Electric lamp and discharge devices – Spark plugs – Shaped electrode chamber – insulator end – shell skirt – baffle...
Reexamination Certificate
2000-01-24
2002-04-16
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Shaped electrode chamber, insulator end, shell skirt, baffle...
C313S141000, C313S135000
Reexamination Certificate
active
06373173
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug used for ignition in an internal combustion engine.
2. Description of the Related Art
FIG. 9
shows a conventional spark plug
300
used for ignition in an internal combustion engine, such as an automobile gasoline engine. The spark plug
300
is mounted on a cylinder head SH of an engine by a male-threaded portion
301
a
formed on an outer circumferential surface of a metallic shell
301
. When the spark plug
300
is mounted on the cylinder head SH, a spark discharge gap g defined by a ground electrode
304
and a center electrode
303
is positioned within a combustion chamber BR and is adapted to ignite a fuel-air mixture. A hexagonal portion
305
(tool engagement portion) is formed on an outer circumferential surface of the metallic shell
301
and is adapted to tighten the male-threaded portion
301
a
by a tightening tool. The metallic shell
301
assumes a substantially cylindrical shape having a bore
306
for receiving an insulator
302
and is conventionally manufactured by cold plastic processing and machining. In many spark plugs, in order to improve manufacturing efficiency, a schematic profile and the bore
306
are formed by die forging, and a final profile including the male-threaded portion
301
a
is finished by machining. Since the metallic shell
301
has a thin-walled portion, the dimensions of the metallic shell
301
must be designed in consideration of a material flow during die forging; otherwise, a working defect is likely to occur.
With a recent tendency toward complication of engine head structure, space allocated around a valve for installation of the spark plug
300
is decreasing. Thus, the hexagonal portion
305
needs to be reduced to increase space for use on the head side. However, reducing the size of the hexagonal portion
305
causes the following problems.
(1) To prevent an excessive reduction in the wall thickness of the hexagonal portion
305
in association with a reduction in the size of the hexagonal portion
305
, a diameter D
1
of a portion (hereinafter referred to as “a major-bore portion”)
306
a
of the shell bore
306
must be reduced. Also, the outside diameter of the insulator
302
must be reduced accordingly. However, when a diameter D
2
of a portion (hereinafter referred to as “an intermediate-bore portion”) of the shell bore
306
corresponding to the male-threaded portion
301
a
is reduced, a forging punch becomes excessively thin, when the intermediate-bore portion
306
b
is formed by forging and thus may be damaged or may cause a working defect when a large working load is applied thereon. This problem arises particularly in the case when the male-threaded portion
301
a
has a long screw reach.
(2) A portion of the insulator
302
positioned within the major-bore portion
306
a
is formed into a flange portion
302
e
. When the metallic shell
306
is swaged onto the insulator
302
, the flange portion
302
e
bears a swaging force. A metallic terminal
313
and a center electrode
303
are connected by a glass seal portion
315
. In the step of the glass seal portion
315
, the flange portion
302
e
bears a pressing force. In particular, the center electrode
303
, a material powder of the glass seal portion
315
, and the metallic terminal
313
are disposed within a through-hole formed in the insulator
302
. Then the insulator
302
is inserted into a bore formed in a seat die such that the flange portion
302
e
rests on an inner seat portion formed on the wall of the bore. In this state, the entire insulator
302
is heated to a temperature equal to or higher than a glass softening point, and the metallic terminal
313
is pressed inwardly in the axial direction to press the material powder with the center electrode
303
, thereby forming the glass seal portion
315
. During this pressing process, the flange portion
302
e
bears a pressing force.
If the outside diameter of the insulator
302
is too small to meet the demand described above in (1), manufacturing the insulator
302
becomes very difficult. Therefore, there is a certain limit to a reduction in the outside diameter of the insulator
302
. As the size of the hexagonal portion
305
is reduced, the diameter D
1
of the major-bore portion
306
a
is reduced accordingly. Thus, the diameter of the flange portion
302
e
, which is accommodated within the major-bore portion
306
a
, is also reduced. Because of a reduction in the size of the hexagonal portion
305
, the diameter of the flange portion
302
e
must be reduced because there is a certain limit to a reduction in the diameter of a portion of the insulator
302
other than the flange portion
302
e
(for example, a portion of the insulator
302
positioned within the intermediate-bore portion
306
b
; hereinafter referred to as “an intermediate-trunk portion
302
a
”). As a result, the amount of a projection of the flange portion
302
e
decreases, causing, for example, a decrease in the area of contact between the flange portion
302
e
and the seat portion of the seat die used in the step of forming the glass seal portion
315
. Consequently, a load concentration causes breakage of the seat die or galling of the insulator
302
and the seat die.
(3) If the diameter of the intermediate-trunk portion
302
a
of the insulator
302
is reduced to meet the demand described above in (2), and also the diameter D
2
of the intermediate-bore portion
306
b
of the metallic shell
306
is set to a rather large value to attain favorable workability during the process in (1), a gap is likely to be formed between the intermediate-bore portion
306
b
and the intermediate-trunk portion
302
a
of the insulator
302
. The presence of this gap tends to cause an eccentric disposition of the insulator
302
within the metallic shell
301
, potentially causing an impairment in spark plug performance (for example, lateral sparking).
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spark plug capable of increasing the degree of freedom with respect to space around a cylinder head on which the spark plug is mounted, through reduction in the size of a tool engagement portion, such as a hexagonal portion, and capable of implementing the following:
(1) in spite of a reduction in the size of the tool engagement portion, a metallic shell can be manufactured efficiently and at high yield;
(2) during formation of a conductive glass seal layer or a resistor by use of a seat die, breakage or galling of the seat die is less likely to occur; and
(3) during incorporation of an insulator into the metallic shell, an eccentric disposition of the insulator within the metallic shell is less likely to occur.
To achieve the above object, the present invention provides a spark plug including an elongate center electrode, an insulator enclosing the center electrode, a metallic shell having open opposite ends and enclosing the insulator, the metallic shell having a male-threaded portion formed on a front-side outer circumferential surface of the metallic shell, and a tool engagement portion formed on the outer circumferential surface of the metallic shell at a rear side with respect to the male-threaded portion, the tool engagement portion projecting circumferentially outwardly, and a ground electrode connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode.
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 insulator has a stepped annular insulator-side engagement portion for engaging with an annular shell-side engagement portion projected inwardly from a portion of an inner surface of the metallic shell corresponding to the male-threaded portion, and |A−E|≦1.5 mm, and 0.4≦(D
2
/E)
2
≦0.6, where A is a dimension of the tool engagement portion represented by a diameter of an in
Finnegan Henderson Farabow Garrett & Dunner LLP
Guharay Karabi
NGK Spark Plug Co. Ltd.
Patel Nimeshkumar D.
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