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Gas Foil Bearings for Rotating Machinery
An introduction to the operation, design and performance of gas foil bearings and their practical integration into oil-free turbomachinery.

INSTRUCTORS
Luis San Andrés & Daniel Lubell

Gas foil bearings (GFBs) are an efficient alternative for load support of high speed (microturbo)machinery (< 400 kW, +1000°F, +3M DN). These bearings are compliant surface hydrodynamic bearings using ambient air as the working fluid media. Oil-free systems have lesser part count, footprint and weight and are environmentally friendly and (nearly) maintenance free, thus saving costs and resources. Current commercial applications include , air cycle machines, cryogenic turbo expanders and micro gas turbines. Other under development applications include auxiliary power units, automotive turbochargers and aircraft gas turbine engines for regional jets and helicopter rotorcraft systems.

Successful implementation of GFBs in commercial rotating machinery involves a three-tier effort with (a) selecting and designing bearing structural components and solid lubricant coatings to improve the bearing load carrying capacity with reduced friction, (b) having accurate performance engineering prediction models anchored to dependable (non commercial) test data, and (c) integration with secondary flow, operating profile, and the rotor-bearing dynamic system. 

Until recently GFB design and construction was largely empirical, each foil bearing being a custom piece of hardware, with resulting variability even in identical units, and limited scalability.  At present, the advances in radial GFB technology (design, construction and predictability) permit OEMs and end users to implement radial GFBs for deployment into novel MTM or to upgrade and improve outdated rotating machinery.

The short course provides engineers with a comprehensive review of existing gas foil bearing technologies including its principle of operation, analysis and experimental verification, comparison with other bearing technologies, as well as its integration into actual rotor-bearing systems (hot and cold). A hands on demonstration kit (foil bearing and rotor) will be available for the attendees learning.

Who Should Attend
The course is designed around the needs of every job function related to integrating oil-free foil bearings into rotating machinery products.  This includes design engineers interested in making their next generation machines or improving current machines with a hybrid oil-lubed and foil bearing system.  Additionally, consumers of oil-free machines will benefit from understanding the technology’s unique characteristics.  This course will include material relevant to oil-free compressors, industrial and aerospace gas turbines with hot section bearings, air cycle machines, and more.  The course will give enough information to rationally select foil bearings understanding their (current) practical limits and planning for further qualification and standardization. The short course will dispel several myths related to GFB performance and stress its unique features.

Why You Should Attend
Foil bearings are an enabling technology that benefits from a novel approach for effective integration into an oil-free rotating machine based on the bearing unique characteristics.  It is taught by engineers with practical, analytical, experimental and commercial experience with oil-free gas turbines and state of the art laboratory modeling and testing. 

The course will utilize common rotordynamics, hydrodynamics, and other physics principals already familiar to the community and show how they apply to foil bearing equipped machines from a theoretical and practical point of view.  In addition, the instructors’ practical experience will prepare the designer for common pitfalls including realistic performance expectations and typical operational failures, streamlining future projects.

Luis San Andrés
ASME Fellow, STLE Fellow, Mast-Childs Tribology Professor, Turbomachinery Laboratory, Texas A&M University

Luis San Andrés has an international reputation as a qualified analyst and experimentalist in the fields of fluid film lubrication and rotordynamics. Dr. San Andrés has provided major advances to the technology of hybrid bearings for applications in high performance turbomachinery. During the last 10 years, Dr. San Andrés has performed research on the analysis and experimental verification of gas foil bearing performance for high temperature oil free turbomachinery and squeeze film dampers for aircraft jet engines. His computational codes, benchmarked against test data, have become standards in the rotating machinery industry. Dr. San Andrés has educated many graduate students currently practicing their skills and providing services and leadership to turbomachinery manufacturers. Dr. San Andrés and his students have published over 150 papers and participate actively in international conferences and journal editorial boards.
Luis San Andrés holds a M.S. Degree from the University of Pittsburgh and a Ph.D. degree  from Texas A&M University. Luis is a named Professor at Texas A&M University and performs research at its Turbomachinery Laboratory. Luis holds honorary academic positions at Universities in The Netherlands, Singapore, South Korea and China.

Daniel Lubell
Principal Engineer of Oil-Free Bearings, Capstone Turbine

Mr. Daniel Lubell, an ASME member, has published papers on squeeze film dampers and foil bearing developments for micro gas turbines.  Daniel joined Capstone Turbine, manufacturer of the only commercially available oil-free gas turbines, in 2000 to be in the oil-free foil bearing group.  His roles have included Manager of Turbomachinery Rotating Systems, Principal Engineer of Oil-Free Bearings, and Manager of Rotordynamics and Air Bearing Technology. as part of a company focus on advancing foil bearing technology.  His responsibilities include bearing and rotordynamic design integration and testing for all Capstone products as well technical lead for all outside foil bearing sales.  Previously he was with Hamilton Sundstrand where he supported rotordynamic design and development testing for vibration requirements on APU's and small thrust jets.  Daniel has a Masters of Science in Mechanical Engineering which he earned at the Texas A&M University Turbo Lab in College Station, Texas.  Prior to that, he earned a Bachelor in Engineering Science from Trinity University in San Antonio, Texas.

Seminar OUTLINE - In progress -

1. Introduction to Gas Bearings (GBs) and Applications
   1. advantages/disadvantages of gas lubricated bearings and rotor gas bearing systems. Why gas bearings now? 
   2. How do gas bearings work – hydrodynamic and hydrostatic bearings
   3. comparison to other oil-free bearings (magnetic, rigid surface, hydrostatic)
   4. limits of GB load performance: static and dynamic
2. Introduction to Gas Foil Bearings (GFBs) and applications
   1. Are GFBs a novelty? How long have they been around? Where can one find foil (flexible) gas bearings in daily life (mundane applications)? How FBs enabled the digital revolution (tapes and hard drives)
   2. Types of GFBs for support of rotating machinery (radial and thrust): overleaf, bump-type, Capstone, and metal mesh
3. Bump-type foil bearings
   1. components, construction, selection of materials.
   2. design/selection of bump strip layers and top foil. Formulas for bump stiffness: dimensions of importance.
   3. The importance of a well designed underspring structure in a foil bearing. The ultimate load capacity of the structure. Hardening (nonlinear effects) and mechanical hysteresis as a measure of energy dissipation.
   4. coatings – why are coatings needed? Coatings of top foil, on rotor or both? How to select them. Is your application hot?
   5. the known (published) performance of GFBs. Do we believe everything we hear in a sales pitch? How to be a discerning customer.
   6. The static load capacity of GFBs: The rule of thumb or the rule of Dumb ? Realistic safety factors
4. Analysis of gas foil bearings
   1. The fundaments of gas film lubrication: Reynolds equation.
   2. Coupling to the underspring (elastic) structure. Simple and complex methods of solution
   3. Tools for analysis: prediction of performance (min film thickness, power losses, gas flow and rotordynamic force coefficients). Effect of excitation frequency on force coefficients.
   4. Simple rotor-bearing system dynamic response analysis. Natural frequencies and typical modes of vibration, imbalance response and rotordynamic stability considerations.
   5. In hot applications, integration to a thermal energy transport equation. How to engineer a successful thermal management.
5. Independent experimental evidence of foil bearing performance
   1. Overview of research activity in the last 15 years. The NASA oil-free program and research abroad (Japan and South Korea). Who is leading the race?
   2. The load capacity and power losses of GFBs. Typical specific loads compared to ultimate loads.
   3. Start up/shut down performance: promoting early rotor lift off and reducing dry friction torque – where coatings matter. Effects of preload (clearance) assembly and temperature
   4. How do GFBs generate damping in excess of its viscous magnitude. The importance of dry-friction sliding among structural components.
   5. The dynamic load response of GFBs: measured vibration responses and appearance of persistent subsynchronous whirl motions. Rotordynamic instability or a forced nonlinear response? Limit cycle performance. Why do rotor-FB systems survive chronic large amplitude vibration problems
   6. How to reduce subsynchronous whirl motions by introducing (1) mechanical preload, (2) management of feed pressure, (3) enhancing material damping (4) reducing instability drivers.
   7. Typical failures of GFBs due to (a) excessive load and poorly designed underspring structures, (b) sudden contact and overheating with immediate parts melting, etc.
   8. Novel foil bearing applications: Capstone bearings, metal mesh bearings, etc.

1. Integration of Foil Bearings into practical rotor bearing systems
   1. Current (proven) applications and envisioned applications.
   2. Limits of operation: specific pressure DN and life?
   3. Some “How to” and “What not to do”.
2. Scaling guidelines to enable FBs into other applications
3. Thermal management
   1. Cooling methods – conduction, convection, and radiation
   2. Budgeting secondary flow capacity
   3. Planning testing into the design
   4. Application factors
4. Questions you are afraid to ask…
   1. What is required for mass production?
   2. What happens in a failure?
   3. How does a wear failure happen?
   4. What happens during a blade-off event?