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GLOSS RY

Absolute fault limit

Acceleration

Accelerometers

Amplitude

Apex

Balance

Ball spin frequency BSF)

Ball-pass inner-race BPFI)

Ball-pass outer-race BPFO)

Broadband

Broadband energy

Broadband trending

The maximum recommended level of overall

vibration accepted in machinery.

The rate of change of velocity with respect to

time.

Use a piezoelectric crystal to convert mechani-

cal energy into electrical signals.

The maximum absolute value attained by the

disturbance of a wave or by any quantity.

The pitch angle is the sum of the pitch lines

extended, which meet at a point.

All forces generated by or acting on the rotating

element of a machine-train are

in

a state of equi-

librium.

The spinning motion of the balls or rollers

within a bearing.

The ballholler rotating speed relative to the

inner race.

The relative speed between the balls or rollers in

a rolling-element bearing and the outer race.

band with a wide range of frequencies.

Provides a gross approximation of machines

condition and its relative rate of degradation.

Vibration analysis technique that plots the

change in the overall or broadband vibration of a

machine-train.

286

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Glossary

87

Centrifugal pump

Chatter

Coastdown

Common shaft

Displacement

Dynamic resonance

machine for moving a liquid, such as water,

by accelerating it radially outward in an impel-

ler to a surrounding volute casing.

An irregular alternating motion of the parts of a

relief valve due to the application of pressure

where contact is made between the valve disk

and the seat.

This occurs when the machines driver is turned

off

and the suspect frequency is recorded as the

speed decreases.

The individual shafts that exist in all machine-

trains.

The change in distance or position of an object

relative to a reference point; the actual distance,

off-centerline, of a rotating shaft as compared to

a stationary reference, usually the machine

housing.

When the natural frequency of a rotating or

dynamic structure, such as the rotor assembly in

a fan, is energized, the rotating element will res-

onate.

Fast Fourier transform

FFT)

Converts a time-domain plot into its unique fre-

quency components using a mathematical pro-

cess.

First mode

Fourth mode

Frequency

Frequency domain

The slightly eccentric rotation (off-center) of a

shaft will generate a low-level frequency com-

ponent that coincides with the rotating speed of

the shaft.

shaft can flex or deform into mode shapes that

will generate running-speed harmonics.

The number of cycles completed by

a

periodic

quantity in a unit time.

plane on which signal strength can be repre-

sented graphically as a function of frequency,

instead of a function

of

time.

Fundamental or first critical speed

Fundamental train frequency

The lowest critical speed.

Generated by the precession of the cage as it

rotates around the bearing races.

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288 Vibration Fundamentals

Gear mesh

Gravity

Harmonic motion

Harmonics

Helical gears

Herringbone gears

Hertz

Hydrodynamic

Imbalance

Laminar

Low frequency cutoff

Machine-train

Narrowband

Narrowband trending

Natural frequency

Frequency is equal to the number of gear teeth

times the running speed of the shaft.

The gravitational attraction at the surface of a

planet or other celestial body.

periodic motion that is a sinusoidal function

of time, that is, motion along a line given by

equation a cos(kt

0 ,

where t is the time

parameter, and a k, and are constants.

sinusoidal component of a periodic wave,

having a frequency that is an integral multiple of

the fundamental frequency. Also known as har-

monic component.

Gear wheels running on parallel axes, with teeth

twisted oblique to the gear axis.

The equivalent of two helical gears of opposite

hand placed side by side.

Unit of frequency; a periodic oscillation has a

frequency of

n

hertz if in

1

second it goes

through n cycles.

The study of the motion of a fluid and of the

interactions of the fluid with its boundaries,

especially in the incompressible inviscid case.

Any change in the state of equilibrium.

Arranged in thin layers. Pertaining to viscous

streamline flow without turbulence.

A frequency below which the gain of a system

or device decreases rapidly.

total machine including the driver, drive train,

and machine.

The total energy within a user-selected range, or

windows. Referring to a bandwith of 3 hertz

or less.

Monitors the total energy for a specific band-

width of vibration frequencies.

The natural frequency of free vibration of a sys-

tem. The frequency at which an undamped sys-

tem with a single degree of freedom will

oscillate upon momentary displacement from its

rest position.

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Glossary

289

Node points

Oil whip

Oil whirl

Peak-to-peak

Rate

Rathbone chart

Resonance

Ringing

Root-cause failure

Rotational frequencies

Running speed

Run-up

Where the shaft flexes into a double bend that

crosses its true centerline.

Occurs when the clearance between the rotating

shaft and sleeve bearing is allowed to close to a

point approaching actual metal-to-metal contact.

ee

lso

Oil whirl.

An unstable free vibration whereby a fluid-film

bearing has insufficient unit loading. The shaft

centerline dynamic motion is usually circular in

the direction of rotation. Oil whirl occurs at the

oil flow velocity within the bearing, usually 40

to 49

of

the shaft speed. Oil whip occurs when

the whirl frequency coincides with (and

becomes locked to) a shaft resonant frequency.

(Oil whirl and whip can occur in any case where

a fluid is between two cylindrical surfaces.)

Amplitude of

an

alternating quantity measured

from positive peak to negative peak.

The amount of change of some quantity during a

time interval divided by the length of the time

interval.

Provides levels of vibration severity that range

from extremely smooth, best possible operating

condition, to an absolute fault limit, or the maxi-

mum level where a machine can operate.

A vibration of large amplitude in a mechanical

system caused by a small periodic stimulus of

the same or nearly the same period as the natural

vibration period of the system.

Method for exciting natural frequencies is to

strike or excite a machine or structure with a

timber or hammer.

Based on machine-train operation and how its

dynamics affect the vibration spectrum.

Related to the motion of the rolling elements,

cage, and rings or races.

The true rotational speed of the shaft or shafts.

The machines driver is turned on and records

the amplitude and phase as the machine acceler-

ates from dead-stop to full speed.

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29 Vibration Fundamentals

Sawtooth waveform)

Second mode

Signature

Signature

(FFT)

Static

Static resonance

Synchronous

Third mode

Velocity

Vibration

Worm gears

X-axis

Y-axis

waveform characterized by a slow rise time

and a sharp fall, resembling a tooth

of

a saw.

s

the shaft rotates, the double bend shape will

create two high spots as it passes the vibration

transducer.

The characteristic pattern of target as displayed

by detection and classification equipment.

Usually applied to the vibration frequency spec-

trum unique to a particular machine or machine

component, system, or subsystem at a specific

location and point of time.

hissing, crackling, or other sudden sharp

sound that tends to interfere with the reception,

utilization, or enjoyment of desired signals or

sounds. Without motion or change.

If the natural frequency of a stationary or non-

dynamic structure, such as a casing, bearing

pedestal, piping or other structure, is energized,

the structure will resonate.

In step or in phase, as applied to two or more

circuits, devices, or machines.

shaft can flex or deform into mode shapes that

will generate running-speed harmonics.

The time rate of change of position of a body; it

is a vector quantity having direction as well as

magnitude.

continuing periodic change in a displacement

with respect to a fixed reference. In its general

sense is a periodic motion. The motion will

repeat itself in all its particulars after a certain

interval of time.

Used for nonintersecting shafts at

90

degrees.

A horizontal axis in a system of rectangular

coordinates.

vertical axis in a system of rectangular coor-

dinates.