Criteria for selecting ball bearings or
sleeve bearings in centrifugal pumps


Centrifugal pumps are usually built with either ball-and-ball bearings, or ball-and-
sleeve
bearings. The term ball-and-ball means that both axial thrust and radial
thrust bearings are made of ball bearings;
ball-and-sleeve means that axial thrust
bearing is made of ball bearing and radial thrust bearings are made of sleeve
bearings.

Two questions that are frequently asked are: What design criteria are used in
selecting the type of bearings to use, and what are the advantages and
disadvantages of ball bearings over sleeve bearings, and vice-versa?


Axial and radial thrust loads

Typically, general-purpose centrifugal pumps and those that are in low-energy
service are built with ball bearings to carry both axial and radial thrust loads
(ball/ball design). Specialized pumps and those in high-energy service are built
with ball bearings to carry axial thrust load and sleeve bearings to carry radial
thrust loads (ball/sleeve design).

Axial thrust loads are thrusts acting in axial direction or parallel to the pump shaft;
radial thrust loads are thrust acting in radial direction or perpendicular to the shaft.
The principal sources of pump thrusts are hydraulic thrust and static thrust due to
the weight of the pump rotor. Manufacturing defects (such as excessive residual
unbalance) and flaws in assembly (such as misalignment) are also contributors.


Ball bearings

Ball bearings are inexpensive, have different thrust load capacities to suit various
applications, and are readily available in most cases. Ball bearings are also
referred to as
anti-friction bearings because they have very low friction loss.

Advantages of ball bearings

Ball bearings generate less heat and require simple lubrication – by oil ring, oil
mist, or oil bath method. They have narrow width and fit in narrow bearing housing
thereby making the shaft bearing span shorter resulting in less shaft deflection.
Depending on its type, a ball bearing can carry both radial and thrust loads. Two,
or more, bearings can be stacked in series to increase their thrust load carrying
capacity.

Disadvantages of ball bearings

The disadvantages of ball bearings are that they have limited thrust capacity and
operating speed. Ball radial bearings are seldom used in centrifugal pumps
running above 3600 RPM. They have more moving parts – balls, cage, and inner
raceway – and are therefore likely to fail sooner and more frequently than sleeve
bearings, although ball bearings are typically sized and selected to provide an L10
bearing life of at least 25,000 operating hours under rated conditions. Defects in,
or failures of, bearing balls, cage, inner raceway, or outer raceway can result in
high and destructive pump vibration.
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A ball bearing is also referred to as anti-friction bearing, or rolling-element bearing
An example of sleeve bearing split in half, also referred to as journal bearing
Sleeve bearings

As the name implies, sleeve bearings are bearings that are shaped like a shaft
sleeve and are primarily used as radial bearings. They are the preferred bearing
type in high DN factor, in high energy density pump service, or where the pump
speed exceeds 3560 revolutions per minute (RPM).

DN factor is defined as the product of pump RPM multiplied by the bearing size in
millimeters. A DN factor in excess of 500,000 is considered a high DN factor.

High energy density service may be defined as an application where the product
of the rated horsepower and pump speed in RPM exceeds 5,400,000.

API Standard 610 (9th edition) and ISO Standard 13709 require that radial sleeve
bearings be used in high DN factor, or high energy density applications.

Advantages of sleeve bearings

Sleeve bearings, also referred to as journal bearings, are virtually unlimited in
their radial thrust capacity, unlike ball bearings, and they are suitable in moderate
to high speed applications. They provide better stiffness than ball bearings. Worn
bearings can be re-babitted and re-used. Because they are split in halves they are
easier to inspect or replaced.

Disadvantages of sleeve bearings

Sleeve bearings are more expensive than ball bearings. They have no axial thrust
load carrying capability and, therefore, have to be used in conjunction with another
axial thrust bearing - either a ball type, or tilting-pad type.

Sleeve bearings are not recommended in very low speed application because the
oil rings that are typically used in lubricating them become less effective. Because
sleeve bearings are wider compared to ball bearings, they have higher friction
loss, generate more heat, and require more lubrication.


R: CRFO-0511
C: design
F: ball and sleeve bearings


Beta version
Photos show a typical ball bearing or anti-friction bearing (left), and a split sleeve
bearing or journal bearing (right). Ball bearings can carry both axial and radial
thrust loads. Sleeve bearings can carry higher radial thrust loads but have no axial
thrust carrying capacity.