Field of the Invention:

This invention concerns improvements relating to electrical switches and more particularly concerns thermostatic electrical switches having an adjustable temperature setting. Backcnround of the Invention:

Thermostatic electrical switches are commonly employed to control the temperature of an environment and often incorporate a bimetallic element arranged to determine the condition of a set of switch contacts in dependence upon the ambient temperature of the bimetal environment. In its simplest form, a bimetallic thermostatic switch comprises a bimetallic strip fixed at one end and carrying at its other end a contact which co-operates in switching operations with an adjacently located fixed contact, the bimetallic strip flexing as its temperature rises and thereby opening- the switch contacts and thereafter resiling as it cools so as again to close the switch. In other forms of bimetallic thermostats, the bimetallic element of the switch does not itself carry the electric current being switched and rather serves as a thermally- responsive actuator determining the status of the

switch contacts through the inter ediacy of a push-rod for example.

To avoid the arcing that is prone to occur with slowly operating switch arrangements, bimetallic thermostatic switches are known which employ intrinsically snap-acting bimetallic elements or employ slow-acting (creep) bimetals in conjunction with snap-acting switch actuators. The M9 thermostatic switch manufactured by us is a typical snap-acting bimetallic thermostat and incorporates a generally rectangular bimetallic blade having a generally U-shaped cut-out which defines a central tongue flanked by a pair of legs which are bridged by a bridging portion of the bimetal which extends between the ends of the legs adjacent to the free end of the tongue. A crimp is formed in the bridging portion so as to draw the ends of the legs towards each other, thereby stressing the bimetal into a generally dished configuration which will move with a snap-action to an oppositely dished configuration when subjected to an appropriate thermal stimulus.

For adjusting the temperature setting of the Otter Controls M9 thermostatic switch, a setting screw is provided which engages with the bridging portion of the bimetallic blade. By adjustment of the setting screw, the stressing of the bimetallic blade can be

adjusted and this in turn determines the temperature at which the blade will snap to its opposite configuration. Bimetallic thermostats employing creep bimetals in conjunction with stressed, spring loaded, switch actuator mechanisms commonly employ adjustment means determining the stressing of the switch actuator- mechanism to determine the temperature setting of the switch. The M9 switch is of a simple and relatively inexpensive construction, but tends to be disadvantageous in that it has only a limited range of adjustment and/or gives rise to different temperature differentials between switch-on and switch-off conditions at different temperature settings. For some applications these disadvantages are immaterial, but for applications requiring a relatively large degree of adjustability with a substantially constant, small differential throughout the adjustment range the M9 switch is not altogether suitable and the more complex and correspondingly more expensive types of adjustable thermostats have been regarded as having a preferable performance capability. However, samples of such more complex adjustable thermostats that we have examined have shown a fundamental problem as regards their capability reliably to achieve a small switching temperature differential of the order, say, of 3 to 5°C over an adjustment range of the order, say,

Of 60°C .

We have found that thermostatic switches as aforementioned, which employ a creep bimetallic movement to cause a stressed, spring-loaded mechanism to move from one stable position to another with a snap action, require a consistent spring bistability and a bimetallic element of high stiffness and having a high rate of bend with temperature change if small differentials are to be achieved over a substantial temperature range. These two requirements tend to be mutually exclusive in practical sizes of bimetallic element. The reason for this is that the spring requires a certain force to change state and when it changes the point of application of the force moves away from the bimetal which generates it by deflecting under load and in response to the change in temperature. When the spring moves away, the bimetallic element follows through because of the load reduction, and this follow through has to be removed by temperature change before the spring can snap back to its original position; this gives rise to the switch differential. A stiff bimetal will not deflect so far under load, so stiffness would appear to be desirable, but stiffness is achieved by increasing the bimetal thickness and this reduces its thermal activity; hence there is an incompatibility.

Objects and Summary of the Invention:

It is therefore a primary object of the present invention to provide an adjustable thermostat which overcomes or at least substantially reduces the abovementioned problems of hitherto known adjustable thermostats.

It is a further object of the present invention to provide an adjustable thermostat which achieves the abovementioned primarily object whilst keeping the thermostat construction relatively simple and inexpensive.

According to the present invention, the abovementioned objects can be achieved by utilization of first and second oppositely-acting creep bimetallic elements to determine the condition of a snap-acting switch actuator.

The first and second oppositely-acting creep bimetallic elements may for example be comprised by two arcuate bimetallic elements arranged with their ends together and their centres spaced apart, one of the bimetallic elements being coupled to the switch actuator and the other being coupled to adjustment means which determines the stressing of the system comprised by the two bimetals and the switch actuator. For example, one of the two bimetallic elements may be coupled, generally at its centre, to the snap-acting

switch actuator by means of a push-rod, and the other bimetallic element may be subject to the action of adjustment means such as a rotary cam or adjustment screw for example which bears upon the second bimetallic element, again generally at its centre.

By use of first and second oppositely-acting creep bimetallic elements, which advantageously are generally similar to each other but need not necessarily be similar, an extended movement for a given force can be obtained for a given temperature change as compared to that obtainable from a single bimetal. With an appropriate arrangement, such a two bimetal system will give approximately twice the thermal activity for a given bimetal stiffness as a single bimetal. There exists an optimum bimetal thickness for a given geometry and force system, and also an optimum bimetal length. Additionally, changes in bimetal width may be arranged to have minimal effect. By designing close to the optimum for both bimetal thickness and bimetal length, the effects of manufacturing tolerances can advantageously be minimised.

For simplicity of manufacture, the snap-acting switch actuator, which could take many different forms, is preferably constituted as a stressed spring member capable of moving between opposite

configurations with a snap action. The spring desirably should be relatively compliant to the forces developed by the bimetallic elements, rather than presenting resistance to such forces, and accordingly the presently preferred spring is constituted by a rectangular blade of spring metal having a cut-out defining a pair of legs, one on each side of the cut¬ out, which are bridged at both ends by bridging portions of the blade, and with one of the bridging portions formed with a crimp such as to pull the adjoining ends of the two legs together. By virtue of this construction, the spring blade is given a generally dished configuration which can snap between concavely and convexly dished orientations. The snap-acting switch actuator, for example the abovementioned spring blade, could be arranged to operate the electrical contacts of the switch by way of a remote coupling, such as a push rod for example, or alternatively could be arranged itself to carry the moving contact of the switch. Thus in the presently preferred arrangement which is described in detail hereinafter and which employs a spring blade as abovementioned, the crimped bridging portion of the spring blade carries a silver contact arranged for switching operation with a fixed contact of the thermostat, and the other bridging portion of the

3 spring blade is welded or otherwise secured to an electrical terminal of the thermostat.

The above and further features of the present invention, together with the advantages thereof, will best be understood from consideration of the following detailed description of a presently preferred but exemplary embodiment of the invention which is illustrated in the accompanying drawings.

Description of the Drawings: Figure 1 shows an exploded perspective view of an adjustable thermostatic switch embodying the present invention;

Figure 2 is a perspective view of the switch of

Figure 1 in assembled condition; and Figure 3 is a side elevation view of the assembled switch.

Detailed Description of the Embodiment:

Referring to Figure 1, the adjustable thermostatic switch as illustrated consists of a moulded plastics body portion or chassis l to which first and second switch terminals 2 and 3 can be affixed, the switch terminals having portions 4 adapted to be fitted into respective slots 5 formed in the body portion 1 and then deformed to retain the terminals to the body portion. A silver contact 6 is spot welded to the switch terminal 3 and constitutes

the fixed contact of the switch.

A switch actuator 7 comprises a rectangular blade of spring metal formed with a cut-out 8 which defines a pair of legs 9 extending between end bridging portions 10 and 11. Bridging portion 10 is plain and is designed to be welded or otherwise affixed to switch terminal 2, and bridging portion 11 is formed with a crimp 12 which has the effect of drawing together the ends of the legs 9 adjacent to the bridging portion 11 and thereby stressing the spring metal blade so that it approximates to a dished configuration which can move between concave and convex orientations with a snap action. The switching of the blade 7 between its concave and convex orientations (or vice versa) is obtained by the application of force to a pair of projections 13 which extend from the legs 9, the force being converted to a torque in the legs which effectively twists the spring from one sense of curvature to the opposite. A silver contact 14 is spot welded to the underside of bridging portion 11 and acts as the moving contact of the switch in switching operations with the fixed contact 6. A member 15 serving as a back-stop for the spring blade 7 in switch-opening operations is arranged to be affixed to the switch body 1 in a manner similar to the fixing thereto of the switch

terminals 2 and 3. The member 15 could if desired be formed as a change-over contact if the thermostat was required to provide a change-over switching action. Although the spring 7 is inherently bistable, its arrangement in the described thermostat is astable in that the back-stop member 15 limits the range of spring movement in a contacts-opening direction and prevents it from going fully over-centre. The spring 7 is thus arranged to be self-resetting and, having been moved from its cold condition to its hot condition by application of an appropriate force to the projections 13, it will reset automatically as the applied force is released.

The switch body portion l is formed with four upstands 16 and with a mounting 17 for a rotary spindle 18 which extends through the thermostatic switch in its assembled condition. First and second creep-acting bimetals 19 and 20 are arranged to be located in operative position on the upper end of the spindle 18 (as it is viewed in the drawings) and to be loosely guided in their movement by the upstands 16. The bimetals 19 and 20 are generally arcuate with turned end portions 21 having respective formations 22 and 23 whereby the bimetal 19 can be set on top of the bimetal 20 with their curvatures oppositely arranged and the formations 22 and 23 engaged with each other.

Openings 24 are formed through the centres of the two creep bimetals for allowing passage of the spindle 18, it being noted that the opening 24 in the upper bimetal 19 is formed with an inwardly directed lug 25. A cam member 26 is adapted to be fitted onto the end of spindle 18 where it will co-operate with the lug 25 for adjusting the setting of the thermostat, and a coupling member 27 is adapted to be loosely mounted on the spindle 18 for coupling the movements of the centre of the lower bimetal 20 to the projections 13 on the spring blade 7, the coupling member 27 having reduced-diameter end portions 28 which are adapted respectively to make a close fit into the opening 24 in the lower bimetal 19 and to sit on top of the opposed ends of the projections 13. The range of movement of the coupling member 27 is desirably limited, by provision for example of a stop (not shown) on the spindle 18, to ensure that excessively high temperatures do not result in the application of such forces to the projections 13 of the spring blade 7 as to cause the switch contacts to reclose.

The axial positioning of the cam member 26 on the spindle 18 is important in that it must be correct in order that the desired temperature adjustment range for the thermostat can be achieved. In the described embodiment, the fit of the cam to the spindle is made

to be stiff so that it can be adjusted with the switch in a temperature-controlled environment. The spindle periphery (and/or the interior of the bore through the cam) can for example be provided with thin ribs designed to collapse as the cam is forced onto the spindle. To set the cam position, the switch is held in an environment at the appropriate temperature and, with the cam and the spindle in a predetermined rotational position, the cam is precision driven onto and along the spindle until the switch contacts change state. The stiffness of the cam/spindle interfit then holds the cam temporarily in this position until it may be fixed in place. The cam and the spindle may advantageously be formed of a thermoplastics material, such as nylon for example, so that the locking of the two together may be achieved by locally melting the two together so as to form a small weld. A peg 29 is formed on the spindle 18 for engaging a projection (not shown) formed on the switch body 1 to provide a limit to the rotation of the spindle.

In assembly of the thus described thermostatic switch, therefore, the following steps are contemplated. First, the spindle 18 is located in a jig and the switch body 1 is positioned over it. The terminal 3 carrying the fixed contact 6 of the switch is then fitted, followed by the terminal 2 carrying

the spring actuator 7 and the moving switch contact 14, these parts having previously been sub-assembled. The back stop member 15 is next fitted and adjusted to give the desired gap between the switch contacts when the switch is open circuit. Next the coupling member 27 is located on the spindle 18 and rests at its lower end on the projections 13 of the spring blade 7, the lower bimetal 19 is located on the spindle 18 and its central opening is engaged with the upper end of the coupling member 27, and the upper bimetal 20 is located on the spindle 18 and sits on the lower bimetal 19. The cam member 26, held in the correct rotational orientation, is then pushed onto the spindle to an initial position, and finally, with the assembled switch in a controlled temperature chamber, the cam is pushed along the spindle until the switch contacts open whereupon the cam may be locked in position as described hereinbefore.

In operation of the thus-described thermostatic switch, the cold condition of the switch is with its contacts closed and the two creep bimetals in a low curvature condition such that there is only a relatively little distance between their centres. As the temperature rises, so the curvature of the bimetals progressively increases which causes a force to be generated between the cam member 26 and the

coupling member 27, until sufficient force is transmitted to the coupling member 27 to cause the bistable spring 7 to change state thereby causing the moving contact 14 of the switch to move away from and break electrical contact with the fixed switch contact 6. The temperature at which this happens may be adjusted by rotation of the cam member 26 through the spindle 18, this adjustment effectively changing the distance between the lug 25 of the upper bimetal 19 and the coupling member 27. In the described embodiment, the cam member 26 provides for a distance adjustment of 3.5mm in order to provide a temperature setting adjustment range of 60°C. The switch contacts having opened, thereby disconnecting the current supply to the load, a fan heater or convector heater for example, the temperature will in due course begin to fall and the creep bimetals will respond by reducing their curvature until the spring 7 resets under its own spring action. By provision of the two bimetals acting in opposition, the thermal activity of the two bimetal system is substantially increased as compared to a single bimetal system with the result that the switch exhibits only a relatively small differential between its "on" and its "off" temperatures throughout the operating adjustment range of the switch.

The invention having been described in the foregoing by reference to a specific embodiment, it is to be appreciated that the embodiment described is exemplary only and that modifications and variations thereto are possible without departure from the scope of the invention as set forth in the appended claims. Thus, for example, whereas the described embodiment has employed two bimetals which are curved in their cold condition and increase their curvature as their temperature rises, the bimetals could be substantially flat in their cold condition so long as they were arranged, for example by having folded ends or by having spacing elements affixed to their ends, so that their centres remained spaced apart from each other over the desired working temperature range of the switch. It should be noted in this regard that once the centres of the two bimetals touch under falling temperature conditions, any further temperature reduction will have no effect upon the status of the switch, so that if the switch contacts are open when the centres of the bimetals make contact with each other, they will not reclose at any lower temperature. Perhaps more significantly in the context of a thermostat, if the switch contacts are closed when the centres of the bimetals make contact with each other, they will not under any circumstances reopen at any

lower temperaturee as has disadvantageously been the case with some prior art thermostatic switches which have exhibited anomalous behaviour at very low temperatures. Furthermore, a step could if desired be provided on the profile of the cam member 26 to provide for the switch to remain in a contacts open (off) condition at all temperatures, the switch then incorporating an on/off facility. The spindle 18 could furthermore (or alternatively) be arranged to be movable axially by means of a separate control for the purpose of providing a fixed or adjustable low temperature setting for night set back applications. The assembly of the bimetals could also be reversed so that a rise in temperature caused their centres to move together rather than apart, such an arrangement allowing the switch actuation to be reversed (the fixed switch contact being located in the position of the back stop of the described switch) or providing a "make on rise" action as opposed to the more usual "make on fall" action of thermostatic switches. Such a thermostat in which the bimetals move apart with falling temperature might usefully find applications in cryogenic systems where exposure of the bimetals to temperatures high above the working range might otherwise cause them to be overstressed. Switches of different working ranges

could simply be manufactured by use of different cam profiles in otherwise identical switch constructions. The spring metal switch actuator blade 7 could furthermore be formed with an integral tongue extending from its bridging portion 11 into the cut¬ out 8, and the contact 14 could be welded to the free end of such tongue for co-operation with the fixed contact 6 of the switch. By virtue of such an arrangement, break-point contact pressures and switching movements in operation of the thermostat can more readily be optimised.