12-12-2012, 05:41 PM
Load Capacity Estimation of Foil Air Journal Bearings for Oil-Free
Turbomachinery Applications
Load Capacity Estimation of Foil Air.pdf (Size: 951.79 KB / Downloads: 49)
SUMMARY
This paper introduces a simple "Rule of Thumb" (ROT) method to estimate the load capacity of foil air journal
bearings, which are self-acting compliant-surface hydrodynamic bearings being considered for Oil-Free turbomachinery
applications such as gas turbine engines. The ROT is based on first principles and data available in the
literature and it relates bearing load capacity to the bearing size and speed through an empirically based load capacity
coefficient, D. It is shown that load capacity is a linear function of bearing surface velocity and bearing projected
area. Furthermore, it was found that the load capacity coefficient, D, is related to the design features of the bearing
compliant members and operating conditions (speed and ambient temperature). Early bearing designs with basic or
"first generation" compliant support elements have relatively low load capacity. More advanced bearings, in which
the compliance of the support structure is tailored, have load capacities up to five times those of simpler designs.
The ROT enables simplified load capacity estimation for foil air journal bearings and can guide development of new
Oil-Free turbomachinery systems.
INTRODUCTION
Foil air bearings are self-acting compliant-surface hydrodynamic bearings that use ambient air (or any process
gas) as their working fluid or lubricant. By utilizing this Oil-Free technology, foil bearing supported turbomachinery
can benefit from design simplicity and reduced weight (no oil system), high speed and temperature capability, and
reduced maintenance. Foil bearings have proven themselves in relatively small lightly loaded applications, like aircraft
air cycle machines (ACM's). Recent advances in foil air bearing design, high-temperature solid lubrication,
and bearing and rotor system analytical modeling enable new applications in Oil-Free turbomachinery (ref. 1).
Foil air bearings were first commercialized in the 1970's in air cycle machines used for aircraft cabin pressurization
(refs. 2 and 3). Since then, new applications in cryogenic turbo-expanders, turbo-alternators and turbochargers
have been demonstrated (refs. 4 to 7). All of these applications relied on an experimental build and test development
sequence. Although relatively time consuming and costly, this development approach is necessary due to the
lack of accurate predictive performance analysis methods for a range of foil bearing sizes and designs. Despite the
analytical and predictive shortcomings, experimental foil bearing characterization continues to add to the foil air
bearing knowledge database. It is anticipated that as more applications are developed and ongoing research continues,
an improved fundamental understanding of foil bearing performance characteristics will be developed to guide
the engineering of new Oil-Free turbomachinery systems.
Three key technical hurdles have impeded the application and widespread use of foil air bearings beyond air
cycle machines into other turbomachinery systems such as gas turbine engines. These technical hurdles are: (1)
adequate load capacity, (2) high temperature start/stop lubricants and (3) reliable predictive performance methods
and design guidelines.
Recent improvements in load capacity have been demonstrated. In a 1994 paper by Heshmat, a twofold increase
in load capacity was reported (ref. 8). This improvement was attributed to the better design of the compliant foil
structure based on elastic and hydrodynamic analytical modeling. Other researchers have indicated similar load
NASA/TM—2000-209782
capacity improvements but have not yet published the data in the open literature. These demonstrated levels of load
capacity help remove the first technical hurdle.
High-temperature (>300 CC) bearing operation has always been a challenging technical hurdle because commonly
used foil lubricant coatings rely on relatively low temperature materials (e.g., PTFE and MoS2) (refs. 5
and 9). These materials are used because, in addition to the good lubrication properties, they are flexible and as foil
coatings they do not significantly alter the compliance and surface morphology of the top foil. These traditional
solid lubricants are temperature limited to use under about 300 °C. Unfortunately, solid lubricants capable of operating
above 300 °C are relatively rigid ceramic-like materials that are difficult to apply and their presence significantly
changes the compliance of the thin and flexible foil members (ref. 10).
Recent research on new high-temperature solid-lubricant coatings applied to bearing shafts (journals) appears to
have overcome this second technical hurdle. Uncoated nickel-based superalloy foil bearings have been successfully
lubricated with PS304 shaft coatings for over 100,000 start/stop cycles at temperatures as high as 650 °C (refs. 1
and 7). PS304 is a plasma sprayed composite solid lubricant that has silver and fluoride eutectic lubricants in a
metal/oxide matrix. During operation the lubricants transfer to the foils creating a thin but effective foil coating layer
(ref. 1). By using PS304, or other similar coatings, high temperature operation with long life is achievable.
The third technical hurdle, reliable predictive performance methods and design guidelines has not yet been
overcome. The reason for this shortfall is that foil bearings are inherently nonlinear and very difficult to model using
relatively simplistic first principle methods (refs. 11 and 12). This modeling difficulty is due to the complex nonlinear
structural, hydrodynamic fluid, and thermal interactions between the compliant foils and the fluid film which
are often influenced by stick/slip frictional contacts between foil elements and the elastic foundation support structure
(e.g., bumps) (ref. 13). In more technologically mature systems, such as rolling element bearings, extensive
experimental data and application based experience has led to empirically based design guidelines (refs. 14 and 15).
For air foil bearings, extensive experimental measurements have not been made, especially at high temperatures, and
thus similar experience based guidelines are not yet available.
In this paper, an empirical or "Rule of Thumb" estimation of journal bearing load capacity is developed as an
aid in feasibility assessments for foil bearing supported rotordynamic systems. The "Rule of Thumb" (ROT) is based
on experimental data and fundamental first principles and is shown to be remarkably effective in making direct
comparisons between bearing designs.
A similar ROT analysis of thrust foil bearings is inhibited by the lack of available thrust bearing load capacity
data. Future work in this area is expected to result in a thrust foil bearing load capacity ROT following a research
path that parallels the one reported in this paper.
Recognizing its limitations in scope and accuracy, the journal load capacity ROT serves as a first step for further
work in developing similar ROT's for thrust bearing load capacity and for bearing dynamic (stiffness and damping)
characteristics. The successful development of additional ROT's will help to overcome the third technological
hurdle and foster the further successful application of foil air bearing technology to Oil-Free turbomachinery
systems.
FOIL BEARING BACKGROUND AND RULE OR THUMB DEVELOPMENT
Foil air bearings operate under self-acting hydrodynamic principles in the same manner as conventional sleeve
type rigid hydrodynamic bearings. However, a major difference is that foil bearings have compliant surfaces relative
to rigid bearings; therefore, foil bearing geometry is not fixed. Figure 1 shows cross sections of two typical journal
foil bearing designs, the overlapping leaf type foil bearing and the bump foil bearing. During operation, the hydrodynamic
film pressure deflects or deforms the foils. The bearing geometry, therefore, is influenced by the operating
conditions such as speed, load, and temperature.
At rest, the top (or inner) foil is spring preloaded against the shaft. There is no clearance as in a rigid sleeve
bearing. As the shaft rotates, viscous air is circumferentially dragged in between the top foil surface and the shaft
generating hydrodynamic pressure. This pressure acts upon the top (inner) foil causing it to separate from or "liftoff
the shaft surface and press against its compliant support structure. The fluid film pressure and foil compliance
interact dynamically to seek an equilibrium state for a given set of conditions. Additionally, pressure changes in the
shearing fluid film and viscous heating can lead to heat generation that may influence fluid film properties or foil
mechanical properties. Because of these significant complex fluid/structural/thermal interactions, modeling of foil
bearings must include fluid film and large deformation elastic effects.