Impact of Whirl and Axial Motion on Ball Bearing Turbocharger Dynamics

Abstract

Ball bearing turbochargers (TCs) incorporate an angular contact ball bearing cartridge to reduce friction and oil consumption, improving the efficiency of internal combustion engines. In this investigation, axial and radial TC rotor motion was experimentally measured and used to develop a model of the TC rotor-bearing system, allowing for a detailed analysis of the TC bearing under varying operating conditions. In order to achieve the experimental objectives of this investigation, eddy-current proximity probes were used to measure the radial motion at the turbine and compressor sides of the TC. The measured radial motion shows whirl with subharmonics at low speed. The measured axial motion of the rotor was found to increase with TC speed. In order to analytically investigate the bearing dynamics, a complete TC model was developed that includes the mass distribution and flexibility of the TC compressor, shaft and turbine, and squeeze-film dampers (SFDs) that support the bearing cartridge. The model was used to replicate the whirl and axial motion of the test rig and then to determine which model parameters could be adjusted to minimize whirl without negatively affecting the bearing dynamics. Simulation results revealed that centrifugal effects cause a change in the bearing internal geometry, with a small clearance becoming a preload at higher speeds. When this effect is coupled with less compliant SFDs, the subharmonics are reduced and the overall sliding and number of large load cycles at the ball–race contacts are minimized, contributing to lower bearing friction and improved overall life.

Keywords:

• Ball bearings
• squeeze-film dampers
• centrifugal gas compressors
• dynamic modeling
• turbocharging
• whirl

Acknowledgement

The authors express their deepest appreciation to METL sponsors for their support of this project.