Analysis and Solution of the Exceedance of the Third Mainshaft Bounce of Yunan 4100QB Crankshaft

**Abstract:** Through a series of comparative process experiments involving thousands of crankshafts and extensive industry research across the country, the root cause of the third main journal runout in crankshafts was identified, and an effective solution was proposed. **Keywords:** Crankshaft third main journal runout; casting internal stress; normalizing; qualitative treatment **Classification number:** TG162.7 **Document code:** B **Article ID:** 1001-2265 (2000) 02-0039-03 The crankshaft is a critical component in diesel engines, responsible for transmitting the necessary force and torque to the engine's motion. Due to its role in assembly, the alignment of the main journals must meet strict tolerances to prevent issues such as "locking" or "burning of bearings," which can lead to serious quality failures. At our factory (Yunnan Internal Combustion Engine Plant), the third main journal runout rate after nitriding has consistently exceeded 20% to 40%, with some cases surpassing 0.08 mm. Initially, many believed the issue stemmed from the nitriding or machining processes. However, even after improving machining precision and strictly controlling nitriding parameters, the problem remained unresolved. **1 Causes of High Runout Rate After Nitriding** To address this persistent issue, we conducted numerous comparative experiments on thousands of crankshafts and gathered insights from similar industries nationwide. The analysis revealed that the primary cause was high internal casting stress before nitriding. Ductile iron typically has higher internal stress than gray iron, and during nitriding at temperatures between 560°C and 600°C, these stresses are released and redistributed, causing deformation and increasing runout. Thus, the nitriding process itself is not the root cause but rather a manifestation of the underlying stress issue. **2 Analysis of Main Causes of High Internal Stress Before Nitriding** By comparing our process with that of Xichai, we found that our normalizing and qualitative steps were suboptimal. Our normalizing used fog cooling, leading to uneven cooling and increased internal stress. Additionally, our qualitative process involved a reverberatory furnace with limited temperature control, resulting in inconsistent heating and stress distribution. **3 Measures Taken** To resolve the issue, we focused on two key areas: improving the uniformity of normalizing and enhancing the qualitative process. We introduced air-cooling and electric furnace tempering, which significantly reduced runout. The experimental results showed a successful reduction in runout to 5.36%, proving the effectiveness of the new approach. Furthermore, we recommend using medium frequency quenching and fillet rolling to correct deformations caused by previous processes. For out-of-tolerance crankshafts, thermal correction methods were applied, and all tested samples met design requirements. These improvements have provided a solid theoretical and practical foundation for addressing long-standing quality issues in our production line.

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