US2175721A - Snap-acting thermostatic switch - Google Patents

Description

Oct. 10, 1939. D. cs. TAYLOR SNAP-ACTING THERMOSTATIC SWITCH Filed Feb. 9, 1938 9 INVENTOR Daniel G. Tql lor ATTO NEY Patented Oct. 10, 1939 UNITED STATES- SNAP-ACTING THERMOSTATIC SWITCH Daniel G. Taylor, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application February 9, 1938, Serial No. 189,565

18 Claims.

This invention relates broadly to thermostatic switches of the open contact snap action type.

In the past considerable difiiculty has been experienced with switches of this type where the switch has been operated by some slowly changing condition, due to what is known as vanishing contact pressure. When the switch has been snapped to closed position due to the response of the condition responsive device, the spring pressure or other force which produced the snap action is holdingthe contacts forcibly closed. As the condition responsive device starts to reverse, it begins to tension this spring, and while the spring is being brought to its dead-center position where it is just about to snap over again the force which has been holding the contacts closed is being reduced to zero, at which time the con tacts are liable to open and shut very rapidly, especially if the switch is subjected to vibrations of any sort. Such action is likely to cause arcing and burn the contacts which will cause inaccurate switch operation.

One of the objects of the present invention is therefore to provide a switch of the snap action type which does not have the above disadvantage.

Another object of the invention is to provide a thermostatic switch wherein the thermostat, when it first starts to move due to response to temperature change, will continue to move the full amount .of its travel, independently of any further change in temperature. f

Another object of this invention is to provide a thermostatic switch wherein the thermostat is .in the form of a bimetal spring, said spring being loaded longitudinally by a second spring which slightly overcompensates the spring rate of the bimetal.

Still another object of the invention is to provide a thermostatic switch which may easily be changed over from summer to winter operation without having to change anycircuit connections. I

These and other objects will become apparent when the specification is read in connection with the accompanying drawing, in which:

Fig. l'is a front elevation of the thermostatic switchfthe contacts being shown in closed position;

Fig. 2 is the same except that the contacts are shown in open position;

Fig. 3 shows the switch in closed position with an adjustable stop reversed so that the switch will close on a change in temperature opposite to that shown-in Figs. 1 and 2, and

Fig. 4 is a detail view of the toggle link taken on the line 4 4 of Fig. 3.

Indicated at H is a base member preferably of insulating material upon which is pivotally mounted at l2 a mounting member l3. Suitably secured to this mounting member, as by rivets l 4,

is a bimetallic blade l5. Also secured to pivot l2 and on the other side of the base member is an arm l6 having a slot I! in the end opposite that which is pivoted to the base member. Also pivoted to the reverse side of the base member I I, as at l 8, is a downwardly extending arm is having a pin 20 thereon which engages in the slot ll of arm l6. Secured to the bottom of the base memher I i is an indicating device 2| having indicia 22 thereon. The lower end of arm l9 has a pointer 23 which cooperates with indicia 22 to indicate the position of arm l9. Upon rotation of arm l9, pin 20 will move back and forth carrying with it arm 16 which will adjust the position of the bimetallic blade l5 through the mounting member l3.

The upper end of the bimetallic blade l5 moves back and forth in response to temperature changes. A plurality of stops are secured to the base member for limiting the movement of blade l5 for a purpose to be later described. Two of these stops 24 and 25 are permanently secured by any suitable means to the base member H at each side of the mid position of blade l5 permitting it to bend in either direction from the normal. Two holes 26 and 21 are also provided in the base member ll into which a third stop 28 may be screwed. Hole 21 is so positioned that when the stop 28 is screwed therein the blade l5 cannot pass beyond the normal or straight up and down position as it moves from stop 24 toward the right, as viewed in Fig. 1. The hole 26 is so positioned that when stop 28 isscrewed therein the blade cannot pass beyond the normal position as it moves from Stop 25 toward the left. The top of the bimetallic element is bent over as at 29 and is provided with a reentrant portion 30 providing a V-shaped seat. As shown more clearly in Fig. 4, there is a slot cut lengthwise of the V-shaped seat 30 for a purpose which will appear later.

Also pivoted to base member I l at 3| is a bellcrank lever 32 having a horizontally extending arm 33. Arm 33 has'a bent in portion 34 forming a V-shaped seat which also has a slot 35 running lengthwise or the V, as in the case of the bimetallic blade, and is more clearly shown in Fig. 4. A connecting means in the form of a toggle 35 cooperates with the seats to connect the bimetallic blade and the lever 32. The toggle member comprises a piece of sheet metal in a form as disclosed in Fig. 4 having atab 31 on the upper end extending through the slot 35 and having a tab 38 on the lower end extending through the slot in the bimetallic element. The shoulders 39 formed by the tabs are in the form of knife edges in order to provide a proper connection between the toggle, thermostat, and lever. Toggle member 36 has two reinforcing members 48 secured thereto by means of rivets ll.

Suitably secured to the free end of'arm 33 is a piece of insulating material 42 which is secured to a spring arm 43 carrying an electrical contact 44. Pin 45 is mounted in base member M and has adjustably mounted therein by means of a screw-threaded connection a stationary contact 46 which is positioned for cooperation with the movable contact 44. Lead wire 41 is secured to the stationary contact 48 by means of a lock nut 48. Secured to the movable contact 44 through the spring arm 43 and rivet 49 is a second lead wire 58. The lever 32 is provided with an upstanding arm 5l having a small hole 52 in the upper end thereof. An upstanding bracket 53 suitably secured to the base member ll is provided with a hole through which projects loosely a screw 54 having an enlarged head 55. Threaded on the screw 54 is ,a nut 56 having a large square head 51 of such size as to prevent rotation of the nut upon rotation of the screw 54. Secured to nut 58 is one end of a tension spring 58. The other end is hooked into hole 52 in arm move it through its entire range because it will 5| of the lever 52. The purpose of this spring is to place a longitudinal load on the bimetallic blade 95.

It is well known that every piece of spring metal has what is known as a spring rate. A strip of spring metal fixed at one end and which is normally in a straight position, will require only a slight force to move the free end a small distance and the farther the free end is moved the greater is the force which must be applied. This is due to the fact that there is a force within the spring metal which constantly tends to return the spring to its straight or normal position, and the farther the spring is bent away from the normal the greater this force tending to return it becomes. This is known as the spring rate of the metal. Itis theoretically possible to so load the spring metal that this spring rate may be exactly compensated. If a load is placed on the spring metal to exert a force longitudinally of the spring metal when it is in its normal or straight up and down position and if this force is adjusted to the proper value, it will exactly balance out the internal force tending to return the spring to its normal position after it has been bent. When the spring is in the normal position, this longitudinal force will, of course, have no bending effect on the spring. If the spring is bent slightly from the normal, the longitudinal load will exert a force through a small moment arm which will further tend to bend the spring in that direction. Assuming that the proper force is chosen to begin with, it will be exactly equal to the force within the spring tending to return it to normal. If the spring is bent farther in the same direction, the force due to the longitudinal load will increase, due to the fact that it will be working through a greater moment arm and will still be equal to the greater force developed within the spring tending to return the spring to normal position. Thus it will be seen that the longitudinal load placed on the spring will exactly balance out the force within the spring tending to return it to normal position, so that theoretically the slightest amount of external force applied to the spring to bend it will be unopposed, the effect of the force tending to return it to normal having been cancelled by the longitudinal load placed upon the spring.

In the present instance, the bimetal blade l5 has the same type of force tending to return it to normal after it has been bent, as the spring above described. The bending force in this case is the unequal expansion of the two metals making up the element, but in addition to the force thus created there is nevertheless the spring force within the metal tending to return it to normal position. The purpose of the tension spring 58 is to place a longitudinal load on the bimetallic element l5 which is slightly in excess of the load necessary to actually balance the spring rate of the bimetallic element. This means that if the bimetallic element were in the position shown in Fig. l, the force would be balanced, that is, there would be no unequal expansion of the two elements making up the bimetallic element. Hence, there would be no bending force. As the element is in its normal position, there would be no spring force within the bimetal itself, and as the load imposed on the element by the spring 58 is exerted longitudinally of the blade it would exert no bending force thereon. Assume now that the temperature rises slightly. This will bend the bimetallic element l5 slightly to the left. The force within the element, due to its spring rate, will now tend to return it to its normal position but the action of spring 58 is to impose a force slightly in excess of the force within the element, which tends to move the blade to the left, thus insuring that after it has once left the stop 28 it will continue to move to the left until it strikes stop 24. This is, in effect, a snap action although the device never moves over center. Conversely, when the bimetal element first leaves stop 24, the longitudinal load will decrease faster than the force within the element and hence the element will rapidly move into engagement with stop 28.

Operation Assuming the parts to be in the position shown in Fig. 1, it will be noted that the bimetallic element l5 is in its normal or straight position, and that it has aligned the toggle 36 with it to rotate the lever 32 to a position where it has caused closing of the contacts 44 and 46 and the circuit controlled thereby. Assuming that the 'metal on the right of the bimetallic element l5,

as viewed in Fig. 1, is the more active metal of the two and that closing the contacts 44 and 46 will cause en'ergization of a heater for heating the air to which the bimetallic element responds. As the temperature increases due to the energization of the heater, the bimetallic element will bend slightly to the left in the direction indicated by the arrow H and, as explained above, after it has once left the stop 28 the force exerted by spring 58 will move it quickly against the stop 24. This position is illustrated in Fig. 2, and it Will be noted that due to the angle between the toggle 36 and the bimetallic element [5 the lever 32 has been rotated in a counter-clockwise direction by the spring 58 causing separation of contacts 44 and 4B and a consequent breaking of the circuit to the heater. The temperature will now begin to drop and the bimetallic element will start to flex or bend to the right in the direction indicated by the arrow C. As it'starts to move to the right, the eifective force exerted by spring 58 on the bimetallic element l5 will decrease more rapidly than the force due to the spring in the bimetal and consequently the blade l5 will move rapidly to the right until stopped spring force within the bimetal winter to summer operation. The operation described above was for the control of a. heater in the winter time. By removing the stop 28 from the hole 21 and screwing it into the hole 26, at the same time making sure that the thermostat or the bimetallic element is located to the right of stop 28 as shown in Fig. 3, the instrument may be changed so that it will close the control circuit on a rise in temperature instead of on a fall in temperature, as in Figs. 1 and 2. Assuming now that the parts are in the position shown in Fig. 3, it will be seen that the blade I5 is in its normal position, at which time the contacts 44 and 46 are in engagement and the control circuit which operates a cooling device is closed. Due to the action of the cooling device, the temperature will begin to drop which will cause the blade I5 to start to bend to the right. As it leaves the stop 28, the force exerted by the spring 58 will cause it to immediately engage the stop 25 due to the fact that the bending force exerted by the spring 58 will increase at a greater rate than the tending to return it to normal position. This will cause the toggle to buckle in a reverse direction to that shown in Fig. 2, and will cause separation of the contacts 44 and 46 and a consequent opening of the control circuit to the cooling mechanism. As a result, the temperature will begin to rise and when it has risen a predetermined amount, the blade I5 will begin to bend to the left, at which time due to the fact that the force exerted by spring 58 is decreasing more rapidly than the force due to the spring of the bimetal, it will immediately move into contact with the stop 28 and again close the control circuit to the cooler. This cycle will be repeated.

It will be noted that this switch is really a snap acting switch in that as soon as the thermostat starts to move, the resultant of the forces acting upon it tending to produce movement, in creases as the movement progresses so that once the thermostat starts to move it continues at an ever increasing rate until it strikes a stop at the end of the desired range. Therefore, it is clear that there are only two positions in which the thermostat is stable, which are the positions in which it rests against one or the other of the two stops between which it operates. Viewing Fig. 1, it will be seen that when the thermostat is in its normal or straight up and down position, the contacts 44 and 46 are held closed through the spring arm 43 which produces a definite contact pressure. The amount of this pressure maybe adjusted as desired merely by screwing contact 46 in or out of its supporting pin 45. Before this contact pressure can be reduced to zero (after it has once been definitely adjusted), it is necessary for the thermostat to bend sufficiently to the left to allow the toggle 36 to buckle an amount sufficient to take up the spring in the arm 43. But in this position the thermostat is unstable and once it leaves stop 28 it continues until it strikes the stop 24. Hence, there can be no vanishing contact pressure here, because there is no stable position of the thermostat at which the contact pressure between contacts 44 and 46 is zero or substantially zero.

The main temperature setting of the device may be varied by moving the setting arm I8 across the scale 22. This rotates the support I3 about its pivot I2 and bodily positions the thermostat I5. The spring 58 exerting the force on the thermostat which compensates its spring rate is also adjustable. It may be adjusted to exert a force which is not great enough to fully com pensate the spring rate of the bimetal. In such,

a case, the switch would not be snap acting. Assuming the parts to be in the position shown in Fig. 1, if the temperature should rise the thermostat would bend slightly to the left, but due to the fact that its spring rate is undercompensated, the internal force tending to return it to normal would increase faster than the force exerted by spring 58 tending to bend it further to the left with the result that the forces would balance after the thermostat had moved a slight amount. The thermostat would therefore have any number of stable positions between the two stops and the position which it assumed would be determined by the existing temperature.

Another adjustment which the spring 58 could be given would be that which would just balance out the spring rate of the bimetal, or what might be called 100% spring rate compensation. Under these conditions, assuming the parts to be in the position shown in Fig. 1, a slight increase in temperature would cause the thermostat to bend towards the left and as the spring rate is exactly balanced there is no opposing force, and therefore the bimetal continues to move at the same rate until it strikes stop 24. That is, of course, if the temperature does not change again in the meanwhile. But the temperature might change and stop the thermostat at any point between the two stops. Therefore, it is clear that with this adjustment of spring 58, the switch is not snap acting. The above discussion of 100% spring rate compensation is based on theory because such conditions could not actually be obtained due to the friction of moving parts, etc.

The preferred adjustment of the spring 58 as above is such that it slightly over compensates the spring rate of the bimetal. This produces a difference between the temperature at whch the contacts open and the temperature at which they close depending upon the amount of over compensation produced by the spring. For example, in Fig. 1, as the temperature increases the thermostat starts to bend to the left and the force exerted by the spring 58 increases faster than the force due to the spring rate of the bimetal. They were balanced at the start of the movement but the force exerted by spring 58 is now substantially greater than the spring force of the bimetal. Now as the temperature drops, the bimetal must overcome the force of this spring 58 before the thermostat begins to move back. This produces the differential of the device. The amount'of th s differential may therefore be adjusted by adjusting the tension of spring 58.

Increasing the force exerted thereby will increase the differential and the force may be decreased to decrease the differential until it is just sufficient to exactly compensatethe spring rate of the bimetal, at which time the differential is theoretically zero.

It will be seen, therefore, that I have provided a snap acting thermostatic switch which attains all of the objects as set forth above and one which is extremely simple and easy to manufacture. It is understood, however, that I am not to be limited' to the specific disclosure made here but that the scope of my invention is to be limited only by the appended claims.

I claim as my invention:

1. An automatic switch comprising in combina- 'tion, a condition responsive device, a switch closed so that it will permit said condition responsive device to move into switch closing position but will prevent it from going beyond said position in one direction, and means for positioning said stop difierently so that it will permit said condition responsive device to move into switch closing position but will prevent it from going beyond said position in the other direction.

2. A mechanism of the class described, comprising a base, a lever pivoted to said base, a spring having one end mounted on said base and the other end connected to said lever tending to rotate it in one direction, a thermostat having a spring rate for rotating said lever in the opposite direction on temperature fall, said spring acting to overcompensate the spring rate of said thermostat, a toggle link connecting said lever to said thermostat, a first stop mounted on said base for preventing said toggle from passingbeyond dead-center on temperature fall, a second stop mounted on said base for limiting movement of said toggle on temperature rise, a stationary contact adjustably mounted on said base, a spring arm insulatingly carried on said lever, a contact carried by the free end of said arm, the arrangement being such that as said thermostat contracts it moves said toggle from engagement with said second stop into engagement with said first stop causing closure of said contacts, and additional mounting means for one of said stops in said base for reversing the action of said thermostat to cause said contacts to close as said thermostat expands.

3. A thermostatic switch comprising in combination, a base, a thermostat mounted on said base, a stationary contact, a movable contact moved into engagement with said stationary contact on temperature fall, a stop cooperating with said thermostat to prevent it from going beyond contact closing position on temperature fall, means for mounting said stop on said base, and another mounting means for said stop on said base, said second mounting means so positioning said stop that the thermostat closes the contacts on temperature rise and the stop prevents the thermostat from going beyond this point on a further rise in temperature.

4. A thermostatic switch comprising in combination, a thermostat, a switch closed by said thermostat in one position thereof, a stop, meansfor positioning said stop so that it will permit said thermostat to move into switch closing position but will prevent it'going beyond said position in one direction, and means for positioning said stop differently so that it will permit said thermostat to move into-switch closing position but will prevent it going beyond said position in the other direction.

5. A thermostatic switch comprising, a base, a bimetallic eleemnt mounted on said base at one end, said other end being free to flex back and forth on temperature changes, a switch operated by said bimetallic element when its free end is in substantially the mid position, two holes in said casing, one on either side of said bimetallic element when it is in its switch closing position, a stop adapted to be positioned in either of said holes, said holes being so positioned that if said stop is in one position said bimetallic element is prevented from passing beyond switch operating position in one direction, and if the stop is in the other position the bimetallic element is prevented from passing beyond said switch operating position in the other direction.

6. A thermostatic switch comprising in combination, a thermostat, means for limiting its movement in one direction due to temperature changes, a switch, movement of said thermostat to its limit position causing switch operation, and means for repositioning said limiting means for limiting the movement of said thermostat to said same limit position but from the opposite direction.

7. A thermostatic switch comprising in combination, a thermostat capable of flexing over a range due to variations in temperature, a switch operable by said thermostat at one position in said range, means for positioning a stop for preventing fiexure of said thermostat to the high side of said range from switch operating position, means for positioning a stop for preventing flexure of said thermostat to the low side of said range from switch operating position, and a stop -member for cooperation with either one of said means.

8. A thermostatic switch comprising in combination, a base, a bimetallic blade mounted at one end to said base, a lever pivoted to said base, a movable contact carried by an arm thereof, means connecting said lever to said blade for rotation thereby, a stationary contact mounted for cooperation with said movable contact, fixed stops for limiting the movement of said blade, movement of said blade from one fixed stop to a position substantially midway between the two causing closing of the contacts, continued movement to the other fixed stop causing said contacts to separate again, and stop means for preventing said blade from passing beyond contact closing position on movement in one direction.

9. A thermostatic switch-comprising in combination, a base, a bimetallic blade mounted at one end to said base, a lever pivoted to said base, a movable contact carried by an arm thereof, means connecting said lever to said blade for rotation thereby, a stationary contact mounted for cooperation with said movable contact, fixed stops for limiting the movement of saidblade, movement of said blade from one fixed stop to a position substantially midway between the two causing closing of the contacts, continued movement to the other fixed stop causing said contacts to separate again, stop means for preventing said blade from passing beyond contact closing position on movement in one direction, and means whereby said stop means may be repositioned for preventing said blade from passing beyond contact closing position on movement in the other direction.

10. In a thermostatic switch, a base, a bimetallic blade mounted at one end thereon, the other end of said blade having a bent over portion forming a seat, a lever pivoted to said base, said lever having a bent portion forming a seat, a toggle element having a knife edge at each end, one end engaging the seat in said blade and the other end engaging the seat in said lever, a spring connected at one end to said base and at the other end to said lever, said spring exerting a force through said lever and toggle element longitudinally of said blade, said force slightly over compensating the spring rate of said blade, a movable contact insulatingly carried by said lever, a stationary contact cooperating therewith, and stops for limiting the movement of said blade.

11. In a thermostatic switch, a base, a bimetalv lic bla-de mounted at one end thereon, the other end of said blade having a bent over portion forming a seat, a lever pivoted to said base, said lever having a bent portion forming a seat, a toggle element having a knife edge at each end, one end engaging the seat in said blade and the other end engaging the seat in said lever, a spring connected at one end to said base and at the other end to said lever, said spring exerting a force through said lever and toggle element longitudinally of said blade, said force slightly overcompensating the spring rate of said blade, a movable contact insulatingly carried by said lever, a stationary contact cooperating therewith, and stops for limiting the movement of said blade, the blade being arranged to flex in a direction approximately at right angles to the direction of rotation of said lever so that upon a continuous movement of said blade in the same direction said lever will be rotated first in one direction to close said contacts and then in the other to open said contacts, one of said stops being positioned to prevent the blade from passing beyond contact closing position.

12. In a thermostatic switch, a base, a bimetallic blade mounted at one end thereon, the other end of said blade having a bent over portion forming a seat, a lever pivoted to said base, said lever having a bent portion forming a seat, a. toggle element having a knife edge at each end, one end engaging the seat in said blade and the other end engaging the seat in said lever, a spring connected at one end to said base and at the other end to said lever, said spring exerting a force through said lever and toggle element longitudinally of said blade, said force slightly overcompensating the spring rate of saidblade, a movable contact insulatingly carried by said lever, a stationary contact cooperating therewith, stops for limiting the movement of said blade, the blade being arranged to flex in a direction approximately at right angles to the direction of rotation of said lever to that upon a continuous movement of said blade in the same direction said lever will be rotated first in one direction to close said contacts and then in the other to open said contacts, means for mounting one of said stops in one position to prevent the blade from passing beyond contact closing position when moving in one direction, and means for mounting said stop means to prevent the blade from passing contact closing direc ion when moving in the other direction, only one of said mounting means being used at one time.

13. In a thermostatic s itch, a base, a member pivoted to said base, a bimetallic blade pivoted at one end to said pivoted member, means for adjustably positioning said pivoted member for adjusting said blade, the other end of said blade having a bentover portion forming a seat, a lever pivoted to said base, said lever having a bent portion forming a seat, a toggle element having a knife edge at each end, one end engaging the seat in said blade and the other end engaging the seat in said lever, a spring connected at one end to said'base and at the other end to said lever, said spring exerting a force through said lever and toggle element longitudinally of said blade, said force slightly overcompensating the spring rate of said blade, a movable contact insulatingly carried by said lever, a stationary contact cooperating therewith, and stops for limiting the movement of said blade.

14. In a thermostatic switch, a base, a member pivoted to said base, a bimetallic blade pivoted at one end to said pivoted member, means for adjustably positioning said pivoted member for adjusting said blade, the other end of said blade having a bent over portion forming a seat, a lever pivoted to said base, said lever having a bent portion forming a seat, a toggle element having a knife edge at each end, one end engaging the seat in said blade and the other end engaging the seat in said lever, a spring connected at one end to said base and at the other end to said lever, said spring exerting a force through said lever and toggle element longitudinally of said blade, said force slightly overcompensating the spring rate of said blade, means for adjusting the force exerted by said spring, a movable contact insulatingly carried by said lever, a stationary contact cooperating therewith, means for adjusting said contact, and stops for limiting the movement of said blade.

15. A thermostatic switch comprising in combination, a bimetallic blade mounted at one end to said base, a contact arm mounted for movement on said base, a movable contact carried thereby, a stationary contact mounted for c0op eration therewith, and means connecting said contact arm to the free end of said blade for rotation thereby, the motion of the movable contact to closed position being at right angles to the motion of the free end of said blade whereby the pressure of the contacts will have no effect upon the motion of the free end of the blade.

16. In a device of the character described, in combination, a movable member, a switch movable to open and closed positions, means controlled by said member for moving said switch to one of said positions at an intermediate position of said member and/to the other of said positions for all other positions of said member, said switch being inoperative to limit the movement of said member, and stop means for preventing said member from moving past said intermediate position in one direction.

17. In a device of the character described, in combination, a movable member, a switch movable to open and closed positions, means controlled by said member for moving said switch to one of said positions at an intermediate position of said member and to the other of said positions for all other positions of said member, said switch being inoperative to limit the movement of 'said member, stop means, and a pair of alternate mounting means for said stop means, said stop means when mounted in one of said mounting means preventing said member from moving beyond said intermediate position in one direction, said stop means when mounted in the other of said mounting means preventing said member from moving beyond said intermediate position in the other direction.

18. In a device of the character described, in combination, a movable member, a switch movable to open and closed positions, means controlled by said member for moving said switch to one of said positions at an intermediate position of said member and to the other of said positions for all other positions of said member, said switch being inoperative to limit the movement of said member, stop means for preventing said member from moving past said intermediate position in one direction, and means for repositioning said stop means to prevent said member from moving past said intermediate position in the

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