A SELF-PROTECTED SHEATHED RESISTOR

Abstract: This invention relates to a temperature limiter particular­ ly useful with electric heater elements of the sheathed type. A switch is supported on the flange of the heater by fulcrum means which allow for the oscillatory movement of the switch relatively to the flange. The oscillation of the switch is actuated by a rod engaging both the sheath of the heater and the switch. The switch comprises a movable control pushbutton engaging the flange.

Description: This invention relates to a temperature limiter particularly useful with electric heater elements of the sheathed type operating in air and/or liquid media. As is known, temperature limiters currently known in the art generally operate either on a liquid expansion, or on the differential thermal expansion between two metals, or even, on changes of electric quantities in electronic components. In actual practice, prior limiters exhibit difficulties of application and thermal contact with the heater elements owing to the formation, over time, of oxide or lime or other heat-insulating materials deriving from the type of utilization, whereby the temperature limiter is either caused to be inoperative over time or to operate at different temperature values from the preset ones. Temperature limiters which comprise a rod to be used in combination with a sheathed resistor are known, e.g. from GB-A-1408388, GB-A-1223165, GB-A-1010375, GB-A-964355, GB-A-591997 and DE-A-3 401 937. The thermal differential expansion between the rod and the sheath of the sheathed resistor results in the opening of the circuit of a switch which interrupts the flow of electric power, thus preventing the resistor from overheating. The solutions proposed to obviate the mentioned prior art shortcomings, in particular those related to the use of sheathed electric heating elements in air and/or liquids, have, however, an excessive cost when compared to the cost of the sheathed electric heater element itself to which the temperature limiter is to be applied. In the light of such problems, it is a primary object of this invention to obviate the above mentioned drawbacks affecting the prior art by providing a temperature limiter which operates on the principle of the thermal differential expansion of the materials constituting the sheathed heating element to which the limiter is to be applied, in particular, sheathed (armored) electric resistance heating elements operating in air and/or liquids, said limiter being adapted to perform its action always at the same temperature over time, without any problems of thermal contact. A further object of this invention is to exploit said temperature limiter as a safety switch adapted to cut off the electric power supply to the sheathed heating element on exceeding a given temperature and/or as a temperature regulator or safety thermostat for the plurality of uses in which sheathed electric heating elements which can cooperate with the temperature limiter of this invention are required to operate. Another object of this invention is to provide a temperature limiter with an extremely simple construction, which can be easily applied to existing sheathed electric heating elements. An important object of this invention is to provide a temperature limiter or safety thermostat which is highly reliable and inexpensive. These and other objects, such as will be apparent hereinafter, are achieved by a heating device, as claimed comprising a sheathed resistance heating element and temperature controlling and limiting means, so arranged to provide for temperature controlling and limiting effects and thereby protect said sheated resistance heating element against overheating. Further features and advantages will be apparent from the following detailed description of a heating device according to the invention, as illustrated by way of example, in the accompanying drawing, where: Figure 1 is a perspective view of the heating device according to the invention and incorporating a sheathed heating element of known design (see, e.g. DE-A-3 401 937); Figure 2 shows diagrammatically a side view of the switch in the closed circuit position thereof and in normal operation; Figure 3 is a side view of the switch of Figure 2 in the open circuit position and with the sheathed resistor in the overheated state thereof; and Figure 4 is a partly sectioned front view of the switch of Figures 2 and 3, wherein the pushbutton drawn in solid lines is indicative of normal operation, while the one shown in dashed lines is indicative of the resistor being overheated and the electric circuit opened. With reference to the cited figures, the sheathed electric resistor 10 comprises a tubular metal sheath 11, a coiled resistance heater 12 enclosed in the sheath 11 and embedded in an insulating material 14. The two electric terminals 15 and 16 are connected at the opposite ends of the sheath 11. Close to the terminals 15 and 16, the opposite ends of the sheath 11 are welded to the flange 17 to provide a rigid form of connection. There is provided a normally closed contact microswitch, whose housing 18 is supported on the flange 17 and a rigid rod 19 connected, at its end 20, to the microswitch housing 18 and, at its end 21, to the point 22 on the sheath. DE-A-3 401 937 discloses a quite similar structure, whereby, however, the microswitch is maintained closed by the pushing action of the rigid rod. Contrary to said known device, whereby the microswitch housing is rigidly connected to the flange, according to the present invention, a fulcrum means is provided to support the switch housing 18 on the flange 17, this fulcrum means permitting the rocking movement of the switch housing 18 relatively to the flange 17. The switch housing 18 comprises a control pushbutton 23 for the switch itself. The pushbutton 23 is movable relative the switch housing 18 and effects, when depressed, opening of the normally closed circuit of the switch. The movement of the rod 19 caused by the differential thermal expansion of the sheath 11 causes the switch housing 18 to rock toward the flange 17 and hence the actuation of the pushbutton 23 which, under the action from the flange 17 and the rod 19, is pushed back into the switch housing 18 and causes the circuit to be opened. The fulcrum means which connects the flange 17 to the switch 18 comprises, for example, two supporting members 31; in the drawing the second supporting member 31 is not visible, being arranged symmetrically. The supporting members 31 may be formed from a small plate bent to a "U", with the base of said "U" attached to the flange 17. The supporting members 31 support a pin 30 which engages pivotably with the switch housing 18. The pin 30 is the fulcrum around which the rocking movement of the switch housing 18 takes place. The rod 19 is connected to the switch housing 18 through a hinge connection which enables the switch housing 18 to rotate around the rod 19. The end of the rod 19 (not shown in the drawing) is configured T-like. The two wings of the "T" engage in two seats provided in the switch housing 18. The point 32 of the switch housing 18 whereto the rod 19 is journalled lies preferably between the pin 30 and pushbutton 23. Electrically conductive elastic means are provided which comprise, for example, a wire 40 bent to a U-like shape, in order to effect the electric connection of the electric terminal 41 of the switch and terminal 16 of the resistance heater 1.2. The elasticity of Me wire 40 bent to a U-like shape enables the switch housing 18 to rock relative to the flange 17 and fulcrum 30. Resetting of the circuit of the switch to the closed condition may take place either manually or automatically. Where resetting is provided automatically, known elastic means not forming a part of this invention, which normally resist actuation of the pushbutton 23, automatically determine, on release of the urge from the rod 19, resetting to the closed circuit conditions. Where resetting occurs manually, according to a preferred embodiment of the invention, the pushbutton 23 further comprises elastic locking means adapted to lock the pushbutton 23 in the open circuit position and to resist the action of the cited known elastic means which would tend to close the circuit on removal of the urge from the rod 19. The elastic locking means according to this invention comprise a pin 50 movable crosswise to the direction of movement of the push-button 23. The pin 50 engages, under the urge of elastic means of its own, comprising for example the spring 51, into at least one seat 52 correspondingly provided in the body of the switch housing 18. The seat 52 is provided at a relative position between the pushbutton 23 and the switch which causes the electric circuit to be opened. Preferably, a second seat 53 for the pin 50 is provided at a relative position between the pushbutton 23 and the switch such that, when pin 50 engages into said second seat 53, the electric circuit is closed.

Claims: A heating device comprising a sheated resistance heating element (10) and temperature controlling and limiting means (18, 19) arranged to provide for temperature controlling and limiting effects and thereby protect said sheathed heating element (10) against overheating, wherein: (a) said sheathed heating element (10) comprises: a tubular metallic sheath (11) enclosing a coiled heating resistor (12) embedded in an insulating material (14), two electric terminals (15, 16) connected to said coiled heating resistor (12) and disposed at the opposite ends of said tubular metallic sheath (11), close to the ends thereof; a supporting flange (17) rigidly connected to said tubular metallic sheath (11) close to the ends thereof; and wherein: (b) said temperature controlling and limiting means comprises: a control switch including a rigid housing (18), a movable control pushbutton (21) projecting peripherally of said housing proximately to one end thereof for selective movement between a normally undepressed position and a depressed position pushed inwardly of said housing, and at least one electric terminal (41) extending form said housing oppositely to said pushbutton for connection with one of said terminals of said coiled heating resistor (12), a rigid rod (19) having one free end (20) operatively connected to said switch and the other end (21) rigidly connected to a point (22) on said tubular metallic sheath (11), said rigid rod (19) being responsible for transmitting to said switch movements resulting from the differential thermal expansion between said tubular metallic sheath (11) and said rigid rod (19), thereby providing for said temperature controlling and limiting effects resulting form said switch being selectively actuated alternatively to its open or closed position; (c) said switch is supported on said flange (17) with said pushbutton (23) oriented towards said flange (17) for movement relative to said housing (18) in a direction substantially parallel to said rigid rod (19); characterized in that: (d) said switch is, in operation, normally closed with said pushbutton in undepressed position, (e) said housing is secured to said flange (17) through fulcrum means (30, 31) attached to said flange and intersecting said housing (18) proximately to one end thereof opposed to said pushbutton (23) in such a manner that said housing (18) is rockable about said fulcrum means to cause said pushbutton (23) to move closer and away from said flange (17), (f) said free end (20) of said rigid rod (19) which is operatively connected to said switch is hinged to said housing (18) at a point thereof (32) located between said pushbutton (23) and said fulcrum means (30, 31), whereby upon movement of said actuating rod (19) in response to a differential thermal expansion of said tubular metallic sheath (10) said housing is caused to pivot towards said flange, and said pushbutton (23) is pressure engaged with said flange thus opening said normally closed switch. A heating device according to Claim 1, characterized in that said fulcrum means (30, 31) comprises two supporting members (31) extending from said flange (17), said supporting members supporting a pin (30), said pin being pivotably connected, with the housing (18) of said switch. A heating device according to Claim 1, characterized in that an electric terminal (41) of said switch and an electric terminal (16) of said coiled heating resistor (12) are electrically connected through electrically conductive elastic means (40) allowing said rocking movement of said housing (18), around said pin (30), relative to said flange (17). A heating device according to Claim 3, characterized in that said electrically conductive elastic means (40) comprises a wire bent to an U-like shape. A heating device according to Claim 1, characterized in that re-closing of the circuit controlled by said switch occurs automatically; said pushbutton (23) being returned automatically, by the action of known elastic means, to its normally closed position upon cessation of the overheating. A heating device according to Claim 1, characterized in that the circuit controlled by said switch is re-closed manually; said pushbutton (23) comprising locking means (50, 51, 52) adapted to lock said pushbutton (23) in its open circuit position, against the action of said known elastic means. A heating device according to Claim 6, characterized in that said locking means (50, 51, 52) comprises a pin (50) movable crosswise to the direction of movement of said pushbutton (23); said pin (50) engaging, under the urge of spring means (51), into at least one seat (52) provided in a wall of the housing (18) of said switch, and corresponding to said open circuit position. A heating device according to Claim 7, characterized in that a further seat (53), corresponding to the closed circuit position, is provided in said wall of the housing (18) of said switch; said movable pin (50) engaging alternately into either of said seats (52 or 53) upon the electric circuit controlled by said switch, being open or closed.

Inventors: CAPRARO, DUILIO
Applicants: I.R.C.A. S.P.A. INDUSTRIA RESISTENZE CORAZZATE E AFFINI
Application year: 1985
Main classification: H05B3
IPC codes: H05B1, H05B3