Abstract
This work presents the bearing design and analysis of radial semi-floating bush oil lubricated bearings for a typical industrial turbocharger configuration. Initially, the stability analysis for a linear rotor/bearing system is evaluated through eigenvalues and eigenvectors. The stiffness and damping coefficients of the inner oil film are obtained for the linear modeling process. The operating speed range of the turbocharger is high enough, at 21,000 to 24,000 rpm, to be unstable, indicating that the analysis should be and is carried out with nonlinear transient modeling. The nonlinear transient analysis evaluates the rotor and bush limit cycle orbits, rotor dynamics, the forces acting on the rotor and semi-floating bush surfaces, the oil flow through the bearing, the oil temperatures, and the power loss of the two oil films. The optimum design of a set of semi-floating bush bearings for this application depends strongly upon the clearances of the bush and squeeze film damper, usually expressed as the non-dimensional clearance to radius ratio. A typical clearance is evaluated to determine the bearing performance in terms of orbit size, forces acting on the bush and squeeze damper surfaces, oil flow through the bearing, power loss, and thermal heating. The nonlinear transient orbit values are evaluated for frequency content using the FFT to determine which orbits show both the synchronous and sub-synchronous vibration components and the associated rotor modes excited. These results are compared to the linear analysis over the operating speed range. The oil flow through the bearing component is much larger than the squeeze film damper. The forces acting on the bush and squeeze damper surfaces are related to the fatigue life of the bearing.
Highlights
Turbochargers are commonly supported in fluid film bearings of several types: (1) single oil film bearings; (2) two oil film, fully floating bush bearings; (3) ball bearings supported by squeeze film damper; and (4) two oil film, semi-floating bush bearings
The linear steady stability analysis of a high speed turbocharger showed that the rotor with the typical fixed-pad bearing design is highly unstable, and the semi-floating bearings were introduced due to the advantage of relatively low cost by using the normal fixed-pad bearing and adding squeeze film dampers outside of the fixed-pad bearing housings
The nonlinear transient analysis shows that the shaft orbits go into limit cycle orbits with acceptable amplitudes
Summary
Turbochargers are commonly supported in fluid film bearings of several types: (1) single oil film bearings; (2) two oil film, fully floating bush bearings; (3) ball bearings supported by squeeze film damper; and (4) two oil film, semi-floating bush bearings (squeeze film damper supported fixed-pad bearings). Small automotive turbochargers are constructed of one of the first three types using fixed-pad bearings, due to cost considerations. All of these bearings usually operate at very high rotating speeds and are commonly linearly unstable. Floating ring bearings (FRBs) are the commonly employed type in the small turbocharger designs used mainly in automotive applications. In these turbochargers, the running speed reaches to well above 100 krpm and FRBs reduce the parasitic power loss as a rotating ring (bush) in their design
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