The basic purpose of a machine bearing is to provide a near frictionless
environment to support and guide a rotating shaft. Two general bearing styles
are utilized at this time: the journal bearing and the rolling element bearing.
For lower horsepower and lighter loaded machines, the rolling element bearing is
a popular choice.
Until the 1940's, the journal bearing was the prevalent style used on
machines. As metallurgy and machining techniques progressed, the rolling element
bearing gained greater usage. Today many of the smaller process support machines
have rolling element bearings.
Rolling Element Bearings have four components: an inner race, an
outer race, a rolling element, and a cage to support, space, and guide the
rolling elements. The rolling elements found in today's rolling element bearings
include: balls, rollers, and tapered rollers. All rolling element bearings have
one thing in common: all parts must be in physical metal to metal contact at all
times. Installation instructions specify the amount of bearing pre-load to
maintain the component contact.
Rolling element bearings have some unique concerns not found in journal
bearings. A rolling element bearing will always force a vibration node at its
location. Because of the metal to metal contact, this bearing will provide very
little vibration damping. Although these bearings are a very precisely machined
part they have a limited lifetime. Each component of the bearing will generate
specific frequencies as defects initiate and become more prevalent.
bearings, as the name implies, utilize spherically shaped balls as the rolling
or load carrying element. The number of balls used in a bearing varies depending
on the application. This rolling element bearing type is designed to carry both
radial and axial loads. By modifying the design, this bearing can also
accommodate large axial loads.
This type of
bearing utilizes cylindrically shaped rollers as the load carrying element. This
bearing type is designed to carry large radial loads. This bearing, in its basic
configuration, is not well suited to counter axial loads. The rollers may
actually be slightly barrel shaped in certain designs. Barrel shaped rollers and
their associated outer race allow for some self alignment of the bearing. Needle
bearings are a special adaptation of the cylindrical roller bearing.
design is a special adaptation of the cylindrical roller bearing. This bearing
is designed to counter axial thrust loads along with carrying radial loads. Due
to the geometrical summation of the radial and axial loads, this bearing has a
lower radial load limit than a similarly sized cylindrical or spherical
Certain applications may employ tapered rollers along with tapered races,
hence the name. Special bearings may have inner and outer races with differing
Vibration Monitoring Applications
element bearings, by their design and installation, provide a very good signal
transmission path from the vibration source to the outer bearing housing. Also,
these bearings require monitoring of the unique bearing frequencies generated by
the various parts of the bearing, in addition to the rotor fault
Bearing Frequency Calculation
rolling element bearings are very precisely machined, they do have micro-defects
which are potential sites for future damage. Due to the precise tolerances,
improper installation practices can reduce bearing life. Extensive information
has been compiled about bearing defect frequencies.
The figure lists the
bearing defect frequency formulas for a defect on the balls or rollers, outer
race, inner race, and cage. The assumption for these formulas is that the outer
race is stationary while the inner race rotates.
If the bearing dimensions are known, the individual bearing defect
frequencies can be calculated precisely, or a general rule of thumb can be
applied. Using the generalized form the inner race frequencies would be N x RPM
x 60% and the outer race frequencies would be N x RPM X 40%. If the bearing
manufacturer model numbers are known several computer programs are available to
calculate the necessary frequencies.
This style of bearing
is typically monitored using a case mounted transducer: an accelerometer or
velocity pickup. A displacement sensor observing the shaft relative vibration
would show little, if any, vibration due to the vibration node created by the
Using signal integration techniques, found in many industrial data
collectors, specific frequency ranges relating to certain defects can be
emphasized. Acceleration signals, obtained from case mounted sensors, emphasize
high frequency sources, while displacement signals emphasize lower frequency
sources, with velocity signals falling between the extremes. Recent innovations
for determining bearing condition are Acceleration Enveloping, Spectral Emitted
Energy (SEE), and Spike Energy. These measure high frequency resonances
generated by bearing defects. As a trended variable, in conjunction with other
parameters such as displacement, velocity or acceleration, they can give the
earliest indication of bearing defects.
The figure depicts the
overall amplitude levels obtained from a bearing as it progresses through
continuing phases of failure. This chart depicts overall vibration levels only.
As time progresses the earliest indication of failure are obtained from filtered
high frequency signals because these signals are generated by the resonance of
the bearing and by bearing component defects.
During the early stages of failure the other three parameters may not
generate enough signal to be detected because these parameters emphasize
progressively lower frequency ranges. As failure continues and the damaged
bearing generates the individual bearing defect frequencies, the other
parameters register signals.
Viewing the four monitoring parameters as spectra, additional information can
be obtained about the failure modes.
This figure shows the spectrum frequency content during four stages of
bearing failure. A normal bearing or newly installed bearing will show no
frequencies except those associated with shaft phenomenon such as balance or
Stage I has
some very high frequency content in the Spike Energy region. This zone is in the
ultrasonic region which requires a sensor specifically designed to detect in
this region. Special circuitry filters pass only those signals. Physical
inspection of the bearing at this stage may not show any identifiable defects.
begins to generate signals associated with natural resonance frequencies of the
bearing parts as bearing defects begin to "ring" the bearing components. A
notable increase in Zones 3 and 4 region signals is associated with this stage.
Beginning signs of defects will be found upon inspection.
condition has the fundamental bearing defect frequencies present. These
frequencies are those discussed previously in this paper. Harmonics of these
frequencies may be present depending upon the quantity of defects and their
dispersal around the bearing races. The harmonic frequencies will be modulated,
or side banded, by the shaft speed. Zone 4 signals continue to grow throughout
Stage IV is
the last condition before catastrophic failure of the bearing. This stage is
associated with numerous modulated fundamental frequencies and harmonics
indicating that the defects are distributed around the bearing races. Due to the
increased degradation of the bearing the internal clearances are greater and
allow the shaft to vibrate more freely with associated increases in the shaft
frequencies associated with balance or mis-alignment. During later phases of
stage IV, the bearing fundamental frequencies will decline and be replaced with
random noise floor or "hay stack" at higher frequencies. Zone 4 signal levels
will actually decrease with a significant increase just prior to failure.