Discussion on the Causes and Preventive Techniques of Diesel Engine Flywheel Imbalance

During the operation of a TY120 bulldozer, the red oil warning light suddenly lit up and the engine made abnormal noises. After stopping the machine for inspection, it was found that the oil level in the engine oil pan was within the standard range, all oil pipes were connected normally, and no other abnormal conditions were found. Therefore, the machine was still used for operation. However, after running for a short while, the engine suddenly stalled and could not be started again.

After removing the oil pan, it was found that the connecting rod shaft neck was severely eroded and the crankshaft bearing bush was melted, thus resulting in a typical "shaft seizure and bearing bush burning" accident. After consultation and analysis, it is believed that this accident was caused by an imbalance resulting from improper installation of the flywheel during engine repair.

The reasons for the imbalance of the flywheel

The function of the flywheel is to increase the rotational inertia of the engine to ensure the uniformity of the rotational speed. In addition, the flywheel is conducive to the starting of the engine, facilitates the smooth passage of the piston through the top dead center, and also enhances the engine's ability to overcome sudden overloading. The materials of flywheels are generally HT200 or ZG270-500, and their main processing techniques are: casting → annealing → machining → static balancing. And before installation, it is essential to carry out dynamic balance calibration of the crankshaft flywheel set on the balancing machine in accordance with the technical requirements of the drawings. Once the flywheel loses balance during use, it must be recalibrated; otherwise, the engine will vibrate more and more severely during operation, causing an "axle seizure and bearing seizure" accident, and sometimes even breaking the related components of the flywheel. This time, during the installation of the flywheel, the unbalance exceeded the specified standard range. As a result, when the external load suddenly changed, it caused intense vibration of the crankshaft. This vibration disrupted the lubrication between the crankshaft and the bearing bush, and a complete oil film could not form on the surface of the bearing bush, resulting in partial dry friction. Due to the adhesive wear caused by dry friction, local high temperatures arise, leading to the melting of the alloy on the bearing bush and resulting in the "seizure of the shaft and burning of the bearing bush" accident.

2. Measures to be taken to prevent the flywheel from losing balance

The fundamental cause of the flywheel losing balance is that the unbalance exceeds the specified value. After a major engine overhaul, a dynamic unbalance test should be conducted on the flywheel. For the 6135 series diesel engines, the maximum allowable unbalance is generally 100g·cm. After the flywheel and the crankshaft are assembled, the total runout at the end of the flywheel plane relative to the crankshaft axis should not exceed 0.2mm.

(2) The flywheel should have no cracks. The working surface should be smooth and clean, and the flatness error should not exceed 0.1mm. Otherwise, it should be flattened. The increase in size should not exceed 1.2mm; otherwise, the thickness of the flywheel after repair will not meet the requirements.

(3) When the working surface of the flywheel is wavy or the groove depth exceeds 0.5mm, it should be polished on the lathe to make it flat. When the depth of the wavy groove does not exceed 0.5mm, no more than two annular grooves are allowed to exist, but burrs should be removed. When the total runout of the end face of the flywheel plane relative to the crankshaft axis exceeds the specified amount, a gasket should be added between the flywheel mating plate and the flywheel for adjustment. It is not allowed to use mechanical processing methods for adjustment.

(4) From the perspective of the technological process, the increase in offset caused by the flywheel casting not being annealed is also an important factor leading to the imbalance of the flywheel. During the casting process of the flywheel, significant internal stress is generated on the spokes. If not annealed, cracks may occur when the spokes in a high-stress state have local defects, uneven thickness or are subject to external force impacts. Meanwhile, for a flywheel that has not been annealed, during processing, balancing, and post-installation use, the internal stress of the flywheel gradually disappears due to natural aging. During the process of stress disappearance, if there are local defects or uneven thickness in the spoke plate, the deformation of the spoke plate along the periphery will be different, resulting in an increase or decrease in the center of mass offset, causing the flywheel to lose its original balance. Therefore, Qing attaches great importance to the annealing process of flywheel castings.