GB2241417A

GB2241417A - Electric element assembly

Info

Publication number: GB2241417A
Application number: GB9102883A
Authority: GB
Inventors: Andrew Hunt
Original assignee: Sheathed Heating Elements Ltd
Current assignee: Sheathed Heating Elements Ltd
Priority date: 1990-02-13
Filing date: 1991-02-12
Publication date: 1991-08-28
Also published as: GB9102915D0; GB2240909A; GB9003259D0; GB9102883D0

Abstract

An electric element (20) for liquid heating purposes is sealingly joined to a metal backplate (22) adjacent each end, and also at a location intermediate its ends to provide adequate heat transfer to actuate a temperature sensitive cut out (not shown). To facilitate use of stainless steel, avoid the need for brazing, and obtain adequate contact area (X), the element (20) is formed intermediate its ends, usually centrally, with at least one substantially planar face, which is then connected to the backplate by welding. Spot welding is preferred along with application of pressure, and, again to facilitate heat transfer, use of an electrode (24) with a notch (25) which fits over the remainder of the element surface is advantageous. <IMAGE>

Description

1 41 :2;2 -ú1 -1- -a 1- -7 1 ELECTRIC ELEMENT ASSEMBLY This invention

concerns an electric element assembly for liquid heating purposes, particularly but not exclusively for an electric kettle.

1 An electric kettle element assembly, of the type with which the present invention is concerned, comprises a substantially flat metal backplate and a metalsheathed resistance heating element which extends from and has its ends sealingly connected to the backplate, with a central portion of the sheath also in direct thermal transfer relation with the backplate. In use, the backplate is sealed relative to an aperture in the kettle wall, with the element extending into the liquid reception vessel. At the reverse side of the backplate, a body is mounted which includes a temperature sensitive cut-out (usually a bimetallic strip) arranged in the vicinity of the connected central portion of the element for activation upon overheating, and power supply connectors which link up with terminal pins projecting from the ends of the element.

The backplate and the element sheath are conventionally formed of copper or an alloy of copper, which are good heat conductors but have the disadvantage of discolouration over a period of use boiling water.

1 SJL070291 2 whilst this dIscolouration is generally tolerated by those who use kettles in the UK, some users, particularly in the European countries, find it unacceptable. Many would prefer a permanently clean (or readily cleanable) metallic surface, which is more aesthetically pleasing and, in the case of a domestic appliance, gives an impression of being more hygienic. To achieve this, element subassemblies in which the backplate and element sheath are made of stainless steel (which is not subject to tarnishing) have recently been introduced.

Conventionally, the ends (legs) and central portion of the element sheath have been connected to the backplate by brazing. This involves applying a jointing or "brazing" material, such as a copper-zinc alloy, (in the manner of a glue) between the regions to be connected and then placing the entire subassembly in a furnace to heat all the material sufficiently to melt the brazing material, and form an effective joint. This would usually be done in two stages, firstly to connect the central element sheath portion, then to connect the legs.

In the case of stainless steel, higher temperatures are required to produce effective joints by SJL070291 3 brazing. This has several drawbacks, not least the higher cost involved in having to heat the furnace to a higher temperature as well as the general difficulty of automating any brazing procedure.

It is an object of the present invention to provide an improved method of production of electric element assemblies as aforesaid, which is particularly applicable to the case where the backplate and element sheath are to be made of stainless steel, but could be used with other materials also.

1 Pursuant hereto, the invention provides a method of producing an electric element assembly for liquid heating purposes wherein the element, adjacent each of its ends, is sealingly joined to a metal backplate, and is further connected to the backplate at a location intermediate its ends, characterised in that the element at said location is formed with at least one substantially planar face, which is connected to the backplate by welding.

Insofar as this specification is concerned, the term "welding" is intended to mean local heating of regions to be joined such that melting and fusion of material in those regions occurs. Generally there is no

SJL070291 4 intermediate jointing material, but that is not precluded.

One specific advantage of using a welding technique instead of brazing is that it becomes possible to insert rubber plugs into the ends of the element sheath beforehand, which plugs receive the terminal pins connecting to the element per se. Previously this was not possible because such plugs would have been damaged in the furnace, and the more laborious technique of subsequent injection of molten rubber was employed to close the ends of the sheath and anchor the terminal pins.

Another advantage of using welding, which involves only local heating, as opposed to brazing, is that silicon can be added to the magnesium oxide within the sheath, around the element, to improve insulation. Again, this was not previously possible owing to the damaging effects of overall heating.

More generally, the welding technique can be carried out with less expensive equipment since there is no need for high capital investment in a furnace for brazing. It is also somewhat easier, less labour intensive, so further saving in costs. Furthermore it lends itself more readily to automation.

SJL070291 5 Since welding of a substantially circular crosssection element to a substantially flat plate would not give a sufficiently reliable connection, nor adequate heat transfer area, a further essential proposal is for the adapting of the cross-sectional shape of the element sheath in its central region to have at least one flat face, which then provides a larger area for connection to the backplate. In this respect, it is fairly easy to deform the circular cross-section of the element in its central region so as to provide a single substantially planar face, or to deform it into a lozenge shape by applying force at opposite sides so as to provide two opposing flat faces, or else to deform it into a triangular shape with three faces. All of these shapes are particularly favourable because as well as being easy to form they provide faces of a relatively large surface area, larger than a shape with more faces. Nevertheless, a rectangular (four sided) shape could be envisaged, and a larger number of faces could certainly be a possibility, even if less favourable.

One of the most commonly employed welding techniques, which is applicable for connecting small areas, is spot welding, wherein respective electrodes are aligned at each side of the welding area, with the two workpieces to be joined sandwiched therebetween, and then a high voltage is applied to cause heating SJL070291 6 sufficient for fusion of the areas between the electrodes.

Spot welding is a suitable technique to be employed in accordance with the present invention, but even then certain problems have to be overcome, as wil:k be explained shortly. In any event, it is preferable, when employing this technique, to exert pressure, via the electrodes, to the locations being joined, at the same time as application of voltage, as this brings about a more reliable connection over a somewhat wider area. Indeed, connection over the entire area of the face of the sheath which is in contact with the backplate should be achievable by use of pressure.

Because the sheath is only a thin metal layer surrounding the element per se and its insulation, there is a problem in spot welding when an electrode is placed at one side of the sheath to effect welding of the other side to the backplate, in that the heat transfer around the sheath may be inadequate, or else the sheath may melt in places. Accordingly, in a highly advantageous development of the invention, wherein the element is formed with at least one substantially planar face, it is proposed that the spot welding is carried out using an electrode which is provided with a notch shaped to fit snugly over the remaining surface portion of the SJL070291 7 element which is not being welded.

In this way, the electrode, on the one hand, delivers maximum heating energy to the face in contact with the backplate, and, on the other hand, serves as a heat sink for the other surface portion of the sheath, preventing any risk of unwanted melting there.

As previously implied, the proposed method is particularly applicable where the backplate and the element sheath are formed of stainless steel. However it could equally well be used when these components are made of other metals, for example, various alloys including nickel, which have recently been introduced for kettle elements.

Although spot welding is probably to be preferred because of its simplicity, and low costs, other welding techniques, such as arc welding, using a tungsten inert gas, or metal inert gas could also be employed for joining one face of the element to the backplate without placing the entire subassembly in a furnace.

Naturally, to maintain the aforesaid advantages of avoiding having to place the element subassembly into a furnace, the legs of the element should also be connected to the backplate by a method which does not SJL070291 8 involve brazing.

One advantageous proposal is that each end region of the element should be sealingly joined to the backplate by way of a resilient sealing ring, means being provided for retaining the ring in sealing abutment around the periphery of the element and against the backplate.

In this respect, the sealing ring may conveniently be retained by an apertured metal plate which is located on the end region of the element, along with the resilient sealing ring either prior to or after insertion of the end region of the element through a respective aperture in the backplate, then secured relative to the backplate either by welding, or by frictional engagement, so as to retain the sealing ring therebetween.

In one particularly favourable embodiment, the backplate may be formed with a recess to receive both the sealing ring and the metal plate, the latter being frictionally engaged at its inner periphery, around the element, and/or at its outer periphery around the recessed region of the backplate.

Alternatively, the metal plate itself may be recessed or (. 1 SJL070291 9 curved to accommodate the sealing ring then welded to the backplate), so as to retain the sealing ring therebetween.

It is possible that both end regions of the element may have their respective sealing rings retainqd in this way by a common apertured plate, which is either welded onto or frictionally engaged relative to the backplate.

In alternative embodiments, the sealing ring around each of the element end regions may be retained by appropriately folded, aperture defining edge margins of the backplate itself, without the need for any additional metal retention plate or plates.

A further proposal, which does not involve additional sealing rings or plate(s) is that the backplate apertures may be so dimensioned and the edges of the backplate apertures may be bent so that these grip the element end regions when the latter are pushed therethrough. Sealing could then be accomplished subsequently by injection of molten rubber or the like, around the element legs. In practice, the apertures in the backplate will be formed of slightly smaller diameter than the element end region and the edge margins defining these apertures will be slightly SJL070291 10 deformed in one direction, the end regions then being pushed through the apertures so that they are resiliently retained by the deformed edge margins, and sealant being subsequently applied around the thus formed joint.

The aforesaid methods of connecting the end regions (or legs) of the element to the backplate may be employed irrespective of the manner of connection of the central sheath region to the backplate.

The invention also encompasses an electric element assembly for liquid heating purposes as produced by any of the methods just described.

The invention will be described further, with reference to the accompanying diagrammatic sketches, in which:

Fig. 1 shows a conventional spot welding technique as applied to joining a metalli element sheath to a backplate; c Fig. 2 illustrates a first preferred spot welding technique in accordance with the present invention; SJL070291 11 Fig. 3 is a corresponding view of a second preferred technique in accordance with the invention; Fig. 4 is a corresponding view of a third preferred technique In accordance with the invention; Figs. 5a and 5b illustrate a first method of joining element legs to a backplate; Figs. 6 to 10 illustrate various other methods of joining element legs to a backplate; and Fig. 11 illustrates an alternative method of joining element legs to a backplate.

Fig. 1 shows how straightforward application of spot welding to joining a central portion of a tubular metal element sheath 10 to a planar backplate 12 involves arrangement of respective electrodes 14,16 at opposing sides of the area A to be joined. The electrodes 14,16 have flat end faces and the element sheath 10 and backplate 12 are sandwiched together between these. A satisfactory connection between the sheath 10 and backplate 12 cannot be obtained in this way. One reason is that the joint area A is little more SJL070291 12 than a line joint, so that subsequent heat transfer between the element and the backplate 12, which is the purpose of the connection, is unlikely to be adequate to activate the element cut-out sufficiently early to avoid irreparable damage to the element. Furthermore, in the welding procedure, energy has to be transferred from the face of the electrode 14 to the area A around the thin, wall of the sheath 10. This may be inadequate to melt the sheath at A, so that the joint is not formed or is excessively weak. Another problem is the possible melting of other parts of the sheath wall.

These problems are all overcome in a similar manner by the welding procedures illustrated in Figs. 2 to 4.

Referring firstly to Fig. 2, the element sheath 20 is deformed to a triangular cross-section, and one substantially planar face thereof then abuts the backplate 22 over an enlarged area X. Moreover, the face of electrode 24 is formed with a V-shaped notch 25, which fits over the other two faces of the sheath 20. As a result, there is rapid and effective energy transfer between the front end portions of the electrode 24 and the sheath face abutting the backplate, whilst the notched area of the electrode acts as a heat sink to remove heat and prevent any deformation or melting of the other two faces. Furthermore, pressure is applied, SJL070291 13 as indicated by the arrows to ensure close contact and welded connection over the entire area X (i.e. avoid gaps at the edges of the abutting sheath face).

Fig. 3 shows a similar technique wherein the element sheath 120 is deformed almost to a semicircular shape, its one planar face abutting the backplate 122 over enlarged area Y. In this case the face of electrode 124 is formed with a rounded U-shaped or trough-shaped notch 125 to match the remaining curving surface portion of the element 120. Pressure is applied, as indicated, and the same advantages are achieved. Fig. 4 shows another alternative wherein the element sheath 220 is squashed by force applied at opposite sides so there are two opposing flat faces, one of which abuts the backplate 222, over area Z. Again, a notched electrode 224 fits over and is used to apply pressure.

By the method illustrated by the examples in Figs. 2 to 4 a reliable and effective connection can be made by welding, avoiding the need for brazing, as discussed extensively in the introductory passages hereinbefore.

Figs. 5a and 5b show how the legs of an element sheath 201 can also be joined to the backplate 22' without the need for brazing, by locating an annular 1 SJL070291 14 metal plate 27 and sealing ring 28 on the end region of the sheath 20', before inserting the latter through an aperture 32 in the backplate 221, then spot welding the edge of the plate 27 to the backplate at several locations B to sealingly retain the ring 28, which is made of rubber or similar, therebetween.

Of course, it is equally possible to locate the sealing ring and the metal plate on the free end of the element at the other side of the backplate, outside the liquid receiving chamber of the appliance, and to weld the plate at that side to sealingly retain the ring.

It is not necessary for the plate to be annular, so long as it is formed with an aperture through which the element end region can be inserted. Moreover, a common plate, having two appropriately positioned apertures, for insertion of the respective end regions of the element therethrough, can be used for sealingly retaining rings around both of the element end regions.

Figs. 6 to 8 show three other ways, similar to each other, whereby the end regions of an element sheath 30 can be joined to a backplate 32 without brazing, by means of a sealing ring 34 retained by an apertured metal plate 36, usually an annular washer. In each of these cases the backplate 32 is recessed to accommodate the sealing ring 34 and the retaining plate SJL070291 15 36 is frictionally engaged.

The same reference numerals have been used in all three drawings to denote the corresponding parts.

In all cases the ring 34 is urged into and deformed into sealing abutment against both the backplate 32 and the element sheath 30 so that liquid cannot leak through the joint. In Fig. 6 the washer 36 is frictionally engaged at both outer and inner peripheries (C and D). In Fig. 7 the washer 36 is frictionally engaged at only its outer periphery (E), but the element 30 is also a friction fit in the backplate aperture (F). In Fig. 8 the washer 36 is frictionally engaged at only its inner periphery (G), but again the element 30 is a friction fit in the backplate aperture (H).

Fig. 9 shows a slightly different manner of retaining a sealing ring 34 in sealing abutment against a flat (unrecessed) backplate 132 by means of an apertured plate 136 which is frictionally engaged (at J) around the element end region 30, and which is itself recessed, or formed as a cupshape, with a peripheral rim, so as to accommodate the ring 34 therebetween. In this case the element 30 again is a friction fit in the backplate aperture (K).

SJL070291 16 By reference to Figs. 8 and 9, it will be appreciated that it may not, in fact, be necessary to provide a recess to accommodate the sealing ring 34 in either the backplate 32 or the washer 136. It would probably be possible to sealingly retain the ring between a substantially flat washer and a substantially flat backplate region so long as the element was a friction fit in both the apertures. However, the recessing of one will give a more reliable seal.

Again in the embodiments of Figs. 6 to 9, or the aforesaid modification it is equally possible to locate the sealing ring and the metal plate on the element at the other side of the backplate. moreover, the apertured metal plate (washer) can be any shape, and a common plate can be used for retaining sealing rings for both legs of the element.

Fig. 10 shows yet another way of retaining a sealing ring 134 in sealing abutment around an element end region 130, this being accomplished directly by the edge margins of the backplate itself being folded back and curled over to accommodate and resiliently urge the sealing ring 134 against the element 130. Again this could be either inside or outside the liquid recei ving chamber, i.e at either side of the backplate 138.

SJL070291 17 Another alternative is shown in Fig. 11 where the edge margin defining the backplate aperture 42 is curvedly bent and grips the element end region 41 as it is inserted to inhibit its release. Subsequently molten rubber or other sealing compound may be injected to complete the seal.

1 SJL070291 18

Claims

1. A method of producing an electric element assembly for liquid heating purposes wherein the element, adjacent each of its ends, is sealingly joined to a metal back plate, and is further connected to the backplate at a location intermediate its ends, characterised in that the element at said location is formed with at least one substantially planar face, which is connected to the backplate by welding.

2. A method as claimed in claim 1 wherein the element is formed with a lozenge shaped cross-section, providing opposing substantially planar faces, in its central region.

3. A method as claimed in claim 1 wherein the element is formed with a substantially hemispherical shaped cross-section, thereby providing a single planar face, in its central region.

4. A method as claimed in claim 1 wherein the element is formed with a substantially triangular crosssection, providing three substantially planar faces, in its central region.

5. A method as claimed in any preceding claim wherein the planar face of the element which is 1 J 1 SJL070291 19 connected to the backplate is connected by spot welding along with application of pressure.

6. A method as claimed in claims 4 and 5 wherein the spot welding is carried out using an electrode with a Vshaped notch which fits snugly over the two faces of the element which are not being welded.

7. A method as claimed in claims 2 or 3, and claim 5, wherein the spot welding is carried out using an electrode with a U-shaped or trough shaped notch, which fits snugly over the surface portion of the element which is not being welded.

8. A method as claimed in any of claims 1 to 4 wherein the planar face of the element which is connected to the backplate is connected by arc welding.

9. A method as claimed in any preceding claim wherein the backplate and the element sheath are formed of stainless steel.

10. A method as claimed in any preceding claim wherein each end region of the element is sealingly joined to the backplate by way of a resilient sealing ring, means being provided for retaining the ring in sealing abutment around the periphery of the element and SJL070291 20 against the backplate.

11. A method as claimed in claim 10 wherein the sealing ring is retained by an apertured metal plate located on the end region of the element and welded to the backplate so as to retain the sealing ring therebetween.

12. A method as claimed in claim 10 wherein the sealing ring is retained by an apertured metal plate located on the end region of the element and secured relative to the backplate by frictional engagement with the element and/or with the backplate so as to retain the sealing ring therebetween.

13. A method as claimed in claim 12 wherein the backplate is formed with a recess accommodating both the sealing ring and the metal plate, the latter being frictionally engaged at its inner periphery, around the element and/or at its outer periphery around the recessed region of the backplate.

14. A method as claimed in claim 11 wherein the metal plate itself is recessed or curved to accommodate the sealing ring.

15. A method as claimed in any of claims 11 to 14 SJL070291 21 wherein the apertured metal plate Is annular.

16. A method as claimed in any of claims 11 to 14 wherein both end regions of the element have respective sealing rings retained by means of a common apertured plate.

17. A method as claimed in claim 10 wherein the sealing ring around each of the element end regions is retained by folded, aperture-defining edge margins of the backplate.

18. A method as claimed in any of claims 1 to 9 wherein each end region of the elements is sealingly joined to the backplate by apertures in the backplate being so dimensioned and edges of these apertures being so bent as to grip the element end regions when pushed therethrough, sealing then being accomplished by injection of molten rubber or the like around the junction between the element end regions and the backplate.

19. An electric element assembly for liquid heating purposes wherein the element, adjacent each of its ends, is sealingly joined to a metal backplate, and is further connected to the backplate at a location intermediate its ends, characterised in that the element at said 1 SJL070291 22 location is formed with at least one substantially planar face, which is connected to the backplate by welding.

20. An electric element assembly as claimed in claim 19 wherein each end region of the element projects through a respective aperture in the backplate and has a resilient sealing ring located thereon, at the side of the backplate remote from the main body of the element, which sealing ring is held in place by an apertured metal plate located on the end region of the element and welded to the backplate.

21. An electric element assembly as claimed in claim 19 wherein each end region of the element projects through a respective aperture in the backplate, wherein the backplate is formed with a recess surrounding each aperture and a respective sealing ring is accommodated in each recess and held in place by an apertured metal plate which frictionally engages with its inner periphery around the respective element and/or with its outer periphery around the respective recessed region of the backplate.

22. An electric element assembly produced by the method claimed in any of claims 1 to 18.

j 1 1 1 1 SJL070291 23

23. A method of producing an electric element assembly substantially as hereinbefore described with reference to any or any combination of Figs. 2 to 11 of the accompanying drawings.

1 Published 1 991 at The Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 1 RH. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cmmfelinfach. Cross Keys. Newport. NP I 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.