Tribological design of nano/magnetorheological fluid journal bearings
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Μπόμπος, Δημήτριος
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Abstract
The journal bearings are components of machines that support rotating shafts. The available damping capacity and their low friction are some of their advantages. Examples of use of the journal bearings extent from micro applications, such as hard disc drives, to large size applications, such as bearings of gas turbine shafts. With the evolution of "smart" materials, i.e. materials with controllable physical properties, the perspective of active adjustments of journal bearings' performance has been made possible. In this work the magnetorheological (MR) and the nanomagnetorheological (NMR) fluids have been examined as possible candidate fluids for use as lubricants in journal bearings.
MR fluids are solutions of ferromagnetic particles into a conventional mineral oil lubricant. A suitably orientated magnetic field aligns the particles into chains, which hinder the lubricant flow. Thus, the apparent viscosity of the MR fluid is increased. In its active state, the MR fluid tends to behave as a viscoplastic solid. Under a certain amount of shear stress, the particle chains break and the MR fluid flows as a liquid. This amount of shear stress is called yield stress. The magnetic field intensity has the ability to adjust the yield stress of the MR fluid, making the control of the rheological behavior of this type of fluid possible.
A subcategory of the MR fluids is the NMR fluids. The major difference between the two is the smaller particle size of the NMR fluids. In the case of MR fluids, the particles may size from 100 to 10 . The major drawback of these fluids is the sedimentation of the particles. NMR fluids employ particles which size less than 10 and up to several nanometers. The main effect of the decreased particle size is a higher viscosity that may offer significant damping. Additionally, the smaller size of the particles prevents the formation of the intermolecular forces that cause sedimentation. Ultimately while the yield stress of the NMR fluids is rather low, i.e. lower of the MR fluids, the control of the properties of the NMR fluids may become easier.
In this PhD thesis, the simulation of a MR/NMR fluid journal bearing is presented. The effect of the field intensity on the performance of the journal bearing is discussed. The main performance parameters such as the relative eccentricity, the normalized friction coefficient, the lubricant flow and the dynamic coefficients are presented for a range of Sommerfeld number values. The rotordynamic behavior of a shaft using a MR and NMR fluid journal bearing is presented. Moreover, the capacity of the MR fluid journal bearing to compensate for wear present on the bushing of the bearing is examined, using the standard wear pattern present in journal bearings, due to contact between the journal and the bushing.
The rheological behavior of the MR and NMR fluids was studied mostly using a macroscopic viscosity model. However, the influence of the particles on the flow of the lubricant was also examined and useful conclusions were drawn concerning the validity of the macroscopic approach.
Ultimately, the journal bearing performance using an in-situ prepared NMR fluid is examined experimentally. The dynamic coefficients are determined and the orbit of the shaft is presented with and without the presence of the magnetic field.
The most important conclusions drawn from the experiments included in this thesis is the ability of the nanomagnetorheological fluid journal bearing to reduce the extent of orbit, increase its damping capacity and activate rapidly. One major drawback that is matter of future research is the ability of the magnetorheological and nanomagnetorheological fluids to be used with some sort of filtering devices in the lubricant circuit. This would enhance the quality of the base fluid and would allow longer periods between maintenance inspections. All in all, despite some practical aspects that remain to be solved, the ability of control of the load carrying capacity, damping and friction coefficient is significant and certainly worth of further development.
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Tribology, Journal bearings, Magnetorheological fluid, Nanomagnetorheological fluid