GB2146843A - Temperature responsive control devices - Google Patents

Abstract

A temperature responsive control device includes a stack of domed bimetal discs (31) which transmit movement via a tappet (35) of top hat shape lightly held against the stack by carrier spring (41) to a pressure point (26) of an overcentre mechanism (23) carrying a movable contact element (28) bridging a pair of fixed contact elements (19) to cause the movable contact element to separate from the fixed contact elements with a snap action at a temperature determined by the setting of an adjustable setting device (37). The tappet (35) is rigid in relation to force transmitted by it from the stack (31) to the pressure point (26) and operates substantially free from frictional constraint. The contact element (28) includes dome-shaped contact faces and the fixed contact elements (19) are sections of a strip which is D-shape in transverse cross-section. The device may alternatively operate a valve. <IMAGE>

Description

SPECIFICATION Temperature responsive control device This invention relates to atemperature responsive control device (hereinafter referred to as being ofthe kind specified) comprising a body, cooperative electrical contact elements supported therefrom and relatively movable between open and closed positions, an operating element providing a mechanical output in response to change oftemperature, and an over-centre mechanism operatively interposed between the operating element and the contacts for causing same to move relatively with snap-action between said open and closed positions.

One of the problems which is encountered in obtaining operation of a temperature control device of the kind specified reliably at a predetermined temperature isthatwhich arises from variation in the frictional resistance to operation. Such frictional resistance may vary significantly between nominally identical devicesofthe kind specified and in a single device ofthe kind specified over the passage oftime.

Relative movement between the cooperative contact elements can be effected by employment of an over-centre mechanism comprising a blade of a configuration such as to snap-overfrom one bowed positionto an oppositely bowed position, the blade carrying the "movable" contact element or elements and the cooperative contact element or elements being "fixed" relativelyto the body (in the sense that it or they are not movable during operation of the switch, although possibly being adjustable in position should this be required). Such an arrangement eliminates to a large degree any variable frictional forces in partofthe mechanism butvariablefrictional forces can arise at the site where pressure is transmitted to a pressure "point" on the blade from the operatingelementwhich provides the mechanical output in response to change of temperature.

Oneofthe objects ofthe present invention is to provide a new or improved structural arrangement by means of which variable frictional force is eliminated or very much reduced.

According to one aspect ofthe invention a tempera ture responsive control device ofthe kind specified is prnvided wherein the operating element comprises a plurality of bi-metal plates each of which is concave and convex at respective opposite faces which are assembled to form a stack in which at least certain of the plates have their concave faces presented towards opposite ends ofthe stack, the heightofwhich,fora given number of plates, varies as a function of temperature.

Itwill be understood that ordinarilythe bi-metal plates will be assembled inthestackin a mannersuch that successive pairs of plates, starting from one end of the stack, have their concave faces presented respectively towards each other. Itwould be possible, however, for the plates to be assembled in successive groups along the stack in such a manner that the plates in each group are mutually nested, while successive groups have the concave faces oftheir nested plates presented in opposite directions towards respective opposite ends ofthe stack.

The bi-metal plates may be aperture in their central region and may be mounted on a pin passing through the apertures. The stack of bi-metal plates may be interposed between the body of the device and a pressure point at the over-centre mechanism atwhich the application of pressure causes itto move through the over-centre position.

According to a further aspect ofthe invention, a temperature responsive control device ofthe kind specified is provided with a tappet to transmit pressure between the operating elements which is operatively interposed between the bodyofthe device and a pressure point ofthe over-centre mechanism, such tappet presenting oppositely directed contact faces at respective opposite ends formed on a body which is rigid in the direction in which it transmits force to the pressure point. The operating element may comprise the stack above referred to.

Such tappet may comprise a bush located in an opening formed in a blade spring applying compressive loading to the operating element.

The bush may fit loosely over the pin at the free end ofthe pin and has internal clearance with respectto the end face ofthefree end of the pin.

With this arrangement it has been found that there is practically no frictional force resisting movement where movement istransmitted from the end of the stack nea rest to the over-centre mechanism to the over-centre mechanism itselfthrough the intermediary ofthe tappet. The blade spring may be so designed as to exert only light compressive pressure in a direction longitudinally of the stack sufficient to maintain successive bi-metal plates in contactwith each other without detracting significantly from the output force which they are able to produce and which is considerably greaterthan that required to cause the over-centre mechanism to move through its centre position.

Afurther problem which is encountered in control devices ofthe kind specified is deterioration due to malfunctioning of the cooperative contact elements.

It is common practice to construct at least one such contact elements of a base part made of a common electrically conductive metal such as brass or copper, e.g. of strip form, to which is secured a contact element proper of a metal selected to withstand marking during repeated opening and closing ofthe contact elements, this metal typically being silver or monel (RTM) metal.

If the relative configurations of the cooperative contact elements are not such that the actual area of engagement is sited and remains sited in the central region ofthesilverormonel (RTM) metal contact element proper but, for example, drifts towards the edge ofthis element proper, then damaging sputtering tends to take place byvapourisation ofthe brass or copper base element and the sputtered metal finds its way to quite distant parts of the device as a whole, typically parts removed by as much as 12 mm. to 15 mm. from the site of the arc.

This effect can be minimised if the contact elements proper made of said selected metal, e.g. silver or monel (RTM), are of dome-shape but such domeshaped contact elements proper are expensive to produce and apply to the base element.

Afurtherobject ofthe invention isto minimise or avoid the destructive vapourisation or sputtering effect without unduly increasing the cost of manufacture ofthe device.

Thus, according to a furtherfeature ofthe invention, the cooperative contact elements comprise one having a dome-shaped contact face and another having a contactface which is of convex arcuate shape in a first cross-sectional plane but substantially rectilinear shape in a second cross-sectional plane at right angles to the first plane. The dome-shaped contact face may be presented by a pressed portion ofthe base metal strip such as copper or brass and the contactface of convex arcuate form may be presented by a silver or otherarc resisting metal contact element proper carried bya base metal member. Such silverorother arc resisting metal contact element proper may comprise a cut-off section of a D-section strip which can be economically manufactured and economically applied to the base metal member in an automatic attachment operation.

It is common practice in a device of the kind specified to provide as the operating element an expansible chamber (herein called "the capsule" which is connected to a remotely positioned tempera ture sensing chamber herein called "the bulb" byway of a conduit with the two chambers and the conduit containing a fluid which undergoes expansion in response to temperature increase. At a certain bulb temperature (the control temperature), the capsule has a wall or diaphragm which then undergoes displacement or exerts pressure on the over-centre mechanism to produce snap over of the contacts between closed and open positions.

In certain cases, it may not be necessary for the control temperature to be sensed art a position remote from the device of the kind specified, and although in such cases it may be practicableto mountthe bulb adjacenttothe bodyofthe device itself, this may be inconvenient because of space and cost considerations.

Further, economies may be achieved by adopting a design, construction, and arrangement, of certain of the components of the device, other than the operating element which can be adopted in common to meet the two requirements, namely where the control temperature isto be sensed ata remote position and atthe position at which the body ofthe device isto be mounted.

In accordance with a further feature ofthe invention, the body and the over-centre mechanism of a device of the kind specified is formed, constructed or arranged in such a mannerastoaffordapocketforthe reception ofthestackofbi-metal elements, or alternativelyforthe reception of an expansible chamber connected, or adapted for connection, to a bulb containing fluid,the pressure of which changes in response to temperature changes.

The invention will now be described by way of example, with reference to the accompanying draw ing wherein:- FIGURE lisa view in side elevation illustrating diagrammatically and partly in cross-section one embodiment of device ofthe kind specified incorpor ating the present invention; FIGURE 2 is a fragmentary view in end elevation showing one form of fixed and movable contact elements in accordance with the invention.

In this embodiment, the device is utilised to operate electrical switch contacts in accordance with a sensed (control) temperature.

The device comprises a body 10 having an upper sheet metal member 11 and a lower sheet metal member 12 connected at one end byan end member 13 also of sheet metal.

At one end of thins structure, a terminal carrier is provided comprising upper and lower blocks 14,15 of insulating material. These blocks are formed with seatings comprising longitudinal grooves for receiving electrically conductive strips 16 and 17, of which the former is provided with a terminal screw 18 externally ofthe body, and the lattercarries a fixed contact element proper 19 within the body. In practice, a plurality of members such as 16 and 17 may be provided in laterally spaced relation having a corresponding number of fixed contact elements proper such as 19 and terminal screws such as 18.For example, in a simple arrangementtherewould be two such sets of strips and therefore two laterally spaced contact elements proper 19 adapted to be bridged by a movable contact when the latter is in its closed position, as hereinafter described.

The upper and lower blocks 14 may be secured in position and clamped between the upper and lower members 11 and 12 ofthe body by one or more screws 20 spaced laterallyfrom the positions occupied by the strips 16 and 17.

Clamped between the upper member 1 ofthe body and the upper block 14 is a strip 21 of spring metal provided at its free end with an inverted U-shaped carrier 22 for an over-centre mechanism 23 comprising a resilient metal blade 24 which has an elongated central aperture in which is mounted a bowed and compressively stressed leaf spring 25.

The blade 24 presents a pressure point 26 against which force may be applied to cause the blade 24to snap overfrom one stable position as shown to another stable position in which its upperfacewill be bowed in a concave manner.

The blade 24 at one end carries a mounting member 27 of insulating material which is secured to the free end ofthe blade in any suitable manner, and which in turn carries a movable contactelement28which may, as previously indicated, bridge a pai;rof the fixed contact elements proper 19 when the closed position shown.

The upper stable postion oftheblade 24, in which the contact element 2aisseparatedfrom and hence open with respect to thecontactelement proper or elements proper 19, is determined by the setting of an adjustable stop element 29 intheform of a screw made of metal and engaging in athreaded bush or opening inthe upper member 11 ofthe body. The lower end of this screw is engaged buy a partofthe mounting member27and between this part and the contact element 28, the mounting member incorpo rates a projecting fin-like barrier portion 30 to increase the length ofthe path over the surface ofthe mounting member between the electrical contact element 28 which may be regarded as "earthed" with respect to the body.

The conductive strips 17 which form the base part of the fixed contact elements may be made of brass, copper or other suitable base metal and the contact elements proper 19 may be made of arc resisting metal such as silver or monel (RTM) metal. The contact elements proper 19 may be sections of a strip of this material which itself of D-shape in transverse cross-section and can, therefore, be formed economically by any suitable manufacturing process such as stamping, rolling or even extruding, the strip incorporating the contact elements proper being fed automaticallyto an attachmentstation at which the base metal strips 17 are also fed, and attachment between cut-off sections of the former strip may be effected automatically at that station.

As previously mentioned, it is importantto confine the contact area between the contact elements proper 19 and the contact elements 28 to the central region of the upwardly presented contact face of the elements 19 in orderto prevent arcing migrating from taking place near edges 1 7a ofthe base strips from which sputtered or vapourised base metal may migrate to other parts ofthe device.

Accordingly the movable contact elements may comprise dome-shaped pressings 28a of a base metal strip made of brass or copper and supported at its centrefromthemounting member27 in a manner such that it can float or rock about mutually perpendicular axes extending respectively lengthwise of, and at right angles to, the strip 24 in the plane of such strip.

Forthis purpose the bridging contacts 28 are supported looselyatthe lower end of a pin 27a having heads at its upper and lower ends. The lower head 27b may be shaped to cooperate with the internal surface of a pocket formed by an upwardly pressed portion 28c of the strip embodying the movable contacts, the shapeofthepocketandthe head being such asto prevent rotation ofthe strip about the axis of the pin 27a.

For operating the over-centre mechanism comprising the parts 24,25 and carrier 22, a stack of bi-metal plates 31 is provided. Each of these plates may be of circularform and domed, and may be provided with a central aperture enabling itto be threadedwith clearance onto a mounting pin 32 fixed to the lower member 12 ofthe body. In the stack of plates 31,31 a indicates the higher expansion material and 31 b the lower expansion material.

As shown, the plates are so arranged that alternate ly positioned platesalong thestackall presenttheir concave faces towards the lower end of the stack, and the other alternate set of plates present their concave faces all towards the upper end ofthe stack. With this arrangement, which successive pairs of plates have their concave faces presented towards each other and their rims in contact, the individual deformations of the plates asa resu It oftemperature change are additive.

Afeature ofthe invention is that upward movement with increasing temperature is transmitted from the operating element, e.g. the topmost bi-metal plate to the pressure point 26 of the over-centre mechanism through the intermediaryofatappet35 located laterally by reception as a loose or clearance fit in an opening 40 in a light blade spring 41, one portion of which is clamped between the lower block 15 and the lower member 12 of the body and which is stressed in a manner such that it exerts light downward compressive loading on the stack of bi-metal plates 31.

The tappet 35 is of hollowform, preferably of cylindrical top-hat shape, so that it has considerable stiffness in relation to force transmitted to the upwardly presented contact face 35a of its crown from the downwardly presented contact face 35b of its rim orflange which will be in contactwith the topmost bi-metal plate. The tappet is able to transmit upthrust fromtheuppersurfaceofthetopmostbi-metal plate, as a function of the temperature of the stack, to the pressure point 26. The dimensions ofthe tappet between its contact faces are constant. The pin 32 projects into the interior of the tappet 35 but has its upper end face spaced from the crown of the tappet when the latter is in its lowermost position corresponding to minimum stack height.The member 24 preferably includes a downwardly extending portion 26a of shallow dome shape which has "point" contact with the cylindrical upwardly presented contact face 35a of the tappet 35. For practical purposes there is no relative movement between the portion 26 and the tappet 35 wherethey are in contact as a result of upward and downward movement ofthetopmost bi-metal plate ofthe stack 31, nor is there any relative movement between the rim orflange of the tappet and the spring 41 in a plane at right angles to the axis ofthe stackand pin.

Thefactorswhich produce opening ofthe contacts 19,28 at a predetermined temperature which, when set (by the adjustment means hereinafter described), is maintained with a very high degree of consistency over a long service life forthe device are a. the virtual elimination of dimensional variations (due to elastic deformation of components) between the pressure point 26 and the output end of the operating element such as stack31 of bi-metal plates, b. the virtual elimination offrictional resistance opposing height variation ofthe stack 31 due to temperature change, and hence the elimination of variable frictional resistance which would be deleterious to consistent opening ofthe contacts at the predetermined temperature.In this respect it is to be noted thatthere is no sliding movement relatively between the tappet and the operating element such as the stack, spring 41, and pressure point 26 in a plane at right angles two the axis of the stack and pin 32, c. the virtual elimination of wear over the contact surfaces through which the requisite force is transmitted to the pressure point 26 from the operating element such as the stack, and hence the avoidance of local contact surface irregularity which could adversely affect consistency, d. selection of an operating element, namely the stack 31, which provides an outputforceverysubstan- tially higherthan the sum of opposing forces exerted atthe pressure point 26 by the over-centre mechanism and the spring 41. Thus any random variation in resistance to expansion of the stack (from whatever cause) is easily overcome without alteration of the temperature at which the contacts open. In this respect it is to be noted that, although the stack31 is capable of exerting a very high output force, it is, in practice, notcalled upon to do so because both the spring 41 and the over-centre mechanism (when the clearance between the contact face 35a and the pressure point 26 is taken up by expansion ofthe stack) exert only relatively low opposing forces so that contact pressure is low contributing to low wear.

In the illustrated arrangement, plates are used singly so far as the exertion of force is concerned but it would bepossibleto useanyselected numberof plates in a force aiding relationship. For example, in the place of each single plate one may ruse a pairof plates nested so that the concave face of one is against the convexface of the adjacent one of the pair.

If desired, any ofthe members ofthe body, for example the lower member 12 and the end member 13. If appropriate, the body may be formed with holes such astoallowairto percolatefreelythroughthe interior ofthe body in the region of the stack of plates 31.

For controlling the position ofthecarrier22,and hence adjusting the magnitude ofthe control temper ature atwhich snap over ofthe mechanism 24takes place, a setting device may be provided comprising a screw34 rotatable by an operating knob (notshown) and extending through a threaded opening in thetop member 11 ofthe body. The screw 34 may be provided with a pointer movable over and relative to a scale or index element as the case may be. A preset adjustment screw 36 may be provided to enable the initial clearance between the tappet 35 and the pressure point to be set when the contacts 19,28 are closed.

The lower member 12 of the body, the end member 13, the over-centre mechanism and its carrier 22 effectively define a pocket 38 which is able to accommodate an operating element 31 in theform of the stack of bi-metal plates, preferably of dome shape as above described. This pocket may be utilised alternatively to accommodate a capsule having a diaphragm wall. This would enable the device described and illustrated to be utilised where the control temperature isto be sensed at a remote position. In this case, a bulb would be mounted at said remote position and connected to the capsule or bellows by a conduit, which is eitherflexible or bendable.

Instead of being utilised to operate control means in the form of electrical contact elements, the device could provide a mechanical outputto operate some furtherform of control means, i.e. a flow control valve such as a flap or damper, for controlling flow of air through orfrom a duct, as in the case of a space heater where heat is supplied bywayofairtraversing a heated core or storage member, preferably electrically heated during off-peak (low tariff) periods of an electrical supply system.

Claims (11)

1. Atemperature responsive control device ofthe kind specified wherein the operating element comprises a plurality of bi-metal plates each of which is concave and convex at respective opposite faces which are assembled to form a stack in which at least certain of the plates have their concave faces pre sentedtowards opposite ends ofthe stack, the height of which, for a given number of plates, varies as a function oftemperature.

2. A device according to Claim 1 whereinthe bi-metal plates are assembled in the stack in a manner such that successive pairs of plates starting from one end ofthe stack have their concave faces presented respectively towards each other.

3. Adevice according to Claim 1 wherein the bi-metal plates are assembled in successive groups along the stack in a manner such that the plates in each group are mutually nested while successive groups have the concave faces of their nested plates presented in opposite directions towards respective opposite ends ofthe stack.

4. A device according to any one of the preceding claimswhereinthe bi-metal plates are apertured in their central region and are mounted on a pin passing through the apertures.

5. A device according to any one ofthe preceding claims wherein the stack of bi-metal plates are interposed between the body of the device and a pressure point of the over-centre mechanism at which the application of pressure causes itto move through the over-centre position.

6. Atemperature responsive control device of the kind specified wherein a tappet is provided to transmit pressure between the operating element which is operatively interposed between the body ofthe device and a pressure point ofthe over-centre mechanism, such tappet presenting oppositely directed contact faces at respective opposite ends formed on a body which is rigid in the direction in which ittransmits force to the pressure point.

7. Adevice according to Claim Swherein atappet is provided to transmit the pressure between the stack and the pressure point, such tappet presenting oppositely directed contact faces at respective opposite ends formed on a body which is rigid in the direction in which ittransmitsforcetothe pressure point

8. A device according to either of Claims 6 and 7 whereinthetappetcomprisesa bush located in an opening formed in a blade spring applying compressive loading to the operating element.

9. A device according to Claim 8 as appendantto Claims 4, 5 and 7 wherein the bush fits loosely overthe pin atthefree end ofthe pin and has internal clearance with respect to the end face of the free end ofthe pin.

10. A device according to either of Claims 8 or 9 wherein the bush is oftop-hatform with its rim or flange intervening between the blade spring and the stack and its crown bearing against the pressure point ofthe over-centre mechanism.

11. A device according to any one of the preceding claims wherein the cooperative contact elements comprise one having a dome-shaped contact face and another having a contactface which is of convex arcuate shape in a first cross-sectional plane but substantially rectilinear shape in a second crosssectional plane at right angles to the first plane.

11. Atemperature control device ofthe kind specified wherein the cooperative contact elements comprise one having a dome-shaped contactface and another having a contact face which is of convex arcuate shape in a first cross-sectional plane but substantially rectilinear shape in a second crosssectional plane at right angles to the first plane.

12. A device according to Claim 11 wherein the cooperative contact elements comprise fixed contact elements spaced apart fro each other and movable bridging contact elements, the latter elements having contactfaces of said dome-shape and the former having contactfaces of said convex arcuate shape.

13. A device according to either of Claims 11 or 12 wherein said contact faces of dome-shape are pressed portions of base metal strip such as copper or brass and said contact faces of said convex arcuate form are presented by silver or other arc resisting metal contact elements proper carried bya base metal member.

14. A device according to any one ofthe preceding claims wherein the body and the over-centre mechanism are formed, constructed or arranged in a manner such asto afford a pocketforthe reception ofthestack of bi-metal elements, or alternatively an expansible chamber connected or adapted for connection to a bulb containing a fluid, the pressure of which changes in response to temperature changes.

15. A device according to any one of the preceding claims wherein the over-centre mechanism comprises a blade of a configuration such as to snap over on one bowed position to the oppositely bowed position, and which carries a mounting member of insulating material on which one of said contact elements is mounted, and wherein there is provided on the body of the device a positionally adjustable stop element made of metal and which is engaged by said mounting member to limitthe travel ofthe contact element carried thereby, moving from the closed position to the open position with respect to the other contact element.

16. A device according to Claim 15whereinthe mounting member of insulating material is shaped to include a barrier portion interposed physically between the contact element carried by said mounting member, and said adjustable stop memberto increase the length ofthe surface path between the contact elements and the screw.

17. A device ofthe kind specified substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

18. A device of the kind specified having any novel feature or novel combination of features disclosed herein and/orshown in the accompanying drawing.

Supersededclaims 1,6,11.

New or amended claims:

1. Atemperature responsive control device of the kind specified wherein the operating element comprises a plurality of bi-metal plates each of which is of domed shape and concave and convex at respective opposite faces, which plates are assembled to form a stack in which at least certain of the plates have their concave faces presented towards opposite ends ofthe stack, the height ofwhich, for a given number of plates, varies gradually as a function of temperature, the plates producing snap action of the overcentre mechanism, which is separate from the plates themselves, at a temperature determined by an adjustable setting means.

6. A device according to any one of the preceding claims wherein a tappet is provided to transmit pressure between the operating element which is operatively interposed between the body of the device and a pressure point of the over-centre mechanism, such tappet presenting oppositely directed contact faces at respective opposite ends formed on a body which is rigid in the direction in which it transmits force to the pressure point.