Page 259 - Machinery Component Maintenance

Balancing of Machinery Components 241

Ideally, rotating parts always should be designed for inherent balance,
whether a balancing operation is to be performed or not. Where low ser-
vice speeds are involved and the effects of a reasonable amount of unbal-
ance can be tolerated, this practice may eliminate the need for balancing.
In parts which require unbalanced masses for functional reasons, these
masses often can be counterbalanced by designing for symmetry about
the shaft axis.
A rotating element having an uneven mass distribution, or unbalance,
will vibrate due to the excess centrifugal force exerted during rotation by
the heavier side of the rotor. Unbalance causes centrifugal force, which
in turn causes vibration. When at rest, the excess mass exerts no centrif-
ugal force and, therefore, causes no vibration. Yet, the actual unbalance
is still present.
Unbalance, therefore, is independent of rotational speed and remains
the same, whether the part is at rest or is rotating (provided the part does
not deform during rotation). Centrifugal force, however, varies with
speed. When rotation begins, the unbalance will exert centrifugal force
tending to vibrate the rotor. The higher the speed, the greater the centrif-
ugal force exerted by the unbalance and the more violent the vibration.
Centrifugal force increases proportionately to the square of the increase
in speed. If the speed is doubled, the centrifugal force quadruples; if the
speed is tripled, the centrifugal force is multiplied by nine.

Units of Unbalance

Unbalance is measured in ounce-inches, gram-inches, or gram-milli-
meters, all having a similar meaning, namely a mass multiplied by its
distance from the shaft axis. An unbalance of 100 g-in., for example,
indicates that one side of the rotor has an excess mass equivalent to 10
grams at a 10 in. radius, or 20 grams at a 5 in. radius. (See Figure 6-2)
In each case, the mass, when multiplied by its distance from the shaft
axis, amounts to the same unbalance value, namely 100 gram-inches. A
given mass will create different unbalances, depending on its distance
from the shaft axis. To determine the unbalance, simply multiply the
mass by the radius.
Since a given excess mass at a given radius represents the same unbal-
ance regardless of rotational speed, it would appear that it could be cor-
rected at any speed, and that balancing at service speeds is unnecessary.
This is true for rigid rotors as listed in Table 6-5 (page 292). However,
not all rotors can be considered rigid, since certain components may shift
or distort unevenly at higher speeds. Thus they may have to be balanced
at their service speed.