IMPROVEMENTS RELATING TO THERMALLY-RESPONSIVE CONTROLS

Abstract: A bimetallic actuator has a dished, snap-acting, bimetal held in a spring metal carrier with its convex surface uppermost in the cold condition of the bimetal, and the carrier is adapted to be biased against a heated surface with a force sufficient to flatten the bimetal. The carrier is arranged to deform under the application of this force and, in doing so, to apply stresses to the bimetal such as to raise its operating temperature above that at which it would operate in stand alone condition. The carrier furthermore spaces the bimetal periphery from the heated surface, even when the bimetal is in its cold condition, and is substantially flattened against the heated surface, so that additional movement is obtained from the bimetal when it snaps to its opposite curvature. A thermally-responsive control incorporating such an actuator is better able to operate with modern thick film heating elements.

Description: Improvements Relating to Thermally-Responsive Controls Field of the Invention: This invention concerns improvements relating to thermally-responsive controls and, more particularly, concerns bimetallic controls as utilised to control the operation of an electric heating element wherein a bimetallic actuator is held in thermal contact with the heating element to be responsive to the temperature thereof. Such bimetallic controls are well known and find application inter alia in water heating appliances such as electric kettles and hot water jugs. Background of the Invention: Heating element overtemperature protection controls are well known in the field of domestic kettles and hot water jugs where they are commonly used . to protect the heating element against overheating in the event of the kettle or hot water jug being switched on without first being filled with water. Such controls commonly employ snap-acting bimetallic actuators comprising a dished bimetallic element which is moveable with a snap-action between two oppositely dished conditions. The movement that is available from a simple dished disc bimetal as it snaps between its oppositely dished configurations is limited by the simplicity of the bimetal geometry. To increase the available movement it has become commonplace in controls that we manufacture to use a bimetal having a cutout which releases from the blade one or more tongue-like portions. Early bimetals of this kind, as described for example in GB 2117568, were rectangular with a generally U-shaped cut-out. Subsequently, as described for example in GB 2097920, a circular blade with a more C-shaped cut-out was employed. More recently, as described for example in GB 2194099 which describes our original XI or Gemini control, a rectangular bimetal having an X-shaped cut-out has been employed. In some controls employing such increased movement bimetals the bimetal has been mounted by its periphery and the movement of the free end of the released tongue has been used in switching operations and in others the bimetal has been mounted by its tongue and the movement of the periphery has been utilised. In the Otter Controls X- series of controls the former has been the case, whereas in controls manufactured by Strix Limited the latter has tended to be the case, for example as in the control described in WO 98/30066. In the following the invention will be described with particular reference to the former case and by reference to our X-series controls, but it is to be understood that the invention is not restricted to such arrangements and is applicable to the alternative mounting arrangement preferred by Strix Limited. In the XI control as described in GB 2194099 with reference particularly to Figures 3A, 3B and 3C thereof, a generally rectangular bimetal having a central X-shaped cut-out was mounted in a plastics material carrier which was spring biased so as, when the control was assembled with a heating element, to be urged towards the heating element. The carrier was made of a material selected to soften or melt at a predetermined temperature so that in the event of the bimetal failing properly to operate its associated switch in a heating element overtemperature situation, so that the heating element temperature continued to rise, the carrier would be collapsed by its biasing springs towards the heating element. The collapse of the carrier was arranged to effect a secondary control function. The XI control and its successors, the X2 and X3 controls, are well known. The X2 control is substantially as described in GB 2283156 and the X3 control is substantially as described in GB 2248724. In the design of the XI bimetal carrier, the bimetal was simply placed with its convex face towards the heating element and movement was achieved when the bimetal snapped to its opposite curvature, with the centre moving towards the control and the edges snapping down onto the heating element surface to provide an abutment against which the output force could be generated. Because of the way that the XI gave compensation for distortion of the heating element head, adequate heat transfer and movement were available to make the control function satisfactorily, although it was always known that the heat transfer would be improved if the bimetal could be pressed more positively against the heater. In the design of the X3 it was elected to place small hooks at the corners of the bimetal to retain it in the carrier, since for use with flat heating elements the snap clips provided in the XI control at the tip of the push rod were preferably not provided so as to avoid interference with the heating element surface. The hooks tended to hold the bimetal away from the heating element, giving the X3 initially a slower response than the XI. To overcome this, shoulders were placed behind the hooks so that the bimetal was trapped between the shoulders and the hooks. This served to prevent the bimetal moving away from the heating element, thus improving the heat transfer back to XI levels. However, it was always believed to be desirable to avoid pressing the bimetal against the heating element in order not to change its set temperature by stressing it. The presence of the hooks gave an incidental benefit in that they supported the corners after the bimetal had reversed curvature, thus giving greater effective movement of the centre. The X-series of controls were originally designed for use with conventional sheathed heating elements used in an immersion or underfioor heating mode. With the advent of thick film heating elements and the X4 control which is described in our British Patent Application No. 9909171.2, the set temperature of the bimetal needed to avoid nuisance operation during the life of the product, especially after the heating element has suffered scaling, has increased to the level where it is difficult to manufacture. In addition, the speed of response of the bimetal depends on its set temperature and the high settings tend to give a slow response. Thus we have a conflict between a bimetal temperature setting low enough for economic manufacture of the bimetal and high enough to avoid nuisance operation, and this has led to tight manufacturing tolerances. In addition the X4 does not have the benefit of a floating carrier to absorb element distortion, so the movement of the bimetal has to be optimised to guarantee opening the contacts. This is especially true of the cycling versions, as opposed to the latched version. In the X4, the comers of the bimetal are retained by a metal clip which is quite thin (0.2mm), and the clip does not add much to the effective bimetal movement, as is the case with the X3. Objects and Summary of the Invention: It is the principal object of the present invention to overcome or at least substantially reduce the above mentioned problem. The invention aims to do this by improving the heat transfer between a bimetallic actuator and also optimising the movement available from the actuator. To overcome these problems as applied to an X4 control we have provided raised pips in the comers of the X4 mounting clip to support the comers of the bimetal and improve the effective movement of the bimetal. In addition we have provided features on the clip which, in use of the control, press the bimetal towards the heating element. The points of action of the forces derived by these features are inboard of the comer pips so that force couples are applied to the bimetal. This tends to increase the curvature of the bimetal, which correspondingly tends to raise its operating temperature. However, the increased curvature is opposed in use by the heating element surface, so that the forces applied to the bimetal result in the bimetal being flattened against the heating element surface. The result of the forces applied to the bimetal and the distortion of the bimetal is twofold: firstly, the effective bimetal set temperature is raised, thereby allowing a more practical, lower, manufacturing setting to be used and still avoiding nuisance operation and, secondly, the flattening of the bimetal against the heater surface results in a much improved heat transfer giving a faster response. In fact the faster response can more than compensate for the rise in set temperature, with the result that a higher resistance to nuisance operation can be achieved with, at the same time, a more rapid operation of the control. More generally, therefore, the present invention proposes a bimetallic control wherein a bimetallic actuator of the control is mounted so as to be subjected by its mounting in use of the control to forces tending to increase its curvature, the mounting furthermore being such as in use of the control with a heating element to enable the actuator to be biased against the heating element with a force sufficient to reduce its curvature. In order that the invention might be fully understood, an exemplary embodiment thereof will be described hereinafter with reference to the accompanying drawings. Description of the Drawings: Figure 1 shows a perspective view of an X4 control as described in our British Patent Application No. 9909171.2; Figures 2A, 2B and 2C are different perspective and side elevation views of a spring metal bimetal mounting carrier for an X4 control modified in accordance with the present invention; and Figure 3 illustrates schematically the forces arising in use of the bimetal mounting carrier of Figures 2A, 2B and 2C in an X4 control as shown in Figure 1. Detailed Description of the Embodiment: Referring first to Figure 1 , shown therein is an X4 control as described in our British Patent Application No. 9909171.2 to which reference may be made for a more detailed description than will be given herein. The control is an integrated 360° inlet connector for a cordless appliance, based upon our C54 inlet (socket) connector and adapted for use with our CP7 output (plug) connector, and heating element protector. The 360° inlet connector is designated in Figure 1 and is integrated with first and second bimetallic overtemperature protection controls designated 2 and 3. The controls 2 and 3 are each adapted to disconnect the electricity supply through the inlet connector 1 to an associated heating element (not shown) in the event of the heating element temperature rising above a certain level, for example because a water boiling vessel fitted with the heating element has been switched on without first being filled with water or, alternatively, has been allowed to boil dry on account of having been boiled with the lid off or open thereby bypassing a steam sensor provided in the vessel. The two bimetals may be set to operate at the same temperature or, more preferably, there may be a temperature difference between the settings of the two bimetals so that one operates normally as a dry boil protector and may be automatically resetting, whereas the other provides a secondary protection facility operable only at a higher temperature in the event of failure of the dry boil protector and may be arranged to latch in its operated condition. As described in our British Patent Application No. 9909171.2 either or both of the controls 2 and 3 may have associated therewith a latching arrangement such as to be reset by the operation of removing the associated cordless appliance from its base and then replacing it; this concept of latching out a bimetallic heating element overtemperature protection control when it operates in response to a sensed overtemperature and enabling it to be reset by disconnection and reconnection of the plug and socket connector set of the appliance was first disclosed in GB 2176055. Each of the controls 2 and 3 has a generally rectangular bimetal 4 formed as shown with a generally X-shaped cut-out 5 and the respective bimetals are each supported in a spring metal carrier 6 which at one (lower) level is secured to a plastics moulding of the X4 control and at another (upper) level has formations at its opposite ends which locate and support opposite ends of the bimetal. This method of supporting the bimetals enables them not only to accommodate dimensional variations between one heating element and another caused by unavoidable manufacturing tolerances, but also enables them to accommodate heating element distortions caused by thermal expansion/contraction effects during operation. Each bimetal has an associated push-rod (not shown) which, when the bimetal snaps from its cold condition (when its curvature is convex towards the surface of an associated heating element) to its oppositely curved hot condition, causes a respective set of switch contacts (not shown) within the control to open thereby breaking the current supply path through the control. In a particularly preferred arrangement one at least of the bimetal mounting springs is formed integrally at its lower level with an overcentre arrangement interfacing with the push-rod so as to latch the push-rod in its operated condition once the bimetal has operated. As mentioned hereinbefore, an arrangement for resetting such a latched out arrangement by lifting and replacing a cordless vessel on its base may be provided. Finally in regard to the X4 control of our British Patent Application No. 9909171.2, it will be seen from Figure 1 that means 7 are provided for the direct connection to the control of a steam sensor control such as our Z5 steam sensor which is described in our British Patent Application No. 9811400.2. Referring now to Figures 2A, 2B and 2C, shown therein is a replacement, configured in accordance with the teachings of the present invention, for the bimetal mounting carrier 6 of the arrangement shown in Figure 1. Figure 2A shows the carrier 10 in perspective view as it would appear on the control, Figure 2B is an inverted perspective view and Figure 2C shows the carrier 10 in side elevation view with a bimetal 4 supported in the carrier. Figure 2A shows the spring metal carrier 10 as comprising a generally rectangular base position 12 having long sides 13, which are apertured at 14 for mounting to the plastics material body of the X4 control, and short sides 15, which terminate in upstanding portions 16 from the upper edges whereof there are generally inwardly extending formations 17, 18 and 19. The portions 17 and 18 are at the comers of the spring carrier 10 and terminate, at their free ends, in a generally flat portion 20 from the outer edge of which there extends a downwardly-depending finger 21. As in the carrier that is described in our British Patent Application No. 9909171.2 aforementioned, the portions 20 overlie the comers of the bimetal when it is mounted in the carrier, and fingers 21 serve to locate the longitudinal edges of the bimetal and the portions 19 underlie the bimetal and retain it in position. This can be seen in Figure 2C where the bimetal is designated 30. Also shown in Figures 2A, 2B and 2C is the provision on the portions 20 of the carrier spring 10 of projections or pips 22 which extend from the portions 20 for contacting the underlying bimetal. These pips 22 serve a dual function in keeping with the teachings of the present invention, namely they increase the available movement from the bimetal and, as described hereinafter, they assist in the flattening of the bimetal when, in use of the control, it is pressed against a heating element with a force sufficient to cause the bimetal to flatten as compared to its normal, rest condition curvature. In this connection, it should be noted that the contact point of the spring portion 19 of the carrier 10 with the bimetal is inwardly (with respect to the longitudinal axis of the bimetal) of the positions whereat the pips 22 contact the bimetal. Referring to Figure 3, this schematically shows the situation when the bimetal 30 is pressed against the heating element surface 50 with a force sufficient to flatten the bimetal curvature. The pips 22 are schematically shown and also schematically shown are the locations 100 where the spring carrier portions 19 apply forces to the bimetal, it being noted that these locations are inwardly of the points of contact of the pips 22 with the bimetal so that schematically-illustrated force couples 200 are applied to the bimetal. Insofar as concerns the added bimetal movement obtained by provisions of the pips 22, it can be seen from Figure 3 that the extremities of the bimetal 30 contact the tips of the pips in the cold condition of the bimetal (which is the condition shown). When the bimetal snaps to its hot condition, designated 30' in Figure 3, it can be seen that the bimetal extremities are lifted by the pips 22 above the heating element surface which results in the bimetal movement being correspondingly increased as compared to the movement it could develop if the pips 22 were not provided. The flattening of the bimetal curvature against the heating element surface is achieved by mounting the control to the heating element so that the bimetal mounting carriers 10 are subjected to substantial forces, in the region of 800 grams, which bends the portions 17 and 18 of the bracket downwardly against the bimetal and at the same time develops the reaction forces 100 from the portions 19 and flattens the bimetal. As mentioned hereinbefore, the effect of the forces applied to the bimetal and its flattening against the heating element cause its effective set operating temperature to be raised, thereby enabling a more practical, lower manufacturing setting to be used whilst still avoiding nuisance operation, and increases the thermal contact of the bimetal with the heating element thereby giving a more rapid response which more than compensates for the rise in set temperature with the result that a higher resistance to nuisance operation is in fact achieved. Nuisance operation, for avoidance of doubt, is when the set operating temperature of the control is so close to the temperatures encountered by the control during normal boiling operation of an associated vessel that the control operates under normal boiling conditions rather than operating only in response to a heating element overtemperature condition. The problem of nuisance tripping of heating element overtemperature protection controls is exacerbated by the build up of lime scale on the heating element, since the lime scale tends to thermally insulate the heating element from the water to be heated so that the heating element has to operate at a higher temperature in order to boil the water, which reduces the differential between the normal boiling temperature of the heating element and the set temperature of the element protection control. The present invention having been described by reference to an exemplary embodiment, it is to be appreciated that modifications and variations thereto are possible without departure from the scope of the invention. For example, whereas the spring metal bimetal carrier of Figures 2A, 2B and 2C does not have an integral overcentre mechanism, as in the embodiments described in our British Patent Application No. 9909171.2, it could have such a feature.

Claims: CLAIMS: 1. A thermally-sensitive bimetallic actuator wherein a dished, snap-acting bimetal is mounted so as in use to be subjected to stresses such as to increase its operating temperature, the mounting enabling the bimetal to be biased against a flat surface with a force sufficient at least partly to flatten the bimetal. 2. An actuator as claimed in claim 1 wherein the bimetal is mounted in a spring metal carrier adapted to be urged against a heated surface with a force such as to deform the carrier in such a manner as to subject the bimetal to stresses. 3. An actuator as claimed in claim 2 wherein the carrier has formations adapted to apply a force couple to the bimetal when the carrier is urged against said surface with said force. 4. An actuator as claimed in claim 3 wherein the bimetal is generally rectangular and the carrier has portions generally at the comers of the bimetal which overlie the comers of the bimetal and further portions which underlie the bimetal, the bimetal being supported between said overlying and underlying portions of the carrier. 5. An actuator as claimed in claim 4 wherein the portions of the carrier which underlie the bimetal make contact with the bimetal at locations spaced from the locations whereat the overlying portions make contact with the bimetal, the spacing of the respective portions being adapted to subject the bimetal to force couples when the carrier is deformed by being urged against the heated surface. 6. An actuator as claimed in any of the preceding claims including means arranged to increase the movement available in operation of the bimetal beyond that determined by the bimetal geometry. 7. An actuator as claimed in claim 7 wherein the bimetal is arranged to be mounted with that surface thereof which is convex in the cold condition of the bimetal contacting the bimetal mounting surface and means are provided in the bimetal mounting which are arranged to support the bimetal periphery at a spacing from said mounting surface. 8. An actuator as claimed in claim 7 as dependent upon claim 4 or 5 wherein said comer portions of the carrier are provided with means to support the comers of the bimetal at a spacing from the heated mounting surface. 9. An actuator as claimed in claim 8 wherein said means to support the bimetal at a spacing from its heated mounting surface comprise projections extending downwardly from said overlying portions and contacting the upper surface of the bimetal. 10. A thermally sensitive bimetallic actuator wherein a dished, snap-acting bimetal is mounted so as, in its cold condition, to present its convex face to a heated surface, and the bimetal mounting has means to space the bimetal periphery from said heated surface even in the hot condition of the bimetal. 11. A thermally sensitive bimetallic actuator substantially as herein described with reference to Figures 2A, 2B, 2C and 3 of the accompanying drawings. 12. A thermally sensitive control incorporating at least one actuator as claimed in any of the preceding claims.

Inventors: WALFORD MARK JOHN, WALFORD, MARK, JOHN
Applicants: OTTER CONTROLS LTD, WALFORD MARK JOHN, OTTER CONTROLS LIMITED, WALFORD, MARK, JOHN
Application year: 2000
Main classification: H01H37
IPC codes: H01H37