GB2088132A

GB2088132A - Thermally responsive switch

Info

Publication number: GB2088132A
Application number: GB8129598A
Authority: GB
Original assignee: Sundstrand Data Control Inc
Current assignee: Sundstrand Data Control Inc
Priority date: 1980-11-21
Filing date: 1981-10-01
Publication date: 1982-06-03
Also published as: IT8149739A0; FR2494898B1; CA1152135A; GB2088132B; JPS57111918A; DE3139723A1; AU528066B2; FR2494898A1; IT1172081B; AU7447181A

Abstract

A thermal switch comprises a contact spring 32 which is movable in response to temperature changes by a bimetal actuator disc 40 through the intermediary of a striker pin 50. The striker pin 50 comprises a ceramic pin member which is fastened by adhesive 53 to the contact spring 32. An end of the ceramic pin member has a cap 51 fastened thereto by adhesive, with a space between the end of the pin member and the cap 51 being filled with the adhesive to establish the effective length of the resulting striker pin 50. An accurately dimensioned striker pin can thus be obtained without the need for grinding or trimming of the ceramic pin member, by placing a mass of adhesive within the cap 51 and pressing the end of the ceramic pin member into the cap to provide flow of adhesive along the ceramic pin member until the desired effective length is obtained. <IMAGE>

Description

SPECIFICATION Thermally responsive switch This invention relates to thermally responsive switches (hereinafter thermal switches), and to a method of manufacture thereof.

In most snap-acting bimetallic disc-type thermal switches, the snap action of the disc is coupled to the contact mechanism by an insulated coupling pin or plunger, commonly referred to as the striker pin. This pin is normally made from vitreous-type material. The length of this pin must be precisely controlled properly to couple the snap travel of the disc to the contacts. Incorrect pin lengths result in an improper switch action and either gross reduction in switching life or susceptibility to intermittent contact closing during vibration. Normal manufacturing tolerances do not allow this pin length to be controlled directly without extraordinarily tight controls on the several parts that make up the assembly.As a result, normal practice has been to manufacture the detail parts to common tolerances, and compensate for the total accumulation of plus and minus tolerances by use of a striker pin fitted to each specific application. Two common methods are now used to fit the pin length to each switch.

Both have limitations and advantages. The new method proposed herein combines the advantages of both and eliminates their fundamental disadvantages.

The most commonly used current procedure utilizes a free-floating coupling pin, manufactured in incremental lengths to cover all possible combinations of tolerance accumulations. Each switch-contact assembly is measured using specialized gauges, which relate the geometry of each assembly to a specific pin size. The specified pin length is selected from available stock and installed in the switch. Since the design approach does not attach the pin to any support, it is free to rattle and bounce within the enclosure, where contamination from rubbing surfaces can be generated. Vibration and shock exposure can also impact the floating pin on the contact assembly, causing inadvertent openings or closings of switch contacts. Fractures of the pin as a result of extreme shock and vibration levels have also been observed.

The other commonly used procedure for obtaining correct pin length has been mechanically to attach a pin of sufficient length to compensate for all combinations of detail parts to a fixed part of the assembly, and then to trim it to the specific dimension required. This design provides superior resistance to high vibration and shock levels, since no "loose" parts are used in the disc-to-armature geometry. However, the trimming operation does, by its nature, create debris in the form of chips or grindings which have the potential for contaminating switch contacts.

Elaborate procedures are sometimes required to be absolutely sure that switches are thoroughly cleaned.

In grinding of a pin to length, a flat lower end is formed which results in harmful abrasive wear of the actuator disc by repeated contact therewith.

Additionally, the grinding leaves a sharp lower edge with the result that particles may break off during use to cause contamination.

An improvement over the procedures described above is shown in our U.S. Patent No. 4,201,967, which shows a thermal switch with a striker pin of ceramic material bonded to a carrier by an adhesive layer of controlled thickness to establish the effective length of the striker pin. The patent also discloses a method of manufacture including a tool used therein. In this thermal switch, there can be problems resulting from direct contact of the striker pin of ceramic material with the bimetal disc.

This invention provides a thermal switch in which a temperature-sensitive actuator is arranged to contact a striker pin on an armature spring in response to temperature changes to effect switching movement of an electrical contact carried on the armature spring, wherein the striker pin comprises a pin member fast to the armature spring with a cap over one end thereof for engaging the actuator, and a space between the one end of the pin member and the cap is filled with adhesive to define an exact effective length for the striker pin. The invention thus utilizes the advantages of prior designs while avoiding the disadvantages thereof. This is accomplished by forming the striker pin from a fixed, pre-formed pin member fast to the armature spring with a cap adjustably fitted thereon.The composite striker pin thus obtained provides a high vibration and shock resistance and avoids sizing by trimming either before or after assembly. The striker pin is thus free of the effects of grinding or other trimming which can leave frangible cracks, slits or nonuniform sharp edges and, preferably, the cap provides the striker pin with an end of a domed or part-spherical shape. The resulting thermal switch is an improvement over that shown in U.S. Patent No 4,201,967 in that the mounting of a preferably cup-shaped cap onto the end of the pin member with a controlled layer of adhesive is accomplished with simpler tooling requiring a lower skill level. Also, there is less potential for tilt misalignment of the striker pin, a requirement for less adhesive and reduced likelihood of voids in the adhesive bond layer.The tilt misalignment is preferably avoided by having the cap formed with a cylindrical portion fitting around a pin member to maintain the two parts in axial alignment and a convex or domed head for contacting the actuator.

The invention also provides a method of assembly of such a thermal switch, wherein the armature spring and striker member are mounted on a header to project into a case of the switch, and before the actuator is fitted to the case and during the setting of the adhesive between the cap and the pin member, a tool is brought into engagement with a base portion of the case to position the cap relative to the pin member and to establish the exact effective length of the striker pin. Preferably the cap is ferromagnetic and is held magnetically by the tool during the setting of the adhesive.During such assembly a mass of adhesive is placed within the cap while it is held by the tool and the adhesive is caused to flow out of the cup and along the pin member as the pin member and cap are brought into final relation by the tool, to leave a controlled thickness layer of adhesive which is then set to fix the cap to the pin member.

From the foregoing, it will be seen that a thermal switch is provided without creation of any contamination within the switch and with a structure and assembly using the controlled adhesive layer thickness whereby all of the pin members of the striker pins can be of one standard, minimal length and each striker pin is automatically given an effective length by the controlled thickness layer of adhesive bonding the cap to an end of the pin member.

Drawings Fig. 1 is a vertical central section of a thermal switch according to the invention; Fig. 2 is a view, similar to Fig. 1, showing a step in assembly of the thermal switch and utilizing the installation tool; and Fig. 3 is a fragmentary, vertical section, taken generally along the line 3-3 in Fig. 2.

The thermal switch has a cylindrical case 10 with a base 11 with a pair of annular steps or lands 14 and 1 5 around the interior thereof and spaced above the base. A header 20 is positioned within the case and spaced from the base by a ring 21 having its lower edge supported on the case land 1 5 and having its upper end interfitting with a peripheral groove 22 on the header 20.The header, by insulating structure, mounts a pair of spaced-apart conductive terminal posts 25 and 26 which have their lower ends extending beneath the header and into the space between the header and the case base 1 The terminal posts provide an electrically-conductive path to a fixed contact 30 carried at the lower end of the terminal post 25 and a movable contact 31 mounted on a carrier 32 in the form of a planar armature spring which is mounted at an end thereof in a cantilever fashion to the lower end of the terminal post 26.

As shown in Fig.1, the contacts 30 and 31 are in engagement. Upward pivoting of the armature spring 32 moves the movable contact 31 out of engagement with the fixed contact 30.

The control of the movable contact 31 is by an actuator 40 in the form of a thermally-responsive bimetal disc having its peripheral edge resting on the case land 14. As shown in Fig. 1 , the disc is upwardly concave, as permitted by a space between the disc and the case base 11. In response to a temperature change of a predetermined degree, the disc will shift to an upwardly convex relation and, through the intermediary of a striker pin 50 fixed to the armature spring 32, the latter will be pivoted upwardly to separate the contacts 30 and 31.

The striker pin 50 is formed of a molded ceramic or plastic material and has a cup-shaped metal cap 51 with a part spherical lower end 52 at the free end thereof. The cap lower end is normally spaced a critical distance from the actuator disc 40 so that slight movement of the disc will not effect contact engagement. The striker pin is fixed to the armature spring by means of an adhesive bond line 53.

As shown in Fig. 3, the cap 51 has an annular wall 54 extending upwardly from the spherical end 52 and which has an internal diameter greater than the output diameter of the striker pin 50 to provide a clearance for adhesive flow. By the process subsequently described, a controllable thickness layer 55 of adhesive within the cap and beyond an end 56 of the striker pin bonds the cap to the striker pin. This provides a striker pin of the desired effective length and with the ceramic material of the striker pin out of direct contact with the bimetal disc 40. Many adhesives are suitable, including a thermosetting epoxy adhesive.

The thermal switch has components assembled by a method or process which embodies the use of an installation tool, shown in Figs. 2 and 3. The installation tool is in the form of a magnetic or magnetized fixture 70 in the form of a plate having a central opening 71 with a bottom wall 72.

In the tool shown, an electromagnet 75 is associated with the fixture 70 to create a magnetic field for holding the cap 51.

Alternatively, the cap could be held by a permanent magnet, gravity, suction, adhesives, or by the resistance of the cap to seat on the striker pin due to hydrostatic pressure caused by the resistance of the adhesive to flow between the cap and the pin or by the resistance of the cap to seat on the pin due to friction between the pin and an expandable cap. The central opening 71 receives the cap 51 and with the lower spherical end 52 thereof engaging the bottom wall 72.

The cap 51 is magnetically held on the tool 70, with a mass of adhesive therein. The header 20 and spacer ring 22 are then moved downwardly to move the end 56 of the striker pin into the cap 51.

Sufficient adhesive has been placed in the cap to make certain that there will always be an excess of adhesive and, as the striker pin moves to final position, the excess adhesive flows along the striker pin 50 in the clearance between the striker pin and the cap wall 51 which is adequate to permit the flow. This results in a controlled thickness layer of adhesive within the cap and beyond the end of the striker pin. The adhesive may then be set while the parts are held in position. Thereafter, the parts can be assembled to form the thermal switch as shown in Fig. 1 There is no need to have striker pins of various lengths. The only requirement is that the striker pin be sized to the short side of the minimum tolerance limits by a minimum thickness of the controlled thickness layer 55 of adhesive.

Claims

1. A thermally responsive electric switch in which a temperature-sensitive actuator is arranged to contact a striker pin on an armature spring in response to temperature changes to effect switching movement of an electrical contact carried on the armature spring, wherein the striker pin comprises a pin member fast to the armature spring with a cap over one end thereof for engaging the actuator, and a space between the one end of the pin member and the cap is filled with adhesive to define an exact effective length for the striker pin.

2. A thermally responsive switch according to claim 1, wherein the armature spring is cantilevermounted by connection to a post at one end portion thereof.

3. A thermally responsive switch according to claim 2, wherein the armature spring carries its electrical contact at the opposite end portion thereof.

4. A thermally responsive switch according to to any preceding claim, wherein the actuator is a bimetallic disc with concave and convex operating conditions, depending on the ambient temperature.

5. A thermally responsive switch according to claim 4, wherein the striker pin is positioned centrally of the bimetallic disc, to be engaged by the disc in one condition of the disc but to be spaced therefrom in the other condition of the disc.

6. A thermally responsive switch according to any preceding claim, wherein the cap of the striker pin is metallic.

7. A thermally responsive switch according to claim 6, whereinthe cap of the striker pin is ferromagnetic.

8. A thermally responsive switch according to any preceding claim, wherein the cap of the striker pin has a cylindrical portion fitting around the pin member and a domed head for contacting the actuator.

9. A thermally responsive switch substantially as described herein with reference to the drawings.

1 O. A method of assembly of a thermally responsive switch according to any preceding claim, wherein the armature spring and striker member are mounted on a header to project into a case of the switch, and before the actuator is fitted to the case and during the setting of the adhesive between the cap and the pin member, a tool is brought into engagement with a base portion of the case to position the cap relative to the pin member and to establish the exact effective length of the striker pin.

1 A method according to claim 10, wherein the cap is ferromagnetic and is held magnetically by the tool during the setting of the adhesive.

- 12. A method according to claim 10 or claim 11, wherein the adhesive is a thermosetting adhesive.