Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1999-01-11
2001-11-20
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S135000, C313S144000
Reexamination Certificate
active
06320302
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to spark plugs having a copper core side wire, and more particularly, to a spark plug wherein the side wire includes a metal sheath welded to a metal spark plug shell using a novel “roll-over” technique.
2. Discussion
Conventional spark plug side wires, also referred to in the art as ground electrodes or outer electrodes, include solid nickel based embodiments. As illustrated in
FIG. 6
, conventional solid nickel-based side wires
200
are normally butt welded with a high welding force ranging from 220 to 300 lbs. to the shell
210
of the spark plug
220
. As can be seen from the weld profile for such a process, the solid nickel side wire
200
does not penetrate the steel shell
210
of the spark plug
220
.
More recently, spark plug side wires have been provided with copper cores in order to enhance thermal conductivity. However, welding such copper core side wires to spark plug shells, which are commonly made from metal such as steel, has been problematic. For instance, U.S. Pat. No. 4,970,426 discloses welding a copper core side wire to the end of the shell with the copper enclosed within the electrode sheath. This technique is used since copper is an electrically highly conductive material which makes it difficult to resistance weld copper directly to steel without causing voids and embrittlement in the copper adjacent to the weld interface.
If the copper core side wire has internal voids and embrittlement, it may easily break off of the metal shell. These problems may also hinder heat transfer and hence fail to keep the firing tip cool to retard erosion (wear) and reduce thermal stress. Although the process of the '426 patent successfully joins the side wire to the spark plug shell, it does not yield optimum thermal conductivity since the copper core does not directly contact the steel shell.
As a further attempt to eliminate the void and embrittlement problem, U.S. Pat. No. 5,210,457 discloses inserting a second core of nickel alloy or iron within the copper core of the side wire and welding that end to the spark plug shell. While somewhat successful, this method also has drawbacks relating to thermal conductivity and complexity.
Still yet another method of welding copper core side wires to spark plug shells is disclosed in U.S. Pat. No. 5,530,313. As illustrated in
FIG. 7
, the '313 patent discloses a “stake” method of welding wherein the side wire sheath
300
penetrates deeply into the steel shell
310
to provide an anchor for enhancing mechanical strength. The weld profile for such a process shows that the penetrating stakes
320
of the sheath
300
penetrate the shell
310
essentially vertically and do not roll under the copper core
330
. While the stake method is effective for joining the side wire to the shell, there is still room for improvement in the art.
In view of the foregoing, it would be desirable to provide a spark plug having a side wire with a copper core surrounded by a metal sheath which partially rolls over the end of the copper core at the weld interface for providing enhanced surface area for bonding and therefore high mechanical strength as well as direct contact between the copper core and the metal shell for enhancing thermal conductivity. It would also be desirable to provide a method of preparing a side wire stock for such a weld and a method for performing the weld.
SUMMARY OF THE INVENTION
The above and other objects are provided by a spark plug with a copper core side wire wherein the elongated copper core is encircled about its side walls or perimeter by a complementary sheath made from a metal such as a nickel-based alloy. The side wire has an open-end wherein the metal sheathing partially rolls over the exposed end of the copper core. As such, the laterally projecting portion of the sheathing overlying the outboard edges of the end of the copper core provides enhanced surface area for bonding to the steel shell of the spark plug thereby enhancing mechanical strength. Further, when the open-end of the copper core side wire is welded to the shell, the central portion of the copper core is in direct contact with the metal shell thereby enhancing thermal conductivity.
Additionally, the present invention includes a process of preparing the side wire for welding to the metal shell. To accomplish this, a piece of copper core side wire stock is pinch-cut using a cutter having a preselected degree of sharpness such that the metal sheath (due to its ductility) is initially rolled over the copper core and is thereafter cut entirely through. In this way, only a central portion of the copper core is exposed at the open end of the side wire while an outboard circumferential edge thereof is overlaid with a rolled over portion of the metal sheath. In a preferred embodiment of the present invention, the rolled over portion of the metal sheath tapers from a maximum thickness at the perimeter of the copper core to a minimum thickness proximate the exposed central portion thereof.
Still further, the present invention includes a process of welding a copper core side wire prepared as described above to the metal shell of a spark plug including the step of welding with a force ranging from about 90 lbs. to about 110 lbs. depending on the size of the side wire. The force is maintained for approximately four alternating current cycles at about 2.25 kiloamps. This process results in the side wire being bonded to the steel shell across a large surface area of the rolled over portion of the metal sheath while a central portion of the copper core is in direct contact with the steel shell in a non-brittle state. Preferably, the welding step ablates a center part of the tapered rolled over portion of the metal sheath adjacent the central portion of the copper core thereby exposing even more of the central portion of the copper core to the metal shell.
REFERENCES:
patent: 4970426 (1990-11-01), Bronchart
patent: 5210457 (1993-05-01), Oshima et al.
patent: 5530313 (1996-06-01), Chiu
patent: 3-149789 (1991-06-01), None
Honeywell International , Inc.
Patel Vip
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