Case Study: Crankpin Bearing Seizure

 

 To investigate the seizure of the crankpin bearing in generator No. 3 of a ship operating we conducted an on-site inspection aboard the ship to identify the root cause. The information provided was as follows:

1. Disassembly and reassembly of covers and pistons are performed for maintenance purposes.
2. After maintenance, the machinery is reassembled.
3. The machine is started and operated for a continuous duration of 15 minutes.
4. Engine connecting rod axial movement is inspected by opening the crankcase covers.
5. An issue of overheating or failure is observed in crankpin bearing No. 1.
   

Figure 1 - From left to right - No. 1 crankpin, No. 1 upper crankpin bearing, No. 1 lower crankpin bearing

Firstly, the No. 1 piston connecting rod, which had been removed by the ship's crew, was tightened without the bearing in place. The inner diameter measurement was taken using a micrometer, and it was determined that there was an ovality of nearly 30%.

Figure 2 - Example Demonstration of Connecting Rod Measurement

 

 

Figure 3 - Connecting Rod Measurement Points

In Figure 3, the measured connecting rod dimensions for points A-A', B-B', and C-C' were found to be 177.80 mm, 178.10 mm, and 177.82 mm, respectively. When assessing ovality the difference between the highest and lowest measured values is taken into account. In this case we can calculate the ovality as 178.10-177.82=0.28. According to the manufacturer's standards for the relevant diesel generator the required value should be 0.08 mm.

Crankpin experiences high temperatures during bearing seizure. Elevated temperatures can lead to undesirable changes in the material's microstructure. One of these changes involves its hardness. Considering this information, hardness measurements were required to observe the alterations in the crankpin due to these effects.

 

Figure 4 -No 1 Crankpin polishing

For accurate hardness measurement, the surface needs to be cleaned first. To achieve this, a polishing process is performed using sandpaper with progressively finer grits, ranging from coarse to fine particles.

Hardness measurements were performed using a portable measurement device. The measurements were conducted in Shore (HS) hardness unit because the unit of HS was the one in which the manufacturer provided hardness standards. As a result of the measurements, an average hardness of 52 HS was determined. Softening was identified in the crankpin, which should ideally fall within the range of 65 to 85 HS.

Figure 5 -No 1 Crankpin Hardness Measurement

 Investigating the cause of these incidents and how the ovality occurred led us to the conclusion that a small but significant detail had been overlooked. The manufacturer places a mark on the both connecting rod and connecting rod cap when they leave the factory. These marks should be in the same direction, and each cylinder's cap should be connected to its corresponding rod. Through inspections, it was determined that the caps of cylinder 1 and cylinder 2 had been swapped. As a result of this mix-up ovality was detected in the rods.

 

Figure 6 - Direction of Set Marks as Shown in the Manufacturer's Manual [1]

Incorrectly attaching the caps causes high ovality, which in turn leads to the disruption of the oil film and the resulting lack of lubrication. This results in elevated temperatures and damage to both interacting surfaces.

 

Figure 7 - Mismatched Marks on Cap and Connecting Rod (2–1 and 2–2, 2–1 should have been mounted on 2–1)

During the assembly of machinery, it's crucial to carefully read and follow the manufacturer's instructions. Rushing through tasks should be avoided. If possible, seeking consultation or advisory services from the manufacturer can help prevent such situations from occurring in the first place.

 

 Source
[1] — Yanmar 8N21 Instruction Book