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Hydrodynamic Lubrication Analysis of a Foil Journal Bearing: Pressure and Lift Build-up

Hydrodynamic Lubrication Analysis of a Foil Journal Bearing: Pressure and Lift Build-up. Jun Shi UTRC. Why Air Bearings ?.

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Hydrodynamic Lubrication Analysis of a Foil Journal Bearing: Pressure and Lift Build-up

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  1. Hydrodynamic Lubrication Analysis of a Foil Journal Bearing:Pressure and Lift Build-up Jun Shi UTRC

  2. Why Air Bearings ? • 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. Boeing 777 with Hamilton Sundstrand’s ACM incorporating patented foil gas bearings Schematic of a Journal Bearing Model

  3. Why FOIL Air 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: • 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. ) • The compliancy of the foils also reduces the requirements of machining tolerance and shaft misalignment, both help to decrease machining and assembly costs. • 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.

  4. Foil Bearing Development History GEN3 > 1990s GEN2 1970~80s GEN1 1960~70s D=F/(L*2*r)*(2*r*w)

  5. How air bearing works: COMSOL simulation Rj=25mm Rb=25.068

  6. 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

  7. Eccentric Journal: eccentricity=0.0375mm Constant pressure due to concentricity, low air density and small gap Highest pressure before the smallest gap Biggest gap=105 mm Mean gap=75 mm Smallest gap=30 mm Total load : 41.5N/m in Y and1.92N/m in X directions Torque on journal: 2.02E-6 N/m

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