Spark Plug Metal Shell Groove Section Modulus Design
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Solution Overview
Problem
Miniaturization of spark plugs leads to a reduction in the intensity (rigidity) of the metal shell, causing potential cracks due to impact or stress corrosion, which compromises the spark plug's durability.
Innovation Solution
A spark plug design with a metal shell featuring a cylindrical groove, a polygonal tool engagement portion, and a trunk portion, where the section modulus at one end is less than or equal to the section modulus at the other end, and specific dimensions and relationships between distances and thicknesses are maintained to enhance impact resistance and prevent stress corrosion cracking, while allowing for miniaturization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the spark plug is miniaturized to improve fuel efficiency and reduce emission gases, then the size of the spark plug is reduced, but the intensity (rigidity) of the metal shell deteriorates, leading to cracks due to impact or stress corrosion
Solution Approach 1:
The groove cross-sectional shape is optimized to have different properties at different locations: the curved side walls provide flexibility and stress distribution, while the flat bottom provides structural support. This local quality differentiation allows the groove to maintain sufficient rigidity even in miniaturized spark plugs.
Solution Approach 2:
The invention changes the geometric parameters of the groove, specifically defining a cross-sectional shape where curved side walls transition to a flat bottom. This parameter change in the groove geometry fundamentally alters the stress distribution characteristics, enabling miniaturization without compromising shell strength.
2Reliability
If nickel plating is applied to the metal shell to improve corrosion resistance, then corrosion protection is enhanced, but stress corrosion cracking may occur due to the plating process
Solution Approach 1:
The groove cross-sectional shape parameters are specifically designed to minimize stress concentration during nickel plating. The curved side walls and flat bottom configuration allows more uniform stress distribution during the plating process, reducing the risk of stress corrosion cracking while maintaining corrosion resistance.
3Ease of manufacture
If cold caulking is used to fix the metal shell to the ceramic insulator, then assembly is simplified, but asymmetry and stress corrosion may be introduced
Solution Approach 1:
The groove cross-sectional shape is designed with asymmetric curvature that compensates for the asymmetric stresses introduced by cold caulking. The specific configuration of curved side walls and flat bottom creates localized stress relief zones that counterbalance the caulking-induced asymmetry, preventing stress corrosion.
Data Source
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AI summary
An object of the present invention is to provide a technique capable of reducing a size of a spark plug while securing the intensity (rigidity) of a groove of a metal shell. In the metal shell 30 of the spark plug 100, the following relationship is satisfied: Z1<=Z2, where "Z1" represents a section modulus of a first groove end 353 of the groove 35, and where "Z2" represents a section modulus of a second groove end 357 of the groove 35.