ELECTRIC COOKING AND TOASTING APPARATUS AND ELECTRIC ELEMENTS THEREFOR

Abstract: The invention relates to an electric heating element (11) in which electrically conductive tape (23) is wound on a non-electrically conductive former (12) and the tape is completely covered by covers (30) of electrically non-conductive but heat conductive material secured to the former. Preferably the covers and former are of Mica or Micanite. The covers are each made from several pieces and are secured to the former and each other by fastening eyelets (26) or staples extending through the covers and former intermediate the edges and at spaced positions along two opposite edges. For use in an upright toaster the tape is wound continuously round the former with each run thereof extending between the grooves in the opposite side edges of the former, the grooves in the upper portion being spaced from one another by 4.6 to 7mm while the grooves in the lower portion of the former are spaced by a smaller distance in the range 3.6 to 5mm. For use in a conveyor toaster the runs adjacent the sides of the conveyor are more closely spaced than those in the centre. The invention extends to toasters and other cooking or heating apparatus incorporating such elements.

Description: ELECTRIC COOKING AND TOASTING APPARATUS AND ELECTRIC ELEMENTS THEREFOR FIELD OF THE INVENTION This invention relates to an electric element and to a toaster or other cooking apparatus (which term includes apparatus for reheating already cooked food) incorporating a plurality of such elements. BACKGROUND OF THE INVENTION Conventionally an element for an upright toaster comprises a former of an electrically non conductive material and an electrically conductive tape wound continuously round the former with each run thereof extending between grooves in the opposite side edges of the board or each half of the board. The conductive tape is exposed except that one or more narrow strips of non-electrically conductive material are frequently located outside the tape running from top to bottom of the former, and spaced from the side edges, to limit the amount of outward bowing of the tape and to prevent the tape flying outwards if it breaks. A toaster comprises a plurality of such elements which could be connected in parallel or in series. Because the conductive tape is exposed and can be easily touched by anything inserted into the toaster slots it is a requirement that the toaster must be double pole switched. This increases the cost of the toaster particularly if any electronics are incorporated in the toaster. It is also a requirement for upright toasters that an array of guard bars to prevent toast contacting the elements and which define the toaster slot are spaced at least 4mm from the tape (since this is likely to bow outwardly when hot). Conveyor toasters or other cooking apparatus in which elements are located horizontally conventionally also have live conducting tape or strip exposed, and suffer from uneven heating across the width of the apparatus. It has been proposed to use mica or micanite sheets as protective covers for the conductive tape. For example UK-A-2324694 shows a mica sheet placed over a heating element and retained by bent over metal tabs. UK-A- 286807 has insulated sheets on opposite sides of a heating element, kept in place by paper fasteners at the four corners. US 4745855 has micanite covers over the heating element held in place by tabs along the top and bottom edges. These methods of attaching a cover are unsatisfactory. One object of the invention is to provide improved construction of such a heating element. SUMMARY OF THE INVENTION In one aspect, the present invention provides an electric heating element in which electrically conductive tape is wound on a non-electrically conductive former and the tape is completely covered by a cover or covers of electrically non-conductive but heat conductive material secured to the former by fastening means, characterised in that the or each cover is formed from two or more pieces, the cover pieces are secured to the former and each other by intermediate fastening means extending through the cover or covers at at least two spaced positions intermediate the sides and not contacting the tape, and by fastening means extending at spaced positions along two opposite edges of the former. The fastening means are preferably rivets extending at spaced intervals down ihe vertical centre line and at spaced intervals down the side edges. Alternatively the fastening means may be staples, similarly arranged. Using such elements in a toaster or other cooking apparatus is safer because the electrically conductive tape cannot be touched so as to make electrical contact by a knife or finger or other object inserted into the toaster or the like. This allows the apparatus to be single pole switched. In addition the number of guard bars in an upright toaster can be reduced because the toast in the slot cannot touch the conductive tape. In addition the arrangement increases the life of the element considerably because most failures are caused by bits of bread, toast, currants or other ingredients falling off and touching the tape. This causes a burn-out which may be explosive. Also instruments inserted can no longer break the tape. In addition because the tape cannot bow outwardly the space between the element and the guard bars in an upright toaster can be reduced. Preferably the cover material is either mica or mica paper (a combination of mica and silicon resin), particularly that known as Micanite, either Muscovite or Phologopite, depending on the temperature requirements. We have found that with covers made of this material in a toasting apparatus, the toasting performance is improved for a particular heat input and is more even over the area of the toast. The former may be made of mica or Micanite so that the heat generated in the tape is absorbed by the former and cover material and then radiated evenly towards the toast. The fastening means, such as eyelets, reduce bowing of the cover away from the former. The eyelets may hold in place washers of electrically insulating but heat conductive material to increase the area over which the covers are held to the former. In one preferred form, where the former is made from a single piece of material, the tape starts adjacent one end of the former, is wound spirally towards the other end using alternate grooves, passes through a slot in the other end of the former to change direction, is wound spirally towards said one end using the remaining grooves and ends adjacent said one end of the former. The former may be made in two parts, each with grooved upright edges with the tape wound first round one part, then the other part and the parts then secured together with abutting edges before connection of the covers. Each part may itself be preformed from one or more pieces connected along horizontal overlapping edges. In a preferred form for an upright toaster, the former has a dimension in the range 1 30 to 1 50 x 1 60 to 1 80mm (preferably 140 x 171 mm) and a thickness in the range 0.3 to 0.7mm. Preferably each cover has a thickness in the range OJ to 0.3mm. Preferably the former and covers have convex rounded or sloped top edge corners corresponding to the shape of many loaves of bread, and to the end profile of the toaster. This allows the height of the former in the central portion to be greater. The invention extends to an electric toaster or other apparatus including a plurality of such elements and which is preferably single pole switched. Preferably the elements are connected in parallel to an electrical supply. With such an arrangement, even if one element fails the remaining elements will continue to function. In addition each element can have a greater (or lesser) length of tape to produce the required watt density and thus produce more heat, giving faster toasting or heating. For toasting the tape should be selected to produce a watt density equal to or less than 3 watts per cm 2 or within the operating band of the tape. With conventional 80/20 nickel chrome alloy tape, and with parallel connection of the elements to a 230 volt supply, the tape for each element can have a length in the range 5 to 10 metres, preferably 7 to 9 metres, in order to achieve this. For a conveyor toaster or other food heater where elements are located horizontally on either side of a continuous conveyor, the elements again provide safer and more even heating, particularly if the runs of tape at the centre of the element are more widely spaced than those adjacent the sides of the conveyor. The elements may also be used in other cooking apparatus such as grills or a pizza oven. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of electrical element and toasters incorporating a plurality of such elements will now be described, by way of example only, with reference to the accompanying drawings of which:- Figure 1 is a front view of a former for an electric element for a toaster, Figure 2 is a front view of a modified former, Figure 3 is a front view of one form of cover for an electric element, Figure 4 is a front view of another form of cover, Figure 5 is a front view of the former of Figure 1 wound with tape, Figure 6 shows a front view of a heating element assembled from the former of Figure 5 and covers similar to that shown in Figure 4, but with only four parts with the cover partly cut away, and showing the fastening rivets. Figure 7 shows a modified former for an element, Figure 8 shows a further modified former, Figure 9 is a diagrammatic view of a toaster incorporating a plurality of such elements in an upright disposition, and Figure 1 0 is a diagrammatic view of a toaster or other food heating or cooking apparatus incorporating elements in a horizontal disposition. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION As seen in Figures 1 and 5, an electric element 1 1 for an upright toaster comprises a former 12 made of mica or Micanite of dimensions 140 x 1 71 x 0.5mm, having upright side edges 1 3, 14 formed with spaced outwardly tapered grooves 1 5 therealong. The former has a top edge 1 6, the outer corners of which are angled or convex rounded so as to approximately conform to the shape of a conventional round topped loaf. A central keyhole slot 1 7 opens to the top edge 1 6. A bottom portion 1 8, below the lowest grooves 15, is formed with holes 20, 21 for providing connection points to an electrical supply. As seen in Figure 5, conventional 80/20 nickel chrome alloy, electrically conducting tape 23 is wound spirally round the former starting at hole 20 and locating in alternate grooves, before passing through the keyhole slot 17, changing direction and being wound spirally downwardly using the remaining grooves and ending in the opening 21 . (The tape on both sides of the board is indicated in Figure 5). The grooves in the lower portion of the former, up to the level 24, are spaced 4mm apart while the grooves in the upper portion, above level 24, are spaced 5mm apart. Two wider portions 25 of 8 to 10mm spacing between grooves are provided to accommodate fastening devices 26, 26A, extending through the former without touching the tape for attaching covers to the former. Preferably the fastening devices are rivets/eyelets or staples which will extend through the covers and former. The former of Figure 5 includes apertures 1 9 for receiving such fasteners. These extend at spaced intervals down the opposite side edges and down the centre line of the former. We have found that with parallel connected elements and a tape length of the order of 7 1 / 2 metres to provide a watt density less than 3 watts per cm 2 and with a voltage of 230 volts, this arrangement of the tape runs gives a radiant heat pattern which provides completely even toasting over the whole toast area. The same spacing between each tape run can be used for all voltages in the range 200-250 volts. For voltages in the range 1 00 to 1 25 volts either only every other groove of the former is used or a former having half the number of grooves with double the spacing between runs is required, as seen in Figure 2. Each side of the former is covered by a respective cover 30, each of the same shape and size as the former with a thickness 0.1 to 0.3mm (preferably approximately 0.2mm), arranged to completely cover the tape on that side of the former so that no live wire is exposed. (The terminals for connection to a power supply may be exposed but will be below the bottom of the toaster slots) . Each cover may be formed from several pieces of material, preferably Mica or Mica paper as seen in Figure 3 or 4, and is provided with vertically spaced apertures 27 on its centre line and along opposite side edges for receiving the fasteners such as 26, 26A. If the former has tape on only one side, only one cover will be required. As seen in Figure 3 one form of the cover 30 is made in two parts 30a, 30b, preferably of Mica material. The parts 30a, 30b overlap at 32 along their central vertical adjacent edges by 8mm (in the range 6-1 Omm). The parts 30a, 30b in the assembled heating element are connected to each other and the former solely by the central fasteners 26 and the fasteners 26 along the side edges. There are no other overlying strips or cover parts so that the two parts 30a, 30b only together form a cover 30 completely covering one side of the former and leaving no tape exposed. The mounting and fastening is extremely simple and provides even heat output. As seen in Figure 4, in another form the cover is made in six parts, 30c, 30d, 30e, 30f, 30g and 30h of Mica material, which in the assembled element overlap over their adjacent vertical and horizontal edges at 33 and 34. The parts are connected to each other and the former by fasteners such as 26 extending through the overlapping portions at positions which coincide with the wider portions 25 or the like on the former. Again the cover parts are secured along their side and upper/or upper and lower edges to the former by fasteners 26A. The six cover parts 30c, 30d, 30e, 30f, 30g and 30h without any other overlying strips form a cover which covers the whole former and the tape and is secured together by the fasteners 26, 26A. The cover might be made of up to ten parts similarly connected. Figure 6 shows a cover 30, as in Figure 4 but with only four parts located over one side of a former as in Figure 5 with fastening means in the form of rivets/eyelets 26 extending through the former and covers along the centre vertical line and horizontal join line of the cover parts, and fastening means 26A extending at spaced intervals along opposite edges. The opposite edges could be the top and bottom rather than the sides. The fastening means could be staples. Where the fastening means 26, 26A are staples apertures 1 9, 27 need not be formed in the covers and former. Figure 7 shows a modified former 1 2A formed in two parts 52, 53, each part formed with tapered grooves 1 5 along its upright edges and the parts abutting along their adjacent upright edges so that the grooves coincide to form a series of slots 54 along the centre of the former, when the parts 52, 53 are connected. The conducting tape 23 is wound first round one part, threaded through the keyhole slots 55 into the other part, wound round the other part and the two parts are then secured together, before the cover or covers 30 are assembled to the former. The parts in this embodiment are secured together by staples 56 extending through the abutting edge portions adjacent the top and bottom of the former; other securing means could be used. Two pairs of adjacent grooves 1 5 on the overlapping edges are seen at 57 to be deeper than the remaining grooves so that the conductive tape is kept away from the central areas where the eyelets or other fastening means 26 will be positioned. This avoids the need for wider portions 25 and wider spaces between the tape runs. Otherwise the former and connections are as in previous embodiments, and may be connected to covers with eyelets 26 or other fasteners as previously described. Some of these are indicated in the drawing but will not be present until the covers are in place. In Figure 7 the tape windings on the front face are shown in full line, while the windings on the back face are indicated in broken line only at the top and bottom of the former. Figure 8 shows a further form of former 1 2B made of Mica and having two parts 52, 53 each with two vertical grooved edges and with the abutting central edges 59 having their grooves aligned as in Figure 7. In this arrangement each part 52, 53 is first assembled from three pieces 60, 61 , 62 having their horizontal edges overlapping and secured together, for example by eyelets 63. (Up to five pieces may be used) . The Mica, in practice, consists of two layers of material. In order to secure the parts 52, 53 together, before the covers are assembled to the former, one of the layers on one of the parts 52, 53 is formed with a tag 64 extending outwardly from its edge 59 and the same layer on the other part is formed with a similar cut out 65 at the corresponding position when the edges 59 of the parts are abutted. Thus at the positions of the tags and cut outs, the first layer of one part overlaps the back layer of the other part and the parts are secured together by fastening means 66, such as eyelets, extending through these overlapping portions. As indicated diagrammatically in Figure 9 a plurality of such elements are located in an upright disposition in a toaster 35 and connected in parallel via lines 36 and 37 and via a single pole switch 38 in the live line to a plug 40 which will connect the toaster to an electrical power source 41 . As indicated diagrammatically in Figure 1 0 a plurality of similar elements 1 1 A of appropriate size and rating are located horizontally in a conveyor toaster or other cooking or heating apparatus 42. Two elements 1 1 A are shown on either side of the top run 43 of a continuous conveyor 44. The elements are connected to power source 41 as in Figure 3. Preferably the runs of tape in the centre of each element are less closely spaced than the runs adjacent the sides of the conveyor or other heating apparatus to provide even heating across the width of the apparatus.

Claims: CLAIMS 1 . An electric heating element (1 1 ) in which electrically conductive tape (23) is wound on a non-electrically conductive former (12) and the tape is completely covered by a cover or covers (30) of electrically non-conductive but heat conductive material secured to the former by fastening means, characterised in that the or each cover is formed from two or more pieces (30a-30f), the cover pieces are secured to the former and each other by intermediate fastening means (26) extending through the cover or covers at at least two spaced positions intermediate the sides and not contacting the tape, and by fastening means (26A) extending at spaced positions along two opposite edges of the former. 2. An element according to claim 1 in which the vertically spaced intermediate fastening means (26) are positioned substantially equidistantly from the vertical sides. 3. An element according to claim 1 in which the adjacent edges of the cover pieces (30a-30f) are overlapped with one another and the fastening means (26) extend through the overlapped edges. 4. An element according to any of claims 1 to 3 in which the former (12) has a thickness in the range 0.3 to 0.7mm. 5. An element according to any of claims 1 to 4 in which the or each cover (30) is made from material having a thickness in the range 0J to 0.3mm. 6. An element according to any of claims 1 to 5 in which the fastening means 26, 26A are rivets and the former and cover pieces are formed with apertures to receive the rivets. 7. An element according to any of claims 1 to 6 for use in an upright toaster in which the former (1 2) has a dimension in the range 1 30 to 1 50 x 1 50 to 180mm, and said opposite sides are upright side edges in use. 8. An element according to any of claims 1 to 7 in which the former (12A) is formed in two parts (52, 53), each part (52, 53) being formed with grooves (15) along its opposite edges with the conducting tape (25) wound continuously round one part and then the other part, the parts being secured together with their central edges (59) abutting one another before the covers are secured to the former. 9. An element according to claim 8 in which the parts (52, 53) are secured together by staples (56). 1 0. An element according to claim 8 in which the former is made of two layers of Mica, one layer of one part (52) of the former having a tag (64) extending outwardly from its edge (59) and the same layer of the other part (53) being formed with a cut out at a corresponding position when the parts have their edges (59) abutted, the parts being connected together by fastening means (66) extending through the tag (64) and other layer of the other parts. 1 1 . An element according to claim 8, 9 or 10 in which some of the grooves (1 5) in the abutting edges (59) are deeper than the adjacent grooves so that the tape (23) is distanced from the fastening means (26). 2. An element according to any of claims 1 to 1 1 for use in an upright toaster in which the former (1 2) and covers (30) have sloped or rounded upper corners. 3. An element according to any of claims 1 to 12 for use in an upright toaster in which the tape (23) is wound continuously round the former with each run thereof extending between grooves (1 5) in the opposite side edges of the former or parts (52, 53) of the former, the runs in the upper portion being spaced from one another by 4.6 to 7mm for a 200 to 250 volt supply and twice this distance for a 100 to 125 volt supply while the runs in the lower portion of the former are spaced from one another by a smaller distance in the range 3.6 to 5mm for a 200 to 250 volt supply and twice this distance for a 100 to 125 volt supply, the length of the tape being in the range 5 to 10 metres for 200 to 250 volt supply and half this length for a 100 to 125 volt supply and the watt density being equal to or less than 3 watts per cm 2 or in accordance with the tape tolerance. 14. An element according to claim 1 3 in which the majority of the grooves (1 5) in the lower part are spaced from one another by a distance in the range 3.6 to 4.5mm while the majority of the grooves in the upper part are spaced by a distance in the range 4.6 to 6mm. 1 5. An element according to any of claims 1 to 6 for use in a horizontal disposition in a toasting or cooking apparatus in which the tape is wound continuously round the former with each run thereof extending between grooves in opposite side edges of the former, the runs adjacent opposite ends of the former being more closely spaced than the runs adjacent the centre of the former. 1 6. An element according to any of claims 1 to 14 including terminals adjacent the bottom edge of the elements for connection to a power supply. 7. An element according to any of claims 1 to 1 6 including washers of non-electrically conducting material around one or more of the fastening means. 8. An element according to any of claims 1 to 1 7 in which the tape is chosen to provide a watt density equal to or less than 3 watts per centimetre 2 .

Inventors: GORT-BARTEN LESLIE, MILTON BRIAN, GORT-BARTEN, LESLIE, MILTON, BRIAN
Applicants: DUALIT LTD, GORT BARTEN LESLIE, MILTON BRIAN, DUALIT LIMITED, GORT-BARTEN, LESLIE, MILTON, BRIAN
Application year: 1999
Main classification: H05B3
IPC codes: A47J37, H05B3