US3839694A

US3839694A - Thermally sensitive electrical switch

Info

Publication number: US3839694A
Application number: US00339000A
Authority: US
Inventors: Rocher G Du; Clure G Mc
Original assignee: Essex International Inc
Current assignee: Lear Corp EEDS and Interiors
Priority date: 1973-03-07
Filing date: 1973-03-07
Publication date: 1974-10-01
Anticipated expiration: 1991-10-01
Also published as: GB1433433A; DE2411022A1; BR7401693D0; FR2220862B1; DE2411022B2; CA1016979A; DE2411022C3; JPS49120176A; IT1008376B; FR2220862A1

Abstract

A thermally sensitive electrical switch comprises a body having a chamber therein occupied at one end by a thermally expansible and contractile material and at the other end by a resilient, deformable, elastomeric switching pad operable in response to the application of compressive force thereto to establish an electrically conductive path between electrical conductors. A deformable force transmitting member preferably is interposed between the thermally expansible material and the switching pad to compress the latter and to assist in decompressing the pad upon contraction of the expansible material.

Description

United States Patent Du Rocher et al.

[ 1 Oct. 1, 1974 THERMALLY SENSITIVE ELECTRICAL SWITCH [75] Inventors: Gideon A. Du Rocher, Mt. Clemens; Gerald L. McClure, Warren, both of Mich.

[731 Assignee: Essex International, Inc., Fort Wayne, Ind.

[22] Filed: Mar. 7, 1973 [211 App]. No.: 339,000

[52] US. Cl. 337/382, 338/114 [51] Int. Cl. H0lh 37/46 [58] Field of Search 337/382, 394, 393, 320;

[56] References Cited UNITED STATES PATENTS 2,744,981 5/1956 Spears 338/114 3,212,337 10/1965 McCarrick 337/394 3,509,296 4/1970 Harshman et a1 338/114 Primary Examiner-Harold Broome Attorney, Agent, or Firm-Learman & McCulloch 5 7 ABSTRACT A thermally sensitive electrical switch comprises a body having a chamber therein occupied at one end by a thermally expansible and contractile material and at the other end by a resilient, deformable, elastomeric switching pad operable in response to the app1ication of compressive force thereto to establish an electrically conductive path between electrical conduct ors. A deformable force transmitting member preferably is interposed between the thermally expansible material and the switching pad to compress the latter and to assist in decompressing the pad upon contraction of the expansible material.

16 Claims, 5 Drawing Figures l THERMALLY SENSITIVE ELECTRICAL SWITCH The invention disclosed herein relates to a thermally sensitive switch operable in response to a predetermined rise in temperature to actuate an electrical signal. A switch of the kind with which the invention is concerned has many applications one of which is that of sensing the temperature of an automotive engine coolant or transmission fluid and actuating a warning signal in the event of overheating.

Switches of the kind to which the invention relates have been proposed heretofore, but not all of them have been altogether satisfactory for a number of reasons. For example, some of the known switches have incorporated one or more bimetallic parts which are responsive to changes intemperature to energize and deenergize a signaling device. Such bimetallic members are expensive to manufacture and require careful calibration if they are to function reliably. The calibration procedure is time consuming, requires considerable skill, and therefore is expensive.

Others of the known switches have included temperature sensitive probes mounted on a vehicle engine or the like and switch contacts mounted at a zone remote from the probes. Such switches thus require a multiplicity of separate parts, together with their mounting means, and electrical connections between the probes and contacts thereby adding to the expense of manufacture and installation.

Although others of the known switches have overcome the diffculties referred to above by providing switch contacts carried by spring arms located within the probe and utilizing a thermally expansible and contractile substance to effect movements of the springmounted contacts, such switches also require calibration. in addition, such contacts are subject to pitting and corrosion due to arcing, thereby adversely affecting the reliability and longevity of such switches.

An object of this invention is to provide a thermally sensitive switch which overcomes the disadvantages of switches used heretofore for similar purposes.

Another object of the invention is to provide a thermally sensitive switchwhich requires little or no calibration and which overcomes the disadvantages normally associated with arcing between contacts.

A further object of the invention is to provide a switch of the character described which is economical to manufacture and install.

Other objects and advantages of the invention will be pointed out specifically or will become apparent from the following description when it is considered in conjunction with the appended claims and the accompanying drawings, in which:

FIG. 1 is an elevational view of a switch constructed in accordance with one embodiment of the invention;

FIG. 2 is a vertical sectional view taken on the line 2-2 of FIG. 1 and illustrating the switch in its open or non-conductive condition;

FIG. 3 is a view similar to FIG. 2, but illustrating the switch in its closed or conductive condition;

FIG. 4 is a vertical sectional view of another embodiment of the invention and illustrating the switch in its open or non-conductive condition; and

HO. 5 is a view similar toFIG. 4, but illustrating the switch in its conductive condition.

A switch constructed in accordance with the embodiment of the invention-shown in FIGS. 1 3 comprises a body 1 formed of electrically conductive metal and having a cylindrical casing 2. One end 3 of the casing tapers and is externally threaded as at 4 so as to be threaded into an opening formed in the block (not shown) of a conventional internal combustion engine, for example. The opposite end of the casing 2 has a flange 5 provided with flats 6 to facilitate the fitting of the body 2 to the cylinder head.

The body 1 defines a multiple part chamber 7 which extends axially of the casing 2. Extending inwardly from the free end of the casing 2 the chamber has a first portion 8 of greatest cross-sectional area which communicates with an intermediate, tapering throat portion 9 which, in turn, communicates with a cylindrical portion 10 of considerably less cross-sectional area than that of the chamber portion 8. The chamber portion 10 communicates with a cylindrical chamber portion 11 which extends to the opposite end of the body 1. An upstanding flange 12 surrounds the chamber portion 11 at the mouth thereof.

Fitted into the chamber portion 11 is a rigid insulator block 13 through which extends an electrical conductor or electrode 14 that is connected by wiring via a signal lamp L to a battery B or other electrical energy source. The electrode 14 has a reduced neck 15 between its ends to prevent relative axial movement of the electrode and the insulator 13, and the latter is prevented from axial movement relative to the body 1 by a shoulder 16 at the juncture of the

chamber portions

10 and 11 and byrpeening the flange 12 over the insulator 13 as is'shown clearly in H68. .2 and 3. The construction and arrangement of the parts thus far described are such that the inner end of the electrode 14 communicates with the chamber portion 10.

The body 1 includes a cup-shaped member 17 formed of a heat conductive metal having a preferably flat bottom wall 18 and an upstanding side wall 19. The diameter of the side wall 19 is enlarged between its ends to form a shoulder 20, the enlarged diameter of the side wall being such as to enable the member 17 to fit snugly within the chamber portion 8. The member 17 is maintained in the chamber portion 8 by a shoulder 21 adjacent the inner end of the chamber 8 and by a peened over flange 22 on the casing 2.

The member 17 contains a quantity of thermally expansible and contractile material which expands volumetrically in response to a rise in its temperature. Preferably, the material 23 is one whose expansion is relatively insignificant until a predetermined, critical temperature is reached, whereupon the material expands quite rapidly. One known thermally expansible and contractile material having the desired characteristics is a microcrystaline wax (either with or without metallic particles) manufactured by Vernay Laboratories, Inc. The temperature at which such wax material liquifies and expands rapidly varies in accordance with the specific formulation of the wax and such waxes having greatly different expansion temperatures may be obtained by specifying the expansion temperature desired. For example, a wax material adapted for use in a device to sense engine coolant temperature may have a critical or expansion temperature of about 200 F. for non-pressurized coolant systems or of about 220 F. for pressurized systems. Waxes currently available have critical temperatures as high as 270 F.

Between the expansible material 23 and the electrode 14 is a force transmitting member or diaphragm 24 formed of electrically insulating, resiliently deformable material such as silicone rubber. The member 24 comprises a body 25 of generally frustoconical configuration having a peripheral flange 26 provided with an annular groove in its lower surface. The free end of the cup wall 19 has an endless rib 27 which fits into the groove and forms a peripheral seal around the open end of the cup member 17. The taper of the body 25 corresponds to the taper of the chamber throat 9 and the body 25 terminates in a short, cylindrical foot portion 28 which extends a short distance into the chamber portion 10.

Interposed between the force transmitting member 24 and the electrode 14 is a molded switching member 29 comprising a resilient, deformable pad of elastomeric, non-conductive material such as silicone rubber throughout which is dispersed a plurality of electrically conductive particles. The particles preferably comprise copper or other base metal spheres coated with a noble metal, such as silver, which has a low electrical resistance and which, if it oxidizes, produces an electrically conductive oxide. v

The cross-sectional area of the switching member 29, when it is not subjected to compressive force, preferably is less than the cross-sectional area of the chamber portion so as to provide a space 30 between the member 29 and the wall of the chamber 10 into which the member 29 may expand when it is subjected to compressive force.

The compresive force to which the member 29 must be subjected to convert it from non-conductive condition to conductive condition depends upon several factors, such as the thickness and durometer hardness of the elastomer, the quantity and diameter of the metal particles, and the pressure under which the member 29 originally was molded. These factors are described in detail in co-pending application Ser. No. 857,941, filed Sept. 15, 1969, and to which reference may be had for a more thorough discussion. Briefly, however, the compressive force required to convert the member 29 from non-conductive to conductive condition is directly proportional to the thickness and hardness of the pad and is inversely proportional to the size and quantity of the particles contained in the pad. Thus, a given switching member may be rendered conductive in response to light or heavy compressive forces, depending upon the operating characteristics desired.

In the embodiment of the invention shown in FIGS. 1 3, the switching member 29 normally is not conductive, but is rendered electrically conductive when it is subjected to an axially compressive force sufficient to deform it radially outwardly into engagement with the wall of the chamber portion 10. In this condition of the pad, a sufficient number of the electrically conductive particles contained in the pad will be moved into engagement with one another to form one or more trains of engaged particles bridging the space between the electrode and the wall of the casing 2, thereby establishing an electrically conductive path between the electrode 14 and that portion of the casing 2 adjacent the switch member 29. The resistance of the current path corresponds to the resistance of the particles.

To condition the apparatus described this far for operation, the body 1 may be threaded into an opening formed in the block of an engine so as to locate the cup member 17 in a position to sense the temperature of the engine coolant, for example. In this position of the body, the casing 2 is electrically grounded. Until such time as the temperature sensed by the material 23 rises to the predetermined, critical temperature, the switch member 29 remains relatively uncompressed and no current path exists through the member 29. When the temperature rises to the critical temperature of the material 23, however, it liquifies and suddenly expands in volume and applies a force on the member 24 displacing the foot 28 toward the member 29 and compressing the latter so as to render it electrically conductive and expand it radially into engagement with the wall of the casing 2. The wall of the chamber portion 10 guides the foot 28 during movement of the latter.

The chamber throat 9 is formed to have an included angle approaching 90, as a result of which a portion of the body 25 is partially extruded into the chamber portion 10 and is simultaneously compressed by the wall of the throat. Significant spring energy thus is stored in the member 24 when it effects compression of the member 29.

When the temperature to which the expanded wax material 23 falls below the critical temperature, the material 23 solidifies and contracts thereby relieving the force applied on the member 24. The spring energy stored in the member 24 then effects sufficient withdrawal of the foot 28 from the chamber 10 to enable the switch member 29 to be decompressed and reassume its non-conductive condition.

The size and number of the discrete conductive particles contained within the switching member 29 will vary in accordance with the value of the current which must be accommodated by the particles. Spherical particles ranging in size from 0.003 inch to 0.008 inch in diameter and constituting between about 93 weight percent of the pad have been found to be satisfactory to accommodate current values encountered in automotive vehicles. Larger size particles may be employed in switches adapted for use in circuits having current values greater than those encountered in vehicles.

Although arcing may occur between adjacent particles upon the making or breaking of a current path through the switching member 29, thereby resulting in pitting or even destruction of one or more of such particles, the pad contains so many particles that particles other than those which may be damaged by arcing will form a conductive train through the pad. As a consequence, the useful life of the switching member 29 greatly exceeds that of conventional contacts which engage and disengage each other repetitively.

The quantity of wax material 23 contained in the cup 17 and the extent of compression required to convert the switching member from its non-conductive condition to its conductive condition are such that the switch is rendered conductive in response to a rise in temperature of the material 23 within only a few degrees of the critical temperature of the wax. Thus, calibration of the apparatus normally is not required inasmuch as it is the critical temperature of the wax which triggers operation of the switch. If it is desired to assure operation of the switch at the critical wax temperature, however, this may be accomplished by indenting the base 18 of the cup 17, as shown in dotted lines in FIG. 3, so as to lightly preload the diaphragm 24.

The embodiment of the invention disclosed in FIGS. 4 and 5 is similar in many respects to the previously described embodiment and differs from the latter primarily in that the body itself forms no part of the electrical current path. The modified embodiment has a body 33 comprising a tapered, externally threaded metal casing shank 34 terminating at one end in a heat conductive probe 35 adapted to sense changes in temperature. The other end of the casing shank 34 terminates in an enlarged sleeve 36 having wrench accommodating flats (not shown) on its outer surface. The sleeve 36 defines a cavity 37 which communicates with a chamber 38 formed in the shank 34. Between the cavity 37 and the chamber 38 is a shoulder having an annular groove 39 therein.

Fitted into the cavity 37 is an insulating block 40 forming part of the body 33 and being retained in the cavity 37 by a peened over flange 41 carried by the sleeve 36. Adjacent the inner end of the block 40 is a chamber 42 which communicates with the chamber 38 via a tapered throat 43 similar to the throat 9.

Extending into the block 40 and communicating with the chamber 42 is a pair of spaced apart, electrically

conductive electrodes

44, 45, the inner ends of the electrodes preferably being cut away on arcs corresponding to the curvature of the chamber 42 so that the electrodes constitute portions of the wall of the chamber.

Occupying the chamber 42 is a switching member 46 similar in all respects to the switching member 29 and having a cross-sectional area less than that of the chamber 42. The switching member is straddled by the

electrodes

44 and 45.

A thermally expansible and contractile wax material 47, similar to the material 23, occupies the chamber 38. Between the switching member 46 and the material 47 is a force transmitting member or diaphragm 48 similar to the member 24 and having a body 49 provided with a peripheral, annularly grooved flange 50 which interfits with the groove 39 to seal the chamber 38. The body 49 tapers complementally to the taper of the throat 43 and terminates in a foot 51 which projects a short distance into the chamber 42 to fonn a seat for the switching member 46. A disc 52 of insulating material positioned at the base of the cavity 42 forms a bearing surface opposite the foot 51. The switching member thus is sandwiched between two insulating members. The disc 52 avoids engagement between the switching member and the electrodes until such time as the switching member has been deformed radially an amount sufficient to bridge the space between the electrodes.

To condition the apparatus for operation, the body 33 is threaded into an opening formed in the engine block or other device whose temperature is to be sensed. The electrode 44 is connected by wiring 53 to a battery B or other source of electrical energy, and the electrode 45 is connected by wiring 54 to a grounded lamp L or other signal. The expansiblematerial 47 is so formulated as to have a normal, non-expanded volume until a predetermined, critical temperature is sensed. In the non-expanded condition of the material 47, the switching member 46 is non-compressed and is electrically non-conductive.

When the temperature of the material 47 rises to its predetermined, critical temperature it expands suddenly, thereby displacing the foot 51 of the force transmitting member 48 into the cavity 42 in the same manner earlier described. The switching member 46 thus is compressed between the foot 51 and the insulating disc 52 so as to render the member 46 electrically conductive and is expanded radially into engagement with the

electrodes

44 and 45. An electrical circuit thus is established between the battery B and the signal L.

When the temperature sensed by the material 47 falls below its critical temperature, the material contracts, thereby enabling the spring energy stored in the stressed force applying member 48 to return the latter to its original condition. The inherent resilience of the switching member 46 decompresses it so as to break the circuit to the signal L.

There may be instances in which it is desired to sense temperatures in excess of the maximum capable of being sensed by presently available waxes. For example, it may be desired to operate a signal when the temperature of a vehicles transmission fluid rises above 280 F. In such a case, a known eutectic solder having a critical temperature of 280 F. or 281 F. may be substituted for the wax material previously described. Such solder functions very much like the wax in that it undergoes very little expansion until its critical temperature is reached whereupon it liquifies and expands suddenly.

The critical temperature at which a device constructed according to the invention operates can be varied in other ways. For example, the stiffer or harder the

diaphragm

25 or 51, the greater the resistance exerted thereby to the expansion of the expansible material. As a consequence, a higher temperature will be required to enable the expansible material to deform the diaphragm and render the switching member conductive. The stiffness or hardness of the diaphragm may be varied by conventional molding techniques, including the introduction of non-conductive powders to silicone resin during the molding of the diaphragm to increase its durometer hardness.

The disclosed embodiments are representative of presently preferred forms of the invention, but are intended to be illustrative rather than definitive thereof. The invention is defined in the claims.

We claim:

1. A thermally sensitive switch comprising a heat conductive body having first and second chambers therein and a tapered throat establishing communication between said chambers; normally unconnected, electrically conductive means in one of said chambers; resilient, compressively deformable switching means occupying said one of said chambers and responsive to the application of compressive force thereto to electrically connect said conductive means; thermally expansible and contractile means occupying the other of said chambers and operable in response to a rise in its temperature to expand in a direction toward said one of said chambers; and resilient, deformable force transmitting means occupying said tapered throat and operable in response to expansion of said expansible and contractile means to engage and transmit compressive force to said switching means, the taper of said throat effecting compression of said force transmitting means and storage therein of spring energy sufficient to effect withdrawal of said force transmitting means from compressive engagement with said switching means in response to contraction of said expansible and contractile means.

2. A switch according to claim 1 wherein said switching means is normally non-conductive and is rendered conductive in response to the application of compressive force thereto.

3. A switch according to claim 1 wherein said other of said chambers has a deformable wall.

4. A switch according to claim'l wherein said electrically conductive means comprises a portion of said body communicating with said chamber and a conductor insulated from said body and communicating with said chamber.

5. A switch according to claim 1 wherein said electrically conductive means comprises a pair of conductors insulated from each other and from said body and communicating with said chamber.

6. A switch according to claim 1 wherein said switching means comprises a pad of non-conductive, elastomeric material containing a plurality of discrete, electrically conductive particles.

7. A switch according to claim 6 wherein said particles are present in such quantity and are of such size that said pad is non-conductive in the absence of the application of compressive force thereto.

8. A switch according to claim 1 wherein said force transmitting means seals said other chamber.

9. A thermally sensitive switch comprising a heat conductive body defining a chamber having a lesser cross-sectional area at one end than at its other end, and a throat establishing communication between the different cross-sectional areas of said chamber and tapering in the direction of said one end of said chamber; spaced apart electrically conductive means communicating with said chamber at said one end thereof; resilient, compressively deformable switching means located at said one end of said chamber and being of less cross-sectional area than that of said one end of said chamber, said switching means beinf deformable in response to the application of compressive force thereto to correspond substantially in cross-sectional area to the cross-sectional area of said one end of said chamber and bridge said conductive means; thermally expansible and contractile means occupying the other end of said chamber and operable in response to a rise in temperat ure to expand in a direction toward said switching means; and resilient, deformable force transmitting means interposed between said switching means and said thermally expansible and contractile means and occupying said tapered throat, said force transmitting means being deformable toward said switching means in response to expansion of said expansible and contractile means for transmitting said compressive force to said switching means, the taper of said throat effecting compression of said force transmitting means and storage therein of spring energy sufficient to effect withdrawal of said force transmitting means from compressive engagement with said switching means in response to contraction of said expansible and contractile means.

10. A switch according to claim 9 wherein said conductive means comprises a conductor in engagement with said switching means, and a portion of said body adjacent said one end of said chamber.

11. A switch according to claim 9 wherein said conductive means comprises a pair of conductors extending into said chamber at said one end thereof and straddling said switching means.

12. A switch according to claim 9 wherein said switching means is sandwiched between electrically insulating members.

13. A switch according to claim 9 wherein said switching means is normally non-conductive and is rendered conductive in response to the application of compressive force thereto.

14. A switch according to claim 9 wherein said switching means comprises a pad of non-conductive, elastomeric material containing a plurality of discrete, electrically conductive particles.

15. A switch according to claim 14 wherein said particles are present in such quantity and are of such size that said pad is non-conductive in the absence of the application of compressive force thereto.

16. A switch according to claim 9 wherein said force transmitting means is composed of a resilient material having a normal hardness and containing a substance in an amount sufficient to vary said normal hardness.

Claims

4. A switch according to claim 1 wherein said electrically conductive means comprises a portion of said body communicating with said chamber and a conductor insulated from said body and communicating with said chamber.

9. A thermally sensitive switch comprising a heat conductive body defining a chamber having a lesser cross-sectional area at one end than at its other end, and a throat establishing communication between the different cross-sectional areas of said chamber and tapering in the direction of said one end of said chamber; spaced apart electrically conductive means communicating with said chamber at said one end thereof; resilient, compressively deformable switching means located at said one end of said chamber and being of less cross-sectional area than that of said one end of said chamber, said switching means beinf deformable in response to the application of compressive force thereto to correspond substantially in cross-sectional area to the cross-sectional area of said one end of said chamber and bridge said conductive means; thermally expansible and contractile means occupying the other end of said chamber and operable in response to a rise in temperature to expand in a direction toward said switching means; and resilient, deformable force transmitting means interposed between said switching means and said thermally expansible and contractile means and occupyiNg said tapered throat, said force transmitting means being deformable toward said switching means in response to expansion of said expansible and contractile means for transmitting said compressive force to said switching means, the taper of said throat effecting compression of said force transmitting means and storage therein of spring energy sufficient to effect withdrawal of said force transmitting means from compressive engagement with said switching means in response to contraction of said expansible and contractile means.