US 3761856 A
A thermal protection device comprises an electric switch mounted in a holder and a contact element movable in the holder into operating engagement with the switch. The contact element is spring-biassed into engagement with the switch but is normally held out of engagement by a washer which abuts against a shoulder formed on each of the contact element and the holder. The washer is fusible at a known temperature and melts at this temperature to allow the contact element to be urged into engagement with the switch.
Description (OCR text may contain errors)
Tlnit Mantelet tateS Patent [191  THERMAL WARNING 0R PROTECTION DEVTCE  Inventor: `lean Mantelet, Paris, France [73'] Assignee: Moulinex, Societe Anonyme,
Bagnolet, France  Filed: Apr. 18, 11972 [2l] Appl. No.; 245,079
 Foreign Application Priority Data Apr. 29, 1971 France 7l 15344  usci. 337/409 [511 intel. Homan/76  riem ofsearch ..337/409,401 408;
 References Cited UNITED STATES PATENTS 3,155,800 ll/l964 Denton....' 337/409  Sept. 25, T973 591,009 10/1897 Robinson et al 337/409 2,934,628 4/1960 Massar et al 337/409 2,707,965 5/1955 Allen 137/75 Primary Examiner-Harold Broome Attorney-Irvin S. Thompson et al.
 ABSTRACT A thermal protection device comprises an electric switch mounted in a holder and a contact element movable in the holder into operating engagement with the switch. The contact element is spring-biassed into engagement with the switch but is normally held out of engagement by a washer which abuts against a shoulder formed on each of the contact element and the holder. The washer is fusible at a known temperature and melts at this temperature to allow the contact element to be urged into engagement with the switch.
4 Claims, 3 Drawing Figures ujm THERMAL WARNING OR PROTECTION DEVICE BACKGROUND OF THE INVENTION` l. Field of the Invention y The invention relates-to thermal protection or warning devices which contain a fusible element the fusing of which results in the opening or closing of an electric contact.
The invention relates more precisely to devices of this kind which comprise a switch, a contactelement which controls said switchl and is mounted in a xed support and which is normally held apart from the switch by a retaining element made of a material which melts at a determined temperature, and a spring interposed between said contact element and a wall of said fixed support, this spring being adapted to bring the Contact element into engagement with the switch when it is freed by the melting of said retaining element.
2. Description of the Prior Art In known devices of this kind the fusible element retaining the contact element is generally in the form of a block of relatively large mass and volume. It follows that when there is an abnormal rise in temperature this fusible element can melt only slowly and release the contact element only with acertain delay, or at least with a certain inaccuracy. In addition, if special precautions are not taken to preserve the contacts of the switch against the action of the molten metal, this material may undesirably restore electrical connection between the contacts.
The aim of the invention is to overcome these disadvantages by providing a protection or warning device of this kind which is rapid, accurate, and reliable in operation.
SUMMARY According to the invention a thermal protection or warning device comprises a switch, a contact element for operating the switch, a holder for the contact element in which the contact element is movable into operating engagement with the switch, spring means in` the holder for biassing the contact element into engagement with said switch, a support shoulder formed on the holder and corresponding support shoulder formed on the contact element, and a retaining element fusible at a known temperature located between said support shoulders normally to hold the contact element out of engagement with the switch against the action of said spring means. The fusible retaining element is thus in the form of a simple key, part of which bears against a shoulder on the fixed support while another part constitutes an abutment for a shoulder on the contact element.
Since a key of this kind can be made with very small dimensions it follows that its mass and volume may be small, so that the aforementioned disadvantages of previously kown devices are avoided.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a view in section of a device according to the invention;
FIG. 2 shows the key constituting the retaining element for the contact element of the device shown in FIG. l;
FIG. 3 illustrates on a smaller scale the application of the device shown in FIG. 1 to the protection of a tank of liquid heated by anl electric resistance from overheating.
DESCRIPTION OF THE PREFERRED EMBODIMENT The thermal projection or warning device ofiFIG. l comprises a switch l0, a contact element l2, which controls the switch and is mounted in a holder or support 114 and which is normally held apart from the switch by a retaining element 16 made of a material, such as bismuth, which melts at a clearly determined temperature, and a spring 118 interposed between the contact element l2 and a wall 20 of the holder 14. As will be understood, the spring 18 is adapted to bias the contact element 12 into engagement with the switch l0 when the contact element is freed due to the melting of the retaining element 16.
The holder 14 is in the form of a tube in which the contact element l2 is mounted for sliding in the form of a piston; one end wall 22 of the tube carries the switch l0 while the other end wall 20 serves as support for the spring 18.
The retaining element 116 is composed of a key having the form of a washer which surrounds the contact element l2 and of which the outer edge 24 bears against a circular shoulder 26 on the wall of the support tube 14, while its inner edge 28 forms an abutment for a corresponding circular shoulder 30 on the contact element.
This shoulder 30 of the contact element is formed by one of the inner faces of a peripheral groove 32 in the contact element. In order to allow the positioning of the washer 16 in the groove 32, the washer is composed of vtwo semi-circular parts which are engaged in the groove by their inner edges, as can be seen in FIG. 2.
The end wall 20 of the tube 14 is composed of a metal lid, disposed over the tube. The washer 16 is imprisoned between this lid and the edge 26 of the adjacent end face of the tube, this edge thus constituting the support shoulder for the washer. The lid 20 is provided at its centre with an inwardly projecting boss 34 which constitutes a centring stud for the spring 18. On its outer face it has a flat annular zone 36 intended to be applied, for example, against a corresponding flat region of the wall 38 of a tank for heated liquid, as illustrated in FIG. 3. The lid 20 is fastened to the support tube by folding its outer edge 40 over an external annular projection 42 provided on the wall of the tube.
The operative portion of the contact element l2 is constituted by an axially directed conical finger 44 made of an electrical insulating material such as porcelain. The switch l0 has two electrically conductive resilient blades 46 `disposed one on each side of an axial plane of the device and normally applied against one another in a contact zone 48 situated in said axial plane. These blades 46 are held in openings 50 in the end wall 22 of the tube 14, which is made of an insulating material, for example porcelain. The blades 46 are respectively provided with inclined faces 52 disposed so as to form a V opening towards the conical finger 44.
As will be understood, the heat received by the metal lid 20 is transmitted direct to the key 16. As soon as the key reaches an abnormal temperature equal to the melting temperature of the material of which it is composed, it melts very quickly due to its small mass and releases the contact element l2. Under the action of the spring I8 the element l2 is thrown towards the switch in the course of this movement the finger 44 engages between the inclined surfaces S2 and moves the blades 46 apart and the contact 4Q between these blades is thus broken. It will be noted that, because of the relatively small dimensions of the key 16, the amount of molten material is very small and that consequently there is no danger of the molten material undesirably restoring contact between the blades 46.
FIG. 3 illustratesfby way of example, the application of the device of the invention to the limitation of the temperature of the vessel 54 of an electric frying appliance. The device, designated generally by 56, is mounted in a circular hole in an arm 58 of a bracket 60 fastened to the vessel. The flat zone 36 of the metal lid is pressed against the wall 38 of the vessel by a spring 62 interposed between the arm 58 and the annular projection 42 on the casing of the device 56. The switch l0 is connected in series in the supply circuit of the heating resistor diagrammatically illustrated at 64. Overheating of the appliance causes the key inside the device to melt, thus releasing the contact element and thereby cutting off current flow to the heating resistor 64.
l. A thermal protection and warning device, comprising a casing in the form of a tube closed by two end walls, a switch mounted in one of said end walls, a switch operating element mounted for piston-like sliding in said tube and biased into operating engagement with said switch by a spring supported by the other end wall, and a retaining element fusible at a known ternperature normally to hold said operating element out of engagement with the switch against the action of said spring, said fusible retaining element being in the form of a washer surrounding the operating element and composed of two separate semi`circular parts with their outer periphery bearing against a circular shoulder formed on the inside surface of the tube and with their inner periphery engaged in a peripheral groove formed in the operating element.
2. A device according to claim l, wherein the end wall for supporting the spring comprises a metal lid disposed over the tube, the washer being imprisoned between said lid and the edge portion of the adjacent end face of the tube, said edge thus constituting the bearing shoulder for said washer.
3. A device according to claim 2, in combination with a vessel for containing hot liquids, the metal lid of the tube being pressed against a wall of said vessel by spring means interposed between a bracket fastened on said vessel and an annular projection provided on the outer wall of said tube.
4. A device according to claim 1, wherein the switch comprises two electrically conductive blades disposed one on each side of an axial plane and normally applied against one another in a zone of contact situated in said axial plane, each of said blades being provided with an inclined surface and said surfaces being mutually disposed to form a V opening towards the operating element, so that when said operating element is freed by melting of the retaining washer the spring will urge said operating element between said inclined surfaces to move the blades apart.
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