CENTRIFUGALPUMP.COM
Criteria for selecting ball or sleeve bearings
in horizontal centrifugal pumps
A ball bearing is also referred to as anti-friction bearing, or rolling-element bearing
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.
An example of sleeve bearing split in half, also referred to as journal bearing
Horizontal centrifugal pumps are mostly built with ball-and-ball or ball-and-
sleeve
thrust bearings. The term ball-and-ball means that both the axial and
radial thrust bearings are of ball bearing type;
ball-and-sleeve means that the
axial thrust bearings are of ball bearing type and the radial thrust bearings
are of sleeve bearing type. Ball bearings are also known as anti-friction
bearings and sleeve bearings as journal bearings.

In instances of exceptionally high speed, high energy density, or high axial
thrust applications,
tilting-pad-and-sleeve bearing design may be used. Both
the tilting-pad and sleeve bearings are considered hydrodynamic bearings
because the manner of their lubrication is based on hydrodynamic principle.
The
tilting-pad-and-sleeve bearing design is discussed in a separate article.

Many small size pumps are built with either ball-and-ball, or ball-and-sleeve
bearings of standard are available with

Two questions that are frequently asked are: What are design criteria 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 and is mostly caused by hydraulic thrust, although in rare instances the
pump driver can also contribute to axial thrust. 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 thrust 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. The thrust bearings can be stacked in
back-to-
back
or face-to-face arrangement to meet different thrust loading condition.

Although sleeve radial thrust bearings are considered to be of more robust
design many pump users still prefer to use ball bearings because of their
adaptability to oil mist lubrication that can be part of a centralized lubricating
system providing lubrication to multiple equipment. Oil mist lubrication
significantly improve bearing life because the ball bearings generate less heat
and are the bearings are less likely to be exposes to contaminants.

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. A recommended DN factor limit is 500,000
where D is the mean bearing diameter in millimeters and N is the rotational
speed in RPM - above this value it is recommended to use sleeve bearing.

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.

Ball bearings also have poor damping and shock-absorbing characteristic

Most overhang pumps have ball-and-ball bearings for the simple reason that
they are relatively of smaller sizes with low to medium energy density.
Overhang pumps are generally limited to 15" impeller diameter at 3560 RPM,
and 26" at 1780 RPM. Beyond these sizes the shaft deflection tend to be
excessive that the pumps have to be of between-bearing design.
Sleeve bearings

As the term sleeve implies, sleeve bearings are bearings that are shaped   
and machined  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.

ANSI / API Standard 610 (11th 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. They provide viscous damping of vibration resulting in smooth
operation. Worn bearings can be re-babitted and re-used. Because they are
split in halves they can be inspected, removed, repaired, or replaced without
removing the rotor from its casing.

Several designs are available to fit every service. Among these are the plain
and axial grooved, the elliptical and lobed design, and the pressure dam
sleeve bearings.

Sleeve bearings provide better bearing stiffness and rotordynamic stability.

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 lubricated by oil rings 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.

For centrifugal pumps made to API 610 and ISO 13709 Standard, the identical
standard has specific requirements under which ball bearings (rolling
element), sleeve bearings, or tilting-pad thrust bearings shall be selected.
The standard generally covers centrifugal pumps for the petroleum,
petrochemical, and natural gas industries.


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