Hydrodynamic Lubrication Analysis of a Foil Journal Bearing:

Pressure and Lift Build-up

Jun Shi

UTRC

•Air bearings with compliant foils are an important class of gas bearing in that they offer a unique set of beneficial characteristics relative to other types of bearings.

•These benefits includes • simple design and no oil contamination issues due to the elimination

of lubrication oil. • the simple design translates directly into low bearing system weight

and low maintenance cost. • The low viscosity of air and its insensitivity to temperature means

that air bearings can operate at high speed without small penalty in transmission loss and in a wide range of temperatures.

Why Air Bearings ?

Inner intermediate foilOuter intermediate foilBump foil

Shaft

Rotation

Rigid Casing

Temporary Trusses

Inner intermediate foilOuter intermediate foilBump foil

Shaft

Rotation

Rigid Casing

Temporary Trusses

Schematic of a Journal Bearing Model Boeing 777 with Hamilton Sundstrand’s ACM incorporating patented foil gas bearings

The foil air bearing offers the additional benefits of tight film thickness over a wide operating conditions due to the foil’s ability to deform under air pressure. The deformation of the foil results in three desirable attributes:

1) It facilitates the build-up of air pressure and the subsequent lift off of the journal from the foil bearing. From general lubrication theory, the thinner the air film, the higher the air pressure and load carrying capacity. (The lower limit of the air film is the journal/bearing surface roughness. Below this limit, surface asperities from the journal and bearing would interact leading to mechanical contact, friction, surface heating and wear. )

2) The compliancy of the foils also reduces the requirements of machining tolerance and shaft misalignment, both help to decrease machining and assembly costs.

3)The relative deformation between foils leads to frictional damping in addition to the squeeze film damping. Therefore, foil bearing offers more damping than rigid casing air bearings and a more stable rotor system.

Why FOIL Air Bearings ?

GEN3 > 1990s

GEN2 1970~80s

GEN1 1960~70s

Foil Bearing Development History

D=F/(L*2*r)*(2*r*w)

How air bearing works: COMSOL simulation

Rj=25mmRb=25.068

Concentric Journal

Constant pressure due to concentricity, low air density and small gap

Linear velocity gradient through gap: 0 on bearing and 130m/s on journal

No net force on journal: no load carrying capacity !Torque on journal: 2.02E-6 N/m

Eccentric Journal: eccentricity=0.0375mm

Constant pressure due to concentricity, low air density and small gap Highest pressure before the smallest gap

Total load : 41.5N/m in Y and1.92N/m in X directionsTorque on journal: 2.02E-6 N/m

Smallest gap=30 mm

Biggest gap=105 mm

Mean gap=75 mm