Vibration Overview
1)
UNDERSTANDING
THE PHYSICAL CHARACTERISTICS OF VIBRATION
2) TYPICAL
VIBRATION SWITCH INSTALLATIONS
3) VIBRATION
SWITCH SENSORS
UNDERSTANDING
THE PHYSICAL CHARACTERISTICS OF VIBRATION
Many
people are aware that the amplitude (displacement) of a machine, resulting
from a vibrating force, is a critical factor in causing mechanical damage
to operating machinery. But many people are not aware that the FREQUENCY
at which the vibratory force is
occurring is of even greater importance in causing mechanical
damage.
In
general terms, vibration can be described as ". . . mechanical motion or
oscillation about a reference point of equilibrium." To some this may only
mean °movement", while to others it might
suggest both movement and the relative number of movements per
unit of time. To most people it may
help to describe, on an operating machine, a condition indicating
impending danger when the "vibration"
appears to be exceeding a level previously considered normal.
DEFINITIONS
OF TERMS
Four simple terms are most frequently used in discussing vibration.
These are:
1.
Displacement.
Also known as
"amplitude" is defined as the magnitude of the motion under study,
generally measured as peak-to-peak or
double amplitude in "mils" (thousandths of an inch). These
measurements are most important in the study of deformation or bending of
structures.
2.
Velocity.
The time rate of
change of displacement, which is the first derivative of displacement with
respect to time. Velocity measurements are most important where the noise
from vibration is of greatest importance.
3.
Acceleration.
The second derivative
of displacement with respect to time.
Its magnitude is proportional to displacement and the square
of frequency. These measurement are
most practical and important where actual mechanical failure of
parts is possible. The unit of
measurement is "gravity" abbreviated by the letter G. The formula is
G = KAF**2, where
K
is a numerical constant
A is the amplitude of motion
F is the frequency at which the amplitude (displacement) is occurring.
Note that
the total G forces (the destructive shock acting upon the
bearings, shafts. etc.) varies directly
with the amplitude (A) but with
the SQUARE of the frequency (F). Therefore, doubling the amplitude
(A) will double the destructive shock; while doubling the frequency (F)
will increase the destructive shock by four times.
WHY MONITOR ACCELERATORY VIBRATION?
Figure 1
shows the resultant wave form of displacement with respect to time of a
pure harmonic motion producing a sinusoidal single frequency wave form.
Robertshaw vibration instruments measure the total acceleratory forces
(shock) and are therefore responsive to both amplitude and frequency of
the resultant vibration. Vibration instruments that are responsive only
to amplitude (displacement) are unable
to detect the total destructive forces" (i.e. "G's") that will tear
costly equipment apart.
Figure 1
The resultant wave shape of displacement with respect to
time of pure harmonic motion — sinusoidal single frequency.
It is important to remember these points:
1. The fact that a particular point on a machine is
being displaced offers little information in regards to its destructive
forces.
2. Acceleration measured at a particular point on a
machine indicates the TOTAL vibratory shock and, therefore, offers a
useful figure of merit of the total destructive forces present.
3. Operating machines' vibrations seldom follow pure
harmonic motions at a single frequency, but rather they are combinations
of a fundamental frequency (basic machine speed) and a number of higher
harmonic frequencies. Since such vibrations result in a complex wave form
(non-sinusoidal), acceleration sensing instruments offer the best measure
of these destructive vibratory forces
See Also:
Understanding the Physical
Characteristics of Vibration
Typical Vibration
Switch Installations
Vibration Switch Sensors
