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

**Abstract:** Through a series of comparative experiments involving thousands of crankshafts and extensive industry research, the root cause of the third main journal runout in crankshafts was identified, along with an effective solution. **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 force and torque necessary for vehicle movement. During assembly, precise alignment between the main journals is essential to prevent issues such as "locking" or "burning" during engine operation. Our factory (Yunnan Internal Combustion Engine Plant) has long struggled with high runout rates—often exceeding 20%–40%—after nitriding, with values beyond 0.08 mm. Initially, we suspected the problem stemmed from the nitriding or machining stages. However, even after optimizing these processes, including strict control of nitriding parameters and improving machining precision, the issue remained unresolved. This led us to investigate further. **1 Causes of High Runout After Nitriding** To address this persistent issue, we conducted extensive comparative experiments on thousands of crankshafts and studied similar cases across the industry. The results revealed that the primary cause of the high runout was the presence of significant internal casting stresses before nitriding. Ductile iron typically has higher internal stress than gray iron, and when nitrided at temperatures between 560°C and 600°C, which falls within the temperature range for stress relief, these stresses were released and redistributed, causing deformation. Therefore, the nitriding process itself is not the root cause but rather a symptom of underlying stress issues in the casting. **2 Analysis of Main Causes of High Internal Stress Before Nitriding** Comparative data from our factory and Xichai showed that while both used similar heat treatment processes, our crankshafts still exhibited higher runout rates. This indicated that our internal stresses were more pronounced. Two key factors were identified: (1) **Uneven Cooling During Normalizing**: Our factory used fog cooling, which resulted in uneven cooling and localized hardness variations. This led to non-uniform pearlite distribution and increased internal stress. For example, some crankshafts showed hardness differences of up to HB60–80 on the same part. (2) **Non-uniform Heating in Qualitative Furnaces**: Our qualitative equipment, a reverberatory furnace, had limited temperature control. With only one thermocouple, it was difficult to ensure even heating, leading to inconsistent stress relief. In contrast, Xichai used a continuous resistance furnace with multiple temperature zones, allowing for more uniform heating and better stress management. **3 Solutions and Improvements** Based on our findings, we proposed two main solutions: (1) **Improve Normalizing Quality**: Ensure uniform cooling and consistent hardness and pearlite content. This reduces internal stress during the normalizing phase. (2) **Upgrade Qualitative Process**: Replace the current reverberatory furnace with a more advanced system, such as a continuous resistance furnace, to allow for better temperature control and uniform heating. Additionally, placing the crankshafts in a way that avoids mutual contact during heating can reduce deformation. We also experimented with alloying the castings by adding copper to increase pearlite content, which helped stabilize the structure during cooling. Using air-cooling instead of water-cooling allowed for more controlled cooling, reducing residual stress. The final results were promising, with a third main journal runout rate of just 5.36%, a significant improvement. In addition, we recommended using medium frequency quenching and fillet rolling to correct any deformation caused in earlier stages. These methods are widely used in major automotive manufacturers and have proven effective. For crankshafts already out of tolerance, thermal correction before tempering was implemented, and all tested parts met design specifications. These improvements provided a solid theoretical and practical foundation for resolving the long-standing issue of high runout in our crankshafts.

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