POWER October 2021 - 18

O&M
Effect of Eddy Grooves on
Tilting Pad Journal Bearings
Tilting pad journal bearings are used
in turbomachinery such as turbo compressors,
turbo gears, gas turbines, or
steam turbines. There is an ongoing and
growing customer demand for bearings
with increased load-carrying capacity or
power density.
It's important these goals be achieved
without an impairment of bearing functionality
and operational reliability. Traditional
bearings or better standard bearing
designs have performance limits, and
these limits often are reached or even
exceeded in turbomachinery applications
for a variety of reasons.
To overcome conservative operational
limits, it is helpful to reduce the oil film
temperatures. This can be done by certain
design modifications or optimizations.
Heat Dissipation
A basic prerequisite for the safe and reliable
operation of bearings is a sufficient
lubricant film thickness, which basically
depends on the occurring load, speed,
and the selected lubricant. During operation,
heat is generated in both the oil and
parts of the bearing. Because of this, the
temperature of the oil and the bearing
are important parameters.
An increase of heat dissipation (Figure
1), which means heat is transferred
from hotter to colder areas-away from
the loaded pads or areas of the bearing-is
key to improve bearing performance.
It is common practice to
accommodate design features to adapt
the bearing accordingly.
The use of features like directed lubrication,
and pad base materials with a
high thermal conductivity, are a common
practice to increase the heat dissipation.
If this approach is still insufficient,
there are geometric options, such as an
increase of the axial length or the bore
diameter of the bearing.
Such changes, though, could increase
oil demand, cause a loss of power,
and also increase space requirements.
That is why in some cases, geometric
modifications of bearings in operation in
turbomachinery are not feasible.
Eddy Grooves Improve Safety,
Performance
Eddy grooves are a hydrodynamic bearing
feature that improve operational
safety and performance. The effectiveness
of this technology was proved experimentally
and a significant reduction
of maximum bearing temperatures due
to eddy grooves was achieved, especially
at higher bearing loads, under otherwise
identical operating conditions. The
reduced bearing temperatures, induced
by eddy grooves, can be utilized to:
■ Increase the safety margin of the
bearing and slow down the lubricating
oil aging.
■ Operate bearings at higher loads without
an increase of maximum temperatures.
■
Decrease the bearing diameter or
the axial length to reduce the power
loss and the necessary rate of oil supply
without an increase of maximum
bearing temperatures.
New technology is being used along
with traditional design adaptations in rotating
equipment. Miba Industrial Bearings
produces hydrodynamic bearings
and labyrinth seals used in turbines,
compressors, generators, motors, and
industrial pumps, along with fluid film
bearings used in power generation as
well as in the oil and gas industry and
the petrochemical industry.
In Miba's designs, robust grooves are
1. New technology for eddy grooves, including
this design from Miba Industrial Bearings,
increases the heat dissipation in turbomachinery
bearings, helping improve performance.
Courtesy: Miba Industrial Bearings
18
machined into a specific area of the bearing
material (Figure 2). The grooves are
optimized to disturb the laminar lubricant
flow, which is converted into a turbulent
flow. This creates a cooling effect by improving
temperature exchange between
cooler oil near the shaft and warmer oil
near the bearing surface.
The foundation of this technology has
www.powermag.com
2. These radial tilting pads are equipped with
Miba-designed eddy grooves, which are machined
into an area of the bearing material.
Courtesy: Miba Industrial Bearings
been known for more than 30 years,
and the possibilities of improvement are
clear. It is possible to push performance
limits upward without changing the bearing,
or the bearing housing design, thus
maintaining or even improving the reliability
and operational safety of the bearing
and the turbomachine.
Performance Improvement
To investigate on the exact design and
position of the grooves, Miba carried out
a study on a journal bearing test rig using
a five-tilting-pad journal bearing with
120-millimeter (mm) shaft diameter at a
speed between 50 meters per second
(m/s) and 110 m/s and a load of 0 Megapascal
(MPa) to 4 MPa.
Temperature sensors close to the sliding
surface enabled Miba to precisely
determine the maximum temperatures.
The results of the study confirmed a
significant surface temperature reduction.
Figure 3 is a Miba eddy groove application
example operating at a speed
of 100 m/s, inlet oil temperature of 55C,
and medium oil flowrate. With the Miba
eddy groove modification of the bearing,
opportunities arise to either use the benefit
solely for a temperature reduction by
12K, thereby increasing the operational
safety of the bearing, indicated by point
2A, or use the full benefit to increase
the bearing load at a constant maximum
temperature, shown as point 2B. A new
point of operation between 2A and 2B
would result, compared to point 1, in a
reduced bearing temperature in combination
with an increased bearing load.
POWER | October 2021
http://www.powermag.com
POWER October 2021

Table of Contents for the Digital Edition of POWER October 2021
