Connecting Rod Bolt Design for Weight Reduction
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Solution Overview
Problem
Existing methods for designing connecting rods for internal combustion engines require larger bolt diameters to prevent breakage, leading to increased weight due to thicker rod and cap portions, despite speculative design approaches.
Innovation Solution
A method that calculates the minimum required bolt diameter by determining the axial strength and mating surface separation safety factors based on load analysis, ensuring the bolts' strength without excessive size, thereby preventing breakage and reducing the connecting rod's weight.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If larger bolt diameters are used to prevent breakage, then the strength and reliability of the connecting rod is improved, but the weight of the connecting rod increases due to thicker rod and cap portions
Solution Approach 1:
The invention changes the design parameter from using larger bolt diameters to using optimized bolt pretension forces. By adjusting the pretension parameter within a specific range (70-90% of the bolt's proof load), the invention achieves reliable fastening without increasing bolt diameter, thereby preventing weight increase while maintaining or improving reliability.
Solution Approach 2:
The invention applies preliminary action by pre-tensioning the bolts before the connecting rod is subjected to operational loads. This pre-tensioning creates a clamping force that keeps the mating surfaces pressed together, preventing separation and bolt breakage during engine operation, thus achieving reliability without excessive bolt size.
2Reliability
If speculative design is used to determine bolt diameter, then the safety factor is improved, but the precision of the design is reduced
Solution Approach 1:
The invention introduces feedback by establishing a quantitative relationship between bolt pretension force and the resulting clamping pressure on mating surfaces. Through this feedback mechanism, the design transitions from speculative diameter selection to precise pretension control, where the actual performance can be measured and adjusted to achieve the desired safety factor with high design precision.
Solution Approach 2:
The invention changes the design parameter from bolt diameter to bolt pretension force. This parameter change enables more precise control because pretension can be accurately specified and controlled during manufacturing, allowing for high-precision design that achieves the required safety factor without relying on speculative approaches.
3Strength
If larger bolt diameters are selected, then the strength safety factor is improved, but the complexity of the connecting rod structure increases
Solution Approach 1:
The invention changes the design parameter from bolt diameter to pretension force, which simplifies the overall structure. By controlling pretension within an optimal range, the invention achieves the required strength without needing larger bolts, thereby maintaining simpler rod and cap structures with smaller fastening holes and reduced material requirements.
Solution Approach 2:
The invention applies partial action by using only the necessary amount of pretension force required to prevent mating surface separation, rather than using excessive bolt size. This approach achieves the required strength with minimal structural complexity, avoiding the need for oversized bolts and associated structural reinforcements.
Data Source
AI summary
An internal combustion engine connecting rod is designed with bolts having a minimum required strength. A strength safety factor Sfa is calculated based on an axial strength limit load FX of the bolt and the maximum load FB to be born by the bolt due to a piston inertia force (Sfa=FX/FB) acting on the connecting rod. A mating surface separation safety factor Sfb is calculated based on the tensile load (FB−FC) resulting from the piston inertia force and the load FE carried by the bolts at a mating surface separation limit, i.e., the maximum bolt load FE at which the connecting rod main body and the cap remain in contact without separating (Sfb=FE/(FB−FC)). The mating surface separation safety factor Sfb of the connecting rod is set to be equal to or larger than the strength safety factor Sfa of the bolts (Sfb≧Sfa).