GB2240909A

GB2240909A - Electric element assembly

Info

Publication number: GB2240909A
Application number: GB9102915A
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: 1991-08-14
Also published as: GB9102915D0; GB9003259D0; GB2241417A; GB9102883D0

Abstract

Each end of the element (20) is sealingly joined to a metal backplate (22), without the need for brazing, by means of a resilient sealing ring (28) which may be held in sealing abutment with the element (20) and the backplate (22) by an apertured metal plate (27) located around the element and either welded to the plate (27) (at B) or frictionally engaged by the element and/or the backplate. In the latter case the backplate may be recessed to accommodate both the sealing ring and the retainer plate. Alternatively, the sealing ring could be held by folded over aperture defining edges of the backplate itself. When brazing is avoided there is no need for the element to be heated in a furnace. This facilitates use of stainless steel for the element sheath and the backplate and also allows use of pre-inserted plugs to retain element terminal pins. <IMAGE>

Description

ELECTRIC ELEMENT ASSEMBLY This invention concerns an electric element assembly for liquid heating purposes, particularly but not exclusively for an electric kettle.

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.

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 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.

In their co-pending application the applicants have set forth a proposal for avoiding a brazing process in production of the relevant type of electric element assemblies by making special provision for welding a central region of the element sheath to the backplate and by connecting the ends of the element sheath to the backplate in various different ways.

The present invention is directed specifically to the latter, i.e. to the various novel ways of connecting the element ends to the backplate, which may still lead to improved methods of production of electric element assemblies whether or not used in conjunction with welding of the central element sheath portion to the backplate.

Pursuant hereto, the invention provides, in its broadest aspect, an electric element assembly for liquid heating purposes wherein the element, adjacent each of its ends, is sealingly joined to a metal 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.

The sealing ring may conveniently be retained by an apertured 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 regions of the element through a respective aperture in the backplate, and is then secured relative to the backplate either by welding, or by frictional engagement, so as to retain the sealing ring therebetween.

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 intermediate jointing material, but that is not precluded.

In one particularly favourable embodiment, the backplate may be formed with a recess to receive both the sealing ring and the apertured 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.

It is possible that both end regions of the element may have their respective sealing rings retained 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.

One specific advantage of avoiding brazing is that it becomes possible to insert rubber plugs into the ends of the element sheath before connection of the sheath to the backplate, which plugs receive the terminal pins connecting to the element per se. With connection by brazing this is not possible because such plugs would be damaged in the furnace, and the more laborious technique of subsequent injection of molten rubber has to be employed to close the ends of the sheath and anchor the terminal pins.

Another advantage of avoiding brazing, is that silicon can be added to the magnesium oxide within the sheath, around the element, to improve insulation.

Again, this is not possible when brazing owing to the damaging effects of overall heating.

More generally, without brazing there is no need for high capital investment in a furnace. Also the various manners of connection now proposed are somewhat easier and less labour intensive and more readily lend themselves to automation.

As previously implied, the proposed methods of connection of the element sheath to the backplate are particularly applicable where both items 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.

When connecting the apertured plate to the backplate by welding spot welding is probably to be preferred because of its simplicity, and low costs, although other welding techniques, such as arc welding, using a tungsten inert gas, or metal inert gas could also be employed.

The invention will be described further, with reference to the accompanying diagrammatic sketches, in which: Figs. 1 and 2 illustrate a first method of joining element legs to a backplate in accordance with the invention; and Figs. 3 to 7 illustrate various other methods of joining element legs to a backplate in accordance with the invention.

Figs. 1 and 2 show how the legs of an element sheath 20 can be joined to the backplate 22 without the need for brazing, by locating an annular 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 22, 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. 3 to 5 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 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. 3 the washer 36 is frictionally engaged at both outer and inner peripheries (C and D). In Fig. 4 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. 5 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. 6 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 cup-shape, 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).

By reference to Figs. 5 and 6, 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. 3 to 6, 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. 7 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 receiving chamber, i.e at either side of the backplate 138.

Claims

1. An electric element assembly for liquid heating purposes wherein the element, adjacent each of its ends, is sealingly joined to a metal 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.

2. An assembly as claimed in claim 1 wherein the sealing ring is retained by an apertured metal plate which is located on the end region of the element and secured to the backplate by welding so as to retain the sealing ring therebetween.

3. An assembly as claimed in claim 1 wherein the sealing ring is retained by an apertured metal plate which is located on the end region of the element and secured relative to the backplate by frictional engagement so as to retain the sealing ring therebetween.

4. An assembly as claimed in claim 3 wherein the backplate is formed with a recess which accommodates both the sealing ring and the apertured 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.

5. An assembly as claimed in claim 2 or 3 wherein the apertured plate is curved, recessed or cup-shaped.

6. An assembly as claimed in any of claims 2 to 5 wherein the apertured plate is annular.

7. An assembly as claimed in any of claims 2 to 5 wherein both end regions of the element have their respective sealing rings retained by a common apertured plate.

8. An assembly as claimed in claim 1 wherein the sealing ring around each element end region is retained by a folded, aperture defining edge margin of the backplate.

9. An assembly as claimed in any preceding claim wherein the backplate and the element sheath are of stainless steel.

10. An electric element assembly for liquid heating purposes substantially as hereinbefore described with reference to and as illustrated by Figs. 1 and 2, or any of Figs. 3 to 5, or Fig. 6 or Fig. 7 of the accompanying drawings.