How long will
a bearing last?
Standardized life
equations help
to answer.
Daniel R. Snyder, P.E.
Director, Applications
Engineering
SKF Industrial Div.
SKF USA Inc.
Kulpsville, Pa.
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Relative effects of contamination and lubrication condition on bearing life with different load levels.
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Experience shows seemingly
identical rolling bearings operated under identical conditions
may not last the same amount of
time. In most cases, it is impractical to test a statistically significant number of bearings, so engineers rely on standardized bearing-life calculations to select and
size bearings for a particular application. These calculations continue to evolve and become more
accurate over time, reflecting the
collective experience of the bearing industry, including recent advances in manufacturing, tribology, materials, end-user condition
monitoring, and computation.
In February of this year, the
International Organization for
Standardization (ISO) published
a revised ISO 281:2007 Standard
for the calculation of bearing
ratings and life. It builds on the
previous Standard ISO 281:2000
to account for such factors as internal stresses from mounting,
residual stresses from hardening and other manufacturing
processes, and material cleanness. Also included are the effects of solid contaminants with various lubricating systems, as
well as bearing material fatigue
stress limits. Before going into
further detail, it's probably a
good time to review the basics
of bearing-life calculations,
starting with the common definitions of life.
Basic life or L10 as defined in ISO
and ABMA standards is the life
that 90% of a sufficiently large
group of apparently identical
bearings can be expected to reach
or exceed. The median or average
life, sometimes called Mean Time
Between Failure (MTBF), is about
five times the calculated basic rating life. Service life is the life of a
bearing under actual operating
conditions before it fails or needs
to be replaced for whatever reason. The so-called specification life
is generally a requisite L10 basic
rating life and reflects a manufacturer's requirement based on experience with similar applications.
CALCULATING LOADS
Engineers typically employ
rolling-contact fatigue models
that compare bearing load ratings
to applied dynamic and static
loads as they impact service life
and reliability.
The basic dynamic load rating
covers dynamically stressed
bearings that rotate under load.
This rating, defined in ISO 281, is
the bearing load that results in a
basic rating life or L10 of 1 million
revolutions. Dynamic loads
should include a representative
duty cycle or spectrum of load
conditions and any peak loads.
The basic static load rating applies to bearings that rotate at
speeds less than 10 rpm, slowly
oscillate, or remain stationary under load over certain periods. Be
sure to include loads of extremely
short duration (shock) because they may plastically deform contact surfaces and compromise
bearing integrity.
Classical mechanics along with
known or calculable external
forces are used to calculate the
loads acting on a bearing. These
external forces may include resultants from power transmission, shaft or housing supports,
or inertia. When calculating loads
on a single bearing, assume the
shaft to be a beam resting on
rigid, moment-free supports.
Basic catalog or simplified calculations typically ignore elastic
deformations in the bearing, housing, or machine frame, as well as
moments produced in the bearing
by shaft deflection. Such calculations may assume loads are constant in magnitude and direction
and act radially on a radial bearing, or axially and centrically on a
thrust bearing. Oftentimes, bearings in actual service see simultaneous radial and axial loads. When
the resultant of radial and axial
loads is constant in magnitude
and direction, calculate an equivalent dynamic bearing load from:
P = XFr + YFa
where P = equivalent dynamic
bearing load, lb; Fr = actual radial
bearing load, lb; Fa = actual axial
bearing load, lb; X = radial load
factor for the bearing; and Y = axial load factor for the bearing.
For single-row radial bearings,
axial load influences P only when
the ratio Fa ⁄ Fr exceeds a certain
limiting value. Conversely, even
light axial loads are significant for
double-row radial bearings. The
above equation also applies to
spherical thrust bearings and
other thrust types that handle
both axial and radial loads. Be
sure to consult manufacturer catalogs for axial-radial thrust bearings because designs can vary
widely. For thrust ball bearings
and other types that carry pure axial loads, the equation simplifies to P = Fa, provided the load
acts centrically.
RATING LIFE EQUATIONS
The equation from ISO 281 or
the American Bearing Manufacturers Association (ABMA) Standards 9 and 11 figures basic, nonadjusted rating life by:
L10 = (C ⁄ P)p in millions of
revolutions
where C = basic dynamic load rating, lb; P = equivalent dynamic
bearing load, lb; p = life-equation
exponent ( p = 3 for ball bearings;
and p = 10/3 for roller bearings)
For bearings run at constant
speed, it may be more convenient
to express the basic rating life in
operating hours:
L10h = (1,000,000/60)nL10
where n = rotational speed, rpm
Predicted bearing life is a statistical quantity in that it refers to
a bearing population and a given
degree of reliability. The basic rating life is associated with 90% reliability of bearings built by modern manufacturing methods from high-quality materials and operated under normal conditions. In
practice, predicted life may deviate significantly from actual service life, in some documented
cases by nearly a factor of five.
Service life represents bearing
life in real-world conditions,
where field failures can result
from root causes other than bearing fatigue. Examples of root
causes include contamination,
wear, misalignment, corrosion,
mounting damage, poor lubrication, or faulty sealing systems.
Ongoing advances in bearing
technology and manufacturing
processes continue to extend
bearing life and reduce sensitivity
to severe operating conditions.
Standard ISO 281 has developed in
step with these advances to predict service life more accurately.
The latest version expands coverage to include bearing material fatigue stress limits, and a factor for
solid contamination effects on
bearing life when using various lubrication systems such as grease,
circulating oil, and oil bath.
The equation calculates modified rating life at n% reliability Lnm in millions of revolutions at constant speed by:
Lnm= a1aISOL10
where a1 = life-adjustment factor for reliability (1.0 for 90% reliability); and aISO = manufacturer life
modification factor according to
ISO 281.
Finding aISO involves the use
of a contamination factor that considers the lubrication system type, cleanliness class,
bearing size, and lubrication operating conditions as defined in
ISO 4406. This contamination
factor, along with the ratio of the
bearing fatigue load limit to the
bearing equivalent load limit,
and the lubrication condition,
determine aISO. In general, better
lubricant conditions and lower
equivalent loads lessen bearing
life sensitivity to contamination
levels. Conversely, high loads
and poor lubricant conditions
raise bearing life sensitivity to
contamination.
SOME ADVANTAGES AND CAUTIONS
Bearing life calculated according to the updated ISO 281:2007
Standard or similar methods
from individual bearing manufacturers is a big step forward in the
ability to more accurately predict
service life based on known operating conditions. With it come
distinct advantages as well as
possible pitfalls.
On the plus side, the additional
information may make it possible
to downsize bearings run under
good operating conditions,
thereby lowering friction, energy
usage, and weight. It can help with
the selection of lubrication and filtration that maximize bearing and
system life. This, in turn, may let
companies extend warranties or
service intervals for bearings run
under controlled operating conditions. The updated Standard also
helps engineers better evaluate
the influence of operating parameters on specific bearing types
and designs.
However, the calculations are
sensitive to changes in load, temperature, lubrication condition,
and contamination level. The
wrong choice of bearing combined with improper assumptions of operating conditions
could lead to premature bearing failure. The methodology and calculations assume modern bearing designs and manufacturing
processes and quality bearing
materials. With today's global
sourcing, that may not be the case. The approach further assumes bearings are properly installed and maintained. Again,
this may not be so. In any case,
it's probably a good idea to consult bearing manufacturers when incorporating bearings into new
designs.
MAKE CONTACT
SKF Industrial Div.,
SKF USA Inc.,
skfusa.com