Abstrict
A sheathed electric heater for use as an engine glow plug or the
like is formed by providing a closed end metal sheath, installing
a centered electric heating element that approaches the sheath closed
end, adding a small amount of sinterable metal powder covering the
end of the heating element at the closed end of the sheath, filling
the remaining volume around the element with granular electrical
insulating material, swaging the assembly to reduce its diameter,
extend its length and compact the insulation, and sintering the
metal powder by passing electric current therethrough to provide
a low resistance electrical connection between the heating coil
and the closed end of the sheath.
Claims
What is claimed is:
1. The method of making an electric heater of the type having an
elongated electrically conductive tubular metal sheath closed at
one end and containing an axially extending heating coil electrically
grounded at one end to the sheath adjacent its closed end and insulated
therefrom elsewhere by heat conductive insulation, said method comprising
the steps of:
(a) forming the sheath with a width substantially larger than the
desired finished size therof,
(b) forming the heating coil with a width dimension smaller than
the interior width of the sheath, the coil being attached at one
end to an electrical conductor, the other end being free,
(c) centering the heating coil and conductor in the sheath with
the free end adjacent to the sheath closed end, the adjacent surfaces
of the sheath interior and the coil free end being free from foreign
substances,
(d) filling the tip portion of the sheath interior with an amount
of sinterable metal powder sufficient to cover the free end of the
heating coil,
(e) filling the remaining open interior of the sheath with granular
high temperature electrical insulating material tightly packed around
and within the heating coil and sealing the open end of the sheath
to retain the insulating material in place,
(f) swaging the heater assembly to reduce the width and extend
the length of the sheath and interior heating coil to finished dimensions,
and
(g) passing an electric current between the heating element and
sheath through the metal powder to heat and sinter the metal powder
in the sheath tip, surrounding the coil free end, whereby a low
resistance electrical connection is formed between the sheath and
the heating coil through the sintered metal.
2. The method of claim 1 and further including the step of preoxidizing
the heating coil, except for the free end thereof, before its assembly
with the sheath, said preoxidizing step being conducted by heating
the coil to oxidizing temperature in air by passing an electric
current therethrough for a predetermined period.
Description BACKGROUND OF THE INVENTION
This invention 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
of the sheath and also connects with an electrode extending from
the sheath open end. The remainder of the sheath interior is packed
with a suitable electrical insulating material such as magnesium
oxide (MgO).
A prior method of manufacturing such heaters has included a step
of hot welding the end of the coiled heating element to the closed
end of the sheath in order to provide a satisfactory electrical
connection between the heating element and the sheath. Even though
this welding is carefully done, localized metallurgical changes
may result in resistance variations and susceptibility to cracking
during a subsequent swaging operation. This may affect the usability
of the resulting part. Another problem of such heaters in service
is caused by internal oxidation of heating element, possibly due
to oxygen generated from the moisture content of the magnesium oxide
of from other external sources. Such oxidation may result in excessive
localized heating and the eventual breakdown of the heating element.
SUMMARY OF THE INVENTION
The present invention provides an improved heater 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 amount of metal 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 electrical current through the
powder between the heating element and the sheath to accomplish
the sintering step without raising the temperatures of the heating
element and sheath to a level causing metallurgical changes in their
structures.
In another optional feature of the invention, the portion of the
heating element not forming the electrical connection may be preoxidized
by heating the element in an oxygen containing atmosphere before
assembly so as to provide a sound 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 preferred embodiment and method
of manufacture taken together with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 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;
FIG. 2 is a longitudinal cross-sectional view of a sheath and heater
in a partial stage assembly;
FIG. 3 is a longitudinal cross-sectional view of a sheath and heater
assembly before swaging;
FIG. 4 is a longitudinal cross-sectional view showing the heater
assembly in the swaging dies at the conclusion of the swaging step;
and
FIG. 5 is a transverse cross-sectional view taken in the plane
indicated by the line 5--5 of FIG. 4 and further illustrating the
swaging dies and heater assembly.
BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
In the drawing, numeral 10 generally indicates a 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 a longitudinal bore 20 in the lower
portion of which there is press-fitted a sheathed heater assembly
generally indicated by numeral 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 extending 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 subsequently described.
The heating coil extends from the closed end of 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 terminal 34. The
terminal extends out through the open end of the sheath 24 and through
the bore 20 to extend outwardly of the hex headed end of the shell
12. A terminal blade 36 is affixed to the exposed end of the center
terminal to receive an electrical attaching clip. The terminal 34
is centered within 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 suitable electrical insulating material 42 such as compressed
granulated magnesium oxide which holds the heating coil and terminal
in their centered positions within the sheath and prevents electrical
contact between them, except at the intentionally joined points
at the closed end of the sheath.
The construction is such that upon installation of the glow plug
in an engine with appropriate electrical connections, 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 FIG. 1 is as follows.
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 heating coil at its points of engagement with the sheath. FIG.
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 FIG. 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.
FIGS. 4 and 5 illustrate the assembly after swaging while still
within the swaging dies 44.
To form a glow plug as shown in FIG. 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 sinterable 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 resistance
of the metal powder in its unsintered form raises its 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 is formed of a high temperature 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 to the center terminal,
by heating the element to about 2,000.degree. 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 contact. 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 protects the coil from subsequent oxidation in
service and thus helps provide a longer lasting assembly.
It should be understood that sheathed heater assemblies of the
type described herein may be utilized for applications 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 invention 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 departing from the inventive concepts
embodied in the foregoing 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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