GB2391111A

GB2391111A - Bimetallic actuator

Info

Publication number: GB2391111A
Application number: GB0315967A
Authority: GB
Inventors: Colin Peter Moughton
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 2002-07-08
Filing date: 2003-07-08
Publication date: 2004-01-28
Anticipated expiration: 2023-07-08
Also published as: GB0315967D0; GB0215755D0; GB2391111B

Abstract

A snap-acting bimetallic actuator has a generally curved shape such that it reverses its curvature when heated to a predetermined temperature. The surface of the actuator has an indentation 10 from the concave side to the convex side which raises the break temperature and the remake temperature.

Description

- 1 2391111

Bimetallic Actuators 5 This invention relates to snap-acting bimetallic actuators. These actuators are used in many applications for sensing temperature, one example being electromechanical controls for liquid heating vessels.

Snap-acting bimetallic actuators are described in 10 GB 1542252, but are now well known in the art. As is well known, the curved shape prestresses the actuator so that as its temperature rises, differential expansion of the two bonded layers causse a counter stress to build up until a critical temperature known as the break 15 temperature, is reached and the actuator reverses its curvature in a snap action. Typically such actuators are provided with a cut-out in the centre thereof defining a tongue. Movement of the edge of the actuator is utilised, typically by means of a push-rod acting on 20 a pair of spring- loaded electrical contacts. Well known actuators are either circular with a single central tongue or rectangular with two central tongues.

It is well known that the 'snapped' configuration of snap-acting bimetallic actuators is also inherently 25 pre-stressed and therefore that they must be cooled significantly below their break temperature before they will snap back to their original curvature. This lower temperature is known as the remake temperature and the difference between the break and remake temperature is 30 termed the temperature differential.

Clearly, in order for these actuators to properly to operate, the remake temperature must be above the normal ambient temperature experienced by the bimetal.

However, it is usually desirable for the temperature 35 differential to be as small as possible since the temperature differential will usually determine the time taken for a thermal cut-out to reset, thereby allowing

I; It d' : l:: l - 2 - the appliance which it is protecting to be reused.

The break temperature of a snap-acting bimetal may be altered by altering the amount of curvature applied to the bimetal during manufacture. As the degree of 5 curvature is increased, the amount of thermal stress required in use to overcome it is similarly increased and therefore the break temperature is raised. However as this has the corresponding effect that a similarly greater degree of stress must be applied to the bimetal 10 upon cooling before it snaps back and therefore the remake temperature is reduced. The combined effect is a significant increase in the temperature differential.

It will be appreciated that for standard snap-

acting bimetallic actuators, the temperature 15 differential is a measure of the amount of stress to which the bimetal is subjected during operation. Thus higher temperature bimetals, having large temperature differentials tend to be less accurate and less consistent in use than lower temperature ones (e.g. up 20 to approx 140 C). The higher curvature which must be imparted to high temperature bimetals also makes them more difficult and unreliable to manufacture.

The overall result is that there is an effective practical limit on the operating (break) temperature and 25 the temperature differential of bimetallic actuators which may be manufactured.

The abovementioned limits to the acceptable amounts of operating stress and curvature also place a limit on the degree of travel between the central tongue of the 30 bimetal and the rim since it is evident that relative movement of the bimetal in reversing its curvature is dependent upon the degree of curvature. This has implications for the tolerance of associated components.

A further problem with existing bimetals is that as 35 the make temperature of the bimetal is increased, the amount of pre-break creep also increases. Pre-break creep is the amount of movement in the bimetal before it

( - 3 - undergoes its full snapping motion. Pre-break creep is undesirable since it increases the amount of travel required to provide a reliable distinction between the heated and unheated configurations with a corresponding 5 impact on the tolerance of associated components.

The present invention seeks to reduce the above-

mentioned problems and provides a snap-acting bimetallic actuator having a generally curved shape such that it reverses its curvature when heated to a predetermined 10 temperature, wherein the surface of the actuator comprises an indentation from the normally concave to the normally convex side of the actuator.

By providing an indentation in the actuator, the break temperature of the bimetal is increased as 15 compared to the same actuator without the indentation.

The reason for this is that the indentation stiffens the structure of the actuator meaning that a higher mechanical force is required to snap the actuator through to its opposite curvature. Thus for a given 20 degree of curvature, the actuator has a higher break temperature than known bimetals.

It is also observed however that actuators in accordance with the invention exhibit a reduced temperature differential. The reason for this is that 25 the indentation is asymmetric in its effect on the actuator it gives the structure of the actuator a directional bias. In effect the indentation preloads the actuator towards its remake point, when it goes from reversed to its normal curvature. In other words, a 30 lesser force is required to remake the actuator which corresponds to a higher remake temperature and thus a lower temperature differential for a given make temperature. It will be appreciated that the resultant effect is 35 that a higher temperature snap-acting bimetallic actuator is achieved, but with a lower rather than a higher temperature differential. The latter corresponds

( - 4 - to a lower operating stress and the higher temperature is increased without the curvature having to be increased. Thus bimetals in accordance with the invention may be made more cost-effectively and can 5 operate more reliably than was heretofore the case.

The indentation may be of any suitable size and/or shape and may be provided on the suitable part of the actuator surface. In preferred embodiments though, the actuator comprises a cut-out region around a central 10 tongue, with the indentation being arranged generally at the base of said tongue.

This is advantageous since it increases the total movement between the tongue and the opposite side of the actuator, i.e. the travel of the rim if the tongue is 15 constrained. The reason for this is that the indentation increases the stiffness around the base of the tongue and therefore reduces movement of the tongue which enhances the difference in movement between the tongue and the unrestricted rim at the opposite side of 20 the actuator.

It will be appreciated that an increase in total travel is advantageous since it tends to relax the required manufacturing tolerances for surrounding parts to ensure positive opening and closing of an associated 25 pair of contacts.

By providing an increased stiffness around the base of the tongue, another advantage is realised since the tongue is where pre-break creep tends to occur. Thus, by stiffening the tongue, the amount of creep is reduced 30 which reduces the amount of travel required to give positive opening and closing of contacts which in turn enables further relaxed manufacturing tolerances as explained above.

The depth of the indentation is preferably 35 approximately the same as the thickness of the material of the actuator.

The area of the indentation is preferably between

( - 5 10 and 50 of the surface area of the actuator, more preferably between 20 and 301.

A preferred embodiment of the present invention will now be described, by way of example only, with 5 reference to the accompanying drawings, in which: Figures la to Id are respectively one plan and three crosssectional views of a known snap-acting bimetallic actuator showing deflection of its rim, shown for reference only; 10 Figure 2 shows typical hysteresis curves of temperature versus rim travel for two known snap-acting bimetallic actuators, shown for reference only; Figure 3 is a perspective view of a bimetallic actuator in accordance with the invention; and 15 Figure 4 is a hysteresis diagram of temperature versus rim travel showing how the embodiment of Figure 3 compares to known bimetallic actuators.

Turning to Figure la, there may be seen a schematic plan view of a snapacting bimetallic actuator. As is 20 well known in the art, the actuator is generally circular but has a central cut-out D defining a central tongue E and a corresponding rim portion F opposite the tongue E. Typically, the actuator may be constrained at the tip G of the tongue E. It may be seen in the cross 25 sectional view of Figure lb, that at room temperature To, the actuator is curved downwardly.

Figure lc shows the corresponding view when the temperature of the blade has reached its break temperature, TBRK. It will be seen that the curvature 30 has reversed.

Figure ld shows Figures lb and lc superimposed to demonstrate how deflection of the rim F is measured.

Turning next to Figure 2, there may be seen the hysteresis curves for two prior art snap-acting

35 bimetallic actuators, commonly known as blades, of different nominal break temperature. Looking firstly at the low temperature blade denoted by curve A, it will be

- 6 seen that as its temperature is increased from room temperature To the rim deflection increases approximately linearly until the break temperature TBRK! is reached. At this point, as is well known in the art, 5 the blade will snap to its reverse curvature giving rise to a sudden increase in the rim deflection. This is shown by the flat upper part of curve A. It will be seen by the cusp at the upper right ha d corner of curve A that further heating of the blade will continue to 10 cause approximately linear deflection.

The blade is then allowed to cool, although the reversed curvature is maintained significantly beyond the break temperature TBRK! and indeed the rim deflection reduces approximately linearly until sufficient stress 15 has built up in the reverse curvature formation to snap the blade back to its original curvature. This is known as the remake temperature TRMK1 and is shown by the lower flat part of curve A. Thereafter the common part C of the curve is followed until the blade has reached room 20 temperature To.

The amount of movement of the rim before the make temperature is reached and the blade snaps, i.e. the pre-break creep, is shown on the abscissa of the graph.

It may be seen that this is a significant proportion of 25 the total deflection of the rim of the blade.

It is known in the prior art that the break

temperature of a snap-acting bimetallic actuator may be increased by increasing the curvature of the actuator.

The hysteresis curve for such a higher temperature 30 actuator is shown by curve B of Figure 2. It will be observed that curve B is the same shape as curve A but it is expanded in each direction. Thus, the desired property of a higher break temperature TBRK2 is achieved but at the price of increased creep and a significantly 35 lower remake temperature TK2.

It will be observed that the latter is undesirable in its own right since the remake temperature is only a

( - 7 - little higher than room temperature. This means that it will take a long time for the actuator to reset once it has been actuated. Furthermore, it will be seen that the temperature differential between the break and 5 remake temperature TBRK2 and TK2 is significantly increased. This corresponds to high operating stress which makes the reliable manufacture and operation of such actuators difficult.

Figure 3 shows a perspective view of a snap-acting 10 bimetallic actuator in accordance with the invention.

In common with known actuators, it is generally circular in shape and has, from the view of Figure 3, a generally convex shape. A central portion 2 of the surface is cut away, thereby defining a central tongue 4. The tongue 4 15 is provided with a hole 6 at its distal end to enable the tongue 4 to be rivetted to a support plate or the like (not shown) to constrain it.

The cut-out region 2 also serves to define a relatively narrow rim region 8 opposite the central 20 tongue 4. In the region of the base of the tongue 4, there is provided an indentation 10 from the concave side of the actuator (not visible in Figure 3) to the convex side of the actuator. The indentation 10 may therefore be seen in Figure 10 to stand proud of the 25 surrounding surface 12.

The indentation 10 serves to stiffen the actuator against its tendency to reverse its curvature in a snap-

action and therefore has the effect of increasing the break temperature of the blade. However, since the 30 indentation is asymmetric, its effect is also non-

asymmetric and it serves to pre-load the blade against snapping back to its original curvature when it is cooled. There is, therefore, not a corresponding decrease in the remake temperature, in fact the remake 35 temperature is increased. In other words a smaller temperature differential between the break and remake temperatures is achieved.

( - 8 - Furthermore, since the indentation lo is located in the region of the base of the tongue it acts to stiffen that region in particular. This has the effect that pre-break creep is reduced but also the total 5 deflection of the rim 8 is increased since it is relatively less stiff than the tongue 4.

The effects described above may be seen on the hysteresis curves shown in Figure 4. These show the relationships between temperature and rim travel for a 10 known snap-acting bimetallic actuator, denoted by curve 14; and for the actuator shown in Figure 3, denoted by curve 16. Both actuators have the same overall curvature, and differ only in the absence or presence respectively of the indentation 10.

15 It will be seen immediately from curves 14 and 16 that the effect of the indentation is to increase the break temperature. However, unlike the higher temperature bimetal corresponding to curve B of Figure 2, there is no corresponding widening of the temperature 20 differential between the break and remake temperatures.

Indeed, this differential is reduced as compared to the standard bimetal, resulting in a significantly flatter hysteresis curve than the known actuator and certainly significant flatter than the known high temperature 25 actuator (curve B of Figure 2). It will also be seen that the pre-break creep 22 of the Figure 3 actuator is reduced as compared to the creep 24 of the known actuator, whilst at the same time the total deflection 26 of the rim 8 is increased as compared to the 30 deflection 28 of the known actuator.

Thus it will be seen that the provision of the indentation allows higher temperature bimetallic actuators to be manufactured. The reduced area within the hysteresis curve 16 corresponds to a lower operating 35 stress which increases the facility and reliability with which the actuators may be manufactured.

It will be appreciated by those skilled in the art

f that many variations and modifications may be made to the embodiment described without departing from the scope of the invention.

The indentation may not be the same size or shape 5 as that shown and may be found on a different part of the actuator. Furthermore, it is not essential that the actuator is circular - it could instead be substantially square or rectangular and there may be more than one central tongue. It will also be appreciated that the 10 invention is not limited to the provision of a single indentation and that more than one could be provided as required to achieve a similar effect.

The principles of the invention may be used to provide higher temperature bimetals or may, 15 alternatively be used to achieve the benefits of a reduction in the temperature differential, reducing pre-

break creep etc. for bimetals within current common temperature ranges.

Claims

- 10 Claims:

1. A snap-acting bimetallic actuator having a generally curved shape such that it reverses its 5 curvature when heated to a predetermined temperature, wherein the surface of the actuator comprises an indentation from the normally concave to the normally convex side of the actuator.

10

2. An actuator as claimed in claim 1 comprising a cut-

out region around a central tongue, the indentation being arranged generally at the base of said tongue.

3. An actuator as claimed in claim 1 or 2 wherein the 15 depth of the indentation is approximately the same as the thickness of the material of the actuator.

4. An actuator as claimed in claim 1, 2 or 3 wherein the area of the indentation is between 10 and 50 of 20 the surface area of the actuator.

5. An actuator as claimed in any of claims 1 to 4 wherein the area of the indentation is between 20% and 30 of the surface area of the actuator.

6. A snap-acting bimetallic actuator substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.