U.S. patent number RE30,126 [Application Number 05/854,057] was granted by the patent office on 1979-10-23 for bilateral heater unit.
Invention is credited to John W. Churchill.
United States Patent |
RE30,126 |
Churchill |
October 23, 1979 |
Bilateral heater unit
Abstract
A heater unit of bilateral construction is formed by forming a
resistor assembly of a resistor helix extending between cylindrical
terminals and overlapping the same, inserting the assembly in a
sheath tube, filling the tube with MgO powder, placing laminated
mica end plugs over the terminals and extending into the ends of
the tube, bending the tube into a U-shape, pressing the legs of the
U together and feeding the pressed unit through swaging dies to
deform the tube over the entire length thereof to provide a heater
unit of substantially circular cross section and having two legs of
substantially semicircular cross section. The resistor helix in the
area of the leg portions has a cross-sectional shape corresponding
to the cross-sectional shape of the sheath and the terminals and
mica plugs within the sheath are flattened to provide a similar
cross-sectional shape thereby locking the terminals within the
sheath.
Inventors: |
Churchill; John W. (Beverly,
MA) |
Family
ID: |
27009712 |
Appl.
No.: |
05/854,057 |
Filed: |
November 23, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
382295 |
Jul 25, 1973 |
398209 |
Sep 21, 1976 |
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Current U.S.
Class: |
219/544; 219/541;
219/552; 338/240; 338/242 |
Current CPC
Class: |
H05B
3/06 (20130101); H05B 3/52 (20130101); H05B
3/48 (20130101) |
Current International
Class: |
H05B
3/06 (20060101); H05B 3/42 (20060101); H05B
3/48 (20060101); H05B 3/52 (20060101); H05B
003/44 () |
Field of
Search: |
;219/523,541,544,552,553
;29/611,613,614,615,617,619 ;338/238,239,240,241,242,333
;174/75R,77R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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164745 |
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Dec 1949 |
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AT |
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59879 |
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Feb 1954 |
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FR |
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466060 |
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May 1937 |
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GB |
|
980466 |
|
Jan 1965 |
|
GB |
|
Primary Examiner: Mayewsky; Volodymyr Y.
Attorney, Agent or Firm: Craig & Antonelli
Claims
I claim:
1. A heater unit comprising an elongated outer metallic sheath
which is semicircular in cross-section along at least a portion of
the length thereof including the end portions, metal terminal means
at respective end portions of the sheath extending within and in
the longitudinal direction of the sheath, said metal terminal means
having a portion lying within the sheath and having a portion
extending outwardly beyond the end of the sheath, said portion of
said metal terminal means lying within the sheath having a
cross-sectional shape corresponding substantially to the
cross-sectional shape of the sheath, an insulating end plug
surrounding said metal terminal means and extending at least
partially within said sheath, a resistor .[.helix.]. .Iadd.means
.Iaddend.disposed within the sheath and secured to portions of said
metal terminal means and extending therebetween, said resistor
.[.helix.]. .Iadd.means .Iaddend.having a cross-sectional shape
corresponding to the cross-sectional shape of the surrounding
sheath, and compacted powder insulation completely filling the
sheath and spacing the resistor .[.helix.]. .Iadd.means
.Iaddend.and a portion of the metal terminal means from the
sheath.
2. A heater unit according to claim 1, wherein said resistor
.[.helix.]. .Iadd.means .Iaddend.is in overlapping relation with
said metal terminal means at the end portions thereof and is
disposed within depressions formed in the end portions of said
terminal means by compaction of said powder insulation.
3. A heater unit according to claim 2, wherein said metal terminal
means is a solid member having a substantially cylindrical portion
extending outwardly beyond the end of said end plug and a portion
having a substantially semicircular cross-section in the area
surrounded by said plug and within the area of said sheath.
4. A heater unit according to claim 2, wherein said metal terminal
means includes an elongated member having an axially extending
hollow portion filled with a flexible lead wire, said flexible lead
wire having a portion extending outwardly beyond the end of said
elongated member and beyond the end of said tubular sheath, said
portion having an insulating coating thereon, and said elongated
member having a cross-sectional shape corresponding to the
cross-sectional shape of said sheath.
5. A heater unit according to claim 2, wherein said heater unit has
a substantially circular cross-section formed by substantially
parallel adjacent leg portions of semicircular cross-section and an
interconnecting portion formed of said elongated sheath bent back
upon itself and formed integrally with the adjacent parallel
extending leg portions, each of said leg portions having said
terminal means extending outwardly from adjacent end portions
thereof.
6. A heater unit according to claim 5, wherein said interconnecting
portion is provided with a substantially zero internal bend radius
at the junction of adjacent leg portions and a substantially
180.degree. return bend at the outer surface of said
interconnecting portion.
7. A heater unit according to claim 5, wherein said resistor
.[.helix.]. .Iadd.means .Iaddend.has a cross-sectional shape
corresponding to the cross-sectional shape of the sheath such that
said resistor .[.helix.]. .Iadd.means .Iaddend.has a semicircular
cross-section in the area of said leg portions and a substantially
circular cross-section at the middle area of said interconnecting
portion between said leg portions.
8. A heater unit according to claim 7, wherein said end plugs are
laminated mica plugs and said powder insulation is MgO powder.
9. A bilateral heater unit comprising an elongated sheath in the
form of two substantially parallel adjacent leg portions
interconnected by a return bend portion formed of said elongated
sheath bent back upon itself and being integral with the adjacent
parallel extending leg portions, each of said leg portions having
terminal means extending outwardly from the adjacent end portion
thereof and spaced from said sheath, a resistor .[.helix.].
.Iadd.means .Iaddend.secured to said terminal means of each of said
leg portions and extending along the sheath between said terminal
means, said sheath between said terminal means being completely
filled with compacted powder insulation spacing said resistor
.[.helix.]. .Iadd.means .Iaddend.from said sheath, each of said leg
portions having a semicircular cross-section formed of a flat
portion and an arcuate portion, said flat portion of said legs
being adjacent one another such that heater unit is provided with a
substantially circular cross-section, said interconnecting portion
forming a substantially 180.degree. return bend along the outer
surface portion of said sheath and a substantially zero radius
return bend along the inner surface portion of said sheath at the
junction of the flat portions of said leg portions.
10. A bilateral heater unit comprising an elongated sheath in the
form of two substantially parallel adjacent leg portions
interconnected by a return bend portion formed of said elongated
sheath bent back upon itself and being integral with the adjacent
parallel extending leg portions, each of said leg portions having
terminal means extending outwardly from the adjacent end portion
thereof and spaced from said sheath, a resistor .[.helix.].
.Iadd.means .Iaddend.secured to said terminal means of each of said
leg portions and extending along the sheath between said terminal
means, said sheath between said terminal means being completely
filled with compacted powder insulation spacing said resistor
.[.helix.]. .Iadd.means .Iaddend.from said sheath, each of said leg
portions having a semicircular cross-section formed of a flat
portion and an arcuate portion, said flat portion of said legs
being adjacent one another such that heater unit is provided with a
substantially circular cross-section, said interconnecting portion
forming a substantially 180.degree. return bend along the outer
surface portion of said sheath and a substantially zero radius
return bend along the inner surface portion of said sheath at the
junction of the flat portions of said leg portions, said resistor
.[.helix.]. .Iadd.means .Iaddend.having a cross-sectional shape
corresponding to the cross-sectional shape of said sheath such that
said resistor .[.helix.]. .Iadd.means .Iaddend.has a semicircular
cross-sectional shape in the area of said leg portions and a
substantially circular cross-section in the middle area of the
interconnecting portion. .Iadd. 11. A heater unit according to
claim 1, wherein said resistor means is a resistor helix.
.Iaddend..Iadd. 12. A heater unit according to claim 2, wherein
said resistor means is a resistor helix. .Iaddend..Iadd. 13. A
heater unit according to claim 7, wherein said resistor means is a
resistor helix. .Iaddend..Iadd. 14. A heater unit according to
claim 9, wherein said resistor means is a resistor helix.
.Iaddend..Iadd. 15. A heater unit according to claim 10, wherein
said resistor means is a resistor helix. .Iaddend.
Description
The present invention relates to a cartridge type or tubular heater
unit having a bilateral construction and a method for constructing
the same.
Tubular or cartridge type heater units have many uses and for
example, may be utilized as insertion units wherein the heaters are
inserted into a drilled hole in a worktable or the like for heating
the table surface. The cartridge heater units are generally
constructed with an outer tubular sheath, an inner core which
supports a resistor winding as well as positioning and anchoring
two terminal studs at the same end of the unit, and MgO powder
filling the annular space between the resistor assembly and the
sheath whereby the unit is reduced in diameter after the ends
thereof are capped. The diameter reduction of the sheath and unit
compacts the MgO and the crushable core providing an increased
density of the MgO and improves both the electrical insulating
value and thermal conductivity of the unit. However, the diameter
reduction and densification of the MgO is limited by the presence
of the core in the unit.
It is also known to form a tubular heater unit by inserting into a
metal tube, magnesium metal strips or the like together with a
coiled resistance wire and placing the assembled element into an
autoclave filled with water so that the tube is covered with water.
The autoclave is then heated to cause crystalline magnesium
hydroxide to be formed in the tube such that the resistance coil is
completely embedded in the hydroxide. Thereafter, the tube is
heated to partially convert the magnesium hydroxide into magnesium
oxide with powder insulation being added and terminals assembled
into both ends of the tube. The tube is then deformed by pressing
between dies to form, for example, a semicircular cross-sectional
shape, with the tube being bent at a later stage into a hairpin
form and pressed together to provide a circular cross section with
terminals at the same end of the element. In this construction, the
tube is again heated to completely convert the magnesium hydroxide
to crystalline magnesium oxide and deformed again to recompact the
crystalline magnesium oxide. This construction has several
disadvantages including the requirement for crystalline magnesium
hydroxide to be formed within the tube and then converted to
crystalline magnesium oxide by means of several heating steps and
additionally requires a pressing deformation operation with
substantially no circumferential reduction and elongation of the
tube as occurs with a swaging operation. Additionally, there is the
requirement for two pressing operations for compacting and
recompacting of the insulation and the bending of the tube has a
tendency to create fissures in the crystalline insulating material
reducing the service life of such unit.
It is therefore an object of the present invention to provide a
heater unit and a method for constructing the same which overcomes
the disadvantages of prior art units and construction methods.
It is another object of the present invention to provide a heater
unit having an improved service life.
It is a further object of the present invention to provide a heater
unit in which a good mechanical and electrical connection is
obtained between the terminals and the resistor wire due to
compaction of powder insulation during the formation of the heater
unit.
It is another object of the present invention to provide a heater
unit in which the terminals become locked within the sheath of the
heater unit as a result of a swaging operation over the entire
length of the sheath of the heater unit.
It is still another object of the present invention to provide a
heater unit with a bilateral construction wherein the resistance
wire is surrounded by a sheath over the entire length thereof and
spaced from the sheath by compacted powder insulating material.
In accordance with the present invention, a heater unit of
bilateral construction is formed by forming a resistor assembly,
inserting the assembly in a sheath tube, filling the tube with MgO
powder, placing laminated mica endplugs over the terminals of the
resistor assembly and extending into the ends of the tube, bending
the tube into a U-shape, pressing the legs of the U together and
feeding the pressed unit through swaging dies so as to deform the
tube over the entire length thereof to provide a heater unit of
substantially circular cross section having two legs of
substantially semicircular cross section.
In accordance with a feature of the present invention, the resistor
assembly is formed by placing each end of an elongated resistor
helix over respective end portions of cylindrical metal terminals
and during the swaging operation, the portion of the metal
terminals within the sheath as well as the resistor helix within
the sheath attains a cross-sectional shape similar to that of the
surrounding sheath whereby the terminals become locked within the
sheath. Additionally, due to the compaction of the MgO powder, the
resistor wire overlapping the end portions of the metal terminals
form depressions in the end portions of the metal terminals and are
disposed in such depressions such that a firm mechanical and
electrical connection of the resistor helix and the terminals is
attained.
In accordance with another feature of the present invention, the
resistor helix in the interconnecting portion which is an extension
of the two leg portions of the heater unit is provided with a
cross-sectional shape corresponding to the sheath of the unit in
the interconnecting portion which varies from a semicircular
configuration to a cylindrical configuration. The interconnecting
portion provides a 180.degree. return bend at the outer portion of
the sheath for the heater unit and has a substantially zero
internal radius return bend at the inner portion of the heater unit
forming the flat portion of the legs of the heater unit.
These and further objects, features and advantages of the present
invention will become more apparent from the following description
when taken in connection with the accompanying drawings which show,
for purposes of illustration only, two embodiments in accordance
with the present invention, and wherein:
FIGS. 1a to 1i are views illustrating the formation of a heater
unit in accordance with the present invention;
FIG. 2 is a partial longitudinal cross-sectional view of the end
portion of a leg of the heater in accordance with the present
invention illustrating the deformation of the metal terminal;
FIG. 3 is a transverse cross-sectional view of the heater leg of
FIG. 2 taken in the plane III--III;
FIG. 4 is a perspective view of a metal terminal construction in
accordance with another embodiment of the present invention;
and
FIGS. 5, 6 and 7 are transverse cross-sectional views of the
interconnecting portion of the heater unit in accordance with the
present invention taken at different angles with respect to the
flattened portion of the leg of the heater unit as indicated in
FIG. 1h.
Referring now to the drawings wherein like reference numerals are
utilized to designate like parts throughout the several views,
there is shown in FIG. 1 the method steps for constructing the
bilateral tubular heater unit in accordance with the present
invention. As shown in FIG. 1a, a cylindrical tube 1 of the
appropriate diameter for forming the sheath of the heater unit, for
example, a 5/16 inch diameter tube for a 3/8 inch heater unit is
selected. In accordance with the present invention and as shown in
FIG. 1b, a resistor assembly is formed consisting of a cylindrical
metal terminal 2, as will be more thoroughly described hereinafter,
attached to the ends of a resistance helix 3 by means of a slight
interference fit such that the resistor helix overlaps or extends
over portions of the cylindrical metal terminal 2. The wire
resistance helix, preferably has a helix diameter to wire diameter
of at least 6 to 1. The assembly, as shown in FIG. 1c, is stretched
through the tube sheath 1 and centrally positioned therein by
fittings 4 with at least one fitting 4 having an opening or the
like therein so as to permit granulated MgO powder 5 to be inserted
into the tube such that the powder 5 fills the tube and passes
through and around the resistor helix. The powder may be densified
somewhat by vibration of the assembled heater so as to ensure that
the MgO powder completely fills the tube 1. As shown in FIG. 1d,
the fittings 4 are then removed and the ends of the tube 1 are
capped by laminated mica plugs 6 which surround the cylindrical
metal terminal 2. The mica plug is sized for a slide fit into the
tube 1 and a slide fit around the terminal 2. It is placed
approximately 1/8 inch into the heater and extends approximately
3/8 inch outside of the heater unit. Alternatively, the mica plug
may extend completely within the tube sheath 1. Additionally, a
metal terminal portion extends outwardly beyond the end of the
laminated plug. The heater unit is then bent into a U-shape in a
manner known in the art, as shown in FIG. 1e, with the legs then
being pressed together, with radius tools 7 as shown in FIG. 1f.
The tool radius is the same as that of the sheath tube in order to
preserve the tube shape and diameter prior to a deforming operation
such as swaging of the heater so that no deformation of the tube
occurs during the pressing of the legs together as well as no
compaction of the MgO. The deformation of the heater during this
step would tend to precompact the MgO in the area of deformation
whereas homogeneous MgO density is necessary for even compaction
during swaging. The heater is then passed as shown in FIG. 1g
through the dies 8 of a rotary swaging machine, or the like,
whereby the heater is progressively deformed over the entire length
of the tube with a diameter reduction and an elongation of the tube
occurring. The heater is progressively deformed into a cylindrical
shape with substantially circular cross section with each leg of
the heater being substantially semicircular in cross section and
having a flat portion 9 and an arcuate portion 10 as shown in FIGS.
1h and 1i. As shown, the heater is provided with an internal radius
bend portion 11 of substantially zero at the junction of the flat
leg portions such that the leg portions at the junction are
substantially in contact while the outer sheath is provided with a
return bend portion 12 of substantially 180.degree.. Additionally,
as shown in FIG. 1i, the mica end plugs 6 are deformed into the
same shape as the surrounding tube sheath 1, i.e., approximately
semicircular, and as a result, become locked into the sheath.
As shown in FIG. 2, which is a partial cross-sectional view of the
end portion of a heater leg in accordance with the present
invention, the deformation of the mica end plugs is also
transmitted to the metal terminals 2 which they surround such that
the terminals are flattened to some extent as a result of the
swaging process in the region A of the terminal, the terminal
assuming a generally semicircular shape in this region. As a result
of the swaging process, the MgO powder insulation 5 is compacted by
the reduction of volume and also transmits pressure to the metal
terminal 2 to flatten and roughen the surface thereof in the region
B which aids in locking the terminal within the heater unit. In the
region C of the resistor wire helix wrap 3 on the metal terminal 2,
the MgO while flattening the terminal, also presses the turns of
the wire wrap into the surface of the terminal creating individual
depressions 13 in the terminal and into which the wire wrap is
disposed. A firm electrical and mechanical connection between the
wire helix and the metal terminal is thus established. The MgO also
transmits pressure to the resistor helix 3, deforming it to a shape
corresponding to that of the shape of the surrounding sheath with
the MgO being compacted.
The metal terminal 2 of the present invention may be in the form of
a solid metal cylindrical stud, a hollow tubular stud, or a stud
having a hollow portion. In any case, the wire helix is wrapped
around the end portion of the terminal as indicated in FIG. 1 and
the swaging of the heater is carried out over the entire length of
the heater sheath such that the terminal 2, in addition to the
heater sheath is flattened or deformed to some extent as shown in
FIG. 2. The starting cylindrical terminal 2 extending within the
sheath area deforms to a shape corresponding to the shape of the
sheath such that it has a substantially semicircular cross section
as shown in FIG. 3, which is a section taken in the plane III--III
in FIG. 2. The solid terminal is generally formed of a ductile
solid rod, for example, a solid nickel cylinder of commercially
pure nickel having a soft temper or Brinnell hardness of 92-120.
The starting diameter of the rod terminal, of course, is variable
in accordance with the dimensions of the heater unit, as well as
with the size of the resistor helix utilized and other desired
properties. The deformation of the starting cylindrical terminal,
as shown in FIG. 3, is such that the terminal is provided with a
width dimension extending parallel to the flattened portion 9 of
the sheath tube as seen in cross section and with a thickness or
height dimension extending in the transverse direction. The
deformation of the terminal, for example, of a solid terminal with
a starting diameter of 0.100 inches in several tests of a heater
unit resulted in an average cross-sectional dimension of 0.079
inches in thickness or height by 0.098 inches in width in the area
C under the wire wrap, 0.099 inches in thickness by 0.100 inches in
width in the area B under the MgO powder, and 0.083 inches in
thickness by 0.103 inches in width in the area A under the mica
plug. It is noted, however, that depending upon the length of the
area of the terminal portion B between the mica plug and the wire
wrap, such portion will deform more or less. However, in general,
there is a greater deformation of the terminal under the mica plug
then under the wire wrap with a smaller deformation under the MgO
powder. The hollow terminal whether a completely hollow tube or a
drilled stud having a hollow portion deforms in substantially the
same manner as the solid terminal with the hollow terminal offering
the advantage that as shown in FIG. 4, a stranded flexible lead
wire 14 may be inserted into the hollow terminal 2' which lead wire
becomes anchored within the terminal due to the deformation of the
terminal. The lead wire is provided with insulation 15 on the
portion thereof extending outwardly from the terminal while the
lead wire within the terminal is bare. For a hollow tubular
terminal 2' having an outer diameter of 0.125 inches and a 0.017
inch thick wall and formed of stainless steel, the average
deformation dimensions of such a terminal in a heater unit, in
accordance with the present invention, resulted in a deformation of
0.069 inches in thickness or height by 0.164 inches in width in the
area C under the wire wrap, 0.073 inches in thickness by 0.165
inches in width in the area B under the MgO, and 0.070 inches in
thickness by 0.163 inches in width in the area A under the mica
plug. Generally, as a result of experimentation, it has been found
that the thickness of the solid terminal is approximately 66
percent of the starting diameter whereas the width is approximately
120 percent of the starting diameter. For a hollow terminal, the
thickness of the hollow terminal is approximately 54 percent of the
starting diameter with the width being approximately 130 percent of
the starting diameter. Thus, in accordance with the present
invention, the metal terminals deform to approximately the shape as
the sheath and by providing a swaging operation in the area of the
terminals such terminals become locked into the sheath as well as
ensuring a good mechanical and electrical connection of the
terminal to the resistor helix.
The resistor helix 3 is deformed by the compaction of the MgO
powder to a shape corresponding to that of the surrounding sheath
such that the resistor helix is also provided with a semicircular
cross section in the area of the leg portions. However, in the area
of the return bend, i.e., the interconnecting portion between the
two leg portions, the shape of the sheath 1 and the corresponding
shape of the resistor helix 3 varies as shown in FIGS. 5-7. As
shown in FIG. 5, which is a cross-sectional view taken at an angle
of 45.degree. with regard to the flattened portion of the leg of
the heater of FIG. 1h, the cross-sectional configuration of the
sheath and helix is approximately semicircular in shape in the
manner of the cross section of the entire leg. As shown in FIG. 6,
which is a cross-section view taken at an angle of 30.degree. with
respect to the flattened portion of the leg and further into the
return bend of the interconnecting portion of FIG. 1h, the sheath
and the resistor helix has an oval or elliptical shape with the
compacted MgO powder surrounding the resistor helix and spacing the
helix from the outer sheath. As shown in FIG. 7, which is a
cross-sectional view taken in the plane of the flattened portion of
the leg or at 0.degree. with respect to this flattened portion of
FIG. 1h, the sheath has a substantially circular cross section
although being deformed somewhat from a true circular cross section
and the resistor helix has a corresponding shape with the MgO
powder uniformly surrounding the resistor helix in order to space
and insulate the same from the sheath. The shape of the sheath and
resistor helix varies in the interconnecting portion since the
shape of the legs of the heater unit is attained by the swaging
operation and the interconnecting portion as well as the legs have
a substantially cylindrical shape prior to the swaging operation.
In this manner, the swaging operation does not cause the helix to
cut through the MgO powder insulation and instead, the helix
deforms to a final shape which at all points closely matches that
of the surrounding sheath. The sharp corners of the coil turns, for
example, match the adjacent sharp corners of the sheath at the
flattened portion of the heater leg whereas at the return bend of
the interconnecting portion, the coil has a different shape
conforming to the sheath cross section. In this manner, there are
no thin spots of MgO insulation and the heater easily withstands a
dielectric voltage test of 2500 vrms.
The construction of a heater unit in accordance with the present
invention provides a heater unit which is readily adapted to be
inserted into a drilled hole of a worktable for heating the table
surface as is known in the art. That is, due to the substantially
circular cross section, a close fit can be obtained in accordance
with the desired size of the drilled hole. Further, due to the
bilateral construction of the heater unit in accordance with the
present invention, upon heating of the unit, the legs have a
tendency to bow out providing for intimate contact with the surface
of the drilled hole in the worktable and better thermal
conductivity therebetween. Additionally, due to the uniform
distribution of the compacted MgO powder between the resistor helix
and the tubular sheath, the heater unit is provided with good
insulation characteristics thereby ensuring a long service life for
the heater unit.
While I have shown and described several embodiments in accordance
with the present invention, it is understood that the same is not
limited thereto, but is susceptible of numerous changes and
modifications as known to those skilled in the art, and I therefore
do not wish to be limited to the details shown and described
herein, but intend to cover all such changes and modifications as
are encompassed by the scope of the appended claims.
* * * * *