| Term | Meaning |
|---|
| Acceleration
|
Acceleration is the rate of change of velocity. It is expressed in m/s² or
gravitational units (gn). 1gn = 9.80665 m/s². Note: m/s² should be written as m
s-2
|
| Accelerometer
|
A transducer (sensor) whose output is proportional to acceleration. The output
is commonly measured in pC/g (pico Coulombs per gravitational unit).
|
| Amplitude
|
Defines the magnitude of a changing level. Its value can be a peak, rms or
average value. When referring to displacement it can be given as a peak or peak
to peak value, which is referenced to a mean or datum point.
|
| Anti-resonance
|
Anti-resonance exists in a system when, with a constant excitation, any change
in the frequency causes a decrease in the response of the system.
|
| Armature structural resonance
|
(electrodynamic vibrators): Armature structural resonance is the resonant
frequency at which the acceleration measured at the centre of the table
becomes 90 out of phase with the exciting force. The armature current is used
as a reference for the exciting force, as the exciting force is always proportional
to the armature current.
|
| Broad band
|
In vibration terms would be considered to be a random noise whose bandwidth is
greater than 500Hz.
|
| Calibration
|
As applied to accelerometers is a method of determining its charge sensitivity,
normally by comparing it to a standard accelerometer whose calibrated value is
traceable back to an international standard. The use of a fopal wedge is only a
method of verification as it relies on the accuracy of the user and the test
equipment used.
|
| Compliance
|
Compliance is the reciprocal of stiffness.
|
| Cross axial motion
|
A motion which is not in the direction of the exciting motion. This is usually
caused by test items and fixtures which are offset.
|
| Crest factor
|
The ratio of the peak to rms value of a waveform. In the case of a sine wave this
ratio is the square root of 2, thus to change from an rms value to a peak value
multiply by square root 2 which is 1.414213562. However in a true random
waveform the crest factor is infinity as theoretically its level can, at some time,
be infinity. For the purpose of vibration testing a value of 3 is generally used
since this will cover 99.73% of the conditions using gaussian distribution.
|
| Damping (material)
|
Is a substance or material used to dissipate vibration energy.
|
| Damping (factor)
|
Is a means of expressing the effectiveness of a damping method.
|
| Decade
|
A decade is the interval between two frequencies having a ratio of 10:1. i.e. 10,
100, 1000, 10000 are decades, but also are 25, 250, 2500.
|
| decibel (dB)
|
Is a logarithmic relationship of two values. Although originally intended for units
of power it is commonly used for non power units such as voltage and gn etc.
|
| Degrees of freedom
|
In mechanical engineering defines the number of directions that an object is free
to move. For example if an object is only able to move up and down it would be
described as having a single degree of freedom.
|
| Displacement
|
Displacement is a vector quantity that specifies the change of position of a body
or point. It can be described as a peak or peak to peak value.
|
| Distortion
|
Distortion is an undesired change in waveform produced by non-linearity in a
system.
|
| Duration
|
Time; of a shock / bump / transient pulse is the period of time to go from zero to
maximum to zero acceleration, usually defined in milliseconds (ms).
|
| Force
|
A force of one Newton is the force required to accelerate one kilogram at one
metre per second per second. Force can also be expressed in pound force (lbf)
or kilogram force (kgf) and can be a peak or rms (root mean square) value. i.e.
lbf rms.
|
| Force (system)
|
Defines the maximum force a vibration system can provide. It is normally derived
from the product of the maximum permitted acceleration (rms or peak) and the
armature mass.
|
| Force (required)
|
Is determined by multiplying the total moving mass (test item, armature, fixture,
etc.) by the acceleration required.
|
| g
|
A term used to describe the level of acceleration. The expression "vibrate at a
level of 5g" should be expressed as "vibrate at a level of 5gn peak", meaning the
maximum value will be 5 gravitational units or in other words 49.03325 m/s² peak.
|
| gn
|
Gravitational unit. The internationally agreed constant 1gn = 9.80665 m/s² , or
in Imperial terms 386.088527 in/sec².
|
| Harmonic
|
A harmonic is a component whose frequency is an integral multiple of the
fundamental frequency. For example, the second harmonic of 100Hz is 200Hz.
|
| Jerk
|
Is a differential of acceleration. i.e. the rate of change of acceleration with time.
|
| Mass
|
Mass is a property of an object which is related to gravity. For example if an
object has a mass of 1kg, its weight on earth would be 1kg, but on the moon its
weight would be 0.165kg but its mass is still 1kg.
|
| Octave
|
An octave is the interval between any two frequencies differing in magnitude by a
factor of two (20 to 40 Hz, 500 to 1000Hz, etc.).
|
| Oscillation
|
Oscillation is the variation, usually with time of the magnitude of a quantity with
respect to a specified reference, when the magnitude is alternatively greater and
smaller than the reference.
|
| Periodic
|
(e.g. sinusoidal motion): Periodic implies that it is repetitive and therefore its
state can be predicted.
|
| Power spectral density
|
PSD describes the power of a random vibration intensity. It is expressed as a
root mean square value of an acceleration with a given band width. The terms
commonly used are gn²/Hz [g²/Hz] (m/s²)²/Hz
|
| Random
|
A varying level, whose level is unpredictable.
|
| Resonance
|
Resonance exists in a system when, with a constant excitation, any change in
the frequency of excitation causes an increase in the response of the system.
|
| Simple harmonic motion
|
The simplest form of vibratory motion, where the amplitude of motion varies
sinusiodally with time.
|
| Stiffness
|
Stiffness is the ratio of change of force (or torque) to the corresponding change in
translational (or rotational) deflection of an elastic element.
|
| Sub-harmonic
|
A sub-harmonic is a component whose frequency is an integral sub-multiple of
the fundamental frequency. For example, the second sub-harmonic of 100Hz is
50Hz.
|
| Velocity
|
Velocity is the rate of change of displacement, in other words speed. It is
expressed in metres per second or millimetres per second (m/s or mm/s), but
should be written as m s-1.
|
| Vibration
|
A body, or point, is said to vibrate when it executes an oscillatory motion about
a position of equilibrium.
|
