Spark Plug Tapered Surface Roughness for Airtight Sealing
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Solution Overview
Problem
Existing spark plug designs with flat and smooth tapered surfaces for improved airtightness are prone to decreased performance due to minute manufacturing marks, leading to inferior productivity and manageability.
Innovation Solution
A spark plug design featuring a tapered surface with an arithmetic mean roughness of 1 μm to 5 μm and annular or helical protruding portions, which ensures reliable contact with the bearing surface, maintaining airtightness while allowing for easier manufacturing and handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a flat and smooth tapered surface is formed to improve airtightness, then contact with the bearing surface is enhanced, but minute marks formed during manufacturing or conveyance cause drastic decrease in airtightness
Solution Approach 1:
The patent changes the surface roughness parameter from extremely smooth (practically impossible to maintain) to a controlled rough range of 0.3μm to 3μm arithmetic mean roughness. This parameter change makes the surface tolerant to manufacturing marks while still achieving reliable airtight contact with the bearing surface.
Solution Approach 2:
The patent anticipates the problem of minute marks by pre-establishing a roughness range that inherently accommodates such defects. By designing the surface to have controlled roughness before manufacturing occurs, the patent cushions against the adverse effects of inevitable manufacturing variations and conveyance marks.
2Reliability
If a flat and smooth tapered surface is formed to improve airtightness, then contact area with bearing surface is increased, but productivity decreases due to extreme caution required during manufacturing and conveyance
Solution Approach 1:
The patent changes the surface roughness parameter to a range (0.3μm to 3μm) that naturally tolerates manufacturing variations, eliminating the need for extreme caution during manufacturing and conveyance. This parameter change directly improves productivity while maintaining airtightness.
3Productivity
If the tapered surface is made rough to allow manufacturing marks, then productivity and manageability improve, but airtightness decreases
Solution Approach 1:
The patent identifies and applies the optimal roughness parameter range (0.3μm to 3μm arithmetic mean roughness) that simultaneously achieves both goals: it is rough enough to tolerate manufacturing marks and maintain productivity, yet smooth enough to ensure reliable airtight contact with the bearing surface.
Data Source
AI summary
A spark plug includes an insulator extending along an axis and a metal shell provided on the outer periphery of the insulator. The metal shell includes a thread portion for threadedly engaging with a mounting hole of an internal combustion engine and a seat portion having a tapered surface whose outer diameter decreases gradually toward a leading end side. The tapered surface contacts with a bearing surface of the internal combustion engine when the thread portion is engaged with the mounting hole. Annular protruding portions or a helical protruding portion are formed within a range on the tapered surface from an outermost peripheral portion to a part forming an outer diameter occupying 95% of the diameter of the outermost peripheral portion. The arithmetic mean roughness of a surface of the tapered surface within the range is set to 1-5 μm on a section including the axis.