1.0 FUNDAMENTALS OF
VIBRATION
1.1 What is Vibration?
Mechanical vibration is a form of motion oscillation. it occurs in all forms of
machinery and equipment. It is what you feel when you put your hand on the hood
of a car, the engine of which is running, or on the base of an electric motor
when the motor is running. Perhaps the simplest illustration of a mechanical
vibration is a vertical spring with weight, as shown in Figure 1. In this
position, the deflection of the spring from its free state is just sufficient to
counterbalance the weight W. This deflection is called the static deflection of
the spring. The position in which the spring is at rest is #1. The spring is
then slowly extended to position #2, and released. The subsequent motion of the
weight as a function of time, when there is negligible resistance to the motion,
is wavy and repetitive as shown in the graph. It exhibits many of the basic
characteristics of mechanical vibrations. The maximum displacement from the rest
or mean position is called the AMPLITUDE of the vibration. The vibratory motion
repeats itself at regular intervals (A1, A2, A3). The interval of time within
which the motion sequence repeats itself is called a CYCLE or PERIOD. The number
of cycles executed in a unit time (for example, during one second or during one
minute), is known as the FREQUENCY. In a high-speed oscillation the frequency is
high and conversely. When, as in Figure 1, the spring-weight system is not
driven by an outside source, the vibration is a FREE VIBRATION and the frequency
is called the NATURAL FREQUENCY of the system.
In general, vibratory motion may or may not be repetitive and its
shape as a function of time may be simple or complex.
Typical vibrations, which are repetitive and continuous, are those
of the base or housing of an electric motor, household fans, vacuum cleaners and
sewing machines, for example. Vibrations of short duration and variable
intensity are frequently Initiated by a sudden impact or shock load; for
example, rocket equipment upon takeoff, equipment subject to impact and drop
tests, a package falling from a height, or a lading in a freight car.
In many machines, the vibration is not part of its regular or
intended operation and function, but rather it cannot be avoided. The task of
vibration isolation is to control this unwanted vibration so that its adverse
effects are kept within acceptable limits.
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