Methodology of evaluating rejects: inspection of crankshaft main journals in repair production

Both the actual size of the part and measurement errors influence the results of tolerance control in repair production. For this purpose, the authors developed the mathematical model of reject estimation by probabilistic characteristics of parts sorting, taking into account the specifics of repair production and the shift of the distribution center relative to the tolerance field. They tested and evaluated the mathematical model of rejects estimation at machining for repair size of crankshaft main journals of the YaMZ‑238 engines. In case of incorrect rejection and incorrect acceptance of main bearing journals, the character of reject losses will be different. Incorrectly rejected crankshafts with journals having irreparable rejects will be sent for recycling. New crankshafts will be purchased to replace them, and there will be reject losses to the left of the scatter zone. Incorrectly rejected crankshafts with journals that have reparable rejects will be sent for remanufacturing. In this case, the loss per crankshaft is determined by the cost of rework, which is the loss to the right of the scatter zone. Incorrect crankshafts with journals having defects to the left and right of the scatter zone will be sent for reassembly. If the largest size limit is exceeded, the liners may rotate and the engine must be rebuilt, which increases losses significantly. In case of violation of the lower size limit, the shaft life will be reduced and losses will occur only for the consumer. In contrast to the existing methodology for calculating the sorting parameters, where the scatter zone coincides with the middle of the tolerance field, the developed methodology for assessing rejects when the scatter zone is displaced relative to the tolerance allowed to form a more differentiated and refined methodological approach to the assessment of losses in disassembly of parts when performing machining operations in repair production.

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