Modeling of dynamically loaded hydrodynamic bearings at low Sommerfeld numbers

Abstract

Current state of the art within the wind industry dictates the use of conventional rolling element bearings for main bearings. As wind turbine generators increase in size and output, so does the size of the main bearings and accordingly also the cost and potential risk of failure modes. The cost and failure risk of rolling element bearings do, however, grow exponentially with the size. Therefore hydrodynamic bearings can prove to be a competitive alternative to the current practice of rolling element bearings and ultimately help reducing the cost and carbon footprint of renewable energy generation. The challenging main bearing operation conditions in a wind turbine pose a demanding development task for the design of a hydrodynamic bearing. In general these conditions include operation at low Reynolds numbers with frequent start and stop at high loads as well as difficult operating conditions dictated by environment and other wind turbine components. In this work a numerical multiphysics bearing model is developed in order to allow for accurate performance prediction of hydrodynamic bearings subjected to the challenging conditions that exist in modern wind turbines. This requires the coupling of several different theoretical fields: • fluid film forces • heat transfer • thermoviscous effects • dynamic response • deformation of structure and components • angular misalignment • wear The multiphysics bearing model is applied for various bearing types in order to study the bearings, their hydrodynamic performance and related phenomena: • a new wear model is proposed which can, with only moderate efforts, be implemented into existing EHD models. • it is discovered that radial tilting pad bearings can exhibit discontinuity effects when subjected high dynamic loads. • the influence of compliant liners on the dynamic response of journal bearings subjected to dynamic loads is studied using a soft EHD model. • the influence of the geometrical design parameters of a radial flexure pad is studied as well as the effect of a compliant liner using an EHD model. • an innovative radial flexure journal bearing designed for operation at heavy angular misalignment is presented. Its hydrodynamic behavior, as well as the effect of a compliant liner, is studied using a TEHD model. • the EHD model is extended to cover 5 degrees of freedom and is applied for a novel compact moment-carrying hydrodynamic bearing.