Assessment of thermal effects on the levitation speed of bump foil bearings made of low cost spring steel

The demand for higher speeds and torque capacity from micro turbines, heat pumps and turbochargers has necessitated the development of high temperature resistant foil bearings. This paper focuses on investigating the effects of successive thermal cycles on the levitation speed of bump foil bearings made up of low cost spring steel. Bump foil bearings were designed for high stiffness of 1.64 MN/m2. Rotor dynamic analysis indicated highest frequency of 4790.5 Hz corresponding to second flexure mode of rotor bearing system up to which it remained stable. The bump foil bearing fabrication procedure was established and rotor was tested under suitably designed bearing rig. The orbital analysis indicated that levitation speed decreased with increase in temperature. During second and third thermal cycle, at lower rotor speeds drastic variations in amplitude of vibrations and uneven waveforms were indicative of unbalance condition of rotor. With further increase in rotor speeds, the rotor - bump foil bearing system attained the balanced state indicative of safe design.