GB2168535A - Thermostats - Google Patents

Abstract

A thermostat has a plate (9) mounted on the end of a rotatable temperature setting spindle (1) and having three projections (10); the projections (10) are angularly spaced and at different radii and engage a non-rotatable intermediate plate (11). The intermediate plate (11) engages an expansible capsule (4) mounted on a pivoted lever (5) and in communication with a temperature sensing file. The lever (5) operates a contact carrier (7) through a calibration screw (6). The intermediate plate (11) has three holes or recesses which receive the three projections (10) at a predetermined angular position of the operating spindle (1) to provide an abrupt change in temperature setting.

Description

SPECIFICATION Thermostats This invention relates to thermostats of the type used for the control of domestic cooking appliances and central heating boilers.

The invention provides a means whereby the substantially linear adjustment to temperature produced by a precision lead screw mechanism is modified to provide an abrupt change in temperature setting by the inclusion of a cam device. Such an abrupt change in temperature setting may be an increase in temperature setting, or a decrease in temperature setting. Similarly both features may be provided at angularly spaced positions of the lead screw mechanism.

A detent or indexing feel may also be provided by the cam mechanism which provides the abrupt change in temperature setting.

In known thermostat constructions the various features described have been achieved by cam mechanisms, without a lead screw, but it is both difficult and expensive to produce moulded or cast cams to the degree of accuracy which is readily attainable with a lead screw mechanism.

Two examples are given in which an abrupt change of temperature setting is required.

On some domestic cooking appliances a grilling heater is built into the same compartment as that which provides normal cooking operations. For such applications the thermostat carries a normal cooking range covering OFF, and low temperatures up to high temperatures. In the OFF position an auxiliary switch is arranged to isolate the oven heater elements, and to make power available to feed a grill element within the same oven space. In this nominally OFF position, the thermally operated contacts of the thermostat need to function at a high temperature so as to limit the temperature which could be attained within the oven if the grill element were to be continuously energised.

When a thermostat is turned to the OFF position on a central heating boiler for example, it is desirable that the contacts should remain open irrespective of the temperature at the sensing phial. The invention disclosed by this specification ensures that the contacts of such a thermostat would remain open in the OFF position irrespective of the temperature at the sensing phial.

Reference to the enclosed figures will clarify the way in which the invention provides the foregoing functions: Figure 1 depicts a conventional thermostat using a lead screw mechanism.

Figure 2 depicts one embodiment of the invention and illustrates the principles involved.

Known thermostats employing lead screw mechanisms utilise the general construction shown in Fig. 1 wherein the operating spindle (1) with lead screw thread (2) is rotatable by the user to select appropriate control temperature settings. The lead screw thread engages a threaded portion of the housing (3).

An expansible capsule (4) engages the end of the operating spindle (1). The expansible capsule (4) communicates with a temperature sensing phial by means of a capillary tube (neither of which are shown), the expansible capsule, phial, and capillary being filled with a heat-expansible fluid whereby an increase in phial temperature causes an expansion of the capsule (4). The expansible capsule (4) is mounted on a pivotted lever (5).

Expansion and contraction movements of the capsule (4) are transmitted by the pivotted lever (5) to the preset calibration screw (6) mounted in the contact carrier (7). Expansion of the capsule (4) causes the contact carrier (7) and stationary contact (8) to become separated when the desired temperature has been attained.

It will be appreciated that the majority of thermostats incorporate a mechanism which ensures quick opening and closing of the contacts. Such mechanisms lie outside the scope of the present invention, and the mechanism already described is adequate for clarity.

Fig. 2 shows an embodiment of the invention within a thermostat having a lead screw mechanism generally as depicted in Fig. 1.

A plate (9) engages the end of operating spindle (1) and is rotatable by the operating spindle. In a preferred construction the plate (9) has three projecting portions (10). The three projecting portions (10) are equispaced in angular relationship, but are positioned at three different radii. An intermediate plate (11) is supported by the three projecting portions (10). The intermediate plate (10) is prevented from rotation by being slideably engaged with a guide, or guides (12).

The centre of the intermediate plate (11) engages the expansible capsule (4). Thus the centre of the intermediate plate (11) provides an engagement for the expansible capsule which is equivalent to the support provided by the end of the operating spindle (1) in Fig. 1.

Rotation of the operating spindle (1) will lift and lower the intermediate plate in accordance with the lead of the thread (2) and thereby vary the temperature at which the contacts separate in a similar manner to that described for the construction of Fig. 1.

The intermediate plate (11) is provided with three holes or recesses (not shown) which receive the three projecting portions (10) at a predetermined angular position of the operating spindle (1). When the operating spindle (1) is rotated to the position in which the three projecting pins (10) engage the holes or recesses in the intermediate plate (11), the intermediate plate (11) moves towards the plate (9).

This movement of intermediate plate (11) changes the level of support for the expansible capsule (4) and causes an abrupt change in the temperature required to separate the contacts (7), (8).

Because the projecting pins (10) and corresponding holes or recesses in the intermediate plate (11) are on radii which match one another for only a limited angular motion of the operating spindle (1), the cam action is achieved when the pins engage the holes or recesses. For other angular positions of the operating spindle (1) and plate (9), the intermediate plate (11) follows the lead of the screw thread (2).

At the moment of engagement and disengagement of the projecting pins (10) and holes or recesses in the intermediate plate (11), a detent or indexing feel may be obtained. This feel may be varied by choice of suitable angles or radii on the projecting pins (10) and around the holes or recesses in the intermediate plate (11).

The construction so far described provides an abrupt raising of the temperature at which the contacts would open with the projecting pins (10) engaging the holes or recesses in the intermediate plate (11).

The intermediate plate (11) may be produced with projecting portions instead of the holes or recesses. Such projecting portions would be formed towards the plate (9), thereby lowering in an abrupt manner the temperature at which the contacts (7), (8), would separate.

When close control is needed of the magnitude of the abrupt change in temperature control, appropriate screw stops may be included.

For example, screw stops may be used to limit the reduction in dimension between intermediate plate (11) and plate (9) when rotated to a position at which the projecting pins (10) engage the mating holes or recesses.

On known thermostats the switching action for such features as isolating oven heaters and introducing grill elements is achieved by means of a cam driven from the rotatable spindle (1), the cam being arranged to operate electrical connections.

By use of the present invention it is possible to utilise the abrupt movement of the intermediate plate (11) to drive an electrical switch. For example, a push rod (13) may be used to operate electrical contacts (14).

The arrangements described are by way of example only, and it will be clear to those skilled in the art that many equivalent constructions are possible whilst retaining the same principles.

Claims (5)

1. A thermostat in which the substantially linear adjustment of temperature produced by a precision lead screw is modified to provide an abrupt change in temperature setting.

2. As Claim 1 in which the abrupt change in temperature setting is produced by a cam mechanism.

3. As Claim 2 in which the cam mechanism consists of a plate driven by the operating spindle co-operating with a non-rotatable plate which transmits positional movements to a thermal actuating mechanism, the plates being formed with mating projections and recesses or holes.

4. As Claim 2, in which the cam mechanism consists of a plate driven by the operating spindle co-operating with a non-rotatable plate which transmits positional movements to a thermal activating mechanism, both plates being formed with mating projections.

5. A thermostat substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

5. As Claim 3, 4, in which movement of the non-rotatable plate is used to operate an electrical switch.

6. As Claim 2, 3, 4, 5, in which the cam mechanism also produces a detent or indexing feel.

7. A thermostat substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

CLAIMS Superseded claims 1 to 7 New or amended claims:

1. A thermostat including a switch, a temperature responsive element coupled to the switch, a lead screw which provides the substantially linear adjustment of the temperature at which the temperature responsive element actuates the switch, and a mechanism through which the lead screw is coupled to the switch and which provides an abrupt change of temperature of the temperature setting in a particular angular position of the lead screw, said mechanism comprising a member coupled to the lead screw and a non-rotatable member coupled to the switch, either of the members being formed with projections and the other member with recesses with which said projections mate in said particular angular position of the lead screw or both members being formed with projections which mate in said particular angular position of the lead screw.

2. A thermostat as claimed in claim 1 in which the members are plates.

3. A thermostat as claimed in claim 1 or claim 2 in which the member coupled to the switch is coupled thereto through the temperature responsive element.

4. A thermostat as claimed in any of claims 1 to 3 in which the mechanism produces a detent or indexing feel.