A multiple objective framework for optimal asymmetric tolerance synthesis of mechanical assemblies with degrading components

In order to produce the mechanical assemblies with the high quality, the finest functionality, and the low cost, the optimal tolerance synthesis can be a useful tool in the design stage. The degradation of components due to some operational or environmental factors (such as the thermal cycling, the mechanical deformation, and the wear) can lead to dimensional variations in components and fall-off of the functionality of the product. In addition, different manners of the degradation on the internal and external dimensions can cause asymmetric deviations in the dimensions. On the other hand, the effect of degradation on the product quality has not been considered in most researches. In this paper, a new multi-objective framework is proposed for optimal asymmetric tolerance synthesis of mechanical assemblies with degrading components. In this method, the optimal tolerances are allocated based on ensuring the fulfillment of the product’s functional requirements, maximizing the product quality, and minimizing the total cost over the lifetime of the product. To incorporate the degradation effect into the loss function concept, the present worth of the expected quality loss (PWL) is formulated in terms of asymmetric tolerances. Accordingly, the functional process capability and manufacturing cost are developed based on asymmetric tolerances and the degradation effects. In order to extract Pareto fronts of optimal solutions, the elitist Non-dominated Sorting Genetic Algorithm II as an evolutionary generating methodology is utilized. In solving multi-criteria tolerance synthesis problem by a generating method, selecting the best tolerances from the obtained optimal Pareto solutions is a significant challenge. In this paper, to find the best asymmetric tolerances from Pareto solutions, a combined Shannon’s entropy-based TOPSIS algorithm is used. Finally, a bi-directional non-back drivable roller clutch assembly as an industrial case study is considered to illustrate the efficiency of the proposed method, and the obtained results are compared and discussed for verification.