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For Want of a Bearing


IN 1998, ENGINEERS WITH PRATT & WHITNEY were running a test at its laboratory in West Palm Beach, Fla., on a prototype engine intended for the next generation of fighter planes. The U.S. military wanted these planes to fly faster and carry more weight, expectations that would require hotter temperatures and create more severe loads on engines.

To meet those conditions, Pratt & Whitney built the engine with turbine bearings made from a new type of stainless steel alloy that had been developed specifically for aerospace applications. So far, they seemed ready to deliver the necessary gains.

Wed gotten these bearings from one of our vendors, and they performed well in rig tests that we had done, recalled Ron Drost, Pratt & Whitneys mechanical systems technology manager. They showed a lot of promise.

But the promise wasnt fulfilled. The prototype engine failed its test when the bearings developed premature wear. Not only did they fall short of goals for the new engine; they actually performed worse than bearings made from conventional steel.

The results were a big surprise and would turn out to have just as much to do with the engine lubricant as it would the bearings. To solve the problem, the military and its contractors would undertake an innovative initiative to develop steel and lube in tandem. A solution is still a few years off, but those involved say it will provide key advances in aviation technology.

The failure of the Florida test was a set-back not just for Pratt & Whitney, but also for an ambitious undertaking by the United States and several allies to develop the next generation of fighter planes. The Joint Strike Fighter program is supposed to provide planes that are faster and more powerful, while at the same time ensuring that these craft will be more affordable to develop, produce and own.

Engine bearings play an important role in that effort. To fly at higher speeds and to carry more weight, the planes engines would have to generate more thrust, and this meant subjecting bearings to greater stress. But bearing performance was starting from a level that was unsatisfactory, even for existing aircraft. A recent study found bearing failure the most common cause of the most serious categories of aircraft accidents, including those resulting in loss of life or aircraft. Surface durability is a major contributor to failures. In addition, corrosion causes high rates of bearing replacement.

Bearing performance has become a bigger problem in recent years, Drost said. In past years, roller bearings would fail after so many cycles, and they would do so in a way that we describe as slow and graceful. That allowed an opportunity to detect the situation and replace the bearings.

Now loads and speeds have increased. These bearings are being designed right to their performance limit, and the deterioration occurs much more rapidly. Sometimes it takes just a couple hours to get to failure, and at that point youre talking about things like an engine going out.

It was clear that the next generation of aircraft needed new bearings, and it seemed that the alloy tested by Pratt & Whitney couldnt do the job. When the engine maker and the military – specifically the Propulsion Directorate of the U.S. Air Force Research Laboratory – investigated, they found it was not just a problem with the alloy. The bearings were receiving less protection from the turbine oil because of incompatibility between the alloy and additives in the lubricant.

The military and its contractors say the problem involves chemical reactions that take place (or dont) when metal surfaces such as bearings and inner or outer races come into contact. With conventional materials, that contact generates low levels of energy that cause chemicals such as phosphorus or sulfur – contained in lubricant additives – to react with iron in the bearings. The result is a slippery film that provides boundary lubrication, as distinguished from, for example, fluid film lubrication that occurs when oil reduces friction by separating surfaces.

With a conventional steel bearing, the iron sites are available in high numbers, so there is ample opportunity for boundary lubrication, said Richard S. Sapienza, chief executive officer of Shoreham, New York-based Metss Corp., which was contracted by the research laboratory. But with new steels, you have higher levels of chrome and the bearing surface is treated with carbon. Apparently the carbon takes up those iron sites, blocking the interactions between the iron and lubricant additives.

Initially, the Propulsion Directorate and Pratt & Whitney tried a different tack, using a hybrid bearing that combined steel races with ceramic balls. That approach did indeed show some advantages, but it also had limitations, so the military and engine maker agreed to try again to find a way to use stainless steel bearings.

The incompatibility between additives and alloy underscored the need for a new way of tackling the problem, a method known as a systems approach. Calls for systems approaches are heard these days in venues such as tribology conferences. In general, the idea is to consider the lubricant an integral component to engines or other types of equipment, and take it into account when designing new models, rather than designing the equipment and expecting oil companies to come up with a suitable product afterward.

In theory it sounds logical, but the approach is seldom put into practice. Another contractor said a systems approach has never been attempted on a project of this scale. If you look at technology development programs, this is the first time in history that a major program has sought to develop the attributes of a new steel and a new oil in conjunction with each other, said Vern D. Wedeven, president of Wedeven Associates, a tribology testing firm in Edgmont, Pa.

To facilitate development of new additive technology, the Materials Directorate of the research lab, which is located at Wright-Patterson Air Force Base near Dayton, Ohio, obtained funding for a Small Business Innovative Research grant, a program that encourages private sector development of enabling technology. In 2003, it hired Wedeven, Metss and a third firm to find additive formulas that would demonstrate an ability to provide boundary lubrication for advanced bearing steels. The following year it retained Wedeven and Metss for a second phase, to develop oils that met all performance criteria. The lab and its partners organized an extensive sequence of tests and then began soliciting samples from industry. Metts performed chemical tests, Wedeven those that dealt with tribology. Oil and additive companies received feedback about performance of their samples and then worked to improve them.

Meanwhile, the search continued for a suitable bearing material. Pratt & Whitney soon honed in on Pyrowear 675, a new corrosion-resistant, carburized stainless steel developed by Carpenter Technology, of Wyomissing, Pa. Pyrowear 675 immediately showed advantages over the earlier alloy, but it still had shortcomings. So Pratt & Whitney and the Propulsion Directorate worked back and forth with Carpenter, getting the steel producer to adjust its process by changing factors such as the temperature at which it carburized, or the amount of time for which it did so.

Because work on the steel was still in progress after the oil component began, Metss and Wedeven had to use a stand-in alloy for tests that included metal components. They chose 440-C, a stainless steel with especially high chrome content. 440-C has characteristics similar to those that the parties were aiming for with Pyrowear 675 – but there was some question whether the progress made with the oils would carry over once the new alloy was ready. There was always a chance that some incompatability would appear, rendering useless the work that had been done to that point.

We thought the 440-C had good similarities, but there was the possibility that it would turn out to be different in some key way and that the work we had done would have to be scrapped, Sapienza said. I was really worried for a while. The contractors did not receive samples of Pyrowear 675 until six months into the second phase of their research grant.

The Materials Directorate says its contractors are about half completed with the oil tests and that several promising products have emerged. Sapienza said candidates that have succeeded so far appear to be using slightly alkaline, phosphorus-based antioxidants. Specific information about additive content is not available, because the program allows oil companies to keep their formulas secret. In fact, Wedeven and Metss receive blind samples, so they do not know the content of what they test.

The additive testing has had its problems. Questions were raised about the applicability of some tests to the field. Also, the program initially contracted three labs to perform oxidation-corrosion tests, but found discrepancies in results. That led to a decision to use just one of the labs. Still, the Materials Directorate and its contractors say the program should produce multiple qualified oils. These oils must also be backward-compatible to serve existing equipment, too.

The program is nowhere near done, and we still have problems that are showing up, Sapienza said. But in general, we believe a new and innovative gas turbine engine oil additive technology is being developed.

All involved seem to agree that the program helped oil marketers to develop a technology in a substantially shorter time-frame than they otherwise would.

We didnt have ready access to these low-corrosion steels, and could not evaluate them in a meaningful anti-wear test, said Pat Godici, vice president of technology for Air BP Lubricants. So it allowed us to test our technology with these metallurgies and to try various formulary approaches to see which ones worked best.

The U.S. military in July unveiled its first edition of the Joint Strike Fighter – now renamed the Lightning II – but it apparently employs conventional bearings and turbine oils. Officials at the Materials Directorate suggest the program wont realize its goals until the new oils are ready for use.

These advanced steels are crucial to meeting the needs of these aircraft, and were told the oils are a critical technology in order for the new bearings to be employed, said Lois Gschwender, senior materials research engineer at the Materials and Manufacturing Directorate. She spoke in June at a fluids and lubricant workshop at Wright-Patterson. In order for the reliability and maintainability of those aircraft to be acceptable, they are going to have to have better oils.

The Small Business Innovative Research program is on schedule to wrap up by late 2008, at which point Pratt & Whitney would test the new oils in another prototype engine built with bearings made from Pyrowear 675. Those involved in the program say that test should go better than the one that took place 10 years earlier.

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