Experimental and analytical investigation of rotor bearing systems

Ankur Ashtekar, Purdue University

Abstract

The objectives of this investigation were to design and construct a high speed turbocharger test rig (TTR) to measure dynamics of angular contact ball bearing rotor system, and to develop a coupled dynamic model for the ball bearing rotor system to corroborate the experimental and analytical results. In order to achieve the objectives of the experimental aspect of this study, a test rig was designed and developed to operate at speeds up to 70,000 rpm. The rotating components (i.e. turbine wheels) of the TTR were made to be dynamically similar to the actual turbocharger. Proximity sensors were used to record the turbine wheel displacements while accelerometers were used to monitor the rotor vibrations. A wireless telemetry based temperature sensor was specifically designed and developed to monitor the bearing internal operating temperature. The TTR was used to examine the dynamic response of the turbocharger under normal and extreme operating conditions. To achieve the objectives of analytical investigation, a discrete element ball bearing model was coupled through a set of interface points with a component mode synthesis rotor model to simulate the dynamics of the turbocharger test rig. Displacements of the rotor from the analytical model were corroborated with experimental results. The analytical and experimental results are in good agreement. The bearing rotor system model was used to examine the bearing component dynamics. Effects of preloading and imbalance were also found to have significant effects on turbocharger rotor and bearing dynamics. The dynamic bearing model was also modified to include the influence of cage flexibility on the force and motions of bearing components (i.e. inner and outer races, and balls). In order to achieve the objectives, a cage model based on the explicit finite element method was developed and combined with a discrete element dynamic bearing model. In this modeling approach, all bearing components, including the flexible cage have 6 degrees of freedom. The 3D explicit finite element approach allows the cage model to elastically deform as the balls impact cage pockets. The ball inner and outer race contacts are modeled using Hertzian equations, while a separate contact algorithm was developed to determine the interacting forces between the cage finite element mesh and discrete bearing components. The discrete and finite element models interact at each time step to determine the position, velocity, acceleration and forces of all bearing components. The ball cage contact forces are used to determine the deformation and stresses generated within the cage. The model results were corroborated with previous experimental results as well as the traditional analytical cage models. The results from the current FEA cage model matched the known cage motion traits observed by previous investigators. However, further investigation using the current model demonstrates that cage flexibility/deflection has a considerable effect on the overall dynamics of the bearing, in particular the ball-cage contact forces. Also, the resulting cage deformation and stress helps to identify the effect of operating and loading conditions on cage stress distribution.

Degree

Ph.D.

Advisors

Sadeghi, Purdue University.

Subject Area

Mechanical engineering

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