A modified Reynolds equation considering the rarefield gas coefficient for the bump type gas foil thrust bearing was established based on the first-order slip velocity boundary condition. The coupled modified Reynolds and lubricating film thickness equations were solved using Newton-Raphson iterative method and finite deference method. The static characteristics including axial load capacity and lift-off speed were obtained. The effects of rarefied gas and bearing structural parameters on the static characteristics were analyzed and discussed. The results indicate that with the influence of rarefield gas

the axial load capacity decreases

and the lift-off speed increases. And the rarefield gas has a slighter effect under a higher operating speed and a larger foil compliance coefficient. When the assembled clearance is smaller than 10 μm

the lift-off speed increases rapidly with increasing of the assembled clearance of the foil thrust bearing. Therefore

the axial clearance should be controlled carefully in assembling the foil thrust bearing.