Optimization of pre-stressed press frames

Pre-stressing is a technique often used in presses, and it is obtained either by wire or tape winding or by bolted rods. Pre-stressing is always performed by combining two materials: one which is tougher and used as the pre-stressing medium and one which is less resistant and used as the pre-compressed casing. Pre-stressing can be applied for several reasons: for reducing the cost of manufacturing and assembling the press frames in case of very large tonnage presses, for reducing the deflections and elongations of the frames during pressing operation, for improving the fatigue resistance of the press components, etc. In this paper a simple analytical model of loads, stresses and deformations will be proposed for pre-stressed columns. The results of the model will be validated and compared to the results of realistic FEM simulations of the behavior of the press frame in operation. The equations of the analytical models will be then used as the constraints of a global optimization algorithm, implemented with Mat lab. The results clearly show that, only if the press frame structure is monolithic, it is possible to obtain a solution which is truly optimal by pre-stressing, i.e. by combination of a more expensive and high performance material with a relatively "poor" material. This conclusion seems to be robust with respect to potential noise or uncertainty issues, which in this case are mainly related to the coefficient of the objective functions.