US2692317A

US2692317A - Snap-acting thermostat

Info

Publication number: US2692317A
Application number: US207382A
Authority: US
Inventors: Bletz Edward
Original assignee: Stevens Manufacturing Co Inc
Current assignee: STEVENS MANUFACTURING COMPANY Inc; Stevens Manufacturing Co Inc
Priority date: 1951-01-23
Filing date: 1951-01-23
Publication date: 1954-10-19
Anticipated expiration: 1971-10-19

Description

O to 19, 1954 E. BLETZ 2 692317 SNAP-ACTING THERMOSTAT Filed Jan. 23, 1951 2 Shees-Sheet l INVEN TOR.

EDWARD B LE 7' Z E. BLETZ 2,692,317

SNAP-ACTING THERMOSTAT Filed Jan. 25, 1951 2 Sheets-$heet 2 IN V EN TOR.

EDWARD BLETZ Patented Oct. 19, 1954 SNAP-ACTING THERMOSTAT Edward Bletz, Lexington,

Ohio, assignor to Stevens Manufacturing Company, Inc., a corporation of Ohio Application January 23, 1951, Serial No. 207,382

(01. 20o 1ss) 1'7 Claims.

The invention relates in general to thermostats and more particularly to snap-acting thermostats. The majority of thermostatic devices are of the creep type wherein the operating portion or electrical contact is gradually moved in accordance with a bimetallic element or other thermally responsive mechanism. The present invention deals with a snap-acting device wherein a spring member is so constructed as to have an overcenter position with alternate positions of static equilibrium on either side of center. Thus as this over-center structure is moved toward the center it will, at some point in its travel, suddenly snap over to the other position of static equilibrium. This makes the thermostatic device a snap-acting form so that when electrical contacts are used there is a very rapid make or break of these contacts. This prevents pitting and burning of the electrical contacts by the arc which occurs especially at the break of the electrical circuit, thus lengthening the useful life of these contacts. Still further, many electrically operated devices having conventional thermostats cause objectionable static interference to radio reception, and with the snap-acting thermostat of the instant invention this radio interference is practically eliminated.

An object of the invention is to provide a snapacting spring member in a switch or thermostat construction which comprises a compression piece and a tension piece which are fastened together near extremities thereof so as to mutually apply the said tension and compression forces.

Another object of the invention is to provide an over-center spring structure wherein first, second, and third spring strips are generally parallel in a single plane and with all strips fastened together near the ends thereof. The outer spring strips are shortened to make them tension members and thus place a compression force on the inner strip.

A still further object of the invention is to provide an overcenter spring structure as recited immediately above wherein an abutment means is provided near the mid-point of the compression member so as to restrict the movement of this member.

A. still further object of the invention is to provide an adjustable abutment on this compression member on the side thereof opposite the afore-mentioned abutment means.

A still further object of the invention is to provide thermally responsivemeans to move one end of the afore-mentioned over-center spring structure.

A still further object of the invention is to provide a flexible contact strip carrying an electrical contact which will cooperate with an electrical contact carried by the over-center spring member and wherein this flexible contact strip has a U-shaped portion carrying an insulating tip which will exert a force on said over-center spring member on the side thereof opposite the electrical contact thereon.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings, in which:

Figure 1 is a side elevational view of an electrical thermostat embodying the invention;

Figure 2 is a plan view of the over-center spring member removed from the completed thermostat;

Figure 3 is a side view of this spring member of Figure 2;

Figure 4 is a View similar to Figure 1 with the contacts in an open position;

Figure 5 is a View similar to Figure 1 with the thermostat set to a lower operating temperature;

Figure 6 is a side elevational view of another modification of the invention;

Figure 7 is a view similar to Figure 6 with the thermostat adjusted to a medium operating temperature;

Figure 8 is a view similar to Figure 7 with the thermostat adjusted to a low operating temperature;

Figure 9 is a further modification of the invention showing a double-throw structure; and,

Figure 10 is a still further modification of the invention showing a manually resettable thermostatic structure.

The Figure 1 generally shows the complete snap-acting thermostat wherein a base is indicated at l5 and a stack is indicated at H5. The base I5 carries a nut ll which is fastened thereto by any suitable means such as welding or swaging. The nut I! has a stop portion It. A shaft H! has a screw portion 20 threadable in the nut l1 and a splined top 2| to receive a manually operable handle, not shown. The shaft I9 has a stop lug 22 which will engage either side of the stop portion E8 to provide limits of rotation of the shaft l9 relative to the nut 11. The lower end of the screw 20 carries an insulating tip 23.

The stack It includes an over-center spring member 24, a flexible contact strip 25, and a flexible bimetallic strip 25. Each of these

strips

24, 25, and 26 are elongated pieces of metal which lie generally parallel and have the left ends thereof firmly fastened into the stack l6. An electrical terminal 2'! is in electrical contact with the spring member 24, and an electrical terminal 28 is in electrical contact with the contact strip 25. A hollow rivet 29 firmly fastens the entire stack l3 to the base 15. The stack l6 includes three insulating washers 36 which mutually insulate the base IS, the spring member 24, the contact strip 25, and the bimetalic strip 26. The over-center spring member 24 is better shown in Figures 2 and 3 wherein this member is shown as being removed from the stack I6. This spring member 24 includes first, second, and third spring strips 3|, 32, and 33 which are all formed from a single piece of metal. Elongated apertures 34 in this spring member 24 cause the separation thereof into these three spring strips. The outer or first and third spring strips 3| and 33 are V-crimped as at 35 in order to slightly shorten the length thereof. This places the strips 3| and 33 in tension and the strip 32 in compression. Hence the strip 32 will bow one side of center as is shown in Figure 3. When the spring member 24 is assembled in the stack [6 the center spring strip 32 is adapted to be contacted by the insulating tip 23 to somewhat flatten the curvature of this strip 32.

An abutment spring 35 is assembled into the stack It just beneath the spring member 24 and this abutment spring 36 has a free end 31 to contact the concave side of the second spring strip 32 at a point approximately opposite the insulating tip 23.

The free end of the spring member 24 carries on the lower side thereof an electrical contact 40, and the free end of the contact strip 25 carries on the upper side thereof a second electrical contact 4|. These contacts 46 and 4| are shown in Figure l as being in physical contact. The free end of the contact strip 25 further carries a U-shaped portion 42 which surrounds the end of the spring member 24. The U-shaped portion 42 has a threaded aperture 43 which carries an adjusting screw 44. The screw 44 has an insulating tip 45 which is disposed near the upper side of the free end of the spring member 24. The bimetallic strip 26 carries an insulating button 46 shown in contact with the lower side of the free end of the contact strip 25.

Operation of Figure 1.-The Figure 1 shows the thermostat with the shaft l9 adjusted for the maximum operating temperature. This is evidenced by the stop lug 22 in engagement with the stop portion l8 in the most clockwise position, looking downward on the shaft [9. The screw is a left hand thread which will mean that the insulating tip 23 is in its upper limit. The second spring strip 32 thus has the greatest curvature that it is permitted to attain when assembled in the thermostat. It will be seen that this curvature is not as great as when the over-center spring member 24 is outside the thermostat, and thus the insulating tip 23 exerts a force on the second spring strip 32 so as to place a compression force in the strip 32 and a tension force in the spring strips 3| and 33. The abutment spring 36 is initially curved so as to exert an upward force on the concave side of the second spring strip 32. Even though the insulating tip 23 is in its upper limit, this abutment spring 36 still exerts an upward force on this strip 32. The bimetallic strip 26 is shown as being straight, that is, horizontally extending toward the right. This is the unstressed position which the bimetallic strip would assume for normal ambient temperatures. As the temperature rises the bimetallic strip 26 is deflected upwardly. The flexible contact strip 25 has an initial stress or bias slightly downward to the right so that this contact strip 25 is always in contact with the insulating button 46 and assures that when the conditions are such that the bimetallic strip 26 moves downwardly with decreasing temperatures, the contact strip 25 will follow and also move downwardly.

The Figure 4 shows the contacts 49 and 4| in an open position, and this is effected by an upward movement of the bimetallic strip 26 sufficient so that the

strips

24 and 25 as a unit move upwardly until the over-center spring member 24 reaches a point where this spring member can snap over-center. Thus the free end of the member 24 will touch the insulating tip 45. This insulating tip 45 acts as a stop so that the spring member 24 will not go too far beyond center. With the contacts 40 and 4| in the open position as shown in Figure 4 it will be assumed that the heat supplied to the bimetallic strip 26 will decrease through some electrical circuit connected to the

terminals

21 and 28, and thus the bimetallic strip 26 will move downwardly. The contact strip 25 follows along, and thus the insulating tip 45 urges the spring member 24 downwardly until it again passes the center point and will snap over center to reclose the contacts 40 and 4| The Figure 5 shows the thermostat adjusted to a lower operating temperature wherein the stop lug 22 is in some intermediate position relative to the stop portion l8. It will be noted that the insulating tip 23 has been moved downwardly to flatten the second spring strip 32. This establishes the spring member 24 in a condition where it is closer to the snap over point, and hence only a smaller movement of the bimetallic strip 26 is required to make the contacts 40 and 4| open.

The structure of Figure 6 shows a modified form of thermostat 50 wherein a different form of abutment on the concave side of the spring strip 32 is provided. In this modification the abutment spring 36 is omitted and in place thereof an insulating spacer 5| is fastened to the flexible contact strip 25 as by the tongue 52 passing through the hole 53. The insulating spacer 5| has suificient height to bear against the underside of the second spring strip 32 and bow the same even though the insulating tip 23 is raised to its upper limit as is shown in the Figure 6. The Figure '7 shows the same thermostat 50 except that the thermostat has been adjusted to a medium operating temperature as is evidenced by the stop lug 22 being rotated counter-clockwise to an intermediate position. In this position the second spring strip 32 is depressed and the curve thereof is flattened. In Figure 6 it will be noted that the free end of the flexible contact strip 25 is spaced from the insulating button 46. In Figure '7 it will be noted that these two elements are still spaced; however, they are much closer together. The Figure 8 shows the same thermostat 50 with the shaft l9 turned to its counter-clockwise limit so that the contacts 40 and 4| are in their open position.

Operation of Figures 6, 7, and 8.-The bimetallic strip 26 in the thermostat 50 is again normally disposed horizontally during the normal ambient temperatures. The contact strip 25 is normally stressed or biased slightly upwardly to the right so that there is always contact pressure between the contacts 40 and 4|. The Figure 6 shows the insulating tip 23 in its upper limit so that the second spring strip 32 is only slightly depressed or flattened. It is caused to be flattened by the downward force of the tip 23 which in turn is opposed by the upward urging of the contact strip 25. When the thermostat 50 is subjected to increasing temperatures the bimetallic strip 26 moves upwardly until the insulating button 46 touches the free end of the contact strip 25. Further upward movement of the bimetallic strip 26 will move the contact strip 25 and the spring member 24. When the spring member 24' has moved sufliciently it will snap over center to open the contacts 40 and 4|. Of course as the bimetallic strip 26 cools it will move downwardly and the insulating tip 45 will move the spring member 24 downwardly until it again snaps over center.

The Figure 7 shows the thermostat adjusted to a medium operating temperature and it will be seen that the insulating button 46 is much closer to the free end of the contact strip 25. Thus a smaller deflection of the bimetallic strip 26 is all that is necessary to cause the contacts 40 and 4| to open. The Figure 8 shows the thermostat at its lower limit and in such position the insulating tip 23 will be in its lower limit to press down the second spring strip 32 sufficiently that the spring member 24 will have snapped over center. It will be seen that the second spring strip 25 is bowed downwardly in this position because the insulating spacer 5| transmits the force of the screw 20 to this contact Strip 25.

The adjustment of the thermostat 50 as seen in Figures 6, 7, and 8 is caused by moving the free ends of

members

24 and 25 as a unit nearer to or farther away from the end of the bimetallic strip 26. The fulcrum point of the contact strip 25 is at the center or at the insulating spacer 5|, and hence this diflers from the construction in Figure 1 wherein the fulcrum of the contact strip 25 is at the stack I6. A further difference is that in the construction of Figure 1 the adjustment for the operating temperature is obtained by compressing or flattening the spring strip 32 which brings the spring member 24 nearer to the snap over point.

The structure of Figure 9 shows a slight modification of the thermostat of Figure 1 which has been modified to include a single pole doublethrow contact structure. A second contact strip 56 has been added in the stack which carries on the free end thereof a third contact 57. The spring member 24 carries on the upper side of the free end thereof a fourth contact 58. The

contacts

58 and 40 are of course electrically and physically connected together. The contacts '5'! and 58 are normally open and the contacts 46 and 4| are normally closed as was the case in Figure 1. In this modification of Figure 9, the insulating tip 45 bears against the upper side of the second. contact strip 56. In the operation of the thermostat of Figure 9, the second contact strip 56 should initially be biased upwardly so that it will always touch the insulating tip 45. As the flexible contact strip 25 moves upwardly the spring member 24 will again move upwardly until the snap over point is reached wherein the contacts 46 and 4| will break and the

contacts

51 and 58 make. As the bimetallic strip 26 cools the contact strip 25 will follow downwardly and the insulating tip 45 will urge the second contact strip 56 downwardly which in turn will urge the spring member 24 downwardly until the snap over point is reached.

The Figure 10 shows a still further modification of the thermostat of Figure 1 and illustrates a thermostat 66 of the manual reset type. The thermostat 60 again includes a base l5 and a stack l6 with the stack carrying a bimetallic strip 26. The insulating button 46 bears against the underside of the free end of a contact strip 6| which carries the contact 4|, The over-center spring member 24 is again provided carrying the contact 46. The abutment spring 36 again contacts the concave side of the second spring strip 32. A second abutment spring 62 is carried in the stack and is normally biased upwardly away from the second spring strip 32. A threaded aperture 63 is provided in the base l5 and carries an adjusting screw 64. The adjusting screw 64 is longitudinally adjustable to exert a variable downward force on the abutment spring 62 thus urging this abutment spring into contact with the convex side of the second spring strip 32. The stack l6 carries a reset spring 65 which has on the free end thereof an insulating button 66. A collar 6'! is carried on the base |'5. A reset button 68 is carried in the collar 61 and abuts the upper side of the reset spring 65. An aperture 69 is provided in the reset spring 65 to allow the adjusting screw 64 to pass through this spring.

The thermostat of Figure 10 operates in essentially the same manner as the thermostat of Figure 1. As the bimetallic strip 26 moves upwardly the contact strip 6| and spring member 24 will move upwardly until the spring member 24 snaps over center. The free end of the spring member 24 will then abut the insulating button 66. The reset button 68 must next be depressed after the thermostat cools in order to reset the thermostat 66. A depression of the button 68 will, through the medium of the insulating button 66, cause the spring member 24 to snap over center so that the contacts 46 and 4| are again closed. The axial adjustment of the screw 64 will cause the abutment spring 62 to oppose the urging of the abutment spring 36 and thus these two abutment springs 62 and 36 will determine the amount of curvature in the second spring strip 32 and thus adjust the operating temperature.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of elements may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A snap-acting thermal device including a base, a flexible member carried by said base and having a movable operating portion, a second operating portion carried by said base for cooperating with said movable operating portion, thermally responsive means for moving said movable operating portion, said flexible member including a tension piece and a compression piece fastened together near the ends thereof so as to mutually apply said tension and compression forces, said forces establishing said flexible member as an over-center structure having alternate positions of static equilibrium on opposite sides of center,

abutment means for deflecting said compression piece in one direction from center, and adjust.- able means acting on said compression piece to place an adjustable force thereon in opposition to that effected by said abutment means.

2. A snap-acting thermal device including a base, first and second flexible members carried by said base and each having a movable operating portion, means for moving the operating portion of said second member toward said first member operating portion with temperature changes in a given sense, said first flexible member including an over-center structure with a tension piece and a compression piece fastened together near the ends thereof so as to mutually apply said tension and compression forces, abutment means for deflecting one of said pieces on one side of center to a position of equilibrium, and adjustable means acting on said one of said pieces to place an adjustable force thereon in opposition to that effected by said abutment means.

3. A snap-acting thermal device including a base, first and second flexible members carried by said base and each having a movable operating portion, means for moving the operating p01- tion of said second member toward said first member operating portion with temperature changes in a given sense, said first flexible member including an over-center structure with a tension piece and a compression piece fastened together near the ends thereof so as to mutually apply said tension and compression forces, abutment means for deflecting said compression piece on one side of center to a position of equilibrium, and adjustable means acting on said compression piece to place an adjustable force thereon in opposition to that effected by said abutment means.

4. A snap-acting thermal device including a base, first and second flexible members carried by said base and each having a free end, thermal means for moving the free end of said second member, said first member being an over-center spring member comprising three paralleled flat spring strips fastened together in substantially a single plane near the ends thereof, the outer two of the spring strips being slightly V-crimped near the center to shorten the length thereof, and hence slightly bow the middle one of said spring strips toward said base, abutment means disposed to engage the concave side of said middle spring strip and urge same toward said base, the free ends of said first and second members carrying cooperating operating portions, an adjustable screw carried by said base and having an insulating tip, said insulating tip disposed to exert a force on the convex side of said middle spring strip and to have a longitudinal travel sufficient to depress the bowed middle spring strip until the outer spring strips snap over center to an alternate position of equilibrium.

5. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first, second, and third strips disposed in that order relative to said base, said third strip being bimetallic and carrying an insulator button on the free end thereof, the free end of said third strip movable toward said base with increasing temperature, said second strip being a spring member, said insulator button adapted to cooperate with the free end of said second strip to move same toward said base with increasing temperature, said first strip being an over-center spring member comprising three paralleled flat spring pieces fastened together at the ends thereof into substantially a single plane, the outer two of the spring pieces being slightly V-crimped near the center to shorten the length thereof, and hence slightly bow the middle one of said spring pieces toward said base, abutment means disposed to engage the concave side of said middle spring piece and urge same toward said base, the free ends of said first and second strips carrying cooperating normally closed electrical contacts, in sulated reset means adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded aperture in said base, an adjusting screw in said aperture and carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to exert a force on the convex side of said middle spring piece and to have a longitudinal travel sufficient to depress the bowed middle spring until the outer spring pieces snap over center to an alternate position of equilibrium.

6. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first, second, and third strips disposed in that order relative to said base, said third strip being bimetallic and carrying an insulator button on the free end thereof, the free end of said third strip movable toward said base with increasing temperature, said second strip being a spring member and having a U-shaped portion at the free end thereof, said insulator button adapted to cooperate with the free end of said second strip to move same toward said base with increasing temperature, said first strip being an over-center spring member comprising three paralleled fiat spring pieces fastened together at the ends thereof into substantially a single plane, the outer two of the spring pieces being slightly V-crimped near the center to shorten the length thereof, and hence slightly bow the middle one of said spring pieces toward said base, abutment means disposed to engage the concave side of said middle spring piece and urge same toward said base, the free ends of said first and second strips carrying cooperating normally closed electrical contacts, the free end of said first strip disposed inside said U-shaped portion, insulating means carried by one leg of said U-shaped portion adapted to exert a force on the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut and carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to engage the convex side of said middle spring piece and to have a longitudinal travel suflicient to depress the bowed middle spring piece until the outer spring pieces snap over center to an alternate position of equilibrium.

7. A snap-acting thermal device including a base, a stack fastened to said base and including a fixed end of each of first, second, and third strips disposed substantially mutually parallel in that order relative to said base, said third strip being a spring member, thermal means for moving the free end of said third strip, said first strip being an over-center spring member including first and second flat spring pieces fastened together near the ends thereof, the first of the two spring pieces being slightly shorter in length than the second thereof to thus slightly bow the second one of said spring pieces, said second strip being a spring member disposed to engage one side of said second spring piece, the free ends of said first and third strips carrying cooperating operating portions, insulating means carried by the 9 free end of said third strip and adapted to engage the free end of said first strip on the side thereof opposite the operating portion thereof, a threaded nut carried by said base, an adjusting screw in said nut carrying an insulating tip and having an axis generally perpendicular to said second spring piece, said insulating tip disposed to engage the side of said second spring piece opposite to said second strip and to have a longitudinal travel sufficient to depress the bowed second spring piece until the first spring piece snaps over center to an alternate position of equilibrium.

8. A snap-acting thermal device including a base, a stack fastened to said base and including a fixed end of each of first, second, third, and fourth strips disposed substantially mutually parallel in that order relative to said base, said fourth strip being bimetallic and carrying an insulator button on the free end thereof, the free end of said fourth strip movable toward said base with increasing temperature, said third strip being a spring member, said insulator button adapted to cooperate with the free end of said third strip, said first strip being an over-center spring member including first and second fiat spring pieces fastened together near the ends thereof, the first of the two spring pieces being slightly shorter in length than the second thereof to thus slightly bow the second one of said spring pieces, said second strip being a spring member disposed to engage one side of said second spring piece, the free ends of said first and third strips carrying cooperating operating portions, insulating means carried by the free end of said third strip and adapted to engage the free end of said first strip on the side thereof opposite the operating portion thereof, a threaded nut carried by said base, an adjusting screw in said nut carrying an insulating tip and having an axis generally perpendicular to said second spring piece, said insulating tip disposed to engage the side of said second spring piece opposite to said second strip and to have a longitudinal travel sufficient to depress the bowed second spring piece until the first spring piece snaps over center to an alternate position of equilibrium.

9. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first, second, and third strips disposed substantially mutually parallel in that order relative to said base, said third strip being a spring member and having a U-shaped portion at the free end thereof, thermal means for moving the free end of said third strip toward said base with increasing temperature, said first strip being an over-center spring member comprising three paralleled flat spring pieces fastened together at the ends thereof, the outer two of the spring pieces being slightly V-crimped near the center to shorten the length thereof, and

hence slightly bow the middle one of said spring pieces toward said base, said second strip being a spring member disposed to engage the concave side of said middle spring piece and urge same toward said base, the free ends of said first and third strips carrying cooperating electrical contacts, the free end of said first strip disposed inside said U-shaped portion, insulating means carried by one leg of said u-shaped portion adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to engage the convex side of said middle spring piece and to have a longitudinal travel sufficient to depress the bowed middle spring piece until the outer spring pieces snap over center to an alternate position of equilibrium.

10. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first, second, third, and fourth strips disposed substantially mutually parallel in that order relative to said base, said fourth strip being bimetallic and carrying an insulator button on the free end thereof, the free end of said fourth strip movable toward said base with increasing temperature, said third strip being a spring member and having a U-shaped portion at the free end thereof, said insulator button adapted to cooperate with the free end of said third strip to slightly deflect same toward said base during normal ambient temperatures, said first strip being an over-center spring member comprising three paralleled flat spring pieces fastened together at the ends thereof, the outer two of the spring pieces being slightly V-crimped near the center to shorten the length thereof, and hence slightly bow the middle one of said spring pieces toward said base, said second strip being a spring member disposed to engage the concave side of said middle spring piece and urge sam toward said base, the free ends of said first and third strips carrying cooperating electrical contacts, the free end of said first strip disposed inside said U-shaped portion, insulating means carried by one leg of said U-shaped portion adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to engage the convex side of said middle spring piece and to have a longitudinal travel sufficient to depress the bowed middle spring piece until the outer spring pieces snap over center to an alternate position of equilibrium.

11. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first, second, third, and fourth strips disposed substantially mutually parallel in that order relative to said base, said fourth strip being bimetallic and carrying an insulator button on the free end thereof, the free end of said fourth strip movable toward said base with increasing temperature, said third strip being a spring member and having a U-shaped portion at the free end thereof, said insulator button adapted to cooperate with the free end of said third strip to slightly deflect same toward said base during normal ambient temperatures, said first strip being an over-center spring member comprising three paralleled fiat spring pieces fastened together at the ends thereof into substantially a single plane, the outer two of the spring pieces being slightly V-crimped near the center to shorten the length thereof, and hence slightly bow the middle one of said spring pieces toward said base, said second strip being a spring member disposed to engage the concave side of said middle spring piece and urge same toward said base, the free ends of said first and third strips carrying cooperating normally closed electrical contacts, the free end of said first strip disposed inside said U-shaped portion, insulating means carried by one leg of said U-shaped portion adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to engage the convex side of said middle spring piece and to have a longitudinal travel sufficient to depress the bowed middle spring piece until the outer spring pieces snap over center to an alternate position of equilibrium.

12. A snap-acting thermal device including a base, first and second flexible members carried by said base and having free ends with cooperating portions thereon, thermal means for moving the free end of said second member, said first member being an over-center spring member comprising three paralleled fiat spring strips fastened together near the ends thereof, the outer two of the spring strips being slightly V-crimped near the center to shorten the length thereof, and hence slightly bow the middle one of said spring strips toward said base, abutment means carried by the center of said second member and disposed to engage one side of said middle spring strip and urge same toward said base, an adjustable screw carried by said base and having an insulating tip, said insulating tip disposed to engage the side of said middle spring strip opposite to said abutment means and to have a longitudinal travel suflicient to depress the bowed middle spring strip until the outer spring strips snap over center to an alternate position of equilibrium.

13. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first and second strips disposed in that order relative to said base, thermal means for moving the free end of said second strip toward said base with increasing temperature, said second strip being a spring member, said first strip being an over-center spring member including first and second fiat spring pieces fastened together near the ends thereof, the first of the two spring pieces being slightly shorterin length than the second thereof to thus slightly bow the second one of said spring pieces, insulating means carried by the center of said second strip and disposed to engage one side of said second spring piece, the free ends of said first and second strips carrying cooperating electrical contacts, insulating means carried by the free end of said second strip and adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut and carrying an insulating tip and having an axis generally perpendicular to said second spring piece, said insulating tip disposed to engage the side of said second spring piece opposite to said insulating means and to have a longitudinal travel sufiicient to depress the bowed second spring piece until the first spring piece snaps over center to an alternate position of equilibrium.

14. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first and second strips disposed in that order relative to said base, said second strip being a spring member and having a U-shaped portion at the free end thereof, ther mal means for moving the free end of said second strip toward said base with increasing temperature, said first strip being an over-center spring member comprising three paralleled fiat spring pieces fastened together at the ends thereof, the outer two of the spring pieces being slightly shorter in length than the middle one of said spring pieces to thus slightly bow the middle one of said spring pieces toward said base, an insulating button carried by the center of said second strip and disposed to engage the concave side of said middle spring piece and urge same toward said base, the free ends of said first and second strips carrying cooperating electrical contacts, the free end of said first strip disposed inside said U-shaped portion, insulating means carried by one leg of said U-shaped portion adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut and carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to engage the convex side of said middle spring piece and to have a longitudinal travel sufficient to depress the bowed middle spring piece until the outer spring pieces snap over center to an alternate position of equilibrium.

15. A snap-acting thermostat including a base, a stack fastened to said base and including a fixed end of each of first, second, and third strips disposed in that order relative to said base, said third strip being bimetallic and carrying an insulator button on the free end thereof, the free end of said third strip movable toward said base with increasing temperature, said second strip being a spring member and having a U-shaped portion at the free end thereof, said insulator button adapted to cooperate with the free end of said second strip and to be spaced therefrom during normal ambient temperatures, said first strip being an over-center spring member com prising three paralleled flat spring pieces fastened together at the ends thereof, the outer two of the spring pieces being slightly V-crimped near the center to shorten the length thereof,

and hence slightly bow the middle one of said spring pieces toward said base, an insulating button carried by the center of said second strip and disposed to engage the concave side of said middle spring piece and urge same toward said base, the free ends of said first and second strips carrying cooperating normally closed electrical contacts, the free end of said first strip disposed inside said U-shaped portion, insulating means carried by one leg of said U-shaped portion adapted to engage the free end of said first strip on the side thereof opposite the electrical contact thereon, a threaded nut carried by said base, an adjusting screw in said nut and carrying an insulating tip and having an axis generally perpendicular to said middle spring piece, said insulating tip disposed to engage the convex side of said middle spring piece and to have a longitudinal travel sufiicient to depress the bowed middle spring piece until the outer spring pieces snap over center to an alternate position of equilibrium.

16. A snap-acting thermal device including a base, first and second flexible members carried by said base and each having a movable operating portion, means for moving the operating portion of said second member toward said first member operating portion with temperature changes in a given sense, said first flexible member including an over-center structure with a tension piece and a compression piece fastened together near the ends thereof so as to mutually apply said tension and compression forces, abutment means carried by said second member and engaging and deflecting one of said pieces on one side of center to a position of equilibrium, and adjustable means acting on said one of said pieces to place an adjustable force thereon in opposition to that efiected by said abutment means.

17. A snap-acting thermal device including a base, a flexible member carried by said base and having a movable operating portion, a second. operating portion carried by said base for cooperating with said movable operating portion, thermally responsive means for moving said movable operating portion, said flexible member including a tension piece and a compression piece fastened together near the ends thereof so as to mutually apply said tension and compression forces, said forces establishing said flexible member as an over-center structure having alternate positions of static equilibrium on opposite sides of center, abutment means carried by said second operating portion and engaging and deflecting said compression piece in one direction from center, and adjustable means acting on said compression piece to place an adjustable force thereon in opposition to that efiected by said abutment means.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,236,699 Riche Apr. 1, 1941 2,264,988 Lee Dec. 2, 1941 2,440,025 Singleton Apr. 20, 1948