Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Reexamination Certificate
1999-06-17
2003-04-15
Ludlow, Jan (Department: 1743)
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
C422S094000, C422S098000
Reexamination Certificate
active
06548023
ABSTRACT:
This application claims the benefit of Japanese Patent Application No. Hei. 10-171438, filed in Japan on Jun. 18, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, HC sensor, or NOx sensor, for detecting a component in gas to be measured,
2. Description of the Related Art Conventional gas sensors have a structure in which a metallic casing accommodates a bar-like or cylindrical sensing element which has a sensing portion formed at its tip end and is adapted to detect a component in gas. The metallic casing includes a combination of a plurality of cylindrical members, such as a metallic shell, a protector, an inner cylindrical member, and an outer cylindrical member. The metallic shell has a screw portion formed on its outer surface that is used for attachment. The protector is connected to the metallic shell in such a manner as to cover the sensing portion of a sensing element which projects from one end of the metallic shell. The inner cylindrical member is connected to the other end of the metallic shell and adapted to cover the sensing element which extends rearward from the metallic shell; i.e., opposite the protector with respect to the metallic shell. The outer cylindrical member is connected to a rear end portion of the inner cylindrical member and allows a lead wire from the sensing element to extend rearward from a rear open end thereof.
The inner cylindrical member and the outer cylindrical member are made of, for example, stainless steel, In many gas sensors, the inner and outer cylindrical members are caulked together. Specifically, an end portion of the outer cylindrical member is fitted onto a corresponding end portion of the inner cylindrical member to thereby form an overlap zone. In the overlap zone, the outer cylindrical member is circumferentially caulked to the inner cylindrical member, thereby forming an annular caulked portion and thus bonding the members in an airtight manner.
Adjusting the hardness of the inner and outer cylindrical members before the inner and outer cylindrical members are caulked as described above is an important measure for attaining a caulked seal which is free from play or loosening and exhibits excellent airtightness. For example, Japanese Patent Application Laid-Open (kokai) No. 9-210953 describes adjustment of the hardness of a stainless steel inner cylindrical member (inner cover) of an oxygen sensor to Hv 150 to 350 on the Vickers scale and the hardness of a stainless steel outer cylindrical member (outer cover) of the oxygen sensor to Hv 100 to 300 on the Vickers scale in order to attain a tightly caulked seal having excellent resistance to vibration. The publication also describes an embodiment in which the outer cylindrical member having a thickness of 0.5 mm a hardness of Hv 150 and the inner cylindrical member having a thickness of 0.7 mm has a hardness of Hv 240.
Gas sensors, e.g., oxygen sensors are often mounted on an exhaust pipe or exhaust manifold through which high-temperature exhaust gas flows, Accordingly, sensor temperature becomes approximately equal to ambient temperature in an idle state, whereas it increases to hundreds of degrees centigrade in high-speed high-load operation. Therefore, the oxygen sensor is repeatedly subjected to a considerably severe thermal shock. The publication described above discusses the effects of the caulked seal being subjected to vibration or water splashes (or submergence), but fails to address in detail the effects of thermal shock. The inventors of the present invention carried out intensive studies, and as a result, found that when the outer and inner cylindrical members assume the values of hardness appearing in the publication, in some cases the caulked portion fails to maintain good airtightness after being subjected to a severe thermal history.
An object of the present invention is to provide a gas sensor having a structure such that an inner cylindrical member and an outer cylindrical member, which cover a sensing element, are caulked together to form a caulked portion, and the caulked portion can maintain good airtightness even in a working environment involving, for example, thermal shocks.
SUMMARY OF THE INVENTION
To achieve the above object, the present invention provides a gas sensor comprising a bar-like or cylindrical sensing element and a cylindrical casing. The sensing element has a sensing portion formed at a tip end portion thereof and is adapted to detect a component in gas to be measured (gas under measurement).
The cylindrical casing covers the sensing element while the gas under measurement is permitted to flow therethrough to the sensing portion. The casing includes at least two axially adjacent cylindrical members. An end portion of one cylindrical member (hereinafter referred to as an inner member) is disposed within a corresponding end portion of the other cylindrical member (hereinafter referred to as an outer member) to thereby form an overlap zone.
The inner cylindrical member has a Vickers hardness H
1
of Hv 250 to 430; the outer cylindrical member has a Vickers hardness H
2
of Hv 160 to 330; and the hardness difference “H
1
-H
2
” therebetween is not less than 30. In the overlap zone, the inner cylindrical member and the outer cylindrical member are circumferentially caulked in an airtight manner.
According to the above-described structure of the gas sensor,the inner cylindrical member and the outer cylindrical member assume hardness values within the above ranges, so that the caulked portion can maintain good airtightness even in a working environment involving, for example, thermal shocks.
A key structural point of the present invention is that the Vickers hardness H
2
of the outer cylindrical member assumes a value of at least Hv 160, which is higher than that disclosed in the aforementioned publication. Through increase in the Vickers hardness H
2
of the outer cylindrical member, the hardness of the caulked portion is increased, thereby suppressing deformation of the caulked portion caused by thermal stress induced therein during exposure to heat cycles, and thus preventing the caulked portion from loosening, while yielding a resultant improvement in airtightness. Further, the Vickers hardness H
1
of the inner cylindrical member is adjusted to not less than Hv 250, and the hardness difference “H
1
-H
2
” between the inner and outer cylindrical members is set to not less than 30. Thus, when the outer cylindrical member having the above high hardness is caulked onto the inner cylindrical member, the inner cylindrical member can sufficiently receive the caulking force, whereby the caulked portion can attain strong adhesion.
When the Vickers hardness H
2
of the outer cylindrical member is less than Hv 160, the thermal shock resistance of the caulked portion deteriorates, resulting in a failure to secure sufficient airtightness upon exposure to repeated heat cycles. When the hardness difference “H
1
-H
2
” between the inner and outer cylindrical members is less than 30, the inner cylindrical member fails to sufficiently receive the caulking force and thus suffers an undesirably intensive deformation, Also, the caulking-related deformation of the outer cylindrical member becomes unsatisfactory. As a result, the caulked portion potentially fails to maintain airtightness. When the Vickers hardness H
1
of the inner cylindrical member is less than Hv 250 or the Vickers hardness H
2
of the outer cylindrical member is in excess of Hv 330, the thermal shock resistance of the caulked portion deteriorates and the caulked portion fails to maintain sufficient airtightness upon exposure to repeated heat cycles. The reason for this is as follows. When the Vickers hardness H
1
of the inner cylindrical member is less than Hv 250, rigidity of the inner cylindrical member becomes insufficient; consequently, conceivably, the inner cylindrical member undergoes undesirably intensive deformation during caulking, resulting in impairment
Akatsuka Shoji
Ishikawa Satoshi
Matsuo Kouji
Ohtsuki Shoichi
Ludlow Jan
NGK Spark Plug Co. Ltd.
Sines Brian
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