Análisis de la influencia de las tolerancias geométricas en el comportamiento del conjunto eje-chumacera mediante análisis numérico del contacto mecánico

[SPA] La línea del eje de la hélice o propulsor de un vehículo sumergible o submarino, tiene como función principal la de transmitir la potencia de la maquinaria propulsora a la hélice o propulsor. El correcto ajuste y mecanizado del eje y los soportes de apoyo del mismo, llamados chumaceras, alargarían el periodo útil de funcionamiento, disminuyendo el riesgo de fallo por fatiga superficial o picadura. En la presente Tesis Doctoral, se estudia y analiza el mejor ajuste del eje del propulsor de un vehículo sumergible a su chumacera, por medio del estudio de la geometría de contacto entre dichos elementos, i.e. sus tensiones subsuperficiales en dicha zona. Para realizar este estudio, se ha aplicado un método semianalítico basado en las normas de la cuadratura de los principales valores de Cauchy, el método de la secante, y un método numérico para la determinación del mapa tensional de las superficies en contacto entre eje y chumacera. Asumiendo que existen imperfecciones de micras en el mecanizado de las superficies en contacto, son estos una posible causa de daños subsuperficiales que favorecen fallos por picaduras (pitting), o formación y el crecimiento de las grietas por fatiga. Estos fallos afectarán no solo al buen rendimiento propulsivo, sino también a las condiciones de estanqueidad con los consiguientes riesgos que ello suponen para la integridad del buque. Los resultados permiten establecer los criterios de diseño en función de los módulos de Young, y la importancia de un correcto mecanizado, y ajuste posterior del sistema eje-chumacera, mejorando por tanto la fiabilidad de dichos elementos. El efecto del arrastre de lubricante mediante el giro del eje, cuando este no sea concéntrico axialmente con la chumacera, y bajo condiciones hidrodinámicas del lubricante ha sido objeto de estudio de esta Tesis. Para conseguir este objetivo se ha desarrollado un método que resuelve la relación de la viscosidad dinámica del lubricante, y el gradiente de presiones, en función del ángulo de desalineamiento del eje con respecto a la chumacera. Por otro lado, la chumacera debe estar lo suficientemente apretada por los tornillos que sujetan las dos partes que la componen. Es por ello necesario estudiar la influencia que tienen los pares de apriete en los tornillos de unión de las dos piezas que forman la chumacera sobre la calidad del mecanizado interior. Para ello, se ha desarrollado un método que permita calcular de una forma directa el reparto de cargas en la superficie de contacto eje-chumacera, usando una formulación matricial basada en condiciones de Kuhn Tücker, y los multiplicadores de Lagrange, para la minimización de la energía de deformación. Los valores teóricos de deformación obtenidos, se han validado en primer lugar, con respecto a los resultados obtenidos mediante el uso del Método de los Elementos Finitos (MEF), y mediante ensayos experimentales. En estos ensayos, se han obtenido resultados de la rugosidad y redondez a partir del medidor de formas compacto y los datos recogidos de un proyector de perfiles, referente a todas las operaciones de mandrinado en las chumaceras bajo distintas condiciones de operación. Finalmente, los métodos anteriormente descritos, relacionados con el reparto de cargas de contacto y el efecto del régimen hidrodinámico, se han aplicado de forma integrada a un ejemplo práctico, con el fin de demostrar la utilidad en el uso conjunto de estos métodos, no existente en la actualidad tanto en el ámbito del diseño de estos elementos como en el desarrollo de nuevos procedimientos de mantenimiento predictivo enfocados al sector naval. Para ello, inicialmente se ha partido de unos valores fijados con anterioridad, y que normalmente pueden venir impuestos en el diseño previo del sistema propulsor.[ENG] The line of the shaft of a submersible vehicle has the function of transmitting the power from the propulsion machinery to the helix. The correct setting and machining of the shaft and the supports, so called bearings, can help to extend their life cycle, thus reducing the risk of surface fatigue failure or pitting. The research work presented in this Thesis, analyzes the best attachment of the propeller shaft of a submersible to his bearing, by studying the contact geometry and the subsurface stresses in the contact area between those elements. For this study, a semi-analytical method based on the quadrature rules of the Cauchy Principal Values, the secant method and a numerical method for determining the tension map, have been applied to the study of the contact surfaces between shaft and bearing. Throughout this study, it is assumed that precision errors in the order of microns when machining the contact surfaces may appear. Those errors can produce subsurface damage, which possibly linked to the pitting and fatigue cracksgrowth. Such failures not only will influence on the proper propulsive performance, but also on the watertightness conditions, which are related to the risk of ship integrity loss. The obtained results let us to set a design criteria, which account for the Young's module and the importance of an adequate machining and further adjustment of clearances on the shaft-bearing system, thus improving the reliability of such elements. In addition, the deformation stresses produced by the hydrodynamic dragging of the lubricant over the rotational axis has been assessed by the determination of a mathematical integration model, which poses an alternative methodology, which helps to establish new criteria for the design of the above mentioned elements. Furthermore, the influence of tightening torque and machining variables, on roundness and surface roughness of the bearing, has been studied by means of a method that directly calculate the load distribution on the contact line between bearing and shaft. This method is based on a matrix formulation based on the Kuhn Tucker conditions and Lagrange multipliers, which minimizes the strain energy. The theoretical strain values obtained by the above methods have been firstly validated with respect to the results obtained by Finite Element Method (FEM) and by additional experimental tests. As a result, measures of roughness and roundness were obtained by different metrological methods for different boring operations carried out on the bearings. Finally, the methods described above related to the distribution of contact loads and the effect of hydrodynamic regime have been integrated and jointly applied to a practical example. It is noted that the use of this integrated methodology can serve as a new predictive maintenance procedure within the naval sector. In order to overcome this objective, a set of design values has been previously defined for the propeller system.Universidad Politécnica de CartagenaPrograma de doctorado en Tecnologías Industriale

Enhanced Positioning Algorithm Using a Single Image in an LCD-Camera System by Mesh Elements' Recalculation and Angle Error Orientation

In this article, we present a method to position the tool in a micromachine system based on a camera-LCD screen positioning system that also provides information about angular deviations of the tool axis during its running. Both position and angular deviations are obtained by reducing a matrix of LEDs in the image to a single rectangle in the conical perspective that is treated by a photogrammetry method. This method computes the coordinates and orientation of the camera with respect to the fixed screen coordinate system. The used image consists of 5 × 5 lit LEDs, which are analyzed by the algorithm to determine a rectangle with known dimensions. The coordinates of the vertices of the rectangle in space are obtained by an inverse perspective computation from the image. The method presents a good approximation of the central point of the rectangle and provides the inclination of the workpiece with respect to the LCD screen reference system of coordinates. A test of the method is designed with the assistance of a Coordinate Measurement Machine (CMM) to check the accuracy of the positioning method. The performed test delivers a good accuracy in the position measurement of the designed method. A high dispersion in the angular deviation is detected, although the orientation of the inclination is appropriate in almost every case. This is due to the small values of the angles that makes the trigonometric function approximations very erratic. This method is a good starting point for the compensation of angular deviation in vision based micromachine tools, which is the principal source of errors in these operations and represents the main volume in the cost of machine elements’ parts.The authors want to thank the University Center of Defense at the Spanish Air Force Academy, MDE-UPCT, for financial suppor

Contact mechanics applied to the machining of thin rings

Precision machining of thin rings is of key importance in the performance of many mechanical components such as bearings, rings, turbines, etc. An important factor to take into account is to know the influence of the clamping forces values at different angular positions of the workpiece in the geometrical tolerances after machining. The lower the clamping force, better tolerances will be achieved, but with the disadvantage of reducing friction force and, therefore, increasing the risk of slipping. Therefore, achieving a minimum but safe clamping force is a key factor to control the process. This paper presents some contributions of contact mechanics to the determination of an optimum clamping force. A subsequent methodology is applied that takes into account model of bulk deformation and local contact stresses and experimental data with the object of obtain the optimum torque applied to the chuck.The authors would like to thank the Basque Government for supporting this work made under the ETORTEK Program within the MARGUNE CRC framework while the first author was a visiting professor at TECNUN