Section 6.1.1 - Centrifugal Pump Bearings: Clearance And Preload

Clearance and Preload   Single-row deep-groove ball bearings are available with several
classes of radial internal clearances. As standard, they are manufactured with Normal
radial internal clearance, but a popular alternative for severe duties, especially applications
involving pumping temperatures higher than ambient, is a larger than Normal, or
C3, radial internal clearance. Pumps designed for petroleum, petrochemical, and natural
gas industries, such as covered by API Standard 610 (ISO 13709) require a C3 internal
clearance as standard. To meet this requirement, bearings should be specified to be in
accordance with ISO 5753, Group 3. Greater internal clearances may reduce the temperature
rise of the bearing lubricant and allow the bearings to run cooler under load. However,
there may be a trade-off in that higher pump vibration velocities may occur with greater
bearing internal clearances. It is important that the temperature rise of the bearings under
maximum load conditions does not exceed the capability of the lubricant to properly perform
and maintain the required viscosity, and thus film thickness, within the bearings.

In centrifugal pumps, bearing preload is a term applied to two bearings mounted in a
pair to handle both radial and axial thrust loads. Tapered roller bearings and angular contact
ball bearings are typically mounted to a predetermined setting or preload. This setting
may be necessary to maintain proper contact of the rolling elements with the bearing
races during operation to avoid skidding of the unloaded bearing in the pair. For angular
contact ball bearings, the setting ranges from various clearances (non preloaded) to various
amounts of preload. The determination of clearance or preload is based on a balance
between providing sufficient clearance to operate at a low temperature and sufficient contact
to avoid skidding of the rolling elements in the lightly loaded bearing in the pair. The
pump manufacturer is responsible for determining the amount of preload or clearance
necessary to meet the requirements of the standard or specification to which the pump is
manufactured and sold.

It should be noted that bearing preload occurs in two other ways: (a) when the bearings
are mounted on the shaft with an interference fit (as required by many standards, particularly
for dynamic loading applications), and (b) when, for hot liquid pumping applications,
the shaft heats more than the bearing races. Also, bearing preload determined by
interference fit must be evaluated as the bearings may heat up in service and it may either
decrease or increase.

The balance between bearing preload and clearance is not always easy to achieve.
To address instances in ball bearings where ball skidding is observed or can be
anticipated—in the unloaded bearing of the duplex-mounted pair, at least one bearing
manufacturer provides a matched set of bearings consisting of one 40^o contact angle
bearing paired with one 15^o contact angle bearing. Properly selected and mounted, the
15^o contact angle bearing should be less susceptible to ball skidding when operating
unloaded (when the axial thrust is in a direction such that it is all absorbed by the 40^o
contact angle mate). Care must be taken with this bearing type to ensure the direction
of axial thrust is known and the bearing mounted accordingly. Also, it must be realized
that the 15^o contact angle bearing has less axial load capability than the 40^o contact
angle bearing, and bearing selection and sizing must account for this difference in capability.
If axial bearing loads in both directions can be of the same magnitude, this option
may not be feasible.

Clearance and Preload   Single-row deep-groove ball bearings are available with several
classes of radial internal clearances. As standard, they are manufactured with Normal
radial internal clearance, but a popular alternative for severe duties, especially applications
involving pumping temperatures higher than ambient, is a larger than Normal, or
C3, radial internal clearance. Pumps designed for petroleum, petrochemical, and natural
gas industries, such as covered by API Standard 610 (ISO 13709) require a C3 internal
clearance as standard. To meet this requirement, bearings should be specified to be in
accordance with ISO 5753, Group 3. Greater internal clearances may reduce the temperature
rise of the bearing lubricant and allow the bearings to run cooler under load. However,
there may be a trade-off in that higher pump vibration velocities may occur with greater
bearing internal clearances. It is important that the temperature rise of the bearings under
maximum load conditions does not exceed the capability of the lubricant to properly perform
and maintain the required viscosity, and thus film thickness, within the bearings.

In centrifugal...