Page 335 - Centrifugal Pumps 2E

300 Centrifugal Pumps: Design and Application

Lubrication of the Bearings

The bearings are normally lubricated with either oil bath or grease.
The oil level is usually at mid-ball. If it is higher than mid-ball, churning
usually occurs resulting in foam in the bearing housing and an increase in
temperature. Disk flingers are used to splash oil within the housing. Roll
pins are also used on the shafts to splash oil. In this case, the initial oil
level is below the balls.
With oil lube, the temperature on the outside skin of the housing is
about 20° cooler than the temperature of the outer race. Skin temperature
of the small pumps ranges between 110°F and 130°F» and on the large
pumps between 140°F and 165°F. At skin temperatures above 185°F» the
unit should be shut down and inspected to determine the cause of high
temperature. It takes approximately 45 minutes to one hour for the tem-
perature of a cradle to stabilize. With roll pin splash, the temperature is
20°F less than the oil bath.
Grease bearings are either replacement grease or seal-for-life bearings.
The problem with the replaceable grease is that excess grease is usually
put into the bearing's cavity causing sharp increase in temperature and
drop in life.
The seal-for-life bearings come with either shields or seals. Shield
bearings are adequate for the majority of applications. In general, the
temperature of grease bearings is approximately 20°F less than the oil
lubrication. The subject of bearing lubrication is discussed in detail in
Chapter 20.

Mounting the Bearing
Finish and dimensions of the shaft should meet the bearing manufac-
turer's recommendations. Typically, the bearing is .000 to .0005 inches
tight on the inner race and ,0005 to .001 inch loose on the outer race.
Refer to the bearing manufacturer's catalog for recommended fit. The
bearings can be pressed on to the shaft, but it is usually better to heat
them to prevent excess stress. They can be heated in an oven or put in an
oil bath up to 240°F. When put in an oven, the bearings should be laid
flat and should not touch each other. For field installation, the bearing
can be set over a light bulb to expand the inner race. If the bearing is too
loose on the outer race, the race will spin within its housing. The bear-
ings are secured to the shaft with a snap ring or a bearing lock nut. The
bearing lock nut is secured with a tab washer. This is preferred to a lock
nut with a nylon type pellet. The outer race axial movement is restricted
in two ways: either within a separate end cover or by having a snap ring
in the outer race clamped between the bearing housing and the end cover.