Nonlinear Analysis of Rotors Supported by Air Foil Journal Bearings – Theory and Experiments

Direct driven compressors supported by air foil bearings (AFB) are gaining increasing popularity, for example within the waste water treatment industry where the demand for larger machines up to 250 kW is growing. In order to keep production costs low, the shaft and bearing design need to be simple and allow manufacturing using conventional materials and production facilities. As a consequence, the assembled rotor weight can be up to 50 kg. The compressors are operated at variable speed and load and are subjected to several starts and stops per day. Therefore, the rotor bearing design must be robust with a good margin of rotordynamical stable operation. To ensure this, good mathematical models, capable of accurately predicting the dynamic behaviour of the rotor-bearing system, are required at the design stage. This thesis focuses on developing and improving existing mathematical models for predicting the nonlinear dynamic behaviour of AFBs, both isolated and coupled with the rotor. Large emphasis is put on the experimental validation of the obtained theoretical results. Two experimental test rigs were designed and manufactured specifically for this purpose. With the first rig, the isolated nonlinear mechanical behaviour of the bump foils was carefully examined. A mathematical model capable of predicting this nonlinear behaviour was developed and compared to the experimental results with good agreement. With the second test rig, the overall nonlinear behaviour of the rotor-bearing system was investigated experimentally, and the linearised stiffness and damping coefficients of the bearings were identified. Results obtained theoretically were compared to these linearised coefficients with reasonably good agreement. Furthermore, the sensitivity to rotor unbalance and the nonlinear response were analysed experimentally and a coupled rotor-bearing mathematical model was compared to these results with very good agreement in terms of the nonlinear rotor unbalance response.