Page 324 - Machinery Component Maintenance

306 Machinery Component Maintenance and Repair

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Special Conditions to Achleve Quality Grades G1 and G0.4

To balance rotors falling into Grades 1 or 0.4 usually requires that the
following special conditions be met:

For Quallty Grade 1:
Rotor mounted in its own service bearings.
No end-drive.

For Quality Grade 0.4:
Rotor mounted in its own housing and bearings.
Rotor running under service conditions (bearing preload, tempera-
ture).
Self-drive.
Only the highest quality balancing equipment is suitable for this work.

Applying Tolerances to Single-Plane Rotors

A single-plane rotor is generally disc-shaped and, therefore, has only a
single correction plane. This may indeed be sufficient if the distance be-
tween bearings is large in comparison to the width of the disc, and pro-
vided the disc has little axial runout. The entire tolerance determined
from such graphs as shown in Figures 6-34 and 6-35 may be allowed for
the single plane.
To verify that single-plane correction is satisfactory, a representative
number of rotors that have been corrected in a single plane should be
checked for residual couple unbalance. One component of the largest re-
sidual couple (referred to the two-bearing planes) should not be larger
than one half the total rotor tolerance. If it is larger, moving the correc-
tion plane to the other side of the disc (or to some optimal location be-
tween the disc faces) may help. If it does not, a second correction plane
will have to be provided and a two-plane balancing operation performed.

Applying Tolerances to Two-Plane Rotors
In general, one half of the permissible residual unbalance is applied to
each of the two correction planes, provided the distance between (in-
board) rotor CG and either bearing is not less than V3 of the total bearing
distance, and provided the correction planes are approximately equidis-
tant from the CG, having a ratio no greater than 3:2.
If this ratio is exceeded, the total permissible residual unbalance (U,,)
should be apportioned to the ratio of the plane distances to the CG. In