SM0486
d. If a rotating part is not concentric with its axis of rotation, imbalance
results.
The extent of the imbalance depends upon the number of gram inches or
ounce inches of imbalance introduced.
As an example, if a 20-pound (320 ounce)
pulley is mounted two thousandths inch (.002") off center, the imbalance is 320 x
.002, or 0.64 ounce inch.
The next section of this chapter discusses one of the
devices used to detect vibrations.
4.
VELOCITY PICKUP 4-102A.
The velocity pickup translates physical vibrations
into measurable electrical signals. The assembly is used to detect vibrations in
any single linear direction regardless of orientation.
To obtain direct readings
of amplitude or average velocity, you must connect the velocity pickup to a
vibration meter.
a. Pickup Construction. Figure 1 shows an exterior view (A), a cutaway (B),
and a schematic (C) of the pickup.
Examining Figure 1, you see a cylindrical
magnet mounted on helical springs.
The magnet assembly responds to Vibration by
moving along the axis of a coil without coming in contact with the winding. The
coil winding is such that the direction is reversed from one end to the other, thus
forming two separate sections connected electrically at the center of the coils.
b. Pickup Operation. The schematic diagram of Figure C, shows how the pickup
coil is wound.
As the pickup vibrates with the member to which it has been
attached, the magnet assembly tends to become stationary in space, though moving
with respect to the coil.
This motion causes the coil windings to cut magnetic
motion. Since the two sections of the coil are wound in opposing directions, the
voltages induced are additive, thus doubling the sensitivity of the velocity pickup
assembly.
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