LINEAR VIBRATION
TESTING AND ADJUSTING
SUBJECT:
Linear Vibration
BUSINESS:
All
PRODUCT/APPLICATION:
Generator Sets
The purpose of this Engine Data Sheet is to:
1.
Review the basic theory and nomenclature of
vibration. A knowledge of the language of this
discipline
will
encourage
a
fundamental
understanding. Such terms as linear, period,
displacement, velocity, and acceleration will be
defined and compared.
2.
Identify causes of engine-related vibration. Major
contributors to engine vibration are described, as
well
as
procedures
to
isolate
and
measure
vibrations.
3.
Provide possible corrective action. Excessive
vibratory motion may be encountered in the engine,
generator, related components, or any part of the
mounting system. If left unresolved this motion may
cause
personnel
discomfort
and/or
equipment
damage. Balance techniques are discussed, and
limits or vibratory forces are defined.
FUNDAMENTALS
Any mechanical system which possesses mass and
elasticity is capable of relative motion. If this motion
repeats itself after a given time period, it is known as
vibration. A simple illustration of vibration is to suspend
a weight (mass) on the end of a spring (elasticity).
As long as no external force is imposed on the system,
the weight remains at rest -- there is no vibration. But
when the weight is moved or displaced and then
released, vibration occurs. The weight will continue to
travel up and down through its original
Figure 1
position until frictional forces again cause it to rest. This
specific illustration is an example of free vibration. If an
external force continues to affect the system while it is
vibrating, it is termed "forced vibration."
The illustration is also an example of linear vibration.
Linear vibration is an "in-line" motion and occurs as an
object moves and returns along a repeatable path.
Linear vibration must not be confused with the torsional
(twisting) vibration motion of the crankshaft and driven
rotating shafts (refer to EDS 202.1 for the subject of
torsional vibration).
The time required for the weight to complete one
complete cycle is called a period.
If the weight needed one second to complete a full cycle,
the vibration frequency of this system would be one
cycle per second. A system that completed its full
motion 20 times in one minute would have a frequency
of 20 cycles per minute, or 20 cpm.
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