LINEAR VIBRATION
TESTING AND ADJUSTING
V Peak = 52.36 D x F x 10-6
Where
V Peak = Vibration velocity in inches
per second peak.
D
= Peak-to-peak displacement in
mils (1 mil = 0.001 in).
F
= Frequency in cycles per
minute (cpm).
Acceleration measurements or "gs" are commonly used
where relatively large forces are applied. At very high
frequencies (60,000 cpm) it is perhaps the best indicator
of vibration.
The vibration acceleration can be calculated as:
g Peak = 1.42 D x F2 x 10-8
Measurement
The exact nature of linear vibration is difficult to define
without instrumentation. The human senses are not
adequate to detect relationships between the magnitude
of displacement of a vibration and its period of
occurrence. For instance, a first order (1 x rpm)
vibration of 0.010 in (0.254 mm) displacement may feel
about the same as a third order measurement of 0.001
in (0.026 mm). However, the severity of vibration does
correlate reasonably well with levels of perception and
annoyance.
Establishing the vibration frequency is necessary when
analyzing this type of problem. It allows identification of
the engine component or mass system which is causing
the vibration. In discussions of vibration, the frequency
of the motion is commonly referred to in terms of "order"
of vibration. In an engine, the order of vibration is the
number of vibratory cycles exhibited by a component
during one revolution of the crankshaft.
Order = Vibration Frequency (cpm)
Engine Speed (rpm)
Overall vibration motion is the vector sum of the motion
of all the orders. In other words, individual order motions
will add or subtract to produce the overall. This
measurement is not used to identify problems or
establish limits, but rather as an indication of the total
linear vibration motion.
A vibration analyzer is a prerequisite to troubleshooting a
problem of this nature. A unit that measures frequencies
and amplitude is suggested. Further, the analyzer
should be rugged and easily understood. Good results
have been obtained with the IRD Mechanalysis Model
320. It is offered by:
IRD Mechanalysis, Inc.
6150 Huntley Road
Columbus, Ohio 43229
All measurements must be made on the main rigid
structural members of the engine and generator. The
instrument pickup must be positioned on the crankshaft
centerline at the locations shown on the sample form.
Measurements (except torque reaction and some
misalignment) can be made at no-load operation. If
measurements are taken while the generator is loaded,
the magnetic field of the generator leads must be
avoided.
If excessive linear vibration motion is present or
suspected, an initial measurement should be made to
identify the source prior to starting corrective action.
These measurements are to be checked against the
applicable limits shown in the graph on Page 107. A
sample form is described in this data sheet for
convenient recording of raw data at various engine
speeds (generator frequencies). This form can be used
for the basic engine, single-bearing generators, and two-
bearing generators.
Identification
Experience has shown that in order of decreasing
importance, the primary sources of linear vibration
motion problems are:
1. Misalignment of engine and driven equipment.
2. Unbalance of rotating parts.
3. Resonance from structural mass (weight) and
stiffness (rigidity) combinations.
4. Torque reaction.
5. Cylinder misfiring.
6. Combustion forces.
7. Unbalance of reciprocating parts.
As test data is reviewed, if excessive vibration motion
exists, it will be due to one of the following causes:
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