U.S. patent application number 13/991369 was filed with the patent office on 2013-09-26 for glow plug.
This patent application is currently assigned to NGK SPARK PLUG CO., LTD.. The applicant listed for this patent is Gaku Hatano, Katsuteru Ito, Hiroyuki Suzuki. Invention is credited to Gaku Hatano, Katsuteru Ito, Hiroyuki Suzuki.
Application Number | 20130248508 13/991369 |
Document ID | / |
Family ID | 46580681 |
Filed Date | 2013-09-26 |
United States Patent
Application |
20130248508 |
Kind Code |
A1 |
Ito; Katsuteru ; et
al. |
September 26, 2013 |
GLOW PLUG
Abstract
A glow plug (1) including a heater (2), a metallic shell (4)
assuming the form of a tube having an axial bore, a center shaft
(3) and a contact member (7). The contact member (7) is configured
such that a contour (70) of a cross section (75) includes a second
contour segment (71) in the form of a straight line, a first
contour segment (72) in the form of a curve swelling radially, and
third contour segments (73) in the form of curves connecting the
second contour segment (71) and the first contour segment (72).
Inventors: |
Ito; Katsuteru;
(Inazawa-shi, JP) ; Suzuki; Hiroyuki;
(Kasugai-shi, JP) ; Hatano; Gaku; (Inuyama-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ito; Katsuteru
Suzuki; Hiroyuki
Hatano; Gaku |
Inazawa-shi
Kasugai-shi
Inuyama-shi |
|
JP
JP
JP |
|
|
Assignee: |
NGK SPARK PLUG CO., LTD.
Nagoya-shi, Aichi
JP
|
Family ID: |
46580681 |
Appl. No.: |
13/991369 |
Filed: |
January 16, 2012 |
PCT Filed: |
January 16, 2012 |
PCT NO: |
PCT/JP2012/050708 |
371 Date: |
June 3, 2013 |
Current U.S.
Class: |
219/270 |
Current CPC
Class: |
F23Q 7/001 20130101 |
Class at
Publication: |
219/270 |
International
Class: |
F23Q 7/00 20060101
F23Q007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2011 |
JP |
2011-013388 |
Claims
1. 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.
2. A glow plug as claimed in claim 1, wherein the contact member is
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.
3. A glow plug as claimed in claim 1, wherein: the center shaft
further comprises 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 further comprises 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;
and the contact member is 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.
4. A glow plug as claimed in claim 3, wherein the contour of the
contact member further comprises 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.
5. A glow plug as claimed in claim 4, wherein the contour forms a
mirror image with respect to a center position in the extending
direction of the second axis.
Description
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 at the
rear end side 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
[0004] Patent Document 1: Japanese Patent Application Laid-Open
(kokai) No. 2007-292444
SUMMARY OF THE INVENTION
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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
[0016] [FIG. 1] Longitudinal sectional view of a glow plug 1.
[0017] [FIG. 2] Sectional view on an enlarged scale of a rear end
portion of the glow plug 1.
[0018] [FIG. 3] View showing, perspectively and in section, a
contact member 7 in a state prior to attachment to the glow plug
1.
[0019] [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.
[0020] [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.
[0021] [FIG. 6] View showing, perspectively and in section, a
contact member 107 in a state prior to attachment to the glow plug
1.
[0022] [FIG. 7] View showing, perspectively and in section, a
contact member 207 in a state prior to attachment to the glow plug
1.
[0023] [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
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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).
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.gtoreq.L2). However, depending on material and detailed
configurational features, this relationship (half of the length
along the axis P>the radial length (L1/2.gtoreq.L2)) may not be
satisfied so long as the relationship L1>L2 is satisfied).
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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."
* * * * *