Page 73 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

58 Practical Machinery Management for Process Plants

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Oil
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level

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Figure 1-25. The sealing film near the bearing end cap.

However, there are at least two considerably more elusive problems involv-
ing bottle oilers.
The first is shown in Figure 1-25. With a relatively viscous oil and close
clearance at the bearing housing end seal-point S-an oil film may exist
between seal bore and shaft surface. Good lube oils have a certain film
strength and, under certain operating conditions, this sealing film near the
bearing end cap may break only if the pressure difference between the bear-
ing housing and atmosphere exceeds % in. of water column.
If the bearing housing is exposed to a temperature increase of a few
degrees, the trapped vapors--usually an air-oil mix-floating above the liq-
uid oil level will expand and the pressure may rise to !4 in. of water column.
While this would not be sufficient to rupture the oil film, the pressure
buildup is sufficient to depress the oil level from its former location near the
center of the bearing ball at the 6 o'clock position, to a new level now barely
touching the extreme bottom of the lowermost bearing rolling element. At
that time, the bearing will overheat and the lube oil in contact with it will
carbonize. An oil analysis will usually determine that the resulting blacken-
ing of the oil is due to this high temperature degradation.14
The second oil-related problem causes the contents of bottle oilers to turn
grayish in color. This problem is primarily observed on ring-oil lubricated
rolling element bearings.
Suppose you have aligned precisely the shafts of the pump and driver.
Nevertheless, shims placed under the equipment feet in order to achieve this
precise alignment caused the shaft system to slant 0.005 in. or 0.010 in. per
foot of shaft length. As a consequence, the brass or bronze oil slinger ring
shown in Figure 1-26 will exhibit a strong tendency to run "downhill."