US3789339A

US3789339A - Thermostatic switch

Info

Publication number: US3789339A
Application number: US00351162A
Authority: US
Inventors: G Wehl
Original assignee: Portage Electric Products Inc
Current assignee: Portage Electric Products Inc
Priority date: 1973-04-16
Filing date: 1973-04-16
Publication date: 1974-01-29
Anticipated expiration: 1991-01-29

Abstract

A thermostatic switch having a bimetallic electric-contactcarrying blade with a dish-shaped snap portion is provided. ''''Creep'''' opening of the contacts prior to snap-open action is substantially eliminated by forming the terminal arm that carries the bimetallic blade and the fulcrum for the dish-shaped portion of a bimetal positioned to deflect in response to temperature change in a direction opposite to the direction in which the bimetallic blade deflects.

Description

United States Patent [191 Web] [ 1 Jan. 29, 1974 THERMOSTATIC SWITCH [75] Inventor: Glenn E. Wehl, North Canton, Ohio [73] Assignee: Portage Electric Products, Inc.,

North Canton, Ohio [22] Filed: Apr. 16, 1973 [21] Appl. No.: 351,162

[52] US. Cl 337/95, 337/38, 337/335,

[51] Int. Cl. H0111 61/00 [58] Field of Search. 337/89, 38, 95, 335, 336, 368, 337/390, 397

[56] References Cited UNITED STATES PATENTS 2,820,870 1/1958 Moksu 337/368 3,443,259 5/1969 Wehl et a1. 337/89 Wells 337/95 Carlson 337/95 Primary Examiner-Bemard A. Gilheany Assistantflirqminer l E. Bell 7 wumm V Attorney, Agent, or Firm-Ryder, McAulay, Fields, Fisher & Goldstein ABSTRACT 5 Claims, 2 Drawing Figures THERMOSTATIC SWITCH BACKGROUND OF THE INVENTION The present invention relates to thermostatic switches such as those used for overload protection purposes with small electric motors, home appliances, etc. Heretofore, attempts have been made to secure positive, quick-opening (and closing) action in thermostats of this general type by providing a dish-shaped portion in the contact carrying, bimetallic blade (see U.S. Pat. No. 2,820,870). Such assemblies, however, are subject to creep which is the tendency for the electric contacts to separate slightly as temperature increases prior to the snap action occurring. Such creep action can cause sparking which is destructive to the contacts and destroys the intended beneficial effect of the snap action, which is to rapidly separate the electric contacts when a predetermined temperature has been reached.

In the past, thermostatic switches of the type described were formed with a pair of rigid arms with one arm carrying a fulcrum designed to bear down on the dish-shaped portion of the bimetallic blade. Various attempts were made to minimize and compensate for the tendency of the contacts to creep by varying the relationship between the fulcrum and dish-shaped portion (see U.S. Pat. No. 3,443,259). The present invention provides a novel approach at solving this same problem.

The principal object of the present invention is to provide an inexpensive thermostatic switch which may readily be mass-produced and is virtually creepless.

SUMMARY OF THE INVENTION In accordance with the present invention, a thermostatic snap switch is provided comprising a mounting member having first and second terminal arms extending out therefrom in spaced relationship with each other. An elongated, bimetallic blade adapted to deflect in response to temperature change has one end coupled to the first arm and the other end free. The blade extends along the side of the first arm directed toward the second arm and carries a contact at its free end. An intermediate portion of the blade is dished to provide a snap-acting portion. The blade carrying arm includes a fulcrum means adapted to bear on the strip and is formed of a bimetal positioned to deflect in response to temperature change in a direction opposite the direction in which the bimetallic blade deflects in response to the same temperature change.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a simplified sectional view of a switch in accordance with the present invention; and,

FIG. 2 is a view similar to FIG. 1 showing an altemative form of construction of the present switch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to the drawings wherein the thermostatic switch of the present invention is illustrated and wherein similar components bear the same reference numerals throughout both views. Accordingly, the switch comprises amounting member 12 formed of a suitable insulating material through which a pair of arms 14 and 16 extend in spaced relationship with each other. The first arm 14 has coupled thereto one end of a bimetallic blade 18. The other end of blade 18 is free and carries a contact 20. If desired, a suitable electrical contact 22 may also be provided on arm 16 for engagement with contact 20.

Intermediate the ends of arm 18 is a dished-out portion 24 of blade 18 which is located fairly close to the center of the blade. Arm 14 carries at its forward end 26 a fulcrum 28 which in the primary embodiment of the present invention contacts the convex side of the dished-out portion. As is well known in the art, and as described in the above cited patents, as the blade 18 heats up, the fulcrum exerts increasing pressure on the convex side of the dished-out portion 24 of blade 18 until the dished-out portion snaps into a concave position at which time contacts 20 and 22 separate. To this end, the bimetallic material chosen for blade 18 is positioned such that an increase in temperature causes it to curl upwardly in the direction indicated by arrow A.

In accordance with the present invention, the first arm 14 which carries blade 18 is formed of a bimetal positioned to creep in the direction indicated by arrow B opposite to the direction of the blades creep. The second arm 16 is formed of a rigid material which remains relatively unchanged by temperature changes. The bimetallic material for arm 14 is chosen so that its creep toward arm 16 is at a rate equal to or greater than the creep of blade 18 away from arm 16. In this manner, the blade contact 20 is maintained in engagement with contact 22 virtually up until the moment the dished portion 24 reverses itself so as to minimize and virtually eliminate any creep and hence sparking at the contacts.

The construction of the alternate embodiment of FIG. 2 is virtually identical with that of the primary embodiment except that in the alternate embodiment the point of fulcrum 28 contacts blade 18 at the forward edge of the dished-out portion 24. The operation of the switch, however, is identical with that described for the primary embodiment.

In operation, as the temperature sensed by the switch increases, the bimetal blade tends to curve upwardly while the bimetal arm tends to curve downwardly at a rate equal to or greater than that at which the blade is curling up thereby causing an increased pressure to be exerted by the fulcrum on the blade. Eventually the pressure of the fulcrum on the dished-out portion causes it to reverse itself so that the contacts open. However, up until virtually the moment of reversal, the contacts are maintained in good electrical engagement.

Accordingly, the above objective is effectively attained.

Having thus described the invention, what is claimed 1. In a thermostatic snap switch of the type comprising: a mounting member; first and second terminal arms supported by said mounting member extending out therefrom in spaced relationship with each other; an elongated bimetal blade adapted to deflect in response to a temperature change having one end coupled to the first arm and the other end free extending along the side of the first arm directed toward the second arm, said blade having a part thereof dished to provide a snap-acting portion; and fulcrum means carried by the first arm and bearing on the strip; the improvement wherein said one arm is formed of a bimetal positioned to deflect in response to temperature change in toward the first arm with the same increase in temperature.

4. The switch in accordance with claim 1 wherein the fulcrum bears on the dished snap-acting portion of the blade.

5. The switch in accordance with claim 1 wherein the fulcrum bears on the dished snap-acting forward edge of the dished snap-acting portion.

Claims

1. In a thermostatic snap switch of the type comprising: a mounting member; first and second terminal arms supported by said mounting member extending out therefrom in spaced relationship with each other; an elongated bimetal blade adapted to deflect in response to a temperature change having one end coupled to the first arm and the other end free extending along the side of the first arm directed toward the second arm, said blade having a part thereof dished to provide a snap-acting portion; and fulcrum means carried by the first arm and bearing on the strip; the improvement wherein said one arm is formed of a bimetal positioned to deflect in response to temperature change in a direction opposite to the direction in which the bimetal blade deflects in response to the same temperature change.

2. The thermostatic switch in accordance with claim 1 wherein the first arm deflects at a rate equal to or greater than the rate at which the blade deflects.

3. The switch in accordance with claim 1 wherein the first arm deflects toward the second arm with an increase in temperature and said bimetal blade deflects toward tHe first arm with the same increase in temperature.