| 摘要 |
<p>A spark plug is provided in which a proof strength against a local stress concentration is increased by adjusting the balance of strength (rigidity) of an insulator based on supported positions of the insulator by a metal shell, so as to prevent the generation of cracks or fractures in the insulator. When the insulator 10 which is held by the metal shell 50 receives an external force in a bending direction perpendicular to an axis O at a rear end side body portion 18, a position C where the insulator 10 is supported by a crimping portion 53 via a packing 6 acts as a fulcrum, and a stress is applied between the position C and a position B where the insulator is supported by a ledge portion 56 via a packing 8. Then, in the event that an insulator 10 is designed in which the balance of size and modulus of section is adjusted so that ÄA which denotes a proof strength against bending between a rear end position A of the insulator 10 and the position C and ÄB which denotes a proof strength against bending between the position B and the position C satisfy 0.71‰¤ÄA/ÄB‰¤1.27, cracks or fractures can be prevented.</p> |
