Whether selecting new or
maintaining existing process equipment, bearings are a critical component in
the equipment’s reliability, efficiency, and life. The majority of rotating
process equipment today relies upon either rolling element or fluid film
bearings to counteract gravity and developed forces in the equipment and allow
for free rotation of the shaft. Selecting the right bearing for the equipment
and application is essential to the successful operation of that equipment.
A key difference between
rolling element and fluid film bearings for personnel concerned about the
maintenance of process equipment is the expected life of the bearings. Rolling
element bearings typically have a predictable life based on the operating
conditions; fluid film bearings, when properly designed and maintained, can
operate for decades. Additional comparisons of rolling element and fluid film
bearing characteristics, such as lubrication needs and the ability to handle
impact loads, are provided in the Bearing Characteristics Comparison chart.
For mechanical design, the
fluid film bearing designer must not only consider the proper sizing of the
bearing to handle the loads and minimize the power loss but also incorporate
features such as preload and offset that optimize the formation of film and
tune the dynamic characteristics of the bearing. Tuning the bearing’s dynamic
characteristics controls the dynamic performance of the process equipment – to
the point where the bearings can limit vibrations.
Lubricant entering the converging geometry of a journal (featured image) or
thrust (above) bearing forms a wedge-shaped film; loads are then supported
through pressure in the film.
BASIC OPERATING PRINCIPLE OF
HYDRODYNAMIC FLUID FILM BEARINGS
The stationary surface of a fluid film bearing is separated from the rotating
surface by a thin film of lubricant, be it oil, air, water or process fluid. In
hydrodynamic fluid film bearings, the film pressure that separates the surfaces
and counters loads (including gravity) is created by the relative motion
(rotation) of the surfaces as the lubricant is pulled into a converging
geometry between the surfaces. Because of this separating force, no contact of
the surfaces occurs during normal operation, reducing wear and power loss
compared to surface-to-surface contact.
Thrust bearings counteract
axial loads (along the axis of rotation), while journal bearings support radial
loads (perpendicular to the axis of rotation). The materials and design of both
thrust and journal fluid film bearings are selected to optimize the converging
geometries and pressure for increased efficiency of the process equipment and
fewer demands on the ancillary equipment.
A combination tilt pad bearing with copper-backed babbitt thrust pads and
steel-backed babbitt journal pads (left) and a combination tilt pad bearing
with solid polymer thrust pads and polymer-lined journal pads (right).
MATERIALS OF CONSTRUCTION
Selecting a fluid film
bearing’s materials of construction relies on two primary factors: application
requirements and operating environment.
Babbitt (in particular
tin-based alloys) is often the material of choice for fluid film bearings in
process equipment when oil is the lubricant. Babbitt exhibits exceptional
conformability, compatibility, and embeddability. These qualities reduce the
likelihood of damage to the shaft during start-up and shutdown, or from upset
conditions, misalignment or the occasional ingestion of contaminants. Because
babbitt loses strength as temperature increases, the surface temperatures of
babbitt bearings are often limited to 266 degrees Fahrenheit (130 degrees
Celsius). Furthermore, the use of babbitt in applications with high dynamic
loads can be limited due to babbitt’s relatively low fatigue strength. For the
most part, however, babbitt has gained wide acceptance in the process equipment
industry because of its many benefits and relatively low cost.
Bronze may be used when operating temperatures are higher than babbitt’s
limits. Oil-lubricated bronze bearings are common in process equipment applications
where the bearing surface can reach 302 degrees Fahrenheit (150 degrees
Celsius), and in some cases even higher. Bronze doesn’t have the conformability
and embeddability of babbitt, however. This can lead to shaft or bearing damage
when contaminants or misalignment are present. In addition, care must be taken
when selecting the material of the rotating surface when using bronze bearings
to limit the potential of damage to the rotating components.
A cermet thrust bearing (left) and collar (right).
For rotating as well as reciprocating process equipment that requires higher
temperature or better conformability and embeddability capabilities than bronze,
aluminum alloyed with tin can be used as the bearing material. Aluminum tin can
operate at higher temperatures than babbitt, up to 320 degrees Fahrenheit (160
degrees Celsius), and doesn’t fatigue as readily. Babbitt and aluminum tin are
applied to steel for added strength or to chromium copper to enhance heat
transfer. Both can be used against a variety of rotating components, including
mild steels.
Engineered polymers, as solid
components or as a lining on steel backing, are also widely accepted in process
equipment. Polymer bearings exhibit embeddability and conformability similar to
babbitt bearings while having the ability to operate at higher temperatures and
loads. Because they have been proven with relatively thin films, polymer thrust
bearings are often used in oil-lubricated applications with unit loads up to 8
MPa (1160 psi)–approximately twice the load typically accepted by babbitt
bearings–and temperatures up to 392 degrees Fahrenheit (200 degrees Celsius).
Polymer bearings have the
additional benefits of being less susceptible to chemical attack than babbitt,
insulating against electrical currents and successfully operating in a variety
of lubricants, including water and process fluids. With the right material
composition, polymer bearings can also withstand film disruptions due to
overload or temporary loss of lubrication.
Combination tilt pad bearing
with polymer-lined thrust pads and bronze journal pads.
When high temperatures,
corrosive materials or contaminated lubricant rule out the use of babbitt,
aluminum tin, bronze, and polymer, fluid film bearings can be manufactured with
extremely hard surfaces. Ceramics, cermets, and diamond have been successfully
deployed when operating temperatures exceed 392 degrees Fahrenheit (200 degrees
Celsius), when films are very thin and/or when abrasives are present. Unlike
babbitt, aluminum tin and polymer, which can operate against mild steels and
other traditional shaft materials, ceramic and cermet require deliberate
material selection for both the rotating and stationary surfaces to obtain the
desired life and performance of the bearings, especially when abrasives are
present or films are extremely thin.
The “best” material for bearings may vary even in the same assembly, and
material alone will not guarantee performance. The carefully selected
material(s) must be paired with the optimum mechanical design.
For mechanical design of a fluid film bearing, the bearing
designer must not only consider the proper sizing of the bearing to handle the
loads and minimize the power loss but also incorporate features such as preload
and offset that optimize the formation of film and tune the dynamic
characteristics of the bearing. In the first half of this series, we examined
the basic operating principle of hydrodynamic fluid film bearings and the
selection of bearing materials. We conclude the series with a closer look at
the options available in the mechanical design of the bearing, including the
method by which lubricant is introduced and removed from the bearing, a factor
that significantly affects bearing performance.
Taper land fixed profile
thrust and tilt pad journal combination bearing.
BEARING DESIGN TOOLBOX
The formation of film is critical to the life and performance
of a fluid film bearing. A viscous lubricant, the relative motion between the
surfaces, and the converging geometry of the bearing are critical factors for
the film’s development. Together, these create the pressure to counteract
gravity and other forces operating on the shaft.
Although axial loads can be supported with flat
bearing faces, typically a geometry is machined into the surface of a fixed
profile thrust bearing to create a converging wedge and increase the bearing
load capacity. A fixed geometry is designed for a specific condition;
therefore, tilt pad thrust bearings are often used to accommodate changing
conditions. Self-leveling tilt pad thrust bearings accommodate misalignment
between the bearing and the collar by equalizing load between pads.
Self-leveling tilt pad thrust
bearing to accommodate misalignment.
A journal bearing, due to the dissimilar diameters of the
bearing and shaft, naturally has a converging wedge when the shaft is not
centered in the bearing. For improved static and dynamic performance, a defined
profile can be machined into the journal bearing to develop the proper film. If
stability is still an issue, if varying conditions must be accommodated, or if other
bearing performance parameters require optimization, journal tilt pads are
often used.
Two of the many defined
profile options for fixed geometry journal bearings: straight bore (left) and
machined-in taper (right).
Thrust and journal bearings are sized to handle application
load requirements and fit the given envelope. Other features can be optimized
to achieve specific dynamic and static performance. Pad arc, the portion of the
pad that develops pressure, can be adjusted to affect power loss and
temperature. The offset, the distance from the leading edge to the pivot
as compared to the pad arc, has a significant effect on the formation of the
film and can impact bearing temperatures, power loss, and, in the case of
journal bearings, dynamic performance.
Journal and thrust pads illustrating pad
arc, leading and trailing edges, and offset pivots. The journal pad (left) is
an integral rib tilt pad; the thrust pad (right) is a line pivot tilt pad.
In tilt pad journal bearings specifically, temperatures,
power loss, and dynamic performance can be greatly affected by the bearing
clearance (the difference between the bearing bore and shaft diameter), by the
length of the bearing, and by preload. Preload represents the deviation of the
pad bore to the bearing bore. It can be positive (pad bore is greater than
bearing bore), zero (pad and bearing bore are the same), or negative (pad bore
is smaller than bearing bore). If the preload is positive, a converging wedge
forms. A negative preload restricts flow into the pad and is not recommended.
An exaggerated view of
positive preload geometry (applicable to journal bearings only).
On both journal and thrust tilt pad bearings, the pivot is
selected based on specific needs, such as the required life of the pivot
(stress), stiffness, and ease of pivot tilt in one or more directions. Common
pivot types are point contact, line contact, ball and socket, and Flexure
Pivot®. Each of these has distinct benefits. For example, the integral
construction of the Flexure Pivot design’s pad and housing achieves tighter
control of clearance and preload, as well as the desired tilt without pivot
wear.
Flexure Pivot tilt pad journal bearing.
Another design option that significantly affects bearing
performance is the method by which lubricant is introduced and removed from the
bearing. By supplying lubricant to the leading edge of the tilt pad and
allowing the lubricant to freely leave the bearing and its housing, ‘Directed
Lubrication’ bearings operate with lower power losses and at lower operating
temperatures than flooded bearings, where the lubricant is typically introduced
into the bearing away from the film and flow out of the bearing is restricted.
Flooded lubrication creates lubricant shearing outside the film that does not
contribute to the load carrying capacity of the bearing.
Examples of trend lines for
power loss and maximum pad temperature with flooded versus ‘Directed’
lubrication.
BRINGING IT ALL TOGETHER
Combining select materials and design features will optimize
performance further than either materials or design features on their own. For
example, combining ‘Directed Lubrication’ with advanced material bearings—which
are capable of handling thinner films, higher temperatures, and therefore,
higher loads—can greatly reduce power loss compared to a flooded design with
traditional bearing materials such as babbitt. Using advanced materials allows
for a smaller bearing and lower viscosity fluid (even process fluid), thus
reducing power loss. If power loss reduction is not required, a higher safety
margin can be achieved.
Switching from a lightly loaded bearing with
flooded lubrication (point A) to an optimized smaller bearing with ‘Directed
Lubrication’ (point B) can significantly reduce power loss.
With the right material(s) and design, fluid film bearings
can offer first-rate dynamic performance, low friction, minimal to no wear, and
long life, making them the right choice to meet the demands of today’s process
equipment.
For More Information:
Barry J. Blair is chief engineer at Waukesha Bearings,
headquartered in Pewaukee, Wisconsin. He has responsibilities for fluid film
bearing research and development, including new products and the refinement of
bearing design tools and methods. For more information, visit www.waukeshabearings.com.
Source: https://modernpumpingtoday.com/
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