Technical Field

The present invention relates to electrical automatic control and regulation devices and is designed for closing or opening an electrical circuit when the environmental temperature deflects from a specified value. Standard requirements to such devices are their small dimensions, electrical safety, hermetic tightness, easy mounting inside the temperature-controlled appliances, and cost efficiency. hi order to obtain good characteristics, such as high accuracy of temperature control, reliability, and fast response, it is necessary to manufacture the sealed relay housing of a strong and thermally conductive material, while the thermally sensitive element must possess low heat capacity, fast response, and preserve its physical/mechanical properties in time.

Many tasks related with electrical automatic control of thermal processes may be reached by using bimetallic snap-action disks as thermally sensitive elements, capable to change abruptly their configuration when a certain critical temperature is reached. hi order to provide the snap switching function, the disks are manufactured in form of a truncated cone, or a spherical part usually called a "dome". The dome changes its shape to opposite with a snap at a certain temperature. In other words, shape inversion of the disk takes place. The bimetallic disk in this case plays the role of a motion activator, the motion then being transmitted to a current-conducting spring contact, which either opens or closes the controlled electrical circuit. Since the disk inversion represents a small displacement, various lever mechanisms are used as motion transmission means from bimetallic disk to the spring contact, in order to provide the desired clearance between the contacts.

Minimization of the relay housing dimensions in the case when it is made of an electrically conductive material requires the electrically interacting elements to be located in positions preventing their discharge on the housing and utilization of small-diameter bimetallic disks. Manufacturing of thermally sensitive bimetallic disks designed for high upper switching temperature and having diameter less than 13 mm must be performed with account of altered physical and mechanical characteristics of such disks. Particularly, such factors should be taken into account as dependence of the critical temperature on the disk thickness / diameter ratio, dome displacement value as a function of its thickness, and bending characteristics of the commercially available bimetallic disks (ranging from 5 10 ^"6 K ^"1 to 22.7 10 ^"6 K ^"1 ), as well as limitations on the admissible bending tension as a function of temperature. Background Art

Known is the bimetallic thermal relay produced by Microtherm [1], in which the displacement of bimetallic disk is transmitted to the current-conducting spring contact by means of a lever. This prior art has the following shortcomings: large dimensions, no sealing means, low value of the upper critical temperature, T = 425° C.

Known are also the bimetallic thermal relays produced by Ehnwood [2] and Microtherm [3], which have acceptable and steady dimensions. However, these relays are complex and expensive, since they use high-precision ceramic parts, are non-hermetic and limited in the upper critical temperature.

Known is the device [4] in which the roles of thermally sensitive and current-conductive elements are combined in a single bimetallic disk having openings thereon connected with the central hole by slots releasing the undesirable bending tensions when the snap-action temperature is reached. The latter device, however, has short service life, due to the following structural deficiencies.

Mechanical load created by an adjustment screw provided in the disk center in order to obtain a reliable contact and set up the desired critical temperature requires manufacturing of thick

bimetallic disks with an obviously higher snap-action temperature. This fact does not release the disk from tensions at low temperatures, which limits the range of operation temperatures.

The closest prior art, selected as a prototype to the present invention, is the device [5], comprising a basic mounting plate made of a metal and a metallic housing, which being fastened together form a sealed chamber. Two electrodes are fixed to the main mounting plate, electrically insulated both from each other and the mounting plate. The external ends of these electrodes form electrical outputs of the device, while one end inside the chamber has a fixed contact attached thereon.

A thermally sensitive switching mechanism is mounted inside the chamber, including a thermally sensitive bimetallic disk and a movable contact attached to one end of a current- conductive spring, having motion transmission elements provided thereon, which transmit the snap action of the disk to the movable contact, either opening or closing an electrical circuit.

The thermally sensitive bimetallic disk is positioned either above or below the current conductive spring disk, allowing free positioning of one disk end, while the other disk end and the second end of the spring disk are attached by a cantilever to the upper part of the housing.

This device, although advantageous over the prior art [1] - [4], has a number of shortcomings. They include a limited range of the upper critical temperature variation, due to the mounting method of the thermally sensitive switching mechanism and electrodes; large dimensions due to utilization of large bimetallic disks; low electrical safety, because electrical connection with the housing exists; complicated structure and adjustment procedure, due to cantilever attachment of the current-conducting spring and bimetallic disk ends to the housing, with further adjustment of the device performed by deformation of the housing. Disclosure of the Invention

The proposed invention has the objectives to increase the upper critical temperature, reduce the relay dimensions, improve its electrical safety, and simplify the relay structure and adjustment procedure.

In order to reach these objectives, the following structural changes were made in the prior art device, comprising: a basic mounting plate and a housing made of a metal, fastened together and forming a sealed chamber, with a fixed contact positioned therein and attached to an end of one of the two electrodes passing through the basic mounting plate and secured thereto, with electrical insulation provided from the basic mounting plate and between the electrodes; and a thermally sensitive switching mechanism including a thermally sensitive bimetallic snap-action disk and a movable contact attached to one end of a current-conducting spring disk, having its second end secured by a console, with motion transmission elements provided thereon, which transmit the snap action of the thermally sensitive disk to a movable contact, either closing or opening the controlled electrical circuit.

The thermally sensitive snap-action bimetallic disk is realized in form of a truncated cone having a round hole in the center of the dome, and at least three openings symmetrically positioned around the truncated cone axis, in the cross-section periphery. The current-conductive spring disk, with the motion transmission elements provided thereon, is realized in form of a round flat disk having an opening in its body, forming a tongue element, symmetric relative to the disk diameter and extending beyond the disk center, with a round hole provided thereon and aligned with the disk center. The thermally sensitive disk has projection stoppers provided on one surface and the opposite ends of the disk diameter coincident with the symmetry axis of the tongue element. The stoppers limit the free motion space of the thermally sensitive snap-action bimetallic disk above the spring disk surface.

The thermally sensitive spring disk is attached by one end - located under the projection stopper near the tongue element base, with the tongue element orientation towards the electrode having a fixed contact - to the end of the other electrode, while the opposite disk end located under the other projection stopper is attached to the movable contact.

An additional support member is introduced in the thermally sensitive switching mechanism, in form of a round disk having two-sided projections, shaped as two cylindrical pins extending normal to the disk surface.

The thermally sensitive snap-action bimetallic disk is freely positioned in the space bounded by the disk surface and stopper pins of the current conductive spring.

The supporting element disk is freely positioned over the tongue element and under the dome of the thermally sensitive bimetallic disk, while its projections pass through the corresponding holes in the tongue element of current conducting spring disk and in the dome center of thermally sensitive bimetallic disk. The proposed relay also has the distinguishing features in that its stopping projections are executed in form of pins protruding normal to the disk surface of the current conducting spring, the said pins having two tags at their ends, bended inside the disk and positioned on the circle of a smaller radius than the disk radius; the current conducting spring disk has a U-shaped tongue element; the electrodes are made solid; the insulators form convex axially symmetric bulges on both surfaces of the basic mounting plate, enclosing the electrodes.

The proposed embodiment of thermally sensitive bimetallic disk makes possible manufacturing of the disks with diameters less than 9 mm, required traverse value of dome, high upper limit of critical temperature and given hysteresis.

Moreover, the said disks may be manufactured from the commercially available bimetallic materials, rather than from the special materials meeting strict requirements.

Indeed, the temperature dependence of the bimetallic disk displacement, also known as the "disk travel", is given by the following relation (see e.g. [6] and [7]):

where, A - is the disk travel; a - is the specific bending; D - is the disk diameter;

Δ T = T _h — Ti _ is the difference between the upper (Ti ₁ ) and lower (Ti) critical snap-action temperatures; s - is the disk thickness; and C - is the parameter given by the Whittacker function, which takes into account the complex nonlinear dependence of the disk travel on different factors, such as the dependence of the ratio A/ s on temperature. It is usually assumed that C has values 4 or 5, however it is not essential for the following considerations.

As it follows from the relation (1), the upper critical temperature Ti, may be increased if the disk diameter D is reduced, at a constant value of the disk travel A. The relation is valid only if its other quantities are independent, or weakly depend on the temperature. However, when the upper temperature limit Tj, is increased, the range of admissible values of the ratio D/s for the snap-action disks satisfying the requirement of temperature independence, is shifted towards higher values. Therefore reduction of the disk diameter D should be accompanied also by reduction of the disk thickness s.

In all cases, and especially in the case of thin disks, one should take into account the temperature dependence of admissible bending tensions. One method of overcoming this dependence is to remove those parts of the disk body where maximal tensions are generated. For example, for the disk shaped as a truncated cone, the body forming the truncation surface is removed, i.e. the disk assumes the form of a washer, or a dome with an opening.

The following relation gives temperature dependence of the travel for such disks:

where, d - is the inner diameter of the washer, with other notations the same as in (1).

As it follows from the relation (2), one can increase the upper snap-action temperature Ti, at a fixed travel value A by utilizing the disks with D/s possessing lower critical temperature, i.e. larger thickness. With this purpose, the hole diameter d should be increased.

However, larger d results in a smaller mass of the thermally sensitive element, which in its turn reduces the amount of thermal energy accumulated by the element, so that the latter may be insufficient for overcoming the elastic force of the spring snap-action mechanism.

The proposed embodiment of a thermally sensitive element shaped as a truncated cone having round holes on the periphery of the truncation face preserves the sufficient mass for snap action. The disk travel versus temperature dependence is given in this case by the same relation (2) in which d ² should be replaced by a squared effective diameter d _e ff . Clearly d ² < d _e /.

Arrangement of such holes also partially releases the undesirable bending tensions in the disk, which occur in the washer.

In order to prevent the creation of torques on the circle of round holes, the holes should be located symmetrically relative to the axis of truncated cone, while the number of such holes should be not less than three.

One may alter the hysteresis curve of the snap-action disk proposed by this invention, since the presence of openings in the periphery not only prevents creation of undesirable bending tensions in the disk, but also changes the interval of snap-action temperature.

The hole in the dome center serves for automatic alignment of the thermally sensitive disk in its location space.

Relays manufactured as embodiments of the present invention used thermally sensitive disks with a diameter D = 8 mm and thickness s = 0.15 mm, having three holes on the periphery and made of bimetal materials TB0621 produced in Russia and 2500 produced in the U.S. The upper critical temperature of snap action T _h was obtained at a value 480 ⁰ C. Attachment of the current-conducting spring disk to the electrically insulated electrode provides electrical safety of the relay, as well as its operation in the cases of occasional deformation of the relay housing. The latter quality was absent in the prototype, since the device was adjusted by deformation of its housing.

The projecting stoppers of the current conducting spring disk limit the free motion space for the thermally sensitive bimetallic snap-action disk over the spring disk surface. The opening made in the body of the spring disk, near its fixed end forms a tongue element, stationary relative to the opposite non-fixed end. The tongue element provides a stationary support to the introduced support member. The support member projection passing through the central hole in the dome of thermally sensitive bimetallic disk automatically aligns and retains the thermally sensitive disk in the limited space. Another projection of the support element passing through the hole in the tongue element retains the support member disk in the space between the tongue element and dome of the thermally sensitive disk. The free positioning of thermally sensitive and support members allows free displacement, without any restrictions, of the support member projections relative to holes in the dome center of thermally sensitive disk and in the tongue element.

These features of the invention allow positioning of the thermally sensitive bimetallic disk without hard fixing units, releasing the disk of all mechanical strains, up to the snap-action temperature. Thus, the factors causing premature aging of the thermally sensitive disk are removed, which provides a fail-safe functioning of the thermally sensitive switching mechanism. Solid structure of electrodes guarantees reliable work of the device at high temperatures.

Indeed, the prior art electrodes embodied in form of cylinder rods made of iron, or its alloys, and having a core made of copper, or its alloys for better conductivity, can decompose at high temperatures, due to different expansion coefficients of the core and coating materials, capable to separate or expand the core. This may damage the relay sealing, affect its electrical circuits and cause failure.

Insulator bulges provide the necessary discharge barrier between the electrodes and basic mounting plate, and thus the electrical safety of the relay. Their presence becomes necessary as the dimensions of the device are reduced.

Further, the essence of this invention is disclosed by means of a specific embodiment. Fig. 1 is a section through the bimetallic thermal relay according to the present invention; Fig.

2 is a section through the current-conducting spring disk; Fig. 3 is a plan view of the current- conducting spring disk; Fig. 4 is a layout of the current-conducting spring disk before bending, in a plan view; Fig. 5 is a section through the thermally sensitive bimetallic disk; Fig. 6 is a plan view of the thermally sensitive bimetallic disk; and Fig. 7 is a section of the support element. The bimetallic thermal relay according to the present invention comprises a metal basic mounting plate 1, a metal housing 2, solid (or monolith) electrodes 3 and 4, insulators 5 having bulges 6 and 7 provided thereon, a fixed contact 8, a thermally sensitive switching mechanism including a movable contact 9, a current-conducting spring disk 10 with motion transmission elements provided thereon, a thermally sensitive bimetallic disk 11, and a support member 12. The basic mounting plate 1, when secured with the housing 2 (e.g. by welding), form a sealed chamber. The electrodes 3 and 4 are secured in the plate 1 by means of insulators 5 having bulges 6 and 7 preventing the electrical discharge on the housing. The end of the electrode 3 located inside the chamber is attached to a fixed contact 8 (e.g. by a spot welding). The current- conductive spring disk 10 having motion transmission elements provided thereon is realized in form of a planar round disk having an opening 13 made in the disk body, forming a symmetrical relative to the disk diameter U-shaped tongue element 14 which extends beyond the disk center and has a round hole 15 with a center coincident with the disk 10 center. Motion stoppers are provided on the opposite sides of the disk diameter coincident with the symmetry axis of U- shaped tongue element 14, on one side of the disk 10, consisting of two projections 16 normal to the disk 10 surface and two tags 17 attached thereon. The stoppers have their edges parallel to the spring disk surface and bended inside the disk along a circle having radius smaller than the disk radius. The end of the electrode 4 located inside the chamber is attached to the end of the current-conducting spring disk 10, with orientation of the U-shaped tongue element 14 towards the fixed contact 8, under the motion stopper near the base of the U-shaped tongue element 14. The other end of the current-conducting spring disk 10 located under the other motion stopper is attached to the movable contact 9. The thermally sensitive bimetallic disk 11 is embodied as a truncated cone having a round hole 18 in its dome center and at least three round holes symmetrically positioned relatively to the cone axis in the periphery of truncation surface. Peripheral domains of the thermally sensitive bimetallic disk 11 are freely positioned in the space bounded by the surface of the spring disk 10, projections 16 and the edges of tags 17 closest to the surface of the spring disk 10. The support member 12 is realized in form of a round disk 20 having two-sided cylinder projections 21 and 22 normal to the disk surfaces. The projection 21 of the support member 12 passes through the hole 18 of the thermally sensitive bimetallic disk 11. The disk 20 of the support member 12 is freely positioned in the space bounded by the surface of U-shaped tongue element 14 of the current-conducting spring disk 10 and the dome of thermally sensitive bimetallic disk 11. The projection 22 of the support member 12 passes through the hole 15 of the U-shaped tongue element 14.

The operation principle of the bimetallic thermal relay presented by this invention is as follows. As the ambient temperature rises, bending extent of the thermally sensitive disk 11 is reduced. As a result, the edge of the thermally sensitive disk 11 located over the surface of spring disk 10 slowly moves, or slides, towards the projections 16 of stoppers. Meanwhile, the dome of the thermally sensitive disk 11 moves towards the disk 20 of the support member 12, the projection 21 of which interacts with the central hole 18 in the dome of the thermally sensitive disk 11 and automatically aligns the latter with the spring disk 10. When the upper critical temperature is reached, the thermally sensitive disk 11 instantly and with a snap changes its shape to the opposite one (i.e. inversion of the disk bending occurs). As a result of snap action, the dome of the thermally sensitive disk 11 is pressed to the surface of disk 20 of the support

member 12, while the disk 20 itself is pressed to the surface of stationary U-shaped tongue element 14. The edges of the thermally sensitive disk 11 are pressed to the edges of tags 17 provided on the stoppers. The edge of the thermally sensitive disk 11 located in domain of the spring disk 10 attachment to the electrode 4, after shape inversion of the disk 11, are pressed to the edges of tags 17 positioned above the movable contact 9, near the loose end of the spring disk 10, moves the contact 9 and disconnects the contacts 8 and 9. The dome part of the thermally sensitive disk 16 located near the fixed end of the spring disk 10 is pressed to the disk 20 of the support member 19. The other dome part of the thermally sensitive disk 16 is deflected from the surface of the disk 20 of the support member 19 (the dome surfaces of disk 16 and disk 20 are positioned at an angle between them). As a result, a structure is formed in which configuration of its parts may be altered only by the instant shape inversion of the thermally sensitive disk 16. Therefore the contacts 8 and 9 remain open until a new shape inversion of the thermally sensitive disk 11.

As the ambient temperature falls to reach the lower snap-action temperature, the contacts 8 and 9 remain disconnected. External disturbances are unable to change the configuration of interacting parts of the spring disk 10, support member 12, and the thermally sensitive disk 11. As soon as the lower critical temperature is reached, the thermally sensitive disk 11 instantly and with a snap changes its shape to the opposite. As a result, the edges of the thermally sensitive disk 11 return to the surface of the spring disk 10 and terminate their pressure on the edges of tabs 17. The dome of the thermally sensitive disk 11 separates from the surface of disk 20 of the support member 12. The current-conducting spring disk 10 is completely released, so that the elastic force of the spring disk 10 closes the contacts 8 and 9. hi view of the above description of the relay embodiment and operation principle, the following will become clear: Manufacturing of relays with different upper snap-action temperature may be standardized.

Indeed, the bimetallic thermally sensitive disks made in form of a truncated cone differ only in the height of dome for various upper snap-action temperatures. In the proposed invention this factor may be taken into account by a simple thickness variation of the disk 20 in the support member 12. Thus the relays may vary only by thermally sensitive disks and disk thickness of the support member. Other units may be assembled on the basic mounting plate beforehand and adjusted. Then the thermally sensitive disk and matching support member shall be mounted. Finally, the housing is attached to the basic mounting plate. It will be seen therefore that the suggested relay has simple structure and adjustment procedure.

The invention task is thus reached by combination of all newly introduced features.

References Cited

1. Microtherm Products Catalogue « Firmensitz: Microtherm GmbH, Taschenstrasse 3». A3 type products.

2. Elmwood Products Catalogue. L260CU98E and Ll 25CS 1996 type products.

3. Microtherm Products Catalogue « Firmensitz: Microtherm GmbH, Taschenstrasse 3». R/20, R/22, and R/40 type products.

4. U.S. Patent no. 5497286. Int. Cl. ⁷ H02H5/04; H01H37/14, 1996.

5. Europatent Application EP no. 1411536, Int. Cl. ⁷ H01H37/16.

6. Kanthal Products Catalogue: "Thermostatic Bimetals", pp. 119 -121.

7. « AUERHAMMER METALLWERK GmbH», "Themostatic Bimetals", Manufacture and Application, Aue/Sachen, 1996.