Page 306 - Machinery Component Maintenance

288 Machinery Component Maintenance and Repair

single correction step, a preliminary correction is made. Then a second
correction method is selected to reduce the remaining unbalance to its
permissible value.

Addition of Mass
1. Addition of solder or two-component epoxy. It is difficult to apply
the material so that its center-of-gravity is precisely at the desired
correction location. Variations in location introduce errors in cor-
rection. Also, this method requires a fair amount of time.
2. Addition of bolted or riveted washers. This method is used only
where moderate balance quality is required.
3. Addition of cast iron, lead, or lead masses. Such masses, in incre-
mental sizes, are used for unbalance correction.
4. Addition of masses by resistance-welding them to a suitable rotor
surface. This method provides a means of attaching a wide variety
of correction masses at any desired angular locations. Care must be
taken that welding heat does not distort the rotor.

Removal of Mass
1. Drilling. Material is removed from the rotor by a drill which pene-
trates the rotor to a measured depth, thereby removing the intended
mass of material with a high degree of accuracy. A depth gage or
limit switch can be provided on the drill spindle to ensure that the
hole is drilled to the desired depth. This is probably the most effec-
tive method of unbalance correction.
2. Milling, shaping, or fly cutting. This method permits accurate re-
moval of mass when the rotor surfaces, from which the depth of cut
is measured, are machined surfaces, and when means are provided
for accurate measurement of cut with respect to those surfaces; used
where relatively large corrections are required.
3. Grinding. In general, grinding is used as a trial-and-error method
of correction. It is difficult to evaluate the actual mass of the mate-
rial which is removed. This method is usually used only where the
rotor design does not permit a more economical type of correction.

Mass Centering
For the definition of mass centering see Appendix A. Such a procedure
is used, for instance, to reduce initial unbalance in crankshaft forgings.
The shaft is mounted in a balanced cage or cradle which, in turn, is ro-
tated in a balancing machine. The shaft is adjusted radially with respect
to the cage, until the unbalance indication for the combined shaft and cra-