Page 288 - Machinery Component Maintenance

270 Machinery Component Maintenance and Repair

Rotors with More than TWO Journals

Rotors which are normally supported at more than two journals may be
balanced satisfactorily on only two journals provided that:

1. All journal surfaces are concentric with respect to the axis deter-
mined by the two journals used for support in the balancing ma-
chine.
2. The rotor is rigid at the balancing speed when supported on only
two bearings.
3. The rotor has equal stiffness in all radial planes when supported on
only two journals.

If the other journal surfaces are not concentric with respect to the axis
determined by the two supporting journals, the shaft should be
straightened. If the rotor is not a rigid body, or if it has unequal stiffness
in different radial planes (e.g., crankshafts), the rotor should be sup-
ported in a (nonrotating) cradle at all journals during the balancing oper-
ation. This cradle should supply the stiffness usually supplied to the rotor
by the rotor housing in which it is finally installed. The cradle should
have minimum mass when used with a soft-bearing machine to permit
maximum balancing sensitivity.

Rotors with Roiling Element Bearings

Rotors with stringent requirements for minimum residual unbalance and
which run in rolling element bearings, should be balanced in their bear-
ings, either in:

1. Special machines where the bearings are aligned and the outer races
held in saddle bearing supports, rigidly connected by tie bars, or
2. In standard machines having supports equipped with V-roller car-
riages.

Frequently, practical considerations make it necessary to remove the
bearings after balancing, to permit final assembly. If this cannot be
avoided, the bearings should be match-marked to the rotor shaft and re-
turned to the location used during balancing. Rolling element bearings
with considerable radial play or bearings with a quality less than ABEC
(Annular Bearing Engineers Committee) Standard grade 3 tend to cause
erratic indications in the balancing machine. In some cases the outer race
can be clamped tightly enough to remove excessive radial play. Only
“fair” or lesser balance quality can be reached when rotors are supported
on bearings of a grade lower than ABEC 3.