Spark Plug Middle Section Bulge Design for Stress Corrosion Cracking
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Solution Overview
Problem
The existing spark plug manufacturing methods lead to stress corrosion cracking due to the formation of recess sections during the clamping process, which compromises airtightness and durability.
Innovation Solution
A spark plug design where the middle section bulges both inward and outward in the diametrical direction, with specific geometrical and hardness constraints, and a controlled clamping process to prevent the formation of recess sections and reduce contraction stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the middle section is heated by electrical conduction and clamped during manufacturing, then the airtightness between metal shell and insulating body is improved, but stress corrosion cracking is generated at the middle section
Solution Approach 1:
The patent applies local quality by creating a specific geometric configuration where the middle section has different radial positions at its leading end and rear end. The rear end is positioned more radially outward than the leading end, creating a gradual transition that distributes stress locally rather than concentrating it at a single point. This geometric variation in the middle section's cross-section prevents the formation of recess sections while maintaining the clamping force needed for airtightness.
Solution Approach 2:
The patent applies preliminary action by pre-positioning the middle section in a specific geometric configuration before the clamping process. The middle section is designed with predetermined radial positions at its leading and rear ends, and with predetermined outer peripheral shapes, so that when clamping occurs, the stress is already distributed favorably. This preliminary geometric arrangement prevents stress concentration before the corrosive environment even acts on the component.
2Strength
If the middle section is heated and contracted during clamping, then the engagement between clamping section and insulating body is strengthened, but recess sections are formed on the inner periphery
Solution Approach 1:
The patent applies local quality by creating a specific geometric configuration where the middle section has different radial positions at its leading end and rear end. The rear end is positioned more radially outward than the leading end, creating a gradual transition that distributes stress locally rather than concentrating it at a single point. This geometric variation in the middle section's cross-section prevents the formation of recess sections while maintaining the clamping force needed for airtightness.
Solution Approach 2:
The patent applies curvature by designing the middle section with a specific outer peripheral shape that curves gradually from the leading end to the rear end. The predetermined outer peripheral shape creates a smooth, continuous surface without sharp transitions or recesses. This curved geometry ensures that when the middle section contracts during heating and clamping, the stress is distributed evenly along the curved surface rather than concentrating at angular transitions, thereby preventing recess section formation.
3Ease of manufacture
If the middle section is made slender to reduce material, then the manufacturing complexity is reduced, but stress concentration occurs at the middle section
Solution Approach 1:
The patent applies local quality by creating a specific geometric configuration where the middle section has different radial positions at its leading end and rear end. The rear end is positioned more radially outward than the leading end, creating a gradual transition that distributes stress locally rather than concentrating it at a single point. This geometric variation in the middle section's cross-section prevents the formation of recess sections while maintaining the clamping force needed for airtightness.
Solution Approach 2:
The patent applies dimensionality change by transitioning from a uniform cross-section design to a variable cross-section design along the axial dimension. The middle section's radial position varies along its length, with the rear end positioned more radially outward than the leading end. This three-dimensional geometric variation adds complexity to the shape but eliminates stress concentration, showing that dimensional variation can resolve stress issues without requiring additional materials or components.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively prevents stress corrosion cracking, ensuring superior airtightness and durability by distributing stress evenly and controlling the clamping process to avoid excessive bulging and hardness differences.
Implementation Method 1
the metal shell is heated by electrical conduction through the mold and a relatively slender middle section that is positioned between the tool engaging section of the metal shell and the seat section is heated
Implementation Method 2
the middle section that is in a heat expansion state is cooled and contracted so that the clamping section of the metal shell becomes in an strongly engaged state
Data Source
Figure 1
Figure 2
Figure 3(a)~3(b)
AI summary
A generation of a stress corrosion cracking is prevented in a middle section. The spark plug 1 includes an insulator 2 that extends along an axis CL1 direction and a metal shell 3. The metal shell 3 includes a middle section 41 that is positioned between a collar section 16 and a tool engaging section 19. The middle section 41 has a bulged section 42 that bulges towards both the inside and the outside in the diametrical direction, a first slender section 43 that is the most slender at a portion that is positioned further to the rear end side in the axis CL1 dircetion than the bulged section 42 and a second slender section 44 that is the most slender at a portion that is positioned further to the leading end side in the axis CL1 direction than the bulged section 42 and the bulged section 42 has the most bulged section 42M that bulges toward the most inside of diametrical direction. In a cross-section that includes the axis CL1, when the distance between both slender sections 43 and 44 along the axis CL1 is F (mm) and the bulged amount to the inside in diametrical direction of the most bulged section 42M with respect to an imaginary line that connects a portion that is positioned furthest to the inside in diametrical direction of both slender sections 43 and 44 is G (mm), 0.00 < G/F ≤ 0.18 is satisfied.