BACKGROUND OF THE INVENTION 
     1. Field of the Prior Art 
     The present invention relates to a snap-action heat responsive device incorporating a bimetallic strip which is adapted for use with a thermostat, a temperature-protecting device or the like. 
     2. Description of the Prior Art 
     A conventional type of heat responsive device employing a bimetallic strip is previously disclosed in Japanese Patent Publication No. 32945/1979 and Japanese Utility Model Laid-open No. 160445/1983. In general, the heat responsive device of the prior art includes a bimetallic strip consisting of two strips each having a concave shape in cross section. Although it is relatively easy to work a large-sized bimetallic strip having such a concave shape, there is a problem in that, as a bimetallic strip to be worked is reduced in size, it becomes difficult to form a concave shape with high precision. This may lead to a problem in that the prior-art thermostat employing a bimetallic strip having a concave cross-sectional shape is unavoidably increased in size due to limitations imposed on its structure. 
     In addition, it is difficult to apply such a bimetallic strip having a concave cross-sectional shape to a heat responsive device of the type used as a small-sized current limiter or the like which is attached, for example, to a printed circuit board for the purpose of handling an electric current of about one ampere. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a snap-action heat responsive device which exibits a high sensitivity with respect to variations in temperature and the size of which can be reduced to that suitable for use as a small-sized current limiter or the like for handling an electric current of about one ampere. 
     The aforementioned object is achieved by the present invention providing a snap-action heat responsive device comprising a bimetallic strip and a stretcher, the bimetallic strip having two elongated portions arranged with a space therebetween and in opposition to each other so that the elongated portions may be deflected in the opposite directions to each other and the stretcher having a size slightly larger than the aforesaid space and rotatably fitted into the two elongated portions. The two elongated portions of the bimetallic strip oppose each other so that they may be deflected in the opposite directions to double the amount of displacement of the bimetallic strip and in addition so that they may be urged in the opposite directions. This enables swift reverse of the bimetallic strip and also production of a small-sized bimetallic device suitable for use as a miniature current limiter for handling an electric current of about one ampere. Accordingly, the snap-action heat responsive device of the present invention is applicable to a thermostat of the general type used for domestic electrical appliances, motor protectors or the like. In addition, since the inventive device can be reduced in size and its performance has improvements over that of the prior-art device, it can find a variety of uses. 
     Further objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings in which preferred embodiments of the present invention are diagrammatically shown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1(A) is a front elevation of a first preferred embodiment of the present invention and showing a bimetallic strip which is not bent; 
     FIG. 1(B) is a side elevation of the first preferred embodiment shown in FIG. 1(A); 
     FIG. 1(C) is a front elevation of a stretcher used in the first preferred embodiment; 
     FIG. 1(D) is a side elevation of the stretcher shown in FIG. 1(C); 
     FIG. 2(A) is a front elevation of the first preferred embodiment of the present invention but showing the bimetallic strip which is bent; 
     FIG. 2(B) is a side elevation of the first preferred embodiment shown in FIG. 2(A); 
     FIG. 2(C) is a bottom view of the first preferred embodiment shown in FIG. 2(A); 
     FIG. 3(A) is a front elevation of the first preferred embodiment but showing a state wherein the stretcher is engaged with the bimetallic strip which is bent; 
     FIG. 3(B) is a side elevation of the bimetallic strip with the stretcher shown in FIG. 3(A); 
     FIG. 3(C) is a bottom view of the bimetallic strip with the stretcher shown in FIG. 3(A); 
     FIG. 4 is a view used as an aid in explaining the relationship between tensile forces acting on the bimetallic strip constituting the first embodiment; 
     FIG. 5 is a bottom view used as an aid in explaining a state wherein the amount of deformation of the bimetallic strip reaches a dead point owing to a variation in ambient temperature; 
     FIG. 6(A) is a front elevation of the first preferred embodiment but showing the bimetallic strip which has completed its swift reverse owing to a variation in ambient temperature; 
     FIG. 6(B) is a side elevation of the first embodiment shown in FIG. 6(A); 
     FIG. 6(C) is a bottom view of the first embodiment shown in FIG. 6(C); 
     FIG. 7(A) is a front elevation of a second preferred embodiment of the present invention and showing a bimetallic strip which is not bent; 
     FIG. 7(B) is a side elevation of the second preferred embodiment shown in FIG. 7(A); 
     FIG. 8(A) is a front elevation of the second preferred embodiment of the present invention but showing the bimetallic strip which is bent; 
     FIG. 8(B) is a side elevation of the second preferred embodiment shown in FIG. 8(A); 
     FIG. 8(C) is a bottom view of the second preferred embodiment shown in FIG. 8(A); 
     FIG. 9(A) is a front elevation of the second preferred embodiment but showing a state wherein a stretcher is engaged with the bimetallic strip which is bent; 
     FIG. 9(B) is a side elevation of the bimetallic strip with the stretcher shown in FIG. 9(A); 
     FIG. 9(C) is a bottom view of the bimetallic strip with the stretcher shown in FIG. 9(A); 
     FIG. 10(A) is a front elevation of the second preferred embodiment but showing the bimetallic strip which has completed its swift reverse owing to a variation in ambient temperature; 
     FIG. 10(B) is a side elevation of the second embodiment shown in FIG. 10(A); 
     FIG. 10(C) is a bottom view of the second embodiment shown in FIG. 10(C); 
     FIG. 11(A) is a front elevation of a third preferred embodiment of the present invention and showing a bimetallic strip which is not bent; 
     FIG. 11(B) is a front elevation of the third preferred embodiment but showing a state wherein a stretcher is engaged with the bimetallic strip which is bent; 
     FIG. 11(C) is a front elevation illustrating a state wherein the amount of deformation of the bimetallic strip reaches a dead point owing to a variation in ambient temperature; 
     FIG. 11(D) is a front elevation of the third preferred embodiment but showing the bimetallic strip which has completed its swift reverse owing to a variation in ambient temperature; 
     FIG. 12(A) is a front elevation of a modification of the third embodiment; 
     FIG. 12(B) is a front elevation similar to FIG. 12(A) but showing another modification of the third embodiment; 
     FIG. 13(A) is a front elevation of a fourth preferred embodiment of the bimetallic strip of the present invention; 
     FIG. 13(B) is a front elevation of the fourth embodiment but showing the state wherein a stretcher is engaged with the bimetallic strip; 
     FIG. 13(C) is a side elevation of the fourth embodiment shown in FIG. 13(B); 
     FIG. 13(D) is a bottom of the fourth embodiment shown in FIG. 13(B); 
     FIG. 14(A) is a front elevation of a fifth preferred embodiment of the bimetallic strip of the present invention; 
     FIG. 14(B) is a front elevation of the fifth embodiment but showing the state wherein a stretcher is engaged with the bimetallic strip; 
     FIG. 14(C) is a side elevation of the fifth embodiment shown in FIG. 14(B); 
     FIG. 14(D) is a bottom view of the fifth embodiment shown in FIG. 14(B); 
     FIG. 15(A) is a front elevation of another example of the stretcher used in the present invention; 
     FIG. 15(B) is a bottom view of the stretcher shown in FIG. 15(A); 
     FIG. 15(C) is a front elevation of another example of the stretcher used in the present invention; 
     FIG. 15(D) is a bottom view of the stretcher shown in FIG. 15(C); 
     FIG. 15(E) is a perspective view of another example of the stretcher used in the present invention; 
     FIG. 15(F) is a perspective view of one example of engagement between a bimetallic strip and the stretcher shown in FIG. 15(E); 
     FIG. 15(G) is a perspective view of another example of engagement between a bimetallic strip and the stretcher shown in FIG. 15(E); 
     FIG. 16(A) is a front elevation of a further example of the stretcher used in present invention; 
     FIG. 16(B) is a bottom view of the stretcher shown in FIG. 16(A); 
     FIG. 17 is a front view showing in part one example of the bimetallic strip used in the present invention; 
     FIG. 18 is a front view showing in part another example of the bimetallic strip used in the present invention; 
     FIG. 19(A) is a perspective view of still another example of the bimetallic strip used in the present invention; 
     FIG. 19(B) is a front elevation showing a state wherein a stretcher is engaged with the bimetallic strip of FIG. 19(A) which is curved; 
     FIG. 19(C) is a front elevation of another example of the bimetallic strip having an S-shaped form; 
     FIG. 19(D) is a front elevation showing a state wherein a stretcher is engaged with the bimetallic strip of FIG. 19(C) which is curved; 
     FIG. 19(E) is a perspective view of an example of a square bimetallic strip having one side on which an elongated portion is formed; 
     FIG. 19(F) a front elevation showing a state wherein a stretcher is engaged with the bimetallic strip of FIG. 19(E) which is curved. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a snap-action heat responsive device of the present invention will be described below with reference to the accompanying drawings. 
     (First Embodiment) 
     Referring to FIGS. 1(A) to 6(C), an elongated bimetallic strip is indicated at 1 and its longitudinal axis extends vertically as viewed, for example, in FIG. 1(A). A hypothetical reference line 2 along which the bimetallic strip 1 is bent is formed about the substantially central portion of the bimetallic strip 1, and the hypothetical referenece line 2 is inclined with respect to a line normal to the longitudinal axis of the bimetallic strip 1. The bimetallic strip 1 is bent such that opposite ends 3 and 3&#39; thereof oppose each other but are offset from each other in the lateral direction as viewed in FIG. 2(A). The bimetallic strip 1 consists of two strips of metal, one strip &#34;H&#34; having a large coefficient of thermal expansion while the other strip &#34;L&#34; has a small coefficient of thermal expansion. When the bimetallic strip 1 is bent, the former strip &#34;H&#34; constitutes the outer side thereof with the latter strip &#34;L&#34; constituting the inner side thereof. As shown, substantially U-shaped cutouts 5 and 5&#39; are in advance formed in the peripheral edge of the bimetallic strip 1. When the bimetallic strip 1 is bent in this manner, two inner edges 4 and 4&#39; of the bimetallic strip 1 oppose each other with the cutouts 5 and 5&#39; facing each other. 
     Referring to FIGS. 1(C) and 1(D), a stretcher indicated at 6 has wedge-shaped cutouts 7 and 7&#39; at its opposite ends. The stretcher 6 is preferably formed of relatively hard metal such as phoshor bronze, german silver, iron, stainless steel or a ceramic material, and has a thickness t as shown in FIG. 1(D). Each of the U-shaped cutouts 5 and 5&#39; has a width a as shown shown in FIG. 2(A), and the thickness t of the stretcher 6 is slightly smaller than the width a, that is, the thickness t is determined such that a&gt;t. A distance p between the opposite bottoms of the wedge-shaped cutouts 7 and 7&#39; of the stretcher 6 is somewhat greater than a distance b between the bottoms of the U-shaped cutouts 5 and 5&#39; of the bimetallic strip 1, that is, the former distance p is determined such that p&gt;b. 
     The thus-formed stretcher 6 is attached to the bimetallic strip 1 by engaging the wedge-shaped cutouts 7 of the former with the U-shaped cutouts 5 and 5&#39; of the latter. Thus, the space between the opposite ends 3 and 3&#39; of the bimetallic strip 1 which is bent is enlarged as shown in FIGS. 3(A) to 3(C). The distance b between the bottoms of the U-shaped cutouts 5 and 5&#39; thereby becomes equal to the distance p between the bottoms of the wedge-shaped cutout 7 and 7&#39; of the stretcher 6, that is, b=p is established and the state shown in FIG. 3(C) is obtained. In this case, the bimetallic strip 1 is bent such that the strip &#34;H&#34; with a large coefficient of thermal expansion may constitute the outer side thereof while the other strip &#34;L&#34; with a low coefficient of thermal expansion constitutes the inner side of the same. Accordingly, as shown in FIG. 3(C), the opposite ends 3 and 3&#39; of the bimetallic strip 1 respectively tend to move in the directions indicated by arrows n and n&#39; in accordance with a rise in temperature. 
     In this state, the bimetallic strip 1 is deformed such that the distance b between the bottoms of the U-shaped cutouts 5 and 5&#39; is enlarged up to a distance q by the motion of the stretcher 6. As shown in FIG. 4, tensile forces m and m&#39; respectively act on the U-shaped cutouts 5 and 5&#39;. The tensile forces m and m&#39; are divided in the vertical and horizontal directions as shown in FIG. 4, and act on the bimetallic strip 1 in the form of vertical component forces l, l&#39; and horizontal component forces k, k&#39;. If temperature rises in this state, the opposite ends 3 and 3&#39; of the bimetallic strip 1 respectively tend to move in the directions indicated by the arrows n and n&#39; as shown in FIG. 3(C). However, since the aforesaid vertical component forces 1 and 1&#39; act on the bimetallic strip 1 as shown in FIG. 4, the opposite ends 3 and 3&#39; are not allowed to easily move in such directions. On the other hand, as temperature rises, the U-shaped cutouts 5 and 5&#39; in the bimetallic strip 1 respectively act in the direction of the arrows n and n&#39;, thereby urging the opposite ends 3 and 3&#39; of the bimetallic strip 1 in the same directions, respectively. In consequence, bimetallic tensile forces j and j&#39; act on the bimetallic strip 1 in the directions of arrows shown by dotted lines in FIG. 4. Subsequently, as this temperature rise further continues, the bimetallic tensile forces j and j&#39; respectively overcome the vertical component forces l and l&#39;. 
     In this state, the opposite ends 3 and 3&#39; of the bimetallic strip 1 starts to move, and are aligned with each other in the lateral direction as viewed in FIG. 5. In this case, the distance b between the bottoms of the U-shaped cutouts 5 and 5&#39; exceeds the aforesaid distance q shown in FIG. 3(C), and increases up to the distance p between the bottoms of the wedge-shaped cutouts 7 and 7&#39; in the stretcher 6. However, after the bimetallic strip 1 has passed the aforementioned laterally aligned state, the distance b starts to decrease. In other words, a position at which the opposite ends 3 and 3&#39; of the bimetallic strip 1 are aligned with each other in the lateral direction as shown in FIG. 5 is a &#34;dead point&#34;. Immediately after the dead point has been exceeded, the respective opposite ends 3 and 3&#39; are swiftly moved in the directions of the arrows n and n&#39; shown in FIG. 5. Simultaneously, the stretcher 6 is rotated in the direction indicated by an arrow i and is swiftly reversed to a position as shown in FIG. 6. In other words, the positional relationship between the opposite ends 3 and 3&#39; shown in FIG. 3 is swiftly reversed to a reverse positional relationship as shown in FIG. 6. 
     It will be appreciated that the aforesaid function is likewise achieved in another case where the bimetallic strip 1 is bent such that one strip thereof with a large coefficient of thermal expansion may constitute the inner side thereof while the other strip with a small coefficient of thermal expansion constitutes the outer side of the same. 
     (Second Embodiment) 
     The second embodiment shown in FIGS. 7(A) to 10(C) differs from the aforesaid first embodiment only in that a bimetallic strip 11 has a crank-shaped form, but they are substantially the same in the other respects. 
     More specifically, a hypothetical reference line 12 along which the bimetallic strip 11 is bent is formed about the substantially central portion of the bimetallic strip 11 having such a crank-like shape as shown in FIGS. 7(A) to 10(C) A. The hypothetical reference line 12 is extended in the direction normal to the longitudinal axis of the bimetallic strip 11. The bimetallic strip 11 is bent in a manner as shown, for example, in FIGS. 8(A) and 8(B). 
     When the bimetallic strip 11 is bent, it is formed in a U-like shape in front elevation and one strip &#34;H&#34; having a large coefficient of thermal expansion constitutes the outer side of the bimetallic strip 11 with the other strip &#34;L&#34; having a small coefficient of thermal expansion constituting the inner side of the same. As shown, substantially U-shaped cutouts 15 and 15&#39; are in advance formed in the peripheral edge of the bimetallic strip 11. When the bimetallic strip 1 is bent in this manner, two inner edges 14 and 14&#39; of the bimetallic strip 11 oppose each other with the cutouts 15 and 15&#39; also facing each other. 
     The stretcher 6 shown in FIGS. 1(C) and 1(D) is engaged with the U-shaped cutouts 15 and 15&#39; of the thus-formed bimetallic strip 11 in the manner shown in FIGS. 9(A) to 9(C). The operation of the bimetallic strip 11 and the tensile forces acting thereon during temperature rise are completely the same as in the case of the first embodiment, and therefore, the description is omitted. 
     It is to be noted that the bimetallic strip 11 finally is swiftly reversed to the reverse position shown in FIGS. 10(A) to 10(C). 
     It will be appreciated that the aforesaid function is likewise achieved in another case where the bimetallic strip 11 is bent such that one strip thereof with a large coefficient of thermal expansion may constitute the inner side thereof while the other strip with a small coefficient of thermal expansion constitutes the outer side of the same. 
     (Third Embodiment) 
     In the third embodiment shown in FIGS. 11(A) to 11(D), a bimetallic strip extends in the lateral direction as viewed in FIG. 11(A). Two hypothetical reference lines 22 and 22&#39; along which the bimetallic strip 21 is bent are formed on the bimetallic strip 21 such that they extend in the direction normal to the longitudinal axis of the same. The bimetallic strip 21 is bent in a manner as shown, for example, in FIG. 11(B). 
     When the bimetallic strip 21 is bent, one strip &#34;H&#34; having a large coefficient of thermal expansion constitutes the outer side of the bimetallic strip 21 with the other strip &#34;L&#34; having a small coefficient of thermal expansion constituting the inner side of the same. A substantially U-shaped cutout 25 is formed in one edge 23 of the bimetallic strip 21, and an aperture 25&#39; is formed in a portion of the bimetallic strip 21 corresponding to the reference line 22, the portion opposing the U-shaped cutout 25 when the bimetallic strip 21 is bent. 
     The distance b between respective opposing edges 25a and 25b&#39; of the cutout 25 and the aperture 25&#39; is smaller than the distance p between the bottoms of the wedge-shaped cutouts 7 and 7&#39; formed in the stretcher 6 shown in FIGS. 1(C) and 1(D). When the stretcher 6 is engaged with the cutout 25 and the aperture 25&#39;, b=p is established and thus a tensile force is produced between the respective opposing edges 25a and 25b&#39; of the cutout 25 and the aperture 25&#39;. 
     Subsequently, as the temperature of the bimetallic strip 21 rises, the bimetallic strip 21 is deformed to the state shown in FIG. 11(C) by the rotary motion of the stretcher 6 so that an angle θ1 is reduced to an angle θ2. Thereafter, as soon as the dead point is passed, the stretcher 6 is swiftly reversed to the position shown in FIG. 11(D). 
     It will be appreciated that the aforesaid function is likewise achieved in another case where, as shown in FIG. 12(A), the bimetallic strip 21 is bent such that one strip &#34;H&#34; thereof with a large coefficient of thermal expansion may constitute the inner side thereof while the other strip &#34;L&#34; with a small coefficient of thermal expansion constitutes the outer side of the same. In addition, even when, as shown in FIG. 12(B), the portions corresponding to the reference lines 22 and 22&#39; are eliminated and such portions are worked in a straight form, it is possible to achieve the same effect. 
     (Fourth Embodiment) 
     In the fourth embodiment shown in FIGS. 13(A) to 13(D), each of two bimetallic strips 31 has one end secured to a non-bimetallic member 31&#39; such that the resultant bimetallic device as a whole is formed in a subtantially U-like shape. Substantially U-shaped cutouts 35 and 35, respectively, are formed in opposing inner edges 34 and 34&#39; of the respective bimetallic strips 31, and the aforesaid stretcher 6 is engaged with the cutouts 35 and 35&#39;. Since this embodiment is the same as the second embodiment in the other respects, the description is omitted. 
     (Fifth Embodiment) 
     In the fifth embodiment shown in FIGS. 14(A) to 14(D), a bimetallic strip 41 formed in a substantially L-like shape is secured to a non-bimetallic member 41&#39;, thereby obtaining a bimetallic device having a substantially U-like shape as a whole. Substantially U-like shaped cutouts 45 and 45&#39; are respectively formed in face-to-face relationship in opposing inner edges 44 and 44&#39; formed in the bimetallic strip 41 and the non-bimetallic member 41&#39;, and the aforesaid stretcher 6 is engaged with the thus-obtained cutouts 45 and 45&#39;. Since this embodiment is also the same as the second embodiment in the other respects, the description is omitted. 
     (Other Embodiments) 
     In the foregoing descriptions of the respective embodiments, the stretcher 6 is shaped as shown in FIGS. 1(C) and 1(D) by way of example. However, as shown in FIGS. 15(A) and 15(B), a leaf spring 56 may be employed, or a coiled spring 66 may be employed as shown in FIGS. 15(C) and 15(D). In either case, it is possible to achieve the same effect. 
     Also, the stretcher 6 can be attached in various manners. For example, the stretcher 6 is formed in the shape shown in FIG. 15(E), and, as shown in FIG. 15(F), a pair of folded portions 55 and 55&#39; are partially formed on the inner opposing edges of the bimetallic strip 11 in an upright projecting manner. Holes are respectively formed in the folded portions 55 and 55&#39;, and the stretcher 6 may be engaged with the holes. In addition, a method as shown in FIG. 15(G) may also be utilized. 
     The respective embodiments and their modifications illustratively refer to an arrangement in which the stretcher 6 (the leaf spring 56 or the coiled spring 66) is engaged with the opposing inner edges of the bimetallic strip 1 (11, 21, 31 or 41). However, as shown in FIGS. 16(A) and 16(B), the stretcher 6 per se may be formed in a substantially U-like shape and engaged with the bimetallic strip 1 from the outside thereof. In this case, the U-shaped cutouts 5 and 5&#39;  are preferably formed in edges of the bimetallic strip 1 such that, when the strip 1 is bent, the respective edges having the cutouts 5 and 5&#39; are located outside. 
     As illustratively described above in the first, second and third preferred embodiments, a single piece of the bimetallic strip 1 (11, 21) is bent at one portion thereof in an arcuated manner. However, the bimetallic strip 1 and a bimetallic strip 1&#39; may be connected as shown in FIG. 17. 
     In addition, as shown in FIG. 18, the two bimetallic strips 1 and 1&#39; may be constructed such that one end of the strip 1 opposes one end of the strip 1&#39;. 
     Also, the bimetallic strip 1 is formed so as to have a square form as shown in FIG. 19(A) and a square aperture is punched therein. The stretcher 6 may be engaged with the thus-obtained bimetallic strip 1 which is curved as shown in FIG. 19(B). In addition, it is preferred that, after the bimetallic strip 1 has been formed in an S-like shape as shown in FIG. 19(C), the stretcher 6 is engaged therewith as shown in FIG. 19(D). Moreover, it is also preferred that, after an elongated portion has been formed on one edge of the square bimetallic strip 1, the stretcher 6 is engaged therewith as shown in FIG. 19(F). 
     It is to be noted that, when the snap-action heat responsive device in accordance with the present invention is to be used with a thermostat or the like, one end of the bimetallic strip is secured to the thermostat body and the other end thereof is employed as a moving contact. In consequence, the amount of displacement of the moving contact can be made two times as large as that of a typical bimetallic strip. 
     As described above, the snap-action heat responsive device in accordance with the present invention incorporates a bimetallic strip having opposing ends capable of moving in the opposite directions to each other. This produces a bimetallic effect equivalent to twice as large as a typical coefficient at which the bimetallic strip is curved in accordance with a rise in temperature. Accordingly, it is possible to achieve a small-sized and high-sensitivity heat responsive device.
This invention provides a snap-action heat responsive device employing a bimetallic strip which is typically used with a thermostat, a temperature-protecting apparatus or the like. Two elongated portions of the bimetallic strip oppose each other so that they may be deflected in the opposite directions to double the amount of displacement of the bimetallic strip and in addition so that they may be urged in the opposite directions. This enables swift reverse of the bimetallic strip and also production of a small-sized bimetallic device suitable for use as a miniature current limiter for handling an electric current of about one ampere. It is therefore possible to produce a snap-action heat responsive device having a high sensitivity with respect to variations in temperature.