Manual for Induction Motors and Generators
104 -
Trouble Shooting
3BFP 000 050 R0101 REV E
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Make a U-shaped 'water lock' on the oil return piping
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Install a breather on top of the bearing housing.
Air pressure outside the bearing
The air pressure near the exit of the shaft from the bearing needs to be verified. This is
especially important if the bearing is flange mounted to the machine, or if the shaft is mounted
inside a cover or other construction which might form a 'centrifugal fan' together with the shaft.
Flange bearings have two canals between the bearing housing and the flange, which normally
are enough to compensate for any under pressure near the exit of the shaft from the bearing
housing. However, if for some reason a very large under pressure is present near this area, the
two canals might not be enough, and some air is additionally sucked from inside the bearing.
This is especially likely to happen to sleeve bearings with axial thrust pads, as the oil flow in
these bearings is larger than in pure radial bearings.
If a large under pressure is noticed or suspected, the air pressure has to be measured near the exit
of the shaft from the bearing housing.
In order to verify that the under pressure outside the bearing can cause the leakage, the pressure
outside the bearing (p0) inside the bearing (p2), and the pressure in the area between the end
shield and the machine seal (p1) must be measured as well. When measuring (p1), the tube has
to be inserted as deep as possible, and the canals must be temporarily closed, see
Verification of air pressure inside and outside of a sleeve bearing
In order to analyze the situation, p1 and p2 have to be compared with p0, which has to be
measured free from any disturbances or turbulence near the machine. The following situations
can appear:
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p0 = p1 = p2. If all pressure readings are the same, the leakage is not caused by pressure
differences. However, bear in mind what has been stated about diesel engines earlier
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p2 > p1(= p0). If the pressure inside the bearing is larger than the outside pressure, there is
only a situation with overpressure inside the bearing
•
p2 (= p0) > p1. If the pressure outside the bearing is smaller the pressure elsewhere, there
is under pressure near the bearing
•
p2 > p0 > p1. If all pressure readings are different, there might be a situation where both
overpressure inside the bearing, and under pressure outside the bearing are present.
