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Small, self-lubricated porous journal bearings are widely applied in all kind of different machinery and applications and produced in enormous quantities. Their low cost, high resistance with respect to impacts and overload make them a very important and successful bearing solution to support a rotating shaft carrying a radial load. For small journal bearings, supporting shafts with diameters in the range of mm, as applied in small machines, standard techniques of friction analysis cannot be used, because the boundary conditions of the specific application cannot be represented. Due to this, different friction values result when compared to the real application and the measurements obtained are not precise enough. This work addresses systematic investigations of small journal bearings lubricated with Ionic Liquids, which present a novel generation of fluids with promising properties and good lubrication performance. A setup was designed and built so as to represent the application of such bearing systems in a small electric motor of 20mm in diameter and imposes identical mechanical boundary conditions as experienced in such motors. A nominal shaft diameter of 3mm was used for the experiments. The important parameter of the radial clearance was altered by using precise shafts of different diameters. The radial loading on the bearings was varied in the range from 0.2N to 10N. A novel procedure for calibration allows to take into account deviations from perfect symmetry, as they are encountered on every part of the mechanical system. This new calibration allows to significantly increase measurement precision. In the case of miniature bearings, as they are used in this work, deviations from the ideal geometry cannot be neglected because measurement precision would suffer dramatically. The setup developed allows for the simultaneous detection of friction and relative displacement between shaft and bearing, a key parameter in hydrodynamic lubrication theory. This gives additional insight into the bearing kinematics and allows to increase the precision of friction measurements even more, if measured values of shaft-bearing eccentricity are considered for the calculation of the friction torque. Using the developed setup, a nominal resolution of 50 nNm could be obtained with displacement measurements recorded at 150 nm nominal resolution. An analysis of the displacement signal in the frequency domain was performed in order to give additional insight into the system dynamics and detection of whirling phenomena, which typically come along with an increase in friction. To avoid any misinterpretation of measured friction data, knowledge of the whirling state is important. Viscous heating in the lubricant film strongly effects the lubricant temperature and therefore its viscosity and was also taken into account. Considering effects of viscous heating is especially important at high speeds. The setup under test allows for rotational speeds up to 25000 rpm. Using a combined experimental, numerical and analytical approach, a thermal model was developed and lubricant temperature and therefore fluid viscosity can be determined for Stribeck measurements under variation of speed. Experiments were performed using Newtonian viscosity standards, which provide constant viscosity over a wide range of shear rates. The effect of porous bearing material on friction was investigated as well, under variation of relevant parameters as rotational speed and radial clearance, applying the corresponding theory and the measurement technique developed. Several selected Ionic Liquids were applied as lubricant in porous, selflubricating journal bearings and compared to an established standard lubricant. Using these new fluids, the measured friction can be described applying the same models as in the case of the Newtonian reference oil. It was found that in the domain of mixed lubrication several low viscosity Ionic Liquids provide lower friction than the highly developed, non-Newtonian standard lubricant taken as reference. |
