Magnetic Fluid Bearings & Seals: Methods, Design & Application

Abstract

The bearing and the seal are two commonly used tribological components since nearly all moving machinery relies on them for proper operation. Even a small improvement in these components can have a big impact on both the market and the environment. The two main problems of these components are wear and friction. In addition, seals suffer from the problem of leakage which is fundamental to both their functioning as well as their performance. The application of magnetic fluids has the potential to be beneficial for these systems. Magnetic fluids consist of a suspension of magnetic particles in a carrier fluid. This gives them the unique properties of being attracted to a magnetic field and changing their rheological behaviour in the presence of a magnetic field. These special properties can give these bearing and sealing systems unique behaviour, potentially improving their performance. Therefore, this thesis has two main objectives. The first objective is to further investigate the potential of magnetic fluids in bearing and sealing system. This part consists of exploratory, fundamental and early stage research. The unique properties of magnetic fluids are eventually used in ferrofluid bearings, ferrofluid seals, bearings with rheological textures and self-healing bearings. The second objective is to develop the necessary knowledge to bring these concepts to society in the form of applications. The research on ferrofluid bearings as described in this thesis consists of different experimentally validated models for the load capacity, torque capacity, out of plane stiffness, rotational stiffness, friction and operational range for both ferrofluid pocket bearings and ferrofluid pressure bearings. These models are suitable to determine the most important bearing parameters which makes it possible to design a bearing system according to desired specifications. The most recent demonstrator shows specifications that are competitive with conventional bearing systems. The research on ferrofluid seals as described in this thesis has resulted in the first seal concept that does not show any leakage over time. The concept relies on a replenishment system that makes sure that that degraded ferrofluid is removed and replaced by fresh ferrofluid. This gives the system a theoretical infinite lifetime, as long as the replenishment system continues to work. The research on bearings with magnetorheological fluids as described in this thesis has resulted in the new design concept of rheological textures. A rheological texture is defined to be a local change in the rheological behaviour of the lubricant in the lubricating film such that a local change in lubricant transport and flow resistance occurs. The idea is to replace geometrical surface textures of more traditional bearings with these rheological textures to enhance the performance of the bearing. This work has in addition provided a new lubrication theory for Bingham plastics that properly models the behaviour. Furthermore, a new experimental method is developed to characterize the rheology of magnetic fluids at high shear rates. The last significant result of this research is the new concept of a self-healing bearing using a lubricant with a suspension of magnetic particles: the application of a local magnetic field with high gradients in the lubricating film causes the particles to settle at locations where the magnetic field gradient is high. This creates geometrical surface textures that will regrow when worn away. In this way we have a bearing with a surface texture that has a theoretical infinite lifetime.