Siemens Induction motors/ generators CGII - Manual (Page 20)

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Maintenance
At the first sign of oil discoloration or contamination,
replace with new oil. Rapid discoloration is caused by
bearing wear, often from vibration or thrust. Change
oil as required to keep clean.
When assembling the bearing, it is possible to foul
the rings so that they will not turn freely. Check ring
operation by rotating shaft by hand after assembly
Oil Seals
The 680, and 800 frames use fixed labyrinth oil seals.
The oil seals should be centered radially on the shaft
and set so that the radial clearance is uniform on all
sides.
The 500, 580 and 1120 frames use self-centering oil
seals that can accommodate small radial movements
in the shaft. These floating labyrinth seals should be
free to move radially in the bearing housing. A
jammed seal could lead to bearing overheating and
poor vibration due to excessive seal rubbing during
operation.
The primary purposes of the oil seal are:
1. To prevent the entrance of dirt into the bearing
cavity.
2. To retain the oil in the bearing cavity.
3. To provide pressure regulation in the bearing
cavity.
To achieve these purposes it is necessary that a
specific diametrical clearance of oil seal to shaft be
held. The cooling fans of the motor tend to create
suction or pressure in the bearing cavity, which, if
large enough, will push or draw oil mist out along the
shaft. The 500, 580 and 1120 designs are equipped
with a close fitting air seal that serves as a barrier
against excessive pressure or suction inside the
bearing.
Standard oil seal clearances
Frame Diametrical Clearance
500 0.009 to 0.014 inches
580 0.015 to 0.019 inches
680 0.015 to 0.019 inches
800 0.015 to 0.020 inches
1120 0.004 to 0.008 inches
The bearing cavities and oil seals on the 680 and 800
frame designs are vented to the outside air in order to
keep the bearing cavity pressure as close to
atmospheric as possible. For these designs, it is
important that the piping and venting for these oil
seals be kept clean.
NOTE
When a sleeve bearing becomes worn and requires
replacement, the labyrinth oil seal should also be
replaced.
Bearing Clearance Too Large
The clearance between the journal and the bearing
will permit the journal center to be slightly below the
bearing center. With the motor operating, the
presence of the oil film between the journal and the
bearing will cause the shaft to rise slightly.
Too large a bearing clearance or clogged oil return
holes in the bearing will permit excessive oil to seep
out the ends of the bearing. This seepage, combined
with the rotation of the shaft, will create an oil mist
inside the bearing cavity that will tend to leak through
the oil seals. Excessive pressure in a force feed
lubrication system can also cause an oil mist to build
up.
Cavity Pressures
Motors are sensitive to the amount of pressure or
vacuum existing in the bearing cavity. The table
below shows the maximum allowable oil cavity
pressure or vacuum in terms of plus or minus inches
of water:
Frame Drive End
Non-drive
End
500 ±0.12 ±0.12
580 ±0.12 ±0.12
680 ±0.12 ±0.12
800 ±0.12 ±0.12
1120 ±0.12 ±0.12
For the correction of a high or low reading it is
important that:
1. The parts and joints around the bearing cavity
and oil seals are sealed.
2. Condensation drains, piping, sight gauges and
breathers are functioning properly.
3. Auxiliary equipment extending into close
proximity of the motor bearing housings is
shielded from the oil seals to prevent a transfer of
air from inside to outside the motor. (ex. motor
coupling)
4. All fixed oil seals are centered properly on the
shaft.