1. Introduction
Marine diesel engines are the heart of ships, powering vessels across the world’s oceans. Among the many components that make up these engines, the connecting rod bolt is a crucial part. It plays a vital role in securing the connecting rod and its associated parts, ensuring the smooth operation of the engine. However, issues such as connecting rod bolt 脱落故障 (bolt falling off failures) can occur, leading to significant problems for the engine and the ship as a whole. This article aims to delve deep into the analysis of such failures and propose effective optimization strategies.
1.1 Importance of Connecting Rod Bolts in Marine Diesel Engines
Connecting rod bolts are responsible for maintaining the structural integrity of the connecting rod assembly. They hold the connecting rod cap in place, which in turn houses the crankshaft journal. A well – functioning connecting rod bolt can withstand the complex forces acting on the connecting rod during the engine’s operation. These forces include the high – pressure gas forces from the combustion process and the inertial forces generated by the reciprocating motion of the piston and the connecting rod itself. Any failure in the connecting rod bolt can disrupt the normal operation of the engine, leading to reduced power output, increased fuel consumption, and in severe cases, engine breakdown.
1.2 Overview of the Problem: Connecting Rod Bolt Falling Off Failures
Connecting rod bolt falling off failures are a common yet serious issue in marine diesel engines. When a bolt falls off, it can cause the connecting rod to dislodge from the crankshaft. This can result in the connecting rod striking other engine components, such as the engine block, cylinder liner, or piston. The consequences can be catastrophic, leading to damage to multiple engine parts, potential oil leaks, and even complete engine failure. Such failures not only cause costly repairs but also lead to significant downtime for the ship, affecting its schedule and profitability.
2. Connecting Rod Bolt Failure Phenomena and Analysis
2.1 Failure Phenomena
2.1.1 Abnormal Engine Shutdown and Component Damage
In many cases, the first sign of a connecting rod bolt falling off is an abnormal engine shutdown. This can happen suddenly during the engine’s operation. After the shutdown, upon inspection, it is often found that the connecting rod has broken free from its normal position. The connecting rod may have hit the engine block, causing cracks or other damage. The piston may also be damaged, and there could be debris in the engine oil sump from the broken parts. For example, in a particular case, a ship’s engine suddenly stopped while at sea. When the engine was opened for inspection, the connecting rod was found to have broken through the engine block, and the piston was severely damaged.
| Incident | Engine Shutdown | Connecting Rod Damage | Piston Damage | Engine Block Damage |
|---|---|---|---|---|
| Case 1 | Sudden stop during operation | Broke through the engine block | Severely damaged | Cracks on the side |
| Case 2 | Gradual loss of power followed by stop | Bent and deformed | Scratched and dented | Minor indentations |
2.1.2 Visual Inspection of Failed Bolts
Visual inspection of the failed connecting rod bolts often reveals characteristic signs of failure. The bolts may show signs of fatigue fracture, such as a smooth fracture surface with beach – mark – like patterns. In some cases, there may also be signs of plastic deformation, where the bolt has been stretched or bent beyond its elastic limit. The threads of the bolt may be damaged, indicating improper tightening or excessive stress. For instance, a failed bolt might have a fractured head, with the fracture surface showing a combination of smooth and rough areas, suggesting a combination of fatigue and overload failure.
| Bolt Failure Sign | Description | Probable Cause |
|---|---|---|
| Fatigue fracture | Smooth fracture surface with beach – mark – like patterns | Repeated loading and unloading |
| Plastic deformation | Stretched or bent bolt | Excessive stress beyond elastic limit |
| Damaged threads | Worn – out or stripped threads | Improper tightening or excessive stress |
2.2 Failure Cause Analysis
2.2.1 Bolt Strength Issues
The strength of the connecting rod bolt is a critical factor in preventing failures. In many cases, the bolt may not be strong enough to withstand the forces acting on it during the engine’s operation. The stress concentration at the thread root is a common problem. During the engine’s operation, the connecting rod bolt is subjected to a combination of forces, including axial tension, shear stress, and bending stress. The rapid vibration of the engine can cause the threads of the bolt and the connecting rod body to wear, which in turn transfers the stress concentration to the thread root. Over time, the cumulative effect of these stresses can lead to fatigue failure of the bolt. For example, if the bolt material has a low fatigue strength, it will be more prone to failure under these cyclic stresses.
| Stress Type | Source | Impact on Bolt |
|---|---|---|
| Axial tension | Combustion pressure and inertial forces | Pulls the bolt along its axis |
| Shear stress | Vibration and misalignment | Causes the bolt to experience a cutting – like force |
| Bending stress | Uneven loading and improper installation | Bends the bolt, creating additional stress |
2.2.2 Bolt Fastening Method Problems
The method of fastening the connecting rod bolt also plays a significant role in its failure. In some engines, the bolts are supposed to be tightened using a specific torque value. However, in practice, manual tightening without proper torque control can lead to inconsistent pre – tightening forces. If the pre – tightening force is too small, the bolt may not be able to hold the connecting rod cap firmly in place, allowing for relative movement between the parts. This can cause additional stress on the bolt and eventually lead to failure. On the other hand, if the pre – tightening force is too large, it can exceed the yield strength of the bolt material, causing plastic deformation and potential fracture. For example, in an engine where the bolts were manually tightened without a torque wrench, some bolts were found to be loose during inspection, while others had signs of over – tightening and were on the verge of breaking.
| Tightening Method | Problem | Consequence |
|---|---|---|
| Manual tightening without torque control | Inconsistent pre – tightening forces | Loose bolts or bolts with excessive stress |
| Incorrect torque value setting | Too small or too large torque | Loosening of the connection or bolt fracture |
3. Simulation Analysis
3.1 Establishment of 3D Models
To better understand the behavior of the connecting rod bolt and the entire connecting rod assembly under different operating conditions, 3D models are created using software such as CREO 2.0. The models include the connecting rod body, the connecting rod end cap, and the connecting rod bolt. In the modeling process, some components like the piston pin and the crankshaft can be omitted for simplicity, while still ensuring that the key aspects of the connecting rod assembly are represented accurately. The material properties of the components are defined based on the actual materials used. For example, a common material for connecting rod bolts is 42CrMoA, and its chemical composition and mechanical properties are input into the model as shown in the following tables.
| Chemical Element | Mass Fraction (%) |
|---|---|
| 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 Property | Value |
|---|---|
| Tensile Strength (MPa) | 1080 |
| Yield Strength (MPa) | 930 |
| Elongation (%) | 12 |
| Reduction of Area (%) | 45 |
| Impact Absorption Energy (J) | 63 |
| Hardness HBW | ≤217 |
3.2 Mesh Generation and Boundary Conditions
After the 3D models are created, the next step is to generate a mesh. For the complex structure of the connecting rod, the Automatic meshing function in ANSYS Workbench Meshing software is used. This results in a total of 27,145 elements for the entire connecting rod assembly model. For the connecting rod bolt, 1,115 elements are generated. Appropriate boundary conditions are then applied. For the connecting rod, a fixed constraint is applied to the inner diameter surface of the large – end bearing shell, and a moment load of 52.8 kN in the vertical direction is applied to the inner hole surface of the small – end bushing. For the connecting rod bolt, a fixed constraint is applied to the top plane of the bolt, and a force load in the direction away from the bolt top is applied to the thread part of the bolt.
3.3 Simulation Results Analysis
3.3.1 Connecting Rod Component Calculation and Verification
By applying the loads and constraints as described above, the stress, strain, and deformation of the connecting rod component are analyzed. The results show that the maximum stress on the connecting rod body is 999.19 MPa, and the maximum stress position is on the connecting rod body. However, this maximum stress is within the allowable range of the material. The maximum equivalent strain of the connecting rod body is 0.00472, and the maximum deformation is 4.183 mm. The calculated safety factor based on the stress and deformation analysis is 3.09, which meets the usage requirements under the maximum working conditions according to the reference value of 1.5 – 2.0.
| Component | Maximum Stress (MPa) | Maximum Strain | Maximum Deformation (mm) | Safety Factor |
|---|---|---|---|---|
| Connecting Rod Body | 999.19 | 0.00472 | 4.183 | 3.09 |
3.3.2 Connecting Rod Bolt Calculation and Verification
For the connecting rod bolt, the simulation results show that the maximum stress is 1,308.7 MPa, which exceeds the ultimate tensile strength of the bolt material. The maximum equivalent strain is 0.00663. Considering the oblique – cut design of the connecting rod big end, the connecting rod bolt is also subjected to shear stress while being stretched. The calculated safety factor of the bolt shank is 1.346, which is less than the allowable safety factor of 1.5 – 2.0. This indicates that the connecting rod bolt has a safety risk when the diesel engine operates at the rated speed and power.
| Component | Maximum Stress (MPa) | Maximum Strain | Safety Factor |
|---|---|---|---|
| Connecting Rod Bolt | 1308.7 | 0.00663 | 1.346 |
4. Connecting Rod Bolt Optimization and Verification
4.1 Bolt Material Replacement
One of the optimization measures is to replace the bolt material. 40CrNiMo material is selected as an alternative to 42CrMoA. The chemical composition and mechanical properties of 40CrNiMo are shown in the following tables.
| Chemical Element | Mass Fraction (%) |
|---|---|
| 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 Property | Value |
|---|---|
| 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 in terms of toughness and hardenability, although its impact absorption energy is slightly lower. This makes 40CrNiMo more suitable for use in connecting rod bolts, as it can better withstand the complex forces acting on the bolts during engine operation.
4.2 Bolt Structure Optimization
The structure of the connecting rod bolt is also optimized. The thread size is increased from M14 to M16, the screw diameter is increased from Φ12 mm to Φ14 mm, the support surface diameter is increased from Φ23 mm to Φ26 mm to meet the standard external hexagon bolt size, and the thread length and the total bolt length are both increased by 5 mm. After the optimization, a 3D simulation analysis is carried out on the bolt. The results show that the maximum stress of the optimized connecting rod bolt under the maximum explosion pressure is 664.6 MPa, which does not exceed the allowable tensile stress. The maximum equivalent strain is 0.0032, and the maximum displacement is 0.015 mm. The optimized connecting rod bolt has a large safety margin.
| Parameter | Original Value | Optimized Value |
|---|---|---|
| Thread Size | M14 | M16 |
| Screw Diameter | Φ12 mm | Φ14 mm |
| Support Surface Diameter | Φ23 mm | Φ26 mm |
| Thread Length | – | +5 mm |
| Total Bolt Length | – | +5 mm |
| Maximum Stress (MPa) | 1308.7 | 664.6 |
| Maximum Strain | 0.00663 | 0.0032 |
| Maximum Displacement (mm) | – | 0.015 |
4.3 Optimization of Connecting Rod Bolt Tightening Torque
The original fixed – torque tightening method has some limitations, as it requires high – precision equipment and skilled operators. An alternative method, torque – plus – rotation – angle tightening, is found to be more effective through experiments. This method can avoid false torque and component deformation. By monitoring the rotation angle during the tightening process, any abnormal conditions can be detected, ensuring that the bolt torque reaches the design requirements.
4.4 Ship – board Verification
After the optimization of the connecting rod bolt and the entire connecting rod component, samples are produced and installed on a diesel engine for ship – board verification. After running the engine for 3000 hours, the connecting rod bolts and other components are disassembled and inspected. No abnormalities are found, indicating that the optimized connecting rod body and connecting rod bolts have passed the ship – board verification. The optimized components have more stable material properties, which greatly improves the operating reliability of the diesel engine and effectively solves the problem of connecting rod bolt failure.
5. Conclusion
In conclusion, the failure of connecting rod bolts in marine diesel engines is mainly caused by factors such as insufficient material mechanical properties and improper bolt tightening methods. Through 3D simulation analysis and strength verification of the connecting rod components and bolts, the reliability of the connecting rod bolts can be improved. When designing the fatigue strength of connecting rod bolts, it is necessary to first reduce the stress on the bolts and adopt a flexible bolt design to reduce the alternating load stress of the bolts under the highest operating conditions and improve their strength. These optimization measures can effectively solve the problem of connecting rod bolt falling off failures, ensuring the safe and reliable operation of marine diesel engines.
6. Future Research Directions
Although significant progress has been made in solving the connecting rod bolt failure problem, there is still room for further research. Future studies could focus on developing new materials specifically designed for connecting rod bolts, with even better mechanical properties and fatigue resistance. Additionally, advanced manufacturing techniques could be explored to improve the precision of bolt manufacturing, reducing stress concentration points. Moreover, real – time monitoring systems for bolt stress and tightness during engine operation could be developed to provide early warnings of potential failures, further enhancing the safety and reliability of marine diesel engines.