TECHNICAL FIELD
[0001] The present invention relates to a glow plug used for assisting in start-up of a
diesel engine.
BACKGROUND ART
[0002] A glow plug used for assisting in start-up of a diesel engine is such that a heater
which has a heat-generating resistor at its forward end portion is held directly or
indirectly at a forward end portion of a tubular metallic shell having an axial bore.
A rodlike center shaft is inserted into the axial bore of the metallic shell and disposed
in a condition electrically insulated from the metallic shell. One end portion of
the center shaft is connected to a rear end portion of the heater, and the other end
portion projects from the rear end of the metallic shell. Two electrodes led from
the heater are electrically connected to the metallic shell and the center shaft,
respectively.
[0003] In the thus-configured glow plug, in order to ensure gastightness through the axial
bore of the metallic shell, an O-ring is disposed between the center shaft and the
inner circumferential surface (wall surface) of the axial bore. Furthermore, an insulation
member for ensuring electrical insulation between the metallic shell and the center
shaft is disposed, on the rear end side of the O-ring, between the center shaft and
the inner circumferential surface of the axial bore. A taper surface is provided in
a region of the inner circumferential surface of the axial bore and/or a region of
the center shaft where the O-ring is disposed. The end surface of the insulation member
presses the O-ring toward the taper surface, thereby ensuring close contact of the
O-ring with three contact surfaces of the inner circumferential surface of the axial
bore, the center shaft, and the insulation member (refer to, for example Patent Document
1).
PRIOR ART DOCUMENT
PATENT DOCUMENT
[0005] Patent Document 1: Japanese Patent Application Laid-Open
(kokai) No.
2007-292444
SUMMARY OF THE INVENTION
[0006] However, in the configuration in which the three contact surfaces of the inner circumferential
surface, the center shaft, and the insulation member surround the O-ring, difficulty
is encountered in providing a space for accepting deformation of a compressed O-ring.
Thus, the positions of disposition of the contact surfaces must be determined accurately
in relation to the O-ring. For example, in the case where the insulation member is
strongly pressed into a space between the center shaft and the inner circumferential
surface of the axial bore, the disposed O-ring assumes a small size. Accordingly,
the O-ring is deformed in such a manner as to enter a narrow gap between the contact
surfaces; thus, internal stress in a deformed portion of the O-ring increases, potentially
resulting in breakage of the O-ring. By contrast, in the case where the insulation
member is loosely pressed into the space, the disposed O-ring assumes a large size.
Accordingly, the O-ring fails to maintain sufficient close contact with the contact
surfaces; thus, a clearance is formed therebetween, potentially resulting in a failure
to ensure gastightness.
[0007] Also, conventional glow plugs employ O-rings having a circular or elliptic cross
section. In the process of assembling a glow plug, when the O-ring is to be disposed
in a narrow gap between the center shaft and the inner circumferential surface of
the axial bore, there has been involved the risk of twist of the O-ring along the
circumferential direction. Also, the O-ring has a circular cross section, or the O-ring
having an elliptic cross section assumes a substantially circular cross section when
disposed in a space surrounded by the three contact surfaces. Thus, since a portion
of the O-ring which intervenes between the center shaft and the inner circumferential
surface of the axial bore assumes a short axial length, when external vibration is
imposed on the center shaft, the effect of restraining oscillation of the center shaft
is unlikely to be exhibited. Joint use of an axially long tube-like vibration insulating
rubber or the like encounters difficulty in disposition between the center shaft and
the inner circumferential surface of the axial bore and increases cost.
[0008] The present invention has been conceived to solve the above problems, and an object
of the invention is to provide a glow plug having a contact member which enhances
close contact with the center shaft and with the inner circumferential surface of
the axial bore of the metallic shell, ensures vibration insulation for the center
shaft, and facilitates attachment thereof.
[0009] According to an embodiment of the present invention, there is provided a glow plug
comprising a heater having, in a forward end portion, a heat-generating resistor which
generates heat through energization; a metallic shell assuming the form of a tube
having an axial bore extending along a first axis which is the axis of the metallic
shell, and holding the heater directly or indirectly at a forward end portion; a center
shaft assuming a rodlike form, disposed in the axial bore of the metallic shell with
a gap formed between the same and an inner circumferential surface of the axial bore,
having one end portion connected to a rear end portion of the heater, and having the
other end portion projecting from a rear end of the metallic shell; and a contact
member formed from an electrically insulative elastic member, assuming an annular
form, inserted into the axial bore at a rear end portion of the axial bore, and disposed
in such a manner as to be in contact with the inner circumferential surface of the
axial bore and with the center shaft; the glow plug being characterized in that a
contour of one of two cross sections resulting from cutting the contact member in
a state prior to attachment to the glow plug by a plane which contains a second axis
which is the axis of the contact member has a first contour segment assuming the form
of a curve extending along the second axis and swelling radially outward with a radius
R1 of curvature, and a second contour segment assuming the form of a straight line
extending along the second axis or the form of a curve extending along the second
axis and swelling radially inward with a radius R2 of curvature which satisfies a
relational expression R1 < R2.
[0010] In the present embodiment, the second contour segment having the radius R2 of curvature
extends along the second axis with a radius of curvature greater than that of the
first contour segment having the radius R1 of curvature. Thus, when the contact member
is pressed in along the second axis, the second contour segment can function as a
core which supports the entire contact member and restrains the contact member from
bending and being dragged inward. Therefore, the contact member is restrained from
bending or wrinkling at the second contour segment.
[0011] Also, when the contact member is disposed between the center shaft and the inner
circumferential surface of the axial bore, the radial thickness of the contact member
perpendicular to the second axis is compressed. Since the first contour segment assumes
the form of a curve swelling outward in a radial direction of the contact member,
compressive deformation can be performed smoothly. Also, since, in the course of deformation,
deformation can be such that material moves from a thick region to a thin region,
a deformed region having high internal stress does not arise locally; thus, even when
the glow plug is subjected to external vibration or the like, the contact member is
unlikely to be broken. Also, the contact member is in contact with two members (two
surfaces); namely, the center shaft and the inner circumferential surface of the axial
bore. Therefore, in view of establishment of gastightness of the axial bore by the
contact member, there is no need to form a complicated seal surface on the metallic
shell and the center shaft, which are counter members of contact with the contact
member, so that machining is facilitated, leading to a reduction in cost.
[0012] In the present embodiment, the contact member may be such that, as viewed on the
one cross section, a length along an extending direction of the second axis is longer
than a length along a direction orthogonal to the second axis. Through employment
of such configuration, when the contact member is disposed between the center shaft
and the inner circumferential surface of the axial bore, there can be increased an
axial intervening length of the contact member intervening between the center shaft
and the inner circumferential surface of the axial bore. Therefore, the center shaft
can be more reliably held in the axial bore; thus, there can be more reliably restrained
oscillation of the center shaft stemming from imposition of external vibration on
the center shaft.
[0013] In the present embodiment, the center shaft may further comprise a rear trunk portion
disposed at such a position with respect to the extending direction of the first axis
of the metallic shell as to face the rear end portion of the inner circumferential
surface of the axial bore, and having a diameter greater than that of the other end
portion, and a shoulder portion connecting the rear trunk portion and the other end
portion in a tapered manner. The metallic shell may further comprise a taper portion
expanding in a tapered manner at the rear end portion of the axial bore from a position
located forward of a forward end of the shoulder portion toward the rear end located
rearward of the position with respect to the extending direction of the first axis.
The contact member may be disposed such that the second contour segment is in contact
with the center shaft and such that the first contour segment is in contact with an
inner circumferential surface of the axial bore located forward of the taper portion.
[0014] For example, even in the case where the first axis of the metallic shell and the
axis of the center shaft fail to coincide with each other, when the contact member
is disposed between the center shaft and the inner circumferential surface of the
axial bore, the taper portion can guide the contact member toward the center of the
axial bore. Therefore, misalignment between the first axis and the axis of the center
shaft can be corrected via the contact member. By specifying that the outside diameter
of the rear trunk portion be greater than the inside diameter of the contact member,
in attachment of the contact member to the glow plug, first, the contact member can
be brought into contact with the axial bore in a condition that the inside diameter
of the contact member is expanded by the shoulder portion. Then, in a state in which
the contact member is in contact with the center shaft (i.e., in a state in which
a big gap is not formed between the contact member and the center shaft), the outside
diameter of the contact member is narrowed along the taper portion; thus, the contact
member is free from inward dragging and, thus, free from twist along the circumferential
direction and wrinkling. Therefore, the contact member can be reliably in contact
with the inner circumferential surface of the axial bore and with the outer circumferential
surface of the center shaft, whereby gastightness through the axial bore can be reliably
established.
[0015] In the present embodiment, the contour of the contact member may further comprise
a third contour segment which is connected, at one end, to the second contour segment;
which extends radially outward from the one end toward the other end thereof while
extending along the second axis; which is connected, at the other end, to the first
contour segment; and whose point of connection to the first contour segment is an
end of the contact member with respect to the extending direction of the second axis.
In attachment of the contact member to the glow plug, the contact member first comes
into contact with the shoulder portion. At the time of this contact, first, the third
contour segment, which extends radially outward while extending along the second axis,
comes into contact with the shoulder portion. Thus, friction of the contact member
against the shoulder portion is small, so that the contact member can be prevented
from being caught by the shoulder portion and dragged inward.
[0016] In the present embodiment, the contour may form a mirror image with respect to a
center position in the extending direction of the second axis. That is, since the
contact member is symmetrical along the extending direction of the second axis, in
attachment of the contact member to the glow plug, the contact member can be fitted,
in either orientation with respect to the direction of the axis P, to a rear end portion
of the center shaft. Thus, there can be omitted labor for checking and correcting
orientation in attachment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
[FIG. 1] Longitudinal sectional view of a glow plug 1.
[FIG. 2] Sectional view on an enlarged scale of a rear end portion of the glow plug
1.
[FIG. 3] View showing, perspectively and in section, a contact member 7 in a state
prior to attachment to the glow plug 1.
[FIG. 4] Sectional view on an enlarged scale of a rear end portion of the glow plug
1 in the process of attaching the contact member 7 to the glow plug 1.
[FIG. 5] Sectional view on an enlarged scale of the rear end portion of the glow plug
1 in the process of attaching the contact member 7 to the glow plug 1.
[FIG. 6] View showing, perspectively and in section, a contact member 107 in a state
prior to attachment to the glow plug 1.
[FIG. 7] View showing, perspectively and in section, a contact member 207 in a state
prior to attachment to the glow plug 1.
[FIG. 8] View showing, perspectively and in section, a contact member 307 in a state
prior to attachment to the glow plug 1.
MODES FOR CARRYING OUT THE INVENTION
[0018] A glow plug according to an embodiment of the present invention will next be described
with reference to the drawings. The entire structure of a glow plug 1 is described,
by way of example, with reference to FIGS. 1 and 2. The drawings referred to herein
are used for explaining technical features which the present invention can employ,
and the configuration, etc., of the glow plug appearing in the drawings are given
by way of illustration and not of limitation. In the following description, the axis
of a metallic shell 4 is referred to as the axis O, and the axis O serves as reference
in describing the positional relationship, orientations, and directions of those component
members of the glow plug 1 which are attached to the metallic shell 4. With respect
to the extending direction of the axis O (hereinafter, may be referred to as "the
direction of the axis O"), a side on which a ceramic heater 2 is disposed (the lower
side in FIG. 1) is referred to as the forward side of the glow plug 1. The axis O
corresponds to the "first axis" in the present invention. An axis P, which will be
described later, indicates the axis of a contact member 7 in a state prior to attachment
to the glow plug 1, and, in the process of assembly and after assembly, the axis O
is used as reference for description. The axis P corresponds to the "second axis"
in the present invention.
[0019] The glow plug 1 shown in FIG. 1 is attached to, for example, a combustion chamber
of a direct-injection-type diesel engine (not shown), and is used as a heat source
for assisting in ignition at start-up of an engine. The glow plug 1 includes the metallic
shell 4, a holding member 8, the ceramic heater 2, a center shaft 3, a connection
terminal 5, an insulation member 6, the contact member 7, and a connection ring 85.
[0020] First, the ceramic heater 2 is described. The ceramic heater 2 assumes the form of
a round bar and has a substrate 21 which is formed from an electrically insulating
ceramic and whose forward end portion 22 is formed into a hemispherical shape. A heat-generating
element 24 formed from an electrically conductive ceramic and having a substantially
U-shaped section is embedded in the substrate 21. The heat-generating element 24 includes
a heat-generating resistor 27 and leads 28 and 29. The heat-generating resistor 27
is disposed in the forward end portion 22 of the ceramic heater 2 and is curved and
bent at opposite ends in a shape resembling the letter U according to the curved surface
of the forward end portion 22. The leads 28 and 29 are connected to opposite ends,
respectively, of the heat-generating resistor 27 and extend substantially in parallel
with each other toward a rear end portion 23 of the ceramic heater 2. The cross-sectional
area of the heat-generating resistor 27 is smaller than that of each of the leads
28 and 29, and, upon energization, heat is generated mainly by the heat-generating
resistor 27. Electrode lead portions 25 and 26 project radially from the leads 28
and 29, respectively, at respective positions located rearward of the center of the
ceramic heater 2. The electrode lead portions 25 and 26 are exposed at the outer circumferential
surface of the ceramic heater 2 at positions deviated from each other in the direction
of the axis O.
[0021] Next, the holding member 8 is described. The holding member 8 is a cylindrical metal
member extending in the direction of the axis O and radially holds a trunk portion
of the ceramic heater 2 within its tubular hole 84. The forward end portion 22 and
the rear end portion 23 of the ceramic heater 2 project from the opposite ends of
the holding member 8. A trunk portion 81 of the holding member 8 has a thick-walled
flange portion 82 formed on a side toward the rear end thereof. The holding member
8 has a stepped metal-shell engagement portion 83 which is located rearward of the
flange portion 82 and is engaged with a forward end portion 41 of the metallic shell
4, which will be described later. Of the electrode lead portions 25 and 26 of the
ceramic heater 2, the electrode lead portion 25 located on a side toward the forward
end is in contact with the inner circumferential surface of the tubular hole 84 of
the holding member 8, whereby the electrode lead portion 25 and the holding member
8 are electrically connected to each other.
[0022] Also, the tubular connection ring 85 of metal is press-fitted to the rear end portion
23 of the ceramic heater 2 projecting rearward from the metallic-shell engagement
portion 83 of the holding member 8. The electrode lead portion 26 of the ceramic heater
2 is in contact with the inner circumferential surface of the connection ring 85,
whereby the electrode lead portion 26 and the connection ring 85 are electrically
connected to each other. As a result of the forward end portion 41 of the metallic
shell 4, which will be described later, being joined to the metallic-shell engagement
portion 83 of the holding member 8, the electrode lead portion 25 is electrically
connected to the metallic shell 4. While the connection ring 85 connected to the electrode
lead portion 26 is disposed within the metallic shell 4, the ceramic heater 2 and
the metallic shell 4 are positioned by the holding member 8 such that the connection
ring 85 and the metallic shell 4 are held mutually in an electrically insulated condition.
[0023] Next, the metallic shell 4 is described. The metallic shell 4 is a slender tubular
metal member having an axial bore 43 extending therethrough in the direction of the
axis O. The inner circumference of the forward end portion 41 of the metallic shell
4 is engaged with the outer circumference of the metallic-shell engagement portion
83 of the above-mentioned holding member 8, whereby the metallic shell 4 is electrically
connected, via the holding member 8, to the electrode lead portion 25 of the ceramic
heater 2. The overlap region of the forward end portion 41 and the metallic-shell
engagement portion 83 is subjected to laser welding, whereby the metallic shell 4
and the holding member 8 are joined together. The metallic shell 4 has an intermediate
trunk portion 44 formed between the forward end portion 41 and the rear end portion
45 and extending long in the direction of the axis O, and the intermediate trunk portion
44 has a mounting portion 42 formed on the outer circumferential surface of a portion
located on a side toward the rear end, the mounting portion 42 having threads for
mounting the glow plug 1 to an engine head of an internal combustion engine (not shown).
The intermediate trunk portion 44 also has a tool engagement portion 46 located rearward
of the mounting portion 42, having a hexagonal cross section, and adapted to allow
a tool to be engaged therewith in mounting the glow plug 1 to the engine head. As
shown in FIG. 2, the rear end portion 45 of the metallic shell 4 has a taper portion
47 formed on the inner circumferential surface of the axial bore 43 and expanding
in a tapered form from the axial bore 43 to the opening of a rear end 48.
[0024] Next, the center shaft 3 is described. As shown in FIG. 1, the center shaft 3 is
a rodlike metal member extending in the direction of the axis O and is inserted into
the axial bore 43 of the metallic shell 4. An intermediate trunk portion 33 located
between a forward end portion 31 and a rear end portion 32 of the center shaft 3 is
smaller in outside diameter than the forward end portion 31 and the rear end portion
32. The forward end portion 31 has a small-diameter ring engagement portion 34 formed
at its forward end so as to be engaged with the inner circumference of the connection
ring 85. As a result of the ring engagement portion 34 being engaged with the connection
ring 85, the ceramic heater 2 and the center shaft 3 are unitarily connected together
along the axis O via the connection ring 85. Although unillustrated, the overlap region
of the forward end portion 31 and the connection ring 85 is subjected to laser welding,
whereby the forward end portion 31 and the connection ring 85 are joined together.
Through this joining, the center shaft 3 is electrically connected to the electrode
lead portion 26 of the ceramic heater 2 via the connection ring 85. As mentioned above,
since the ceramic heater 2 and the metallic shell 4 are positioned by the holding
member 8, the center shaft 3 and the metallic shell 4 are held mutually in an electrically
insulated condition in the axial bore 43.
[0025] As shown in FIG. 2, the rear end portion 32 of the center shaft 3 has a connection
end portion 36 projecting from the rear end 48 of the metallic shell 4, and a connection
base portion 37 which connects the connection end portion 36 and the intermediate
trunk portion 33. The connection end portion 36 has a lock portion 39 formed by knurling
its outer circumferential surface. The connection end portion 36, including the lock
portion 39, is smaller in outside diameter than the connection base portion 37. A
shoulder portion 38 is formed between the connection end portion 36 and the connection
base portion 37 for connecting, in a tapered manner, the connection end portion 36
and the connection base portion 37.
[0026] The contact member 7 and the insulation member 6 are disposed on the rear end portion
32 of the center shaft 3. The contact member 7, which will be described later, is
disposed between the inner circumferential surface of the axial bore 43 of the metallic
shell 4 and the connection base portion 37 of the center shaft 3, holds the center
shaft 3 in the axial bore 43 to thereby restrain oscillation of the center shaft 3,
and maintains gastightness of the axial bore 43.
[0027] The insulation member 6 is a tubular member formed from a heat-resistant, electrically
insulative material; for example, nylon (registered trademark), for preventing short
circuit which could otherwise result from contact between the metallic shell 4 and
the center shaft 3 or the connection terminal 5 (which will be described later). The
insulation member 6 is fitted to the center shaft 3 such that the rear end portion
32 of the center shaft 3 is inserted through the insulation member 6; is positioned
such that a taper portion 63 provided on its outer circumference is in contact with
the taper portion 47 of the metallic shell 4; and maintains an electrically insulating
condition between the metallic shell 4 and the center shaft 3. In this condition,
a rear end 65 of the insulation member 6 projects rearward from the rear end 48 of
the metallic shell 4, and a flange portion 51 (which will be described later) of the
connection terminal 5 is in contact with the rear end 65, whereby the connection terminal
5 and the metallic shell 4 are held mutually in an electrically insulated condition.
[0028] The connection terminal 5 is fixedly attached to the connection end portion 36 of
the center shaft 3. The connection terminal 5 has a cap-like trunk portion 52 which
is fitted externally to the connection end portion 36, and a pin-like protrusion 53
protruding rearward from the trunk portion 52. The trunk portion 52 has a flange portion
51 provided at its forward open end in such a manner as to radially project along
the entire circumference. When the connection terminal 5 is fitted externally to the
connection end portion 36 of the center shaft 3, the flange portion 51 comes into
contact with the rear end 65 of the insulation member 6. Also, in a state in which
the connection terminal 5 is pressed forward with respect to the direction of the
axis O, the trunk portion 52 is crimped radially inward, whereby the inner circumferential
surface of the trunk portion 52 is firmly locked to the lock portion 39 of the connection
end portion 36. Since the lock portion 39 is knurled, the force of fixation is enhanced
for the trunk portion 52 which is crimped to the lock portion 39, whereby the connection
terminal 5 and the center shaft 3 are unitarily fixed and electrically connected to
each other.
[0029] In mounting the glow plug 1 to the engine head (not shown), a plug cap (not shown)
is fitted to the protrusion 53 of the connection terminal 5. The heat-generating element
24 (see FIG. 1) of the ceramic heater 2 generates heat through application of electricity
between one end of the heat-generating resistor 27 which is grounded to the engine
via the holding member 8 and the metallic shell 4, and the other end of the heat-generating
resistor 27 which is connected to the plug cap via the connection terminal 5 and the
center shaft 3.
[0030] Next, the contact member 7 is described. As mentioned above, the contact member 7
is a member disposed between the inner circumferential surface of the axial bore 43
of the metallic shell 4 and the connection base portion 37 of the center shaft 3,
holds the center shaft 3 in the axial bore 43 to thereby restrain oscillation of the
center shaft 3, and maintains gastightness of the axial bore 43. As shown in FIG.
3, the contact member 7 is formed into a cylindrical shape from a heat-resistant,
electrically insulative material; for example, fluororubber, acrylic rubber, or silicone
rubber. Preferably, the contact member 7 has a Knoop hardness of 60 to 80.
[0031] Specifically, the contact member 7 has such a cylindrical shape as to have a tubular
hole 76 extending in the extending direction of its axis P (hereinafter, may be referred
to as "the direction of the axis P"), and is formed in such a manner as to have a
substantially D-shaped cross section in a state prior to attachment to the glow plug
1. More specifically, when the cylindrical contact member 7 is cut (split) into two
pieces by a plane which contains the axis P, each of the pieces has two cross sections.
In the present embodiment, when attention is focused on one 75 of two cross sections
of the piece, a contour 70 of the cross section 75 assumes the following form.
[0032] The contour 70 has three kinds of contour segments (line segments which constitute
the contour); namely, a second contour segment 71, a first contour segment 72, and
third contour segments 73. The second contour segment 71 extends in the form of a
straight line along the axis P. The first contour segment 72 assumes the form of a
curve extending along the axis P and swelling radially outward in a direction orthogonal
to the axis P. The third contour segments 73 connect the second contour segment 71
and the first contour segment 72 at their ends located on the same side with respect
to the axis P, and each of the third contour segments 73 assumes the form of a curve
extending toward the outside of the cross section 75 while swelling. Also, connections
between the first contour segment 72 and the third contour segments 73 are upper and
lower (forward and rear) ends of the contact member 7 with respect to the direction
of the axis P.
[0033] Also, the first contour segment 72 is disposed radially outward of the second contour
segment 71 and is longer in length along the direction of the axis P than the second
contour segment 71. Therefore, each of the third contour segments 73 extends radially
outward from the end of the second contour segment 71 and is connected to the end
of the first contour segment 72. Furthermore, the contour 70 forms a mirror image
with respect to the center position (represented by the dash-dot-dot line A-A in FIG.
3) in the direction of the axis P. That is, the contact member 7 has a symmetrical
shape along the direction of the axis P.
[0034] The following provisions are made for the contact member 7 having the above-mentioned
cross-sectional shape, with respect to the shapes of the second contour segment 71
and the first contour segment 72. The first contour segment 72 assumes, as mentioned
above, the form of a curve swelling outward in a radial direction perpendicular to
the axis P and has a radius R1 of curvature. The second contour segment 71 assumes
the form of a straight line; however, assuming that the second contour segment 71
assumes the form of a curve having a radius R2 of curvature, the second contour segment
71 can be considered as a curve having infinite R2. Therefore, the present embodiment
specifies that the radius R2 of curvature of the second contour segment 71 and the
radius R1 of curvature of the first contour segment 72 satisfy the relational expression
R1 < R2. In other words, the first contour segment 72 swelling radially outward is
greater in radial swelling than the second contour segment 71 in the form of a straight
line (according to the above assumption, swelling radially inward). Also, the present
embodiment specifies that the cross-sectional shape of the contact member 7 is such
that a length L1 along the extending direction of the axis P is longer than a length
L2 along a direction orthogonal to the extending direction of the axis P (i.e., along
a radial direction) (i.e., the cross-sectional shape satisfies the relational expression
L1 > L2).
[0035] The contact member 7 having such a shape can be manufactured by an ordinary method
for manufacturing an O-ring except that an employed die or mold differs from that
employed in the ordinary method. For example, the contact member 7 can be manufactured
by compression forming; specifically, upper and lower dies having shapes corresponding
to the shape of the contact member 7 are pressed, from above and from underneath,
against a sheet of fluororubber. The manufacturing method is not limited thereto.
Injection molding can be utilized; specifically, a material, such as fluororubber,
is injected into a split-type mold whose cavity has the shape of the contact member
7. Alternatively, machining can be utilized; specifically, an annular member (ring)
of fluororubber or a like material is machined into the shape of the contact member
7.
[0036] The glow plug 1 having such a structure is assembled as outlined below. A material
composed of an electrically conductive ceramic powder, binder, etc., is injection-molded
into an element green-body which is to become the heat-generating element 24 of the
ceramic heater 2. Also, an electrically insulating ceramic powder is die-pressed into
substrate green-body halves which are collectively to become the substrate 21 of the
ceramic heater 2. An assembly of the substrate green-body halves with the element
green-body accommodated therein in a sandwiched condition is subjected to press compression.
The compressed assembly is subjected to a debindering process, a firing process, such
as hot pressing, and then a surface polishing process, thereby yielding the rodlike
ceramic heater 2 having a hemispherical forward end. The method of manufacturing the
ceramic heater 2 may be modified as appropriate. For example, The substrate green-body
may be manufactured as follows: a previously formed substrate green-body half is placed
in a die; the element green-body is placed on the substrate green-body half; the electrically
insulating ceramic powder is charged into the die; and press compression is performed.
[0037] The ceramic heater 2 is press-fitted into the connection ring 85 formed by forming
a steel material, such as stainless steel, into the shape of pipe, thereby establishing
electrical connection between the connection ring 85 and the electrode lead portion
26. Similarly, the ceramic heater 2 is press-fitted into the holding member 8 formed
into a predetermined shape, thereby establishing electrical connection between the
holding member 8 and the electrode lead portion 25. Meanwhile, the center shaft 3
is formed as follows: a rodlike member formed by cutting an iron-based material (e.g.,
Fe-Cr-Mo steel) into a predetermined dimension is subjected plastic working, cutting,
etc. In a state in which the ring engagement portion 34 of the center shaft 3 is engaged
with the connection ring 85 fitted to the ceramic heater 2, the overlap region is
subjected to laser welding, thereby uniting the center shaft 3 and the ceramic heater
2.
[0038] The tubular metallic shell 4 is formed from an iron-based material, such as S45C,
and threads are formed on the mounting portion 42 by rolling. Furthermore, the taper
portion 47 is formed, by cutting or the like, on the inner circumferential surface
of the axial bore 43 at the rear end portion 45 of the metallic shell 4 in such a
manner as to expand in a tapered form from the axial bore 43 to the opening of the
rear end 48. The center shaft 3 united to the ceramic heater 2, etc., is inserted
through the axial bore 43 of the metallic shell 4. The overlap region of the metallic
shell 4 and the holding member 8 is subjected to laser welding, whereby the metallic
shell 4 and the holding member 8 are joined together.
[0039] Next, the contact member 7 is fitted to the rear end portion 32 of the center shaft
3 projecting from the rear end 48 of the metallic shell 4. As mentioned above, since
the contact member 7 has a symmetrical shape (mirror image) along the direction of
the axis P, the contact member 7 can be fitted in either orientation with respect
to the direction of the axis P. Also, as shown in FIG. 4, an inside diameter C1 of
the tubular hole 76 of the contact member 7 is greater than an outside diameter C2
of the connection end portion 36 of the center shaft 3. Thus, when the connection
end portion 36 is inserted through the contact member 7, there can be prevented damage
to the tubular hole 76 (the inner circumferential surface of the contact member 7)
which could otherwise result from rubbing between the inner circumferential surface
of the tubular hole 76 and the lock portion 39 of the connection end portion 36.
[0040] With respect to the direction of the axis O, a forward end position B1 (corresponding
to the boundary between the shoulder portion 38 and the connection base portion 37)
of the shoulder portion 38 of the center shaft 3 is located rearward of a forward
end position B2 (corresponding to the starting position of expansion of the taper
portion 47 of the axial bore 43) of the taper portion 47 of the metallic shell 4.
Therefore, after the connection end portion 36 is inserted through the contact member
7, the contact member 7 reaches the shoulder portion 38 of the center shaft 3 before
coming into contact with the taper portion 47 of the metallic shell 4.
[0041] Also, the inside diameter C1 of the tubular hole 76 of the contact member 7 is smaller
than the outside diameter C3 of the connection base portion 37 of the center shaft
3. Therefore, when the contact member 7 which has reached the shoulder portion 38
is further pressed in forward along the direction of the axis O, the tubular hole
76 is expanded along the taper of the shoulder portion 38. At this time, the third
contour segment 73 of the cross section 75 of the contact member 7 (see FIG. 3) is
pressed against the tapered shoulder portion 38. Since the third contour segment 73
assumes the form of a curve swelling toward the outside of the cross section 75, the
inside diameter of the contact member 7 can be smoothly expanded along the taper of
the shoulder portion 38.
[0042] When the contact member 7 whose tubular hole 76 is expanded by the shoulder portion
38 is pressed in forward along the direction of the axis O in a state in which the
inner circumferential surface of the tubular hole 76 is in contact with the outer
circumferential surface of the connection base portion 37, next, the outer circumferential
surface of the contact member 7 comes into contact with the taper portion 47 of the
metallic shell 4. When the contact member 7 is further pressed in, as shown in FIG.
5, a portion of the contact member 7 on a side toward the outer circumferential surface
is elastically deformed along the taper portion 47, and the contact member 7 is inserted
forward beyond the forward end position B2 of the taper portion 47. The first contour
segment 72 assumes the form of a curve swelling outward in a radial direction of the
contact member 7 with the radius R1 of curvature. Therefore, as compared with the
second contour segment 71 assuming the form of a straight line (assuming that the
second contour segment 71 assumes the form of a curve, the radius R2 of curvature
is infinite), the first contour segment 72 can be more smoothly deformed such that
the radial thickness of the contact member 7 is compressed. The contact member 7 is
first expanded in inside diameter by the shoulder portion 38 of the center shaft 3
and then reaches a state in which the inner circumferential surface of the tubular
hole 76 is in contact with the outer circumferential surface of the connection base
portion 37. While the contact member 7 is in this state (i.e., in a state in which
a big gap is not formed between the contact member 7 and the center shaft 3), the
outside diameter of the contact member 7 is narrowed by the taper portion 47 of the
metallic shell 4; thus, the contact member 7 is free from inward dragging and, thus,
free from twist along the circumferential direction and wrinkling. Meanwhile, a jig
may be used, or, as shown in FIG. 5, the insulation member 6 may be utilized for pressing
the contact member 7 into a space between the connection base portion 37 and the inner
circumferential surface of the axial bore 43.
[0043] As mentioned above, the second contour segment 71 of the contact member 7 assumes
the form of a straight line along the axis P. Thus, when the contact member 7 is pressed
in along the axis O such that the pressing force acts mainly along its axis P, the
second contour segment 71 can function as a core which supports the entire contact
member and restrains the contact member from bending and being dragged inward. Therefore,
the contact member 7 is restrained from bending or wrinkling at the second contour
segment 71.
[0044] Also, when the contact member 7 is disposed between the connection base portion
37 and the inner circumferential surface of the axial bore 43, the contact member
7 is guided toward the center of the axial bore 43 by the taper portion 47 of the
metallic shell 4. Thus, for example, even when misalignment arises between the axis
O of the metallic shell 4 and the axis of the center shaft 3, the center shaft 3 is
guided toward the center of the axial bore 43 via the contact member 7; therefore,
misalignment between the axis O and the axis of the center shaft 3 can be corrected.
[0045] In this manner, since the cross section of the contact member 7 has the first contour
segment 72 which swells radially outward with the radius R1 of curvature, the contact
member 7 can be smoothly elastically deformed. Furthermore, since the cross section
has the second contour segment 71 in the form of a straight line, the contact member
7 is reliably disposed between the connection base portion 37 and the inner circumferential
surface of the axial bore 43 without involvement of bending and wrinkling. Also, the
contact member 7 whose cross section before attachment has the D-shaped contour 70
is deformed such that its radial thickness is compressed to thereby impart, to the
contact member 7, a flat profile along the axis P. Thus, deformation is such that
material in a thickest region (located near the center with respect to the direction
of the axis P) moves toward thin regions (opposite end regions with respect to the
direction of the axis P). Therefore, in the contact member 7, a deformed region having
high internal stress does not arise locally; thus, even when the glow plug 1 is subjected
to external vibration or the like, the contact member 7 is unlikely to be broken.
As viewed after attachment, the contact member 7 does not maintain the form of a mirror
image along the direction of the axis P, and such deformation reliably establishes
close contact of the contact member 7 with the inner circumferential surface of the
axial bore 43 and with the connection base portion 37. Therefore, sufficient reaction
is generated against the outer circumferential surface of the connection base portion
37 and against the inner circumferential surface of the axial bore 43. Thus, the contact
member 7 can reliably hold the center shaft 3 in the axial bore 43 and thus can restrain
oscillation of the center shaft 3 when the glow plug 1 receives external vibration
or the like.
[0046] Also, in a space between the outer circumferential surface, encircling the axis O,
of the connection base portion 37 of the center shaft 3 and the inner circumferential
surface, encircling the axis O, of the axial bore 43 of the metallic shell 4, the
contact member 7 has two radially separated contact surfaces for contact with the
connection base portion 37 and for contact with the inner circumferential surface
of the axial bore 43. For example, the position of the disposed contact member 7 may
shift due to vibration generated as a result of operation of the glow plug 1. However,
for the contact member 7 having the two radially separated contact surfaces for contact
with the connection base portion 37 and for connection with the inner circumferential
surface of the axial bore 43, such a positional shift emerges merely as shifts of
positions of contact with the two members, and the sizes of the contact surfaces can
be maintained intact. Additionally, since the contact member 7 can maintain, through
elastic deformation, a state of contact with the inner circumferential surface of
the axial bore 43 and with the connection base portion 37, gastightness through the
axial bore 43 can be reliably ensured. Also, the contact member 7 is in contact with
two surfaces; namely, the inner circumferential surface of the axial bore 43 and the
outer circumferential surface of the connection base portion 37. Therefore, in view
of establishment of gastightness of the axial bore 43 by the contact member 7, there
is no need to form a complicated seal surface on the metallic shell 4 and the center
shaft 3, which are counter members of contact with the contact member 7, so that machining
is facilitated, leading to a reduction in cost.
[0047] Since the contact member 7 satisfies the relational expression L1 > L2, when the
contact member 7 is disposed between the center shaft 3 and the inner circumferential
surface of the axial bore 43, there can be increased the intervening length, along
the direction of the axis O, of the contact member 7 which intervenes between the
center shaft 3 and the inner circumferential surface of the axial bore 43. Therefore,
the center shaft 3 can be more reliably held in the axial bore 43; thus, there can
be more reliably restrained oscillation of the center shaft 3 stemming from imposition
of external vibration on the center shaft 3.
[0048] After the contact member 7 is disposed between the connection base portion 37 and
the inner circumferential surface of the axial bore 43, as shown in FIG. 2, the insulation
member 6 is fitted to the rear end portion 32 of the center shaft 3. In a state in
which the insulation member 6 is positioned such that the taper portion 63 of the
insulation member 6 is in contact with the taper portion 47 of the metallic shell
4, the insulation member 6 is fitted to the rear end portion 32 of the center shaft
3. The trunk portion 52 of the connection terminal 5 is crimped, whereby the connection
terminal 5 is fixed to the connection end portion 36 of the center shaft 3, and the
glow plug 1 is completed.
[0049] The present invention can be modified in various forms. In the embodiment described
above, the connection member 7 is formed such that connections between the first contour
section 72 and the third contour sections 73 are its upper and lower (forward and
rear) ends. However, the present invention is not limited thereto. For example, the
proportion of the third contour sections 73 to the contour 70 may be increased such
that the third contour segments are the upper and lower ends with respect to the direction
of the axis P.
[0050] Also, for example, the contact member may be formed as in the case of a contact member
107 shown in FIG. 6; specifically, a contour 170 of a cross section 175 does not have
the third contour segments and has a shape resembling the letter D such that a second
contour segment 171 and a first contour segment 172 are directly connected. Also,
the contact member may be formed as in the case of a contact member 207 shown in FIG.
7; specifically, a contour 270 of a cross section 275 does not assume the form of
a mirror image with respect to the center position in the direction of the axis P;
i.e., the contour 270 is asymmetric along the direction of the axis P.
[0051] Also, for example, the contact member may be formed as in the case of a contact member
307 shown in FIG. 8; specifically, a contour 370 of a cross section 375 has a second
contour segment 371 assuming the form of a curve swelling radially inward with the
radius R2 of curvature. In this case, similar to the present embodiment, it suffices
for the radius R1 of curvature of a first contour section 372 swelling radially outward
and the radius R2 of curvature of the second contour section 371 to satisfy the relational
expression R1 < R2. Through employment of this relationship, when the contact member
307 is disposed between the center shaft 3 and the inner circumferential surface of
the axial bore 43, the first contour segment 372 larger in the degree of swelling
than the second contour segment 371 can be smoothly elastically deformed. The second
contour segment 371 smaller in the degree of swelling and closer in shape to a straight
line than the first contour segment 372 can function as a core which supports the
entire contact member and restrains the contact member from bending and being dragged
inward.
[0052] The contact members 107, 207, and 307 shown in FIGS. 6, 7, and 8, respectively, are
similar in preferred features to the contact member 7 of the present embodiment shown
in FIG. 3. Specifically, preferably, the cross section is such that the length L1
along the extending direction of the axis P is longer than the length L2 along a direction
orthogonal to the extending direction of the axis P (along a radial direction); i.e.,
the cross section has a short radial length so as to have a flat profile (L1 > L2).
More preferably, the cross section is such that the radial length L2 is half the length
L1 along the direction of the axis P or less (L1/2 ≥ L2). However, depending on material
and detailed configurational features, this relationship (half of the length along
the axis P > the radial length (L1/2 ≥ L2)) may not be satisfied so long as the relationship
L1 > L2 is satisfied).
[0053] The glow plug 1 has the ceramic heater 2. However, the present invention is not limited
thereto. The glow plug 1 may have a sheath heater configured such that a coil-like
heat-generating resistor and a controlling resistor are disposed within a metallic
sheath tube whose tip is hemispherically closed. Also, the glow plug may be such that
the forward end portion 41 of the metallic shell 4 directly holds the ceramic heater
2 or the sheath heater without use of the holding member 8. The ceramic heater may
be of a so-called surface heat-generation type in which the heat-generating element
24 is externally disposed around the substrate 21.
[0054] The connection base portion 37 of the center shaft 3 may have, at a position located
toward its forward end, a stopper in the form of a flange or a protrusion for forming
some level difference. Through provision of such a stopper, even when the contact
member 7 is positionally shifted as a result of reception of vibration or the like,
the stopper prevents movement of the contact member 7 to the intermediate trunk portion
33 of the center shaft 3. Also, the contact member 7 may be disposed between the connection
base portion 37 and the inner circumferential surface of the axial bore 43 in a noncontacting
manner in relation to the insulation member 6.
[0055] The contact member 7 of the present embodiment is expected primarily to ensure gastightness
of the axial bore 43 and is also expected to hold the center shaft 3 to thereby restrain
radial oscillation of the center shaft 3. Therefore, preferably, the contact member
7 is in close contact with the inner circumferential surface of the axial bore 43
and with the outer circumferential surface of the center shaft 3, along the full circumference
around the axis O, since vibration isolation and gastightness can be ensured.
[0056] In the present embodiment, the forward end portion 31 corresponds to "one end portion,"
and the connection end portion 36 corresponds to "the other end portion." The connection
base portion 37 corresponds to "a rear trunk portion."
1. Glühkerze (1), umfassend:
einen Heizer (2), der in einem vorderen Endabschnitt (22) einen wärmeerzeugenden Widerstand
(27) aufweist, der Wärme durch Erregung erzeugt;
eine metallische Hülle (4), die die Form eines Rohres mit einer axialen Bohrung (43)
annimmt, die sich entlang einer ersten Achse (O) erstreckt, die die Achse der metallischen
Hülle (4) ist, und die den Heizer (2) direkt oder indirekt an einem vorderen Endabschnitt
(41) hält;
einen zentralen Schaft (3), der eine stabförmige Form annimmt, die in der axialen
Bohrung (43) der metallischen Hülle (4) mit einem Spalt angeordnet ist, der zwischen
diesem und einer inneren Umfangsfläche der axialen Bohrung (43) gebildet ist, und
der einen Endabschnitt (31) aufweist, der mit einem hinteren Endabschnitt (23) des
Heizers (2) verbunden ist, und der einen anderen Endabschnitt (36) aufweist, der von
einem hinteren Ende (48) der metallischen Hülle (4) vorsteht; und
ein Kontaktelement (7), das aus einem elektrisch isolierenden elastischen Element
gebildet ist, das eine ringförmige Form annimmt, und das in die axiale Bohrung (43)
an einem hinteren Endabschnitt der axialen Bohrung (43) eingesetzt ist und so angeordnet
ist, dass es mit der inneren Umfangsfläche der axialen Bohrung (43) und mit dem zentralen
Schaft (3) in Kontakt steht;
die Glühkerze (1) ist
dadurch gekennzeichnet, dass:
eine Kontur (70) eines (75) von zwei Querschnitten, die sich aus dem Schneiden des
Kontaktelements (7) in einem Zustand vor der Befestigung an der Glühkerze (1) durch
eine Ebene ergibt, die eine zweite Achse (P) enthält, die die Achse des Kontaktelements
(7) aufweist,
ein erstes Kontursegment (72) aufweist, das die Form einer Kurve annimmt, die sich
entlang der zweiten Achse (P) erstreckt und mit einem Krümmungsradius R1 radial nach
außen quillt, und
ein zweites Kontursegment (71) aufweist, das eines der folgenden a) oder b) annimmt:
a) die Form einer geradlinigen Durchgangslinie, die sich entlang der zweiten Achse
(P) erstreckt;
b) die Form einer Kurve, die sich entlang der zweiten Achse (P) erstreckt und radial
nach innen mit einem Krümmungsradius R2 anschwillt, der einer Ungleichung R1 < R2
genügt.
2. Glühkerze (1) nach Anspruch 1, wobei das Kontaktelement (7) so beschaffen ist, dass
wenn man auf den einen Querschnitt (75) sieht, eine Länge entlang einer sich erstreckenden
Richtung der zweiten Achse (P) länger ist als eine Länge entlang einer Richtung orthogonal
zur zweiten Achse (P).
3. Glühkerze (1) nach Anspruch 1 oder 2, wobei:
Der zentrale Schaft (3) ferner umfasst
einen hinteren Rumpfabschnitt (37), der an einer solchen Position in Bezug auf die
Längsrichtung der ersten Achse (O) der metallischen Hülle (4) angeordnet ist, dass
er dem hinteren Endabschnitt der inneren Umfangsfläche der axialen Bohrung (43) zugewandt
ist, und der einen größeren Durchmesser als der des anderen Endabschnitts (36) aufweist,
und
einen Schulterabschnitt (38), der den hinteren Rumpfabschnitt (37) und den anderen
Endabschnitt (36) in sich verjüngender Weise verbindet;
die metallische Hülle (4) ferner einen Kegelabschnitt (47) umfasst, der sich am hinteren
Endabschnitt der axialen Bohrung (43) von einer Position, die vor einem vorderen Ende
des Schulterabschnitts (38) angeordnet ist, in Richtung des hinteren Endes (48), das
rückwärts von der Position in Bezug auf die Längsrichtung der ersten Achse (O) angeordnet
ist, verjüngt; und
das Kontaktelement (7) so angeordnet ist, dass das zweite Kontursegment (71) mit dem
zentralen Schaft (3) in Kontakt steht und dass das erste Kontursegment (72) mit einer
inneren Umfangsfläche der vor dem Kegelabschnitt angeordneten axialen Bohrung (43)
in Kontakt steht.
4. Glühkerze (1) nach Anspruch 3, wobei die Kontur (70) des Kontaktelements (7) ferner
ein drittes Kontursegment (73) umfasst, das an einem Ende mit dem zweiten Kontursegment
(71) verbunden ist; das sich von einem Ende zum anderen Ende hin radial nach außen
erstreckt und sich entlang der zweiten Achse (P) erstreckt; und das am anderen Ende
mit dem ersten Kontursegment (72) verbunden ist; und dessen Verbindungspunkt mit dem
ersten Kontursegment (72) ein Ende des Kontaktelements (7) in Bezug auf die Erstreckungsrichtung
der zweiten Achse (P) ist.
5. Glühkerze (1) nach Anspruch 4, wobei die Kontur (70) ein Spiegelbild in Bezug auf
eine zentrale Position (A-A) in Längsrichtung der zweiten Achse (P) bildet.