US3258580A - Hold-down means for convoluted sheathed heater - Google Patents

Description

June 28, 1966 R. DILLS 3,

HOLD-DOWN MEANS FOR CONVOLUTED SHEATHED HEATER Filed Dec. 20, 1965 F'lG.l

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l-HS ATTORNEY United States Patent 3,258,580 HOLD-DOWN MEANS FOR CONVOLUTED SHEATHED HEATER Raymond L. Dills, Louisville, Ky., assignor to General Electric Company, a corporation of New York Filed Dec. 20, 1963, Ser. No. 332,037 Claims. (Cl. 219463) The present invention relates to electrical surface heating units such as are used with electric ranges and particularly to a means and method of assembling a convoluted sheathed heating element to a supporting spider.

It is a general practice today to manufacture metal sheathed heating elements with a central resistance conductor inserted into a metallic sheath and embedded in a compacted layer of heat conducting, electrically insulating material, such as powdered magnesium oxide. Such a heating element is wound in the form of a flat coil to define a flat cooking surface on which a cooking vessel or skillet is to be placed for heating. Such a heating coil is supported from below by an underlying spider that is adapted to hold the turns or convolutions of the heating coils in a flat horizontal position in a circular opening in the top cooking surface of the range. Frequently, the convolutions are held in place by positioning the convolutions in notches or slots in the spider and flattening the heating coil therein to position and form a rigid attachment thereto. As there are expansions and contractions of the heating element as its temperature is raised and low- .ered, considerable strains can be set up in the metallic sheath which may result in buckling or warping of the heating element so that it is no longer in a flat horizontal position or plane.

Other fastening means that have been utilized in the past are welded straps looped under the spider and having the ends of the straps welded directly to the sheath of adjacent convolutions. This technique can be hazardous if not properly mastered on a repetitive basis because an inept welder or a poorly designed automatic welding machine might burn through the sheath or at least weaken the sheath to a point where it might fail prematurely during normal use of the surface heating unit.

A third type of hold-down means which has been used extensively is the formation of tabs on the spider which are crimped around the convolution. This requires that the crimping pressure must be accurately governed otherwise the sheath might be fractured by poor workmanship during the crimping operation or due to the expansion and contraction of the heating element during normal use. Also, the high operating temperature of the sheath in contact with the crimped tabs causes erosion of the tabs so that in time they do not hold the sheath effectively.

The principal object of the present invention is to provide a design of a convoluted sheathed heater held to an underlying spider by strap means without using any mechanical bond to the sheath.

A further object of the present invention is to provide a surface heating element with thin strap means for holding the element to a supporting spider while allowing slight relative movement between the sheathed element and spider so as not to set up undue stresses that would tend to warp the element out-of-fiat when heated.

A further object of the present invention is to provide a simple strap means for holding a plurality of convoluted heating elements to a supporting spider so that when replacement is necessary each element may be replaced separately.

A still further object of this invention is to provide a convoluted sheathed heating element with looped straps which position the convolutions with respect to the supporting spider so the convolutions are always spaced apart for good heat transfer conditions.

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The present invention, in accordance with one form thereof, pertains to a resistance type of sheathed heater formed with flat coils or convolutions for presenting a fiat cooking surface. The heater rests on a supporting spider having generally radial arms. The convolutions are to be held down to the spider at one or more points so as to steady the heating element thereon while permitting the necessary expansion and contraction of the element during its use. Strap means is located adjacent a radial arm of the spider and it has an intermediate portion tightly looped over at least one convolution, while the two end portions are immovably secured to the arm. Means are also provided to countersink the strap means into the sheathed heater so as to present a flat cooking surface.

My invention will be better understood from the following description taken in conjunction with the accompanying drawing and its scope will be pointed out in the appended claims.

FIGURE 1 is a top plan view of a surface heating unit embodying the present invention;

FIGURE 2 is a front elevational View partly in crosssection of the surface heating unit of FIGURE 1 taken on line 2-2 of FIGURE 1;

FIGURE 3 is a fragmentary view showing a wire member fastening a single convolution onto a radial arm of a supporting spider and taken on the line 3 3 of FIG URE 2;

FIGURE 4 is a fragmentary view partly in cross-section showing a modification of this invention wherein a single wire member fastens three adjacent convolutions to the underlying spider;

FIGURE 5 is another modification of this invention showing a fragmentary view of a deep-drawn type spider in place of the intersecting metal strip type spider of FIG- URES 1-4, where the strap means has its two ends fastened to the top surface of the spider;

FIGURE 6 is another view of the modification of FIG- URE 5 with an improved showing of the transverse configuration of the spider taken on the line 6-6 of FIG- URE 5.

Turning now to a consideration of the drawing, and in particular to FIGURE 1, 10 represents a portion of a top cooking surface whether it be an electric range or a portable electrical appliance or hotplate. The cooking surface or cooktop 10 includes a circular recess 11 within which is supported a surface heating unit 12 embodying the present invention. This heating unit 12 includes three main elements; namely, the metal sheathed heating element 13 that is wound into a flat coil or series of convolutions, an underlying supporting framework or spider 14, and an encircling trim ring 15 that is associated with the spider for supporting the spider from the edge of the circular recess in the cooktop, as will be explained in detail hereinafter.

The convoluted heating element 13 is of standard construction in that it has a central resistance element 9 that is helically wound longitudinally of the heating element. A suitable electrical insulating material such as powdered magnesium oxide 16 is packed around the resistance element and the overall assembly is covered with a thin metal tube or sheath 17 The heating unit shown in FIGURE 1 has two coils, an inner coil 19 of small diameter and an outer coil 20 of larger diameter. As an example, this inner coil 19 has four convolutions while the other coil 20 has only two. Each coil 19 and 20 has a pair of electrical terminals 21 and 22, respectively. The heated portion of each coil stops in the flat plane of the coil, and the terminals are best described as cold terminals in that they do not include the helical resistance heater, but instead a rod-like terminal pin (not shown) as is standard in this art. These terminals 21 and 22 are formed downwardly and then radially in parallel directions at one side 6 of the heating unit for connection with a suitable terminal block or combined terminal block and hinge means (not shown).

The inner coil 19 is about 4 inches in overall diameter and it is used for low heating requirements and for small size cooking utensils, sauce pans and the like. The outer coil 20 is about 6 inches in diameter and it is used either alone or in combination with the inner coil for larger sized sauce pans and skillets, it being understood that when these two coils are connected with either rated voltage or 236 volts or one-half voltage that many different heating rates or wattages can be obtained covering the whole scale of heating conditions from simmer to quick boil. Admittedly, this invention is not limited to any particular size of surface heating element but may be used with any of them.

It is necessary to provide some support means beneath the heating element because the metal sheath 17 is not so rigid as to preclude the various convolutions from moving up and down with respect to each other. Also, it would be undesirable to reinforce the sheath by increasing the thickness or gauge of the metal, because this would tend to slow down the heating rate by increasing the mass of the sheath and causing slow heat up and cool down periods, and it also would be inefficient from a power usage standpoint. Many different types of support means are used in this art and two examples are given in the accompanying drawing. FIGURE 1 shows a supporting framework or spider 24 that is formed of intersecting metal strips 25 and 26 which are set on edge and are shown as two intersecting right angular diagonal plates. A second modification of spider is shown in FIGURES and 6 of the drawing which will be explained hereinafter. These intersecting metal strips 25 and 26 form a group of four radial arms 27, 28, 29 and 30. Various means may be used for fastening the two strips together at the center such as welding, riveting or using lanced tabs, but since this fastening means forms no part of the present invention such means have not been specifically illustrated. A metal disk 32 of ornamental design is shown fastened over the center of the spider to serve as an embellishment as wellas a partial closure of the center opening in the coils of the heating element. Usually the manufacturers name and/or registered trademark for the surface unit is imprinted on this disk.

Now that the heating element 12 is resting on the spider 24, some means must be provided for supporting the spider from the cooktop 10. In the modification of FIG- URES l4 this support means is exemplified by an annular trim ring that is of generally angular cross-section as best seen in FIGURE 2 with an inner periphery formed by a vertical rim 34 and a top portion formed by a generally horizontal or slightly downwardly inclined flange 35. The vertical rim 34 is adapted to fit within the circular opening 11 in the cooktop, while the top fiange 35 is adapted to rest upon the edge or periphery of the cooktop opening 11. Suitable slots are formed in the vertical rim 34 in alignment with the radial arms 27-30, and the outer end of each such arm is provided with a tab such as 36 in FIGURE 2 that extends into the slot of the trim ring and is supported thereby.

Now that the heating element 13 is resting on the spider 24, and the spider is supported from the trim ring 15, and the trim ring is supported on the cooktop 10, some means must be provided to prevent the heating element 12 from being displaced from the spider in a horizontal direction. Strap means have been provided so as to have an intermediate portion looped over at least one of the convolutions, while the two ends of the strap are immovably secured to a radial arm of the spider. Specifically in FIG- URE 2, a strap means in the form of a wire member 38 of short length is tightly looped over the convolution of the sheathed heater and the two ends of the wire are twisted slightly to extend down fiat along the side of the radial arm 30 of the spider. Various means may be used 4 for immovably securing the two ends of the wire member to the spider. In FIGURES 2 and 3, a pair of slots 39 are formed in the arm 30 and the ends 40 of the wire member are folded and extended through the slots and then folded back on itself in the manner of a bent tab. Still looking at FIGURE 2, it will be understood that alternate convolutions are provided with the wire member 38 and that these wires are alternately arranged on opposite sides of the radial arm 30. Also to be recognized is the location of these Wire straps. They are positioned adjacent the radial arm 30 that is opposite the terminal ends 21 and 22 of the inner and outer coils 19 and 20 of the heating element 13. Of course, it is conceivable that the strap means might be located over the remaining three arms 27-29 of the spider, but in that case it would be preferable not to have more than one strap means for a given turn or convolution in order to allow some freedom of movement of the heating element during the inherent expansion and contraction movement caused by the heating up and cooling down of the element. It would be disadvantageous if the strap means extended above the generally flat surface of the plurality of convolutions. Hence, the area of the convolution located beneath each strap is countersunk as at 42 as is best seen in FIGURE 3.

Turning to a consideration of the modification of FIG- URE 4 where the same elements are identified by the same reference numerals as found in the first modification of FIGURES 1 and 3, there is illustrated a Welded technique for fastening the ends of a strap means 44 to the radial arm 30 of the spider 24. Moreover, the strap 44 is of the multiple type in that it is looped over a plurality of convolutions; for example, three convolutions which, of course, would simplify the assembly operation and lower the cost thereof. Notice that there are raised tabs 46 between adjacent convolutions. These tabs serve as spacers to prevent the convolutions from being drawn together in a bunch by the tension of the strap 44 and creating an overheated condition which might bring about early failure of the heating element. Similar spacers 46 could be used with the modification shown in FIGURES 1-3, where single wire straps are used for the various convolutions. Looking at FIGURE 2 it will be noticed that slight indentations 47 are formed in the radial arm 30 beneath the alternate convolutions which are held down by the straps 38.

Now, in turning to the last modification of FIGURES 5 and 6, the first important feature is the difference in the design of the spider 48 which has three or more radial arms of inverted channel shape as is best seen in FIGURE 6. This spider is of deep drawn sheet metal construction having radial arms which are not necessarily mechanically fastened to the trim ring but instead merely rest on a ledge 49 at the bottom of the vertical rim 44 of the trim ring 34. In other words, the spider 48 is loosely supported within the trim ring. Again the top surface of the radial arm is provided with a series of indentations 50 in which the adjacent convolutions are seated, and a strap member 51 of the multiple type is tightly looped over the convolutions and rigidly fastened to the spider by means of bent-over tabs 52 which extend through suitable slots in the top surface of the spider arm in a manner somewhat similar to the bent tab construction of FIGURE 2. Again, the area of the convolutions beneath the strap is countersunk as at 42 in FIGURE 6. Of course, it will be appreciated that instead of a multiple strap 51, single straps for single convolutions could be used.

Having described above several modifications of the invention of a novel hold-down means for a sheathed heater it will readily be apparent to those skilled in this art that I have provided a positive hold-down means where the coils are always held snugly against the spider without the use of crimped spider tabs, peened sheathed portions or strap members that are welded directly to the sheath from below. The various coils or convolutions are free to expand in diameter when heated. This allows the convolutions to remain fiat. Primarily, no mechanical bond is made to the sheath nor is any excessive mechanical pressure exerted by the use of crimped or peened holding means. Moreover, since the various coils of the heating element may be individually tied down to the spider, they may be removed separately within the home by the appliance Serviceman rather than having to replace the entire heating unit. Also, since the straps are formed of such fine wire, the cost is relatively unimportant and the strap can be made of expensive high temperature resistant metal such as Incoloy which will better withstand corrosion and erosion.

Modifications of this invention will occur to those skilled in this art therefore it is to be understood that this invention is not limited to the particular embodiments disclosed but that it is intended to cover all modifications which are within the true spirit and scope of this invention as claimed.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical heating unit comprising a convoluted sheathed heater, the convolutions of which are disposed in a single plane for defining a fiat cooking surface, a supporting framework in engagement with the underside of the heater, the framework having spacer means between at least some of the convolutions for holding them apart and strap means for holding the convolutions to the framework, said strap means having an intermediate portion looped over at least one convolution and a pair of end portions immovably secured to the underlying framework, the convolution being indented beneath the strap means for lowering the strap means beneath the level of the flat cooking surface.

2. An electrical surface heating unit comprising a convoluted sheathed heater, the convolutions of which are disposed in a plane for defining a flat cooking surface, a supporting spider in engagement with the underside of the heater, the top surface of the spider having raised portions between at least some of the adjacent convolutions for holding them apart, and strap means for tying down the convolutions to the spider, said strap means having an intermediate portion looped over at least one convolution and the two end portions immovably secured to the underlying spider, whereby no mechanical bond is made between the strap means and the sheath.

3. An electrical surface heating unit comprising a convoluted sheathed heater, the convolutions of which are disposed in a plane for defining a fiat cooking surface, a supporting spider having generally radial arms on which the heater is rested, the heater having at least two terminals which extend beneath the heater at one side thereof, and strap means for tying down the convolutions to the spider, said strap means being located adjacent a radial arm of the spider diagonally disposed from the terminals of the heater, said strap means including at least one small gauge wire with an intermediate portion tightly looped over at least one convolution and the two end portions thereof immovably secured to the underlying arm of the spider, the portion of the convolution beneath the looped wire being countersunk to receive the wire beneath the top cooking surface of the heater.

4. A method of attaching a generally fiat metal sheathed heating element to a spider having supporting arms; comprising, the steps of indenting the top surface of at least one of the arms, seating the heating element across the indentation, countersinking the portion of the element located within the indentation, and tightly looping a wire member over the countersunk portion of the element, and rigidly fastening the two ends of the wire member to the underlying arm of the spider, whereby the wire member does not protrude above the flat surface of the heating element.

5. In a method of attaching a convoluted sheathed heater to a supporting spider having radial arms; comprising the steps of countersinking certain intermediate portions of the heater in the area overlying the radial arms, tightly looping a short wire member over each countersunk portion, and rigidly fastening the two ends of the wire member to the underlying arm of the spider.

References Cited by the Examiner UNITED STATES PATENTS 2,357,150 8/1944 Vogel 219463 2,662,157 12/1953 Vallorani 219463 2,881,302 4/1959 Ammerman 219-463 2,943,178 6/ 1960 Aldons 219-463 3,031,735 5/1962 Jepson 29-1555 3,032,861 5/1962 Foster 29155.5 3,042,785 7/1962 Hanson 2l9463 RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

Claims (1)

1. AN ELECTRICAL HEATING UNIT COMPRISING A CONVOLUTED SHEATHED HEATER, THE CONVOLUTIONS OF WHICH ARE DISPOSED IN A SINGLE PLANE FOR DEFINING A FLAT COOKING SURFACE, A SUPPORTING FRAMEWORK IN ENGAGEMENT WITH THE UNDERSIDE OF THE HEATER, THE FRAMEWORK HAVING SPACER MEANS BETWEEN AT LEAST SOME OF THE CONVOLUTIONS FOR HOLDING THEM APART AND STRAP MEANS FOR HOLDING THE CONVOLUTIONS TO THE FRAMEWORK, SAID STRAP MEANS HAVING AN INTERMEDIATE POR-

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