Spark Plug Shell Geometry for Sealing Without Insulator Cracking
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Solution Overview
Problem
Conventional spark plugs face a trade-off between reducing gas leakage and preventing cracking in the insulator, where increased crimping force minimizes leakage but may cause cracking, and decreased force minimizes cracking but allows gas leakage.
Innovation Solution
A spark plug design with a tubular metal shell featuring a specific axial-line-direction distance and thread thickness configuration, along with an annular packing, to apply an appropriate compressive force and reduce both cracking and gas leakage, where the axial-line-direction distance between the rearward facing surface and seating surface is within 17.2 to 28.2 mm, and the thickness obtained by subtracting the inner diameter of the trunk portion from the pitch diameter is between 2.6 and 0.1 times the distance plus 1.48, ensuring adequate force and pressure on the packing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If force for pressing the insulator to the front side by the crimping portion is increased, then leakage of gas through a gap between the frontward facing surface of the insulator and the rearward facing surface of the metal shell is reduced, but cracking is likely to occur in the root of the frontward facing surface of the insulator
Solution Approach 1:
The patent applies parameter changes by precisely controlling the axial-line-direction distance X (17.2≤X≤28.2 mm) and thickness Y (Y≥2.6 mm, Y≤0.1X+1.48) of the metal shell. These parameter optimizations ensure that the crimping portion applies sufficient force to prevent gas leakage while avoiding excessive force that would cause cracking in the insulator root.
Solution Approach 2:
The patent introduces an annular packing as an intermediary element between the insulator and the metal shell. This packing material serves as a mediator that enhances the sealing effect between the insulator frontward facing surface and the metal shell rearward facing surface, allowing adequate sealing force to be applied without directly transmitting excessive crimping force to the insulator root, thereby preventing cracking.
2Strength
If force for pressing the insulator to the front side by the crimping portion is decreased, then cracking is less likely to occur in the root of the frontward facing surface of the insulator, but gas is likely to leak through a gap between the frontward facing surface of the insulator and the rearward facing surface of the metal shell
Solution Approach 1:
The annular packing acts as a mediator that maintains gas sealing even when crimping force is reduced. The packing material fills and seals the gap between the insulator frontward facing surface and the metal shell rearward facing surface, allowing the crimping portion to apply only the force necessary to compress the packing without applying excessive force that would crack the insulator.
Solution Approach 2:
The patent optimizes the metal shell dimensions (distance X and thickness Y) to ensure that even with reduced crimping force, the structural integrity of the metal shell is sufficient to maintain sealing pressure on the packing, preventing gas leakage while protecting the insulator from cracking.
Data Source
AI summary
Provided is a spark plug (10) with which it is possible to reduce cracking of an insulator and leakage of gas. The spark plug comprises an insulator (11) having a distal end facing surface (15) provided on an outer circumference thereof, and a cylindrical main metal fitting (20) disposed at the outer circumference of the insulator, wherein: the main metal fitting is provided with a body portion (21) having a rear end facing surface (22), which is in contact with the distal end facing surface either directly or via another member (31), provided on an inner circumference thereof, and a male thread (23) provided on an outer circumference thereof, a flange portion (24) which includes a seating surface (25) adjacent to a rear end of the body portion, and which protrudes to the outside of the male thread, and a crimped portion (28) which presses the insulator toward the distal end side; and if X (mm) is a distance between the rear end facing surface and the seating surface in an axial direction, and Y (mm) is a thickness obtained by subtracting an inner diameter of the body portion at the position of the seating surface from an effective diameter of the male thread, the following relationships are satisfied: Y≤0.1X+1.48, 17.2≤X≤28.2, and Y≥2.6.