Force measurement of bimetallic thermal disc

FIELD OF THE INVENTION

The present invention relates generally to methods for manufacturing thermally responsive bimetallic members, and in particular to methods for determining the snap energy generated by snap-action bimetallic members during transit between first and second states of stability.

BACKGROUND OF THE INVENTION

Thermally responsive bimetallic members that exhibit a snap-action response are commonly utilized to actuate overheat protection and thermostatic switching mechanisms. One type of such mechanisms is a thermostatic switch that utilizes an actuator formed of a bimetallic material having materials of relatively low and high thermal expansion coefficients joined together along a common interface. Snap-action bimetallic switching mechanisms typically exhibit two states of stability with each of these states being responsive to a predetermined threshold or set-point temperature. When the switching mechanism senses a temperature that is below a first lower of these predetermined set-point temperatures, the thermally responsive member is in one of the two stable states. Accordingly, when the sensed temperature is above a second higher predetermined set-point temperature, the thermally responsive member snaps to a second of the two stable states and remains in this second state while the sensed temperature remains at or above this second higher set-point temperature. Should the sensed temperature be reduced to the first lower temperature, the temperature of the member is lowered correspondingly. As a result, the thermally responsive member snaps back to the first lower temperature state. The difference between the two predetermined set-point temperatures corresponding to the respective first and second states of stability is known as the “differential temperature” of the thermally responsive member.

A known method of manufacturing thermally responsive snap-action switches of the variety described above has included a forming operation in which a pre-sized blank of the thermally responsive bimetallic member is positioned between two opposingly positioned shaping or die members. The shaping members are actuated to engage the bimetallic member, thereby providing the bimetallic member with the desired configuration needed to achieve snap-action at each of the two predetermined set-point temperatures. Such a configuration usually consists of a knee and/or corresponding bowed portion, a dimpled portion or portions, or a series of ridges. Examples of such of formations are described in U.S. Pat. No. 3,748,888 and U.S. Pat. No. 3,933,022, each of which is incorporated herein by reference in its entirety, wherein a thermally responsive snap-action bimetallic disc is provided.

U.S. Pat. No. 3,748,888 also describes a smoothly formed prior art disc-shaped snap-action bimetallic member, as illustrated in side view in

FIG. 1. A

bimetallic member

1

is formed using a disc of material formed of two materials

2

,

3

having different thermal expansion coefficients joined together along contiguous surfaces. One of the members

2

is formed of a material having a relatively high coefficient or rate of thermal expansion, while the other member

3

is formed of a material having a low coefficient or rate of thermal expansion relative to that of the first member

2

. The difference in thermal expansion coefficients between the two members

2

,

3

is a factor in determining the set-point temperature at which the resulting bimetallic disc actuator

1

operates and in an actuation force F produced by the snap-action. The disc-shaped bimetallic member

1

is often circular and, in some instances, is provided with a small, centrally located aperture therethrough (not shown). Bimetallic discs of this type are generally formed by “bumping” a flat circular disc blank with a punch-and-die set to stretch the bimetallic material of the disc into the concave structure having a depth H

1

, as illustrated by full line

4

in FIG.

1

. The bimetallic disc

1

is formed, for example, with a substantially planar peripheral hoop portion

5

surrounding a central portion

6

stretched into a concave configuration. The central portion

6

is mobile relative to the peripheral hoop portion

5

, the central portion

6

moving from one side of the peripheral hoop portion

5

to the other as a function of temperature. The set-point operation temperature and the force F applied by the snap-action are thus physical characteristics of the two members

2

,

3

that form the bimetallic member

1

.

Generally, when the bimetallic disc

1

is intended to operate at a temperature above ambient temperature, the disc

1

is bumped on the high expansion rate side

2

to form the central stretched portion

6

, whereby the central portion

6

is stretched to space the inner concave surface thereof to a depth H

1

away from the plane P of the peripheral hoop portion

5

, as illustrated by the full line configuration

4

. The depth of penetration of the punch during the bumping operation determines the depth H

1

and thus is another factor in determining both the upper set-point temperature and the force F applied by the snap-action operation of the disc

1

. The set-point operation temperature and the force F applied by the snap-action are thus also structural characteristics of the bimetallic member

1

, as also described in above-incorporated U.S. Pat. No. 3,748,888.

The bimetallic disc

1

is illustrated in

FIG. 1

in full line

4

in one of its two states of stability. Assuming the bimetallic disc

1

is intended for operation at a set-point temperature above ambient temperature, the high expansion rate side is located on the surface

2

and the low expansion rate side is along the surface

3

. If the bimetallic disc

1

is intended for operation at a set-point temperature below ambient temperature, the bimetallic disc

1

is formed in the opposite shape with the low expansion side located on the surface

2

and the high expansion rate side along the surface

3

. For purposes of explanation only, the bimetallic disc

1

shown in

FIG. 1

is assumed to be intended for operation at a set-point temperature above ambient temperature. Accordingly, at a temperature well below the upper set-point temperature the bimetallic disc

1

is configured with the central stretched portion

6

in an upwardly concave state, as shown by the upper dotted line

7

.

As the temperature of the bimetallic disc

1

is raised to approach its upper set-point operating temperature, the high expansion rate material

2

begins to stretch, while the lower expansion rate material

3

remains relatively stable. As the high expansion rate material

2

expands or grows, it is restrained by the relatively more slowly changing lower expansion rate material

3

. Both the higher and lower expansion rate sides

2

,

3

of the bimetallic disc

1

become distorted by the thermally induced stresses, and the central mobile portion

6

of the bimetallic disc

1

changes configuration with a slow movement or “creep” action from the upper dotted line configuration

7

to the full line configuration

4

. The inner concave surface of the central mobile portion

6

is spaced the depth H

1

away from the plane P of the peripheral hoop portion

5

. The full line configuration

4

is considered herein to be a first state of stability.

As soon as the temperature of the bimetallic disc

1

reaches its upper predetermined set-point temperature of operation, the central stretched portion

6

of the disc

1

moves with snap-action downward through the unstretched hoop portion

5

to the second state of stability with the inner concave surface of the central mobile portion

6

spaced a distance H

2

away from the plane P of the peripheral hoop portion

5

, as shown by the phantom line

8

. If the temperature of the bimetallic disc

1

is raised to a still higher temperature, the high expansion rate material

2

continues to expand at a greater rate than the relatively lower expansion rate material

3

joined thereto. As a result of this continued differential expansion, the central mobile portion

6

of the bimetallic disc

1

continues to creep toward a state of even greater downward concavity, as shown by the second lower dotted line configuration

9

.

As the temperature of the bimetallic disc member

1

is reduced from the high temperature toward the lower predetermined set-point temperature of operation, the central mobile portion

6

of the bimetallic disc

1

moves from the state of extreme concavity, as shown by the lower dotted line

9

, toward the second state of stability indicated in phantom

8

. As the temperature of the bimetallic disc

1

is reduced below the second or lower predetermined set-point temperature of operation, the material

2

having the relatively larger thermal coefficient also contracts or shrinks more rapidly than the other material

3

having the relatively smaller thermal coefficient. The bimetallic disc

1

changes configuration with a similar slow movement or creep action from the state of greatest downward concavity toward the second state of stability indicated in phantom

8

. As the bimetallic disc

1

reaches the lower set-point temperature, the central stretched portion

6

snaps back through the unstretched hoop portion to the first state of stability, as shown by the upper full line

4

. If the temperature is decreased still further, the differential expansion between the high and low rate materials

2

,

3

causes the central mobile portion

6

to continue to creep toward the state of greatest upward concavity, as shown by the upper dotted line

7

.

Many thermal switch designs use one of the bimetallic discs

1

that snap into a different state of concavity at a predetermined threshold or set-point temperature, thereby closing a contact or other indicator to signal that the set-point has been reached. The speed at which the bimetallic disc actuator

1

changes state is commonly known as the “snap rate.” As the term implies, the change from one bi-stable state to the other is not normally instantaneous, but is measurable. A slow snap rate means that the state change occurs at a low rate of speed, while a fast snap rate means that the state change occurs at a high rate of speed. Accordingly, in some known configurations of switch and indicator devices, a slow snap rate results in arcing between the operative electrical contacts. Slow snap rates thus limit the current carrying capacity of the thermal switch or indicator device. In contrast, a fast snap rate means that the change in state occurs rapidly, which increases the amount of current the thermal switch or indicator device can carry without arcing. The temperature rate of change affects the snap rate. A slower temperature rate of change tends to slow the snap rate, while a faster temperature rate of change usually results in a faster snap rate. While some applications provide fast temperature rates, switches and indicators experience very slow temperature rates in many other applications. In some applications, the temperature rates may be as low as about 1 degree F. per minute or less. For long-term reliability the device must operate in these very slow temperature application rates without arcing.

Furthermore, a minimum force F is required to actuate the contacts. As described above, the force F is thermally induced in the bimetallic disc

1

as the result of both the depth H

1

of the concavity formed in the disc

1

, and the differential thermal expansion rate between the high and low expansion rate sides

2

,

3

thereof. The force F produced during transit from one state of stability to the other state must be sufficiently powerful to overcome the contact restoring force in order to actuate the device. For example, the force F must be sufficient to overcome a restoring spring force in a flexible switch contact. If a bimetallic disc

1

with insufficient snap force F is installed into a thermal switch or other indicator device, the switch or device may fail prematurely, requiring replacement of the bimetal disc

1

or replacement of the entire mechanism.

Typically, the snap force F generated by the individual bimetallic disc

1

is tested prior to installation in the using device. For example, the bimetallic discs

1

are pre-tested under maximum load to ensure that each exerts sufficient snap force F at temperature application rates of about 1 degree F. per minute or less to actuate the device's contact without arcing. One known method of ensuring the snap quality of the bimetallic disc

1

is testing of the force F produced during actuation of the snap in situ. Pre-testing is thus accomplished by placing the disc

1

in the intended device and testing the fully assembled thermal switch or other indicator mechanism. Pre-testing is thus cumbersome and time consuming. Furthermore, the present in situ testing process is typically a simple go/no-go test in which marginally performing bimetallic discs

1

may remain undiscovered. The manufacturer may thus be forced to employ excessively conservative quality control measures.

SUMMARY OF THE INVENTION

The present invention is a method and means for determining an amount of energy released by a thermally responsive snap-action bimetallic actuator. The method of the invention includes forming a bimetallic disc having a mobile center portion surrounded by a substantially immobile peripheral portion; qualifying an energy released by transit of the mobile portion from a first side of the peripheral portion to a second opposite side of the peripheral portion during operation of a snap action; and subsequently assembling the disc into operative relationship with a movable indicator portion of a sensing device.

According to another aspect of the invention, the method of the invention includes presenting a thermally responsive snap-action bimetallic actuator to a sensing portion of a force sensing device while the actuator is configured in a first pre-snap state wherein a mobile portion of the actuator is spaced away from the sensing portion of the force sensing device, and determining a force generated by the actuator during transit to a second post-snap state wherein the mobile portion of the actuator is moved into forceful contact with the sensing portion of the force sensing device.

According to one aspect of the invention, presenting the actuator to the sensing portion of the force sensing device includes thermally activating the actuator to transit to the second post-snap state.

According to another aspect of the invention, presenting the actuator to the sensing portion of the force sensing device includes placing the actuator on a support structure configured to support the actuator.

According to another aspect of the invention, determining a force generated by the actuator includes detecting a peak force generated by moving the mobile portion of the actuator into forceful contact with the sensing portion of the force sensing device.

According to another aspect of the invention, presenting the actuator to the sensing portion of the force sensing device includes positioning the actuator in proximity to a thermal stage, and activating the thermal stage. Activating the thermal stage includes activating the thermal stage in a controlled manner. According to another aspect of the invention, determining a force generated by the actuator includes determining an energy-temperature rate relationship exhibited by the actuator.

According to still another aspect of the invention, the method of the invention also includes assembling the actuator into operative relationship with a movable indicator portion of a thermal sensing device.

According to other aspects of the invention, the invention provides an energy measuring device having a means for supporting a bimetallic member in a first pre-snap state; a means for qualifying an energy released by the bimetallic member during transit from the first pre-snap state to a second post-snap state, the qualifying means being positioned relative to the supporting means to be engaged by the bimetallic member in the second post-snap state; and a means for thermally activating the bimetallic member, the thermally activating means being positioned relative to the supporting means for thermally activating the bimetallic member to transit from the first pre-snap state to the second post-snap state.

According to another aspect of the invention, the means for qualifying the released energy includes means for measuring a force generated by the bimetallic member, and may also include means for measuring a peak force generated by the bimetallic member during the transit from the first pre-snap state to the second post-snap state.

According to another aspect of the invention, the thermally activating means of the device includes means for thermally activating the bimetallic member in a controlled manner, including for example, means for heating or cooling the bimetallic member at a controlled rate of temperature change.

According to another aspect of the invention, the means for supporting the bimetallic member in the first pre-snap state includes means structured to support a substantially immobile peripheral portion of the bimetallic member while a substantially mobile portion of the bimetallic member that is located centrally to the peripheral portion is disengaged from the qualifying means.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1

illustrates a known bimetallic actuator disc;

FIG. 2

is a top plan view of the thermally responsive device of the present invention embodied as a snap-action thermal switch;

FIG. 3

is a cross-sectional view of the snap-action thermal switch illustrated in

FIG. 2

, wherein the electrical contacts form a closed circuit;

FIG. 4

is another cross-sectional view of the snap-action thermal switch illustrated in

FIG. 2

, wherein the electrical contacts form an open circuit;

FIG. 5

illustrates the thermally responsive bimetallic member realized by the method of the invention embodied as a bimetallic disc actuator;

FIG. 6

illustrates the testing apparatus of the invention embodied as a disc snap-energy tester;

FIG. 7

illustrates the sizing of an intermediary drive pin positioned between a sensitive operational portion of a force indicator of the testing apparatus of the invention shown in FIG.

6

and an actuated bimetallic disc that is configured in a second post-snap state; and

FIG. 8

illustrates the disc snap-energy tester of the invention embodied without an intermediary drive pin.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the Figures, like numerals indicate like elements.

The present invention is an apparatus and method for determining the snap energy or snap force F generated by a bimetallic actuator during transit between first and second states of stability. The invention further provides an apparatus and method for determining of the threshold or set-point temperature of the bimetallic actuator during transit between bi-stable states. Accordingly, the apparatus and method provide for directly measuring both the snap force F and the set-point temperature of the bimetallic actuator during transit.

FIG. 2

is a top plan view and

FIGS. 3 and 4

are cross-sectional views of the thermally responsive device of the present invention embodied as a snap-action thermal switch

10

. The snap-action is driven by a thermally responsive snap-action actuator of the present invention embodied as a snap-action bimetallic disc actuator

12

, wherein the bimetallic disc actuator

12

includes a minimum snap force F generated during transit between bi-stable states at a predetermined set-point temperature as determined according to the method of the invention. For example, the method is operated using the apparatus of the invention. The thermal switch

10

also includes a pair of electrical contacts

14

,

16

that are relatively movable under the control of the disc actuator

12

. The electrical contacts

14

,

16

are mounted on the ends of a pair of spaced-apart, electrically conductive terminal posts

20

,

22

that are mounted in a header

24

such that they are electrically isolated from one anther. For example, terminal posts

20

,

22

are mounted in the metallic header

24

using a glass or epoxy electrical isolator

26

.

As illustrated in

FIGS. 3 and 4

, the electrical contacts

14

,

16

are moveable relative to one another between an open state (

FIG. 4

) and a closed state (FIG.

3

). For example, the movable contact

16

is affixed to an electrically conductive carrier

28

that is embodied as an armature formed of an electrically conductive spring material. The armature

28

is affixed in turn in a cantilever fashion to the electrically conductive terminal post

22

such that a spring pressure S of the armature

28

operates to bias the movable contact

16

toward the fixed contact

14

to make electrical contact therewith, as shown in FIG.

3

. The electrical contacts

14

,

16

thus provide an electrically conductive path between the terminal posts

20

,

22

such that the terminal posts

20

,

22

are shorted together.

The disc actuator

12

is spaced away from the header

24

by a spacer ring

30

interfitted with a peripheral groove

32

. A cylindrical case

34

fits over the spacer ring

30

, thereby enclosing the terminal posts

20

,

22

, the electrical contacts

14

,

16

, and the disc actuator

12

. The case

34

includes a base

36

with a pair of annular steps or lands

38

and

40

around the interior thereof and spaced above the base

36

. The lower edge of the spacer ring

30

abuts the upper case land

40

. A peripheral edge portion

42

of the disc actuator

12

is captured within an annular groove created between the lower end of the spacer ring

30

and the lower case land

38

. The disc actuator

12

operates the armature spring

28

to separate the contacts

14

,

16

through the distal end

44

of an intermediary striker pin

46

fixed to the armature spring

28

. Separation of the contacts

14

and

16

creates an open circuit condition.

As shown in

FIG. 3

, while the thermal switch

10

is maintained below a predetermined set-point temperature, the disc actuator

12

is maintained in a first state with the bimetallic disc actuator

12

withdrawn into a space

47

between the lower case land

38

and the case end

36

. In this first state, an inner concave surface

48

of an arcuate or dish-shaped central mobile portion

49

of the bimetallic disc actuator

12

is spaced away from the intermediary striker pin

46

, whereby the actuator force F is removed from the armature

28

. The relatively moveable electrical contacts

14

,

16

are moved together under the spring pressure S supplied by the armature

28

and thereby form a closed circuit. The spacing between the inner concave surface

48

of the bimetallic disc actuator

12

and the distal end

44

of the striker pin

46

is sufficient to prevent slight movement of the actuator disc

12

effecting contact engagement.

In

FIG. 4

, the armature

28

is operated under the control of the bimetallic disc actuator

12

, which inverts the central mobile portion

49

with a snap-action as a function of a predetermined set-point temperature between bi-stable states of opposite concavity. As shown in

FIG. 4

, in response to an increase in the sensed ambient temperature above the predetermined set-point, the central mobile portion

49

inverts in a high speed, forceful snap-action into a loaded relationship with the electrical contacts

14

,

16

, whereby the inner concave surface

48

is inverted into an outer convex surface

48

that rapidly engages the distal end

44

of the intermediary striker pin

46

. The snap-action of the bimetallic disc actuator

12

operates at the predetermined set-point temperature to rapidly generate a force F that is predetermined to be sufficient to overcome the spring pressure S of the armature

28

. Accordingly, operation of the bimetallic disc actuator

12

flexes the movable contact

16

away from the fixed contact

14

. For example, the disc actuator

12

operates through the intermediary striker pin

46

fixed to the armature spring

28

to pivot the armature spring

28

upwardly, thereby separating the contacts

14

,

16

. Separation of the contacts

14

,

16

creates an open circuit condition.

The snap rate and force F with which the central mobile portion

49

of the bimetallic disc actuator

12

changes state determine the speed with which the contacts

14

,

16

are allowed to come together to make the circuit, or are separated to break the circuit. The make and break speeds determine how much current can be carried without undesirable arcing between the contacts

14

,

16

. Faster make and break speeds increase the amount of current the thermal switch

10

can carry without arcing, and thus increase reliability while extending the useful life of the thermal switch

10

.

According to one embodiment of the invention, the bimetallic disc actuator

12

is fabricated to transit or snap the central mobile portion

49

at a high rate while exerting at least a minimum force F. Accordingly, the snap-action of the bimetallic disc actuator

12

changes state within about 1 millisecond while exerting sufficient force F to overcome the spring pressure S of the armature

28

to break the circuit. The movable contact

16

is thus flexed away from the fixed contact

14

rapidly, so that little arcing occurs. The current carrying capacity of the thermal switch

10

is thereby maximized.

When the ambient temperature sensed by the bimetallic disc actuator

12

is reduced below the predetermined set-point, the fast snap rate rapidly returns the central mobile portion

49

to the spaced-away, noninterference relationship with the electrical contacts

14

,

16

, as shown in FIG.

3

. The relatively moveable electrical contacts

14

,

16

are rapidly moved together again under the spring pressure S of the armature

28

. A closed circuit between the two terminal posts

20

,

22

is thereby formed. Accordingly, one embodiment of the invention provides a snap-action that changes state of the bimetallic disc actuator

12

within about 1 millisecond. The spring pressure S of the armature

28

causes the movable contact to follow the retreating central mobile portion

49

of the disc actuator

12

. The movable contact

16

is thus flexed into contact with the fixed contact

14

rapidly, so that arcing is minimized and the current carrying capacity of the thermal switch

10

is maximized.

The thermal switch

10

is sealed to provide protection from physical damage. The thermal switch

10

is optionally hermetically sealed with a dry Nitrogen gas atmosphere having trace Helium gas to provide leak detection, thereby providing the contacts

14

,

16

with a clean, safe operating environment.

FIG. 5

illustrates the thermally responsive bimetallic member realized by the method of the invention embodied as the bimetallic disc

12

. The central mobile portion

49

of the bimetallic disc

12

generates a predetermined minimum snap force F at a predetermined set-point temperature during temperature application at a predetermined rate as determined by a test performed in a prescribed manner according to the method of the invention. For example, the method is operated using the apparatus of the invention. The bimetallic disc actuator

12

according to the invention is initially fabricated according to generally known methods, as described in connection with FIG.

1

. For example, a thermally responsive bimetallic material

50

, such as ASTM-1, is selected according to known criteria for forming a bimetallic actuator. Such thermally responsive bimetallic material includes a first metallic material

52

having a first coefficient or rate of thermal expansion and a second metallic material

54

having a second relatively higher rate of thermal expansion. The first and second metallic materials

52

,

54

of the thermally responsive bimetallic material

50

are bonded together along one contiguous surface

56

.

The bimetallic material

50

is formed into a blank of desired shape and size. For example, a flat, round disk-shaped blank is formed having a diameter D sized to move freely within the annular groove created in the thermal switch assembly

10

between the lower end of the spacer ring

30

and the lower case land

38

.

The disk-shaped blank is subjected to a forming or “bumping” operation in which the blank of thermally responsive bimetallic material is positioned between two opposingly positioned shaping members (not shown). The shaping members are actuated to engage the disk-shaped blank of bimetallic material

50

, thereby forming the bimetallic disc

12

with the central mobile portion

49

having a configuration that achieves forceful snap-action at each of the two predetermined set-point temperatures. For example, the disk-shaped blank is placed in a female die which supports the blank along its peripheral edge portion

42

. A male punch having a spherical end is pressed against the center of the disc to stretch the metal and form the central mobile portion

49

having the inner dish-shaped concave surface

48

. The peripheral edge portion

42

either retains its substantially planar initial shape, or is formed by the shaping members with a substantially planar shape. Examples of such dish-shaped discs are illustrated in U.S. Pat. Nos. 2,717,936 and 2,954,447, each of which is incorporated herein by reference in its entirety. The formed bimetallic disc may be subsequently subjected to a heat treatment operation in order to achieve forceful snap-action of the central mobile portion

49

at each of the two predetermined set-point temperatures.

The method and apparatus of the present invention reduce or eliminate reiterative processes of screening the snap force F by installing the bimetallic disc actuators

12

into fully assembled thermal switches

10

or other thermally responsive indicator mechanisms. In contrast to current reiterative screening methods, the method and apparatus of the present invention determine the energy, i.e. the snap force F, generated during the snap-action transit of the central mobile portion

49

of the bimetallic disc

12

before it is assembled into a sensor mechanism. Low energy bimetallic discs

12

are identified and removed from a pool of usable hardware. The method and apparatus of the present invention thereby result in predictable product delivery since the measurement of the snap force F prevents imbalanced mechanisms from reaching customers. Improved predictability satisfy customer requirements for quality and reliability while reducing manufacturing costs.

FIG. 6

illustrates the testing apparatus of the invention embodied as a disc snap-energy tester

60

. The snap energy of the central mobile portion

49

of each individual disc

12

is effectively determined by thermally inducing the snap-action state transformation under controlled conditions and measuring the snap force F prior to installation into a sensor mechanism.

According to one embodiment of the invention, the disc energy tester

60

is an apparatus having a support structure

62

upon which a measuring device

64

is mounted. The measuring device

64

includes a supporting means

66

for supporting one of the bimetallic discs

12

in a manner substantially similar to the intended application. For example, the supporting means

66

is embodied as a stand or column formed to resemble a portion of the intended thermal switch

10

or other indicator device, such as the cylindrical case

34

shown in

FIGS. 3 and 4

. The supporting means

66

is, for example, embodied having a first annular step or land

68

around the interior thereof and spaced above a base or lower edge

70

thereof. The land

68

is sized to support the peripheral edge portion

42

of the bimetallic disc

12

above the base

70

. A suitable force indicator

72

, such as a conventional pressure-sensing transducer, is spaced above the bimetallic disc

12

. For example, the force indicator

72

is suspended from an arm portion

73

of the measuring device

64

overhanging the support structure

62

and the supporting means

66

. The force indicator

72

is of a type that senses a maximum or peak force F

P

of the snap and displays the value of the peak force F

P

in a useful manner. Optionally, the force indicator

72

is positioned in sufficiently close proximity to the bimetallic disc

12

that the snap force F is measured directly. Alternatively, the force indicator

72

is spaced away from the bimetallic disc

12

and the snap force F is measured through an intermediary member

74

.

A means

76

for thermally inducing the snap-action transformation in a controlled manner is provided. For example, the support structure

62

is embodied as containing a thermal stage

78

capable of inducing a predetermined, adjustable and controllable rate of temperature change in the bimetallic disc

12

under test. The thermal stage

78

includes adjustable and controllable heating sources. For example, the heating sources are thermoelectric devices such as commercially available electrical resistive devices. For actuation temperatures above ambient, controlled cooling is provided by alternate energizing and de-energizing of the controllable heating sources. Alternatively, cooling sources are provided as a manifold formed in the support structure

62

and filled with a coolant such as liquid nitrogen (LN2) or carbon dioxide (CO2) derived from a conventional external source (not shown). Temperature control is provided external to the energy tester

60

, for example, by a conventional programmable controller

80

such as are well-known for providing precise control of thermal application rate and steady-state temperatures.

The disc energy tester

60

shown in

FIG. 6

is embodied for testing the bimetallic disc

12

intended to actuate the contacts

14

,

16

in a thermal switch

10

. Accordingly, the supporting means

66

is embodied as a hollowed column structure formed in a tube or sleeve configuration similar to the cylindrical case

34

shown in

FIGS. 3 and 4

. The material of the hollowed column supporting means

66

is a thin stainless steel, which is either the same or a similar material used to form the cylindrical case

34

. The sizes and relative spacings of the land

68

around the interior of the hollowed column supporting means

66

and the base

70

thereof are similar to the land

38

and the base

36

, respectively, of the cylindrical case

34

. Furthermore, the supporting means

66

embodied as a hollowed column structure includes a second annular step or land

82

around the interior thereof. The second land

82

is paired with the land

68

and spaced above it relative to the base

70

. The pair of annular lands

68

,

82

are formed to resemble the pair of annular lands

38

,

40

around the interior of the thermal switch case

34

. The spacing between the land

82

and each of the respective land

68

and base

70

is similar to the spacing between land

40

and the respective land

38

and base

36

of the case

34

. The supporting means

66

is thus substantially identical to the support structure provided by the intended device.

As embodied in

FIG. 6

, the force indicator

72

is spaced away from the bimetallic disc

12

. The snap force F is applied to the force indicator

72

through the intermediary member

74

, which is embodied as a stiff, lightweight drive pin formed of titanium or another suitable material. The intermediary drive pin

74

may be hollowed to reduce its weight. A lighter weight drive pin

74

has less effect on the measurement of the snap force F. A first end portion

84

of the intermediary pin

74

is configured similarly to the striker pin

46

and contacts the inner concave surface

48

of the bimetallic disc

12

when it is configured in a first pre-snap state, wherein mobile central portion

49

of the bimetallic disc

12

is withdrawn into a space

85

between the first annular land

68

and the base

70

of the hollowed column structured supporting means

66

, its inner concave surface

48

being spaced away from the force indicator

72

. Alternatively, the first portion

84

of the intermediary pin

74

is spaced slightly away from the surface

48

of the bimetallic disc

12

under test, whereby the weight of the intermediary pin

74

is absent from the bimetallic disc

12

in the first pre-snap state. Accordingly, the first portion

84

of the intermediary drive pin

74

is spaced slightly away from the inner concave surface

48

of the bimetallic disc

12

. The intermediary pin

74

is thus positioned similarly to the striker pin

46

of the thermal switch

10

when the central mobile portion

49

of the bimetallic disc

12

is withdrawn into the space

47

between the lower case land

38

and the case end

36

so that the contacts

14

,

16

are configured in a closed circuit condition.

A second end portion

86

of the pin

74

is configured in a shape suitable for striking the force indicator

72

. The pin

74

has a length sized to substantially but not completely fill the space between the between the bimetallic disc

12

and the force indicator

72

when the bimetallic disc

12

is configured in the first pre-snap state having the inner concave surface

48

of its central mobile portion

49

spaced away from the force indicator

72

, whereby the snap force F is absent from the force indicator

72

. In other words, the pin

74

is short enough that it does not press on the force indicator

72

when the bimetallic disc

12

is withdrawn into the space

85

between the first annular land

68

and the base

70

of the hollowed column structured supporting means

66

.

FIG. 7

illustrates that the pin

74

is further sized to drive against a sensitive operational portion

88

of the force indicator

72

when the bimetallic disc

12

is configured in a second post-snap state, wherein the inner concave surface

48

of the central mobile portion

49

is inverted into an outer convex surface

48

directed toward the force indicator

72

. Accordingly, the intermediary drive pin

74

completely fills the space between the bimetallic disc

12

and the force indicator

72

and transmits the snap force F of the bimetallic disc

12

to the sensitive operational portion

88

of the force indicator

72

for the measurement thereof. Stated differently, during testing the central mobile portion

49

of the bimetallic disc

12

is inverted into an outer convex surface

48

that rapidly engages the first end

84

of the intermediary drive pin

74

. The drive pin

74

transmits the energy in the snap of the bimetallic disc

12

through the second end

86

of the pin

74

into the sensitive portion

88

of the force indicator

72

.

In summary, the support means

66

provides the annular land

68

to support the bimetallic disc along its peripheral edge

42

and within the space

85

between the annular land

68

and the base

70

of the hollowed column structure. In its first pre-snap state the central mobile portion

49

of the bimetallic disc

12

is withdrawn into the space

85

so that its inner concave surface

48

is spaced away from the force indicator

72

. The snap force F is thus absent from the force indicator

72

. This first pre-snap state is consistent with the closed circuit condition of the thermal switch

10

wherein the inner concave surface

48

of the central mobile portion

49

is spaced away from the intermediary striker pin

46

and the actuator force F is removed from the armature

28

, which permits the contacts

14

,

16

to close. During testing the bimetallic disc

12

is inverted into its second post-snap state wherein the inner concave surface

48

of the central mobile portion

49

is inverted into an outer convex surface

48

that rapidly engages the pin

74

and presses it into the force indicator

72

. This second post-snap state is consistent with the open circuit condition of the thermal switch

10

wherein the outer convex surface

48

rapidly engages the intermediary striker pin

46

and the actuator force F is applied to the armature

28

, which forces the contacts

14

,

16

open. The apparatus of the invention thus tests the snap force F applied by the central mobile portion

49

of the bimetallic disc

12

under test during transit from the first pre-snap state to the second post-snap state.

According to one embodiment of the invention, a cylindrical spacer

90

is mounted on the second land

82

formed around the interior of the hollowed column structured supporting means

66

adjacent to the bimetallic disc

12

. The spacer

90

operates similarly to the spacer ring

30

of the thermal switch

10

. The spacer

90

cooperates with the first land

68

to form an annular groove within which the peripheral edge portion

42

of the bimetallic disc

12

is captured. The spacer

90

is also provided with an aperture

92

sized to slidingly engage the drive pin

74

. The aperture

92

provides a track for guidance of the drive pin

74

. When the drive pin

74

is installed in the aperture

92

of the cylinder

90

, the first end

84

contacts the central mobile portion

49

of the bimetallic disc

12

, and the second end

86

is positioned for striking the sensitive portion

88

of the force indicator

72

. According to one embodiment of the invention, the spacer

90

is formed of a thermally insulating material, such as glass or ceramic.

Spacing adjusting means

94

are optionally provided for adjusting the position of the first end

84

of the intermediary drive pin

74

to compensate for differences in the thickness of the bimetallic disc

12

under test. For example, the spacing adjusting means

94

are embodied as shims provided between the cylindrical spacer

90

and the second end

86

of the drive pin

74

.