Electric lamp and discharge devices – Spark plugs – With specific joint structure
Reexamination Certificate
2000-01-20
2002-07-02
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Spark plugs
With specific joint structure
C313S141000, C313S143000, C313S145000, C313S135000, C313S137000, C313S118000
Reexamination Certificate
active
06414420
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The present application claims foreign priority from Japanese Patent Application No. Hei 11-013515, filed on Jan. 21, 1999, the content of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug used as a source of ignition in an internal combustion engine, and more particularly, to a spark plug having a small-sized metallic shell for installation in a narrow space.
2. Description of the Related Art
Some conventional spark plugs employ a cushion material formed from talc powder and filled into a cylindrical space defined by an outer circumferential surface of an insulator and an inner circumferential surface of a metallic shell to improve impact resistance. Other conventional spark plugs do not employ such a cushion material (talc) but are configured such that the insulator is secured directly by the metallic shell through thermal swaging. These conventional spark plugs include a screw diameter of 14 mm (M14) or 12 mm (M12), and a hexagonal tightening portion with which a plug wrench is engaged having a distance of 20.8 mm or 16 mm between two parallel, diagonally opposed surfaces of the hexagonal tightening portion (“width-across-flats”).
With recent improvement in engine control technology a tendency toward employing a multi-valve type combustion chamber, the number of components mounted on and around an engine has been increasing. Particularly, in the case of a direct-injection-type engine, which is becoming popular, a volume allot ed to a spark plug on a cylinder head is small. Accordingly, the width-across-flats of a tightening portion of a metallic shell has been required to be decreased from the conventionally employed width of 16 mm to not greater than 14 mm.
When the width-across-flats is reduced to not greater than 14 mm, the wall thickness of a metallic shell decreases accordingly. As a result, the volume of the metallic shell decreases, and thus, the metallic shell decreases in strength. A spark plug having a width-across-flats not greater than 14 mm and not employing a cushion material (talc) suffers impairment in impact resistance and a considerable reduction in airtightness after exposure to impact.
Also, since the wall thickness of a tightening portion decreases, a load imposed on the tightening portion during swaging causes swelling of the tightening portion. As a result, the width-across-flats may fail to fall within a predetermined tolerance, potentially causing a failure to establish engagement between the tightening portion and a plug wrench.
The aforementioned engagement problem will be described specifically with reference to
FIGS. 2 and 3
. An insulator
1
is fixedly attached to a metallic shell
5
by swaging in the following manner. A swaging die is applied from underneath to a seat portion
5
F of the metallic shell
5
, while another swaging die is applied from above to a tightening portion
5
A and a swaging portion
5
C. The upper swaging die exerts a downward force to buckle a curved portion
5
D by about 0.5 mm to 0.8 mm, whereby the insulator
1
is strongly pressed against an inner stepped portion
5
E of the metallic shell
5
via a packing member
6
. In this manner, the insulator
1
is fixedly attached to the metallic shell
5
through swaging. During such swaging, a strong force exerted by the upper swaging die causes plastic deformation of not only the curved portion
5
D but also the tightening portion
5
A. As a result, the tightening portion
5
A swells slightly. This swelling has not raised a problem with respect to a conventional spark plug having a width-across-flats W of not less than 16 mm, since a wall thickness P of the tightening portion
5
A is sufficiently thick so that the tightening portion
5
A has a sufficient strength.
However, a spark plug having a width-across-flats of not greater than 14 mm encounters a difficulty in bringing the width-across-flats W within a predetermined tolerance, since the wall thickness P of the tightening portion
5
A is thin and results in significant swelling of the tightening portion
5
A. Unless the width-across-flats W falls within a predetermined tolerance, a plug wrench cannot be engaged with the tightening portion
5
A. By contrast, when, in order to reduce the swelling of the tightening portion
5
A, the wall thickness of the curved portion
5
D is reduced so that a forced required to buckle the curved portion
5
D can be reduced, the strength of the curved portion
5
D of a spark plug becomes insufficient for enduring a tightening torque exerted when the spark plug is mounted on an engine. Alternatively, when a thickness M of talc
9
serving as a cushion material is reduced to accordingly increase the wall thickness P of the tightening portion
5
A, the effect of the talc
9
as a cushion material is diminished, resulting in an impairment in impact resistance.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug capable of exhibiting high impact resistance even when the width-across-flats of a tightening portion of a metallic shell is small, and capable of maintaining airtightness even after subjection to strong impact.
Another object of the present invention is to provide a spark plug having further improved impact resistance and capable of bringing the width-across-flats of a tightening portion into a predetermined tolerance through suppression of swelling of the tightening portion.
A further object of the present invention is to provide a method of manufacturing a spark plug as mentioned above.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To achieve the above objects, the present invention provides a spark plug including an insulator having a center through-hole formed therein, a center electrode held in the center through-hole, a metallic shell holding the insulator by swaging, and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion formed on the outer circumferential surface of a front end portion of the metallic shell, and a tightening portion formed on the outer circumferential surface of the metallic shell, located at a rear side with respect to the male-threaded portion. In the specification, the term “front” refers to a spark discharge gap side with respect to an axial direction of the center electrode, and the term “rear” refers to a side opposite the front side.
The tightening portion is used to screw the male-threaded portion into a female-threaded hole formed in an internal combustion engine. The distance between two opposed parallel surfaces of the tightening portion (hereinafter referred to as a width-across-flats W) is not greater than 14 mm (W≦14.0 mm).
A cushion material is charged into a cylindrical space defined by an outer surface of the insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion. The cushion-material charged portion has an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm≦L≦10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm≦M≦1.3 mm). The cylindrically filled cushion material eases impact exerted on the metallic shell, thereby preventing loosening of swaging between the metallic shell and the insulator even when the width-across-flats is not greater than 14 mm. Even when swaging between the metallic shell and the insulator loosens to some extent and thus the pressure produced at the packing potion between the metallic shell and the insulator decreases with a resultant leakage of combustion gas through the packing portion, the cushion-ma
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
NGK Spark Plug Co. Ltd.
O'Shea Sandra
Ton Anabel
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