GB1595340A - Thermal switches - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THERMAL SWITCHES (71) We, LIMITOR AG, a Corporation organised under the laws of Switzerland, of Seestrasse 55, 6052 Hergiswil (Switzerland), do hereby declare that the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to improvements in thermal switches. Thermal switches are used particularly as safety switches, which interrupt the current to motors, transformers or electrical household equipment when the temperature of the machine or appliance rises above a permissible value. In the design and construction of such switches, a number of problems have to be solved. The switch must interrupt the current circuit at a preset, as far as possible accurately reproducible temperature, and when the switch is actuated the contact studs shall be separated sufficiently rapidly and far from each other for pratically no burn-up to occur at the contact surfaces. Furthermore, in particular those thermal switches intended for use in household appliances should desirably be of the smallest possible dimensions and capable of being made at an economical cost. On account of the many possibilities of their use, many thermal switches constructed upon different principles are already known. For example, in U.S. Patent Specification 2,777,032 (Burch), a thermal switch comprising a directly heated switching element which initiates the interruption of an electrical circuit is described.

In this switch, the switching element is formed as an E-shaped flat spring made from a bimetal. The two free ends of the outer arms of this spring are urged towards each other in the general plane of the spring and are secured in this condition to a mounting plate or to the switch housing. As a result the central arm is deflected out of the general plane of the spring and possesses a stable position on each side of this general plane. Furthermore, the spring is installed in the switch in such a manner that the bending force produced when the bimetal is heated up is opposite to the spring force and, when a temperature adjustable by the form and assembly of the spring is exceeded, is greater than this spring force. On the free end of the central arm of the flat spring a first contact stud is mounted, which, at normal ambient temperature, bears in the first stable position against a second and stationary contact stud and closes the electrical circuit. As soon as electric current flows through the spring and the electrical heat thereby generated rises above a predetermined value, the bending force of the bimetal exceeds the spring force and the central arm of the spring jumps out of its first stable position into the second stable position and separates the contact studs. In this switch, the force, by which the first contact stud is pressed against the second contact stud, is equal to the difference between the practically constant spring force and the temperature-dependent bending force in the bimetal. When the switch heats up, the contact pressure therefor decreases and, when the switching temperature is exceeded, the two contact studs separate from each other relatively slowly. It will readily be understood that with this method of operation, small electric arcs and a relatively intense burn-up of the contact surfaces cannot be avoided. If it is desired to prevent the switch, when it has been opened by exceeding of the switching temperature, from automatically closing again when it has cooled to below the switching temperture, then the spring force and the bending force of the switching element must be set by a very careful control of material, finishing and assembly, which renders inexpensive manufacture of this thermal switch practically impossible.

In Swiss Patent Specification 537,088 (Sprecher & Schuh), a further thermal switch is described. In this switch also, an Eshaped flat spring is used, which is prestressed in the same manner as described above and possesses two stable positions. By contrast to the already described known form of construction, however, the spring is not made from a bimetal, and the movable contact stud is mounted not upon an arm but upon the web which joins the three arms of the spring together. For triggering this switch, a bimetal is used, which is directly or indirectly heated up by the current of the circuit to be monitored and the bending of which is transmitted by means of a plunger onto the free end of the central arm of the spring, in order to deflect this arm. As soon as the central arm has been deflected beyond a specific point, the spring jumps over, the contact stud disposed upon the spring web thus becoming disengaged from another, stationary contact stud.

In this thermal switch also, the central arm of the flat spring, which causes the springing over of the spring and thus the disengaging of the one contact stud from the other, is displaced slowly and continuously into its switched-over position as the temperature rises. This has the consequence that the desired switch-over point is extended over a switching range, and in addition in this switch also the contact pressure is decreased with increasing temperature and especially in the switch-over range. The practical disadvantages of such an arrangement have already been described above.

Furthermore, from German Patent Specification 2,121,802 (Thermik-Gerätebau) and German Patent Application 21 34 652 (Micro-Therm), two thermal switches are known, which comprise a spring possessing only one stable position. A contact stud is attached to this spring or the spring presses against a contact bridge provided for connecting together two contact studs. Furthermore, in each of these switches, a cup-shaped bimetallic snap disc is provided, which bears directly or indirectly against the spring and springs over at a predetermined temperature and deforms the spring and opens the switch.

Both of these switches posses at least two serious disadvantages. On opening of the switches, the switching range of the movable contact head or of the contact bridge is at best as long as the working stroke of the cupshaped bimetallic snap disc, which limits the current-carrying capacity of the switch. When the bimetallic snap disc cools down and springs back, the switches are of necessity closed again under the action of the spring which posseses only one stable position which can lead to a continuing switching on and off of the switch, a state which normally is not permissible for a safety switch.

It is an object of the present invention to provide an improved thermal switch, which possesses an accurately reproducible switching temperature, the contact surfaces of which are pressed together with a substantially constant pressure until the switching temperature is reached and, when opened, are abruptly separated from each other, the switching range being sufficiently large for interrupting with reliability medium -value currents under, commonly encountered voltages and which after it has cooled down, does not again automatically switch on.

According to the present invention there is provided a thermal switch comprising a substantially flat stressed spring member having a portion capable of assuming either one of a first and a second stable position by flexing about a bending region of the member and carrying a first contact stud spaced from the bending region and engageable in the first stable position with a second contact stud, a bimetallic temperature sensor having a part spaced from the spring member and capable of a snapping action under the influence of temperature between a first position and a second position in which the sensor part is displaced in the direction away from said spring member, and a connecting member extending between said sensor part and a region of the spring portion closer to the bending region of the spring member than the first contact stud, said connecting member being operable to transmit displacement of the sensor part to said region of the spring portion only when the spring portion is in the first stable position and the sensor part is displaced from the first to the second position to cause the spring portion to assume the second stable position.

In a preferred embodiment of the invention a resetting device is provided operable to apply an external force to the spring member and return the spring portion to the first stable position when the sensor part is in the first position.

With advantage, the temperature sensor is in the form of a disc and it is possible by suitable selection of the material and dimensions and shape of the bimetallic snap disc, for the switching temperature to be very accurately set, and, even after a very long period in operation and very frequent switching, this temperature remains practically unaltered. Because the force which causes the spring portion to assume the second stable position is brought into action suddenly and not continuously as the switching temperature is reached, the contact pressure remains constant until the contacts open and even in the immediate vicinity of the switching temperature. Also, because the working stroke of the bimetallic snap disc is utilised only for deflecting the spring portion from its first stable position and the spring portion then springs over by its won force from the first stable position to the second stable position, a considerably greater switching range can be attained than the working stroke of the bimetallic snap disc. The ratio of switching range to working stroke is still further improved by the larger distance according to the present invention between the contact stud and the bending region of the spring member in comparison with the distance between the point of action of the connecting member and the bending region.

As the transmission of movement from the bimetallic snap disc to the spring member is effective in one direction only, assurance is also provided that the switch, after switching off, cannot again automatically switch on.

Finally, the switch, on account of the favourable ratio of working stroke of the bimetallic snap disc to the switching range of movable or first contact can be made from individual components, the dimensional accuracy of which lies within normal tolerances, and in the assembly of the switch also, no additional adjustment or compensating procedures are necessary, in order to attain the desired accuracy and reliability of switching. It is thus possible to manufacture the new switch in large numbers and economically.

One embodiment of the invention will now be described by way of example, reference being made to the accompanying drawings in which: Fig. 1 is a plan of an E-shaped spring member installed in a cylindrical housing and having a contact stud secured to its central arm, Fig. 2 is a section through a thermal switch incorporated in a cylindrical housing with the contacts closed and Fig. 3 is a similar section to Fig. 2 showing the contacts open.

In the example illustrated in Fig. 1, there is shown an E-shaped spring member 10 having the free ends of the two outer arms 11, 12 fixed to a base plate 15 by pins 13 and 14 with the free ends of the arms urged towards each other in the plane of the spring member, so that the spring member is always stressed.

This has the effect that the central arm 17 is bent out of the general plane of the spring member. Approximately in the centre of this central arm 17 there is a bore 16 through which an upper cylinder 32 (Figs. 2 and 3) of a connecting member 29 is guided. A contact stud 18 is attached to the free end of the central arm 17. The bending zone of the central arm 17 is situated in the region of the connecting web 19 of the three arms of the spring.

Fig. 2 shows a section through a preferred form of embodiment of the new thermal switch. The switch possesses a pot-shaped housing 21, which is made from a material possessing good thermal conduction. The lower part of the housing wall adjacent to the bottom 22 of this housing has a smaller internal diameter than the upper part of the wall to provide a step 23 on which a base plate 15 is placed and held down by means of a beading 24, swaged in the upper part of the housing wall. The beading 24 serves not only for fixing the base plate 15 but also makes possible improved thermal conduction from the housing wall to the base plate. The base plate 15 is formed with a central bore 25, through which the connecting member 29 is guided. The lower part of the housing and the plate define a space or chamber 26, in which is disposed a cup-shaped bimetallic snap disc 27 having a central bore 30 through which a lower cylinder 28 of the connecting member 29 is guided. The diameter of this lower cylinder 28 is enlarged at its free end, for example by upsetting, so that a beading 31 is produced which prevents the snap disc 27 from being lifted off the connecting member 29. The free end of upper cylinder 32 of the connecting member 29 is also enlarged to form a beading 33, which prevents the spring arm 17 from being lifted off the connecting member 29. The distance between the mutually facing surfaces of the upper and lower beadings 33 and 31 is substantially equal to the distance between the middle of the central arm 17 of the spring member 10 when the arm 17 is in its first stable position shown in Fig. 2 and the snap disc 27 when the latter faces with its convex surface towards the base plate 15 as shown in Fig. 2. The movable contact stud 18 bears against a stationary contact stud 35 which is fixed to an electrical conductor, which in the example illustrated is formed as a pipe or tube 36, into which a conducting wire (not shown) can be soldered. The pipe 36, is cast with an insulating casting material 37 into a cylinder 38, which is pressed into the upper part of the housing. In the middle part of the housing is disposed a displaceable ring 40, the lower edge 41 of which is intended to rest upon the external edges of the ends of the two outer arms 11 and 12 of the spring members 10 which are fixed by the pins 13 and 14. The upper edge of the ring 40 possesses three extensions, disposed at intervals of 1200 about the axis of the housing and of which only the extensions 42 and 43 can be seen in Fig. 2. These extensions extend through and are guided by longitudinal recesses 38a in the external wall of the cylinder 38 and are provided for displacing of the ring 40 in the axial direction, as described in more detail below.

The method of functioning of the new thermal switch is described below with the assistance of Fig. 3. For this purpose, the lower part of the thermal switch shown in Fig. 2 is illustrated again in Fig. 3 and in the same section as in Fig. 2, but with the contacts 18 and 35 opened. So long as the ambient temperature and in particular the temperature in the space 26 is below a predetermined switching temperature, the bimetallic snap disc 27 possesses the shape shown in Fig. 2, and the contact stud 18 is pressed by the central arm 17 of the spring member 10 into engagement with the contact stud 35. An electrically conducting connection then exists from the metallic housing 21 via the base plate 15, the two pins 13, 14, the E-shaped flat spring member 10, the movable contact stud 18, the stationary contact stud 35 to the tubular conductor 36. As soon as the ambient temperature has risen to a value which also causes the temperature in the space 26 to rise above the switching temperature, the cup-shaped bimetallic snap disc 27 springs over, so that it then adopts the position shown in Fig. 3, in which the concave surface of the snap disc is towards the base plate 15. When the snap disc 27 springs over, the external edge of the disc bears against the base plate 15 and the centre of the disc is lifted away from the base plate 15. The connecting member 29 is thus abruptly pulled down by the stroke of the cup-shaped snap disc 27. This displacement of the connecting member 29 is transmitted to the central arm of the flat spring member , which arm is thereby pulled downwards out of its first stable position shown in Fig. 2 on the upper side (in the figures) of the general plane, of the spring member) through the switching point, and as a consequence of the spring stress jumps into its second stable position on the lower side of the general plane of the spring member. The movable contact stud 18 is thereby abruptly separated from the stationary contact stud 35 and the above-described electrically conducting connection between the housing 21 and the tubular conductor 36 is interrupted. It will be readily understood that, with the arrangement described, the switching range of the contact stud 18 is considerably larger than the working stroke of the connecting member 29 produced by the springing over of the bimetallic snap disc 27, because the distance of the contact stud 18 from the bending zone of the central spring arm 17 is about twice as large as the distance of the point of action of the connecting member 29 from the bending zone, because the working stroke only needs to deflect the spring arm 17 out of the position shown in Fig. 2 to just beyond the switching point and because the spring arm 17, on account of the spring stress, does not have the form of a straight lever but possesses in each of its two stable positions a bending which increases the deflection.

The two cylinders 28, 32 at the ends of the connecting member 29 are together somewhat longer than the working stroke of the bimetallic snap disc 27. Consequently, as the snap disc cools to below the switching temperature and the disc snaps back into the position shown in Fig. 2, the connecting member is displaced by the working stroke of the snap disc 27, but the central arm of the E-shaped spring member remains in its second stable position shown in Fig. 3. The switch is consequently necessarily opened as the switching temperature is exceeded, but is not again closed as the temperature falls below the switching temperature.

For the purpose of closing the switch, the displaceable ring 40 is provided. As already described above, this ring possesses three extensions 42, 43, projecting out of the housing. If these extensions are pressed into the housing, then the ring 40 is pushed downwards and presses in the region of the two pins 13, 14 onto the outer edges of the flat spring member. As a result of the consequent displacement of the spring member 10 opposite its points of attachement, the spring arm 17 jumps over into its first stable position and again presses the contact stud 18 into the contact stud 35.

The thermal switch described can be made with various dimensions and for different circuit-breaking capacities. One preferred embodiment is intended for incorporation into household appliances and in particular into electrically heated water tanks. This embodiment has an external diameter of only 6 3 mm and reliably switches a current of 5 amperes at an operating voltage of 220 volt A.C. The switching temperature is about 1200C. The set spring stress produces a contact pressure of at least 0 4 N and the mutual arrangement of the individual constituent elements is so selected that a displacement of about 0- 15 mm transmitted by the connecting member 29 to the central arm 17 of the spring member is sufficient for opening the switch, the switching range of the movable contact stud 18 being about 1-0 mm. The working stroke of the bimetallic snap switch is about 0 25 mm and is therefore about twice as large as that which is necessary for deflecting the spring member and causing the spring arm to spring over. This measure makes not only for very high switching reliability but also makes possible an economic production of the switch.

It will be understood that the described construction of the new thermal switch can be modified when the switch is intended for special purposes. For example, the switch can be pushed into an electrically insulating sleeve 50 (illustrated in Fig. 2 by a dot-anddash line), which advantageously consists of nomex-aramyde paper. In order that the electrical contact resistance between the stationary contact stud 35 and the tube 36 used as conductor shall be as small as possible, the contact stud 35 can be rivetted to the tube by means of a tool inserted into the tube. For fixing the tube 36 in the cylinder 38, a hardened glass smelt can with advantage be used instead of the described cast material.

It is also possible to dispense with the cylindrical jacket 38 and to bond the cast material or hardened glass smelt directly to the ring 40. In this form of embodiment, when the spring arm is switched back from the second stable position corresponding to the open contact into the first stable position corresponding to the closed contact by means of the ring 40, the stationary contact stud 35 is also displaced and, after the spring arm has sprung over, the stationary contact stud and the ring are pushed back into their starting position by the spring force transmitted through the movable contact stud 18. Although, in the simplest form, the ring 40 is manually displaced for the purpose of resetting the switch, it is also possible for mechanical elements such as pistons, cylinders, diaphragms or levers to be brought into action for this purpose. Finally, it is well-known to any skilled person that the temperature at which the bimetallic snap disc 27 springs over can be adjusted through a wide range by appropriate selection of the bimetal, of its dimensions and of the form of the disc.

It will be appreciated from the above description that the connecting member 29 is of rod-like or cylindrical form with reduced diameter neck portions 28 and 32 at opposite ends and enlarged retaining head portions 31 and 33 at or towards the respective ends of the member. The neck portion 32 extends freely through the bore 16 in the spring arm 17, the general body portion of the connecting member 29 extends freely through the bore 25 in the base plate 15 and the neck portion 28 extends freely through the bore 30 in the bimetallic snap disc 27. As the distance between the opposed faces of the retaining head portions 31 and 33 is substantially equal to the spacing between the upper face of the spring arm 17 in its first stable position illustrated in Fig. 2 and the lower face of the marginal area about the bore 30 in the snap disc 27 in its first stable position shown in Fig. 2, it will be seen that snapping of the disc 27 to its second stable position shown in Fig. 3 correspondingly displaces the connecting member 29 and this in turn displaces the spring arm 17 to its second stable position.

However, if the disc 27 then returns to its first stable position, the axial length of the reduced diameter neck portions 28 and 32 together, provides a sufficient lost motion to prevent any resulting displacement being transmitted to the spring arm 17 which remains in its second stable position until it is restored to the first stable position by operation of the ring 40.

WHAT WE CLAIM IS 1. A thermal switch comprising a substantially flat stressed spring member having a portion capable of assuming either one of a first and a second stable position by flexing about a bending region of the member and carrying a first contact stud spaced from the bending region and engageable in the first stable position with a second contact stud, a bimetallic temperature sensor having a part spaced from the spring member and capable of a snapping action under the influence of temperature between a first position and a second position in which the sensor part is displaced in the direction away from said spring member, and a connecting member extending between said sensor part and a region of the spring portion closer to the bending region of the spring member than the first contact stud, said connecting member being operable to transmit displacement of the sensor part to said region of the spring portiononly when the spring portion is in the first stable position and the sensor part is displaced from the first to the second position to cause the spring portion to assume the second stable position.

2. A thermal switch according to Claim 1 in which the spring member is of substantially E-shape with the two outer arms urged towards each other and secured to a support plate to produce stress under the influence of which the central arm which comprises the spring portion is capable of being deflected transversely of the general plane of the spring member between the first and second stable positions which are disposed on opposite sides of said general plane.

3. A thermal switch according to Claim 2 in which the two outer arms of the E-shaped spring member are secured to one face of the support plate, the sensor in the form of a disc is operably associated with the other face of the support plate and the connecting member extends freely through the centre of the disc, the support plate and the central arm of the spring member and is enlarged at or towards its ends to limit the maximum spacing between said region of the spring portion and the central area of the disc which central area constitutes said sensor part.

4. A thermal switch according to Claim 2 or 3 including a pot-shaped housing having a base which, together with the support plate defines a chamber in which the temperature sensor is located, and a closure piece mounted in the housing and spaced from the support plate with the spring member therebetween, the second contact stud being carried on the closure piece.

5. A thermal switch according to any one of the preceding claims including resetting means operable to apply an external force to the spring member and return the spring portion to the first stable position when the sensor part is in the first position.

6. A thermal switch according to Claims 4 and 5 in which the closure piece is formed with a plurality of circumferentially spaced longitudinally extending recesses and the resetting means is provided with a corresponding number of extensions which extend through and are guided by said recesses for the application of said external force.

7. A thermal switch according to Claims 4 and 5 in which the resetting means and the closure piece are secured together and are

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. the ring 40, the stationary contact stud 35 is also displaced and, after the spring arm has sprung over, the stationary contact stud and the ring are pushed back into their starting position by the spring force transmitted through the movable contact stud 18. Although, in the simplest form, the ring 40 is manually displaced for the purpose of resetting the switch, it is also possible for mechanical elements such as pistons, cylinders, diaphragms or levers to be brought into action for this purpose. Finally, it is well-known to any skilled person that the temperature at which the bimetallic snap disc 27 springs over can be adjusted through a wide range by appropriate selection of the bimetal, of its dimensions and of the form of the disc. It will be appreciated from the above description that the connecting member 29 is of rod-like or cylindrical form with reduced diameter neck portions 28 and 32 at opposite ends and enlarged retaining head portions 31 and 33 at or towards the respective ends of the member. The neck portion 32 extends freely through the bore 16 in the spring arm 17, the general body portion of the connecting member 29 extends freely through the bore 25 in the base plate 15 and the neck portion 28 extends freely through the bore 30 in the bimetallic snap disc 27. As the distance between the opposed faces of the retaining head portions 31 and 33 is substantially equal to the spacing between the upper face of the spring arm 17 in its first stable position illustrated in Fig. 2 and the lower face of the marginal area about the bore 30 in the snap disc 27 in its first stable position shown in Fig. 2, it will be seen that snapping of the disc 27 to its second stable position shown in Fig. 3 correspondingly displaces the connecting member 29 and this in turn displaces the spring arm 17 to its second stable position. However, if the disc 27 then returns to its first stable position, the axial length of the reduced diameter neck portions 28 and 32 together, provides a sufficient lost motion to prevent any resulting displacement being transmitted to the spring arm 17 which remains in its second stable position until it is restored to the first stable position by operation of the ring 40. WHAT WE CLAIM IS

1. A thermal switch comprising a substantially flat stressed spring member having a portion capable of assuming either one of a first and a second stable position by flexing about a bending region of the member and carrying a first contact stud spaced from the bending region and engageable in the first stable position with a second contact stud, a bimetallic temperature sensor having a part spaced from the spring member and capable of a snapping action under the influence of temperature between a first position and a second position in which the sensor part is displaced in the direction away from said spring member, and a connecting member extending between said sensor part and a region of the spring portion closer to the bending region of the spring member than the first contact stud, said connecting member being operable to transmit displacement of the sensor part to said region of the spring portiononly when the spring portion is in the first stable position and the sensor part is displaced from the first to the second position to cause the spring portion to assume the second stable position.

2. A thermal switch according to Claim 1 in which the spring member is of substantially E-shape with the two outer arms urged towards each other and secured to a support plate to produce stress under the influence of which the central arm which comprises the spring portion is capable of being deflected transversely of the general plane of the spring member between the first and second stable positions which are disposed on opposite sides of said general plane.

3. A thermal switch according to Claim 2 in which the two outer arms of the E-shaped spring member are secured to one face of the support plate, the sensor in the form of a disc is operably associated with the other face of the support plate and the connecting member extends freely through the centre of the disc, the support plate and the central arm of the spring member and is enlarged at or towards its ends to limit the maximum spacing between said region of the spring portion and the central area of the disc which central area constitutes said sensor part.

4. A thermal switch according to Claim 2 or 3 including a pot-shaped housing having a base which, together with the support plate defines a chamber in which the temperature sensor is located, and a closure piece mounted in the housing and spaced from the support plate with the spring member therebetween, the second contact stud being carried on the closure piece.

5. A thermal switch according to any one of the preceding claims including resetting means operable to apply an external force to the spring member and return the spring portion to the first stable position when the sensor part is in the first position.

6. A thermal switch according to Claims 4 and 5 in which the closure piece is formed with a plurality of circumferentially spaced longitudinally extending recesses and the resetting means is provided with a corresponding number of extensions which extend through and are guided by said recesses for the application of said external force.

7. A thermal switch according to Claims 4 and 5 in which the resetting means and the closure piece are secured together and are

displaceable as a unitary assembly to apply said external force to the spring.

8. A thermal switch constructed, arranged and adapted to operate substantially as herein described with reference to the accompanying drawings.