This article presents a new approach in which the explicit finite element method (EFEM) and the discrete element method (DEM) are coupled to investigate dynamics of flexible rotor systems supported by deep-groove ball bearings. In this investigation, DEM is used to develop the bearing (dynamic motion) model in which all of the components of the bearing (i.e., inner and outer race, balls, and cage) have 6 degrees of freedom. The flexible shaft is modeled with a full 3D elastic formulation using the EFEM rather than the reduced form, which implements component mode synthesis. The EFEM and DEM were combined to investigate the dynamics of flexible shaft rotor systems supported by ball bearings. Rotor and inner races of the bearings are fully coupled such that both translation and rotation of the flexible rotor are transmitted to the bearings. At each time step, the translational motion and rotation/tilt angle of the rotor cross section at the location of an inner race are applied to the inner race of the bearing. The resulting reaction forces and moments calculated in the dynamic bearing model are in turn applied to the nodes of the shaft. The combined model is used to investigate the motions of the inner races and the resulting reaction forces and moments from the supporting bearings due to an applied load on the shaft. In the current coupled modeling approach, the deformation of the shaft affects the internal components of the bearing by altering the orientation of the inner race, which results in ball spin and slip.

The objective of this experimental investigation was to design and develop a high speed turbocharger test rig (TTR) in order to critically examine the whirl and frictional characteristics of floating ring and ball bearing turbochargers. In order to achieve the objective, a high speed TTR was designed and developed with the capability of reaching speeds in excess of 100,000 rpm and was equipped with speed and displacement sensors to obtain the necessary results for comparison between the two turbocharger models. The TTR was used to compare and contrast the whirl and friction characteristics of two identical turbochargers differing only by the support structure of the rotor system: one containing a floating ring bearing turbocharger (FRBT) and the other a ball bearing turbocharger (BBT). The TTR is driven by an industrial compressor powered by a six cylinder 14 liter diesel engine. This configuration closely resembles turbocharger operation with an actual engine and was able to operate in both nominal and extreme operating conditions. A pair of displacement sensors was installed to measure the whirl of the rotor near the end of the compressor. Whirl results indicated that the BBT was significantly more rigid and stable than the FRBT. Waterfall plots were used to compare the frequency response of the two turbochargers over the full range of operating speeds. The majority of motion for the BBT was the whirl of the synchronous excitation due to a negligible inherent imbalance with some larger motions caused by vibrational modes. The whirl of the FRBT consists of not only the synchronous motion but also subsynchronous motions as a result of oil film instabilities throughout the entire operating range of speeds. The TTR was also used to compare frictional losses within the bearings. A study of the run-down times after the pressurized air supply was removed indicated that the BBT has significantly lower frictional losses under all operating conditions tested.

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