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New Coatings Improve Bearing Life

New Coatings Improve Bearing Life

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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