Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2002-12-24
2004-10-26
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S118000, C313S135000, C313S144000
Reexamination Certificate
active
06809463
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug used for igniting an internal combustion engine.
2. Description of the Related Art
The metallic shell of a spark plug is fixedly attached to an insulator by means of crimping. Specifically, the insulator is inserted into the metallic shell formed into a tubular shape, and then by use of dies a compressive load is applied to the peripheral edge of a rear end portion (a portion to be crimped) of the metallic shell. By this procedure, the portion to be crimped is curved toward a flange-like protrusion formed on the outer circumferential surface of the insulator to thereby become a crimped portion, whereby the insulator is fixed in place. The metallic shell is generally formed from a steel material such as carbon steel.
A method for firmly joining the insulator to the metallic shell by means of the crimped portion is specifically carried out in the following manner. As shown in FIG.
2
(
a
), when a portion-to-be-crimped
1
d
′ is axially compressed by means of crimping dies
110
and
111
, the portion-to-be-crimped
1
d
′ is plastically deformed radially inward in a compressed condition. Packings
60
and
62
and a filler material
61
such as talc are usually disposed between the portion-to-be deformed
1
d
′ and a flange-like protrusion
2
e
(in some cases, the filler material may be omitted, with only a single thick packing disposed). When compressive deformation of the portion-to-be-crimped
1
d
′ increases, a load begins to be imposed on the packings
60
and
62
, the filler material
61
, and the flange-like protrusion
2
e
(hereinafter, these are generically and collectively called a “portion to be compressed”). While the portion to be compressed undergoes compressive deformation, plastic deformation of the portion-to-be-crimped
1
d
′ proceeds further. Then, as shown in FIG.
2
(
b
) which is a step following the step shown in FIG.
2
(
a
), when a final value for a compression stroke for crimping is reached, unloading is performed to thereby complete the crimping process (the portion-to-be-crimped
1
d
′ becomes a crimped portion
1
d
). The unloading induces some springback of the crimped portion
1
d
. However, since the crimped portion
1
d
is plastically deformed, the crimped portion
1
d
retains the compressed portion in an elastically deformed condition, thereby inducing a fastening force for firmly joining the insulator
2
to the metallic shell
1
.
3. Problems Solved by the Invention:
Along with a recent tendency of an engine toward complex arrangement around heads and an increase in valve diameter, spark plugs show a marked tendency towards a decrease in diameter and increase in length. However, decreasing the diameter of a spark plug requires employing a metallic shell having a small diameter and a thin wall. As is apparent from the above-described principle, a force for fastening the insulator against the metallic shell is induced by reaction from the crimped portion
1
d
. Since a reduction in the diameter and wall thickness of the metallic shell is accompanied by a reduction in the cross-sectional area of the crimped portion
1
d
, bringing stress arising on the cross section of the crimped portion
1
d
to the same level as a conventional one requires a reduction in compression stroke for crimping. Thus, total fastening force decreases by an extent corresponding to the reduction in the cross-sectional area. As a result, gastightness established between the metallic shell and the insulator is deteriorated. Particularly, when harsh vibrations act on a spark plug as in high-speed, high-load driving, crimping of the spark plug may be loosened, and thus gastightness is more likely to be deteriorated.
By contrast, an attempt to maintain the total fastening force at the same level as a conventional one involves an increase in stress by an extent corresponding to a decrease in the cross-sectional area of the crimped portion
1
d
; as a result, the strength of the crimped portion
1
d
fails to endure the stress, thereby leading to a failure to maintain gastightness.
SUMMARY OF THE INVENTION
An object of the present invention is to enable, in a spark plug configured such that a metallic shell is joined to an insulator through crimping, the metallic shell to be firmly joined to the insulator by means of a sufficient fastening force even when the diameter of the spark plug is reduced, to thereby enhance gastightness and vibration resistance.
The above object of the present invention is achieved by providing a spark plug comprising a rodlike center electrode, a rodlike insulator surrounding the center electrode and having a protrusion at a central portion thereof, a metallic shell assuming an open-ended, tubular shape and surrounding the insulator, and having two protrusions and a thin-walled portion formed on an outer surface thereof at a central portion thereof with respect to the direction of said axis, the thin-walled portion being located between said two protrusions and being thinner than said two protrusions; and a ground electrode facing the center electrode and defining a spark discharge gap in cooperation with the center electrode, and characterized in that:
an insulator insertion hole into which the protrusion of the insulator is inserted is formed in the metallic shell while extending in the direction of an axis (O); when a side toward the spark discharge gap with respect to the direction of the axis is taken as a front side, a rear end portion of the metallic shell is crimped by a cold crimping step toward the insulator to form a curved, crimped portion; and, in order to achieve the above object,
the inside diameter of the insulator insertion hole of the metallic shell is 8-12 mm as measured at a position where the inner wall surface of the insulator insertion hole transitions to the inner wall surface of the crimped portion with respect to the direction of the axis of the metallic shell; and the cross-sectional area S of the metallic shell as measured when the metallic shell is cut at the position by a plane perpendicular to the axis, and the carbon content of a steel material used to form the metallic shell satisfy either of the following conditions A and B:
condition A: 15≦S<29 mm
2
and a carbon content of 0.20%-0.50% by weight; and
condition B: 29≦S<35 mm
2
and a carbon content of 0.15%-0.50% by weight.
When a side toward a spark discharge gap with respect to the direction of the axis is taken as a front side, a tool engagement portion (a so-called hexagonal portion) is usually formed on the metallic shell of the spark plug to be located adjacent to and on the front side of the crimped portion of the metallic shell. When the spark plug is to be mounted into a plug attachment hole formed in an internal combustion engine, a tool such as a wrench is engaged with the tool engagement portion. Conventionally, the tool engagement portion of a spark plug has dominantly employed an opposite side-to-side dimension of 16 mm or more, so that the cross-sectional area of the crimped portion can be 40 mm
2
or more. However, the previously mentioned tendency to decrease the diameter of a spark plug is also bringing about increasing demand for reducing the size of the tool engagement portion, for, for example, the following reasons: employment of a direct ignition method-in which individual ignition coils are directly attached to upper portions of corresponding spark plugs-narrows an available space above a cylinder head; and the previously mentioned increase in area occupied by valves forces a reduction in the diameter of plug holes. As a result, the opposite side-to-side dimension of the tool engagement portion is forced to be reduced to, for example, 14 mm or less from a conventionally available dimension of 16 mm or more. Condition A or B of the present invention provides the range of the cross-sectional area of the crimped portion in view of employing a metallic shell whose diameter is reduced such that the
NGK Spark Plug Co. Ltd.
Patel Vip
LandOfFree
Spark plug and method for manufacturing the spark plug does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Spark plug and method for manufacturing the spark plug, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Spark plug and method for manufacturing the spark plug will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3316971