Page 325 - Machinery Component Maintenance

Baloncing of’ Machinery Components 307

other words, the larger portion of the tolerance is allotted to the correc-
tion plane closest to the CG; however, the ratio of the two tolerance por-
tions should never exceed 7:3, even though the plane distance ratio may
be higher.
For rotors with correction plane distance (b) larger than the bearing
span (d), the total tolerance should be reduced by the factor d/b before
any apportioning takes place.
For rotors with correction plane distance smaller than V3 of the bearing
span and for rotors with two correction planes outboard of one bearing, it
is often advisable to measure unbalance and state the tolerance in terms
of (quasi-) static and couple unbalance. Satisfactory results can generally
be expected if the static residual unbalance is held within the limits of
Ustaw 5 UpJ3

and the couple residual unbalance within

(where d = bearing span)
If separate indication of static and couple unbalance is not desired or
possible, the distribution of the permissible residual unbalance must be
specially investigated, taking into account, for instance, the permissible
bearing loads.4 It may also be necessary to state a family of tolerances,
depending on the angular relationship between the residual unbalances in
the two correction planes.
For all rotors with narrowly spaced (inboard or outboard) correction
planes, the following balancing procedure may prove advantageous if
U,, is specified in terms of residual unbalance per correction plane.

1 . Calibrate respectively the balancing machine to indicate unbalance
in the two chosen correction planes I and I1 (see Figure 6-36).
2. Measure and correct unbalance in plane I only.
3. Recalibrate or set the balancing machine to indicate unbalance near
bearing plane A and in plane 11.
4. Measure and correct unbalance in plane I1 only.
5. Check residual unbalance with machine calibrated or set as in 3.
Allow residual unbalance portions for the inboard rotor as dis-
cussed above (inversely proportional to the correction plane dis-
tances from the CG), considering A and I1 as the correction planes;
for the outboard rotor allow no more than 70 percent of U,, in
plane 11, and no less than 30 percent in plane A.