Analysis and Treatment of the Causes of Easy fracture of Engine Crankshafts

A CPB14RHH Japanese heavy-duty truck with a Nissan NE6 engine model was found to have burned the connecting rod bearing of the second cylinder due to lubricating oil issues after 300,000 kilometers of driving. After being inspected by a flaw detector, no damage was found on the crankshaft. The engine was repaired after the crankshaft was enlarged and corrected. When driving about 20,000 kilometers, the crankshaft broke at the connection between the third cylinder connecting rod journal and the fourth main shaft journal. We believe that this was caused by poor traffic conditions, where the crankshaft could not bear the rated load after being enlarged, and trucks often operate at rated load or even overloaded. Therefore, a new crankshaft was replaced. After driving over 20,000 kilometers, the crankshaft broke again at the connection between the second cylinder connecting rod journal and the third main shaft journal. This accident has drawn our attention.

The causes of crankshaft fracture

The causes of crankshaft fracture can be summarized as follows:

When overloaded, the engine is often in a state of high-speed or overspeed operation. When driving at high speed, sudden braking is often carried out. The road conditions were poor and the driver operated improperly. The material of the crankshaft is uneven and has poor fatigue strength. The crankshaft itself has hidden damage. The lubricating oil is of poor quality and there is a phenomenon of bearing seizure. The main bearing is loose. The main bearing housing hole is deformed.

Analysis and Detection

1. Inquire

Based on the reasons for the crankshaft fracture mentioned above, we had a conversation with the driver and his assistant to carefully understand the usage of the vehicle. The driver reported that after the engine of the vehicle was repaired, it was powerful, quiet, did not emit black smoke, and the fuel consumption was normal. Recently, the driver has been driving on urban roads at a low speed and has never experienced sudden braking. This driver is a veteran of our company with 20 years of driving experience and is highly skilled. And based on the recent business operation of our company, it can be proved that the above conversation is true. Therefore, reasons such as "overloading, sudden braking, and poor road conditions" can be ruled out.

2. Detection

Apart from the above-mentioned "uneven crankshaft material and poor fatigue strength" which cannot be detected for the time being, after the engine was dissected and observed from the fracture surface of the crankshaft, no old scratches were found. The wear of the mating surface between the connecting rod bearing and the connecting rod bearing is normal, and no bearing seizing phenomenon is observed. Remove the main bearing bush, install the main bearing bush cover, tighten the bolts as per the specified force, and measure the hole diameter with a dial indicator. The deviation is less than 0.01mm, but there are a few scratch marks on the third and fourth seat holes. The main bearing bush cover was removed again. Taking the oil passage hole as the reference, the deviation of the seven holes was measured with a steel ruler. It was found that the third and fourth holes differed by 0.5mm compared with the other holes. It can be seen from this that the cause of crankshaft fracture is the scratch on the hole seat and the deformation of the hole seat.

Repair and treatment

This vehicle has been driven for nearly 400,000 kilometers. Considering that it is approaching the end of its service life, it is not economically viable to replace it with a new cylinder block. However, from the perspective of the overall performance of this vehicle, it still has some utilization value. Therefore, we adopted the machining method: we purchased a new crankshaft and enlarged the main bearing seat hole of the cylinder block by 2mm. According to the enlarged crankshaft hole seat by 2mm, we reconfigured a pair of thickened main bearing shells by 2mm, and processed their inner diameters according to the standard parts of the crankshaft. After the engine was repaired, the vehicle has traveled over 40,000 kilometers. Judging from aspects such as the engine's sound, smoke color, power, and fuel consumption, the performance of this car is normal.