Heshmat, Hooshang and Chen, H.M. "Principles of Bearing Design."Compressor Handbook (2001)

Heshmat, Hooshang and Chen, H.M. “Principles of Bearing Design.”Compressor Handbook (2001).

 

Gas turbines are used in a wide array of stationary, marine, and aerospace applications. Land, sea, and air-based generating systems produce anywhere from a few tens of kilowatts to multi-megawatts of power. Drives for natural gas pipeline compressors and land and marine propulsion systems produce thousands of shaft horsepower. The simplicity of the gas turbine, its efficiency, and ability to use many different fuels make s it attractive for these applications. the gas turbine operates on the principal of the Brayton cycle where compressed air is mixed with fuel in a combustor, burned under constant pressure conditions and then expanded through a turbine to perform work. Due to more stringent environmental legislation, the viability of alternative forms of power generation, and demands for increased power density, improvements in gas turbine efficiency, emissions, and life cycle costs are necessary and are being implemented. Since these gains cannot be allowed to sacrifice operability or reliability, modern gas turbines require bearings and lubricants that can handle extreme speed, temperature, and other stress without breaking down.
It has been apparent for some time and especially since the mid-1980’s when the Department of Defense, NASA, and industry established the Integrated High Performance Turbine Engine Technology (IHPTET) that tribological limitations associated with the bearings, seals, and lubricants represent major obstacles to achieving significantly improved performance and live in advanced gas turbines. For example, rolling element bearings (REB) used in modern gas turbine engines may operate at ND values as high as 2.5 to 3.0 million ( where D is bearing bore diameter in mm and N is shaft speed in rpm). One of the outcomes of the Tribological Limitations in Gas Turbine Engines: A Workshop to identify the Challenges and Set Future Directions was projections that indicated that operating temperature will continue to rise and bearing DN values above 4.0 million will be required in the coming decades. Present bearings even those using ceramic rolling elements material and lubricants will be inadequate for the task at hand mere advances to existing practices will not be sufficient to meet the life and performance improvement goals. rather, entirely new lubricants, baring materials, and bearing designs will need to be conceiv3eed, developed, and tested. Areas already under development include:

  • High-temperature liquid lubricants such as polyphenylethers and perfluoralkylethers
  • Hybrid ceramic-steel materials
  • Solid film lubricant coatings
  • Advanced alternative lubrication systems such as vapor phase or powder lubricants
  • Magnetic and auxiliary bearings
  • High-performance gas bearings capable of operating at temperatures up to 1500°F either alone or in combination with magnetic bearings

Industry and government identified tribological problem areas for existing systems included:

  • Corrosion of bearing materials due to oil breakdown
  • Lubricant limitations in life and temperature capabilities
  • Contamination of the lubricant and the bearings
  • Breakdown in elastomeric seals and components
  • Reliability and service life of the bearing, lubricant, and lubrication systems
  • Lubricant bulk and spot temperature limitations
  • Seal wear
  • High thrust loads and the impact on bearing and lubricant life

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