GB2235257A

GB2235257A - Pivot for electric switch

Info

Publication number: GB2235257A
Application number: GB8919282A
Authority: GB
Inventors: Robert Andrew O'neill
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1989-08-24
Filing date: 1989-08-24
Publication date: 1991-02-27
Anticipated expiration: 2009-08-24
Also published as: GB8919282D0; GB2235257B

Abstract

A pivot of an electric switch such as a steam sensor for a water-boiling vessel comprising a trip lever 4 and a bimetal element 5 in an overcentre arrangement comprises a V-edge 20 of the trip lever engaged with a V-groove 10 in the main body of the switch and in order to prevent the application of undue manual force to the trip lever from possibly damaging the bimetal the V-groove has a semi-cylindrical recess at the base of one of its faces to receive a complementary part-cylindrical projection 23 of slightly lesser radius formed on the V-edge for opposing translational movement of the V-edge along the face of the groove. <IMAGE>

Description

IMPROVEMENTS RELATING TO THERMALLY RESPONSIVE ELECTRIC SWITCHES This invention concerns improvements relating to thermally responsive electric switches and switch actuators, and has particular though not exclusive application to the field of thermally responsive electric switches for use in electrically heated water boiling vessels such as kettles, jugs, urns, pans, laboratory equipment and the like for switching off or reducing the supply of power to an electric heating element of the vessel when water or another liquid boils in the vessel and the vapour generated impinges upon a thermally-responsive switch-actuating element.

In our British Patent Application No. 8827340.4 (Serial No. 2212664) there is disclosed in Figs. 3A to 3M-1 and 3M-2 a steam sensing module designed to be plugged into an element protection unit, that is to say a unit for protecting the electrically powered heating element of a water boiling vessel from overheating, so as to convert the unit additionally into a steam sensing unit for automatically switching off the heating element when water boils within the vessel. The manner in which the steam sensing module plugs into the element protection unit is more particularly described with reference to Figs. 5A to 5E of the application, and in Figs. 9A, 9B and 10 a modified form of the steam sensor module is shown.

As described in our abovementioned application, the steam sensor module comprises a main moulding formed with a chamber which serves to accommodate a switch sub-assembly, a trip lever adapted to be mounted in the main moulding so as to cooperate with the switch sub-assembly and having a part formed for cooperation with a limb of the main moulding which is shaped to provide a knife-edge pivotal mounting for the trip lever, and a bimetal blade mounted between spaced-apart abutments in the trip lever and the main moulding so as to define with a portion of the trip lever an overcentre bridge capable of moving with a snap-action in response to a temperature change and with the spring force necessary for causing such snap action developed by the bimetal.

Whilst the steam sensor module of our abovementioned application has many significant advantages, we have identified a potential problem with its design in that if excessive force is applied to the trip lever, for example as the result of a vessel manufacturer desiring to incorporate the module into his vessels and fitting a customised cosmetic knob to the trip lever, a sliding movement of the trip lever relative to its knife edge pivot in the main moulding can result. This movement has a component which exerts a force on the bimetal and could be capable of buckling the bimetal and thereby impairing the action of the control. The same potential problem could arise if the knife edge portion of the pivotal mounting of the trip lever to the main moulding were to be provided on the trip lever for cooperation with a V-shaped groove in the main moulding.

The object of the present invention therefore is to retain the simplicity of assembly of a simple Vtype knife edge pivot whilst avoiding the abovementioned problem.

According to the present invention the inclined plane defining one face of a V-groove into which a Vshaped knife edge engages in pivotal fashion is provided with a local reversal in its slope such as to define an abutment preventing movement of the knife edge up the face of the V-groove under the action of forces directed within a predetermined range of directions. The V-shaped knife edge may be complementarily shaped for more positive movement inhibiting engagement with the reversed slope portion of the V-groove.

Thus, for example, in one embodiment of the invention which will be described in detail hereinafter, a V-shaped groove defined by first and second faces angularly spaced apart from each other by an angle of 900 has a semi-cylindrical recess formed in the first face and with the cylindrical surface of the recess tangential with the second face.A Vshaped knife edge defined by first and second faces angularly spaced apart from each other by an angle 6 less than 900 so as to allow pivotal movement of the V-edge in the V-groove has a part cylindrical projection the axis of which lies in the first face of the V-edge and the cylindrical surface of which is tangentially intercepted by the second face of the Vedge, and the radius of the part cylindrical projection is slightly less than the radius of the semi-cylindrical recess in the first face of the Vgroove with both radii being of the order of half a millimetre so that pivotal movement of the V-edge in the V-groove is accomplished without any significant frictional penalty to the operation of the pivot.The provision of the recess in one face of the V-groove and of the complementary projection in the corresponding face of the V-edge restricts the degrees of freedom of the pivotal connection whilst retaining the simplicity of assembly abovementioned.

The present invention will best be understood from consideration of the following description of an exemplary embodiment given with reference to the accompanying drawings wherein: Figure 1 is a side view, similar to that shown in Fig. 10 of our British Patent Application No.

8827340.4 (Serial No. 2212664), of a steam sensing module incorporating a modified pivotal connection according to the present invention, and including a 10 times enlarged view of the pivot; and Figures 2A and 2B are schematic views illustrating the range of contained forces with a simple V-pivot and the improved range achieved by use of the arrangement of Figure 1.

Referring to Figure 1, the steam sensing module shown comprises a main moulding 1 within a chamber of which there is mounted a switch sub-assembly having a tail portion 2 with electrical contacts one of which is shown at 3. A trip lever 4 is mounted for pivotal movement within the main moulding by virtue of the provision of a pivotal mounting which is shown in enlarged view and will be described further hereinbelow. A bimetal 5 is mounted at its lowermost end (as viewed in Figure 1) in a V-groove formed in the upper end of a part 6 of the trip lever 4 and at its upper end in a groove formed in a compliant spring part 7 captured in the main moulding 1.The bimetal 5 together with the trip lever part 6 form an overcentre bridge which is movable with a snap action in the event of a sufficient temperature change at the bimetal, and the movements of the bridge (and more particularly of the trip lever part of the bridge) cause the trip lever to operate the contacts set provided in the switch sub-assembly. Further details of the construction and operation of the steam sensing module are given in GB 2212664 abovementioned.

The steam sensing module of Figure 1 has a modified pivot arrangement as compared to that of either steam sensing module described in GB 2212664.

In the prior arrangements a knife-edge shaped extension from the main moulding provided a pivotal mounting for a V-groove formed in the trip lever. In the present arrangement, the main moulding 1 is formed with a V-groove 10 and the trip lever 4 has a portion 20 defining a V-shaped knife edge engaged with the groove 10. As can best be seen in the enlargement portion of Figure 1, the V-groove 10 in the main moulding has first and second faces 11 and 12 and a semi-cylindrical recess 13 is formed in the base of the first face 11 so as to meet the second face 12 tangentially. The knife edge portion 20 likewise has first and second faces 21 and 22 and has a partcylindrical projection 23 from the base of the first face 21, the axis of such cylindrical projection 23 being on the plane of the face 21 and the surface of such cylindrical portion merging tangentially with the second face 22.

In the exemplary embodiment shown the faces 11 and 12 of the V-groove 10 are angularly spaced by 900, the faces 21 and 22 of the V-edge 20 are angularly spaced by an angle e less than 900, and the angle subtended by the part-cylindrical projection 23 is 2700be. The radius of the recess 13 is 0.50mm and the radius of the projection 23 is 0.45mm, the difference between these radii providing a working clearance for the pivot and their small size avoiding any significant frictional penalty. The geometry of the two components of the pivot is arranged so that the axes of the two cylindrical surfaces are coincident and lie at the intersection of the original knife edge and V-groove which the arrangement of the invention replaces.

The effect of the improved pivot arrangement, caused by the interaction of the two described part cylinders, is to extend the range of directions of applied forces against which the pivot prevents lateral motion by a further 900, ie. from a force normal to the plane of the lower knife edge face and towards it to a force in the plane of the lower edge face and away from the fulcrum. This is shown in Figures 2A and 2B.

The above described embodiment provides optimum conditions, according to our current thinking. The invention as thus described is however susceptible to modification and variation without departure from the spirit and scope thereof, particularly in the following respects, namely: 1. the V-groove could be greater than or less than 900, though a V-groove of less than 900 might present assembly problems; 2. the cylindrical surfaces of the recess 13 and the projection 23, or of either of them, need not be tangential to the second face of the respective part, though such an arrangement might be of reduced strength but otherwise as effective; and 3. the axis of the part cylinders, or either of them, need not lie in the plane of the first face of the respective part, though such an arrangement might also be of reduced strength but otherwise effective.

Finally, whilst the invention has been partic'ularly described in relation to a specific form of steam sensor module and has significant advantages in relation thereto, the invention clearly has wider application to any kind of knife edge/V-groove pivotal connection where the mechanical stability of the pivot is an important consideration.

Claims

CLAIMS:

1. A pivotal arrangement wherein a member having a generally V-shaped edge portion is received in a generally V-shaped groove in another member for relative pivotal movement between the members and wherein a face of the V-groove is formed with a local slope reversal adjacent the base of the groove for limiting translational movement of the V-edge across the said face.

2. An arrangement as claimed in claim 1 wherein the slope reversal in the said face of the V-groove defines a recess in the said face and a complementary projection is provided on the V-edge for engagement with the said recess.

3. An arrangement as claimed in claim 2 wherein the recess and the projection are part-cylindrical in cross-section.

4. An arrangement as claimed in claim 3 wherein the recess is semi-cylindrical.

5. An arrangement as claimed in claim 3 or 4 wherein the axis of the part cylindrical surface of the recess lies in the plane of the said face of the V-groove.

6. An arrangement as claimed in claim 3 or 4 or 5 wherein the part cylindrical surface defining the recess merges tangentially with the other face of the V-groove.

7. An arrangement as claimed in any of claims 3 to 6 wherein the axis of the part cylindrical surface of the projection lies in the plane of the face of the Vedge which faces said face of the V-groove.

8. An arrangement as claimed in any of claims 3 to 7 wherein the part cylindrical surface of the projection merges tangentially with the face of the V-edge furthest from said face of the V-groove.

9. An arrangement as claimed in any of claims 2 to 8 wherein the complementary projection is slightly smaller than the recess.

10. An arrangement as claimed in claims 3 to 9 wherein the axes of the part cylindrical surfaces defining the recess and the projection are coincident and lie at the position where the V-edge and V-groove would engage if the recess and the projection were not provided.

11. A thermally-responsive electric switch or switch actuator incorporating a pivotal arrangement as claimed in any of the preceding claims.

12. A thermally-responsive electric switch or switch actuator as claimed in claim 11 and wherein the switch/switch actuator includes a manually operable control interacting with the pivotal arrangement, and the pivotal arrangement by virtue of its construction is such as to resist the application of excess force to the control.

13. A thermally-responsive electric switch or switch actuator as claimed in claim 12 wherein the control defines part of an overcentre bridge including a bimetallic element and the pivotal arrangement is such as to resist the application of undesired compressive forces to the bimetallic element.

14. A thermally-responsive electric switch or switch actuator as claimed in claim 13 and wherein the overcentre bridge is arranged between two spaced apart positions and incorporates the bimetallic element as part of the length of the bridge for generating snap acting overcentre movement of the bridge in response to a change in temperature, the bimetallic element furthermore being arranged by itself to provide the spring force necessary for causing the bridge to move overcentre with a snap action.

15. A thermally responsive electric switch or switch actuator as claimed in claim 14 and adapted for use as a steam sensor in an electrically heated water boiling vessel.

16. A thermally responsive electric switch or switch actuator as claimed in claim 15 and wherein the steam sensor is substantially as herein described with reference to the accompanying drawings.

17. An electrically heated water boiling vessel incorporating a thermally-responsive electric switch or switch actuator as claimed in any of claims 11 to 16.