Tribo-dynamics of high speed precision spindle bearings
2013-08-13T14:14:19Z (GMT) by
The demand for higher productivity and improved quality of the machined surfaces requires spindle designers to aim for higher spindle speeds and feed rates. For instance, in routing of non-ferrous metals, wood and plastics, spindle speeds of the order of 60,000 rpm have been achieved with the help of better assembly aiming for higher operational accuracy, and increased component quality. The limitations on attainable speeds and quality of surface finish are partly governed by rolling element bearing vibrations. Dynamic performance of a rolling bearing has been of considerable interest. Although fatigue life is sometimes a significant parameter, it has been fairly well established that in the case of many practical applications, bearing failure is caused by dynamic instability in the motion of the rolling elements. Ball bearings are widely used for high speed spindles, where high speeds, high temperatures, and heavy load/forces are encountered, because they offer low friction, appropriate stiffness characteristics and minimum starting torque. Dynamic stiffness of a bearing is very important characteristic in bearing design for achieving precise tolerances, which has been the concern of industry, especially wood machining industry. Due to poor machining the energy consumed in manufacturing better quality product is very high. Higher energy consumption and the time to carry out extra finishing processes is a major concern. This thesis provides a numerical model incorporating inertial dynamics of vertical routing spindle with five degrees of freedom simulating an existing routing spindle, which can run at speeds up to 60,000 rpm (Le. 2.4 million DN). An experimental rig is also devised to investigate the vibration spectra the high speed precision 7.5 kW power routing spindle. The vibrations generated in the high speed precision spindles have amplitudes in the order of microns. Fine measurement of spindle vibration characteristics are carried out using laser vibrometry. Use of this technique is quite novel for high speed applications, with precise resolution of rigid body motions with fine alignment of a single laser beam with respect to an optically smooth surface of a specially designed tool held in the spindle collet. The experimental spectra are compared with the numerical model predictions with very good agreement. . Numerical model for grease lubrication· capable of solving thermal elastohydrodynamic lubrication of rough surfaces with combined entraining and squeeze motions is developed. For this a modified Reynolds equation is derived from basic principles considering grease as a Bingham solid using the Herschel Bulkley flow model. Heat generation and the power loss in contact conjunction due to viscous shear and compressive action is quantified by solving the energy equation. Boundary interactions due to adhesive and ploughing friction are taken into account. The power lost due to friction and viscous shear was found to account for 16% of the total input power at the speed of20,000 rpm.