Ball and roller bearings are kinds of rolling bearing and commonly used machine elements. They are generally used to support rotating motion of shafts in simple commercial devices such as bicycles, in-line skates, and electric motors as well as in complex mechanisms such as aircraft gas turbines, wind mills, power transmissions and main spindle of machine tools.
In rotating machine system such as a main spindle of machine tools, the rigidity and the positional accuracy of the bearing are very important. Operating characteristics of the bearing significantly affects to the performance of machine tools, so it is very important to select the proper bearing at the design stage of machine tools. For the appropriate selection of the bearing for machine tools, it must be considered the bearing load capacity, the driving speed and driving conditions such as temperature. In addition, for the long-term use of the bearing without damage, bearing characteristics must be well identified and the appropriate lubricant and the lubrication conditions have to be applied.
Ball bearings reduce friction during the rotation of the radial and axial direction to support the load. However, the heat generation by friction, the wear, and the power loss are occurred because of the relative motion between the metal materials. Internal friction in the bearing leads to a temperature rise. If the heat generated from inside the bearing is not adequately removed, the system temperature will be continuously increased. When the lubricant is deteriorated due to the temperature rise, the bearing premature breakage will occurs in the end. In addition, the heat generation caused by the friction of the bearing is the main factor of interference for the speedup of the main spindle in machine tools. And it is an important issue in terms of the energy efficiency as like the bearing friction loss at initial operation (starting torque) and the increase of the friction loss due to the driving speed increases (driving torque). Thus, the study on frictional losses and the heat generation caused by the friction of the bearing is extremely important.
Most studies on the calculation and the measurement of the frictional torque are limited to the single bearing behavior, so there is a gap between studies and actual load and operating conditions of the bearing in the mechanical system. In particular, for the bearing supporting the main spindle of machine tools, the increased rigidity due to the preload at the mounting process give a big impact to the performance and accuracy of the machine, therefore it is necessary to study on the frictional torque calculation and the measurement in consideration with actual operating conditions for the spindle system.
Here, we describe a method to estimate the frictional torque in bearings using an empirical formula developed using a method based on bearing analysis tool and the measured frictional torque in a spindle system. The frictional loss was estimated by the obtained frictional torque, through which heat generation characteristics would be predicted. Considering the heat generated by the friction on bearings a thermal analysis has been done with the modeling of the entire system considering convection and conduction conditions. As a result of the analysis the temperature distribution of the entire system including bearings has been obtained. The actual spindle system was built and then the frictional torque and the bearing temperature were measured in the experiment. Results of finite element analysis were compared to the measured temperature by the actual experiment for the verifying the validity of the FEA.