Page 305 - Machinery Component Maintenance

Balancing of’ Machinery Component$ 287

piece. Since the UI component of K1 now adds to the reversed
workpiece unbalance U2, indication I2 will be opposite U1 and twice
its magnitude.
4. Depress switch “U + K2” and adjust compensation voltage K2 until
I2 is zero.
5. Depress switch “U + 1/2K2.” This divides compensation voltage K2
,
in half. The remaining indication is U1 or the true initial unbalance
in the workpiece. The tooling error E remains compensated by KI
and thus has no more influence on this reading or on readings taken
on subsequent workpieces of the same type. If the workpiece type
changes, the double compensator procedure described above must
be repeated for a new setup.

Just as the compensator is used to correct for unwanted errors, it can
also be used to bias tooling, thereby producing a specified unbalance in a
part. A typical example would be a crankshaft for a single or dual piston
pump which might call for a given amount of compensating unbalance in
the counterweights.
Before using a compensator for this purpose, the required accuracy for
the bias must be evaluated. For large biases with tight tolerances, it may
be necessary to add precisely made (and located) bias masses to the tool-
ing. An error analysis and statistical evaluation (see earlier chapters) may
then be required to take into account all error sources such as weight of
bias mass, its CG uncertainty due to unbalance and mounting fit toler-
ance, distance of bias mass to the shaft axis of the arbor, angular location,
etc.

Unbalance Correction Methods

Corrections for rotor unbalance are made either by the addition of
mass to the rotor, by the removal of material, or in some cases, by relo-
cating the shaft axis (“mass centering”). The selected correction method
should ensure that there is sufficient capacity to allow correction of the
maximum unbalance which may occur. The ideal correction method per-
mits reduction of the maximum initial unbalance to less than balance tol-
erance in a single correction step. However, this is often difficult to
achieve. The more common methods described below, e.g., drilling,
usually permit a reduction of 10: 1 in unbalance if carried out carefully.
The addition of mass may achieve a reduction ratio as large as 20:l or
higher, provided the mass and its position are closely controlled. If the
method selected for reduction of maximum initial unbalance cannot be
expected to bring the rotor within the permissible residual unbalance in a