Identification of the dynamic characteristics of turbulent journal bearings using active magnetic bearings

In the field of chemical and environmental industry the demands on leakage free, so called hermetic pumps are becoming more and more stringent. One possible way to realise this demands is to use process fluid lubricated journal bearings in combination with canned or wet motor pumps or magnetic drive pumps (horizontal and vertical type). The processing fluid has often a very low viscosity which causes turbulence in the lubricating film of the bearing. The turbulence in the lubricating film changes the static and dynamic behaviour of the bearing and therefore of the whole pump system. Up to now, calculation tools for the design of turbulent bearings are not state of the art. In the presence of a sufficient static preload (e.g. horizontal machines), the dynamic of the journal bearing can be linearized around a equilibrium point. The description of the dynamic characteristics is done using rotordynamic coefficients. When the lubricating film becomes turbulent, the stiffness and damping coefficients differ strongly from these of laminar bearings, and moreover, inertia coefficients can no longer be neglected. Within the presented research project, active magnetic bearings (AMBs) are used to identify the dynamic characteristic of hydrodynamic, turbulent journal bearings. The AMBs represent a mechatronic tool to levitate and to position a test rotor without any mechanical contact, to generate relative motions of the rotor inside the journal bearing and to measure the generated displacements and forces during rotation in a very accurate way. The paper describes the experimental technique and typical results of the static and dynamic behaviour of a cylindrical journal bearing operated in the turbulent regime are presented. (Author)