Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
This inventi,on relates to electric heaters,and their
manufacture and more particularly to tubular heaters of a type
usable as engine glow plugs or for other localized heating
applications and having a conductive tubular sheath surrounding
an electrical heating"coil.
It is known in the electric heater art to provide
heaters for use as engine glow plugs and the like which have a
closed end tubular protective metal sheath surrounding an
axially extending heating element. The element is electrically
connected with the closed end o the sheath and also connects
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, with an electrode extending from th0 sheath open end. The
';~-'~ remainder of the sheath interior is packed with a suitable
~,,' ; Plectrical insulating material such as magnesium oxide ~MgO).
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; ~ , A prior method of manufacturing such heaters has in-
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i, , 30 cluded a step of hot welding the end of the 'coiled heating element
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to -the closed end of ~he sheath in or~er ~o provide a sa-~is-
factory electrical connection between -the heating element and
the sheath-. Even though this welding lS carefully done,
localized metallurgical changes may result in resistance
variations and susceptlbility -to cracking during a subse~uent
swaginy operation. This may affec-t the usability of the - .
resulting part. Another problem o~ such heaters in service is
caused by internal oxidstion of heatlng element, possibly due
to oxygen generated from the moisture content of the magnesium
oxide or from other external sources~ Such oxida~ion may
result in excessive localized heating and the eventual break- ;
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down of the heating el.ement.
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Summary of the Invention : -
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, : ,The present invention provides an improved hea-ter
construction and method of manufacture which reduces the above
mentioned problems. This is accomplished in part by eliminating
- the welding step-and utilizing instead a small amoùnt of metal
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powder which is placed in the closed end of the sheath, in ',
-contact with the end of the,heating element, and sintered to
provide.a low resistance electrical connection,between the
'` sheath and the element. The'sintering step may be accomplished
by passing an electric current through the powder between the
heating elèment and the sheath to accomplish the sintering
step without raising the temperatures o~ the heating element
and she,ath to a level'causiny metallurgical changes in their
structures. ...................... .. . .
' - ' In another'optiQnal feature of the invention, the
portion of the heatlng element not formlng the electriGal con- .
nection may be preoxidized by heating the element in an oxygen
containing atmosphere before assembly so as to`p'rovide a sound'
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oxide layer that protects the element from further oxidation
in use after assembly.
These and other features of the invention will be
more fully understood from the following description of a pre~
ferred embodiment and method of manufacture taken together
with the accompanying drawing.
Brief Description of the Drawing
In the drawing:
Figure 1 is a side elevational view of a diesel engine
glow plug formed according to the invention and having portions
broken away to show the interior construction;
Figure 2 is a longitudinal cross~sectional view of a
sheath and heater in a partial stage assembly;
Figure 3 is a longitwdinal cross-sectional view of a
sheath and heater assembly before swaging;
Figure 4 15 a longitudinal cross-sectional view show-
ing the heater assem~ly in the swaging dies at the conclusion
of the swaging step; and
Figure 5 is a transverse cross-sectional view taken
in the plane indicated by the line 5~5 of Figure 4 and further
illustrating the swaging dies and heater assem~ly.
Brief Description of the Illustrated Embodiment
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In the drawing, numeral lO generally indicates 2
diesel engine glow plug having the features of and made in
accordance with the method of this invention.
Glow plug 10 includes a conventional metal outer -~
shell 12 having a conical sealing surface 14 at one end, a
threaded portion 16 intermediate the ends and a hexagonal head
18 at the end opposite the sealing surface. The shell includes
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a longitudinal bore 20 in the lower portion of which there is
press-fitted a sheathed heater assembly generally indicated
by nume.ral 22.
Heater assembly 22 includes a tubular metal sheath 24
having an upper open end portion 26 fixed within the bore 20
and an elongated closed end portion 28 extending outwardly of
the shell along the axis of the bore 20
Centered within the sheath is a longitudinally extend-
ing electrical resistance heating coil or element 30, one end 31
of which engages the closed end of the sheath~ At this end, the
sheath and heating coil are mechanically and electrically joined
by a small amount of sintered metal 32 which is emplaced in
accordance with the invention in a manner to be subsequen-tly
described. ;
The heating coil extends from the closed end oE the
tubular sheath up to about its center, at which point it is
attached, preferably by flash welding, to the end of a center
rod or -~erminal 3 A . ~he terminal extends out through the
open end of the sheath 24 and through thé bore 20 to extend
outwardly of the hex headed end of the shell 12. A termi.nal
blade 36 is affixed to the exposed end of the center terminal
to receive an electrical attaching clip. The terminal 34 is
centered withi~ and insulated from the shell 12 and sheath 24
by a phenolic insulator 38 between the terminal and shell and
a compressed rubber o-ring 40 between the terminal and the open
end of the sheath. The remaining space within the sheath is
filled with a suitabl0 electrical insulating material 42 such
as compressed granulated magnesium oxide which holds the heat-
ing coil and terminal in their centered positions within the
sheath and preven-ts electrical contact between them, except at
the intentionally jo.ined points at the closed end of the sheath.
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The construction is such that upon installation of
the glow plug in an engine with appropriate electrical connec-
tions, a current may be passed from the blade 36 through the
terminal and heating coil to the closed end of the sheath, and
therethrough back to the shell which is grounded to the engine,
causing the heating coil to raise to operating temperature the
exposed end of the sheathed heater assembly.
A preferred method of making an electric heater of
the type used in the glow plug illustrated in Figure 1 i5 as
~ollow~.
First, the closed end tubular sheath 24 is formed
with a diameter about 30 percent larger and a length about 30
percent shorter than desired finished dimensions. The wire
heating coil 30 is then wound and attached by flash welding to
one end of the center terminal 34 to form a terminal and coil
assembly. This assembly is then inserted into the open end of
the sheath 24 and centered therein with the lower end of the
heating coil engaging the inner surface of the closed end of
the sheath. A small amount of the sinterable metal 32 is then
poured into the sheath, covering the lower portion of the heat-
ing coil at its points of engagement with the sheath. Figure 2
illustrates the partially complete heater at this stage in the
assembly process.
Subsequently, the open areas within the tube at the -~
locations of the heating coil and part way up the center
terminal are filled with the insulating material 42, which is
preferably granulated magnesium oxide. The loose material is
retained in place by the placement of the o-ring 40 in the
annular space between the terminal 34 and sheath 24, spaced
slightly inward from the open upper end of the sheath, as shown
in Figure 3.
Thereafter, the assembly is swaged to reduce its
diameter and extend its length. Preferably, the swaging
operation is done in multiple steps, with the open end of the
sheath being swaged first in order to compress the o-ring 40
and positively prevent the escape of any of the insulating
material 42. The assembly is then swaged from its closed end
until the whole length of the assembly has been reduced and
extended to the final dimensions of the heater assembly 22.
Figures 4 and 5 illustrate the assembly after swaging while
still within the swaging dies 44~
To form a glow plug as shown in Figure 1, the
sheathed heater assembly 22 is pressed into the shell 12 and
the phenolic insulator 38 and terminal blade 36 are attached.
At this point, or immediately after the swaging step, the
sintexable metal 32 is sintered into a solid mass by passing
an electric current through the heater assembly. The current
is preferably controlled at about the same level as the normal
current rating of the heater in operation. The high resis-
tance of the metal powder in its unsintered form raises its
2Q temperature to a point where the particles adhere together in
a more solidified mass, providing a low resistance electric
contact between the end of the sheath 24 and the heater coil 30.
The various elements of the heater assembly and the
glow plug may be made from any of the known materials capable
of accomplishing the desired purpose. In one version of the
illustrated embodiment, the sheath i5 formed of a hi~h tempera-
ture corrosion resistant nickel alloy of the Inconel type,
while the center terminal and heating coil are made of high
nickel alloys and the sinterable metal is nickel powder.
In an optional step in the method of assembly, the
heating element may be preoxidized, preferably after attachment
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to the center terminal, by heating the element to about 2,000 F
in air for a period of from 15 minutes to two hours so that an
insulating layer of oxide is formed on the surface of the wire.
The electrical connection should be made in such a way that the
very end turn of the heater coil, which engages the inner end
of the sheath upon assembly, is not heated. This end turn -then
remains free of the oxide coating and is capable of making a
low resistance electrical connection with the sheath through
the sintered metal 32.
Glow plugs and other sheathed heater devices made in
accordance with the invention as described have the advantage
that welding of the heater coil to the sheath is replaced by
the sintering of metal powder to provide the electrical con-
tact. This process is accomplished at much lower temperatures
than the usual welding process and thus avoids undesirable
metallurgical changes at the tip of the sheath and heating
coil. Further, the optional preoxidizing of the heater coil
pro`tects the coil from subsequent oxidation in service and
thus helps provide a longer lasting assembly.
It should be understood that sheathed heater assem-
blies of the type described herein may be utilized for appli-
cations other than engine glow plugs. One such application
presently contemplated for such a sheathed heater is to provide
heat to exhaust gas sensor devices used in engine exhaust
systems and connected with electronic air-fuel ratio controls.
While the inven-tion has been described by reference
to a specific embodiment chosen for purposes of illustration,
it should be understood that numerous changes could be made in
the structure and method of assembly described without depart-
0 ing from the inventive concepts embodied in the foregoing
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disclosure. Accordingly, it is intended that the invention not
be limited to the specific embodiment and method disclosed, but
have the full scope permitted by the language of the following
claims.
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