GB2347271A

GB2347271A - Thermal controls

Info

Publication number: GB2347271A
Application number: GB0004564A
Authority: GB
Inventors: Robert Andrew O'neill; Paul Boundy
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1999-02-25
Filing date: 2000-02-25
Publication date: 2000-08-30
Anticipated expiration: 2020-02-25
Also published as: EP1163690A1; GB0004564D0; GB9904391D0; AU2815500A; GB2347271B; CN1339168A; WO2000051151A1

Abstract

A steam control for switching off the heating element of a water boiling vessel comprises a snap-acting bimetal 16 arranged to determine the condition of an overcentre mechanism via a push-rod 11. The overcentre mechanism comprises a trip lever 3 and a stressed spring 2 arranged mechanically in series between spaced-apart abutment (4, 5, Fig. 1). To avoid loading the bimetal by the push-rod and the overcentre mechanism as the bimetal approaches its operating temperature, the push-rod is made relatively short so that it does not apply force to the overcentre mechanism, and correspondingly load the bimetal, until after the bimetal itself has moved overcentre. The trip lever is movable for resetting the bimetal beyond its normal cold condition rest position against spring bias, the additional trip lever movement providing additional movement to the push-rod for resetting the bimetal.

Description

IMPROVEMENTS RELATING TO THERMAL CONTROLS Field of the Invention: This invention concerns improvements relating to thermal controls and has particular, though not exclusive, application to thermal controls utilized in electrically heated water boiling vessels such as domestic kettles and jug kettles for example where the generation of steam when water boils in the vessel is sensed by a bimetallic actuator which changes state and causes an overcentre trip lever to operate thereby opening a set of switch contacts and disrupting the electrical supply to the heating element of the vessel. Such thermal controls are well known and are commonly referred to as"steam controls".

Background of the Invention: Hereinafter the present invention will be described by reference to the steam control that is described in GB-A-2 331 848, but the invention is applicable to other steam controls wherein a snap-acting bimetallic actuator determines the condition of an overcentre mechanism, for example the controls that are described in GB-A-2 248 520, GB-A-2 218 029 and GB-A 1 470 336.

The control that is described in our British Patent Application No.

9811400.2 (hereinafter referred to as the Z5 steam control) comprises a snap-acting bimetallic actuator in the form of a circular disc-shaped blade having a generally U-shaped cut-out which releases a tongue from the blade, the blade being dished so as to be movable between oppositely curved configurations with a snap action. Such bimetallic actuators are well known and when mounted into a control by the periphery of the disc-shaped blade provide a substantial amount of movement at the free end of the tongue as the dished blade moves between its oppositely dished configurations. In the Z5 control the bimetallic blade is thus mounted to a cover moulding of the control and the free end of the tongue co-operates with an overcentre mechanism via a push rod. The overcentre mechanism comprises a trip lever and a spring which are mounted in series with each other between spaced-apart abutments in a body part of the control, the overcentre arrangement being movable with a snap action between stable positions on either side of an unstable central position. In the Z5 control the spring has integral contact-carrying extensions which move with the spring as the overcentre mechanism moves between its two stable positions and either make contact with or break contact from fixed switch contacts provided in the control, but in other arrangements the trip lever has a switch-operating part which in dependence upon the position of the trip lever, this being dependent upon the status of the overcentre mechanism, determines the status (open or closed) of a set of switch contacts.

In operation of the Z5 steam control as thus described, steam generated when water is boiled in a vessel fitted with the control impinges upon the bimetal and causes it to switch to its oppositely curved state, this movement causing the free end of the blade tongue to move inwardly with respect to the body of the control, thereby urging the push-rod inwardly and, by abutment of the push-rod with the trip lever, causing the trip lever to pivot so that the overcentre mechanism is moved into and through its central unstable position and snaps to its other stable position. The spring extensions follow the movement of the overcentre mechanism and the contacts on the ends of the spring extensions move apart from the fixed contacts in the control, thereby disrupting the supply of current through the control to the vessel heating element. To reset the control, the trip lever has to be operated manually so as to restore the overcentre mechanism to its original state, this movement also serving to depress the push-rod and push the tongue of the bimetal towards the position it occupied in the original cold condition of the bimetal and thus seek to reset the bimetal. Of course, the bimetal will seek to oppose this resetting action if its temperature has not cooled.

We have found that the length of the push rod has a critical effect upon the operation of the Z5 control. If the push rod is too short, the movement available from the bimetal may be insufficient to effect reliable operation of the overcentre mechanism when the bimetal is subjected to steam generated by water boiling in the vessel. Additionally, the movement available from the overcentre mechanism may be insufficient to guarantee that the bimetal can be reset without its temperature having had to reduce by an excessive amount.

On the other hand, if the push rod is too long the response of the bimetal to rising temperature can be undesirably dependent upon mechanical loading of the bimetal by the push rod and by the overcentre mechanism, at least as the bimetal approaches its snap point, and this can raise the snap temperature of the bimetal towards a level where operation of the control may not be guaranteed, particularly if the vessel is operated at an elevated location where the boiling point of water is less than 100 C.

The abovementioned difficulties can be successfully accommodated by appropriate design of the operating components of the control, but this has cost penalties. If the bimetal has to be selected to have a defined operating temperature within a 10 C range and the push rod length must be controlled to better than 0.1 mm, for example, then significant cost penalties can arise.

Obiects and Summary of the Invention: It is, accordingly, the principal object of the present invention to overcome or at least substantially reduce the abovementioned problems.

According to the present invention there is provided a thermally responsive control comprising a snap-acting bimetallic actuator arranged to determine the condition of an overcentre mechanism and wherein the arrangement is such that the overcentre mechanism does not or does not significantly load the movement of the bimetallic actuator from an initial cold condition to and through its intermediate unstable position in response to a temperature rise.

Preferably, the overcentre mechanism is movable between first and second, cold and hot positions in response to operation of said bimetallic actuator and, for resetting the bimetallic actuator to its cold condition, the overcentre mechanism is manually movable beyond its cold position, in a direction from its hot to its cold position, against spring bias.

According to exemplary embodiments of the present invention which are described hereinafter the length or effective length of a force transmitting member acting between the bimetal and the overcentre mechanism, a push rod for example is selected so that the bimetal operating temperature is not subject to being affected by loading of the bimetal by the push rod and its associated overcentre mechanism until the bimetal has moved to and through its unstable centre position, the consequent shortness in the push rod length, considered from the viewpoint of resetting the bimetal, being accommodated by providing additional resetting movement, against spring bias, in the overcentre mechanism.

In a control constructed in accordance with the teachings of the present invention, the trip lever of the overcentre mechanism thus has two free rest positions corresponding to the two stable positions of the overcentre mechanism and there is further provided a resilient stop limiting the movement of the trip lever from its operated (hot) position to and through its reset (cold) position into a third unstable position past its reset (cold) position, the resilient stop returning the trip lever to its reset (cold) position when the reset force is released. From the viewpoint of a user, a vessel provided with such a control will have an ON/OFF knob attache to the trip lever for manual operation by the user and in operating the knob to reset the vessel after it has boiled and automatically switched off, the user will be required to push the knob from its OFF position to an ON position as is conventional, but there will be a slight resistance to the final movement of the knob and upon release of the manual operating force, the knob will resile slightly; the user will hardly notice the difference.

The resilient stop can be provided in any convenient manner and in the following several exemplary arrangements will be described, namely one in which the stop is defined by an abutment in the switch body which co-operates with the spring component of the overcentre mechanism, another in which a separate spring associated with the trip lever performs a similar function and another in which the stop is defined by a resilient tongue released from a body moulding of the control much in the same fashion as a tongue is released from the bimetal. In yet another embodiment an intermediate member is provided between the C-spring and the trip lever and is arranged to be pivotally movable, the intermediate member having a part which functions as a push-rod.

The above and further features of the present invention are set forth in the appended claims and, together with advantages thereof, will become clear from consideration of the following detailed description of exemplary embodiments which is given with reference to the accompanying drawings.

Description of the Drain : Figures 1 and 2 are side elevation views, partly in phantom, of a first embodiment of steam control according to the present invention, Figure 1 showing the embodiment in its contacts-closed rest position and Figure 2 showing that the trip lever of the embodiment can be pushed forward so as to effect additional bimetal resetting movement of the push-rod; Figures 3,4 and 5 are sectional side elevation views of a second embodiment similar to the first, Figure 3 showing the ON (cold) condition of the embodiment, Figure 4 showing the OFF (hot) condition and Figure 5 showing the forced reset condition; Figures 6 to 9 show a third embodiment, Figure 6 showing the embodiment in its contacts-closed rest position, Figure 7 being similar to Figure 6 but showing that the trip lever can be pushed forward so as to effect additional bimetal resetting movement of the push-rod, Figure 8 being a perspective view of a moulded plastics body part of the embodiment showing a resilient part released therefrom, and Figure 9 being a perspective view of the body part of Figure 8 viewed from a different direction; and Figures 10 to 21 show a fourth embodiment, Figures 10 to 13 being, respectively, a sectional side elevation view (Figure 10), a perspective view (Figure 11) showing the upper side of the embodiment, a perspective view (Figure 12) showing the under side of the embodiment and an enlarged view (Figure 13) of the portion designated C in Figure 12, all with the embodiment in its ON (contacts closed) condition, Figures 14 to 17 being similar views all with the embodiment in its forced reset condition, and Figures 18 to 21 being similar views all with the embodiment in its OFF (contacts open condition.

Detaled Describption of the Embodiments: The embodiments of the present invention that are described hereinafter are variations of the steam switch for a kettle or hot water jug that is particularly described in GB-A-2 331 848 and will be described hereinafter only in sufficient detail to provide an understanding of the present invention.

Reference may be made to GB-A-2 331 848 for a more detailed understanding of the construction of the embodiments.

Referring to Figures 1 and 2, the switch shown therein comprises a moulded plastics body part 1 which supports an overcentre mechanism constituted by a spring 2 and a trip lever 3 mechanically in series with each other between spaced-apart abutments 4 and 5. The spring 2 is E-shaped, as more particularly described in GB-A-2 331 848, and has a central element 6 in the form of a C-spring which extends between elongate side elements 7 carrying at their free ends moving contacts 8 of the switch. The moving contacts 8 co-operate in switching operations with fixed contacts 9 that are provided on metal terminal parts 10 of the switch.

A push-rod 11 locates in a bore formed in the body part 1 of the switch and co-operates with a snap-acting bimetallic switch actuator (not shown) to determine the condition of the overcentre mechanism. As is well known, a snap-acting bimetallic actuator is a bimetal, commonly of dished configuration, which is movable between stable positions on opposite sides of an intermediate unstable position. More particularly, the push-rod 11 is movable in its locating bore in dependence upon the condition of the bimetal, the push-rod bearing upon an undersurface of the trip lever 3 as shown so as to cause the trip lever to pivot in a clockwise direction about the abutment 5 when the push-rod 11 is pushed upwards (as viewed in the drawings) by the bimetallic switch actuator changing shape, with a snap action and at a substantially predetermined temperature, from its cold condition to its hot condition when steam impinges upon it in use. The clockwise pivotal movement of the trip lever 3 causes the overcentre mechanism to move with snap action to and through its central unstable position and into its contacts-open condition, such movement causing the elongate side elements 7 of the spring 2 to pivot in an anti-clockwise direction about the abutment 4 and opening the moving contacts 8 from the fixed contacts 9.

To enable the switch, once operated from its cold, contacts-closed condition to its hot, contacts-open condition, to be reset, and also to enable the switch to be operated manually, the trip lever 3 is provided with an operating knob 12. To reset the switch, the knob 12 is moved in the direction of the arrow shown in Figure 2, this causing the overcentre mechanism to return to its original contacts-closed condition. The resetting, anticlockwise movement of the trip lever 3 depresses the push-rod 11 and this movement is employed to reset the bimetal. In accordance with the teaching of the present invention, in this embodiment the push-rod 11 is of a length much shorter than in conventional arrangements and just less than the distance between the location on the underside of the trip lever 3 where it is abutted by the push-rod in switching operation and the actuating part of the bimetal when it has just moved overcentre between its oppositely dished configurations. By virtue of this arrangement, it is ensured that the switching operation of the bimetal is not loaded by the push-rod cum overcentre mechanism such as to affect (elevate) the operating temperature of the bimetal. Namely, the bimetal has already moved to and through its intermediate position by the time it has to operate the overcentre mechanism via the push-rod, albeit it has not completed its movement.

Given that the push-rod 11 has only limited length, the need arises to enable the trip lever 3 to be pivoted anticlockwise beyond its free rest position in the cold condition of the switch so as to enable the depressed push-rod to push the snap-acting bimetal overcentre and cause it to reset. In the embodiment of Figures 1 and 2 this additional movement is achieved by virtue of the geometry of the trip lever 3 which has an abutment 13 which engages the body moulding 1 to limit the extend to which the trip lever can be pivoted in an anticlockwise direction. As shown in Figure 1, this abutment 13 is spaced from the body moulding 1 when the overcentre mechanism is in its free, cold (low temperature) condition. The C-spring element 6 is arranged to abut a surface 14 of the body moulding 1 when the overcentre mechanism is in its free cold condition, as shown in Figure 1, and this provides a return force tending to restore the trip lever to its free cold condition when, in order to reset the switch, the trip lever is pivoted anticlockwise so as to cause the abutment 13 to contact the body moulding 1 as shown in Figure 2. Thus, when the trip lever 3 is released following a manual reset operation, it will be returned by the resilience of the C-spring element 6 to its free position where, as previously explained, the bimetal is not loaded by the push rod and overcentre mechanism so that its operating temperature is assured.

Figures 3 to 5 show a second embodiment of the invention which is similar in many respects to the first embodiment. The same reference numerals are used in Figures 3 to 5 as were used to identify the same or like parts in Figures 1 and 2. The main difference between the first and second embodiments resides in the provision on the underside of the trip lever 3 of the second embodiment of a spring 20 which serves the function that in the first embodiment is served by the bottoming of the C-spring 6 on the surface 14 of the body moulding. The right hand end (as viewed) of the spring 20 is rigidly engaged with the trip lever moulding so that no relative movement therebetween is possible. The left-hand tip of the spring 20 locates on a ledge 21 on the underside of the trip lever 3, to give a positive, known, position which acts as a rotation stop when the trip lever is moved to its"on"position.

The form of the spring is such that, as it is located, it always applies a force to the ledge which is high enough to give a positive feel to the trip lever as it is rotated beyond its"on"position to its"reset"position. The three views of Figures 3,4 and 5 show the three possible positions of the trip lever. In the "on"position shown in Figure 3, with the trip lever resting on a backstop 22 which acts through the spring 20 the push rod 11 has the correct geometry to work with the bimetal 16 when it operates. In the forced reset position shown in Figure 5 the trip lever is rotated anticlockwise, against the force of the spring 20 resting on the backstop 22. The spring 20 therefore is lifted off its ledge 21, and the push rod 11 is pushed towards the bimetal 16 to ensure itresets. When the force on the trip lever is removed, it returns to the"on" position. In the"off"position shown in Figure 4, the trip lever 3 is rotated clockwise, the spring 20 remains seated on its ledge 21, and the contacts are opened. By selecting the force between the spring 20 and its support ledge 21 it is possible to provide a positive"on"position, which can be moved to the forced reset position by the application of a small additional force, which generally will not be noticed by the user as he or she switches the kettle on.

A third embodiment of the invention is shown in Figures 6 to 9 which again use the same reference numerals to designate parts the same as or similar to those of the preceding embodiments. Referring first to Figures 6 and 7 these show the switch body moulding 1 together with the overcentre arrangement consisting of spring 2 and trip lever 3 and the push-rod 11 assembled into an outer body moulding 15 which, as described in GB-A-2 331 848 aforementioned, provides a mounting for a snap-acting bimetallic switch actuator 16 in the form of a dished circular bimetal blade having released therefrom a tongue 17 the free end of which serves to operate the push-rod 11 as the bimetal switches between its oppositely curved configurations. As with the previously described embodiments the trip lever 3 is formed with an abutment 13, but in the present embodiment the abutment 13 co-operates with a plastics material spring member 18 which is released from the bottom surface of the inner body moulding 1 of the device and is limited in the extent of its available movement by the underlying part of outer body moulding 15 as shown in Figure 7.

Figures 8 and 9 are perspective views, from above and below respectively, of the inner body moulding 1 of the switch of Figures 6 and 7 and show how the spring member 18 is formed as a tongue released from the base of the inner body moulding 1 and capable of flexing by an amount which is precisely determined by the underlying surface of the outer moulding 15 when the inner and outer mouldings are assembled together.

The action of the embodiment of Figures 6 to 9 is the same as that of the previous embodiments except that the resilience of the spring member 18 replaces the resilience of the C-spring element 6 in the first embodiment and of the spring 20 in the second embodiment. Figure 6 shows the overcentre mechanism in its free rest condition, and Figure 7 shows the trip lever 12 pivoted anticlockwise so as to depress the spring member 18 into contact with the underlying surface of outer body moulding 15 thus limiting the extent of anticlockwise movement of the trip lever.

Figures 10 to 21 show a fourth embodiment and again, wherever possible, the same reference numerals as were used in the drawings showing the previous embodiments are used in Figures 10 to 21 to designate the same or similar parts. The principal feature distinguishing the fourth embodiment from the previous embodiments is its use of an intermediate member 30 which is located between the C-spring 6 and the trip lever 3, the intermediate member being formed with a V-notch 31 on one side for engagement with the respective end of C-spring 6 and with a V-projection 32 on its other side which is received in a V-notch 33 of the trip lever 3, the V-notch 33 and the V-projection 32 being sized to permit the intermediate member 30 a limited degree of pivotal movement relative to the trip lever 3. The intermediate member 30 furthermore has first and second depending nose portions 34 and 35 which, in central cross-section as shown in Figures 10,14 and 18 give the intermediate member 30 an inverted, generally U-shaped appearance, one of the limbs of the inverted U-shape defined by nose portion 34 extending through an opening 36 in the forward (left-hand as shown) end of the trip lever 3 and downwards for interaction with the bimetallic actuator 16 in the manner of a push-rod, and the other limb, defined by nose portion 35, being outside of the opening and acting as a backstop for movement of the intermediate member. The operation of this embodiment will be clear from the following explanation.

Referring first to Figures 10 to 13, these show the embodiment in its "ON"condition. The action of the C-spring 6 tends to rotate the auxiliary moulding 30 in an anticlockwise direction, because of the relative positions of the three fulcrum points (a, b, c). This tends to lift the operating nose 34 of the auxiliary moulding away from the bimetal 16, giving the correct push rod gap for the break action of the bimetal. The backstop nose 35 of the auxiliary moulding 30 acts as a resilient stop for the rotation of the trip lever 3, and the shape of the two mouldings 36,3 ensures that their relative rotation is limited to give the correct overall geometry, by means of the stop shown at 37.

Figures 14 to 17 show the"Forced reset"state, when the trip lever 3 is rotated anticlockwise beyond its normal"on"position, in order to ensure that the bimetal 16 resets. In this position the auxiliary moulding 30 has been rotated clockwise against the force of the C-spring 6, which results in its operating nose 34 moving towards the bimetal 16, beyond the natural push rod gap position, which tends to reset the bimetal to its cold state curvature. The reset rotation is limited by the forwarded end of the trip lever abutting the chassis moulding as shown. On subsequently releasing the trip lever, it rotates slightly clockwise under the force of the C-spring, to return to the"ON"state.

The difference between the two positions is a small relative rotation of the trip lever and the auxiliary moulding, about the vee of the trip lever, resulting in a downward movement of the vee, which in turn reduces the push rod gap. The direction of the force of the C-spring ensures that the rest position is that corresponding to the"ON"state.

Figures 18 to 21 show the"OFF"state, with the trip lever 3 rotated clockwise and with the force of the C-spring 6 transmitted through the auxiliary moulding 30 which has no other function in this position.

The invention having been explained in the foregoing by reference to particular embodiments, it is to be well appreciated that the embodiments are exemplary and are susceptible to modification and variation without departure from the spirit and scope of the appended claims. For example, whereas in the third embodiment described hereinafter a plastics material spring in a body part of the device co-operates with a rigid trip lever to provide resilience in the resetting part of the trip lever movement, the plastics material spring could just as well be provided on the trip lever for co-operation with a rigid part of the body moulding of the device, or there could even be co-operating springs on both parts. Clearly with all such sprung arrangements there should be a rigid stop feature limiting the amount of resetting movement available to the trip lever to ensure that the bimetal is not overstressed during reset operation.

A further possibility which exhibits some advantages would be to form the push-rod integrally with the trip lever; at the very least this might facilitate assembly of the device by virtue of negating the requirement for separate fitting of the push-rod.

A control which has a push-rod formed integrally with a trip lever in a bimetal operated overcentre arrangement is the boil control part of a Strix U-Series control substantially as described in WO-A-9534187 with reference to Figures 15, 16A and 16B thereof. As presently configured, the trip lever in this control is a moulded plastics member which is pivotally mounted with respect to a moulded plastics chassis and pivotal movement of the trip lever in a direction to cause the push-rod to reset the bimetal is limited by engagement of the trip lever with the chassis. The teachings of the present invention could be applied to this construction simply by provision of resilient stop (s), for example as in the embodiment of Figures 6 to 9, on either or both of the trip lever and the chassis moulding, the resilient stop (s) defining the normal rest position of the trip lever relative to the chassis and being manually deformable to enable a shortened push-rod to reset the bimetal. The present invention, though exemplified in the foregoing by specific reference to steam controls substantially as described in GB-A-2 331 848, thus is not limited to such controls and is capable of much wider application.

It would also be possible to provide the trip lever with a see-saw action by providing the inner moulding with an upwardly extending abutment about mid-way between the v-notch on the forward end of the trip lever which engages the free end of the C-spring element 6 and the trip lever pivotal abutment 5. Alternatively a downwardly extending abutment could be provided on the trip lever or co-operating abutments could be provided on the two parts. In operation of the device in response to the sensing of steam by the bimetal, the trip lever movement would be irrespective of the see-saw abutment (s), but in resetting operating the arrangement would be such that the see-saw abutment (s) came into play, causing the trip lever to rock forwardly (from right to left as viewed in the figures) about the abutment (s) with its right-hand end displacing slightly from the abutment 5 against the action of the C-spring element 6.

Finally, in the embodiments of Figures 1 to 9 the push rod will load the bimetal insignificantly in the cold condition of the control if the control is used in a disposition as shown in the drawings. It would be a simple matter to arrange that this was not the case, as in the embodiment of Figures 10 to 21, for example by having the head of the push rod engaged loosely in an accommodating formation in the trip lever or having a weak spring acting on the push rod so that the push rod was held up away from the bimetal in the normal cold condition of the control.

Claims

Claims: 1. A thermally responsive control comprising a snap-acting bimetallic actuator arranged to determine the condition of an overcentre mechanism and wherein the arrangement is such that the overcentre mechanism does not or does not significantly load the movement of the bimetallic actuator from an initial cold condition to and through its intermediate unstable position in response to a temperature rise.
2. A thermally responsive control as claimed in claim 1 wherein the overcentre mechanism is movable between first and second, cold and hot positions in response to operation of said bimetallic actuator and, for resetting the bimetallic actuator to its cold condition, the overcentre mechanism is manually movable beyond its cold position, in a direction from its hot to its cold position, against spring bias.
3. A thermally sensitive control as claimed in claim 2 comprising a snap-acting bimetallic actuator movable between stable positions on opposite sides of an intermediate unstable position, said bimetallic actuator being arranged to move an overcentre mechanism between predetermined cold and hot positions substantially at a predetermined temperature by movement of a force transmitting member in a first direction, the force transmitting member being arranged so that it does not or does not significantly load the movement of the bimetallic actuator to and through its intermediate unstable position in response to a temperature rise to said predetermined temperature, and the overcentre mechanism being manually operable for moving the force transmitting member in an opposite direction to reset the bimetal and having provision for resetting movement beyond against spring bias.
4. A thermally sensitive control as claimed in claim 3 wherein the effective length of the force transmitting member is less than the distance between those parts of the bimetallic actuator and of the overcentre mechanism which interact therewith when the overcentre mechanism is in its predetermined cold position.
5. A thermally sensitive control as claimed in claim 4 wherein the force transmitting member comprises a push-rod which is axially movable in a bore in a body part of the control, the length of the push-rod being insufficient to bridge the distance between the operative parts of the bimetallic actuator and the overcentre mechanism when the overcentre mechanism is in its predetermined cold condition.
6. A thermally sensitive control as claimed in claim 4 wherein the force transmitting member comprises an intermediate component of the overcentre mechanism which has a limited degree of movement in the overcentre mechanism against the spring bias of the overcentre mechanism for enabling resetting of the bimetallic actuator.
7. A thermally sensitive control as claimed in claim 6 wherein said intermediate component has a part which functions as a push rod and a part which functions as a fulcrum for levered movement of the intermediate component under manual operation of the overcentre mechanism and against its spring bias, such levered movement serving to reset the bimetal by movement of said push-rod component beyond its normal cold position.
8. A thermally sensitive control as claimed in any of the preceding claim wherein the overcentre mechanism comprises a spring and a trip lever arranged mechanically in series between spaced-apart abutments.
9. A thermally sensitive control as claimed in claim 8 wherein the spring comprises a C-spring element.
10. A thermally sensitive control as claimed in claim 9 wherein the spring is generally E-shaped with the C-spring element constituting a central limb of the E and being flanked by elements carrying at their free ends moving contacts arranged to co-operate in switching operations with fixed contacts provided in the control.
11. A thermally sensitive control as claimed in any of the preceding claims wherein the overcentre mechanism comprises a spring and a trip lever arranged mechanically in series between spaced-apart abutments and the spring of the overcentre mechanism provides said spring bias.
12. A thermally sensitive control as claimed in any of claims 1 to 10 wherein said spring bias is provided by means other than the overcentre mechanism.
13. A thermally sensitive control as claimed in claim 12 wherein the overcentre mechanism comprises a spring and a trip lever arranged mechanically in series and a further spring associated with the trip lever provides said spring bias.
14. A thermally sensitive control as claimed in any of claims 1 to 10 wherein the overcentre mechanism comprises a spring and a trip lever arranged mechanically in series and the trip lever and a the body part of the control are formed to abut each other in a resilient manner whereby to provide said spring bias.
15. A thermally sensitive control as claimed in claim 14 wherein said spring bias is provided by a resilient abutment which is released from the trip lever and/or from the body part of the control.
16. A thermally sensitive control as claimed in any of the preceding claims wherein the bimetallic actuator comprises a dished bimetallic blade movable between oppositely curved configurations with a snap action and the push member is such as not to load the bimetallic actuator until, in moving from its cold to its hot conditions, it has moved through a central unstable position.
17. A thermally sensitive control as claimed in any of the preceding claims adapted and arranged for use as a steam-sensitive control.
18. A thermally sensitive control substantially as herein described with reference to any of the accompanying drawings.
19. An electrically heated water boiling vessel including a thermally sensitive control as claimed in any of the preceding claims.
20. A steam control for switching off the heating element of a water boiling vessel, said steam control comprising a snap-acting bimetal arranged to determine the condition of an overcentre mechanism via a push-rod and the overcentre mechanism comprising a trip lever and a stressed spring arranged mechanically in series between spaced-apart abutments, and wherein, To avoid loading the bimetal by the push-rod and the overcentre mechanism as the bimetal approaches its operating temperature, the push-rod is relatively short so that it does not apply force to the overcentre mechanism, and correspondingly load the bimetal, until after the bimetal itself has moved overcentre, the arrangement furthermore providing for resetting of the bimetal by manual operation of the trip lever by virtue of the trip lever being movable for resetting the bimetal beyond its normal cold condition rest position against spring bias, the additional trip lever movement providing additional movement to the push-rod for resetting the bimetal.