GB2175140A - Switching device - Google Patents

Abstract

In a thermal switching device for electric cooking appliances, a single expansion rod (16) operates a first snap-action contact (25) at a lower temperature and a second snap-action contact (26) at a higher temperature. Each snap-action contact (25,26) is provided with its own operating temperature adjusting member (41,42) mounted in the housing (1) of the device. The expansion rod (8) actuates rocker (18) which is pivotable at a point (23) between first and second contact actuating portions (20,22). The rocker (18) is urged against the head (16) of the expansion rod by means of a pressure spring (24) which is so dimensioned that it is capable of operating one of the snap-action contacts (25). <IMAGE>

Description

SPECIFICATION Switching device This invention relates to a switching device for electric cooking appliances wherein a single expansion rod switches, through a component which can be pivoted by it and is mounted on the housing, a first snap-action contact at a lower temperature and a second snap-action contact at a higher temperature, for which purpose the component has two effective actuating portions which are spaced apart from one another and each of which corresponds to a striking point of one of the snap-action contacts, and wherein each of the snap-action contacts is provided with its own adjusting member.

Such a switching device is mounted within the radius of action of the heating element of the cooking appliance. The expansion rod preferably extends between the heating element and the cooking surface, for example a ceramic hot-plate. By means of the first snap-action contact, an indicating device is actuated which delivers a signal as soon as the hotplate or the respective region of the hot-plate can no longer be touched without danger. The heating element is actuated by the second snap-action contact.

A switching device of the type mentioned at the beginning is described in DE-OS 31 00 758. In this, the pivotable component is formed by a resilient support, fixed at one side, on which the expansion rod acts. The adjusting member for the first snap-action contact is mounted directly on the support.

This adjusting member serves to operate the snap-action contact at the same time. It is therefore adjacent to a part of the snap-action contact which is moved during the switching.

As a result, when the adjusting member is adjusted, not only the response point but also the snap action is influenced.

In DE-OS 31 00 758, the support is held on the expansion rod in both its directions of deflection. Canting of the expansion rod therefore leads to a displacement of the support.

Special means must be provided on the support to insulate the two snap-action contacts electrically from one another.

A similar switching device is described in DE-OS 34 23 086. In this, a two-stage, spring-loaded push-rod, driven by the expansion rod, is provided for the actuation of the snap-action contacts. A displacement of the expansion rod is proposed for the adjustment of the second snap-action contact. Since the operating point of the first snap-action contact is also altered as a result, its contact carrier has to be adjusted by bending. Such an adjustment is inaccurate.

Both in DE-OS 31 00 758 and in DE-OS 34 23 086, the snap-action contacts lie in different planes. The consequence of this is that the housing of the switching device becomes correspondingly wide and projects far beyond the heating element of the cooking appliance in the direction of its radius. This necessitates a correspondingly large installation space.

In DE-OS 34 23 086, a switching device is described wherein the expansion rod is pivotable without impairing the switching accuracy of the snap-action contact. The head of the expansion rod has a radius which is considerably smaller than the radius of a ball socket.

It is the object of the invention to provide a switching device of the type initially mentioned wherein the adjustment of the two snap-action contacts is possible independently of one another and wherein a component which is flexible in itself is superfluous for the actuation of the snap-action contacts.

According to the invention, the above problem is solved in a switching device of the type referred to above in that the pivotable component is formed from an inherently stable rocker which is mounted on the housing for rocking between the actuating portions, that the head of the expansion rod rests on the rocker and the rocker is urged towards the head by means of a pressure spring, the spring force being so great that it is capable of actuating one of the snap-action contacts, and that both adjusting members are mounted on the housing and are arranged each adjacent to the snap-action contact in question in such a manner that the position of adjustment thereof is not moved by the rocker during the switching.

The adjustment of each of the snap-action contacts can be carried out independently by means of the adjusting members without the adjustment of the one snap-action contact having any effect on the other snap-action contact. During the adjustment, no part of the particular snap-action contact which moves during the switching is displaced. As an inherently stable component, the rocker can be produced from a temperature-stable and electrically insulating material. As a result, special insulation means between the two snap-action contacts are superfluous.

As a result of the arrangement of the rocker, it is possible to dispose the two snapaction contacts in the same plane. This permits a small installation width of the switching device. The two adjusting members are preferably accessible from the same side of the housing.

The assembly and mounting of the switching device are simple. Component tolerances can be compensated for by the said possibilities for adjustment.

Advantageous developments of the invention can be seen from the sub-claims and the following description of preferred examples that are illustrated in the accompanying drawings, in which: Figure 1 is a plan view of a switching de vice according to the invention, Figure 2 is a partial view of the switching device of Figure 1, Figure 3 is a view on the line Ill-Ill of Figure 2, Figure 4 is a view similar to Figure 1 of a second embodiment of switching device according to the invention, Figure 5 is a view similar to Figure 1 of a third embodiment of switching device according to the invention, Figure 6 is another similar view of yet a further embodiment of switching device according to the invention.

Referring to Figures 1-3 of the drawings, the switching device comprises a housing 1 in which a guide passage 2 is formed. At the outside of the housing, the guide passage 2 is covered by a plate 3 which comprises a dome-shaped middle region 4. The plate 3 is secured to the housing by means of bolts 5 and nuts 6 inserted in the housing 1.

An expansion rod 8 with a high coefficient of thermal expansion, which is ilustrated shortened in the Figures, projects through a wide central aperture 7 in the middle region 4 of the plate 3. It projects substantially over the circular heating zone 9, illustrated in chain line in Figure 1, of a heating element of a cooking appliance not otherwise illustrated. At the outside of the housing, a bush 10 of ceramic material is seated on the dome-shaped middle region 4 and rests with annular contact on the middle region 4. Supported in the bush 10 is a quartz tube 11 with a low coefficient of thermal expansion which bears, at its end 12 remote from the bush 10, against a stop 13 formed on the expansion rod 8.Inside the guide passage 2, a compression spring 14 is placed round the expansion rod 8 and is supported, on the one hand against the inside of the plate 3 and on the other hand, via a guide sleeve 15, on a head 16 of the expansion rod 8. The guide sleeve 15 is urged against the head 16 by the compression spring 14.

The guide sleeve 15 has a cylindrical (see Figures 1 and 5) or spherical (see Figure 6), surface 17 which lies inside the guide passage 2 in every expansion position of the expansion rod 8 and the diameter of which has only a slight clearance in relation to the free-space sectional area of the guide passage 2. As a result, the effect is achieved that in the event of tilting of the expansion rod 8 with the quartz tube 11, the head 16 is only slightly displaced. The guide sleeve 15 ensures that, in the event of such a horizontal swing of the expansion rod 8, the pivot point lies in the region of the head 16. In the event of a horizontal swing of the expansion rod 8, the annular contact area of the bush 10 on the dome-shaped middle region 4 is displaced. A measure to compensate the displacement of the head 16, which is slight because of the slight clearance, is described below.

Mounted in the housing 1 is an inherently stable, electrically non-conducting rocker 18, for example of ceramic or a high-heat-resisting plastics material, which is provided, on its one arm 19, with an actuating surface 20 and on its other arm 21, with an actuating surface 22. Between the two actuating surfaces 20 and 22, the rocker 18 rests on a support 23 formed on the housing 1. The arm 19 of the rocker 18 is acted upon, at its side remote from the actuating surface 20, by a hairpin spring 24 which urges the rocker 18 against the head 16 at the side of the arm 21 remote from the actuating surface 22. The distances of the actuating surfaces 20, 22 from the support 23 are substantially equal. The distance of the head 16 from the support 23 is less than the distance of the actuating surface 22 from the support 23.As a result, the movement distances of the actuating surfaces 20 and 22 are greater than a travel of the head 16 depending on temperature, the movement distance of the actuating surface 20 being considerably greater than that of the actuating surface 22.

Mounted in the housing 1 are a first snapaction contact 25 and a second snap-action contact 26. The two snap-action contacts 25, 26 extend beside one another in a common plane E. Thus they are not one above the other. As a result, the housing 1 has a narrow width B.

Each snap-action contact 25, 26 comprises a movable contact blade 27 which is braced, by a spring clip 28 formed thereon, against an angled portion 29 of a contact carrier 30. The contact blade 27 is rigidly connected, at its end remote from a contact head 31, to a free adjusting end 32 of the contact carrier 30 which is rigid in comparison with the contact blade 27. Formed on the contact blade 27 between the adjusting end 32 and the angled portion 29 is a hump-like striking point 33.

The striking point 33 of the snap-action contact 25 is situated opposite the actuating surface 20. The striking point 33 of the snapaction contact 26 is situated opposite the actuating surface 22.

At their ends remote from the adjusting ends 32, the contact carriers 30 end in clips 34. The clip 34 of the snap-action contact 25 is riveted to a plug 35. The clip 34 of the snap-action contact 26 is riveted to a plug 36.

The snap-action contact 25 further comprises a fixed contact 37 with a plug 38. The snap-action contact 26 correspondingly comprises a fixed contact 39 with a plug 40. The plugs 35 and 38 of the snap-action contact 25 are situated at the one side of the housing 1. The plugs 36 and 40 of the snap-action contact 26 are situated at the opposite side of the housing 1.

Associated with each of the two adjusting ends 32 is an adjusting screw 41 and 42 respectively which are guided in nuts 43 and 44 respectively let into the housing 1. On adjustment of the adjusting screws 41, 42, the particular contact carrier 30 bends.

In the embodiment shown in Figure 1, the rocker 18 is held against slipping off the support 23 by means of webs 45 and 46, one at each side, which are formed on the housing 1.

In the embodiment shown in Figure 4, the rocker 18 is superimposed on the support 23 by means of a groove 47 formed therein so that it cannot slip.

In the embodiment shown in Figure 5, the head 16 lies in a depression 48 in the rocker 18 so that here, too, this cannot slide off the support 23.

In the embodiment shown in Figure 6, the hairpin spring 24 comprises, at one end, a bent portion 51 which engages in a notch 52 in the arm 19. A further bent portion 53 at the other end of the hairpin spring 52 engages in a depression 54 in the housing 1.

The rocker 18 is also prevented from being able to be displaced on the support 23 by this means.

A cover 49 (see Figure 3), not illustrated in Figures 1, 2, 4 5 and 6, is provided for the housing 1 and also covers the guide passage 2 and forms a stop for the guide sleeve 15.

In the embodiments shown in Figures 1, 2 and 4, the head 16 comprises a rounded portion 50 with a radius of curvature Ra about a centre which lies in the domeshaped middle region 4 (see Figure 2). The dome of the dome-shaped middle region 4 has a radius of curvature Rb about a centre which lies in the region of the head 16 (see Figure 2). The two radii are substantially equal. As a result, the effect is achieved that when the head 16 is deflected horizontally within the scope of the slight clearance mentioned above, its point of contact with the rocker 18 is not displaced. In the embodiment shown in Figures 5 and 6, the radius Ra of the rounded portion 50 of the head 16 is smaller in comparison.

The mode of operation of the device described is substantially as follows: The first snap-action contact 25 actuates an indicating device which delivers a signal above a certain temperature at which it is dangerous to touch the hot-plate. This temperature may be 60"C for example. The second snap-action contact 26 switches off the heating element above a specified temperature, for example 700"C. Below this temperature, it is closed. In the Figures, an operating position of the rocker 18 is illustrated at an intermediate temperature at which the hot-plate is switched on and the indicating device is delivering a signal. The actuating surfaces 20 and 22 of the rocker 18 are not touching the striking points 33 of the snap-action contacts 25 and 26.

If, starting from the position of the rocker illustrated in the Figure, the hot-plate is further heated, the expansion rod 8 becomes longer so that it deflects the rocker 18 in counterclockwise direction. The actuating surface 22 then impinges on the striking point 33 so that the snap-action contact 26 opens and switches off the heating element. The snap-action contact 25 remains closed. After a certain drop in temperature, the snap-action contact 26 closes again. The indicating device still remains switched on. The temperature of the hot-plate oscillates about the limiting value.

When the heating element of the hot-plate is switched off by means of a further control device not illustrated, the temperature will gradually drop. In the course of this, the expansion rod 8 contracts as a result of which the rocker 18 is deflected in clockwise direction under the action of the hairpin spring 24. At a temperature beyond which it is possible to touch the hot-plate without danger, the contact surface 20 impinges on the striking point 33 of the snap-action contact 25 so that this opens under the action of the hairpin spring 24. As a result, the indicating device is switched off.

In order to adjust the response temperature of the snap-action contact 25, the expansion rod 8 could be brought to the temperature in question and the adjusting screw 41 be adjusted so that the snap-action contact 25 responds. Preferably, however, the switching distance is adjusted with a gauge which corresponds to the known travel of the expansion rod 8 at the response temperature. The adjustment of the snap-action contact 26 to its switching temperature is carried out in a corresponding manner by means of the screw 42. The adjustment of the two snap-action contacts 25 and 26 is independent of one another. Any tolerances in the components are also compensated for by the adjustment.

A tilting of the expansion rod 8 has practically no influence on the adjustment.

The adjustment does not alter even after a long period of operation, since the operating points are not dependent on components which alter their shape as a result of ageing.

In particular, a sliding disc of Teflon (RTM) between the quartz tube 11 and the domeshaped middle region 4, which disc would settie down in the course of time, is superfluous.

Any desired transmission ratio between the travel of the head 16 and the actuating surfaces 20 and 22 respectively can be achieved by the selection of the arm lengths of the rocker 18, of the position of the support 23 and the point of action of the head 16. In the illustrated examples, the lengths of travel of the actuating surfaces 20, 22 of the rocker are stepped up in comparison with the travel of the expansion rod 8. An increase in sensitivity is afforded by a corresponding transmission ratio in such a manner that with only a relatively short travel of the temperature-sensi tive expansion rod 8, a comparatively greater length of travel of the rocker 18 is achieved at its actuating surfaces 20, 22. With appropriate dimensioning of the transmission ratio, therefore, the necessary length of travel can be achieved with expansion rods of different length or different lengths of travel can be achieved with expansion rods of the same length.

The construction of the switching device described is easily understood and easy to handle because the expansion rod is disposed at the one side of the housing and the two adjusting screws 41 and 42 are disposed at the opposite side and the plugs 35, 38 and 36, 40 are provided at the other two sides. In addition, the assembly of the components is easily possible from the one open side of the housing 1. The rocker 18 is also inserted through this open side.

Claims (14)

1. A switching device for electric cooking appliances wherein the arrangement is such that a single expansion rod switches, through a component which can be pivoted by it and is mounted on a housing of the device, a first snap-action contact at a lower temperature and a second snap-action contact at a higher temperature, for which purpose the component has two actuating portions which are spaced apart from one another and each of which is associated with a striking point of one of the snap-action contacts, and wherein each of the snap-action contacts is provided with its own adjusting member, the said pivotable component being formed by an inherently stable rocker which is mounted for rocking on the housing at a point located between the actuating portions, a head of the expansion rod resting on the rocker and the rocker being urged towards the head by means of a spring, the force of the spring being such that it is capable of switching one of the snapaction contacts, and both the adjusting members being mounted on the housing and being contiguous with the snap-action contact in question at a point which is unmoved during the switching by the rocker.

2. A switching device as claimed in Claim 1, wherein both snap-action contacts extend in a common plane.

3. A switching device as claimed in Claim 1 or 2, wherein the adjusting members are both disposed at the same side of the housing, which side is situated opposite to a side of the housing from which the expansion rod extends.

4. A switching device as claimed in any one of the preceding Claims, wherein the arrangement is such that the adjusting members each act on an adjusting end of the snap-action contacts, to which end a contact blade with a contact carrier is connected.

5. A switching device as claimed in any one of the preceding Claims, wherein, in the region of its head, the expansion rod is mounted with little play in a guide passage of the housing by means of a guide sleeve.

6. A switching device as claimed in any one of the preceding Claims, wherein the head of said expansion rod rests on the rocker with a rounded portion the radius of curvature of which is substantially equal to the radius of curvature of a dome-shaped middle region of a plate on which the expansion rod is supported on the housing by way of a tube.

7. A switching device as claimed in any one of the preceding Claims, wherein the said rocker consists of a non-electrically conducting ceramic or plastics material.

8. A switching device as claimed in any one of the preceding Claims, wherein the faces of the actuating portions of the rocker are so arranged that they do not touch the striking points of the first snap-action contact and of the second snap-action contact at temperatures between the lower temperature at which the first snap-action contact operates and the higher temperature at which the second snapaction contact operates.

9. A switching device as claimed in any one of the preceding Claims, wherein the arrangement is such that between its actuating portions the rocker lies flat on a support of the housing and the two arms of the rocker are guided laterally by webs.

10. A switching device as claimed in any one of the preceding Claims, wherein the rocker has a groove engaging a support of the housing.

11. A switching device as claimed in one of Claims 1 to 9, wherein the head of the expansion rod engages in a depression in the rocker.

12. A switching device as claimed in one of Claims 1 to 9, wherein the said spring has a bent portion which engages in a notch in the rocker.

13. A switching device substantially as described herein with reference to Figures 1-3, Figure 4 or Figure 5 of the drawings.

14. A switching device substantially as described herein with reference to Figure 6 of the drawings.