SM0486
opposite direction, it moves through one complete cycle of motion.
We admit the
waveform is not essentially that of a sine wave; nevertheless, its movement is
measured in cycles per second or vibrations per second.
c. The term "amplitude" has the same meaning for the vibration waveform that
peak value has for in AC voltage. The term "amplitude" is directly associated with
the term "displacement".
Displacement values in vibration measurements are
comparable to peak-to-peak AC voltage values.
Since the acceleration associated
with vibration may be expressed in feet per second per second (or feet per second
squared), you may suspect that the force of gravity is involved.
d. In aircraft work, gravity is a fundamental quantity, and the acceleration
of gravity is a unit of acceleration.
This unit is called a G; one G equals
approximately 980 centimeters per second per second, or 32.2 feet per second per
second. The accelerations are specified in G's.
3.
TYPES OF VIBRATION. Vibration measurements deal with a type of vibration known
as steady-state vibrations.
Steady-state vibrations are not transient in nature
and continue until the primary cause of the vibration is shut down or eliminated.
a. One of the fundamental causes of vibration in machinery and rotating
equipment is imbalance in the rotating components.
This imbalance may be
considered as a heavy spot, or weight, for which there is no compensation.
Such
imbalance can be reduced by having an equivalent weight located on the opposite
side of-the rotating component directly opposite the heavy spot. You have probably
experienced this imbalance in the tires on your car. In such cases you merely have
your wheels balanced by the proper placement of weights. If you have watched the
device used in wheel balancing, you saw how a stroboscopic instrument was used to
position the weight and remove the imbalance.
b. Imbalance may also be though of in the sense that each rotating component
has a mass center. If this mass center is not physically located at the same point
as the rotating center, the part is out of balance when rotating. Thus it may be
seen that balance is affected by concentricity as it is affected by heavy spots.
c. Imbalance involves both a weight and a distance function. Standard terms
which are most often used when referring to imbalance are "ounce inches", "gram
inches", "micro ounce inches", and "pound inches". If a 2-gram weight is added to
a part which has previously been balanced and the weight is placed 5 inches from
the center of rotation, an imbalance of 10 gram inches exists in the rotating part.
If a weight of 10 grams is added at a radius of 1 inch, the imbalance is 10 gram
inches.
An imbalance of 10 gram inches manifests itself in the same manner and
causes the same amount of vibration without regard to whether a large weight was
added at a small radius or a small weight was added at a large radius. The number
of gram inches determines the severity of the unbalanced condition and the
resulting vibration.
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