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MOTORINDIA
l
June 2012
capabilities of Timken’s debris
resistance offering by addressing
life-limiting wear such as scoring,
smearing, micropitting, and fret-
ting. Conditions that promote these
life-limiting wear modes are com-
monplace in underground mining
applications, including continuous
miners, long wall shears, and shut-
tle cars as well as in above ground
haulage and excavation equipment
applications such as mine truck
wheels, dragline fairlead sheaves,
boom point sheaves and hoist and
drag drum gearboxes.
In addition to bearings, this new
coating can also be applied to shafts,
gears, seal riding surfaces and more.
Just as adverse conditions and en-
vironments impact bearings, they
also impact other key components
operating in mining applications.
These components can also benefit
from this new coating technology to
improve performance in the overall
system.
Technology impact
Since roller bearings seldom op-
erate in well-lubricated application
environments, they do not often
experience the number of cycles
for which they were designed. In
low lambda situations, asperities on
the rollers and raceways come into
contact and bearing life is reduced.
Lambda is the ratio between the lu-
bricant film thickness and the com-
posite surface roughness of the roll-
ers and raceways.
Timken observed that during op-
eration, the new coating on the roll-
ers of debris resistant bearings cre-
ates very smooth ring raceways that
significantly reduce asperity interac-
tions. This polishing has been found
to continue until the roller and race-
way surfaces are fully separated by
the lubricant film and the bearing is
no longer operating in a low lambda
situation.
Interruption of the supply of lu-
bricant to bearings can result in
severe adhesive wear between the
rollers and contacting surfaces on
the rings. Depending upon the loads
and speeds, the adhesive wear rates
increase until scuffing, scoring or
galling occurs. The roller coat-
ing will not participate in adhesive
wear with steel, but if the loads and
speeds in the contacting areas are
large enough and the lubricant inter-
ruption is long enough, the coating
on the rollers can wear. Once the
coating is worn away, adhesive wear
can ensue, but while the coating is
wearing, it allows the bearing to re-
main operational.
Debris particles, which pass
through worn seals, that were not
removed after manufacture, or gen-
erated by wear of other components
like gears can damage bearing sur-
faces if the particles are larger than
the thickness of the lubricant film.
Depending upon the hardness and
brittleness of the particle, the parti-
cles can generate dents on the race-
way and/or roller surface. During
the denting process, displaced mate-
rial creates shoulders around debris
craters. These shoulders can create
very high subsurface stresses and fa-
tigue cracks can initiate at relatively
low stress cycles.
Because the new roller coating
is almost twice as hard as the steel
raceways, it removes these shoul-
technology
Based on The Timken Com-
pany’s experience and un-
derstanding of mining ap-
plications
and
operating
environments, the validation
of these enhanced debris re-
sistant bearings is just another
step toward providing custom-
ers increased performance and
improved machine uptime even
in the world’s most extreme
operating conditions.