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Full text of "Modern Mechanical Engineering Vol-I"

260                               HEAVY  MACHINERY
(i.e. where the wire sling grips the rotor) is moved until4he balance is obtained.                   '?
Sometimes, in order to achieve this, it is necessary to hang a set of chain
blocks on the crane hook and support the pedestal slightly by tightening
up the chain blocks. Having thus got the rotor slung level, it is an easy
matter to thread the piece through the stator until the rope sling supporting                   ,
the rotor, &c., is nearly up against the stator winding. A support of steel
beams or baulks of timber should previously have been arranged to support                   i
the turbine end of the generator rotor, and the rotor is then carefully lowered;
some ten or twelve, or more, steel rollers should be placed under the out-
board pedestal, and the latter lowered down on to them. These rollers
should be made of \ in. diameter steel rod, in lengths a little greater than
the width of the pedestal. The sling is then removed from the body of
rotor, and the turbine end of the rotor shaft is supported by the crane, and
a slight endwise pull applied by the crane. The pedestal end will roll on
the small rollers, and, when the generator and turbine couplings are together,
the generator bearing (turbine end) can be put in pgsition and the rotor
lowered into it. It only remains to lift the outboard pedestal end of the
rotor with the crane and remove the small rollers. It is the practice nowa-
days to place a sheet of insulating material, e.g. fuller-board, leatheroid, &c.,            . ,
under the outboard pedestal, and to insulate from the pedestal, by means
of insulating tubes and washers, the bolts which hold the pedestal down                   I
to the bedplate. This is done to prevent the circulation of stray currents
through the rotor shaft, pedestals, and bedplate. Under certain circum-
stances these stray currents reach high values, and the effect on the generator %
is to cause pitting of the journals and white-metal bearings, and the breaking
up of the oil passing through the bearings, with the formation of acid, which
in turn causes further corrosion.
Before closing up the bearings and bolting down the bearing pedestal                   \
covers, it is most important that the clearances between the bearing and
journal for oil be accurately measured, and if necessary increased to a safe
figure, and at the same time the fact be definitely established that the cover
is actually binding down the bearing inside it. The white-metal lining of
the bearings should be scraped away carefully at the sides (see fig. 15) for
a sufficient distance down, so as to leave the actual bearing area—that area
contained in an angle of about 120°; this side clearance should not be less
than five-thousandths of an inch, and it should be possible to get a feeler
gauge down on each side, all along the bearing. The clearance between
the top of the journal and the bearing is obtained by putting two or three
strands of soft lead wire across the journal and bolting the top and bottom                   j
half-bearings tightly together; on opening out again, the lead will be found
to have been flattened out to the exact clearance on the top of the bearing,
and this thickness can then be accurately measured by a micrometer gauge.
This clearance varies with different makers; an average figure is about
i mil. ('ooi in.) per inch of journal diameter, and if necessary the inside of
the top half-bearing should be scraped away carefully to obtain the necessary
uniform clearance.