2.18. LINING UP GENERATOR WITH ENGINE
The proper operation of a generator set requires that the armature shaft and frame be in line with
the engine crankshaft, and that the air gaps under each pole be equally spaced. The eccentricity at the
coupling should be held to a minimum as this directly affects both balance between the parts and the
wear of brush and bearing. The airgap of the generator must be uniform within plus or minus 10 percent
from average under each main pole, under each commutating pole, and from front to rear. The
recommended place for alining the airgap and coupling is at the engine end of the generator.
a. Alining airgap. Where the generator installation permits, measure the airgap under each pole
at the coupling end. This is done with a long feeler gage inserted from the commutator end. Since the
generator has a nonuniform airgap under the main poles, measurements must be made under the tip of
the pole piece. The radial clearance between the edge of the fan and the generator frame may be used;
one point on the fan is used for reference and the fan rotated so that the measurements can be made
between this point and the frame at the top, bottom, and each side. If the difference in these
measurements 180 degrees apart is less than 0.020 inch, the airgap is satisfactory.
b. Alining coupling. The coupling is alined by an indicator fastened to a support extending
from the armature flange through a hole in the fan blade. The indicator is attached to this support so that
the indicator plunger rides against the face of the generator fan. The dial of the indicator should be set at
zero at the top position and readings should be taken 90 degrees apart. An allowance of plus or minus
0.010 inch at the bottom and plus or minus 0.005 on each side will result in satisfactory alinement if the
coupling is concentric with the crankshaft and armature shaft.
c. Final operations. Since any movement of the generator frame affects both the coupling
alinement and airgap, readings must be repeated for both after setting. Only full-length shims should be
used under each generator mounting pad. Not more than three shims should be used under each
mounting pad with one shim not to exceed 0.060 inch in thickness; taper the thickest shim to make the
airgap and coupling conform to specified limits. After the generator is coupled to the engine, place the
generator frame so that the single bearing at the commutator end is located axially to avoid thrust load in
either direction. The generator has a float of approximately 3/16 inch between the outer race of the
bearing and the housing, and is coupled to the engine with no clearance between the bearing outer
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