In the design phase of manufacturing, it is necessary to specify the dimensions with critical importance or what is known by the Key Characteristics (KCs). A mechanical system involves multiple functional requirements. In this study each functional requirement considered includes a chain of dimensions having a certain relation. In industrial practices the use of the Failure Mode Effects and Criticality Analysis (FMECA) allows the identification of the critical functional requirement, and then all the involved parts are set as KCs. The production of KCs costs higher since they need special attention, so it is required to reduce their number and specify only the necessary ones. There is not a common method for the identification of the KCs in a critical functional requirement. The strategies followed for this purpose differs from a company to another.

The strategy to be followed for the identification of KCs should take into consideration the tolerance intervals, the capability requirements and the mathematical formulation of the topological loop comprising the studied dimensions. The method proposed in this article consists of considering that all the dimensions involved in a critical functional requirement have equal sensitivities, and then estimate the corresponding tolerance intervals to match what is called “Iso-sensitivity tolerance allocation”. This is done under the conditions that are required by the companies for a high quality production. In this study the condition to be satisfied is that the Non Conformity Rate (NCR) should be equal to a target value. So this is an optimization problem with an objective to have equal sensitivities and equality constraints on the NCR to estimate the tolerance intervals. First a design is proposed and then an optimal design is found after applying the proper optimization algorithm. The estimated tolerance intervals are then compared to reference tolerance values that can be achieved by the manufacturer. So when a tolerance interval for a corresponding dimension cannot be achieved, it is considered as a KC.