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

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