Development of the thermo-mechanical finite element model of the rotating units

Abstract

High-speed machining is a particularly important industrial process during which material is removed using a cutting tool with rotational and translational motion. The rotating unit is a major part of the machine tool, and is composed of a bearing, spindle, and associated surrounding components. During machining, the rotating unit is exposed to large mechanical loads, as well as significant heat generated due to friction in the bearings, which can lead to thermal deformation and failure of the spindle. To achieve reliability of the spindle system, an accurate description of the thermo-mechanical behavior of the system is useful to predict the thermal effects during the design phase. Existing methods to simulate the thermo-mechanical behavior of rotating parts require significant computational resources because of the geometric complexity of the models. In this dissertation we discuss a finite element method (FEM) approach for calculating the thermo-mechanical behavior of the rotating unit using the commercially available software package ANSYS. We modeled the heat generated in angular contact ball bearings, and applied the heat transfer of a rotating cylinder to calculate thermal load. We explored the characteristics of the spindle system, specifically angular contact ball bearings, which are commonly used in machine tools. We carried out a geometric simplification process for angular contact ball bearings out to overcome the divergence phenomenon in the FEM simulation, which may occur because of contact between rolling elements and bearing races. We used Matrix27 elements to simulate the stiffness/damping characteristics of ball bearings, and used the ‘close gap’ function in the contact elements to implement heat transfer between the inner and outer races of the bearing. We used the thermal contact conductance to describe heat transfer between the surfaces of the bearing using the close gap function. We also determined a constant describing the thermal contact conductance for several types of bearing using a parametric simulation study, and compared the results with experimental data to determine the value of the constant. We determined the fraction of the heat flux to the inner and outer races empirically via comparisons with experimental data. Next, we carried out simulations of the rotating unit using the results of this simulation. We analyzed the thermo-mechanical behavior of the rotating unit, and compared the simulated temperature distribution and thermal deformation of the spindle tip with experimental data. We investigated three types of angular contact ball bearings, each with three different preloads. We verified the simulation results by comparing them with experimental data from a motorized rotating unit.