1. Introduction
Marine diesel engines play a crucial role in the operation of ships, powering vessels across the world’s oceans. Among the various components of a marine diesel engine, the connecting rod bolt is of particular importance. It serves as a vital link between the connecting rod and other parts, ensuring the smooth transfer of forces during the engine’s operation. However, issues such as connecting rod bolt 脱落故障 (falling off failures) can lead to serious consequences, including engine damage, ship breakdowns, and potential safety hazards. This article aims to comprehensively analyze the causes of marine diesel engine connecting rod bolt falling off failures, propose effective optimization strategies, and provide valuable insights for improving the reliability and performance of marine diesel engines.
2. The Role and Importance of Connecting Rod Bolts in Marine Diesel Engines
2.1 Function within the Engine System
Connecting rod bolts are essential components in marine diesel engines. They are responsible for securely fastening the connecting rod cap to the connecting rod body, maintaining the integrity of the connecting rod assembly. This connection is crucial as it enables the conversion of the reciprocating motion of the piston into the rotational motion of the crankshaft. During the engine’s operation, the connecting rod bolt is subjected to a complex combination of forces, including the high – pressure gas forces from the combustion chamber, the inertial forces generated by the reciprocating motion of the piston and connecting rod, and the dynamic forces resulting from the rotation of the crankshaft. A well – functioning connecting rod bolt ensures that these forces are properly transmitted and that the connecting rod assembly remains stable.
2.2 Impact on Engine Performance and Safety
Any failure of the connecting rod bolt can have a significant impact on the engine’s performance. If a connecting rod bolt falls off or breaks, it can cause the connecting rod to become loose or even separate from the crankshaft. This can lead to severe engine damage, such as scoring of the cylinder walls, damage to the piston and valves, and even complete engine seizure. In addition to engine damage, a connecting rod bolt failure can also pose a serious safety risk to the ship and its crew. In extreme cases, it can cause the engine to stop suddenly, leaving the ship adrift in potentially dangerous waters. Table 1 summarizes the potential consequences of connecting rod bolt failures.
| Consequences of Connecting Rod Bolt Failures | Details |
|---|---|
| Engine Damage | Scoring of cylinder walls, piston and valve damage, engine seizure |
| Performance Degradation | Reduced power output, increased fuel consumption, engine vibrations |
| Safety Hazards | Engine stoppage, ship adrift, potential for collisions or sinking |
3. Common Failure Modes of Connecting Rod Bolts
3.1 Fatigue Fracture
Fatigue fracture is one of the most common failure modes of connecting rod bolts. It occurs when the bolt is repeatedly subjected to cyclic loading beyond its fatigue limit. In marine diesel engines, the connecting rod bolt experiences cyclic loading during each engine cycle due to the alternating forces acting on it. Over time, these cyclic loads can cause cracks to initiate and grow in the bolt, eventually leading to fracture. The fatigue cracks often start at areas of high stress concentration, such as the thread roots or the fillets of the bolt. Figure 1 shows a typical fatigue – fractured connecting rod bolt.
[Insert an image of a fatigue – fractured connecting rod bolt here]
3.2 Plastic Deformation and Fracture
Plastic deformation and fracture can occur when the connecting rod bolt is subjected to excessive loads that exceed its yield strength. This can happen if the bolt is over – tightened during installation or if it experiences abnormal forces during engine operation. When the bolt is over – tightened, the pre – load in the bolt can be much higher than the design value, leading to increased stress levels. During engine operation, if the bolt is subjected to sudden impact loads or high – frequency vibrations, it may also experience plastic deformation. Once the plastic deformation exceeds the bolt’s ductility limit, fracture will occur. Figure 2 shows a connecting rod bolt that has undergone plastic deformation and fracture.
[Insert an image of a plastically deformed and fractured connecting rod bolt here]
3.3 Corrosion – Induced Failures
Marine environments are highly corrosive due to the presence of saltwater, humidity, and other corrosive substances. Connecting rod bolts are exposed to these corrosive conditions, which can lead to corrosion – induced failures. Corrosion can weaken the bolt’s material, reducing its strength and increasing the risk of fracture. In addition, corrosion can also cause pitting on the surface of the bolt, which can act as stress raisers and accelerate the fatigue crack initiation process. Figure 3 shows a corroded connecting rod bolt.
[Insert an image of a corroded connecting rod bolt here]
4. Case Study: Connecting Rod Bolt Falling Off Failure in a Marine Diesel Engine
4.1 Failure Incident Description
A specific case of a marine diesel engine experiencing a connecting rod bolt falling off failure is presented. The engine was operating on a ship during a long – distance voyage when it suddenly stopped. Upon inspection, it was found that one of the connecting rod bolts had fallen off, and the connecting rod had caused significant damage to the engine components. The engine’s technical parameters are shown in Table 2.
| Engine Technical Parameters | Values |
|---|---|
| 爆发压力 (爆发压力，实际为 Burst Pressure)/MPa | 13 |
| 转速 (转速，实际为 Rotational Speed)/(r/min) | 1500 |
| 角速度 (角速度，实际为 Angular Velocity)/(rad/s) | 157 |
| 曲柄半径 (曲柄半径，实际为 Crank Radius)/mm | 105 |
| 连杆中心距 (连杆中心距，实际为 Connecting Rod Center Distance)/mm | 400 |
4.2 Failure Analysis Process
4.2.1 Visual Inspection
The first step in the failure analysis process was a visual inspection of the failed components. The connecting rod bolt was found to be broken, and the fracture surface showed signs of fatigue and plastic deformation. The connecting rod and other engine components also showed signs of damage, such as scoring and deformation.
4.2.2 Material Analysis
The material of the connecting rod bolt was analyzed to determine its chemical composition and mechanical properties. The results showed that the bolt was made of 42CrMoA material, and its chemical composition was within the specified range. However, the mechanical properties, such as the tensile strength and yield strength, were found to be slightly lower than the standard values.
4.2.3 Stress Analysis
A stress analysis was conducted to determine the stress distribution in the connecting rod bolt during engine operation. This was done using finite element analysis (FEA) software. The results of the stress analysis showed that the maximum stress in the bolt occurred at the thread roots, which was higher than the allowable stress limit. This indicated that the bolt was operating under high – stress conditions, which could have contributed to its failure.
5. Simulation Analysis of Connecting Rod Bolts
5.1 Three – Dimensional Model Establishment
To further understand the behavior of the connecting rod bolt under different operating conditions, a three – dimensional (3D) model of the connecting rod bolt, connecting rod body, and connecting rod end cap was established using 3D modeling software such as CREO 2.0. The model was simplified by omitting some non – essential components, such as the piston pin and crankshaft, to facilitate mesh generation and analysis. The material of the components was defined as 42CrMoA, and its chemical composition and mechanical properties are shown in Table 3 and Table 4.
| Chemical Composition of 42CrMoA Material (Mass Fraction/%) | Values |
|---|---|
| C | 0.38 – 0.45 |
| Si | 0.17 – 0.37 |
| Mn | 0.50 – 0.80 |
| Cr | 0.90 – 1.20 |
| Mo | 0.15 – 0.25 |
| Mechanical Properties of 42CrMoA Material | Values |
|---|---|
| Tensile Strength/MPa | 1080 |
| Yield Strength/MPa | 930 |
| Elongation/% | 12 |
| Reduction of Area/% | 45 |
| Impact Absorption Energy/J | 63 |
| Hardness HBW | ≤217 |
5.2 Mesh Generation and Boundary Conditions
The 3D model was then imported into ANSYS Workbench Meshing software for mesh generation. The Automatic meshing method was used, and a total of 27,145 elements were generated for the entire model. For the connecting rod bolt, 1,115 elements were generated. Appropriate boundary conditions were applied to the model. The inner diameter surface of the large – end bearing shell of the connecting rod was fixed, and a moment load of 52.8 kN was applied to the inner hole surface of the small – end bushing in the vertical direction. For the connecting rod bolt, the top – plane of the bolt was fixed, and a force load in the direction away from the top – plane was applied to the thread part of the bolt.
5.3 Simulation Results and Analysis
5.3.1 Connecting Rod Component Calculation and Verification
The simulation results for the connecting rod components showed the distribution of equivalent stress, equivalent strain, and total deformation. The maximum stress in the connecting rod body was 999.19 MPa, which occurred at the rod body. However, this stress was within the allowable range of the material. The maximum equivalent strain of the connecting rod body was 0.00472, and the maximum deformation was 4.183 mm. The calculated safety factor based on the stress and deformation analysis was 3.09, which met the requirements for the maximum operating conditions. The results of the connecting rod component analysis are summarized in Table 5.
| Connecting Rod Component Analysis Results | Values |
|---|---|
| Maximum Stress in Connecting Rod Body/MPa | 999.19 |
| Location of Maximum Stress | Connecting Rod Body |
| Maximum Equivalent Strain of Connecting Rod Body | 0.00472 |
| Maximum Deformation of Connecting Rod Body/mm | 4.183 |
| Calculated Safety Factor | 3.09 |
5.3.2 Connecting Rod Bolt Calculation and Verification
The simulation results for the connecting rod bolt showed that the maximum stress in the bolt was 1308.7 MPa, which exceeded the material’s limit tensile strength. The maximum equivalent strain of the bolt was 0.00663. The calculated safety factor for the bolt rod body was 1.346, which was less than the allowable safety factor range of 1.5 – 2.0. This indicated that the connecting rod bolt had a safety risk during the engine’s operation at the rated speed and power. The results of the connecting rod bolt analysis are summarized in Table 6.
| Connecting Rod Bolt Analysis Results | Values |
|---|---|
| Maximum Stress in Connecting Rod Bolt/MPa | 1308.7 |
| Maximum Equivalent Strain of Connecting Rod Bolt | 0.00663 |
| Calculated Safety Factor of Connecting Rod Bolt Rod Body | 1.346 |
6. Optimization Strategies for Connecting Rod Bolts
6.1 Material Replacement
One of the optimization strategies was to replace the bolt material. 40CrNiMo material was selected as a replacement for 42CrMoA. The chemical composition and mechanical properties of 40CrNiMo are shown in Table 7 and Table 8.
| Chemical Composition of 40CrNiMo Material (Mass Fraction/%) | Values |
|---|---|
| C | 0.37 – 0.44 |
| Si | 0.17 – 0.37 |
| Mn | 0.50 – 0.80 |
| Cr | 0.60 – 0.90 |
| Mo | 0.15 – 0.25 |
| Ni | 1.25 – 1.65 |
| Mechanical Properties of 40CrNiMo Material | Values |
|---|---|
| Tensile Strength/MPa | 980 |
| Yield Strength/MPa | 785 |
| Elongation/% | 10 |
| Reduction of Area/% | 45 |
| Impact Absorption Energy/J | 55 |
| Hardness HBW | 241 |
Compared with 42CrMoA, 40CrNiMo has a higher hardness and better mechanical properties, although its impact absorption energy is slightly lower. This material change can improve the strength and durability of the connecting rod bolt.
6.2 Structure Optimization
The structure of the connecting rod bolt was also optimized. The thread size was changed from M14 to M16, the screw diameter was increased from Φ12 mm to Φ14 mm, the support surface diameter was increased from Φ23 mm to Φ26 mm, the thread length was increased by 5 mm, and the total bolt length was increased by 5 mm. These changes were made to meet the standard external hexagon bolt dimensions and to improve the bolt’s load – carrying capacity. After the structure optimization, a 3D simulation analysis was conducted again. The results showed that the maximum stress in the optimized bolt under the maximum burst pressure was 664.6 MPa, which did not exceed the allowable tensile stress. The maximum equivalent strain was 0.0032, and the maximum displacement was 0.015 mm. The optimized bolt had a larger safety margin. The comparison of the performance of the original and optimized bolts is shown in Table 9.
| Comparison of Original and Optimized Bolt Performance | Original Bolt | Optimized Bolt |
|---|---|---|
| Maximum Stress/MPa | 1308.7 | 664.6 |
| Maximum Equivalent Strain | 0.00663 | 0.0032 |
| Maximum Displacement/mm | – | 0.015 |
| Safety Margin | Small (Safety Factor < Allowable Range) | Large |
6.3 Tightening Torque Optimization
The original tightening method of the connecting rod bolt was a fixed – torque method, which had high requirements for personnel, mechanical equipment, and the processing accuracy of bolts and mating components. To address this issue, a torque – plus – rotation – angle tightening method was proposed. This method can avoid false torque and component deformation and can monitor abnormal situations during the tightening process through the rotation angle, ensuring that the bolt torque meets the design requirements.
7. Verification of Optimization Results
7.1 Simulation Verification
After implementing the optimization strategies, a new round of simulation analysis was carried out. The results showed that the safety factor of the connecting rod bolt at the thread root was 3.07, which was greater than 2. This indicated that the optimized connecting rod bolt was safe and reliable under the highest operating conditions.
7.2 Ship – Based Verification
The optimized connecting rod components were produced as samples and installed on a marine diesel engine for ship – based verification. The engine was operated for 3000 hours, and then the connecting rod bolts and other components were disassembled and inspected. No abnormalities were found, indicating that the optimized connecting rod body and bolts passed the ship – based verification. The optimized components had more stable material properties, significantly improved the operating reliability of the diesel engine, and effectively solved the connecting rod bolt falling off failure problem.
8. Conclusion
This article comprehensively analyzed the connecting rod bolt falling off failure in marine diesel engines through a combination of theoretical analysis, simulation, and practical verification. The main causes of connecting rod bolt failures, including material mechanical properties and improper bolt tightening methods, were identified. By conducting 3D simulation analysis and strength verification of the connecting rod components and bolts, effective optimization strategies were proposed, including material replacement, structure optimization, and tightening torque optimization. The optimized connecting rod bolts passed the ship – based verification, improving the reliability and performance of the marine diesel engine. These findings can provide valuable references for solving similar problems in marine diesel engines and ensuring the safe and stable operation of ships. Future research can focus on further improving the design and manufacturing processes of connecting rod bolts, as well as exploring new materials and technologies to enhance their performance and durability in harsh marine environments.