Gas foil bearings (GFBs) have been successfully introduced in the field of high speed turbo machineries. Low drag friction, high speed operation and the omission of an oil system are some advantages of bump type foil bearings. However, experimental and numerical investigations have shown sub synchronous vibrations, which affect the rotor dynamic behavior. Several methods and devices have been introduced to decrease these vibrations (e.g. viscoelastic foil bearings, shims and side feed pressurization). This current paper examines the effect of different bump foil configurations on the rotor dynamic performance. Different bump stiffness distributions in axial and circumferential direction are considered for a set of loadings (5, 15, 30 and 60 N) and rotor speeds (6,000–70,000 rpm). To evaluate the onset speed of sub synchronous vibration a linear stability analysis is applied. It uses the linearized bearing parameters stiffness and damping. The results show, that a variation of stiffness distributions may enlarge the stability range. Simulations indicate that a non-uniform circumferential stiffness distribution is very effective to avoid sub synchronous vibrations, due to smaller cross coupling effects.

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