GB2175141A - Switching device - Google Patents

Abstract

A thermal switching device for electric cooking appliances comprises a spring biased expansion rod (16) which extends in a tube (11) pivotally mounted on a domed region (4) disposed on a housing (1). In order to prevent a horizontal deflection of the expansion rod (16) from having an effect on the point of support on a contact actuating member (18), the expansion rod (16) is mounted with little play in the region of its head, by means of a guide member (15) located in a guide passage (2) in the housing (1). <IMAGE>

Description

SPECIFICATION Switching device This invention relates to a switching device for electric cooking appliances wherein, disposed on a housing is an expansion rod which extends in a tube which is pivotally mounted on a dome-shaped region of the housing, the expansion rod comprising, in the housing, a head which bears against a member of a contact mechanism and is held prestressed by means of a spring disposed in a guide passage in the housing.

Such a switching device is described in the DE-OS 34 23 085. In such switching devices, the tube with the expansion rod should be pivotable in relation to the housing within certain limits. The dome-shaped region on which the tube is supported is provided as a bearing. During the horizontal deflection, the fragile tube should not break. In the DE-OS 34 23 085, therefore, the end of the tube standing on the dome is of funnel-shaped construction.

It was also proposed to lubricate the domeshaped region or to place a sliding disc of Teflon (RTM) thereon. Such a sliding disc has the disadvantage that it settles down in the course of time as a result of which the operating point of the switching device is altered.

In the known switching devices, the head of the expansion rod participates in the horizontal deflection movement. As a result, its point of support on the member of the contact mechanism is altered. This leads to a displacement of the operating point actually desired. Such a displacement takes place during the mounting of the switching device on the cooking appliance. This is particularly undesirable because the operating points of the switching device are adjusted before installation and, in itself, a readjustment after the installation should be superfluous.

It is the object of the invention to provide a switching device of the type initially mentioned wherein a horizontal deflection of the expansion rod is possible without substantial influence on the point of support of the head on the member of the contact mechanism.

According to the invention, the above problem is solved in a switching device of the type referred to in that the expansion rod is mounted with little play in the region of its head, by means of a guide member in the guide passage. The necessary horizontal deflection of the expansion rod with the tube in relation to the housing is not eliminated as a result. The effect is achieved, however, that the head does not participate in the deflection.

The guide member ensures that the pivot point of the deflection lies defined in the region of the head. The guide member does not hamper the movement of the head effected, depending on temperature, in the longitudinal direction of the expansion rod.

The invention can be used with the most varied switching devices. For example, it can be used in those switching devices in which the member of the contact mechanism is a switching rocker or a switching push-rod. The advantages of the invention are effective in switching devices in which either only one switch contact is provided or two switch contacts are provided.

Advantageous developments of the invention are apparent from the sub-claims and the following description of preferred examples illustrated in the accompanying drawings, in which: Figure 1 is a plan view of a switching device according to the invention, Figure 2 is a partial view of the switching device of Fig. 1, Figure 3 is a view on the line Ill-Ill of Fig. 2, and Figure 4 is a view corresponding to Fig. 2 of another embodiment of the invention.

Referring to 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 domeshaped 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 illustrated shortened in the Figures, projects through a wide central opening 7 in the 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. Mounted on the dome-shaped region 4, at the outside of the housing, is a bush 10 of ceramic material which makes annular contact on the 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 ?0, against a stop 13 formed on the expansion rod 8.

Laid round the expansion rod 8, inside the guide passage 2, is a compression spring 14 which is supported, on the one hand, at the inside, against the plate 3 and, on the other hand, through a guide sleeve 15, against a head 16 of the expansion rod 8. A bottom 17' of the guide sleeve 15 is urged against the head 16 by the compression spring 14. A central bore in the bottom 17' engages closely round the expansion rod 8 adjacent to the head 16.

The guide sleeve 15 has a cylindrical peripheral surface 17 (see Figues 1, 2) which still 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 cross-sectional area of the guide passage 2. In the embodiment shown in Fig. 4, the peripheral surface 51 is spherical in construction. 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 at most slightly displaced. The guide sleeve 15 ensures that the pivot point lies in the region of the head 16 in the event of such a deflection of the expansion rod 8. During a horizontal deflection of the expansion rod 8, the annular contact surface of the bush 10 on the domeshaped region 4 is displaced.A measure to compensate for the slight displacement of the head 16 as a result of the slight play is described below.

Mounted in the housing 1 is an inherently stable, electrically non-conducting rocker 18, for example of ceramic material or a high-heatresistant synthetic 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. At its side remote from the actuating surface 20, the arm 19 of the rocker 18 is acted upon 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 shorter than the distance of the actuating surface 22 from the support 23.As a result, the lengths of movement of the actuating surfaces 20 and 22 are greater than a travel of the head 16, depending on temperature, the length of movement 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 do not lie 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 against an angled portion 29 of a contact carrier 30 by a spring clip 28 formed thereon. At its end remote from a contact head 31, the contact blade 27 is rigidly connected to a free adjusting end 32 of the contact carrier 30 which is rigid in comparison with the contact blade 27. A hump-like striking point 33 is formed on the contact blade 27 between the adjusting end 32 and the angled portion 29.

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.

The contact carriers 30 end in clips 34 at their ends remote from the adjusting ends 32.

The clip 34 of the snap-action contact 25 is riveted to a plug 35. The clip 34 of the snapaction 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 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. When the adjusting screws 41, 42 are adjusted, the particular contact carrier 30 bends.

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.

Provided on the housing 1 is a cover 49 (see Fig. 3) which also covers the guide passage 2 and forms a stop for the guide sleeve 15.

The head 16 comprises a rounded portion 50 with a radius Ra about a centre which lies in the dome-shaped region 4 (see Fig.2). The dome of the dome-shaped region 4 has a radius Rb about a centre which lies in the region of the head 16 (see Fig. 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 play mentioned above, its point of contact with the rocker 18 is not displaced.

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 touching the hot-plate is dangerous. 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 a switch 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 18 illustrated in the Figure, the hot-plate is heated further, then the expansion rod 8 becomes longer so that it deflects the rocker 18 in counter-clockwise 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 she 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 relevant temperature 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 operating temperature is correspondingly effected 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. Tilting of the expansion rod 8 has practically no influence on the adjustment. Thus the adjustment can be carried out before the switching device is mounted on the cooking appliance. Any horizontal deflection or tilting of the expansion rod 8 with the tube 11 resulting during this does not alter the adjustment.

Claims (9)

1. A switching device for electric cooking appliances, comprising an expansion rod which extends in a tube which is disposed on a housing of the device and pivotally mounted on a dome-shaped region of the housing, said expansion rod including, in the housing, a head which is spring urged towards a contact mechanism by means of a spring disposed in a guide passage in the housing, and the arrangement being such that the expansion rod is mounted with little play, in the region of its head, by means of a guide member located in the guide passage.

2. A switching device as claimed in Claim 1, in which said guide member is formed by a guide sleeve which comprises a peripheral surface and a bottom portion, the peripheral surface bearing against the inside of the guide passage and the bottom portion being urged against the head of said expansion rod by the said spring.

3. A switching device as claimed in Claim 2, in which the peripheral surface of said sleeve is so dimensioned that it lies in the guide passage in every expansion position of the expansion rod.

4. A switching device as claimed in any preceding Claim, in which the guide passage is covered by a cover which forms a stop for the said peripheral surface of the guide sleeve.

5. A switching device as claimed in any preceding Claim, in which the head of said expansion rod has a rounded portion which engages a member of the said contact mechanism with the radius of curvature of said rounded portion being substantially equal to the radius of curvature of the said domeshaped region of the housing.

6. A switching device as claimed in Claim 2 or 3 or Claim 4 or 5 as appended thereto, in which the peripheral surface of said guide member is spherical.

7. A switching device as claimed in Claim 2 or 3 or Claim 4 or 5 as appended thereto, in which the peripheral surface of said guide member is cylindrical.

8. A switching device substantially as described herein with reference to Figs. 1-3 of the accompanying drawings.

9. A switching device substantially as described herein with reference to Figs. 1-3 as modified by Fig. 4 of the accompanying drawings.