In this session, we will present an approach developed to set optimum low pressure blade tolerances in order to reduce machining cost and satisfy appropriate blade reliability and aero performance requirements. The core of the newly developed methodology is the method of Geometry Deviations Decomposition. This method allows to decompose any complex blade manufacturing geometry deviations into a set of Basic Blade Geometry Deviations (BBGD). Sensitivity matrixes for features of the blade design deemed critical to bucket reliability and aero performance (Critical To Quality, CTQs) are calculated by finite element codes using each BBGD. Combining them into simple matrix equations enables simulation of behavior of all the buckets and bucket channels in the row. Deviations of the bucket design CTQs for all possible combinations of bucket geometry deviations are evaluated by superposition of responses from each BBGD for each blade in the blade row. Applying random geometry deviations that are within manufacturing tolerances gives random output for design CTQs for the whole bucket row. Post-processing of the output by statistical tools allows calculation of the quality level for each CTQ and comparison to established requirements. An inverse problem has also been solved: determination of the optimum tolerance ranges that satisfy blade and blade row quality levels (CTQ specification limits). So obtained maximum values of tolerances reduce machining process cost.

Key Words: Manufacturing Tolerance Ranges, Long Buckets Reliability, Aero Performance