Page 277 - Machinery Component Maintenance

Balancing of Machinery Components 259

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(8) HARD-BEARING BALANCING MACHINES

Figure 6-16. Phase angle and displacement amplitude versus rotational speed in soft-
bearing and hard-bearing balancing machines.

Bearings (and the directly attached support components) vibrate in uni-
son with the rotor, thus adding to its mass. Restriction of vertical motion
does not affect the amplitude of vibration in the horizontal plane, but the
added mass of the bearings does. The greater the combined rotor-and-
bearing mass, the smaller will be the displacement of the bearings, and
the smaller will be the output of the devices which sense the unbalance.
As far as the relationship between unbalance and bearing motion is
concerned, the soft-bearing machine is faced with the same complexity as
shown in Figure 6- 1 1.
Therefore, a direct indication of unbalance can be obtained only after
calibrating the indicating elements for a given rotor by use of test masses
which constitute a known amount of unbalance.
For this purpose the soft-bearing balancing machine instrumentation
contains the necessary circuitry and controls so that, upon proper calibra-
tion for the particular rotor to be balanced, an exact indication of amount-
of-unbalance and its angular position is obtained. Calibration varies be-
tween parts of different mass and configuration, since displacement of
the principal axis of inertia in the balancing machine bearings is depen-
dent upon rotor mass, bearing and suspension mass, rotor moments of
inertia, and the distance between bearings.