Demonstrator setup for semi-active control of a hydrodynamic journal bearing lubricated with magnetorheological fluid

Abstract

In a hydrodynamic journal bearing, the lubricant is pressurized by the relative movement of the bearing surfaces, creating a load-carrying capacity. The load-carrying capacity can be increased by using a lubricant with higher viscosity, at the cost of increased friction. A proposed method to increase the maximum load-carrying capacity, while not increasing the friction at lower loads (or at higher speeds), is by using magnetorheological (MR) fluid as lubricant and electromagnets to generate a magnetic field. MR fluid consists of a carrier fluid with micron-sized magnetic particles suspended. Under the influence of an external magnetic field, the viscosity of the fluid increases as the particles form chain-like structures. Using electromagnets, the magnetic field strength can be adjusted based on the operating conditions of the bearing and the bearing can be semi-actively controlled. The objective of this thesis is to create a visual demonstrator setup, intended for educational purposes, that can be used to investigate the pressure distribution and generated forces in a hydrodynamic half journal bearing using MR fluid and electromagnets. Furthermore, the research objective is to increase the maximum load capacity for a specified minimum film thickness, while not increasing the friction in the hydrodynamic lubrication regime. The test setup was based on an existing setup made by GUNT which is able to show the pressure distribution in a half journal bearing using pressure tubes for various eccentricity ratios. Adaptations to this setup were made to measure the generated forces (horizontal force, vertical force and friction force, defined as the frictional moment divided by the radius of the bearing) and to implement an electromagnet. First, experiments using hydraulic oil were performed to test and improve the performance of the setup. Afterwards, experiments using MR fluid were performed with varying magnetic field strengths. Furthermore, a numerical model based on the two-dimensional Reynolds equation was created and validated with the experimental results. Both the numerical and experimental results showed comparable trends of increasing pressure, horizontal force and vertical force, meaning an increasing load capacity, for increasing eccentricity ratio and applied current. The numerical friction force also increased with increasing eccentricity ratio and applied current, while the experimental friction force only increased with increasing applied current and did not show a clear trend with increasing eccentricity ratio. The magnitude of the forces differed significantly between the experimental and numerical results, where the largest differences were observed for the vertical force and the friction force. Many plausible causes for the differences are identified, such as bearing tolerances, uncertainties in eccentricity and modeling simplifications. A combination of these causes is believed to lead to the observed differences. The increase in pressure, horizontal force and vertical force due to an increase in applied current was found to be much smaller experimentally than numerically. This was thought to be caused by a larger temperature increase in the experimental tests than was modeled and by simplifications used in the numerical model such as neglecting the shearthinning characteristics of MR fluids. The friction force differed even more significantly between the numerical and experimental results. Additionally, this is caused by the small magnitude of the friction force in combination with a test setup that is not accurate enough to measure small forces consistently. The created setup can show the change in pressure distribution when the applied current or eccentricity is changed. It therefore serves as a visual demonstrator setup for an MR lubricated hydrodynamic half journal bearing which can be used for educational purposes. However, demonstrator performance was found to be far from ideal as the changes in fluid column height were very slow because of the high MR fluid viscosity. Performance can potentially be improved by using a different or diluted MR fluid. Despite the large differences between the numerical and experimental results, both can be used to display the potential of semi-active control using MR fluid and electromagnets. It is shown that the maximum load capacity can be increased while not increasing the friction coefficient in the hydrodynamic lubrication regime. Several recommendations have been made for changes to the demonstrator setup to improve performance while keeping the setup simple and elegant. The main recommendations include improving the bearing tolerances, testing with a different or diluted MR fluid, implementing capacitive distance sensors and performing experiments at a more consistent ambient temperature.