Page 284 - Machinery Component Maintenance

266 Machinery Component Maintenance and Repair

balance on the first rotor. For subsequent rotors of the same kind, read-
ings may be obtained in a single run but must be manually entered into
the calculator and then suitably manipulated.

Class 11: Calibratable Balancing Machines Requiring a Balanced Prototype.
Machines in this class are of the soft-bearing type using instrumentation
which permits plane separation and calibration for a given rotor type, if a
balanced master or prototype rotor with calibration masses is available.
However, the same trial-and-error procedure as for Class I machines is
required for the first of a series of identical rotors.

Class 111: Callbratable Balancing Machines Not Requiring a Balanced Proto-
type. Machines in this class are of the soft-bearing type using instrumen-
tation which includes an integral electronic unbalance compensator. Any
(unbalanced) rotor may be used in place of a balanced master rotor with-
out the need for trial and error correction. Plane separation and calibra-
tion can be achieved in one or more runs with the help of calibration
masses.
This class also includes soft-bearing machines with electrically driven
shakers fitted to the vibratory part of their rotor supports.

Class iV Permanently Calibrated Balancing Machines. Machines in this
class are of the hard-bearing type. They are permanently calibrated by
the manufacturer for all rotors falling within the weight and speed range
of a given machine size. Unlike the machines in other classes, these ma-
chines indicate unbalance in the first run without individual rotor calibra-
tion. This is accomplished by the incorporation of an analog or digital
computer into the instrumentation associated with the machine. The fol-
lowing five rotor dimensions (see Figure 6-21) are fed into the computer:
distance from left correction plane to left support (a); distance between
correction planes (b); distance from right correction plane to right sup-
port (c); and rl and r2, which are the radii of the correction masses in the
left and right planes. The instrumentation then indicates the magnitude
and angular position of the required correction mass for each of the two
selected planes.
The compensation or “null-force” balancing machine falls into this
class also. Although no longer manufactured, it is still widely used. It
balances at the natural frequency or resonance of its suspension system
including the rotor.