-
EXTENDING BEARING LIFE
Bearings
fail
for
a
number
of
reasons
，
but
the
most
common
are
misapplication
，
contamination
，
i
mproper lubricant
，
shipping
or handling damage
，
and
misalignment. The problem
is often not
difficult to diagnose because a failed bearing
usually leaves telltale signs about what
went wrong
．
However
，
while
a
postmortem
yields
good
information
，
it
is
better
to
avoid
the
process
altogether by specifying the bearing
correctly in The first
place
．
To do
this
，
it is useful to review
the
manufacturers sizing guidelines and
operating characteristics for the selected
bearing.
Equally critical is a study of
requirements for noise, torque, and runout, as
well as possible
exposure to
contaminants, hostile liquids, and temperature
extremes. This can provide further clues
as to whether a bearing is right for a
job.
1 Why bearings fail
About 40% of ball bearing failures are
caused by contamination from dust, dirt, shavings,
and
corrosion.
Contamination
also
causes
torque
and
noise
problems,
and
is
often
the
result
of
improper
handling
or
the
application
environment
．
Fortunately,
a
bearing
failure
caused
by
environment or handling contamination
is preventable
，
and a simple
visual examination can easily
identify
the cause
．
Conducting a postmortem il1ustrates
what to look for on a failed or failing bearing
．
Then
，
und
erstanding the mechanism behind the failure, such
as brinelling or fatigue, helps eliminate the
source of the problem.
Brinelling is one type of bearing
failure easily avoided by proper handing and
assembly. It is
characterized by
indentations in the bearing raceway caused by
shock loading
－
such as when a
bearing is dropped-or incorrect
assembly. Brinelling usually occurs when loads
exceed the material
yield point(350,000
psi in SAE 52100 chrome
steel)
．
It may also be caused
by improper assembly,
Which places a
load across the
races
．
Raceway dents also
produce noise
，
vibration
，
and increased
torque.
A similar defect is a pattern of
elliptical dents caused by balls vibrating between
raceways
while the bearing is not
turning
．
This problem is
called false brinelling. It occurs on equipment in
transit or that vibrates when not in
operation. In addition, debris created by false
brinelling acts like
an abrasive,
further contaminating the bearing. Unlike
brinelling, false binelling is often indicated
by a reddish color from fretting
corrosion in the lubricant.
False
brinelling is prevented by eliminating vibration
sources and keeping the bearing well
lubricated.
Isolation pads
on the equipment or a separate foundation may be
required to reduce
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environmental vibration. Also a light
preload on the bearing helps keep the balls and
raceway in
tight contact. Preloading
also helps prevent false brinelling during
transit.
Seizures
can
be
caused
by
a
lack
of
internal
clearance,
improper
lubrication,
or
excessive
loading. Before seizing, excessive,
friction and heat softens the bearing steel.
Overheated bearings
often
change
color
，
usually
to
blue-black
or
straw
colored
．
Friction
also
causes
stress
in
the
retainer
，
which
can break and hasten bearing
failure
．
Premature
material
fatigue
is
caused
by
a
high
load
or
excessive
preload
．
When
these
conditions
are
unavoidable
，<
/p>
bearing
life
should
be
carefully
calculated
so
that
a
maintenance
scheme can be
worked out
．
Another solution for fighting premature
fatigue is changing
material
．
When standard
bearing
materials
，
such as
440C or SAE 52100
，
do not
guarantee sufficient
life
，
specialty materials can
be
recommended. In
addition
，
when the problem is
traced back to excessive
loading
，
a higher capacity
bearing or different configuration may
be used
．
Creep is
less common than premature
fatigue
．
In
bearings
．
it is caused by
excessive clearance
between bore and
shaft that allows the bore to rotate on the
shaft
．
Creep can be expensive
because
it causes damage to other
components in addition to the
bearing
．
0ther
more likely creep indicators are
scratches
，
scuff
marks
，
or discoloration to
shaft and
bore
．
To
prevent creep damage
，
the
bearing housing and shaft fittings should be
visually checked
．
Misalignment
is
related
to
creep
in
that
it
is
mounting
related
．
If
races
are
misaligned
or
cocked
．
The
balls
track
in
a
noncircumferencial
path
．
The
problem
is
incorrect
mounting
or
tolerancing
，
or
insufficient squareness of the bearing mounting
site
．
Misalignment of more
than
1/4·
can cause an early
failure
．
Contaminated
lubricant
is
often
more
difficult
to
detect
than
misalignment
or
c
reep
．
Contamination shows as
premature wear
．
Solid
contaminants become an abrasive in the
lubricant
．
In
addition
。
insufficient
lubrication between ball and retainer wears and
weakens the
retainer
．
In this
situation
，
lubrication is
critical if the retainer is a fully machined
type
．
Ribbon or
crown
retainers
，
in
contrast
，
allow lubricants to
more easily reach all
surfaces
．
Rust
is
a
form
of
moisture
contamination
and
often
indicates
the
wrong
material
for
the
application
．
If
the material checks out for the
job
，
the easiest way to
prevent rust is to keep bearings
in
their packaging
，
until just
before installation
．
2 Avoiding failures
The best
way to handle bearing failures is to avoid
them
．
This can be done in the
selection
process
by
recognizing
critical
performance
characteristics<
/p>
．
These
include
noise
，
starting
and
- 2 -
running
torque
，
stiffness
，
nonrepetitive
runout
，
and radial and axial
play
．
In some applications,
these items are so critical that
specifying an ABEC level alone is not
sufficient
．
Torque requirements are determined by
the lubricant
，
retainer
，
raceway quality(roundness
cross curvature and surface
finish)
，
and whether seals or
shields are used
．
Lubricant
viscosity must
be
selected
carefully
because
inappropriate
lubricant
，
especially
in
miniature
bearings
，
causes
excessive
torque
．
Also
，
different
lubricants
have
varying
noise
characteristics
that
should
be
matched
to the application. For
example
，
greases produce more
noise than oil
．
Nonrepetitive runout(NRR)occurs during
rotation as a random eccentricity between the
inner
and outer
races
，
much like a cam
action
．
NRR can be caused by
retainer tolerance or eccentricities
of
the raceways and
balls
．
Unlike repetitive
runout, no compensation can be made for NRR.
NRR is reflected in the cost of the
bearing
．
It is common in the
industry to provide different
bearing
types and grades for specific
applications
．
For
example
，
a bearing with an
NRR of less
than
0.3um
is
used
when
minimal
runout
is
needed
，
such
as
in
disk
—
drive
spindle
motors
．
Similarly
，
machine
—
tool spindles tolerate only
minimal deflections to maintain precision
< br>cuts
．
Consequently,
bearings are manufactured with low NRR just for
machine-tool applications
．
Contamination
is
unavoidable
in
many
industrial
products
，
and
shields
and
seals
are
commonly used to protect
bearings from dust and dirt
．
However
，
a perfect bearing
seal is not
possible
because
of
the
movement
between
inner
and
outer
races
．
Consequently
，
lubricati
on
migration and contamination are
always problems
．
Once a bearing is contaminated, its
lubricant deteriorates and operation becomes
noisier
．
If it
overheats
，
the
bearing can seize
．
At the
very least
，
contamination
causes wear as it works between
balls
and the raceway
，
becoming
imbedded in the races and acting as an abrasive
between metal
surfaces
．
Fending
off
dirt
with
seals
and
shields
illustrates
some
methods
for
controlling
contamination
．
Noise
is
as
an
indicator
of
bearing
quality
．
Various
noise
grades
have
been
developed
to
classify bearing performance
capabilities
．
Noise analysis is done with an
Anderonmeter, which is used for quality control in
bearing
production and also when failed
bearings are returned for analysis. A transducer
is attached to the
outer ring and the
inner race is turned at 1,800rpm on an air
spindle. Noise is measured in andirons,
which represent ball displacement in
μm/rad.
With experience,
inspectors can identify the smallest flaw from
their sound. Dust, for example,
makes
an irregular crackling. Ball scratches make a
consistent popping and are the most difficult to
identify. Inner-race damage is normally
a constant high-pitched noise, while a damaged
outer race
makes an intermittent sound
as it rotates.
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