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MOTORINDIA
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June 2012
93
ders through the same kind of pol-
ishing action that was described
above. As a result, the stress risers
that can cause early fatigue crack
initiation are greatly reduced, al-
lowing the bearing to operate much
longer than it would otherwise.
If high loads are applied to skid-
ding rollers in low lambda situa-
tions, the frictional heat from roller/
raceway sliding can increase the
surface temperature to the point that
the steel melts. This melting and
subsequent resolidification proc-
ess weakens the steel and creates a
smeared appearance when it occurs
on bearing raceways. In laboratory
tests designed to produce smearing
wear on cylindrical and spherical
roller bearings, Timken has not been
able to produce smearing in the bear-
ings where the new coating has been
applied to the rollers. This result is
attributed to the extremely high du-
rability of the roller coating and its
low friction coefficient against steel.
Cyclic shear stresses from skid-
ding rollers in low lambda condi-
tions can create bearing damage
known as low cycle micropitting.
Rollers with the new coating can
greatly reduce the shear stresses
from skidding rollers that cause low
cycle micropitting to occur in bear-
ings.
As described above, the coated
rollers create very smooth raceways
during bearing operation through a
dynamic micropolishing action. The
effect of this polishing creates race-
ways with roughness values much
lower than those obtained with tra-
ditional finishing processes. As a
result, bearings with ultra-smooth
raceways operate at higher lambda
ratios with the same amount of lu-
brication compared to conventional
bearings.
A common engineering practice to
increase bearing life in low lambda
conditions is to use lubricants with
high viscosities to create thicker
lubricant films. But
high
viscosity
lubricants can
subs t an t i a l l y
decrease the
efficiency of
a mechanical
system. On the
other hand, bear-
ings with this
new coating
on the rollers
can achieve much
higher lambda
ratios with lower
viscosity lubricants, thereby real-
izing efficiency improvements that
have been measured as high as 15
per cent.
These advanced bearings are use-
ful in almost every market space that
Timken currently serves. For exam-
ple, the durability of this coating
and its ability to provide protection
during periods of interrupted lubri-
cation has enabled the development
of a new, high-efficiency turbine en-
gine for commercial jets.
Timken also sees this technol-
ogy being used systemically. For
another example, if gears were to
use this coating, it may be possible
to eliminate EP additives from lu-
bricants. That could enable the use
of low torque polymer-type cages
in gearbox bearings, increase the
life of elastomer seals and provide
a cost saving by using less expen-
sive and “greener” lubricants. This
technology offers a host of reasons
and benefits for using it, and opens
up new areas for exploration and de-
velopment.
Timken debris-resistant bearings
are the result of breakthroughs in the
materials and process improvements
associated with the development of
this new coating. The functional-
ity of the coating in these bearings
may establish a new paradigm for
understanding tribological coatings.
Coatings are typically thought of
as a “defensive” measure, and this
coating certainly functions that way.
However, this new coating also pro-
duces significant benefits by work-
ing offensively improving or repair-
ing the surfaces that it runs against.
This is how the large boost in low
lambda fatigue life, the lower rolling
torque or friction and the “debris tol-
erance” attributes of debris-resistant
bearings are achieved.
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technology