Spark Plug Insulator Sealing via Ledge Geometry
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Solution Overview
Problem
The existing spark plug assembly process generates excessive residual stress in the packing, leading to deformation and potential cracking of the insulator, which can result in misfire or engine stop due to combustion failure.
Innovation Solution
A spark plug design featuring a ledge portion on the metal shell and a step portion on the insulator with a specific angle and narrowing distance, combined with zinc plating, to manage stress and prevent excessive deformation of the packing, ensuring gastightness while maintaining sufficient residual stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the opening portion of the metal shell is crimped strongly to increase residual stress in the packing, then gastightness is improved, but the packing is deformed excessively causing the inside diameter portion to be squeezed out inwards and press the insulator, generating cracks
Solution Approach 1:
The patent applies local quality by creating a ledge portion with a specific geometric configuration (inclined surface at 1-10 degrees angle, stepped structure) at the sealing location. This local structural modification concentrates the crimping stress on the ledge portion rather than distributing it uniformly, allowing strong sealing pressure to be applied to the packing without transmitting excessive force to the insulator, thus preventing insulator cracks while maintaining gastightness.
Solution Approach 2:
The ledge portion is pre-formed on the metal shell before assembly with the insulator and packing. This preliminary structural preparation ensures that when crimping is applied during assembly, the stress is immediately directed to the predetermined ledge portion, preventing unpredictable deformation patterns and ensuring the packing is compressed uniformly without squeezing outwards or inwards excessively.
2Reliability
If the outside diameter portion of the packing is squeezed out excessively from the rear end portion of the ledge portion, then the packing enters between the metal shell and insulator, but this causes press cracks to form
Solution Approach 1:
The stepped configuration of the ledge portion creates a localized stress concentration zone. The first step portion and second step portion with different radial positions and axial lengths create distinct stress distribution zones, confining the packing deformation to specific areas and preventing the outside diameter portion from squeezing outwards excessively into the gap between metal shell and insulator, thereby preventing press cracks.
3Reliability
If the angle between the ledge portion and step portion is too small, then stress concentration is insufficient and gastightness is compromised, but if the angle is too large, then excessive deformation of the packing occurs
Solution Approach 1:
The patent optimizes the geometric parameters of the ledge portion, specifically setting the inclination angle of the inclined surface between 1-10 degrees and defining specific dimensional relationships between the first and second step portions. This parameter optimization creates the ideal balance where the angle is sufficiently large to concentrate stress effectively for gastight sealing, yet not so large as to cause excessive packing deformation or displacement.
Data Source
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AI summary
A spark plug is provided which can ensure gastightness between a metal shell and an insulator while preventing the cracking of the insulator. There is provided a spark plug in which a ledge portion 30 is formed on the metal shell 11 whose inside diameter is reduced gradually towards a front end portion, a step portion 32 is formed on an insulator 12 whose outside diameter is reduced gradually towards the front end portion and which confronts the ledge portion 30, and packing 34 is disposed between the ledge portion 30 and the step portion 32, characterized in that a distance between the ledge portion 30 and the step portion 32 gets narrower as the ledge portion 30 and the step portion 32 extend radially inwards, and in that an angle θ formed by the ledge portion 30 and the step portion 32 is one degree or larger and 10 degrees or smaller.