Bimetal Snap Disc

Abstract

A bimetal snap disc is disclosed for use in thermal controls. The disc is formed of a bimetal material having a positive deflection curve both above and below a predetermined temperature. The disc can be formed to operate at one temperature while being incapable of resetting in response to thermal stresses for use in manually resetting devices or the like. The disc can also be formed for higher temperature operation with relatively small temperature differentials. Also, the disc can be formed to provide one or more operating temperatures both above and below the predetermined temperature.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to thermally responsive devices employing bimetal as a thermal operator and more particularly to a novel and improved bimetal snap disc and to a novel and improved device employing a bimetal snap disc as a thermal operator.

PRIOR ART

Thermally responsive devices employing a bimetal snap disc operator are well known. In such devices, the bimetal element is formed with a shallow dished shape which causes it to move between two positions of stability with snap action when the bimetal snap disc reaches predetermined operating temperature normally referred to as a calibration temperature. In instances in which the bimetal element senses the ambient temperature, the device is generally referred to as a thermostat. In other instances, the temperature of the bimetal element is changed in response to other conditions and the devices are used to sense such other conditions. For example, the device may be a motor controller, a motor protector or a relay. Usually in such latter types of devices, the bimetal element is heated by an adjacent heater or by current flow through the bimetal itself.

The conventional bimetal material used to form such snap discs has been of a type which provides a deflection curve which is positive throughout the temperature range of its use. Generally with such conventional material, the chord height or depth of the disc increases as a direct function of the difference between the normal room ambient and the operating temperature of the disc. For example, a disc formed of a given material of such type to provide a high temperature of operation at about 400.degree.F. has a greater depth or chord height than a disc formed of a similar material having a high temperature of operation of 200.degree.F.

When attempting to form a disc of such conventional material with high operating temperatures, difficulty is encountered because the depth of the disc of chord height must be relatively large. Therefore, the disc must, during operation, be subject to relatively high thermally induced stresses which tend to produce a permanent deflection of the material and result in changes in the calibration temperature of the disc. Further, such discs tend to fatigue and fail because of such high thermally induced stresses.

Further, difficulty is often encountered in forming a relatively high temperature disc with a relatively low differential temperature. The differential temperature of a disc is the difference between the temperature at which 4 the disc snaps in one direction and the temperature at which the disc snaps in the other direction. This difficulty tends to occur because the differential temperature of a given disc tends to be a direct function of the depth or chord height of the discs.

Further, difficulty is often encountered when attempting to form a relatively high temperature operating disc which will not automatically reset in the system in which it is installed. Such discs are generally formed with a sufficiently high differential temperature so that the disc does not encounter one of its operating temperatures in the installed system. For example, if the disc is to operate on increasing temperature at 200.degree.F., it is provided with a sufficiently high differential temperature or lower operating temperature so that it does not encounter its lower operating temperature in use. Such a disc, for example, would be provided with a lower operating temperature below 0.degree.F. and requires a differential temperature in excess of 200.degree.F. When attempting to form discs for relatively high temperature operation, for example, above 500.degree.F., this presents severe problems since the differential temperature required to prevent nonautomatic reset can substantially exceed 500.degree.F.

SUMMARY OF THE INVENTION

There are a number of aspects to the present invention. In accordance with one aspect of the present invention, a snap disc may be formed of bimetal which cannot under any circumstance snap back to its reset condition as a result of thermally induced forces. In accordance with another aspect of this invention, it is possible to form a relatively high temperature bimetal snap disc having a relatively low differential temperature. In accordance with still another aspect of this invention, a bimetal snap disc may be provided which snaps from a first position of stability to a second position of stability at two different temperatures, and when the disc is formed for automatic resetting, it snaps back to the first position of stability at two different temperatures.

In accordance with the present invention, a snap disc is formed of a bimetal material having a positive deflection both above and below a predetermined temperature. One such material is manufactured and sold by Texas Instruments, Inc. under identification number 1,513. It is believed that this material is provided with an alloy on the high expansion side composed of 32 percent nickel, 1 percent colbalt, 1 percent molydenum and the balance iron. It is further believed that the material forming the low expansion side is 32 percent nickel, 15 percent colbalt, 1 percent molydenum and the balance iron. Such material has a positive deflection on increasing and decreasing temperatures above and below a temperature of about 230.degree.F.

A snap disc in accordance with one aspect of this invention utilizing this particular material can be formed to operate for example at temperatures in excess of 550.degree.F. with sufficient differential temperature to ensure that it will not automatically reset. Such disc will not automatically reset so long as the internal stresses required to produce reset are not reached above about 230.degree.F. since further cooling below such temperature will tend to return the disc to a thermally stressed condition which approaches the initial operating condition.

In accordance with another aspect of this invention, the disc may be formed at relatively high temperatures, for example in the order of 550.degree.F. with a relatively narrow temperature differential, for example in the order of 50.degree.F. This is accomplished because the material need not be as deeply formed, when compared to conventional bimetal materials, to provide the calibration temperature of 550.degree.F. Further, in accordance with the present invention, it is possible to form a disc which is provided with two operating temperatures, both above and below the predetermined temperature of the material.

Because discs formed in accordance with the present invention need not be as deeply drawn in most instances as discs comparable temperatures formed of conventional bimetal material, the tendency for the disc to fail by fatigue resulting from high thermally induced stresses is minimized. Further, the disc tends to maintain calibration to a greater degree because the disc is not required to experience as great a thermally induced stress.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation in longitudinal section of a simple, manual reset thermostat provided with a novel and improved snap disc in accordance with the present invention; and

FIG. 2 is a graph illustrating in full line the deflection characteristics of the bimetal material used to form a snap disc in accordance with the present invention, and in dotted line the deflection curves of more conventional bimetal materials.

DETAILED DESCRIPTION OF THE DRAWINGS

For purposes of simplification, the snap element incorporating the present invention is referred to as a snap disc and the illustrated embodiment is formed as a circular disc. However, it should be understood that as used herein, the term "snap disc" is intended to encompass other forms of snap elements which may be, for example, noncircular in shape, but which are formed with a dished portion causing the disc to be provided with two positions of stability between which it moves with snap action in response to predetermined temperature conditions of the material forming the disc.

FIG. 1 illustrates a manual reset thermostat of the type illustrated in the U.S. Pat. No. 3,621,434. Such thermostat is illustrated merely to disclose a thermostatic device of the general type which might be used in combination with a bimetal snap disc in accordance with the present invention. It should be understood, however, that the novel and improved snap disc in accordance with the present invention could be utilized in other types of devices which might or might not be, strictly speaking, thermostats. Further, it should be understood that the bimetal snap disc can be used in devices wherein the disc operates a valve or other type of device in response to some thermal condition.

In the illustrated embodiment, a body assembly 10 includes a main body 11, a guide plate 12, and a disc retaining cap 13. The body member 11 and the guide plate 12 cooperate to define a switch chamber 14 which encloses a switch consisting of a fixed contact 16, and a movable contact 17. The movable contact 17 is supported on a contact support arm 18 formed of resilient material. Suitable terminals (not illustrated) are connected to the fixed contact 16 and the movable contact support arm 18 so that an electrical connection is provided therebetween when the two contacts engage as illustrated.

A bimetal snap disc 19 in accordance with the present invention is supported in position by the guide plate 12 and the disc retaining cap 13. The disc 19 is formed with a shallow dished shape so that it is provided with two positions of stability and operates to snap between the two positions of stability in the manner discussed in greater detail below. A bumper 21 is guided within an opening 22 formed in the guide plate 12 and is proportioned so that when the disc is in the position of stability illustrated, the two contacts 16 and 17 engage. However, when the disc snaps from the illustrated position to the operated position, the bumper 21 is moved toward the movable contact support arm until it engages the support arm and thereafter causes the movable contact 17 to move out of engagement with the fixed contact 16. This causes the switch mechanism to be opened and occurs with a snap action.

Also provided in the device is a reset pin 23 which is guided for longitudinal movement in an opening 24 formed in the body member 11. When the disc 19 operates to open the switch, the reset pin 23 is engaged by the movable contact support arm and raised from the position illustrated. After operation of the switch, the device may be manually reset by pressing on the surface 26 causing the reset pin 23 to push the disc back toward its reset condition illustrated. When the disc is pushed to an unstable intermediate position by the reset pin, it snaps on through to the reset position and the switch is reclosed. In manufacturing a manual reset thermostat of the type illustrated, of course it should be understood that the material selected to form the various elements must be chosen to be capable of withstanding the temperatures expected to be encountered.

Referring to FIG. 2, the disc 19 is formed of a bimetal material having a deflection curve of the type illustrated at 27. This material which may be of the type designated above has a deflection curve which is positive as the temperature increases and decreases from a predetermined temperature of about 230.degree.F. as illustrated at 28. Such material deflects in a positive direction as the temperature is increased above the predetermined temperature of the material, and also deflects in the same positive direction when the temperature is decreased below the temperature 28.

The dotted curves 29 and 31 illustrate the types of deflections which are provided by the bimetals heretofore used to form snap discs. It should be noted that both of the materials 29 and 31 have a positive deflection on temperature rise throughout their temperature range of use. Conversely, the deflection is in the opposite direction or negative on temperature drop throughout their temperature range of use. Conversely, the deflection is in the opposite direction or negative on temperature drop throughout their temperature range of use.

When a snap disc is formed of the material represented by the curve 29 and is required to operate on increasing temperature at a relatively high temperature, for example a temperature in the order of 400.degree.F. - 600.degree.F., it is necessary to deeply form the disc and provide it with a relatively large chord height. If the disc is to be nonautomatic and capable of remaining in the operated condition, it must be provided with a very large differential temperature. For example, if the disc must remain in the operated condition when its temperature drops to about 0.degree.F. and operates at a temperature in the order of 550.degree.F., the disc must be formed to provide a differential temperature of at least 550.degree.F. It is extremely difficult to form such a disc with conventional materials. In any event, when it is possible to form the disc with such a wide differential, difficulty is usually encountered due to fatigue failures of the disc or due to permanent thermally induced distortions when the disc is cycled through such a wide temperature range which causes loss of calibration.

When a disc is formed in accordance with the present invention, however, for operation at a temperature in the order of 500.degree.F. - 600.degree.F. as illustrated by the bracket 32, it is only necessary to provide the disc with a shape and a depth so that it will not reset or snap back to its reset condition upon reaching a temperature at 28. Therefore, if the disc has an operating temperature of 600.degree.F., and the material forming the disc has a minimum deflection at a temperature of 230.degree.F., the required differential temperature to ensure that the disc will not automatically reset is about 370.degree.F.

Since the disc material experiences a positive deflection below the predetermined temperature at 28, it cannot thermally reset regardless of the temperature encountered if it does not reset before reaching the temperature at 28. As the temperature of the disc drops below the temperature at the point 28, the positive deflection of the disc material tends to relieve the stresses of the disc. Therefore, the disc is not exposed to excessive thermally induced stresses, so the tendency for fatigue failure is reduced, and the tendency for loss of calibration is minimized.

In accordance with another aspect of this invention, a disc formed of the material having a deflection curve 27 may be formed to operate at relatively high temperatures for example in the order of 550.degree.F. with a differential temperature of as low as about 50.degree.F. Such a disc would not, of course, be nonautomatic, but would snap in one direction upon reaching its upper temperature of 550.degree.F. and would automatically snap back to its first position when the temperature of the disc dropped to a temperature in the order of 500.degree.F. Such disc could be used in a device generally as illustrated in FIG. 1, but such device would not be provided with means to manually reset the disc because its automatic operation would cause resetting.

In accordance with still another aspect of the present invention, a disc can be formed to operate from a first position of stability to a second position at two different temperatures, one of which is above the temperature 28 and the other which is below the temperature 28. Such disc, for example, can be formed to operate at about 450.degree.F. at the point 35 on the curve 27. Since the same amount of positive deflection occurs at the point 33 when the temperature of the disc reaches a temperature of about -75.degree.F., such disc will snap from a first position of stability to a second position of stability at upon reaching either of the temperatures at 35 and 33. If the differential temperature of the disc is sufficiently great, such disc will not, under any circumstances, reset thermally since the thermally induced stressed required for resetting cannot occur. In some instances, a disc is provided with only one operating temperature. If the required thermal stress for operation cannot be obtained above absolute zero and below the predetermined temperature, the disc cannot operate below the predetermined temperature. Such disc could, however, have a calibration temperature at which it operated above the predetermined temperature.

If the disc is provided with a sufficiently low differential temperature, for example about 50.degree.F., it would snap on increasing temperature from the first position of stability to the second on reaching 450.degree.F. and would snap back to its first position of stability when reaching 400.degree.F. as indicated by the point 34. Such a disc would have two operating temperatures below the predetermined temperature 28 and would snap in one direction on reaching a temperature of about -75.degree.F. and would return or automatically reset to its initial position on reaching a point 36 on the curve at 0.degree.F.

Generally speaking, a disc formed of a material having the deflection curve 27 should be calibrated to operate at above 400.degree.F. or below 0.degree.F. since the activity of the material, i.e., the deflection produced by a given change in temperature, is relatively low between about 0.degree.F. and 400.degree.F. as indicated by the relatively flat slope of the curve 27 between these two points.

It should be understood that a disc in accordance with the present invention may be formed of other bimetal materials so long as the material exhibits a positive deflection curve and on both increasing and decreasing temperatures from a predetermined temperature. It should also be understood that a nonautomatic disc in accordance with the invention can also be used in devices which are not provided with means to reset the disc once it operates. Such device is often referred to as a thermal fuse since it can only operate once.

Although preferred embodiments of this invention are illustrated, it should be understood that various modifications and rearrangements of parts may be resorted to without departing from the scope of the invention disclosed and claimed herein.

Claims

What is claimed is:

1. A snap disc comprising a piece of bimetal formed of a material having a deflection curve which is positive as the temperature of the material varies from a predetermined temperature in either direction said disc having a portion formed with a shallow dished shape, said disc providing two positions of stability between which it moves for snap action, said disc when in said first position snapping to said second position upon reaching a calibration temperature different than said predetermined temperature.

2. A snap disc as set forth in claim 1 wherein said disc provides first and second calibration temperatures, one of which is above said predetermined temperature and the other of which is below said predetermined temperature.

3. A snap disc as set forth in claim 2 wherein said disc is incapable of snapping back to said first position solely in response to thermally induced forces.

4. A snap disc as set forth in claim 2 wherein said disc operates to snap back to said first position upon reaching third and fourth calibration temperatures, one of which is above said predetermined temperature and the other of which is below said predetermined temperature.

5. A snap disc as set forth in claim 4 wherein said deflection curve includes portions of relatively low activity and portions of substantially greater activity and said calibration temperatures are located at points along said portions of substantially greater activity.

6. A snap disc as set forth in claim 1 wherein said calibration temperature is above said predetermined temperature, and said snap disc operates to snap back to said first position upon reaching a second calibration temperature which is also above said predetermined temperature.

7. A snap disc as set forth in claim 6 wherein said disc snaps from said first position to said second position at a temperature of at least about 550.degree.F. and has a differential temperature in the order of 50.degree.F.

8. A snap disc as set forth in claim 1 wherein said calibration temperature is above said predetermined temperature, and said disc is incapable of snapping back to said first position solely in response to thermally induced forces.

9. A snap disc as set forth in claim 1 wherein said bimetal is formed with a high expansion side consisting of about 32 percent nickel, 1 percent colbalt, 1 percent molydenum and the balance iron; and a low expansion side consisting of 32 percent nickel, 15 percent colbalt, 1 percent molydenum and the balance iron.

10. A snap disc device comprising body means, switch means or the like on said body means, and a bimetallic snap disc formed of a material having a positive deflection as the temperature of the material varies away from the predetermined temperature in either direction a predetermined temperature, said disc having first and second positions of stability and operating to snap from said first position to said second position at a temperature above said predetermined temperature, said disc after snapping to said second position remaining in said second position in all temperatures encountered including said predetermined temperature, said switch means or the like being operated when said disc is snapped from said first position to said second position.