New Coatings Improve Bearing Life

Anumber of years ago, Timken and other bearingcompanies started using coatings on rollers for niche bearing applications. Themost widely used coating for bearing rollers was a tungsten carbide containingamorphous hydrocarbon coating commonly called tungsten diamond-like carbon. Inlate 2008, Timken started performing extensive application testing of bearingswith this coating, which was commercially available from a number of differentsources.

Whatwe found was that this coating was not durable enough to provide performanceimprovements for many bearing applications. We wanted to understand why thiswas the case, so we performed an in-depth analysis of the coating andidentified a defect that we thought might be responsible for the limiteddurability of the coating. A second study focused specifically on eliminatingthat defect during the coating deposition process and a new coating withoutthis defect was created.

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

Thefunctionality of the coating in wear-resistant bearings mayestablish a new paradigm for our understanding of tribological coatings. Thatis, coatings are typically thought of as a "defensive" measure. However, wehave also observed that this new coating works offensively by improving or repairingthe surfaces that it runs against. This results in a large boost in low lambdafatigue life, lower rolling torque or friction, and debris tolerance attributes(these issues are discussed in detail under "Tackling the Issue of BearingWear" below).

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

Unanticipatedwear modes appear to be responsible for the limited lives of many main shaftand gearbox bearings. These wear modes are low-cycle micropitting, smearing andinclusion-generated brittle flaking. When we looked at the root causes of windturbine bearing failures, all of them were related in some way to high-shearforces created by the roller/raceway sliding. What we were able to determine isthat by reducing these shear forces, these wear modes could be inhibited oreliminated.

Inresponse, Timken launched a product line called Wear Resistant Bearingsfeaturing this new coating that specifically addresses the life-limiting issuesfaced by wind turbines. However, these new bearings also have broad usagepotential in other markets.

Potential Applications

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

Thiscoating technology also holds potential for use on industrial systems. Forexample, if this coating were applied to gears, it should be possible toeliminate extreme pressure (EP) additives from the lubricants. Doing that couldenable the use of low torque polymer-type cages in gearbox bearings, increasethe life of elastomer seals, and provide a cost savings by using less expensiveand greener lubricants.

Tackling the Issue of Bearing Wear

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

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

Debris particles thatpass through worn seals not removed after manufacture, or generated by wear ofother components like gears, can damage bearing surfaces if the particles arelarger than the thickness of the lubricant film. Depending on the hardness andbrittleness of the particle, they can generate dents on the raceway and/orroller surface. During the denting process, displaced material createsshoulders around debris craters. When these shoulders come into the contactzone of a bearing, very high subsurface
stresses are generated and fatiguecracks can initiate at relatively low stress cycles. Because the roller coatingis twice as hard as the steel raceways, it removes these shoulders through thesame kind of polishing action described above. As a result, the stress risersthat can cause early fatigue crack initiation are removed, allowing the bearingto operate much longer than it would otherwise.

When the lubricantfilm is insufficient to keep loaded steel surfaces in relative motion fromcoming into contact, adhesive and abrasive wear occurs. If high loads areapplied to skidding rollers, the frictional heating from the interaction ofcontacting asperities can increase the temperature in the contact zone to thepoint the steel melts. This melting and subsequent resolidification processweakens the steel and creates a smeared appearance when it occurs on bearingraceways. At Timken, we have not been able to produce smearing in wear-resistantbearings, and we attribute that to the high durability of the roller coatingand its low friction coefficient against steel.

Shear stresses frommoderate loads applied to skidding rollers can create bearing damage known aslow-cycle micropitting. On the other hand, very high transient loads applied toskidding rollers can also generate high shear stresses on non-metallicinclusions, creating cracks that propagate and remove thin pieces of theraceway. This type of damage is known as brittle flaking. The coating on thewear-resistant bearing rollers provides a barrier against the ability ofraceway asperities to bond to the roller, and reduces the shear stresses fromskidding rollers that contribute to these bearing damage modes.

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