Industrial machinery with high horsepower and high loads, such
as steam turbines, centrifugal compressors, pumps and motors, utilize journal
bearings as rotor supports.
One of the basic purposes of a bearing is to provide a
frictionless environment to support and guide a rotating shaft. Properly
installed and maintained, journal bearings have essentially infinite life.
A journal bearing, simply stated, is a cylinder which
surrounds the shaft and is filled with some form of fluid lubricant. In this
bearing a fluid is the medium that supports the shaft preventing metal to metal
contact. The most common fluid used is oil, with special applications using
water or a gas. This application note will concentrate on oil lubricated journal
Hydrodynamic principles, which are active as the shaft rotates,
create an oil wedge that supports the shaft and relocates it within the bearing
clearances. In a horizontally split bearing the oil wedge will lift and support
the shaft, relocating the centerline slightly up and to one side into a normal
attitude position in a lower quadrant of the bearing. The normal attitude angle
will depend upon the shaft rotation direction with a clockwise rotation having
an attitude angle in the lower left quadrant. External influences, such as
hydraulic volute pressures in pumps or generator electrical load can produce
additional relocating forces on the shaft attitude angle and centerline
An additional characteristic of journal bearings is damping.
This type of bearing provides much more damping than a rolling element bearing
because of the lubricant present. More viscous and thicker lubricant films
provide higher damping properties. As the available damping increases, the
bearing stability also increases. A stable bearing design holds the rotor at a
fixed attitude angle during transient periods such as machine startups/shutdowns
or load changes. The damping properties of the lubricant also provides an
excellent medium for limiting vibration transmission. Thus, a vibration
measurement taken at the bearing outer shell will not represent the actual
vibration experienced by the rotor within its bearing clearances.
Journal bearings have many differing designs to compensate for
differing load requirements, machine speeds, cost, or dynamic properties. One
unique disadvantage which consumes much research and experimentation is an
instability which manifests itself as oil whirl and oil whip. Left uncorrected,
this phenomenon is catastrophic and can destroy the bearing and rotor very
quickly. Oil whip is so disastrous because the rotor cannot form a stable oil
wedge consequently allowing metal to metal contact between the rotor and the
bearing surface. Once surface contact exists the rotor begins to precess, in a
reverse direction from rotor rotation direction, using the entire bearing
clearance. This condition leads to high friction levels which will overheat the
bearing babbit metal that leads to rapid destruction of the bearing, rotor
journal, and the machine seals.
Some common designs employed are lemon bore, pressure dam, and
tilt pad bearings. These designs were developed to interrupt and redirect the
oil flow path within the bearing to provide higher bearing stabilities.
installed in industrial machinery today generally fall into two categories: full
bearings and partial arc bearings. Full bearings completely surround the shaft
journal with many differing geometries such as elliptical, lobed, or pressure
dam configurations and usually are two pieces, mated at a split line. Partial
arc bearings have several individual load bearing surfaces or pads and are made
up of numerous adjustable components.
The bearing inner surface is covered with a softer material,
commonly called babbit. Babbit, which is a tin or lead based alloy, has a
thickness that can vary from 1 to 100 mils depending upon the bearing diameter.
A babbit lining provides a surface which will not mar or gouge the shaft if
contact is made and to allow particles in the lubricant to be imbedded in the
liner without damaging the shaft.
The plain bearing
is the simplest and most common design with a high load carrying capacity and
the lowest cost. This bearing is a simple cylinder with a clearance of about 1-2
mils per inch of journal diameter. Due to its cylindrical configuration it is
the most susceptible to oil whirl. It is a fairly common practice during
installation to provide a slight amount of "crush" to force the bearing into a
slightly elliptical configuration.
The lemon or elliptical bore bearing is a variation on the plain
bearing where the bearing clearance is reduced on one direction. During
manufacture this bearing has shims installed at the split line and then bored
cylindrical. When the shims are removed the lemon bore pattern is results. For
horizontally split bearings, this design creates an increased vertical pre-load
onto the shaft.
This bearing has a lower load carrying capacity that plain
bearings, but are still susceptible to oil whirl at high speeds. Manufacturing
and installation costs are considered low.
A pressure dam bearing is basically a plain bearing which has been
modified to incorporate a central relief groove or scallop along the top half of
the bearing shell ending abruptly at a step. As the lubricant is carried around
the bearing it encounters the step that causes an increased pressure at the top
of the journal inducing a stabilizing force onto the journal which forces the
shaft into the bottom half of the bearing.
This bearing has a high load capacity and is a common
correction for machine designs susceptible to oil whirl. Pressure dam bearings
are a unidirectional configuration.
Another unidirectional bearing configuration is the offset
bearing. It is similar to a plain bearing, but the upper half has been shifted
horizontally. Offset bearings have increasing load capacities as the offset is
Tilting pad bearings is a partial arc design. This configuration has
individual bearing pads which are allowed to pivot or tilt to conform with the
dynamic loads from the lubricant and shaft. This type of bearing is a
unidirectional design and is available in several variations incorporating
differing numbers of pads with the generated load applied on a pad or between
supported by journal bearings will move relative to the bearing housing as
various forces are imposed onto the shaft. A vibration transducer is required
which can monitor the relative motion between the shaft and the bearing. Higher
vibration frequencies are not of prime concern since they would not be
transmitted through the oil film reliably.
The only sensor available that can measure relative
measurements of the shaft is the non-contacting pickup, sometimes called a
displacement, eddy current, or proximity pickup. This type of sensor measures
the relative vibration of the shaft and, also, the relative position of the
shaft with respect to the bearing clearances. High frequencies such as blade
passage and cavitation would be attenuated by the lubricant. Case mounted
sensors would not provide an accurate indication of the vibration due to the
inherent damping offered by the lubricant between the shaft and the bearing. For
more information about installation and theory of operation of NCPUs, see the
STI Application Notes: Eddy Current Transducer Installation, Part 1-Radial