Page 275 - Machinery Component Maintenance

Balancing of Machinery Components 257

Gravity Balancing Machines

First, consider the simplest type of balancing-usually called “static”
balancing, since the rotor is not spinning.
In Figure 6-14A, a disc-type rotor on a shaft is shown resting on knife-
edges. The mass added to the disc at its rim represents a known unbal-
ance. In this illustration, and those which follow, the rotor is assumed to
be balanced without this added unbalance mass. In order for this balanc-
ing procedure to work effectively, the knife-edges must be level, parallel,
hard, and straight.
In operation, the heavier side of the disc will seek the lowest level-
thus indicating the angular position of the unbalance. Then, the magni-
tude of the unbalance usually is determined by an empirical process, add-
ing mass in the form of balancing wax or clay to the light side of the disc
until it is in balance, Le., until the disc does not stop at the same angular
position.
In Figure 6-14B, a set of balanced rollers or wheels is used in place of
the knife edges. Rollers have the advantage of not requiring as precise an
alignment or level as knife edges; also, rollers permit run-out readings to
be taken.
In Figure 6-14C, another type of static, or “nonrotating”, balancer is
shown. Here the disc to be balanced is supported by a flexible cable, fas-
tened to a point on the disc which coincides with the center of the shaft
axis slightly above the transverse plane containing the center-of-gravity.
As shown in Figure 6-14C, the heavy side will tend to seek a lower level
than the light side, thereby indicating the angular position of the unbal-
ance. The disc can be balanced by adding mass to the diametrically op-
posed side of the disc until it hangs level. In this case, the center-of-grav-
ity is moved until it is directly under the flexible support cable.
Static balancing is satisfactory for rotors having relatively low service
speeds and axial lengths which are small in comparison with the rotor
diameter. A preliminary static unbalance correction may be required on
rotors having a combined unbalance so large that it is impossible in a
dynamic, soft-bearing balancing machine to bring the rotor up to its
proper balancing speed without damaging the machine. If the rotor is
first balanced statically by one of the methods just outlined, it is usually
possible to decrease the initial unbalance to a level where the rotor may
be brought up to balancing speed and the residual unbalance measured.
Such preliminary static correction is not required on hard-bearing bal-
ancing machines.
Static balancing is also acceptable for narrow, high speed rotors which
are subsequently assembled to a shaft and balanced again dynamically.
This procedure is common for single stages of jet engine turbines and
compressors.