New Coatings Improve Bearing Life

By: 
September 01, 2010

A
number of years ago, Timken and other bearing
companies started using coatings on rollers for niche bearing applications. The
most widely used coating for bearing rollers was a tungsten carbide containing
amorphous hydrocarbon coating commonly called tungsten diamond-like carbon. In
late 2008, Timken started performing extensive application testing of bearings
with this coating, which was commercially available from a number of different
sources.

What
we found was that this coating was not durable enough to provide performance
improvements for many bearing applications. We wanted to understand why this
was the case, so we performed an in-depth analysis of the coating and
identified a defect that we thought might be responsible for the limited
durability of the coating. A second study focused specifically on eliminating
that defect during the coating deposition process and a new coating without
this defect was created.

When
we tested bearings with this new coating and process on the rollers, the
bearings performed far better than any bearings Timken had ever tested. As an
example, we are seeing a 3.5 to four times improvement in the fatigue life of
Timken's premium roller bearings.

The
functionality of the coating in wear-resistant bearings may
establish a new paradigm for our understanding of tribological coatings. That
is, coatings are typically thought of as a "defensive" measure. However, we
have also observed that this new coating works offensively by improving or repairing
the surfaces that it runs against. This results in a large boost in low lambda
fatigue life, lower rolling torque or friction, and debris tolerance attributes
(these issues are discussed in detail under "Tackling the Issue of Bearing
Wear" below).

While
this research was underway, we became aware of widespread bearing problems in
wind turbines, specifically with main shaft spherical roller bearings and
spherical and cylindrical roller bearings in the gearboxes. Although these
bearings were supposed to last 30 years, wind farm operators were telling us
that if you get five years out of them, you are doing well.

New Coatings Improve Bearing Life

Unanticipated
wear modes appear to be responsible for the limited lives of many main shaft
and gearbox bearings. These wear modes are low-cycle micropitting, smearing and
inclusion-generated brittle flaking. When we looked at the root causes of wind
turbine bearing failures, all of them were related in some way to high-shear
forces created by the roller/raceway sliding. What we were able to determine is
that by reducing these shear forces, these wear modes could be inhibited or
eliminated.

In
response, Timken launched a product line called Wear Resistant Bearings
featuring this new coating that specifically addresses the life-limiting issues
faced by wind turbines. However, these new bearings also have broad usage
potential in other markets.

Potential Applications

The
durability of the coating and its ability to provide protection during periods
of interrupted lubrication has enabled the development of a new,
high-efficiency turbine engine for commercial jets.

This
coating technology also holds potential for use on industrial systems. For
example, if this coating were applied to gears, it should be possible to
eliminate extreme pressure (EP) additives from the lubricants. Doing that could
enable the use of low torque polymer-type cages in gearbox bearings, increase
the life of elastomer seals, and provide a cost savings by using less expensive
and greener lubricants.

Tackling the Issue of Bearing Wear

Since roller bearings
seldom operate in fully lubricated environments, they do not often experience
the number of cycles for which they were designed. In low lambda situations
(the ratio between the lubricant film thickness and the composite surface
roughness), asperities on the rollers and raceways come into contact and
bearing life is therefore reduced. The coating on the rollers of wear-resistant
bearings polishes the ring raceways and reduces or eliminates the asperity
interaction. This polishing usually continues until the contacts are fully
separated by the lubricant film and the bearing is no longer operating in a low
lambda situation.

Interruption of the
supply of lubricant to bearings can result in adhesive wear between the rollers
and contacting surfaces on rings. Depending upon the loads and speeds, the
adhesive wear rates increase until scuffing, scoring, or galling occurs. The
roller coating will not participate in adhesive wear with steel, but if the
loads and speeds in the contacting areas are large enough and the lubricant
interruption is long enough, the coating on the rollers can wear. Once the
coating is worn away adhesive wear can ensue. However, while the coating is
wearing, it allows the bearing to remain operational.

New Coatings Improve Bearing Life

Debris particles that
pass through worn seals not removed after manufacture, or generated by wear of
other components like gears, can damage bearing surfaces if the particles are
larger than the thickness of the lubricant film. Depending on the hardness and
brittleness of the particle, they can generate dents on the raceway and/or
roller surface. During the denting process, displaced material creates
shoulders around debris craters. When these shoulders come into the contact
zone of a bearing, very high subsurface
stresses are generated and fatigue
cracks can initiate at relatively low stress cycles. Because the roller coating
is twice as hard as the steel raceways, it removes these shoulders through the
same kind of polishing action described above. As a result, the stress risers
that can cause early fatigue crack initiation are removed, allowing the bearing
to operate much longer than it would otherwise.

When the lubricant
film is insufficient to keep loaded steel surfaces in relative motion from
coming into contact, adhesive and abrasive wear occurs. If high loads are
applied to skidding rollers, the frictional heating from the interaction of
contacting asperities can increase the temperature in the contact zone to the
point the steel melts. This melting and subsequent resolidification process
weakens the steel and creates a smeared appearance when it occurs on bearing
raceways. At Timken, we have not been able to produce smearing in wear-resistant
bearings, and we attribute that to the high durability of the roller coating
and its low friction coefficient against steel.

Shear stresses from
moderate loads applied to skidding rollers can create bearing damage known as
low-cycle micropitting. On the other hand, very high transient loads applied to
skidding rollers can also generate high shear stresses on non-metallic
inclusions, creating cracks that propagate and remove thin pieces of the
raceway. This type of damage is known as brittle flaking. The coating on the
wear-resistant bearing rollers provides a barrier against the ability of
raceway asperities to bond to the roller, and reduces the shear stresses from
skidding rollers that contribute to these bearing damage modes.

Wear-resistant
bearings have very smooth raceways because the coatings on the rollers
dynamically polish them. This polishing goes beyond traditional finishing
processes and allows these bearings to operate at higher lambda ratios with the
same amount of lubrication. Wear-resistant bearings can achieve small lambda
denominators. In some applications, more lubrication can be used to increase
the numerator but it can create other losses associated with the viscosity of
the fluid. Wear-resistant bearings can operate with low viscosity fluids and
achieve rolling friction reductions that we conservatively estimate at 5-15
percent, which is quite large for some applications.

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