Page 320 - Machinery Component Maintenance

302 Machinery Component Maintenunce and Repair

1940 on “Balance Quality of Rotating Rigid Bodies” which, in the mean-
time, has also been adopted as S2.19-1975 by the American National
Standards Institute (“ANSI,” formerly USASI and ASA). The principal
points of this standard are summarized below. Balance tolerance nomo-
grams, developed by the staff of Schenck Trebel Corporation from the
composite IS0 metric table, have been added to provide a simple-to-use
guide for ascertaining recommended balance tolerances (see Figures
6-34 and 6-35).

Balance Quality Grades

We have already explained the detrimental effects of unbalance and the
purpose of balancing. Neither balancing cost considerations, nor various
rotor limitations such as journal concentricity, bearing clearances or fit,
thermal stability, etc., permit balancing every rotor to as near zero unbal-
ance as might theoretically be thought possible. A tolerance must be set
to allow a certain amount of residual unbalance, just as tolerances are set
for various other machine shop operations. The question usually is, how
much residual unbalance can be permitted while still holding detrimental
effects to an insignificant or acceptable level?
The recommendations given in IS0 1940 will usually produce satisfac-
tory results. The heart of the Standard is a listing of various rotor types,
grouped according to “quality grades” (see Table 6-5). Anyone trying to
determine a reasonable balance tolerance can locate his rotor type in the
table and next to it find the assigned quality grade number. Then the
graph in Figure 6-33 or the nomograms in Figures 6-34 and 6-35 are used
to establish the gram inch value of the applicable balance tolerance
(i . e., “permissible residual unbalance” or Upr).
Except for the upper or lower extremes of the graph in Figure 6-33,
every grade incorporates 4 bands. For lack of a better delineation, the
bands might be considered (from top to bottom in each grade) substan-
dard, fair, good, and precision. Thus, the graph permits some adjustment
to individual circumstances within each grade, whereas the nomograms
list only the median values (centerline in each grade). The difference in
permissible residual unbalance between the bottom and top edge of each
grade is a factor of 2.5. For particularly critical applications it is, of
course, also possible to select the next better grade.

CAUTION: The tolerances recommended here apply on1.y to rigid ro-
tors. Recommendations for flexible rotor tolerances are contained in IS0
5343 (see Appendix 6-C) or in Reference 2.
(En continued on page 306)