GB2044540A - Thermal cut off switch - Google Patents

Abstract

A thermal cut-off switch for breaking electrical continuity between two lead wires (12a, 12b) utilises the voluminal expansion on melting of a temperature-sensitive material (17) sealed in a space within a housing (11) to move a contact (14) from a first position, in which electrical connection between the two lead wires (12a, 12b) is made via the contact (14), to a second position in which contact between the contact (14) and at least one of the lead wires (12a, 12b) is broken. Optionally, a resilient means (15) is arranged to exert a force on the contact (14) to urge the contact to return from the second position to the first position, in order to provide a self-restoring thermal cut-off switch. The temperature- sensitive material (17) is sealed within the housing space by a slidable resilient bushing (16), a slidable conductive rubber contact or by a resilient diaphragm. <IMAGE>

Description

SPECIFICATION Thermal cut-off fuse This invention relates to a thermal cut-off fuse for beaking electric continuity between two lead wires at the time that the ambience of the fuse reaches a prescribed temperature.

Many kinds of thermal cut-off fuses have heretofore been disclosed to the art. As far as securing electric continuity between two lead wires under the normal working conditions below the prescribed temperature is concerned, those thermal cut-off fuses which use solid contacts are relatively safe and inexpensive. However, such conventional thermal cutoff fuses have required some kind of mechanical actuating means for dependably moving and isolating a contact means from at least one of the leads at the time the temperaturesensitive member reaches the prescribed temperature and melts. Generally, these actuating means incorporate at least one spring means.

The incorporation of such actuating means, therefore, has notably added to complication of the assemblage of smaller thermal cut-off fuses (those which are 3 to 5 mm in diameter and 7 to 10 mm in length, for example.

Further from the operational point of view, variation in the charge imparted to the spring means has resulted, not infrequently, in mechanical failures. Moreover, the incorporation of these actuating means has the disadvantage that the total number of component parts involved are inevitably increased.

In the conventional thermal cut-off fuses using a temperature-sensitive member, the member melts upon reaching the prescribed temperature and the molten material flows into the gaps in the thermal cut-off fuse housings and, owing to its viscosity, locally retards the movements of the mechanical components so much as to impede normal functions of the fuses. As a results, even when the member melts at the prescribed temperature, the separation of the contacts of the fuse may occur only with some time lag.

In the extreme cases, there is the possibility that the component parts of such fuses may fail to function normally to effect separation of the contacts as required.

Having issued from the realization of the state of affairs described above, the present invention has as one of its objects the provision of a thermal cut-off fuse which has a temperature-sensitive member and directly functions to effect separation of its contacts without use of a spring operated actuating means and which, therefore, functions safely after many years of service.

Another object of this invention is to provide a highly reliable thermal cut-off fuse which has a simple construction and uses a small number of component parts.

According to the present invention, a ther mal cut-off fuse for breaking electric continu ity between first and second lead wires at a prescribed temperature comprises a housing, an electrically conductive contact means within said housing movable from a first position to a second position, the contact means in the first position being in contact with first and second terminal portions electrically con nected within said housing respectively to said first and second lead wires, and the contact means in the second position being separated from at least one of the said two terminal portions, and the thermal cut-off fuse further including a temperature-sensitive material tightly sealed in a space within the housing adjacent to one of the contact means when in its first position with a surface of said material opposed to the contact means, said material being adapted to remain solid under normal conditions and at the prescribed temperature being adapted to melt and undergo voluminal expansion, whereby the melting and voluminal expansion of the temperature-sensitive material at said prescribed temperature causes that surface of the temperature sensitive material opposed to the contact means to be moved in the direction of the contact means until the contact means is moved to its second positions and the electrical continuity between the terminal portions and thus the first and the second lead wires is broken.

As described above, the contact means of the thermal cut-off fuse of the present invention is disposed inside the housing and is adapted to be moved from the first position to the second position by the voluminal expansion of the temperature-sensitive member. The thermal cut-off fuse, accordingly, enjoys the advantage that the number of component parts is small and that it provides the required breakage of electric continuity without fail.

In an alternative embodiment of the thermal cut-off fuse of the present invention, a resilient element is disposed within the housing and acts to urge the contact means to return from the second position to the first position when the temperature-sensitive member resolidifies.

The inclusion of a resilient element acting in this way provides a self-restoring thermal cutoff fuse which will break the electrical continuity between the lead wires when the ambient temperature rises above a prescribed value, and will re-establish electrical continuity when the temperature falls back below that value.

The characteristic features of the present invention will become more apparent from the description given in further detail hereinbelow with reference to the accompanying drawings, in which: Figure 1 is a sectioned view of a first embodiment of the thermal cut-off fuse according to the present invention.

Figure 2 (A) is a perspective view of an embodiment of the contact means for use in the thermal cut-off fuse of Fig. 1.

Figure 2(B) is a side view of the contact means of Fig. 2(A).

Figure 3 is a sectioned view illustrating the thermal cut-off fuse of Fig. 1 after electrical contact has been broken.

Figure 4 is a sectioned view of a second embodiment of the thermal cut-off fuse according to the present invention.

Figure 5 is a sectioned view of the third embodiment of the the thermal cut-off fuse according to the present invention.

Figure 6 is a longitudinal sectioned view of a first embodiment of a self-restoring thermal cut-off fuse, held in the normal state.

Figure 7(A) is a perspective view of the contact means used in the embodiment of Fig. 6.

Figure 7(B) is a side view of the contact means shown in Fig. 7(A).

Figure 8 is a longitudinally sectioned view of the thermal cut-off fuse of Fig. 6, in the state assumed after the temperature of the ambience has reached the prescribed temperature of the fuse.

Figure 9 is a longitudinally sectioned view of the second type of self-restoring thermal cut-off fuse according to the present invention, in the state assumed after the ambient temperature has reached the prescribed temperature.

Figure 10 is a longitudinally sectioned view of the third type of self-restoring thermal cutoff fuse according to the present invention, in the normal state.

Figure 11 is a longitudinally sectioned view of the thermal cut-off fuse of Fig. 10, in the state assumed after the ambient temperature has reached the prescribed temperature.

Figure 12 is a longitudinally sectioned view of the fourther type of self-restoring thermal cut-off fuse according to the present invention, in the normal state.

Figure 13 is a longitudinally sectioned view of the thermal cut-off fuse of Fig. 12, in the stae assumed after the ambient temperature has reached the prescribed temperature.

The accompanying Figs. 1 to 3 illustrate a first embodiment of the thermal cut-off fuse according to the present invention. The thermal cut-off fuse 1 of this embodiment is of the so-called tubular type, having a pair of lead wires 3a, 3b extending out through the opposite ends of the housing 2.

The housing 2 has a hollow space within and contains within this hollow space electrically conductive contact means 4 adapted to be slidably movable from the first position shown in Fig. 1 to the second position shown in Fig. 3. The contact means 4 serves the purpose of establishing electric continuity between the two lead wires 3a, 3b at the first position and breaking the electric continuity at the second position. For this purpose, the two lead wires 3a, 3b possess terminal portions 5a, 5b respectively exposed in the hollow space of the housing.In the case of the present embodiment, one lead wire 3a enters the housing interior and the leading end thereof constitutes the terminal portion 5a, and the other lead wire 3b is fastened by caulking to the housing 2 which is made of an electrically conductive material so that the inner wall of the housing itself constitutes the terminal portion 5b.

In this arrangement, when the contact means 4 is at the first position, it remains in contact with the terminal portion 5a at the end of the lead wire 3a and the inner wall 5b of the housing to maintain electric continuity between the two lead wires. When the contact means 4 moves to the second position, however, it still remains in contact with the inner wall 5b of the housing but is separated from the terminal portion 5a of the lead wire 3a and, therefore, breaks the electric continuity between the two lead wires.

One typical shape of the contact means 4 to be used in the present embodiment is as illustrated in Figs. 2(A), (B), for example. This contact means 4 is provided at the center thereof with a circular perforation 4a of a diameter such as to permit the terminal portion 5a of the lead wire 3a to be inserted therethrough with moderate pressure and further provided around the periphery thereof with a plurality of circumferentially spaced petal-like portions 4b rising so much as to come into contact with the inner wall 5b of the housing with moderate pressure. Thus, the contact means 4 has a general appearance of a chrysanthemum-shaped seat. The individual petal-like portions 4b are shaped so as to slant outwardly in the radial direction (as indicated by the solid line in Fig. 2(B)).At the time that the contact means 4 is inserted into the hollow space of the housing, the leading ends of these petal-like portions are slightly bent inwardly in the radial direction (as indicated by the chain line in Fig. 2(B)) and, after the insertion, the petal-like portions arepressed against the inner wall 5b of the housing by virtue of their own resilience.

In this case, since the housing 2 is cylindrical in shape, the contact means 4 is in the general shape of a disc. When the housing has some other cross-sectional shape such as a square, for example, the contact means 4 should, of course, possess the shape of a square in conformity with the square cross section of the housing interior.

In the course of the maufacture of the thermal cut-off fuse of this invention, the terminal portion 5a of the first lead wire 3a is inserted by the exertion of slight pressure into a perforation 4a at the centre of the contact means 4 and the contact means, with the petal-like portions on the periphery thereof bent inwardly, is inserted to the position in the housing interior as illustrated in Fig. 1.

This is the first position for the contact means 4. In the condition illustrated herein, the contact means 4 establishes electric continuity between the two lead wires 3a, 3b.

The hollow space within the housing is divided into the space 6a on the side of the contact means 4 from which the lead wire 3a extends and the space 6b on the other side of the contact means. The space 6a on the side of the contact means adjacent the first lead wire 3a is filled with a temperature-sensitive material 7 well known to the art.

As the temperature-sensitive material 7 is in the form of an ordinary pellet at normal temperature, it can be compression moulded or melt moulded to match the shape of the space 6a in the housing. Once the temperature-sensitive material 7 is inserted into the space 6a, its peripheral surface 7b and rear surface 7c are surrounded by fixed walls and the only surface thereof not in contact with a fixed wall is the surface 7a facing the contact means 4.

To be specific, the peripheral surface 7b of the material 7 is surrounded by the inner wall 5b of the housing and the rear surface 7c thereof is held back by a cover made of a synthetic resin, for example, and adapted to seal the opening of the housing through which the lead wire 3a is inserted after the internal component parts (such as the terminal portion 5a, the contact means 4, the material 7, and a bushing to be described afterward) have been incorporated in the housing.

Between the contact means 4 and the surface 7a of the temperature-sensitive material opposed to the contact means 4, there is interposed a bushing 8 made of rubber or some other suitable resin which remains in intimate contact with the inner wall 5b of the housing, possesses a perforation at the centre for permitting passage therethrough of the lead wire 3a and serves to preclude escape of the material 7 when it is melted at the prescribed temperature. Since the bushing 8 is not fastened to the wall of the housing, it is allowed to perform a sliding motion relative to the housing. When it is thus moved to the lefthand side with reference to the drawing, it can push the contact means 4 from the first position to the second position.

In one aspect, the idea underlying the present invention depends on a special physical property of temperature-sensitive materials in general use. Many kinds of temperature-sensitive materials adapted for the purpose are characterised by undergoing discernible voluminal expansion (generally variable with the range of from to 3 to 8%) at the time they change from the solid to the melted state.

In the thermal cut-off fuse of the construction described above, therefore, when such a material 7 is tightly sealed in the space 6a at the first position of the contact means 4, the thermal cut-off fuse fulfills its function advantageously by operating in the manner to be described below.

The state which the termal cut-off fuse of the present embodiment assumes upon completion of its assembly or during the normal condition existing prior to the elevation of the ambience to the prescribed temperature has so far been described as illustrated in Fig. 1.

While the thermal cut-off fuse remains in this normal state, a continued path for electric current is formed between the lead wire 3a and the lead wire 3b via the terminal portion 5a, the contact means 4 and the terminal portion 5b (the inner wall of the housing).

Thus, the electric appliance incorporating this thermal cut-off fuse is allowed to operate normally.

When the ambient temperature of the thermal cut-off fuse reaches the prescribed temperature (melting point of the temperaturesensitive material) and the electric appliance must be protected by breaking its path of electric current, the material 7 quickly melts.

Consequently, the material undergoes voluminal expansion, which occurs only in the direction of the free surface 7a. The force of this voluminal expansion is transformed into a force which pushes the interposed bushing 8 to the lefthand side with reference to the drawing.

As the bushing is pushed as described above, the contact means 4 which is held in contact with the bushing is similarly pushed to the left and the terminal of the lead wire 3a entering into the housing interior is released from its pressed engagement with the central perforation 4a (Fig. 3). By thus breaking the electrical continuity between the two terminals 5a, 5b, the thermal cut-off fuse discharges its primary object.

Since the temperature-sensitive material 7 is tightly sealed so that it is allowed to expand only in the direction of the surface 7a opposed to the contact means 4, a thermal cutoff fuse of a very simple construction using a small number of component parts can provide the required circuit breakage without fail. The surface 7a of the material which is moved when the material is melted and voluminally expanded is required to keep the molten material from leaking and entering the space 6b on the side of the second position of the contact means until the contact means is in the second position. The part which fulfills this requirement is the bushing 8 in the present embodiment.

In the embodiment set forth above, the contact means 4 is described as being formed of metal. If, however, in place of a metallic contact means, there is used a contact means made of a material having both electrical conductivity and elasticity such as conductive rubber, the bushing 8 can be eliminated. A modified arrangement employing electrically conductive rubber as the contact means 4 is shown in Fig. 4. Here the conductive rubber contact means 4 is positioned to maintain intimate contact with both the terminal face of terminal 5a and the surface 7a of the temperature-sensitive material 7 and is formed to the proper dimensions for maintaining appropriate pressure contact with the inner wall of the housing.This arrangement assures that electrical continuity is maintained between the leads 3a and 3b at normal temperature and that, since the temperature-sensitive material 7 is snugly fitted in the space 6a, the contact means 4 will be moved ta the second position ta break this electrical continuity when the ambient temperature rises to the melting point of the temperature-sensitive material to cause it to melt and undergo voluminal expansion.

In the case of the above mentioned two embodiments, impacts such as external vibrations which are exerted upon the thermal cutoff fuse after the fuse has fulfilled its function of breaking the continued electric path may possibly shake the contact means and bring it into accidental contact with- the terminal portion 5a which has been separated from the contact means.To preclude such an accident, the thermal cut-off fuse of this invention is provided in the second position of the contact means shown in Fig. 3 or Fig. 4 with. a groove 1Q formed in the inner wa[l 5b of the housing, so that after the contact means 4 has been moved to this point, the petal-like peripheral portions 4b (or the periphery of the electrically conductive rubber) of the conact means 4 are allowed, by virtue of their resiliency, to snap into fast engagement with the groove and hold the contact means in the second position.

When the thermal cut-off fuse is provided with such a check member as described above, a coiled spring or other similar means may be contained in the space on the side of the contact means adjacent its second position, to keep the contact means 4, under the normal conditions, pressed toward the first position and, with the force of this pressure, allow the contact means in a form devoid of the perforation 4a to be kept in tight contact with the end face of the terminal 5a of the lead wire so as to establish required electric continuity af the terminals under normal conditions.In this arrangement, when the temperature-sensitive member is melted at the prescribed temperature and the force of the voluminal expansion of the molten member overcomes the force of the spring coil and moves the contact means to the second position illustrated in Fig. 3, the contact means which has been consequently brought into fast engagement with the groove 10 cannot return to its original position despite. the resiliency of the spring coil.

The present invention contemplates not merely thermal cut-off fuses of the axial type as described above, but also thermal cut-off fuses of the vertical or radial type having a pair of lead wires extended from the housing in the same direction. One example of this type af thermal cut-off fuse will be described with reference to Fig. 5. The component parts of this thermal cut-off fuse which are identical or similar to those of the type described above are denoted by the same numerical symbols as used in the preceding embodiment.

The housing 2 in the present embodiment may be moulded of an electrically insulating resin. The two lead wires 3a, 3b extend out of the housing in the same direction and they possess leading ends 5a, 5b which are both within the housing. These leading ends serve as their respective terminal portions. The contact means 4 possesses perforations 4a, 4a', adapted to permit pressed insertion of the aforementioned terminal portions 5a, 5b respectively. The position indicated by the solid line in the drawing wherein the two perforations contain the two terminals is the first position for establishing electric continuity between the two lead wires.

The space 6a on the side of the first position of the contact means 4 is similarly filled with a temperature-sensitive material 7 and sealed tightly. Although the material is tightly sealed in. this space, it is pressed down by the bushing 8 which is slidably set in position within the housing. When the member 7 is melted and is voluminally expanded at the prescribed temperature, it applies pressure to the bushing 8 and eventually causes the contact means 4 which abut the opposite surface of the bushing to be pushed to its second position (indicated by the chain line in the drawing). Consequently, the two terminals 5a, Sb are released from their pressed insertion or contact with the contact means to effect requiced breakage of the electric continuity.

In this case, the contact means 4 is adapted so that it will separate simultaneously from the terminals 5a, 5b of the two lead wires.

Optionally, one of the lead wires may be attached to the housing 2, which would then be made of an electrically conductive material so that the inner wall of the housing will serves as the terminal portion of that lead wire and the contact means 4 may be adapted so that its periphery will remain in contact with the inner wall of the housing and its central portion will come into contact with the terminal of the other lead wire similarly to the construction of the first or second embodiment. When the contact means is made of electrically conductive rubber so as to enjoy freedom of sliding motion on the inner wall of the housing and, at the same time, provide a tight seal to the material then the bushing 8 may be eliminated and the contact means 4 may be directly opposed to or held in intimate contact with the open surface of the material.

Also in the case of the embodiment illustrated in Fig. 5, the cross section of the housing may be circular or rectangular.

The thermal cut-off fuses illustrated in the remaining Figures are modifications of the initial concept of the invention, in that they provide a self-restoring action as distinct from the single cut-off action of the previously described embodiments.

As distinct from the action of the previously described fuses, which break contact once and must then be removed and a new fuse substituted if contact is to be re-made, the fuses to be described below have a selfrestoring action which breaks contact when the temperature rises but remakes contact when the temperature subsequently falls.

A self-restoring thermal cut-off fuse is shown in Figs. 6 to 8, and is of a tubular type having a pair of lead wires 1 2a, 1 2b extending axially in opposite directions from a housing 1. First lead wire 1 2a extends into the housing through an opening in one of its ends, and the end of the lead wire forms a terminal portion 1 3a.

By contrast, the second lead wire 1 2b is attached caulkingly to the other axial end of the housing which is made of a suitable electrically conductive metal, The inner wall 1 3b of the housing, therefore, serves as the second terminal portion for the second lead wire 1 2b.

Within the housing, there is provided contact means 14 which is adapted to remain in contact with both the terminals 1 3a, 1 3b and consequently establish electrical continuity between the first and second lead wires 1 2a, 1 2b while the thermal cut-off fuse is in the normal state, namely when the ambient temperature is below the melting point of the temperature-sensitive material.

Since the housing has a cylindrical shape, the contact means 1 4 is formed substantially in the shape of a disc and is provided on the periphery thereof with a plurality of circumferentially spaced petal-like pieces 1 4a each slanted outwardly in the radial direction as seen in Figs. 7A and 7B. The contact means is inserted into the hollow space of the housing with the petal-like pieces slightly bent inwardly and kept outwardly resilient. The resilient force adds to the security with which the electric contact is maintained between the contact means 14 and the inner wall 1 3b of the housing.

The contact means 1 4 is set in position in such a manner during the assembly of the thermal cut-off fuse that the central portion 1 4b of the disc comes into contact with the end surface of the terminal portion 1 3a of the lead wire inserted into the housing as illustrated in Fig. 6. This is the first position for the contact means 1 4. To ensure perfect contact between the contact means 14 and the terminal portion 1 3a, spring means 1 5 capable of applying a resilient force as described below is disposed in a precompressed state between the rear surface of the contact means and the surface 1 a of the housing opposed to the aforementioned rear surface.

The spring means 15, even in this state, exerts a force in the direction of pressing the contact means 14 against the terminal portion 13a.

On the opposite surface of the contact means 14 is disposed a resilient bushing 1 6 made of rubber or resin and, therefore, able to produce a sealing action in co-operation with the inner wall of the housing and to slide on the inner wall of the housing. Behind the bushing 1 6 is sealed in a temperature-sensitive material 1 7 which is normally solid, and melts at temperatures above the prescribed temperature. The end of the housing is tightly closed with a suitable seal means 1 8 made of resin, for example.

The temperature-sensitive material 17, therefore, is held inside the space on the side of the first position with reference to the contact means as the boundary. The surface of the temperature-sensitive material 1 7 opposed to the contact means 14, though sealed tightly is allowed by the function as described in the previous embodiments to move in the direction of pushing the bushing 1 6. The other surfaces of the temperaturesensitive material are enclosed by the fixed wall surfaces.

The temperature-sensitive materials used for existing thermal cut-off fuses may be used in the present invention. In this case, since the material is generally formed as a pellet, assembly of the fuse can advantageously be effected if the material is preformed by compression moulding or melt moulding to match the shape of the chamber for accommodating the material.

Now, the function of the temperature-sensitive member in the present embodiment is similar to that described in relation to the nonself-restoring fuses described in relation to Figs. 1 to 5 of the drawings, and will be be repeated here except insofar as the operation of the self-restoring fuses illustrated in Figs. 6 to 1 3 differs from the operation of the nonself-restoring fuses.

Thus, after electrical contact has been broken in response to a rise in ambient temperature, the fuse of Fig. 6 assumes the configuration shown in Fig. 8. Two requirements requiring specical attention here are the requirement that even after the contact means 1 4 has been moved to the second position in consequence of the breakage of the electrical continuity, the temperature-senstive material in its molten state is tightly sealed lest it should leak from the space on the side of the first position past the contact means, and the requirement that the spring means 15 acting upon the contact means is fully compressed.

The first requirement is met by forming the outer edge of the flange 1 2c supporting the terminal portion 1 3a of the lead wire 1 2a and the inner edge of the opening 16' of the bushing 16 in size permitting the two edges to form a perfect seal. The second requirement is easily satisfied by using spring means 1 5 possessed of a suitable energizing force.

As the temperature of the ambience falls the temperature-sensitive material 1 7 is gradually solidified with proportional loss of volume. The spring means 55, therefore, pushes the contact means and the bushing back in the direction of the first position in proportion to the loss of volume of the temperature-sensitive material.

By the time the ambient temperature has sufficiently fallen and the temperature-sensitive material has solidified throughout, the contact means 14 again comes into contact with the terminal portion 1 3a as illustrated in Fig. 6, with the result that there is re-established the electrical continuity between the two lead wires.

The alternate breaking and making of the electrical continuity between the lead wires- is repeated each time the temperature of the ambience rises to the prescribed temperature and falls from it.

The self-restoring thermal cut-off fuse, therefore, can be used for the same purpose as the conventional thermostat and enjoys the advantage that the number of component parts is small and the reliability of the operation is high. This invention can provide a miniature self-restoring thermal cut-off fuse approximately 9mm in length and 4 mm in diameter, for example.

When the contact means is made of a known material like electroconductive rubber which combines electric conductivity and resiliency, it can concurrently fulfill the roles of contact member and bushing. As the result, the bushing may be eliminated and the material 1 7 may be held in direct contact with the contact means 14 as illustrated in Fig. 9.

In this case, the breakage of the electric continuity between the two lead wires I 2a, 1 2b which occurs when the material 1 7 is melted and voluminally expanded can be more readily effected by allowing a recess 14' formed in the contact means 14 for the purpose of engagement with the terminal portion 73a and the flange 1 2c to be formed in a blind construction and forming a glange 1 2c of an insulating material to have a size capable of maintaining a sealed state between itself and the recess 14'. of the contact means 14 even when the material 1 7 is in a molten state. Use of a spring means 15 of high strength ensures safe reestablishment of the electrical continuity.

Ample addition to the strength of the spring 1 5 has no adverse effects. This is because the energy of expansion produced by a substance contained in a tightly sealed space is quite large. The reliability of the establishment of electrical continuity, therefore, can be ensured by amply increasing the strength with which the contact means and the terminal 1 3a are pressed against each other while the thermal cut-off fuse is in its normal state or when the electric continuity is being re-established.

Optionally, a resilient piece of rubbery substance may replace the coil spring in the hollow space of the housing adjacent the second position of the contact means. The resilient force exerted by this resilient piece acting as the spring means 1 5 may be utilized for the purpose of moving the contact means between the two positions. Incorporation of this spring means 15 is seen in the embodiment to be described afterward.

The embodiment illustrated in Figs. 10 and 11 represents a self-restoring thermal cut-off of the vertical or radial type having two lead wires 1 2a, 1 2b extended in one and the same direction from the housing 11. In the present and following embodiments, the component parts fulfilling the same functions as the corresponding parts in the embodiment just described are denoted by the same numerical symbols.

The shape of the housing 11 may be freely chosen. For example, the housing may be in the shape of a cylinder or a rectangular column. The present embodiment will be described in terms of a cylindrical housing.

Through one axial end of the housing 11, a pair of lead wires 1 2a, 1 2b extend in the same direction. The two lead wires are both inserted into the hollow space of the housing.

Their leading ends are bent at right angles toward each other so that their tips are radially opposed to each other across a small gap.

They are provided with enlarged portions 1 3a, 1 3b which are intended as lower-side contact faces.

The contact means 14 upwardly confronts these two portions 1 3a, 1 3b. This is the first position for the contact means. The contact means is at all times biased in the direction of the first position by the spring means 1 5. In this case case the spring means 15 is formed of the first coil spring means 1 spa upwardly pressing the peripheral portion 1 4c of the contact means and the second coil spring means 1 sub similarly pressing the central portion 1 4d of the contact means to ensure uniform distribution of the biasing force. It is naturally permissible to use just one coil spring means instead.

In the present embodiment, the contact means 14 itself is formed of two parts i.e. the peripheral portion 1 4c and the central portion 1 4d. By having these two parts assembled through pressed insertion, the contact means is both integrated and solidified functionally.

The central portion 1 4d is upwardly extended in the shape of a shaft through the gap between the two terminal portions 1 3a, 1 3b.

Its upper end terminates in a head portion 1 4e of a slightly increased diameter.

The head portion 1 4e of the contact means 14 is abutted by a diaphragm 16' made of a resilient substance such as rubber and sup ported along the periphery thereof by a sup port 1 9 made of relatively rigid rubber. Be hind the diaphragm 16', there is sealed in a temperature-sensitive material 1 7 of which the surface 1 7a opposed to the contact means is tightly closed with the aforementioned diaphragm 16'.

The lower surface of the housing is sealed with a sealer member 1 8 which is formed of the lid portion 1 8a serving to support the other end of the spring means 15 and the peripheral seal portion 1 8b.

When the thermal cut-off fuse of the construction described above has its temperature elevated to the prescribed temperature, the diaphragm 16' is exposed to the force of expansion of the material 17, so that the portion of the diaphragm held in contact with the head portion 1 4e of the contact means in gradually distended downwardly.

Since the force of this expansion is fairly strong as described above, the contact means 14 is pushed downwardly in spite of the biasing force exerted by the spring means 15, with the result that the electrical continuity between the two lead wires 1 2a, 1 2b is eventually broken (Fig. 11).

When the ambient temperature falls eventually below the present temperature, the material 1 7 begins to solidify with loss of volume. The force generated by the spring means 1 5 moves the contact means 14 upwardly in conjunction with the diaphragm 16' which is in the process of returning to its original position by its own resiliency. Consequently, the contact means is returned to the first position indicated in Fig. 10 and the electrical continuity between the two lead wires is completed.

The diaphragm 16' is allowed to expand only in its central position for the reason that the solidification of the material 1 7 gradually proceeds inwardly from the periphery thereof and consequently, the central portion is cooled last. This limited expandability of the diaphragm 16' is aimed at equalizing the speed of return of the diaphragm 16' with that of the contact means during the solidification of the pellet.

When the diaphragm possesses a very strong resilient force, it can be relied on to fulfil additionally the function of the resilient spring 1 5. In this case, therefore, the spring means 1 5 is completely eliminated and the head portion 1 4e of the contact means is fastened to the central portion of the diaphragm 16' or the diaphragm 16' may be integrally mouled with the cylindrically shaped central portion 1 4e of the contact means.

As a further alternative, the spring means 1 5 may be positioned to act on the side of the contacts 1 3a and 1 3b adjacent the temperature-sensitive material, and be arranged to pull the contact means 1 4 upwards as seen in Figs. 10 and 11 towards the first position.

Further in the present embodiment, the peripheral portion 1 4c of the contact means 1 4 exposed to contact with the terminal portion may be made of the aforementioned electrically conductive rubber material instead of the ordinary metal material. Particularly in the case of the peripheral portion which is adapted to be pressed into union with the shaft portion 1 4d, the functional integration of these two parts can be attained with increased reliability by making the peripheral portion with the electrically conductive rubber and considerably decreasing the diameter of the hole formed for admitting the insertion of the shaft portion 1 4d so that the resilience of the material barely permits the forced insertion of the shaft portion through the hole, This arrangement also contributes to the convenience of the thermal cut-off fuse 6f this invention.

The embodiment illustrated in Figs. 1 2 and 1 3 represents a modification given to the thermal cut-off fuse of the present invention by replacing the coil spring of the resilient spring means by an elastic block of rubber.

The two terminals 1 3a, 1 3b, each have the appearance of a flat heat as though they were shaped by crushing the leading ends of the lead wires inside the housing. The undersides of the terminal portions 1 3a, 1 3b of an increased diameter are kept in contact with the contact means 1 4. This is the first position of the contact means 14. The resilient springs means 1 5 which serves to bias the contact means at all times against the aforementioned undersides of the terminal portions is formed of a block of resilient material such as rubber.

Another difference due to the modification is that the contact means has the shape of a simple, flat disc and, instead, the diaphragm 16' is disposed at a position low enough to come into contact with the remaining sides of the lead terminal portions 1 3a, 1 3b respectively. The two terminal portions, therefore, additionally act as the support portion 1 9 involves in the embodiment described above in relation to Figs. 10 and 11.

Because of this constructions, when the material 1 7 tightly sealed by the diaphragm 16' inside the space on the side of the first position of the contact means within the hollow space of the housing melts and voluminally expands at the preset temperature, the diaphragm 16' is allowed to distend exclusively through the opening between the two flat-headed terminal portions 1 3a, 1 3b to come into contact with the contact means 14 and push the conact means downwardly and by overcoming the resilient force of the resilient block 1 5 and deforming the resilient block downwardly, bring the contact means to its second position, namely the position separated from the two terminal portions 1 3a, 1 3b (Fig. 13).

When the ambient temperature begins to fall and the material consequently begins to solidify, the diaphragm 16' by its own resilient force regains its original shape as illustrated in Fig. 1 2. On the other hand, the contact means 1 4 is also caused by the resilient force of the resilient block 1 5 to return to its first position.

In the case of the present embodiment, the construction of the thermal cut-off fuse is particularly simple. In this construction, the number of component parts can further be decreased by having the contact means 14 made of an electrically conductive rubber material and integrally moulded with the resilient block as the spring means. In this case, the resilient block illustrated in the drawing may be formed in an annular shape with the interior hollowed out.

In the case of the last two embodiments illustrated the housing 11 may be made of an electrically conductive material or electrically insulating material such as a resin. When an electrically conductive material is chosen, one of the lead wires may be directly attached caulkingly to the housing so that the inner wall of the housing will serve as the terminal portion of that lead wire and, between this terminal portion and the terminal of the other lead wire inserted into the hollow space of the housing, the contact means 14 will make and break the electric continuity, in a manner similar to that of the embodiemnt of Figs. 6 to 9.

In any of the self-restoring fuses described, the lead wires 1 2a, 1 2b may be in any of the various shapes including those of circular wires. Optionally, the shell of the housing itself may be used as one of the lead wires.

As described above, the present invention can provide self-restoring type thermal cut-off fuse of very easy fabrication. For the purpose of applications heretofore fulfilled by thermostats, these self-restoring type thermal cut-off fuses by far excel the thermostats in terms of space requirement, costs, reliability of performance and accuracy of operation.

In any event, the present invention offers a thermal cut-off fuse using the smallest possible number of component parts an directly utilizes the property of a temperature-sensitive material for the effective operation of the thermal cut-off fuse. Thus, it can provide a highly reliable thermal cut-off fuse which is free from the failures ascribable to mechanical complication. The temperature at which the electric continuity is broken may be determined by suitably selecting the temperaturesensitive material to be used.

The coefficient of voluminal expansion of the member can, if desired, be easily achieved by incorporation of a suitable foaming agent and, optionally, an agent capable of promoting the action of the foaming agent as will be clear to any person of ordinary skill in the art.

The expression "lead wires" is used in the present invention so comprehensively as to embrace additionally those in the form of terminals.

Claims (11)

1. A thermal cut-off fuse for breaking electrical continuity between first and second lead wires at a prescribed temperature, which comprises a housing, an electrically conductive contact means within said housing movable from a first position to a second position, the contact means in the first position being in contact with first and second terminal portions electrically connected within said housing respectively to said first and second lead wires, and the contact means in the second position being separated from at least one of the said two terminal portions, and the thermal cut-off use further including a temperature-sensitive material tightly sealed in a space within the housing adjacent to one of the contact means when in its first position with a surface of said material opposed to the contact means, said material being adapted to remain solid under normal conditions and at the prescribed temperature being adapted to melt and undergo voluminal expansion, whereby the melting and voluminal expansion of the temperaturesensitive material at said prescribed temperature causes that surface of the temperature sensitive material opposed to the contact means to be moved in the direction of the contact means until the contact means is moved to its second position and the electrical continuity between the terminal portions and thus the first and the second lead wires is broken.

2. A thermal cut-off fuse according to claim 1, wherein the contact means and the surface of the temperature-sensitive material opposed to said contact means have interposed therebetween an electrically insulating bushing capable both of sealing the inner wall of the housing and performing a sliding motion on the inner wall.

3. A thermal cut-off fuse according to claim 1, wherein the contact means is made of electrically conductive rubber, and is capable of sealing the inner wall of the housing, and the surface of the temperature-sensitive material opposed to the contact means is held in direct contact with the contact means.

4. A thermal cut-off fuse according to any preceding claim in which the contact means is held in the second position when resolidification of the temperature-sensitive material occurs.

5. A thermal cut-off fuse according to claim 4 in which the contact means is substantially disc-shaped and is axially movable within a cylindrical housing, a groove on the inner walls of the housing being engaged by the edge of the contact means when the latter is in its second position to retain the contact means in that position when resolidification of the temperature-sensitive material occurs.

6. A thermal cut-off fuse according to any of claims 1 to 3 in which a resilient element is disposed within the housing to urge the contact means to return from the second position to the first position, when the temperaturesensitive material resolidifies.

7. A thermal cut-off fuse according to claim 6, in which the resilient member is a coil spring or comprises a number of coil springs.

8. A thermal cut-off fuse according to claim 6 in which the resilient means comprises a piece of resilient material.

9. A thermal cut-off fuse according to any of claims 6 to 8 in which there is interposed between the temperature-sensitive material and the contact means a diaphragm the edge of which is rigidly supported by the housing, deformation of the diaphragram causing the contact means to move from the first to the second position.

1 0. A thermal cut-off fuse according to any preceding claim wherein the first terminal portion is the leading end of the first lead wire inserted in the hollow space of the housing, and the housing is electrically conductive, the second terminal portion being the inner wall of the housing.

11. A thermal cut-off fuse according to any of claims 1 to 9, wherein the first and second terminal portions connected respectively to the first and second lead wires are the leading ends of the respective lead wires inserted into the housing.

1 2. A thermal cut-off fuse substantially as herein described with reference to Figs. 1 and 3, Fig. 4, Fig. 5, Figs. 6 and 8, Fig. 9, Figs.

10 and 11, or Figs. 12 and 13 of the accompanying drawings.