Study On Mechanical Properties Of Fluid Lubrication And Viscosity Of Magnetic Fluids

The synthesis and application of nanometer-sized particles have received considerable attention in recent years because of their different physical and chemical properties from those of the bulk materials or individual molecules. When nanometer-sized particles mixed with lubricating oil, the nanometer particles can improve the lubricating oil's abilities of extreme pressure, anti-wear, reduce frictional resistance and repair the friction surface. Research results show that the magnetic fluid is really a new kind of lubricant that is reliable and economical. And there is no leakage and pollution in the process of lubrication under the effect of magnetic field.However, few experimental investigations on the tribological properties of lubricating oils with nano-ferromagnetic particles have been performed under the effect of magntic field. In this paper a controllable and variable magnetic field was got by improving the oil cup of the MS-800 four-ball tester. By this improved four-ball tester, the load capacity of Mn_0.78Zn_0.22Fe₂O₄ magnetic fluid was tested with the magnetic field and without the magnetic field. The results show that, tribological characteristics of the lubricant adding Mn_0.78Zn_0.22Fe₂O₄ nanoparticles improve considerably with the effect of magnetic field. First, the viscosity of magnetic fluids increase with the increasing of magnetic field,the comprehensive load capacity is improved greatly, especially the composite wear value of Mn_0.78Zn_0.22Fe₂O₄ magnetic fluid is 1.43 times of the base oil. Then, Zn element contented in the Mn_0.78Zn_0.22Fe₂O₄ nanoparticles improve the load carrying capacity PB and PD of lubricant, and 26% increasing of PB, 100% increasing of PD is tested. Moreover, the 46# turbine oil doped with 6wt% Mn_0.78Zn_0.22Fe₂O₄ nanoparticles show the best tribological properties among the tested oil samples. When the mass percentage is 6wt%, it is found that 22mT magnetic induction is the optimum magnetic induction.In addition, more investigations were performed by using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) to interpret the possible mechanisms of lubrication and wear with the nanoparticle. It was found that: First, at the effect of short force such as adsorption, faintness electron magnetism field. The thickness and intensity of oil films will be increased which will improve the tribological properties of magnetic fluid. Secondly, new oil film was built because of tribochemical reaction concerned with nanoparticles ernriched in surface under the effect of friction and wear. Thirdly, the worn surface was optimized by nanoparticles, for instance filling the tiny hole and crackle on the surface. Under the effect of magnetic field Mn_0.78Zn_0.22Fe₂O₄ particles are easily deposited and adsorbed on the worn surface, which then formed a protection film due to the shearing effect. This film can smooth and repair the wear scar then improve the wear resistance, load carrying capacity and antifriction ability of Mn_0.78Zn_0.22Fe₂O₄ magnetic fluid.At last, theoretical analysis and numerical study of the lubrication performance for magnetic fluid journal bearings are presented. A general Reynolds equation based on magnetic fluid model is obtained, which can be easily extended to other non-Newtonian fluids and this equation can provide theoretical basis for hydrodynamic analysis of magnetic fluid journal bearings. For the case of static loaded magnetic journal bearings, the influence of magnetic fluid effects on the lubrication performance is studied under various eccentricity ratios, magnetic intensity and concentration. The numerical results show that: with the increasing of concentration, the bearing capacity is obviously increased; the increase magnitude is larger when the eccentricity ratio is large. Under the effect of magnetic field, the bearing capacity increasing with the increasing of magnetic field intensity. When the eccentricity is small, the side leakage is highly decreased. It can be completely eliminate by appropriately designing the bearing geometry and the magnetic field which can't be existed in normal journal bearings. Under the effect of magnetic field little change of the attitude angle is obtained.