US3291390A

US3291390A - Thermally operated switch

Info

Publication number: US3291390A
Application number: US435643A
Authority: US
Inventors: Ralph E Solomon
Original assignee: Deutsch Controls Corp
Current assignee: Deutsch Controls Corp
Priority date: 1965-02-26
Filing date: 1965-02-26
Publication date: 1966-12-13
Anticipated expiration: 1983-12-13

Description

Dec. 13, 1966 R. E. SOLOMON 3,291,390

THERMALLY OPERATED SWITCH Filed Feb. 26, 1965 :5 Sheets-Sheet 1 CL H F l G. 5 RAMP/7' 1 g' f bw 1966 R. E. SOLOMON THERMALLY OPERATED SWITCH 5 Sheets-Sheet 2 Filed Feb. 26, 1965 l N V! {N 'l '0] EA; PH E. $0.4 OMO/V Z /m K Arm/avers Dec. 13, 1966 SOLOMON 3,291,390

THERMALLY OPERATED SWITCH Filed Feb. 26, 1965 5 Sheets-Sheet 3 FIG. IO

INVENIOR. QALPH E. d'oLoMo/v United States PatentOfii ce 3,291,399 Patented Dec. 13, 1966 3,291,390 THERMALLY OPERATED SWITCH Ralph E. Solomon, Los Angeles, Calif., assignor to Deutsch Controls Corp., Los Angeles, Calif, 21 corporation of California Filed Feb. 26, 1965, Ser. No. 435,643 Claims. (Cl. 236-21) This invention pertains to a switching device operated by a temperature change and, in particular, to a normally closed limit switch opened when elevated to a predetermined temperature.

The switch of this invention finds particular use with water heater controls of the type shown and described in my copending application Serial No. 425,158, filed January 13, 1965, for Energy Cutoff Device. Briefly, the device of the copending application is a safety control to prevent overheating of the water heater in the event of malfunction of the thermostat valve. It acts as an energy cutoff device to shut down the source of heat so that the water in the tank will not reach an unsafe temperature. In accomplishing the safety control. a normally closed switch is series-connected in the magnetic circuit of the safety pilot valve. This circuit is energized by a thermocouple positioned to receive heat from the pilot burner. It includes a coil capable of producing sufficient force to hold a shutoff valve in an open position so long as the thermocouple is receiving heat and can generate current. In conventional operation, if the pilot should go out, the current flow will cease so that a spring then will close the shutoff valve because the coil no longer exerts a holding force. This shuts off the main gas supply to the burner. According to the invention in my copending application, the normally closed temperature-sensitive switch is placed with the rod-and-tube element in the water storage tank. Consequently, the switch is sensitive to the temperatures within the tank. If the temperatures should reach a predetermined unsafe value,- the switch is opened, thereby interrupting the current flow through the coil so that the main shutoff valve will be moved to its closed position by its biasing spring.

It is essential that the temperature switch utilized in such a safety control have absolute reliability of operation. If this is not the case, there is no real safety in the use of the device. Moreover, it should be opened at an exact temperature on each actuation of the switch. If it does not have this characteristic, the danger exists that it can open too late, allowing a hazardous overheating of the water in the tank. If it opens too soon, on the other hand, it gives a false shutdown, incorrectly indicating that the thermostat had malfunctioned. Another requisite for a switch to be used with such invention is that it be miniaturized so that it can fit in the space between the rod and tube of the thermostat without modification of the rod-and-tube construction. This enables the switch to be positioned within the tank, where it will be subjected to ambient temperatures in that region.

The present invention accomplishes these results by the provision of a switch incorporated in an annular body that can be dimensioned to circumscribe the rod and fit .Within the narrow confines of the rod-and-tube element.

One end of the bimetal is fixed, while the opposite end an extremely small amount of movement of the bimetal element. Consequently, the spring rate factor is negligible and contact pressure is almost constant in the range of closed contact operation. Its operational characteristics are similar to a snap switch, and it is easily constructed to provide a break point within 1 F. at any calibration point. The semicircular configuration of the bimetal element provides optimum contact wipe Without wear of the contacts.

Accordingly, it is an object of this invention to provide a reliable thermally operated switch device.

Another object of this invention is to provide a thermal switch that will operate at a precise set temperature.

A further object of this invention is to provide a thermal switch of compact and low cost construction.

An additional object of this invention is to provide a miniaturized thermal switch adapted to fit in an annular space such as that between the rod and tube of a rodand-tube thermostat element.

Yet another object of this invention is to provide a thermal switch that is adjustable so that it can be set to open at a desired temperature.

These and other objects will become apparent from the following detailed description taken in connection with the accompanying drawing in which:

FIGURE 1 is a side elevational view, partially broken away, of a water heater control utilizing the thermal switch of this invention;

FIGURE 2 is a schematic diagram of the electrical circuit in which the switch is incorporated;

FIGURE 3 is an enlarged fragmentary sectional view of the portion of the rod-and-tube thermostat in which the switch is installed;

FIGURE 4 is an enlarged transverse sectional view taken along line 44 of FIGURE 1;

FIGURE 5 is an enlarged transverse sectional view taken along line 5-5 of FIGURE 1;

FIGURE 6 is an exploded perspective view of the components of the switch;

FIGURE 7 is a perspective view of the switch body as seen from the end opposite from that shown in FIG- URE 6;

FIGURE 8 is a side elevational view, partially in section, of a different embodiment of the switch of this invention in which the bimetal is in a different plane;

FIGURE 9 is a transverse sectional view taken along line 99 of FIGURE 8; and

FIGURE 10 is an axial sectional view taken along line 1010 of FIGURE 9.

With reference to the drawing, a typical storage-type water heater, as shown in FIGURE 1, will include a thermostat control 10 from one side of which extends a boss 11 that is threaded into a coupling 12 that extends through the insulated wall 13 of the water storage tank. This positions the rod-and-tube probe 14 within the interior of the tank where it is subject to the temperature of the water. The unit 14 includes a tube 15 of a material of a high coefficient of thermal expansion, such as copper, the inner end of the tube being fixed to the shank 11 which is also fixed to the housing of the thermostat assembly 10. At its outer end, the tube 15 carries a rod 16 that is coaxial with the tube. The rod 16 is of a material of low coefficient of thermal expansion, such as Invar. Consequently, when the temperature of the temperature of the water rises in the tank, the rod 16 will be moved outwardly, or to the right as illustrated, by the more rapidly expanding tube 15. Conversely, upon reduction in temperature, the rod 16 is moved inwardly, or to the left.

The inner end of the rod is associated with the thermostat valve (not shown) that controls the flow of gas to the burner. Thus, when the tube expands from an increase in temperature in the tank, the outward movement of the rod 16 causes the thermostate valve to close to shut off the supply of gas to the burner. When the water temperature cools and the tube contracts, the inward movement of the rod 16 opens the thermostat valve to again supply heat to the contents of the tank.

The safety control system with which the thermal switch of this invention is particularly adapted for use is illustrated schematically in FIGURE 2. A thermocouple 17 is positioned adjacent the pilot 18 for the gas burner. The heat from the pilot causes the current to flow from the thermocouple 17 through its lead 19 to a coil 20. This is a holding coil that generates a force sufficient to keep an associated shutoff valve 21 in an open position against a spring biasing the valve closed. This allows gas to flow from inlet line 22 into outlet line 23 that leads to the burner. Thecoil 20 is grounded through a conductor 24. Included in this conductor is a thermal switch 25 normally closed and opened upon elevation to a predetermined temperature. When this temperature is reached, therefore, and the switch 25 opens, the circuit that includes the thermocouple 17 and the coil 20 is interrupted so that current can no longer flow. As a result, the shutoif valve 21 is moved to the closed position and gas can no longer flow from the inlet line 22 to the outlet line 23 so that the burner ceases operation. This, of course, stops the temperature rise in the tank.

In accordance with the provisions of this invention, the switch 25 is made to an annular configuration so that it fits in the space 27 between the rod 16 and the tube 15 of the probe 14 of the thermostat assembly (see FIGURE 1). As illustrated in the enlarged views of FIGURES 3, 4 and 5, as well as the perspective views of FIGURES 6 and 7, the switch 25 includes an annular body 28 of dielectric material, such as a suitable plastic. In one radial face 29 of the body 28 is an arcuate recess 30 that normally extends through an arc of around 240. Extending through the body 28 parallel to its axis are two diametrically opposed apertures 31 and 32 that communicate with the recess 30. The opening 32 extends into a relatively shallow portion 33 of the recess 30. A rivet 35 extends through the opening 32, carrying at its end a bimetal element 36. The latter member is welded to the rivet 35 and has an arcuate configuration so that it fits within the recess 30. Both the bimetal element 36 and the recess 30 are coaxial with the body 28 and hence the rod and tube. The bimetal element 36 is arranged so that its more rapidly expanding side 37, which is one of its principal flat surfaces, is toward the inner wall of the recess. Consequently, as temperature increases, the bimetal will expand so that its distal end will become deflected to the right from the position shown in FIGURE 3. In other words, when the temperature increases, the bimetal bends outwardly with respect to the recess 30, so that its outer end moves generally parallel to the aXis of the switch.

At its outer end, the bimetal element 36 carries a contact 38, preferably of gold material. The contact 38 is on the inner surface 37 of the bimetal element 36 that faces toward the bottom of the recess 30. In the normal position, the movable contact 38 engages a second contact 39, which also may be of gold and carried on the end of a brass screw 40. The latter element threads into the opening 31 in the body 29. The screw 40 has a head 41 provided with a cross slot 42 to permit the axial position of the screw 40 to be adjusted. This, in turn, controls the temperature at which the contacts become separated upon expansion of the bimetal element from heat.

A sheet metal resilient grounding member 44 intercom nects the head 41-of the screw 40 and the rod 16. The member 44, preferably of spring beryllium copper alloy, is bent to substantially the shape of a figure 8 with an upper portion 45 of relatively small diameter wrapped around the head 41 and a larger lower section 46 cir cumscribing the rod 16.

The member 44 provides a dual function. One aspect of it is to position the switch assembly within the space 27 between the rod and the tube so that it will remain substantially axially fixed and will move with the rod 16 through its small increment of travel. This mechanically connecting effect is accomplished by the gripping force on the screw head 41 and the rod 16 by the upper and

lower sections

45 and 46 of the member 44 due to the resilience of the member 44. Additionally, the member 44 provides an electrical connection between the screw 40 and the rod 16. In this respect, it acts as the grounding connection in the electrical circuit with which the limit switch is associated. The rod 16 is grounded so that the limit switch is associated. The rod 16 is grounded so that the member 44 provides an electrical circuit from the contact 39 through the screw 40 to ground. Obviously, with the switch closed and the

contacts

38 and 39 in engagement, there is a ground connection through the switch.

The conductor 24 makes its connection to the switch 25 through a washer plate 47 that fits in a recess 48 in the radial face 49 of the switch body 29. The washer plate 47 is located beneath the upset head 50' of rivet 35, holding the plate in the recess 48 and making an electrical connection to it. The end of the conductor 24 is soldered, brazed or welded to a tab 51 that projects outwardly from the washer plate 47. As a consequence, there is a circuit through the tab 51, the washer plate 47, the rivet 35 and the bimetal element 36 to the contact 38. This circuit is completed through the contact 39, the screw 40, the grounding element 44 and the rod 16. It can be seen, therefore, that this switch normally provides a closed circuit by which the coil 20 of the safety valve 21 can be energized when the thermocouple 17 is subjected to heat.

However, in the event that a predetermined temperature is reached in the tank, the bimetal element 36 will expand so as to move the contact 38 axially outwardly away from the contact 39. As a result, the switch is opened, the circuit is interrupted and the coil no longer will be energized. Upon reaching the critical temperature, therefore, the valve 21 will be closed so that energy no longer will be supplied to the burner. When the energy source is shut off in this manner, there will be no further temperature rise in the tank.

The switch of this invention operates with complete reliability with practically no variation in the temperature at which the contacts are opened. The semicircular configuration of the bimetal element assures that a good electrical connection can be effected through the

contacts

38 and 39, while allowing the contacts to separate rapidly when the limiting temperature is reached so that a quick interruption at exactly the desired temperature value is achieved. When the contacts are closed, a typical unit will have less than .005 ohm resistance. Thus, even for the very small current flow of a thermocouple circuit, the device will not provide undue resistance to the current. Its rate of movement is only S 1O* F. This means that there is virtually no elfect on the switch from the spring rate of the bimetal, which becomes a negligible factor. Contact pressure, therefore, is practically constant in the range of closed contact operation.

Another arrangement of the switch of this invention may be seen in FIGURES 8 and 9, where again the device is of compact annular construction, but in which the plane of the bimetal is rotated from the previously described arrangement. The annular body 52 is of dielectric'material and generally similar to the body 28 described above. It includes an arcuate recess 53 in one radialend face 54. The recess receives a bimetal element 55 which has its more rapidly expanding surface 56 facing the axis of the body. On one end, the bimetal element is fixed, being suitably connected to a pin 57. The latter element extends through the body past the opposite radial face 58. At this location, it is attached to the electrical conductor 24.

At its free outer end, the bimetal 55 carries an electrical contact 59. This normally engages a fixed contact 60 carried by a post 61 extending through the body from the recess 53 to a location beyond the end 58 of the body. The outer end of the element 61 may be connected to ground by a member similar to the resilient connector 44-, or by other means. Contact pressure is controlled by a screw 62 extending radially into the body 52 from its circumferential surface to engage the midpoint of the post 61 in an enlarged recess 63. The screw 62 can hold the post 61 toward one side or the other of the recess 63 to thereby control the force of engagement of the

contacts

59 and 60.

Operation of the embodiment of FIGURES 8 and 9 is basically the same as in the previously described embodiment, with the bimetal expanding upon increase in temperature to lift the contact 59 away from the stationary contact 60. As before, the device is particularly adapted for use in a safety system for a water heater, fitting in the small space between the rod and tube of the thermostat probe.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

Iclaim:

1. A thermally operable switch comprising a body of dielectric material,

said body having an arcuate configuration and including a radially extending face,

an electrically conductive bimetal element,

said bimetal element being substantially fiat in one plane so as to provide a duality of principal surfaces,

said bimetal element being arcuate,

one of said principal surfaces of said bimetal element being adjacent said radially extending face of said body,

said one principal surface being more rapidly expansible under increasing temperature than the other principal surface, means attaching one end portion of said bimetal element to said body,

said attaching means including an electrical termination means for connection to an electrical circuit, and an electrical contact carried by said body at said radially extending face thereof,

said electrical contact being adapted for connection to an electrical circuit,

said electrical contact being substantially diametrically opposed from said attaching means,

said bimetal element having an end portion in electrically conductive engagement with said electrical contact under ambient conditions below a predetermined temperature,

said bimetal element being deflectable upon reaching said predetermined temperature to separate said end portion from said contact,

said bimetal element being substantially semicircular intermediate said end portion and said attaching means.

2. In combination with a rod-and-tube assembly of a thermostat, said rod-and-tube assembly including a rod of a material of relatively low coefficient of thermal expansion and a tube of a material of relatively high coefficient of thermal expansion circumscribing said rod so as to provide an annular space between said rod and said tube, a thermally actuated switch comprising an annular body of dielectric material circumscribing said rod within said space,

said body having a radially extending face on one end thereof,

6 a bimetal element,

said bimetal element being curved about the axis of said rod in a plane normal to said axis and being substantially flat in said plane, said-bimetal element having a first principal surface on one side of said bimetal element adjacent said radially extending face of said body, and a second principal surface on the opposite side of said bimetal element,

said first principal surface being expansible under rising temperature at a greater rate than the opposite principal surface of said bimetal element, means attaching one end of said bimetal element to said body,

said attaching means being adapted for electrical connection in an electrical circuit, and a screw threadably received in said body and extending substantially parallel to the axis thereof,

said screw having a contact surface on one end thereof adjacent said radially extending face,

said contact surface of said screw being substantially diametrically opposed to said attaching means, said bimetal element having an electrically conductive contact portion on said first principal surface at the opposite end portion of said bimetal element normally engaging said contact surface of said screw for forming an electrical connection therewith at ambient temperatures below a predetermined temperature, said opposite end portion of said bimetal element being movable outwardly away from said contact surface at said predetermined temperature as a result of said more rapid expansion of said first principal surface for interrupting said electrical connection at said predetermined temperature. 3. A thermally actuated switch comprising an annular body of dielectric material,

said body having a radial face at either end therethe first of said radial faces having an arcuate recess therein coaxial with the axis of said body, 7 a bimetal element in said recess,

said bimetal element being of sheet metal construction having a first principal surface facing the bottom of of said recess and a second principal surface facing the exterior of said recess,

said principal surfaces being substantially normal to the axis of said body. said bimetal element being arcuate and coaxial with said body, means fastening one end of said bimetal element to said 'body in said recess,

said fastening means being adapted for connection to an electrical conductor, and a screw threadably engaging said body and extending therethrough substantially parallel to the axis thereof,

one end of said screw extending into said arcuate recess adjacent the opposite end of said bimetal element, the opposite end of said screw extending outwardly beyond th second of said radial faces,

said opposite end of said screw being adapted for connection to an electrical conductor, said one end of said screw having an electrical contact portion normally engaging the opposite end portion of said bimetal element for forming an electrical connection therewith,

said bimetal element being deflectable upon increase in temperature to lift said opposite end portion substantially axially away from said contact portion on said screw for interrupting said electrical connection when a predetermined temperature is reached,

said electrical contact portion on said screw being substantially diametrically opposed to said fastening means,

said bimetal element being substantially semicircular intermediate said opposite end portion and said fastening means.

4. In combination with a storage-type water heater having a tank, a source of gaseous fuel, a burner for the combustion of said gaseous fuel for imparting heat to the contents of said tank, a thermostat control including an expansible rod-and-tube element received in said tank at the bottom portion thereof and sensitive to the temperature at said portion, and a thermostat valve operable by said rod-and-tube element for normally controlling the supply of said gaseous fuel to said burner for maintaining the contents of said tank within a predetermined temperature range, a safety arrangement for preventing overheating of the contents of said tank in the event of malfunction of said thermostat control, said arrangement comprising a shutoff valve for shutting off the supply of said gaseous fuel to said burner independently and irrespective of said thermostat valve,

means biasing said valve to a closed position,

an electromagnetic coil for holding said valve open in opposition to said biasing means,

and a thermally operated switch,

said switch including an annular body of dielectric material located within said tube and circumscribing said rod so that said switch is subjected to the temperature in said tank at said bottom portion, an arcuate bimetal element substantially fiat in a plane normal to the axis of said rod-and-tube element,

said bimetal element including a first principal surface portion adjacent one end of said body and a second principal surface portion opposite to said first principal surface portion, said first principal surface portion being expansible in response to rising temperature at a greater rate than said second principal surface portion, means attaching one end portion of said bimetal element to said body at said one end of said body, a fixed contact carried by said body at said one end thereof, the opposite end portion of said first principal surface portion of said bimetal element normally engaging said fixed contact for completing a circuit through said attaching means to said fixed contact, and conductor means series-connecting said fixed contact and said fastening means with said electromagnetic coil,

said bimetal element being deflected to lift said opposite end portion away from said fixed contact and interrupt said circuit and prevent current flow through said electromagnetic coil upon reaching a predetermined temperature, said fixed contact being substantially diametrically opposed to said attaching means, said bimetal element being substantially semicircular intermediate said opposite end portion and said attaching means and coaxial with said rod.

5. A device as recited in claim 4 including in addition means for adjusting the position of said fixed contact axially with respect to said body,

for thereby adjusting the temperature at which said circuit is interrupted.

References Cited by the Examiner UNITED STATES PATENTS 2,312,479 3/1943 Ray 23621 2,454,552 11/1948 Cobean ZOO-122.1 2,600,692 6/ 1952 Rothwell et a1 200138 X 2,825,785 3/1958 Taylor 200--113.8 3,231,193 1/1966 Coffey 23621 EDWARD J. MICHAEL, Primary Examiner.

Dedication 3,291,390.lfulph E. Solomon, Los Angeles, Calif. THERMALLY OPERAT- ED SWITCH. Patent dated Dec. 13, 1966. Dedication filed Oct. 13, 1969, by the assignee, Controls Company of Awwrica. Hereby dedicates to the Public the entire terminal part of the term of said patent.

[Official Gazette January 20, 1.970.]

Claims

2. IN COMBINATION WITH A ROD-AND-TUBE ASSEMBLY OF A THERMOSTAT, SAID ROD-AND-TUBE ASSEMBLY INCLUDING A ROD OF A MATERIAL OF RELATIVEL LOW COEFFICIENT OF THERMAL EXPANSION AND A TUBE OF A MATERIAL OF RELATIVELY HIGH COEFFICIENT OF THERMAL EXPANSION CIRCUMSCRIBING SAID ROD SO AS TO PROVIDE AN ANNULAR SPACE BETWEEN SAID ROD AND SAID TUBE, A THERMALLY ACTUATED SWITCH COMPRISING AN ANNULAR BODY OF DIELECTRIC MATERIAL CIRCUMSCRIBING SAID ROD WITHIN SAID SPACE, SAID BODY HAVING A RADIALLY EXTENDING FACE ON ONE END THEREOF, A BIMETAL ELEMENT, SAID BIMETAL ELEMENT BEING CURVED ABOUT THE AXIS OF SAID ROD IN A PLANE NORMAL TO SAID AXIS AND BEING SUBSTANTIALLY FLAT IN SAID PLANE, SAID BIMETAL ELEMENT HAVING A FIRST PRINCIPAL SURFACE ON ONE SIDE OF SAID BIMETAL ELEMENT ADJACENT SAID RADIALLY EXTENDING FACE OF SAID BODY, AND A SECOND PRINCIPAL SURFACE ON THE OPPOSITE SIDE OF SAID BIMETAL ELEMENT, SAID FIRST PRINCIPAL SURFACE BEING EXPANSIBLE UNDER RISING TEMPERATURE AT A GREATER RATE THAN THE OPPOSITE PRINCIPAL SURFACE OF SAID BIMETAL ELEMENT, MEANS ATTACHING ONE END OF SAID BIMETAL ELEMENT TO SAID BODY, SAID ATTACHING MEANS BIENG ADAPTED FOR ELECTRICAL CONNECTION IN AN ELECTRICAL CIRCUIT, AND A SCREW THREADEDLY RECEIVED IN SAID BODY AND EXTENDING SUBSTANTIALLY PARALLEL TO THE AXIS THEREOF, SAID SCREW HAVING A CONTACT SURFACE ON ONE END THEREOF ADJACENT SAID RADIALLY EXTENDING FACE, SAID CONTACT SURFACE OF SAID SCREW BEING SUBSTANTIALLY DIAMETRICALLY OPPOSED TO SAID ATTACHING MEANS, SAID BIMETAL ELEMENT HAVING AN ELECTRICALLY CONDUCTIVE CONTACT PORTION ON SAID FIRST PRINCIPAL SURFACE AT THE OPPOSITE END PORTION OF SAID BIMETAL ELEMENT NORMALLY ENGAGING SAID CONTACT SURFACE OF SAID SCREW FOR FORMING AN ELECTRICAL CONNECTION THEREWITH AT AMBIENT TEMPERATURES BELOW A PREDETERMINED TEMPERATURE, SAID OPPOSITE END PORTION OF SAID BIMETAL ELEMENT BEING MOVABLE OUTWARDLY AWAY FROMSAID CONTACT SURFACE AT SAID PREDETERMINED TEMPERATURE AS A RESULT OF SAID MORE RAPID EXPANSION OF SAID FIRST PRINCIPAL SURFACE FOR INTERRUPTING SAID ELECTRICAL CONNECTION AT SAID PREDETERMINED TEMPERATURE.