Abstrict
A sheathed electric heater for use as an engine glow plug or the
like is formed by providing a closed end metal sheath with a diameter
larger than the finished diameter, Installing in the sheath a centered
electric heating element having an enlarged diameter portion that
engages the sheath adjacent to its closed end, the sheath and heating
element being formed of compatible metals capable of being welded
together by cold working, filling the sheath interior around the
element with granular electrical insulating material, and swaging
the assembly to reduce its diameter, extend its length, compact
the insulation and cold work the engaged end portions of the sheath
and heating coil so as to increase their engagement pressure and
thereby establish a low resistance electrical connection between
them.
Claims
I claim:
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 thereof,
(b) forming the heating coil in a width dimension smaller than
the interior width of the sheath except at one end where a tightly
wound portion is formed with an enlarged outer width dimension about
equal to the sheath inner width dimension, the coil other end being
attached to an electrical conductor,
(c) placing the heating coil and conductor in the sheath with the
enlarged portion engaging the sheath adjacent to its closed end,
the engaging surfaces of the sheath interior and the coil enlarged
portion being free from foreign substances, and centering the remainder
of the coil within the sheath with the attached conductor extending
from the open end,
(d) filling the 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, and
(e) swaging the heater assembly to reduce the width and extend
the length of the sheath and interior heating coil to finished dimensions
said sheath and said heating coil being formed of compatable metals
whereby said swaging step causes the engaging portions of the sheath
and heating coil to establish a low resistance electrical connection
therebetween.
2. 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 diameter substantially larger than
the desired finished size thereof,
(b) winding the heating coil in a helix smaller than the interior
diameter of the sheath except at one end where a tightly wound portion
is formed with an enlarged outer diameter about equal to the sheath
inner diameter, the coil other end being attached to an electrical
conductor,
(c) placing the heating coil and conductor in the sheath with the
enlarged diameter portion engaging the sheath adjacent to its closed
end, the engaging surfaces of the sheath interior and the coil enlarged
diameter portion being free from foreign substances, and centering
the remainder of the coil within the sheath with the attached conductor
extending from the open end,
(d) filling the 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, and
(e) swaging the heater assembly to reduce the diameter and extend
the length of the sheath and interior heating coil to finished dimensions,
said sheath and said heating coil being formed of compatable metals
capable of being welded together by cold working under pressure,
whereby said swaging step causes the engaging portions of the sheath
and heating coil to establish a low resistance electrical connection
therebetween.
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 susceptability 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
or 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 heating construction
and method of manufacture which reduces the above mentioned problems.
This is accomplished in part by eliminating the hot welding step
and providing the required low resistance electrical connection
by mechanical swaging of the sheath to a smaller diameter while
the coil end turns are in engagement with the sheath. This action
compresses both the sheath and the heating coil and causes cold
working of their engaged portions under pressure. When the sheath
and heating coil are formed of compatable materials capable of being
welded together by cold working, the swaging action causes the engaged
portions of the sheath and heating coil to establish a low resistance
electrical connection between them.
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 heater assembly
before swaging;
FIG. 3 is a longitudinal cross-sectional view showing the heater
assembly in the swaging dies at the conclusion of the swaging step;
FIG. 4 is an enlarged view of a portion of FIG. 3 showing the engaged
portions of the sheath and heating coil after swaging; and
FIG. 5 is a transverse cross-sectional view taken in the plane
indicated by the line 5--5 of FIG. 3 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 comprises
an enlarged portion that engages the sheath near its closed end.
At this end, the sheath and heating coil are mechanically and electrically
joined in a low resistance connection through cold working under
pressure in a swaging step 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
adjacent 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
in a helix smaller than the interior diameter of the sheath, except
at the enlarged end portion 31 which is tightly wound with another
diameter about equal to the sheath inner diameter. The smaller end
of the coil 30 is then 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 enlarged diameter lower end 31 of the heating coil
engaging the inner surface adjacent the closed end of the sheath.
The contacting surfaces of both the coil and the sheath should be
free of foreign substances at this time.
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 compacted and 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. 2.
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. 3-5 illustrate the assembly after swaging while still within
the swaging dies 44.
The swaging operation deforms the outer sheath in a cold working
process while compressing and extending the heating coil and surrounding
insulation within the sheath. The tightly wound end portion 31 of
the heating coil, which is in engagement with the sheath interior,
is squeezed to a smaller diameter by the reduction in sheath diameter.
This action causes the sheath material to flow partially around
the coil wires as is best shown in FIG. 4. At the same time, an
intimate joining of the surfaces takes place, forming a low resistance
electrical connection between the sheath and heating coil. This
is believed to occur in the manner of cold welding of components
under pressure.
To best accomplish the desired electrical connection, the materials
of the sheath and heating coil should be made from compatable materials
capable of being cold welded. Many such materials are capable of
being so used. For example, high temperature alloys of about 80
percent nickel and 20 percent chromium are suitable for both the
sheath and the heating coil. Such alloys are sold under trade names
Chromel AA and Chromel A. Other nickel based alloys may also be
used, such as Nichrome V for the heating coil and Inconel 601 for
the sheath.
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.
If desired, however, the sheathed heater assembly may be used in
other than glow plug applications. One such application presently
contemplated for such heaters is to provide heat to exhaust gas
sensor devices used in engine exhaust systems and connected with
electronic air-fuel ratio controls.
Glow plugs and other sheathed heater devices made in accordance
with the invention as described have the advantage that high temperature
welding of the heater coil to the sheath is eliminated, thus avoiding
undesirable metallurgical changes at the tip of the sheath and heating
coil.
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. pg,8
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