimed at providing the shaker control system

with needed information about the testing system and the test article.Pretest characterization (at reduced intensity), repeated several

times iteration is commonly performed. Increasingly, adaptive

characterization modifying the control system, based upon

observing responses, is being performed during the test.

Charge amplifier. An amplifier which converts a charge input signal

(as from an accelerometer) into an output voltage; a charge-to-

voltage converter.

Circuit card. A flat board that holds chips and other components on

the top side and has printed electrically conductive paths in multiple

layers for the components on its bottom side.

Closed Loop Control. Responses are measured and fed back to the

control system so as to refine or modify drive signals in order to bring

responses closer to the reference or desired motions. See iterative

closed loop control.

Coherence. A measure of the similarity of vibration at two locations,

giving insight into possible cause and effect relations.

Comparison. A term applied to calibration (e.g. of an accelerometer)

in which sensitivity is tested against a standard. See also ab s o l u t e

calibration .

Compliance . The reciprocal of stiffness, i.e. displacement divided by

force.

Condition monitoring (CM). The measurement, recording and

analysis of machinery parameters (such as acceleration) to determine

machinery health. Todays condition is compared with earlier condition,

as when a machine was new. Also called machinery health monitoring,

MHM.

Constant-bandwidth filter. A bandpass filter whose bandwidth isindependent of center frequency. Filters simulated digitally by an FFT

process are constant bandwidth.

Constant percentage filter. A bandpass filter whose bandwidth

relates (1/3 x, 1/10 x, etc.) to center frequency. May be synthesized

digitally.

Contact Bounce. The intermittent opening of relay contacts during

closure.

Critical frequency. A particular resonant frequency (see r e s o n a n c e )

at which damage or degradation in performance is likely.

Critical machines. Machines (in a plant) that are vital to continued

operation. Vibration should be monitored continuously.

Critical speeds. Any rotating speed which results in high vibration

amplitudes. Often these are speeds which correspond to systemnatural frequencies.

Cross-axis sensitivity. See transverse sensitivity.

Crossover frequency. In sinusoidal vibration testing, the unique

forcing frequency at which the required displacement yields the

desired acceleration and vice versa.

Cross-talk. Interference or noise in a sensor or channel, coming from

another sensor or channel.

Cycle. The complete sequence of instantaneous values of a periodic

event, during one period.




5/22

back

- D -

D/ A converter. A device that converts a digital signal (discretevalues) into an analog voltage.

Damp (not dampen). To reduce the Q or maximum resonant

response, by extracting energy, usually by converting motion into

heat. Your automobiles shock absorbers damp body motion.

Damper. One form adheres or is sprayed onto relatively thin metal

panels so as to extract energy when panels flex. Lowers "Q" of each

panel mode. Goal: to reduce air flow (over the panel)-induced

vibration and radiated noise.

Damping. Dissipation of oscillatory or vibratory energy with motion or

with time. Critical damping Cc

is that value of damping that provides

most rapid response to a step function without overshoot. Damping

ratio is some fraction of Cc.

Decade . The interval between two frequencies which differ by exactly

10:1.

deciBel. Ratios of identical quantities are expressed in decibel or

deciBel or dB units. The number of dB is ratiod against some standard

or reference value in terms of the base 10 logarithm of that ratio. In

measuring acoustic or vibration power (as in PSD or ASD of random

vibration), the number of dB = 10 log10 P/Po. P

o, the reference level,

equals 0 dB. In measuring the more common voltage-like quantities

such as acceleration, the number of dB = 20 log10 E/Eo

Eo, the

reference level, equals 0 dB.-

Degrees of freedom. In mechanics, the total number of directions of

motion (of all the points being considered) on a structure being

modeled or otherwise evaluated. Example: a platform experiencing

heave, fore-and-aft, left-and-right motions as well as roll, pitch and

yaw, is said to have six degrees of freedom. Statistical degrees of

freedom: In random vibration measurement, twice the number of

averages being used.

Dependability. A measure of the degree to which an item is operable

and capable of performing its required function at any (random) time

during a specified mission profile, given item availability at the start of

the mission. (This definition is significantly different from the definition

of dependability used by most other US and international organizations

dealing with reliability e.g., IEC and SAE. E.g. IEC 50 Chapter 191:

"The collective term used to describe the availability performance and

its influencing factors: reliability performance, maintainability

performance and maintenance support performance." As such, its use

is restricted to general descriptions in non-quantitative terms).

Dependability is related to reliability; the intention was that

dependability would be a more general concept then reliability.

Design limit. The operational limit of a product, beyond which it not

required to function properly.

Design ruggedization. See Accelerated Life Testing.

Deterministic vibration. A vibration whose instantaneous value at

any future time can be predicted by an exact mathematical expression.

Sinusoidal vibration is the classic example. Complex vibration is less

simple (two or more sinusoids).

Differentiation. Representation in terms of time rate of change.

Example: differentiating velocity yields acceleration. In a computer,

this is accomplished by multiplying the velocity signal by j, where

is frequency multiplied by 2.

Differential I nputs. Two inputs, where the measured signal is the

difference between them. Any voltage common to both is rejected.




6/22

Differential inputs can reduce noise picked up by the signal leads.

Displacement specifies change of position, or distance, usually

measured from mean position or position of rest. Usually applies to

uniaxial, less often to angular motion.

Distortion . In mechanics, any unwanted motion. If sinusoidal motionwere desired at a fundamental frequency, distortion is any motion at

harmonics or subharmonics of that frequency, or any mechanical

"hash" (perhaps due to parts colliding). In electronic measurements,

distortion is any unwanted signal; e.g. amplifiers may generate

unwanted signals.

Drift. Slow variation of a performance characteristic such as gain,

frequency, or power output; for instance, due to temperature or aging.

Usually, drift only is significant when measuring low-level signals (a

few millivolts) over long periods of time or in difficult environmental

conditions.

DSP - Digital Signal Processor. A microprocessor optimized for digital

signal manipulations.

Duration of a shock pulse is how long it lasts. For "classical" pulses

(not found in the real world), time is usually measured betweeninstants when the amplitude is greater that 10% of the peak value.

Durability. A measure of useful life (a special case of reliability).

DUT. Device under test. See also UUT or unit under test (being

tested).

Dynamic motion. Movement, as compared with nonmoving or static

position. Dynamic motion is sensed with displacement or velocity

pickups or with accelerometers.

Dynamic Range. The ratio of a specified maximum level of a

parameter, such as power, current, voltage, or frequency, to the

minimum detectable value of that parameter.

Dynamic signal analyzer DSA. Vibration analyzer using digital signal

processing and the Fast Fourier Transform (FFT) to display vibration

frequency components. May also display the time domain and thephase spectrum. Usually interfaced to a computer.

back

- E -

Eccentricity, mechanical. Variation of shaft surface radius when

referenced to the shaft's true geometric centerline. Out-of-roundness.

Eddy current. Electrical current generated (and dissipated) in a

conductive material (often a rotor shaft) when it intercepts the

electromagnetic field of a displacement or proximity probe.

Engineering units. Units that are decided upon by an individual user

or by agreement among users. Examples include inches/second,

mm/s, g, Hz, Tu, etc.

Environment . The aggregate of all external and internal conditions

(such as temperature, humidity, radiation, magnetic and electric fields,

shock vibration, etc.) either natural or man made, or self-induced, that

influences the form, performance, reliability or survival of an item.

Environmental engineering specialist. One whose principal work

assignment lies in the technical area of natural and induced

environments and their relation to military equipment. A person who

has expertise in measuring and analyzing field environmental

conditions, formulating environmental test criteria, specifying

laboratory simulation of environments, and evaluating the effects of

environments on equipment.




7/22

Environmental stress screening (ESS). A post-production process

in which produced units are subjected to stresses more severe than

anticipated in service. The object is to precipitate latent defects into

recognizable failures, so that that particular unit does not proceed

further in production nor reach the customer.

Environmental testing. Subjecting a sample of products to a

simulation of anticipated storage, transport and service environments

(such as vibration, shock, temperature, altitude, humidity, etc.)

Equal Loudness Curves. Graphs of pure tone (constant or steady)

sound pressure levels (labeled as to loudness level in phons) vs.

frequency, with each graph representing equal loudness.

Error. The difference between the indicated and the true values of a

variable being measured.

Essential machinery. See Critical Machinery.

Excitation . The voltage or current applied to a transducer.

back

- F -

Failure. The event, or inoperable state, in which any item or part of

an item does not, or would not, perform as previously specified.

Failure analysis. Subsequent to failure, the logical systematic

examination of an item, its construction, application and

documentation to identify the failure mode made and determine the

failure mechanism and its basic course.

Failure catastrophic. A failure that can cause loss.

Failure effect. The consequence(s) a failure mode has on the

operation, function, or status of an item. Failure efforts are classified

as local effect, next higher level, and end effect.

Failure mechanism. The mechanical, chemical, physical or otherprocess that results in failure.

Failure mode analysis. A procedure aimed at determining why a

failure occurred.

Failure mode and effects analysis (FMEA). A procedure by which

each potential failure mode in a system is analyzed to determine the

resulting effects thereof on the system and to classify each potential

failure mode according to its severity.

Failure rate. The total number of failures within an item population,

divided by the total number of life units expended by that population,

during a particular measurement interval under stated condition.

Fatigue life. The amount of time under defined operational conditions

that a product is expected to survive before wearout.

Fault. Immediate cause of failure (e.g. maladjustment, misalignment,defect, etc.)

FFT or Fast Fourier Transform . A popular computer method of

shifting data from the time domain to the frequency domain.

Feedback signal is a response measurement that is used in a closed-

loop control process.

Filter. An electronic device to pass certain frequencies (pass band) but

block other frequencies (stop band). Classified as low-pass (high-

stop), high-pass (low-stop), band-pass or band-stop.

Finite element modelin g or finite element analysis or FEA. A

computer-aided design technique for predicting the dynamic behavior




8/22

of a possible future mechanical system.

First order vibration. Rotating machine vibration caused by shaft

unbalance. Frequency in hertz (Hz) is calculated by shaft RPM/60. Also

called 1x vibration. Additional orders, 2x, 3x .... 36x, etc. are caused

by other mechanisms

Fixture. The intermediate structure that attaches a device under test

(DUT) to a shaker or shock test machine.

FMEA. Failure Mode and Effects Analysis. A procedure by which each

potential failure mode of a system is analyzed to determine the effects

on the system and classify each potential failure mode according to its

severity.

Forced vibration. The vibratory motion of a system caused by some

mechanical excitation. If the excitation is periodic and continuous, the

response motion eventually becomes steady-state.

Forcing frequency. In sinusoidal vibration testing or resonance

searching, the frequency at which a shaker vibrates.

Forcing function, A climatic or mechanical environmental input to an

item of equipment that affects its design, service life or ability tofunction. (Also referred to as an environmental condition or an

environmental stress.

Fourier. See Fast Fourier Transform or FFT. Term honors

mathematician Baron Jean Baptiste Joseph Fourier 1768-1830.

Fragility . The maximum load an equipment can stand before failure

(malfunction, irreversible loss of performance or structural damage)

occurs.

Fragility test. Expensive but highly useful dynamic tests of several

samples (to account for variations in tolerances, material properties

and manufacturing processes) at potentially destructive frequencies, to

determine fragility.

Free vibration. Free vibration occurs without forcing, as after a reed

is plucked.

Frequency. The reciprocal of the period T in seconds (of a periodic

function) (1/T). Usually given in hertz (Hz), meaning cycles per second

(cps).

Frequency range - see bandwidth.

Frequency response. The portion of the frequency spectrum over

which a device can be used, within specified limits of amplitude error.

Fundamental frequency. The number of hertz or cycles per second

of the lowest-frequency component of a complex, cyclic motion. (See

also Harmonic and Subharmonic .)

Frequency spectrum. A description of the resolution of any electrical

signal into its frequency components, giving the amplitude (sometimes

also phase) of each component.

Fundamental mode of vibration. That mode having the lowest

natural frequency

back

- G -

g The acceleration produced by Earth's gravity. By international

agreement, the value for 1 gravitational unit is 9.80665 m/s =

386.087 in/sec = 32.1739 ft/sec.

gal. 1 gal (seems to mainly be used in Japan) is an acceleration of 1

cm/sec. A more or less typical earthquake measures around 250 gal




9/22

near the epicenter, so 400 gal is sometimes specified for earthquake

testing. 980.6 gal = 1g.

Gearmesh frequency. A potential vibration frequency on any

machine employing gears. Multiply the number of teeth on a gear

times its RPM, then divide by 60

g units or gravitational units . A way to express an acceleration, in

terms of a ratio. Divide a given acceleration by the appropriate value

(on Earth use 9.80665 m/s or 386.087 in/sec or 32.1739 ft/sec).

Graduation mark, The marks that define the scale intervals on a

measuring instrument are known as graduation marks.

Ground Loop. A current loop created when a signal source and a

signal measurement device are grounded at two separate points on a

ground bus through which noise currents flow. These currents

generate voltage drops between the two ground connections, causing

measurement errors.

back

- H -

HALT. Highly accelerated life test. See accelerated life test.

HASS. Highly accelerated stress screening. See environmental stress

screening (ESS).

Hard failure. A product under test ceases to work correctly. It does

not resume correct operation, even when the stressing environment is

eased. Differs from soft failure.

Harmonic. A sinusoidal quantity having a frequency that is an integral

multiple (2, 3, etc.) of a fundamental (1) frequency.

Harmonic Distortion. In the output signal of a device, distortion at

those frequencies caused by the presence of frequencies not present in

the input signal.

Hash. Distortion (usually non-harmonic) on a signal. May be viewed

on an oscilloscope trace. (slang).

hertz. (lower-case h, but abbreviated Hz) The unit of frequency.

Formerly cps for cycles per second.

Hysteresis. Also called deadband. That portion of a measuring

systems response where a change in input does not produce a change

in output.

back

- I -

Imbalance. Unequal distribution of weight or mass on a rotor. The

geometric center of the mass is not where it should be: along the shaft

centerline.

Impact. A collision between masses.

Impact test (bump test). A broad frequency range of structural

responses is caused by a deliberate impact.

Impulse. The integral of force over a time interval.

Induced environments. Conditions generated by operating some

equipment, as opposed to natural environments.

Inertance (or accelerance.) The ratio of acceleration to force.

Inertially-referenced. Motion that is referenced to free space or to a




10/22

fixed point in space. A sensor (such as an accelerometer) which

measures such motion.

Input. The mechanical motion, force or energy applied to a

mechanical system, e.g. the vibratory input from shaker to test item.

Or an electrical signal, e.g. from a shaker controller to the power

amplifier driving a shaker.

Input control signal. Originates in a control sensor; sometimes

selected between or averaged between several sensors. Used to

regulate shaker intensity. (May originate in a force sensor for force-

controlled testing.)

Input Impedance. The shunt resistance and capacitance (or

inductance) as measured at the input terminals, not including effects

of input bias or offset currents.

Integrator. Circuitry which converts an acceleration signal to a

velocity signal or a velocity signal to a displacement signal.

Integration. Representation inversely related to time rate of change.

Example: integrating velocity yields displacement. In a computer, this

is accomplished by dividing the velocity signal by j, where is

frequency multiplied by 2.

Intensity. The severity of a vibration or shock. Nearly the same

meaning as Amplitude, defined earlier, but less precise, lacking units.

Isolation. A reduction in motion severity, usually by a resilient

support. A shock mount or isolator attenuates shock. A vibration

mount or isolator attenuates steady-state vibration.

Iterative closed loop control precalculates drive signals but then

modifies those signals based upon resulting motion, in order to better

match measured with desired motions. Evaluation and modifications

take place after each excitation, repeating until the match is

acceptable.

Itch. Sometimes called glass itch or lace itch. An automotive sound

caused by metal sliding on glass.

back

- J -

Jerk. The rate of change of acceleration with time.

back

- K -

Keyphasor. A signal used in rotating machinery measurements,

generated by a sensor that observes a once-per-revolution event.

Used in phase measurements for analysis and for balancing. Bentley-

Nevada trade name.

back

- L -

LabView. A programming language (National Instruments) for

developing data acquisition software.

Latent defect. A flaw (in a part or assembly) and/or workmanship

that is dormant, not immediately apparent visually or by electrical

test, yet can result in failure. See Patent Defect.

Lateral sensitivity. See transverse sensitivity.




11/22

Leakage. An unfortunate result of sampling with finite intervals.

Results in smearing of frequency components. Improved by windowing

(e.g. Hanning).

Level. The (usually base 10) logarithm of the ratio between a quantity

and a reference quantity. For acoustic measurements the standardreference quantity is 20 micropascals. For acceleration measurements,

the reference might be 1g or 1 micro-g.

Life Cycle Testing. Subjecting products to stresses similar to those

anticipated in actual service while collecting engineering data related

to life expectancy, reliability, specification compliance, or product

improvements. Usually aimed at determining the products' mean time

between failures or MTBF.

Life cycle history. A time history of events and conditions associated

with an item of equipment from its release from manufacturing to its

removal from service. The life cycle should include the various phases

that an item will encounter it its life, such as: handling, shipping and

storage prior to use; mission profiles while in use; phases between

missions, such as standby time or storage, transfer to and from repair

sites and alternate locations; and geographical locations of expected

deployment.

Life units. A measure of use duration applicable to the item. Measures

include time, cycles, distance, rounds fired, attempts to operate, etc.

Linear system. A system is linear if its magnitude of response is

directly proportional to its magnitude of excitation, for every part of

the system.

Linearity . The closeness of a calibration curve to a specified straight

line, preferably passing through zero. Departures from that straight

line are commonly specified as a % of full scale.

LN2. Liquid nitrogen, often used for rapid cooling of environmental

test chambers.

Longitudinal or Compressional or Axial Vibration is in the same

direction as the transfer of energy. Transverse Vibration is

perpendicular to that direction.

Loudness. The human ranking of an auditory sensation, usually in

terms ranging from soft to loud, expressed in sones (not in deciBels).

back

- M -

Machinery health monitoring (MHM). See Condition Monitoring

Magnetostriction. Slight changes in the dimensions of iron or steel

components resulting from changes in the magnetic fields acting on

these components.

Mass. A physical property, dynamically computed as acceleration

divided by force. Statically computed as W (which can be measured on

a butcher scale) divided by the acceleration due to gravity. Ordinary

structures are not pure masses as they contain reactive elements, i.e.

springs and damping.

Mean. A value intermediate between quantities under consideration. A

shaker's mean acceleration must be zero - no steady-state

acceleration. But a vehicle can have steady-state motion.

Mean-Time-Between-Failure (MTBF). A basic measure of reliability

for repairable items: The mean number of life units during which all

parts of the item perform within their specified limits, during a

particular measurement interval under stated conditions.

Mean-Time-To-Failure. A basic measure of reliability for non-




12/22


13/22

a receiver.

Notch. Minimum spectral value, at a natural frequency. Also, the

deliberate reducing of a portion of a test spectrum (random vibration

testing).

back

- O -

Octave. The interval between two frequencies differing by exactly 2:1.

OEM - original equipment manufacturer.

Open Loop control provides precomputed or preconceived drive

signals to the exciter system without modifying or refining those

signals based on observation of the resulting motion. See also Closed

Loop.

Operational environment. The aggregate of all external and internal

conditions (such as temperature, humidity, radiation, magnetic and

electric fields, shock vibration, etc.) either natural or man made, or

self-induced, that influences the form, operational performance,reliability or survival of an item.

Operational limit. The extremes beyond which a product is not

expected to operate.

Orbit. The path of a shaft centerline during rotation. The orbit is

usually observed on an oscilloscope connected to x- and y-axis

displacement sensors. Sometimes called a Lissajous pattern.

Order. See First order vibration. Vibration at multiples of a machine's

shaft speed.

Order tracking. Control of a computer's data sampling rate, such that

display of vibrations at multiples of the shaft's rotating speed (x1, x2,

x3, etc.) are enhanced and more easily.

Oscillation . Variation with time of a quantity such as force, stress,

pressure, displacement, velocity, acceleration or jerk. Usually implies

some regularity (as in sinusoidal or complex vibration).

back

- P -

Patent defect. A flaw (in a part or assembly) and/or workmanship

that has failed under test or screen. See Latent Defect.

Peak. Extreme value of a varying quantity, measured from the zero or

mean value. Also, a maximum spectral value.

Peak-to-peak value. The algebraic difference between extreme

values (as D = 2X).

Performance-based requirements (specifications). Requirementsthat describe what the product should do, how it should perform, the

environment in which it should operate, and interface and

interchangeability characteristics. They should not specify how the

product should be designed or manufactured.

Period. The interval of time over which a cyclic vibration repeats

itself.

Periodic vibration. (See also Deterministic vibration.) An

oscillation whose waveform regularly repeats. Compare with

probabilistic vibration.

Phase. (Of a periodic quantity), the fractional part of a period between




14/22

a reference time (such as when displacement = zero) and a particular

time of interest; or between two motions or electrical signals having

the same fundamental frequency.

Phase reference probe. A device for giving a once-per-shaft-

revolution signal.

Pickup. See transducer .

Piezoelectric (PE) transducer. One which depends upon

deformation of its sensitive crystal or ceramic element to generate

electrical charge and voltage. Many present-day accelerometers are

PE.

Piezoresistive (P R) transducer. One whose electrical output

depends upon deformation of its semiconductor resistive element,

offering greater resistance change than does the wire of a strain-gage

transducer, for a given deformation.

Pitch. Rotation in the plane of forward motion, about the left-right

axis. In music, relates to frequency.

Plastic Reinforcement. (example: inside a box section acting as a

beam) raises local stiffness without adding much weight.

Platform. Per MIL-STD-810, any vehicle, surface or medium that

carries an equipment. For example, an aircraft is the carrying platform

for internally-mounted avionics equipment and externally-mounted

stores. The land is the platform for a ground radar set, and a man for

a hand-carried radio.

Pow er spectral density or PSD. Describes the power of random

vibration intensity, in mean-square acceleration per frequency unit, as

g/Hz or m/s. Acceleration spectral density or ASD is preferred

abroad.

Precision. The smallest distinguishable increment (almost the same

meaning as resolution); deals with a measurement system's possible

or design performance.

Probability distribution. An evaluation of the magnitude of events,

stresses, strengths, etc. They don't all have the same value. Oftenthey are distributed in some sort of "bell shaped" graph of percentage

vertical vs. value horizontal. Most are clustered around the mean

value.

Probabilistic vibration. (As compared to Deterministic vibration),

one whose magnitude at any future time can only be predicted on a

statistical basis.

Probe. A machinery-mounted sensor (usually internal). May sense

vibration.

Proof of screen. A process aimed at showing that a screen is

effective in identifying existing defects in a product. The screen must

not damage good products.

Proximity sensor. Usually a displacement sensor for measuring the

varying distance between a housing and a rotating shaft.

PSD. Power Spectral Density. The power of random vibration intensity

in mean-square acceleration per frequency unit, as g/Hz or m/s.

back

- Q -

Quadrature motion. (Or side or lateral motion or crosstalk), any

motion perpendicular to the reference axis. Shakers are supposed to

have zero quadrature motion.

Quadrature sensitivity. (Or side or lateral motion or crosstalk




15/22

sensitivity) of a vibration sensor is its sensitivity to motion

perpendicular to the sensor's principal axis. Commonly expressed in %

of principal axis sensitivity.

back

- R -

Radial. A direction perpendicular to a shafts centerline.

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.

Range. A statement of the upper and lower limits over which an

instrument works satisfactorily.

Rate gyro. A kind of gyroscope that measures rotational velocity

(degrees or radians per second) around a fixed axis.

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

and bolts.

Real-time closed loop control resembles iterative closed loop

control but continuously modifies drive signals throughout the test.

Redundancy. 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).

Reliability . The likelihood or probability that an equipment will "do itsob" 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 Guru.net) 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".

Here are some examples I found on the Web: Rolls-Royce cars arefamous for their quality and reliability. My car is seven years old but

it's still fairly reliable. The reliability of this smoke detector is

guaranteed.

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

Repeatability. (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




16/22

change in other test conditions). Do not confuse with accuracy.

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.

Replication. Testing that reproduces a specified desired motion

history (time domain) or waveform.

Response. 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.

Resolution . 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.

Resonance. Forced vibration of a true SDoF system causes resonance

when the forcing frequency equals the natural frequency, when anyforcing 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.

Ringing. 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.

RM S 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 Responding. A measurement equal to the rms value of the inputsignal for all waveforms within the specified frequency range and crest

factor limit.

Roll. Rotation about the axis of linear motion.

Rolling element ( anti-friction) bearings 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.

back

- S -

Sampling rate. The number of readings an A/D converter takes per

second or per minute.

Screening. The process of stressing products so that defective units

can be identified, then repaired or replaced. A winnowing out.

Seismic. Having to do with earth motion, as earthquakes. A kind of

sensor that depends upon the inertia of an internal mass to generate a

signal, as an accelerometer or velocity pickup.

Sensitivity. Of a mechanical-to-electrical sensor or pickup, the ratio

between electrical signal (output) and mechanical quantity (input)..




17/22

Sensor. (See Transducer .)

Self-induced vibration. Also called self-excited vibration, results

from conversion of non-oscillatory energy into vibration, as wind

exciting telephone wires into mechanical vibration.

Shock machine. Or shock test machine, a device for subjecting a

system to controlled and reproducible mechanical shock pulses.

Shock pulse. An event that transmits kinetic energy into a system in

a relatively short interval compared with the system's greatest natural

period. A natural decay of oscillatory motion follows. The event is

usually displayed as a time history, as on an oscilloscope.

Shock pulse, classical. Classical shock pulses (mechanical shock

machine). Unless the procedure requires the use of a classical shock

pulse, the use of such a pulse is not acceptable unless it can be

demonstrated that measured data is within the tolerances of the

classical shock pulses. Only two classical shock pulses are defined for

testing in the method the terminal peak sawtooth pulse, and the

trapezoidal pulse. The terminal peak sawtooth pulse along with its

parameters and tolerances are provided on Figure 516.6-10, and is an

alternative for testing in Procedure I - Functional Shock and Procedure

V - Crash Hazard Shock Test.

The trapezoidal pulse along with its parameters and tolerances is

provided on Figure 516.6-11, and is an alternative for testing in

Procedure II - Materiel to be Packaged, and Procedure III - Fragility.

Shock pulse, haversine. A practical variation on the obsolete half-

sine pulse, whose abrupt transitions at beginning and end cannot be

achieved in test labs. Practical testing requires some rounding, and the

result in called a haversine pulse. Another definition: an inverted

cosine offset by half its amplitude. A continuous haversine resembles a

sine wave.

Shock response spectrum (or SRS). A plot of maximum responses

of imagined SDoF systems vs. their natural frequencies, as they

respond to an applied shock.

Signal conditioner. An amplifier following a sensor, which preparesthe signal for succeeding amplifiers, transmitters, readout instruments,

etc. May also supply sensor power.

Simple harmonic motion. Periodic vibration that is a sinusoidal

function of time.

Single-Ended I nput. An analog input measured with respect to a

common ground.

Slew rate. The maximum rate at which an instrument's output can

change by some stated amount.

Sneak circuit analysis. An analytical procedure for identifying latent

paths that cause occurrence of unwanted functions or inhibit desired

functions, assuming all components are operating properly.

SNR. Signal-to-Noise Ratio. The ratio of the amplitude of the desired

signal to the amplitude of noise signals at a given point in time. Thelarger the number the better.

Solid-state sensor. One which has no moving parts.

Sone. A unit of loudness.

Soft failure. A product under test ceases to operate correctly, but

resumes correct operation when the stressing environment is eased.

Differs from hard failure.

Sound. (1.) An oscillation in pressure, capable of evoking the

sensation of hearing. (2.) The sensation of hearing

Sound intensity. In a specified direction, the average rate of sound




18/22

energy flow through a unit area perpendicular to that direction.

Sound level. The quantity in dB measured by a standardized Sound

Level Meter. The reading is 20 log10

of the ratio between a given

sound pressure and 20 micropascals.

Source follower. A device for converting a high impedance electricalsignal to low impedance. Also referred to as an "impedance converter."

Generally has a voltage gain of unity.

Source Impedance. The combination of resistance and reactance

that a source presents to the input terminals of a measuring

instrument.

Spall. A flake or chip of metal; removed from one of the races of a

rolling-element bearing. That bearing is nearing the end of its useful

life.

Specifications . Documents (the USA military service once had as

many as 28,000 specifications) that describe products or services, for

the purpose of procurement. Differs from standards.

Spectral map. A three-dimensional plot ofamplitude (Z axis) vs. time

(or shaft speed) (Y axis) vs. frequency (X axis)

Spectrum analyzer. An instrument which displays the frequency

spectrum of an input signal, usually amplitude vertical vs. frequency

horizontal.

Squeak. A sound resembling that of an unlubricated hinge. A sound

made by PVC or other material rubbing on glass, i.e. a windshield.

Spectrum. See frequency spectrum.

Standards . Documents (the USA military services had as many as

1,700 standards) that prescribe engineering disciplines, impose

particular management practices, reporting and auditing requirements.

Differ from specification.

Standard deviation. A statistical term: , the square root of the

variance 2, i.e., the square root of the mean of the squares of the

measured deviations from the mean value.

Standing w ave. A wave that is characterized by lack of vibration at

certain points (nodes), between which are areas of maximum vibration

(antinodes). Standing waves are produced at certain forcing

frequencies when the resulting resonant vibratory response is confined

within boundaries, as in the vibrating string of a musical instrument or

the wing of an airplane or the whip antenna on your automobile. Also

called "stationary wave".

Stationarity. A property of probabilistic vibration if the PSD (or ASD)

and the probability distribution remain constant.

Steady state vibration. Periodic vibration for which the statistical

measurement properties (such as the peak, average, RMS and mean

values) are constant.

Stepped sine testing. Sine shaking in a series of dwells. Frequency isincrementally increased or decreased.

Step stressing. Increasing stresses in a series of preselected

increments.

Stiffness. The ratio of force (or torque) to deflection of a spring-like

element.

Strain-gage transducer. A changing-resistance sensor whose signal

depends upon sensitive element deformation. In an unbonded wire

strain-gage accelerometer, inertia affects a mass supported by

nichrome wires; the wires change resistance in proportion to

acceleration. The term may include piezoresistive accelerometers.




19/22

Stress. Intensity of applied load, usually at the site of a failure.

Stress Screening. A modern electronics production tool for

precipitating latent defects such as poorly-soldered connections.

Utilizes random vibration + rapid temperature ramping.

Subharmonic . A sinusoidal quantity having a frequency that is an

integral submultiple (1/2, 1/3, etc.) of a fundamental (1)

frequency.

Subsychronous . Components of a vibration signal whose frequency is

less than 1x shaft speed.

Supply current. The typical current that must be supplied to a sensor

(along with the supply voltage) to make it function properly.

Swept-sine testing. Sine shaking whose frequency is smoothly and

continuously varied. Commonly required for sequentially identifying

resonances. By contrast, see stepped sine testing.

Synchronous . Vibration components (on rotating machinery) that are

related to shaft speed.

Synchronous sampling. Control of a computer's rate of datasampling to achieve order tracking.

System. A composite of equipment and skills, and techniques capable

of performing or supporting an operational role, or both. A complete

system includes all equipment, related facilities, material, software,

services, and personnel required for its operation and support to the

degree that it can be considered self-sufficient in its intended

operational environment.

System effectiveness. (a) For repairable systems and items: the

probability that a system can successfully meet an operational demand

within a given time when operated under specified conditions. (b) For

"one-shot" devices and non-repairable items: the probability that the

system will operate successfully when called upon to do so under

specified conditions.

back

- T -

TAAF. Test, analyze and fix. See Accelerated Life Testing.

Tailoring . Selecting or altering test procedures, conditions, values,

tolerances, measures of failure, etc., to simulate or exaggerate the

environmental effects of one or more forcing functions.

Temperature range. The temperatures between which a sensor will

operate accurately.

THD. Total Harmonic Distortion. For a signal, the ratio of the sum of

the powers of all harmonic frequencies above the fundamental

frequency to the power of the fundamental frequency.

Thermal cycling. Subjecting a product to predetermined temperaturechanges, between hot and cold extremes.

Thermistor. An electrical device used for temperature measurement.

Thermocouple. An electrical device used for temperature

measurement. Two dissimilar metals joined together.

Threshold. The smallest change in a measured variable that gives a

measurable change in output signal.

Thrust position. Location in direction of a shaft centerline. See axial

position.

Tracking filter. A narrow bandpass filter whose center frequency




20/22

follows an external synchronizing signal.

Time constant. The interval needed for an instrument's output to

move 63% of its ultimate shift as a result of a step change in its input.

Transducer (or pickup or sensor). A device which converts some

mechanical quantity into an electrical signal. Less commonly, thereverse conversion.

Transient vibration Short-term vibration of a mechanical system.

Transmissibility . In steady-state vibration, Tr is the non-dimensional

ratio of response motion/input motion: two displacements, two

velocities or two accelerations. The maximum Tr value is the

mechanical "Q" of a system. At resonance, Tr is maximum.

Transverse sensitivity. The unfortunate sensitivity of a sensor (or

pickup or transducer) in a direction perpendicular to the advertised or

stated sensitive axis. Also called cross-axis or lateral sensitivity.

back

- U -

Unit. Every measurement is expressed as a multiple or fraction of

some appropriate, well-defined unit quantity such as centimeter, volt,

etc.

Unbalance (imbalance). Unequal mass distribution on a rotor. The

mass centerline does not coincide with the rotation or geometric

centerline.

UUT. Unit under test. See also DUT or device under test (being

tested).

back

- V -

Velocity. Rate of change of displacement with time, usually along a

specified axis; it may refer to angular motion as well as to uniaxial

motion.

Vibration. Mechanical oscillation or motion about a reference point of

equilibrium.

Vibration machine (or exciter or shaker). A device which produces

controlled and reproducible mechanical vibration for the vibration

testing of systems, components and structures.

Vibration meter. An apparatus (usually an electronic amplifier,

detector and readout meter) for measuring electrical signals from

vibration sensors. May display displacement, velocity and/or

acceleration.

Vibromyography. Also called mechanomyography or acoustic

myography. The measurement of muscle vibration when a musclecontracts.

back

- W -

Waterfall plot. A series of spectral maps taken at regular intervals of

time, regularly spaced shaft speeds, etc. Early maps move down the

display, followed by later maps, something like the flow of a waterfall.

Wave. Nature's mechanism for transporting energy without

transporting matter.




21/22

Waveform. A presentation or display of the instantaneous amplitude

of a signal as a function of time, as on an oscilloscope or oscillograph.

In the time domain.

Wavelet, as used for shock testing on shakers, are half-sine

windowed (amplitude modulated) sinusoids of specific frequencies,with the window chosen so the resulting waveforms have an odd

number of half cyles. Usually, ten to thirty wavelets, each with

different basis frequencies, e.g., 10 Hz to 2000 Hz, and different

numbers of half cycles, e.g., three to 31 or so, are superimposed to

synthesize a transient oscillatory signal (pulse) with a prescribed

Shock Response Spectrum that can be used as a reference waveform

for a shaker shock test.

Weight. That property of an object that can be weighed, as on a

scale; the gravitational force on an object.

Weighting Emphasis or attenuation applied to sound measurements

at certain frequencies. C weighting is essentially flat. A weighting

attempts to compensate for the non-constant sensitivity of human

hearing at certain frequencies. See Equal Loudness Curves.

White random vibration. That broad-band random vibration in which

the PSD (ASD) is constant over a broad frequency range.

back

- X -

(empty)

back

- Y -

Yaw . Rotation about the vertical axis.

back

- Z -

Zwicker loudness. A sound measurement methodology. Loudness is

measured in linear units called sones as opposed to decibels.

Zero-G drift. The amount that a sensor's no-acceleration signal shifts

over some temperature range.

Zero-G output. The accelerometer output that is read when the

sensor is not accelerating.

Zero-to-peak. See peak, as in peak value. Half of the peak-to-peak

value.

back

- # -

6DOF machines. Often called Repetitive Shock Machines. That class

of non stationary random vibration producing machines that use air-

driven impact hammers to excite a vibrating plate or table onto which

products are either attached directly or fixtured. The resulting

excitation is a combination of impulse and table modal response, or an

intermodulation of both.




22/22

Home | Training | Consulting | Resources | About ERI

Copyright 2000-2011 Equipment Reliability Institute. All rights reserved.ERI's Privacy Statement and Terms of Use

Equipment Reliability Institute :: 1520 Santa Rosa Avenue, Santa Barbara, CA 93109 :: Tel (805) 564-1260 :: Fax (805) 966-7875
