Oxygen sensor

An oxygen sensor includes a tubular metal shell, a tubular housing having an end attached to the metal shell and another end sealed by a seal member. A solid electrolyte tube is attached to the metal shell and has on inner and outer surfaces thereof electrode layers and lead layers connected to the electrode layers, respectively. A pair of lead wires extend through the seal member between inside and outside of a housing assembly constituted by the metal shell and housing while being sealingly held by the seal member. A pair of metallic leads are disposed inside of the housing assembly for connection between the lead layers and the lead wires, the metallic leads having at one end thereof electrode connecting portions having a spring property, the electrode connecting portions being attached to the inner and outer surfaces of the solid electrolyte tube and held in contact with the lead layers under spring pressure. A stopper is interposed between the seal member and the open end of the solid electrolyte tube so as to obstruct axial movement of the electrode connecting portions of the metallic leads relative to the solid electrolyte tube when the lead wires are pulled outward of the housing assembly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an oxygen sensor used, for example, for 
detecting an oxygen content in the exhaust gases emitted from an internal 
combustion engine. 
2. Description of the Prior Art 
An oxygen sensor of this kind is made up of an oxygen content detecting 
element having a tubular shape with one end closed and made of solid 
electrolyte and a metallic housing assembly including a metal shell for 
holding the element thereon. The oxygen sensor is installed on an exhaust 
pipe of an internal combustion engine by way of the metal shell to bring 
an electrode layer (i.e., reference electrode layer) on the inner surface 
of the element in contact with a reference oxygen gas (i.e., atmospheric 
air) and an electrode layer (i.e., measurement electrode layer) on the 
outer surface of the element in contact with the exhaust gases. The oxygen 
sensor is adapted to cause an electromotive force (i.e., potential 
difference) between the electrodes in accordance with the difference of 
the oxygen content between the inner and outer surfaces of the element and 
output a signal representative of the electromotive force to a control 
circuit for thereby detecting the oxygen content in the exhaust gases and 
controlling the same. 
The electrode layers formed on the inner and outer surfaces of the element 
of the oxygen sensor are connected by way of metallic leads to an output 
lead wire for connection to a control circuit and a grounding lead wire. 
The metallic leads have at one end thereof electrode connecting portions 
having a spring property. The electrode connecting portions are pushed 
axially of the element to fit in and on the element and held in contact 
with the lead layers under spring pressure. The metallic leads are 
connected at another end thereof to lead wires. 
FIG. 13 shows a prior art oxygen sensor of the type utilizing metallic 
leads for connection between lead layers and lead wires. A metallic lead 
51 is employed for connection between a lead layer 14 connected to an 
electrode layer on the inner surface of an oxygen content detecting 
element 3 and an output lead wire 12. The metallic lead 51 is constructed 
as shown in FIG. 14 and is formed from a metal plate having a spring 
property. The metallic lead 51 has an electrode connecting portion 52 at 
one end or lower end thereof. The electrode connecting portion 52 is 
formed into a tubular shape having a part-circular cross section. The 
electrode connecting portion 52 has, in an unloaded or unstressed state, 
an outer diameter that is a little larger than the inner diameter of the 
tubular lead layer 14 formed on the inner surface of the element 3. The 
metallic lead 51 further has a nearly vertical intermediate or stem 
portion 53 and a crimp-style terminal portion 54 of a U-like cross section 
at another end or upper end thereof. The crimp-style terminal portion 54 
is provided for connection to lead wires. The intermediate portion 53 is 
provided between the electrode connecting portion 52 and the crimp-style 
terminal portion 54 for interconnecting therebetween. The electrode 
connecting portion 52 has a smaller diameter lower end portion for forming 
a ceramic heater gripping portion 55 of a part-circular cross section. The 
ceramic heater gripping portion 55 is smaller in diameter than the 
electrode connecting portion 52 and adapted to grip or hold a ceramic 
heater 18 under spring pressure. The ceramic heater gripping portion 55 
has, in its unloaded or unstressed state, an inner diameter smaller than 
the outer diameter of the ceramic heater 18. With such a metallic lead 51, 
the crimp-style terminal portion 54 is connected by crimping to an end 
portion 12a of a lead wire 12 extending through a sealing member 16, and 
the ceramic heater gripping portion 55 grips a heater 18. Under the above 
condition of the metallic lead 51, the electrode connecting portion 52 is 
pushed axially into the element 3 and brought in contact with the inner 
lead layer 14 under spring pressure or by spring force. In the meantime, 
61 in FIG. 13 is an insulator made of a ceramic material, or the like and 
has a tubular shape or hollow cylindrical shape. The insulator 61 is 
arranged in contact with the bottom surface of the seal member 16 and has 
through holes consisting of smaller diameter through hole sections 62 
aligned with respective through holes 16b of the seal member 16 and lower 
larger diameter through hole sections 63. By accommodating the crimp-style 
terminal portion 54 of the metallic terminal 51 within one of the through 
hole sections 63, the insulator 61 insulates the crimp-style terminal 
portion 54 of the metallic lead 51 from a crimp-style terminal portion 74 
of a metallic lead 71 and metallic housing inner and outer tubes 64 and 
65. 
The metallic leads 71 is provided for connection between a lead layer 15 
connected to an electrode layer on the outer surface of the element 3 and 
a grounding lead wire 13. The metallic lead 71 is constructed as shown in 
FIG. 15 and basically similar to the above described output metallic lead 
51. That is, the metallic lead 71 is formed from a metal plate having a 
spring property and has an electrode connecting portion 72 at one or lower 
end thereof. The electrode connecting portion 72 is formed into a tubular 
shape having a part-circular cross section. The electrode connecting 
portion 72 has, in an unloaded or unstressed condition, an inner diameter 
smaller than the outer diameter of the tubular lead layer 15 formed on the 
outer surface of the element 3. The metallic lead 71 further has an 
intermediate or stem portion 73 and a crimp-style terminal portion 74 of a 
U-like cross section. The crimp-style terminal portion 74 is provided for 
connecting to the grounding lead wire 13 extending through the seal member 
16. The intermediate portion 73 is provided between the electrode 
connecting portion 72 and the crimp-style terminal portion 74 for 
interconnecting therebetween. Under the condition of the crimp-style 
terminal portion 74 being connected to the inner end 13a of the grounding 
lead wire 13, the electrode connecting portion 72 is pushed axially of the 
element 3 to fit on the element 3 and is brought in contact with the lead 
layer 15 under spring pressure. 
By the above structure, electrical connection between the metallic leads 51 
and 71 and the electrode connecting portions 52 and 72 and attachment or 
fixing of them can be attained with ease since such electrical connection 
and attachment or fixing only requires to push the metallic leads 51 and 
71 against the element 3 and make them fit in and on the open end of the 
element 3. 
In such an oxygen sensor, the lead wires 12 and 13 may possibly be 
subjected to a tensile force even after installation of the sensor on an 
exhaust pipe for various reasons. The seal member 16 is thus required to 
not only sealingly surround the lead wires but fixedly hold the lead 
wires. However, the force exerted by the seal member for fixedly holding 
the lead wires is not sufficiently large. While in the prior art oxygen 
sensor, the insulator 61 is provided in contact with the seal member 16, 
such an insulator 61 can scarcely exert a force for obstructing axial 
movement of the lead wires. 
For the above reason, the above described prior art oxygen sensor has a 
possibility that when the lead wires 12 and 13 are pulled outward by a 
relatively large force, the electrode connecting portions 52 and 72 
connected to the inner and outer surface lead layers 14 and 15 of the 
element 3 are moved upward of the respective lead layers 14 and 15 as 
shown by the two-dot chain lines in FIG. 13 or may be separated from the 
element 3 in case the force is excessively large. Thus, the above 
described prior art oxygen sensor has a problem on the reliability and 
durability, i.e., a problem that unstable electrical connection and 
unstable outputting may occur or malfunction may occur due to 
disconnection by the separation, thus disabling accurate detection and 
control of oxygen content in the exhaust gases. 
SUMMARY OF THE INVENTION 
According to an aspect of the present invention, there is provided an 
oxygen sensor that comprises an oxygen content detecting element made of 
solid electrolyte and in the form of a circular tube with one end closed. 
Electrode layers are provided to inner and outer peripheral surfaces of 
the oxygen content detecting element, and a lead layer is provided to the 
oxygen content detecting element and connected to one of the electrode 
layers. A metallic lead having at one end thereof an electrode connecting 
portion having a spring property is provided, the electrode connecting 
portion being attached to the oxygen content detecting element and held in 
contact with the lead layer under spring pressure. The metallic lead is 
connected at another end thereof to a lead wire. A seal member sealingly 
holds the lead wire while allowing the same to extend outward thereof. A 
stopper is engaged with the electrode connecting portion of the metallic 
lead to prevent the electrode connecting portion from being separated from 
the lead layer when the lead wire is pulled outward to subject the 
metallic lead to a tensile force. By the above structure, even when the 
lead wire, which is connected to either of the lead layer on the inner 
surface of the oxygen detecting element or the lead layer on the outer 
surface of same, is pulled outward of the sensor to subject the metallic 
lead to a tensile force, the stopper effectively prevents the electrode 
connecting portion from being separated from the lead layer of the oxygen 
detecting element. Thus, the oxygen sensor of this invention enables the 
sensor to effect a stable operation in electrical conduction and 
outputting. As a result, the sensor of this invention can effect an 
improved accuracy in detection of oxygen content. 
According to another aspect of the present invention, the stopper is 
interposed between the oxygen content detecting element and the seal 
member and has a hollow cylindrical portion surrounding the metallic lead 
radially thereof. By such a stopper, it becomes hard for liquid or gas to 
enter the inside of the oxygen content detecting element. For this sake, 
pollution of the reference oxygen sensor can be prevented with more 
efficiency, thus lowering of the accuracy in detection of oxygen content 
being prevented. 
According to a further aspect of the present invention, the stopper is made 
of a ceramic material. This is desirable since it can effect an improved 
insulation of the metallic lead and an improved heat resistant property. 
According to a further aspect of the present invention, there is provided 
an oxygen sensor comprising an oxygen content detecting element made of 
solid electrolyte and in the form of a circular tube with one end closed. 
Electrode layers are provided to inner and outer surfaces of the oxygen 
content detecting element and lead layers are provided to the inner and 
outer surfaces of the oxygen content detecting element and connected to 
the electrode layers, respectively. A pair of metallic leads having at one 
end thereof electrode connecting portions having a spring property are 
provided, the electrode connecting portions being attached to the oxygen 
content detecting element and held in contact with the lead layers under 
spring pressure, respectively. The metallic leads are connected at another 
end thereof to lead wires. A seal member sealingly holds the lead wires 
while allowing the same to extend outward thereof. A stopper is engaged 
with the electrode connecting portions of the metallic leads to prevent 
the electrode connecting portions from being separated from the lead 
layers when the lead wires are pulled to subject the metallic leads to a 
tensile force. 
According to a further aspect of the present invention, there is provided 
an oxygen sensor comprising a tubular metal shell. A tubular housing is 
attached at one end thereof to the metal shell to constitute a housing 
assembly. A seal member is attached to another end of the tubular housing 
for sealingly closing the same. A solid electrolyte tube with one open is 
attached to the metal shell with the open end positioned inside of the 
housing assembly. Electrode layers are provided to inner and outer 
surfaces of the solid electrolyte tube and a lead layer is provided to the 
solid electrolyte tube and connected to one of the electrode layers. A 
lead wire extends through the seal member between inside and outside of 
the housing. While sealingly held by the seal member. The lead wire having 
an inner end is disposed inside of the housing. A metallic lead is 
disposed inside of the housing assembly for connection between the lead 
layer and the inner end of the lead wire, the metallic lead having at one 
end thereof an electrode connecting portion having a spring property. The 
electrode connecting portion is attached to one of the inner and outer 
surfaces of the solid electrolyte tube and is held in contact with the 
lead layer under spring pressure. A stopper is interposed between the seal 
member and the open end of the solid electrolyte tube and engaged with the 
electrode connecting portion to obstruct axial movement of the electrode 
connecting portion relative to the solid electrolyte tube when the lead 
wire is pulled outward of the housing assembly. 
According to a further aspect of the present invention, the oxygen sensor 
further comprises a separator interposed between the sealing member and 
the stopper for insulating another end of the metallic lead relative to 
the housing assembly. The stopper is formed integral with or separately 
from the separator. 
According to a further aspect of the present invention, the electrode 
connecting portion of the metallic lead has a part-circular cross section 
and has an axial end face flush with a face of the open end of the 
electrolyte tube. The stopper is hollow and cylindrical for allowing 
passage of the metallic lead therethrough and has an axial end abuttingly 
engaged with the open end of the solid electrolyte tube. 
According to a further aspect of the present invention, the electrode 
connecting portion of the metallic lead is fitted on the open end of the 
solid electrolyte tube, the stopper having at the axial end a cross 
section including a four-cusped figure-like central opening for passage of 
the metallic lead, and a solid peripheral portion for abutting engagement 
with the axial end face of the electrode connecting portion. 
According to a further aspect of the present invention, there is provided 
an oxygen sensor comprising a tubular metal shell, a tubular housing is 
attached at one end thereof to the metal shell. A seal member is attached 
to another end of the tubular housing for sealingly closing the same. A 
solid electrolyte tube with one open end is attached to the metal shell 
with the open end positioned inside of the housing assembly, the solid 
electrolyte tube having on inner and outer surfaces thereof electrode 
layers and lead layers connected to the electrode layers, respectively. A 
pair of lead wires extend through the seal member between inside and 
outside of the housing assembly while sealingly held by the seal member. 
The lead wires having inner ends are disposed inside of the housing. A 
pair of metallic leads are disposed inside of the housing for connection 
between the lead layers and the inner ends of the lead wires, the metallic 
leads having at one end thereof electrode connecting portions having a 
spring property. The electrode connecting portions are attached to the 
inner and outer surfaces of the solid electrolyte tube and are held in 
contact with the lead layers under spring pressure, respectively. A 
stopper is interposed between the seal member and the open end of the 
solid electrolyte tube and is engaged with the electrode connecting 
portions to obstruct axial movement of the electrode connecting portions 
relative to the solid electrolyte tube when the lead wires are pulled 
outward of the housing. 
According to a further aspect of the present invention, the oxygen sensor 
further comprises a separator interposed between the sealing member and 
the stopper for insulating another end of the metallic leads relative to 
the housing assembly. The stopper is formed integral with or separately 
from the separator. 
According to a further aspect of the present invention, each of the 
electrode connecting portions of the metallic leads has a part-circular 
cross section and has an axial end face flush with a face of the open end 
of the electrolyte tube. The stopper is hollow and cylindrical for 
allowing passage of the metallic leads therethrough and has an axial end 
abuttingly engaged with the open end of the solid electrolyte tube. 
According to a further aspect of the present invention, the stopper has at 
the axial end a cross section including two circular openings for passage 
of the metallic leads, a radially inner solid portion for abutting 
engagement with the axial end face of one of the electrode connecting 
portions fitted in the open end of the solid electrolyte tube and a 
radially outer solid portion for abutting engagement with the axial end 
face of the other of the electrode connecting portions fitted on the open 
end of the solid electrolyte tube. 
According to a further aspect of the present invention, the oxygen sensor 
further comprises a heater accommodated in the housing. A pair of lead 
wires are connected to the heater and extend between inside and outside of 
the housing through the stopper and the sealing member while sealingly 
held by the seal member. The stopper has at the above mentioned end a 
cross section including a four-cusped figure-like opening including a 
central opening section for disposition of the heater, a four-cusped 
figure section for disposition of the lead wires connected to the lead 
layers and the lead wires connected to the heater, a radially outer 
peripheral portion for abutting engagement with the axial end face of one 
of the electrode connecting portions fitted on the open end of the 
electrolyte tube, and four radially inwardly protruded portions for 
abutting engagement with the end face of the other of the electrode 
connecting portions fitted in the open end of the solid electrolyte tube. 
According to a further aspect of the present invention, the stopper has a 
plurality of axial slits in such a manner as to allow the four cusped 
figure section opening radially outward so that the cross section of the 
stopper at the above mentioned end thereof consists of four independent 
solid portions. 
The above structure is effective for solving the above noted problems 
inherent in the prior art device. 
It is accordingly an object of the present invention to provide a novel and 
improved oxygen sensor that is highly reliable in operation and durable. 
It is a further aspect of the present invention to provide a novel and 
improved oxygen sensor of the above described character that enables 
electrode connecting portions of metallic leads to be assuredly held in 
contact with lead layers of oxygen detecting element even when the lead 
wires are pulled outward of the sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIGS. 1 to 7, an oxygen sensor is generally indicated by 
1 and shown as being of the type equipped with a ceramic heater, i.e., of 
the type having four lead wires. The oxygen sensor 1 includes a tubular 
metal shell 2 having a threaded portion 2a for attachment to an exhaust 
manifold or pipe of an internal combustion engine, and a tubular oxygen 
content detecting element or solid electrolyte tube 3 with one end closed 
and another end open. The solid electrolyte tube 3 has at a nearly 
longitudinal center thereof an outward flange 3a received inside the metal 
shell 2. The solid electrolyte tube 3 is fixedly attached to the metal 
shell 2 by interposing therebetween a tubular insulation member 4 made of 
a ceramic material, a seal member 5 made of powdered talc, a seal pipe 6, 
packings 7 and 8, and a housing inner tube 9 that cooperates with a 
housing outer tube 11 to constitute a tubular housing and further with the 
metal shell 2 to constitute a metallic housing assembly. The housing inner 
tube 9 is disposed at the rear end (i.e. the upper end in FIG. 1) of the 
metal shell 2 and has a flanged lower end 9a fixedly attached to the metal 
shell 2 by interposing between the packing 8 and a bent or crimped upper 
end 2b of the metal shell 2. By this arrangement, an insulation between 
the metal shell 2 and the solid electrolyte tube 3 can be attained, 
together with a seal between the metal shell 2 and the solid electrolyte 
tube 3 (i.e., between an exhaust gas side and a reference gas side) and a 
seal between the metal shell 2 and the housing inner tube 9 (i.e., air 
tight seal or liquid tight seal of the sensor 1). For attaining such a 
seal, the rear or upper end 2b of the metal shell 2 is bent so as to be 
laid over the lower end flange 9a of the housing inner tube 9 and crimped 
by pressing axially of the sensor 1. In the above manner, the solid 
electrolyte tube 3 is attached to the metal shell 2 with the open end 
thereof accommodated in the housing assembly constituted by the metal 
shell 2, the housing inner tube 9 and the housing outer tube 11. In the 
meantime, 10 is a protective cap attached to the front or lower end of the 
metal shell 2 for covering a portion of the solid electrolyte tube 3 
protruding from the metallic housing assembly for thereby protecting the 
solid electrolyte tube 3. 
The housing outer tube 11 is fitted on the upper end portion of the housing 
inner tube 9 to constitute the above described housing. An output lead 
wire 12 and a grounding lead wire 13 have inner ends disposed inside the 
housing and connected to an inner surface lead layer 14 and an outer 
surface lead layer 15 by way of the same metallic leads 51 and 71 as those 
shown in FIGS. 14 and 15. The lead layers 14 and 15 are formed on the 
inner and outer surfaces of the solid electrolyte tube 3 at a location 
adjacent the open end of the solid electrolyte tube 3. The lead layers 14 
and 15 are connected to the electrode layers 3b and 3c formed on the inner 
and outer surfaces of the solid electrolyte tube 3, respectively. The 
metallic leads 51 and 71 have at one end thereof electrode connecting 
portions 52 and 72 for connection to the inner surface lead layer 14 and 
the outer surface lead layer 15 and at another end crimp-style terminal 
portions 54 and 74 for connecting to the lead wires 12 and 13. The 
electrode connecting portions 52 and 72 are respectively pushed against 
the open end of the solid electrolyte tube 3 and fitted in and on the 
inner surface lead layer 14 and the outer surface lead layer 15 of the 
solid electrolyte tube 3 in such a manner that upper end faces 52a and 72a 
of the electrode connecting portions 52 and 72 are flush with the face of 
the open end or upper end of the solid electrolyte tube 3. The lead wires 
12 and 13 have inner ends 12a and 13a disposed inside of the housing and 
connected to the crimp-style terminal portions 54 and 74 of the metallic 
leads 51 and 71. The lead wires 12 and 13 extend outward of the housing 
through a seal member 16 whilst being sealingly held by the seal member 
16. In the meantime, indicated by 17 is a pair of lead wires, though only 
one is shown in FIG. 1, which is connected to a ceramic heater 18 for 
heating the solid electrolyte tube 3. The lead wires 17 extend outward of 
the housing through the seal member 16 while sealingly held by the seal 
member 16. The seal member 16 is attached to the upper end of the housing 
outer tube 11 to sealingly close the same. The seal member 16 is in the 
form of a disc and has a recess 16a at a nearly central portion of its 
bottom. The seal member 16 has four axial through holes 16b for allowing 
passage of the lead wires 12, 13 and 17 therethrough. The through holes 
16b are arranged in a circular array with equal intervals. It is desirable 
to form the seal member 16 of a fluorine resin having a high heat 
resistant property. In this embodiment, the seal member 16 is made of PFA 
(tetrafluoroethylene-perfluoroalkyvinylester). 
An important feature of the present invention resides in a stopper 21, 
which will now be described. The stopper 21 has a hollow cylindrical body 
30 hanging from a lower end of a separator 22. That is, the stopper 21 in 
this embodiment is formed integral with the separator 22 to constitute a 
single piece. 
In FIGS. 4A and 4B, the portion above the two-dot chain line represents the 
separator 22. The separator 22 is similar to that of the above described 
prior art oxygen sensor and made of a ceramic material. The separator 22 
has an upper end 24 fittingly engaged in the recessed bottom of the seal 
member 16. The separator 22 is in the form of a disc and adapted to fit at 
an upper end 24 in the recess 16a of the seal member 16. The separator 22 
has four stepped through holes axially aligned with the through holes 16b 
of the seal member 16 and includes upper smaller diameter through hole 
sections 25 and lower larger diameter through hole sections 26. The larger 
diameter through hole sections 26 are adapted to accommodate therewithin 
the crimp-style terminal portions 54 and 74 of the metallic leads 51 and 
71 to connect thereto the inner ends 12a and 13a of the lead wires 12 and 
13, respectively. The separator 22 has at a location adjacent the upper 
end thereof an outward flange 27 having an upper end face 28 brought into 
abutting engagement with the bottom face of the seal member 16. The 
separator 22 has at a location axially under the flange 27 a hollow 
cylindrical portion 29 of the same outer diameter as the hollow 
cylindrical body 30. 
The stopper 21 has a lower end face 23 in abutting engagement with the 
upper end faces 52a and 72a of the electrode connecting portions 52 and 72 
of the metallic leads, i.e., in abutting engagement with the face of the 
open end of the solid electrolyte tube 3. The stopper 21 further has a 
through hole consisting of axial hole sections 32 larger in diameter than 
the through hole sections 26 and axially aligned with the same, 
respectively and a center hole section 33 concentric with the hollow 
cylindrical body 30. Thus, the stopper 21 of this embodiment has at the 
lower end thereof a cross section including a four-cusped figure-like 
opening including a central opening section 33 for disposition of the 
ceramic heater 18, a four-cusped figure section for disposition of the 
metallic leads 51 and 71 connected to the lead layers 14 and 15 and the 
metallic leads 18b connected to the ceramic heater 18, a radially outer 
peripheral portion 23a for abutting engagement with the axial end face 72a 
of the electrode connecting portion 72 fitted on the open end of the solid 
electrolyte tube 3, and four radially inwardly protruded portions 23b for 
abutting engagement with the end face 52a of the electrode connecting 
portion 52 fitted in the open end of the solid electrolyte tube 3. 
In the oxygen sensor 1, when the lead wires 12 and 13 are pulled to subject 
the electrode connecting portions 52 and 72 of the metallic leads 51 and 
71 to a tensile force so that the electrode connecting portions 52 and 72 
are urged for removal or separation from the lead layers 14 and 15, axial 
movements of the electrode connecting portions 52 and 72 are stopped or 
blocked by the lower end face 23 of the stopper 21 so that separation or 
removal of the electrode connecting portions 52 and 72 from the lead 
layers 14 and 15 is prevented. Furthermore, as shown in FIG. 7, the outer 
peripheral part 23a of the lower end face 23 of the stopper 21 is engaged 
almost at its entirety with the upper end face 72a of the electrode 
connecting portion 72 connected to the outer surface lead layer 15, 
whereas the four radially inwardly protruded portions 23b are engaged, 
with equal intervals, with the upper end face 52a of the electrode 
connecting portion 52 connected to the inner surface lead layer 14 so that 
each radially inwardly protruded portions 23b effect an equal blocking 
action. Accordingly, inclination of the electrode connecting portions 52 
and 72 does not occur, thus making it possible to obtain stable and 
assured conduction between the electrode connecting portions 52 and 72 and 
the lead layers 14 and 15. 
Further, the stopper 21 with the hollow cylindrical body 30 is adapted to 
surround the metallic leads 51 and 71 radially Whereof and at a location 
between the open end of the solid electrolyte tube 3 and the crimp-style 
terminal portions 54 and 74 of the metallic leads 51 and 71, and the lower 
end face 23 of the stopper 21 is adapted to be matched with or joined with 
the open end face of the solid electrolyte tube 3. By this arrangement, 
even if deterioration occurs in the seal between the solid electrolyte 
tube 3 and the lower part of the metal shell 2, thus allowing oil, water, 
exhaust gases or gasoline to intrude into the inside of the housing inner 
tube 9, it is difficult for such liquid and gas to further intrude into 
the inside of the oxygen sensor 1 due to the provision of the stopper 21 
so that the reference oxygen gas (open air) does not become polluted or 
contaminated. Accordingly, the oxygen sensor 1 can prevent deterioration 
of the oxygen content detecting ability and corrosion of the inner surface 
electrode layer 14 and can effect a reliable operation for an elongated 
period. 
The oxygen sensor 1 is assembled as follows. 
Firstly, the solid electrolyte tube 3 and the housing inner tube 9 are 
fixed to the metal shell 2 to prepare a front end side subassembly. 
On the other hand, the ends 12a and 13a of the lead wires 12 and 13 are 
connected to the inner face and outer face lead layers 14 and 15 of the 
solid electrolyte tube 3 by way of the metallic leads 51 and 71 in the 
following manner. That is, the end 12a of the output lead wire 12 is let 
to pass through the through hole 16b of the seal member 16 and the through 
holes 25, 26 and 32 of the separator 22 and the stopper 21. The end 12a of 
the lead wire 12 is then connected by crimping to the crimp-style terminal 
portion 54 of the metallic lead 51. The ceramic heater 18 is gripped by 
the heater gripping portion 55. The end 13a of the grounding lead wire 13 
is let to pass similarly through the through hole 16b of the seal member 
16 and the through holes 25, 26 and 32 of the separator 22 and the stopper 
21 and is connected by crimping to the crimp-style terminal portion 74 of 
the metallic lead 71. The end portions of a pair of lead wires 17 
connected to the ceramic heater 18 are let to pass through the through 
holes 16b of the seal member 16 and the through holes 25, 26 and 32 of the 
separator 22 and the stopper 21 and connected by soldering to the 
connecting terminals 18a of the heater 18 by way of intermediate metallic 
leads 18b. The thus obtained assembly is fitted in the housing outer tube 
11 previously installed on the lead wire side in such a manner that the 
seal member 16 is abuttingly engaged with the upper bent end 11a of the 
housing outer tube 11 for thereby sealingly closing the same and the upper 
end 24 of the stopper 21 is fittingly engaged in the recess 16a of the 
seal member 16, whereby a rear end side subassembly is prepared. 
Then, the rear end side subassembly and the front end side subassembly are 
assembled in such a manner that the housing outer tube 11 is fitted on the 
housing inner tube 9 and the electrode connecting portions 52 and 72 are 
pushed axially into and onto the open end of the solid electrolyte tube 3 
so as to be brought in contact with the inner face and outer face lead 
layers 14 and 15 under spring pressure and thereby electrically connected 
to same. The upper end portion of the above described housing outer tube 
11 is pressed axially by a predetermined compression force and heated to a 
predetermined temperature so that the seal member 16 is deformed so as to 
seal the interface between the lead wires 12, 13 and 15 and the seal 
member 16, and the lower end portion of the housing outer tube 11 and the 
inner tube 9 are driven inward so as to reduce in diameter and thus joined 
together by caulking. In the meantime, a packing 34 made of fluororesin is 
disposed between the lower face of the flange 27 of the separator 22 and 
the upper end portion of the inner tube 9 to provide a seal therebetween. 
In the prior sensor as shown in FIG. 13, there is not provided between the 
separator 61 and the electrode connecting portions 52 and 72 any structure 
for transmitting a pushing force therebetween. Thus, in order to push each 
electrode connecting portions 52 and 72 into the respective lead layers 14 
and 15, a particular metallic jig for assemblage is used to insert the 
electrode connecting portions 52 and 72 into the housing inner tube 64 and 
thereafter the separator 61 and the seal member 16 are installed in 
sequence. In contrast to this, in this embodiment, the stopper 21 can 
transmit a pushing force for pushing the electrode connecting portions 52 
and 72 against the respective lead layers 14 and 15. By previously 
assembling the seal member 16, the stopper 21 and the metallic leads 51 
and 71 into a single unit so that they are inserted into the inner tube 9 
all at once, it becomes possible to push the electrode connecting portions 
52 and 72 into the respective lead layers 14 and 15 located at the 
innermost part of the housing inner tube 9 without using a particular jig. 
Accordingly, the oxygen sensor 1 of this embodiment can effect easier 
assemblage. 
In the foregoing, it is to be noted that the stopper 21 is not limited to 
what has been described and shown in the above embodiment. For example, 
the stopper 21 can be designed to have another shape and structured so 
long as it is held engaged with the electrode connecting portions 52 and 
72 and prevents them from being separated from the lead layers 14 and 15 
of the solid electrolyte tube 3 when the lead wires 12 and 13 are pulled 
outward to subject the metallic leads 51 and 71 to a tensile force. 
Accordingly, although an action of preventing, in case oil or the like 
enters into the housing inner tube 9, such oil or the like from intruding 
further into the inside of the oxygen sensor 1 as is obtained in the above 
described embodiment, is eliminated, slits 35 may be formed in the 
cylindrical body 30 of the stopper 21 in the above described embodiment 
with suitable intervals. By the provision of the slits 35, the sensor can 
be made lighter. In this connection, the stopper 21 in FIG. 8 has a 
plurality of axial slits 35 in such a manner as to allow the four-cusped 
figure section 32 to open radially outward so that the cross section of 
the stopper 21 at the lower end thereof consists of four independent solid 
portions. 
Further, while in the above described embodiment the stopper 21 is formed 
integral with the separator 22, they can be separated in the place 
indicated by the two-dot chain line in FIG. 4 so that the separator 22 and 
the stopper 21 are formed separately from each other as shown in FIG. 9. 
In FIG. 9, the lower end outer portion of the separator 22 is formed into 
a stepped shape and the upper end inner circumferential portion of the 
stopper 21 is stepped correspondingly so that they can be fitted together. 
Except for this, the stopper 21 and separator 22 are the same as that 
shown in FIG. 2 so similar reference characters are used to indicate 
similar parts and portions. 
Further, in the foregoing, the oxygen sensor 1 has been described and shown 
as being of the type having four leads, i.e., a pair of output and 
grounding lead wires 12 and 13 connected by way of the metallic leads 51 
and 71 to the inner surface and outer surface lead layers 14 and 15 and a 
pair of lead wires 17 for the ceramic heater 18. However, this is not for 
the purpose of limitation but the oxygen sensor can be of the type not 
equipped with any heater, i.e., of the type having two leads, which is 
constructed so as to dispense with such a ceramic heater 18, intermediate 
lead 18b, lead wires 17, and the gripping portion 55 of the metallic lead 
51 otherwise provided to the above described embodiment. However, in such 
a case, the above described seal member 16 needs to be of such a variant 
that does not have any through holes for the lead wires 17. In the 
meantime, regarding the stopper 21, while such one as shown in FIGS. 2, 8 
or 9 can be used as it is, it can be of such a variant as for example 
shown in FIG. 10, i.e., can be of such a variant that has a lower end face 
(the face for engagement with the electrode connecting portion of each 
metallic lead) 23 having only four through holes 32. Further, as shown in 
FIG. 11, the stopper can be of such one that has two through holes 32. In 
the meantime, in FIGS. 10 or 11, two-dot chain lines indicate the upper 
ends of the electrode connecting portions 52 and 72. 
Further, in the foregoing, the oxygen sensor can be of the three lead type 
or of a so-called case grounded type in which the inner surface lead layer 
14 of the solid electrolyte tube 3 is electrically connected to the output 
lead wire 12 for thereby being connected to a control circuit whilst the 
outer surface electrode layer 15 of the solid electrolyte tube 3 is 
electrically connected to the metallic housing assembly for thereby being 
grounded by way of the exhaust pipe. That is, the oxygen sensor can be of 
the type in which the lead layer (normally, outer surface lead layer) 
connected to one of the inner surface and outer surface electrode layers 
of the solid electrolyte tube is adapted to be grounded by way of the 
exhaust pipe and the lead layer connected to the other of the electrode 
layers is connected to the metallic lead which is in turn connected to the 
output lead wire. In the meantime, in such a case, it will suffice to 
prevent only the metallic lead for connection with the output lead wire 
from being separated from the solid electrolyte tube. Naturally, the 
oxygen sensor can be of one lead type not equipped with any heater and is 
connected by only an output lead wire. In case the oxygen sensor of the 
case ground type is employed and the above described metallic lead is 
connected to the outer surface output lead layer, it is not necessary, as 
shown in FIG. 12, for the end face 23 of the stopper 21 to have any 
radially inwardly protruded portions since the upper end of the electrode 
connecting portion 72, as indicated by two-dot chain line in FIG. 12, can 
engage almost at its entirety the lower end of the stopper 21.