Non-Resettable Heat Responsive Safety Switch

1. Field of the Invention

The present invention relates to electrical safety components for the protection of equipment and appliances against overheating, and more particularly to non-resettable safety switches which, when triggered by excess heat, shut down the source of overheating by interrupting an electrical line, and which thereafter must be replaced, in order to reconnect that electrical line.

2. Description of the Prior Art

One non-resettable heat responsive safety switch of this type is disclosed in U.S. Pat. No. 3,821,685. This known device comprises a glass cylindrical envelope having lead-in wires on each end, a conical end closure being connected to one of the lead-in wires, and a matching longitudinally slotted contact shell being connected to the other lead-in wire. This contact shell remains engaged against the conical end closure as long as a packing of fusible granulate, which backs up the contact shell against a compression spring, remains solid and resists the bias of the compression spring. The latter, when allowed to expand as a result of softening and/or fusion of the granulate packing, withdraws the contact shell from the conical end losure, thereby permanently interrupting the connection between the lead-in wires.

This type of non-resettable thermal safety switch is intended to eliminate the risk encountered with conventional bimetallic switches, where the contact points can fuse together as a result of arcing, or where the temperature setting can be deliberately or accidentally distorted so as to render the device ineffective. One shortcoming of the mentioned prior art device is that, under borderline temperatures, the contact between the cooperating conical end closure and the shell, relying on the radial resiliency of the latter, becomes indefinite, instead of remaining intact or being permanently interrupted.

Additional shortcomings of the prior art device relate to its complexity of design, requiring comparatively expensive component parts and stringent quality controls in manufacture and assembly.

It is a primary objective of the present invention to provide an improved safety switch of the above-mentioned type which, while being reliable in operation, is extremely simple in design and inexpensive to manufacture as a mass-produced standardized component.

In order to attain the above objective, this invention suggests a non-resettable heat responsive safety switch in which substantially identical contact sleeves reach into an insulating outer jacket of the device, and a packing of conductive granulate normally extends into the end portions of both sleeves, thereby establishing the normal operating contact. Inside one of the two sleeves is arranged a plunger of nonconductive material, biased against the metal granulate packing by a compression spring so that, when the fusible granulate becomes compressible after acquiring a given temperature, the spring loaded plunger extrudes the metal granulate packing from the associated conductive sleeve, thereby breaking contact.

A particular characteristic of the present invention is that a packing of metallic granulate is used to establish normal contact between the axially spaced contact sleeves. The heat responsive trigger element may be a fusible granulate packing arranged axially behind the conductive granulate packing, in the sense of the spring bias. Alternatively, the fusible granulate and the conductive metal granulate may form a single composite granulate packing.

In another preferred embodiment of the invention, the heat responsive triggering element is mounted on the forward end of a cavity, in a mounting arrangement which yields at the crucial temperature, whereupon the triggering element and the conductive granulate packing normally held in place by it are displaced into that cavity under the action of the compression spring.

In comparison with the known prior art device, the proposed novel component is simpler and more compact in design, using the sleeves around the lead-in wires as conductors and eliminating the special contact components (end closure and shell) which require flexibility of at least one of them, in order to establish a good contact. The component parts of the proposed safety switch are thus extremely simple and few in number, with the result that they can be economically massproduced. The use of a packing of conductive granulate as a contact body, taking the place of multiple contact interfaces in the prior art device, accordingly eliminates several important manufacturing and assembly tolerances, thereby also greatly simplying the assembly operation.

As part of the facilitation of assembly, the present invention further suggests that the two contact sleeves be provided with peripheral collars in the form of integral wall pleats which serve to axially anchor the contact sleeves inside the insulating outer jacket. The latter is conveniently produced from a thermo-setting plastic material, in an injection molding operation. The outer end portions of the contact sleeves are preferably arranged to protrude from the jacket. Into these protruding portions are inserted the ends of the lead-in wires, the latter being permanently secured in a swaging operation. The resulting structure is extremely simple and robust, being also very compact in its overall dimensions.

Further special features and advantages of the invention will become apparent from the description following below, when taken together with the accompanying drawings which illustrate, by way of example, several embodiments of the invention, represented in the various figures as follows:

FIG. 1 shows an enlarged longitudinal cross section of a heat responsive safety switch, representing a first embodiment of the invention;

FIG. 2 shows a similar, second embodiment of the invention;

FIG. 3 shows a third embodiment of the invention; and

FIG. 4 shows a fourth embodiment of the invention.

Referring first to FIGS. 1 and 2, which are similar in many respects, there can be seen two non-resettable heat responsive safety switches representing two embodiments of the present invention. The device consists typically of an outer, generally cylindrical jacket 1 of non-conductive material, holding in its longitudinal axis a contact sleeve 2 and a somewhat longer contact sleeve 3, both sleeves being metal tubes and therefore electrically conductive. The two contact sleeves are arranged in coaxial alignment in the longitudinal center axis of the jacket 1, being axially spaced apart at their inner extremities, while an outer end portion of each sleeve protrudes axially from the jacket 1. Except for their different lengths, the two sleeves are identical in structure, it being also possible, of course, to utilize entirely identical contact sleeves on both sides of the device. Each sleeve has a circumferential collar 9 near its inner extremity formed as an outwardly folded pleat of the sleeve wall.

For production and assembly purposes, the insulating jacket 1 and the two contact sleeves 2 and 3 form a sub-assembly, the jacket 1 being preferably injection molded around the two contact sleeves which, during this operation, are seated on a suitable mandrel. The jacket 1 must be electrically non-conductive and heat resistant. Preferred among materials with these characteristics are thermo-setting synthetic plastics, materials such as glass and ceramics being likewise suitable. Into the protruding end portion of each contact sleeve is inserted an end portion of a lead-in wire 4, the sleeve end portion being thereafter swaged over the lead-in wire, thereby creating a solid mechanical and electrical connection between the wires 4 and the contact sleeves 2 and 3, respectively.

The extremities of the lead-in wires 4 confine between them the actual switching parts of the device consisting of a compression spring 5, a cylindrical plunger 6 of non-conductive material, and packings of conductive metallic granulate 7 and non-conductive fusible granulate 8. The conductive granulate 7 occupies in each case an axial length portion of both contact sleeves, thereby establishing an electrical connection between them. The pressure exerted by spring 6 against the granulate packing assures the maintenance of a good contact.

As the drawing indicates, the only difference between the embodiments of FIG. 1 and FIG. 2 resides in the arrangement of the fusible granulate 8 in relation to the conductive metal granulate 7. In FIG. 1, the two granulates are provided in the form of a granulate mixture 7/8; in FIG. 2 they are provided in the form of separate axially abutting packings. In the latter case, the conductive granulate packing 7 occupies the center of the jacket cavity, engaging short end portions of each contact sleeve, while the fusible granulate packing 8, preferably in the form of a solid pellet, occupies the axial space between the conductive packing 7 and the extremity of one of the lead-in wires 4.

The electrical current flows in both cases from one of the lead-in wires 4 to its associated contact sleeve 2 and 3, respectively, and over the conductive granulate packing 7 to the other contact sleeve and associated lead-in wire 4. This conductive state is normally maintained indefinitely, so long as the temperature of the device remains below a critical temperature at which the fusible granulate packing 8 loses its mechanical resistance through softening and/or fusion.

The establishment of a temperature at or above the fusing temperature of the granulate packing 8 causes the latter to yield under the compressive force exerted against it by spring 6, whereupon the fusible granulate 8 undergoes a reduction in volume, both in the case of the composite granulate 7/8 of FIG. 1 and in the case of the separate fusible granulate packing 8 of FIG. 2. This volumetric change in the fusible granulate packing allows the compression spring 5 to expand against the packing, pushing the plunger 6 ahead of it, and thereby extruding the conductive granulate packing 7 from the contact sleeve 3, to break the electrical contact between the two sleeves. The change in volume of the fusible granulate packing 8 requires either a vacuum between the granular particles, or the possibility for air to escape from the device, or to shift within the device. The latter is made possible through the permeability of the conductive granulate packing 7 and through a radial clearance between the plunger 6 and the surrounding contact sleeve 3. Still another advantageous possibility of operation, suited especially for the embodiment of FIG. 1, foresees the reverse extrusion of the fused non-conductive material from the packing 8 through the annular gap between the contact sleeve 3 and the advancing plunger 6 into the space occupied by the compression spring 5. The fused non-conductive material thereby performs a spark extinguishing function, as the plunger 6 extrudes the conductive granulate packing from the sleeve 3, moving beyond the extremity of the latter and interrupting the current flow.

The composite granulate packing 7/8 has the additional advantage of good heat conductivity from the outside to the particles of the fusible granulate 8, because of the intimate contact between the latter and the particles of the conductive granulate 7. This feature enhances the speed of response to a heat build-up in the area surrounding the safety switch.

The embodiment of FIG. 2, featuring separate packings of conductive granulate 7 and fusible granulate 8, responds to overheating in a way which is similar to that previously described in connection with the embodiment of FIG. 1. Depending upon the structure of the two granulate packings, the volumetric compressibility of the fusible packing 8 may be the result of air escaping from the packing either across the conductive granulate packing 7, or axially to the outside along the lead-in wire 4. Alternatively, the fused material of the packing 8 may itself penetrate into the interstices between the particles of the metal granulate 7.

In FIG. 3 is illustrated a third embodiment of the invention, where the previously employed non-conductive fusible packing 8 has been replaced by a fusible trigger disc 10. As long as it remains in the solid state, the trigger disc 10 serves as an axial barrier and abutment member for the conductive granulate packing 7 which again occupies a central position between the contact sleeves 2 and 3, reaching into the end portions of both. The trigger disc 10 is so arranged that a small peripheral radial overlap configuration with a groove arranged inside the contact sleeve 2 holds the disc in place, against the axial pressure of spring 5. Once the critical temperature is reached or exceeded, the thrust supporting peripheral portion of the trigger disc 10 quickly yields, whereupon the axial column consisting of the disc itself, the conductive packing 7, and the plunger 6, is thrust in the direction of spring pressure, for a clean and quick interruption of the current. The radial groove supporting the fusible trigger disc 10 inside the contact sleeve 2 is conveniently associated with the collar pleat of the latter. The disc itself may be made of a eutectic solder metal which softens and fuses at the critical temperature.

In a fourth embodiment of the invention, illustrated in FIG. 4, there is again provided a trigger body forming an axial barrier against the conductive granulate packing 7, at a distance from the lead-in wire 4. But, instead of using a peripheral groove to axially retain the trigger disc 10', the latter is supported on a central stem extending axially from the disc 10' to a similar disc engaging the end face of the lead-in wire 4. This trigger body 11 has in this case the shape of a double mushroom, axially spaced head portions being linked by a central stem. Responding to overheating, the trigger body 11 simply collapses axially at its stem, the disc 10', the conductive granulate 7, and the plunger 6 executing again a rapid contact breaking motion under the effect of spring 5. In this context, it will be readily understood that the double-mushroom-shaped trigger body 11 could be provided as a composite part, using fusible material for the stemp portion only, while the disc portions on both extremities serve to center the stem portion and to confine the granulate packing 7.

Instead of using a metal granulate for the conductive packing 7, it is also possible to use a filling of small metal spheres, the latter having the advantage of responding to the spring-generated axial preload by creating an even contact pressure at all points of engagement with the contact sleeves 2 and 3. Alternatively, it is also possible to use a contact body of solid metal, provided such a body is radially flexible to the extent of establishing contact with both contact sleeves, while permitting sliding disengagement from one of the sleeves. Such flexibility is obtainable, for example, with a longitudinally slotted tubular body.

Lastly, it should be understood, that the foregoing disclosure describes only preferred embodiments of the invention and that it is intended to cover all changes and modifications of these examples of the invention which fall within the scope of the appended claims.