AFRL experts develop additives aimed at improving aircraft engine bearings

  • Published
  • By Mindy Cooper
  • AFRL Materials and Manufacturing Directorate
Researchers for the Air Force Research Laboratory's Materials and Manufacturing
Directorate managed a multi-contract to help improve advanced aircraft engines.

As part of the Small Business Innovation Research (SBIR) program, this newly developed additive will be used with the fully formulated gas turbine engine oils on the new aircraft engine bearings. Development of these additives will allow bearings with improved characteristics to be used in advanced aircraft engines, according to Lois
Gschwender, the fluid and lubricant expert who managed the program.

The tempo of the Air Force's operational environment requires the aircraft to fly faster and carry more weight than is currently possible with the current qualified oils. Pursuing higher speeds and loads results in increased temperatures and fatigue for aircraft engines. In addition, because increased speeds and weight will require engines to produce more thrust, an increased amount of stress is expected on steel bearings in the engines.

Bearings are designed for maximum load carrying and minimum friction. They consist of smooth metal balls or rollers, and a smooth inner and outer metal surface, called races, for the balls to roll against. These balls or rollers and races bear the load, allowing a device to spin smoothly. M-50, a tool steel, is currently the widely used bearing material for gas turbine engines and other applications. While it is a good bearing material, it is
not very corrosion resistant. It also has some limitations at higher temperatures, so researchers are seeking replacement materials with characteristics similar to M-50, but with improved corrosion resistance and improved load carrying ability at high temperatures. The M-50 bearings, despite their drawbacks, have worked well for a number of years.

One of their greatest attributes is their ability to react with currently used lubricity additives when metal to metal contact between the balls or rollers and the metal races occurs. M-50 reacts with these additives under these boundary lubrication conditions to provide a lubricious film that prevents severe wear, and subsequent bearing failure.

One of the main obstacles of replacing the bearings made of M-50 steel is that the newer metal alloys do not produce enough of a chemical reaction to create lubricious films. Experts in the directorate's Fluids and Lubricants laboratory recognized that the additives in current engine oils were the phosphates known to react with the iron on the surface of the M-50 bearing metals.

In order to replace M-50 bearings, new additives would be required to generate a similar reaction with the new alloy elements that are present on the surfaces of these new bearing alloys. The Materials and Manufacturing Directorate proposed developing new
additives to formulate into the gas turbine engine oil that would be totally soluble and allow the additive to be carried within the oil to the new bearings. These new additives would react with the surfaces in the same way that the previous additives, primarily aryl
phosphates, reacted with the iron surfaces of the M-50 steel to create the necessary lubrication.

Researchers received funding for three Phase I SBIR contracts, which are government contracts with small businesses to develop technologies that are useful to the Air Force. The three participating companies include: METSS Corp., Westerville, Ohio; UES Inc., Dayton, Ohio; and Wedeven Associates, Inc., Edgmont, Pa. The companies were asked to develop and screen a number of additives for anti-wear properties, as well as other critical properties such as thermal stability, coking, etc.

Experts in the Materials and Manufacturing Directorate provided close guidance during Phase I and interfaced with both oil companies (NYCO S. A., ExxonMobil, AirBP and Hatco Corp.) and engine companies (Pratt & Whitney and Rolls- Royce), which allowed open lines of communication and information sharing among the companies working on the additives. The oil company, NYCO S.A., agreed to provide their oil for testing and assessment of suitability with the additive candidates. This collaboration was successful, and Wedeven Associates Inc. and METSS Corp. both received Phase II contracts.

"This program is unique because the oil is being customized for modern bearing steels rather than, as usually done, with the hardware being made to work with existing oil formulations developed decades ago," Ms. Gschwender said.

This research will result in more suitable gas turbine engine oil for aircraft engines that will help prolong the life and enhance the performance of advanced aircraft engines. The goal of Phase II is to demonstrate a technology for additives that will provide the necessary anti-wear protection for the new bearing materials and other critical properties while operating at the temperatures anticipated for advanced aircraft engines. This critical, improved technology is planned for insertion into future engine technology.