چكيده به لاتين
Machine tool lubricated linear guideways, along with their general design, are one of the most complex systems in the engineering sciences. In this thesis, by stating the history, the importance of identifying and extracting nonlinear behaviors of sliding contacts is discussed in order to accurately explain the dynamic behavior of a real machine tool. In order to better design the machine tool and ensure that the machine does not fail under different operating conditions and has good machining quality, in the design process, it is necessary to consider sliding contacts, which are the best source of deformation and energy loss. Therefore, the dynamic behavior of gantry machine tools will be highly dependent on their sliding and moving contacts. Also, changes in the dynamic characteristics of machine tools strongly affect the quality of machining of parts, the occurrence of the phenomenon of self-excitation vibrations and the stability of the machine. Therefore, it is necessary to carefully examine the dynamic characteristics of the machine tool along with the sliding connections. For this purpose and to have an accurate and complete description of the nonlinear dynamic behavior of lubricated machine tool contacts, in this thesis a new method of modeling and identification of nonlinear behaviors in normal direction is presented. This nonlinear normal model, developed based on the expansion of the Taylor series of oil films and roughness forces. Using the least squares technique, the Taylor series coefficients of the nonlinear model in the time domain are identified. The subset selection method was used to select the sentences that have the greatest impact on the nonlinear forces of the oil film. Also, the effects of friction in pre-sliding and sliding situations are analyzed. During machining of machine tools and due to the presence of shear forces, sliding contacts are exposed to oscillating excitations and therefore it is necessary to study sliding properties in addition to pre-sliding behaviors. In this model, the effects of coupling effects between tangential and normal forces are considered. To validate the performance of the proposed model, an experimental test was performed on a lubricated linear guideway. Comparing the results between the model and the experiment shows that the modified LuGre model in this thesis can well predict the dynamic frictional behaviors.
Linear guideways play a crucial role in determining precision of machine tools. Understanding their dynamic response is essential for objectively controlling their behavior and performance in operation. Due to highly loaded lubricated contacts, mixed-elastohydrodynamic (mixed-EHL) regime is dominant. The mixed-EHL film maintains the coupling between horizontal degree of freedom (feed velocity) and vertical degree of freedom (loading direction). This paper presents a novel tribo-dynamic solution for linear guideways, taking in to account the lubricant effects and coupling between horizontal and vertical degrees of freedom. An analytical tribology model is used implicitly within the dynamic model. For in-depth tribological quantities including pressure and film thickness distribution, an explicit full numerical solution for mixed-EHL is utilized. Results show that the coupled solution of vertical and horizontal degrees of freedom taking in to account lubricated contacts is essential. It is shown that at moderate and light loads, the effect of this coupling and presence of lubricant is more pronounced.
Finally this thesis presents an experimentally validated tribo-dynamic model for linear guideways considering coupling between transverse and vertical motions and taking into account lubricant starvation. The dynamic model is coupled with an implicit tribological method for the starved lubricated contacts. The friction in this implicit tribo-dynamic model is calculated considering thermal and non-Newtonian effects. The inlet starvation is considered by incorporating potential flow method. After completing the implicit tribo-dynamic model with starvation effects, load and contact kinematics are provided to a full numerical mixed-elastohydrodynamic analysis of the roller to obtain detailed pressure and film thickness distributions. This explicit model also considers the effect of starvation by adjusting the inlet for computational domain to the one calculated by starvation approach. Results show that starvation has pronounced effect on the film thickness, friction and contact load by up to 32%. The severity of starvation effect is frequency dependent. It is also revealed that the starvation effect can be controlled by the amount of preload on linear guideway.
