Abstract

The connecting rod assembly transmits the force exerted on the piston to the crankshaft, converting it into torque and transforming the piston's reciprocating motion into the crankshaft's rotational motion. The connecting rod assembly consists of the connecting rod body (small end, rod shank, big end), connecting rod cap, connecting rod bolts, and connecting rod bearings. During operation, the connecting rod bearings endure the impact load of in-cylinder combustion pressure and the dynamic load from the inertial forces of the piston and connecting rod assembly, accompanied by high relative sliding speeds. Therefore, the bearings must exhibit high mechanical strength (fatigue resistance), heat resistance (thermal hardness, thermal strength), and adequate anti-friction properties. Common failure modes include metal layer peeling, melting, adhesion, tearing, uneven wear, and cracking.

Generally, two main factors lead to the fracture of connecting rod bearings:

1. Bearing Quality Defects: Connecting rod bearings typically employ forced lubrication. Pressurized lubricating oil forms a high-pressure wedge-shaped oil film through the relative motion between the journal and bearing, transmitting pressure and shear forces to the bearing's alloy layer. If there are initial cracks in the alloy layer or steel backing, fractures may occur after a short period of operation.

2. Fatigue-Induced Cracks in the Alloy Layer: If the bonding between the alloy layer and the backing is poor, periodic alternating loads can create microcracks due to excessive compressive and bending stresses. Over time, these microcracks may develop into macroscopic cracks.

However, not all bearing fractures result from the aforementioned causes. This paper analyzes the fracture of connecting rod bearings in a specific medium-speed diesel engine and proposes improvement measures.

1. Fault Description
In 2018, during the maintenance of a medium-speed diesel engine, cracks and crushing were found on the upper connecting rod bearings. Indentations and collapse were observed around the oil holes and grooves of the connecting rod body. All upper connecting rod bearings and two connecting rods were deemed unusable and scrapped (see Fig. 1).

The affected engines had operated for approximately 6,000 hours. Even the intact portions of the upper bearings showed localized bright bands of wear, with bending deformation and wear marks on the backing.

Similarly, in 2018, returned connecting rod bearings from the same engine type, which had operated for around 6,200 hours, exhibited cracks on the steel backing of the upper bearing. Bending deformation and wear marks were also present near the oil holes and grooves, while the wear on the bearing surface was uneven. Areas corresponding to the grooves and oil holes showed no wear due to deformation, but the regions outside the oil holes exhibited significant wear.

2. Fault Cause Analysis
(1) Structure of Oil Holes and Grooves on the Connecting Rod
The pistons in this engine utilize free-spray cooling. Cooling oil flows through the crankshaft main journal oil holes, connecting rod journal oil holes, connecting rod bearing oil holes, circumferential grooves in the connecting rod big end, connecting rod shank oil holes, and finally into the cooling chamber via the small-end nozzle.

However, structural analysis reveals that the placement of grooves and holes in high-load areas of the upper bearing reduces its load-bearing capacity to one-fourth of its ungrooved counterpart. The grooves and holes should ideally be positioned in low-load areas to ensure smooth oil flow without compromising bearing strength.

Simulations confirm that the maximum stress in the upper bearing's load-bearing area reaches 535.11 MPa, far exceeding the steel backing's allowable stress of 250 MPa, leading to deformation and fractures.

(2) Bearing Thickness
The thickness ratio of the bearings in this engine is lower than standard thin-wall bearings, making them prone to bending deformation under stress. This deformation exacerbates the formation of cracks and subsequent mechanical failures, especially in areas of stress concentration around grooves and holes.

3. Improvement Measures
Based on the analysis, the following improvements were made:

• The design of oil holes and grooves was revised to avoid high-load areas. A transverse oil hole configuration was adopted to reduce stress in the main load-bearing zones.

• Simulations show a 56.85% reduction in maximum stress, decreasing from 535.11 MPa to 230.92 MPa.

Durability tests confirmed the effectiveness of these modifications, as no fractures were observed in the bearings after testing.

4. Conclusion

1. The root cause of the bearing fractures was the improper placement of grooves and oil holes in high-load areas.

2. Bending stress induced by thin bearing walls led to early wear, stress concentration, and eventual fractures.

3. Redesigning the lubrication system to avoid high-stress areas effectively resolves the issue.