U.S. patent number 3,820,050 [Application Number 05/378,843] was granted by the patent office on 1974-06-25 for thermal switch.
This patent grant is currently assigned to Robertshaw Controls Company. Invention is credited to Richard K. Mitts, Hugh Jean Tyler, Denis G. Wolfe.
United States Patent |
3,820,050 |
Tyler , et al. |
June 25, 1974 |
THERMAL SWITCH
Abstract
A thermal switch has a pair of conductors with a longitudinally
movable contact element engaging a side of at least one of the
conductors and forming an electrical connection between the
conductors. The contact element is biased against a thermally
yieldable member such that when a predetermined temperature is
reached the member yields and the element is moved longitudinally
off an edge of the one conductor to open the electrical connection
between the conductors.
Inventors: |
Tyler; Hugh Jean (Santa Ana,
CA), Wolfe; Denis G. (Santa Ana, CA), Mitts; Richard
K. (Fullerton, CA) |
Assignee: |
Robertshaw Controls Company
(Richmond, VA)
|
Family
ID: |
23494768 |
Appl.
No.: |
05/378,843 |
Filed: |
July 13, 1973 |
Current U.S.
Class: |
337/408;
337/414 |
Current CPC
Class: |
H01H
37/766 (20130101); H01H 1/38 (20130101) |
Current International
Class: |
H01H
37/00 (20060101); H01H 1/38 (20060101); H01H
1/12 (20060101); H01H 37/76 (20060101); H01h
037/76 () |
Field of
Search: |
;337/408,409,407,403,413,414,401,402,186 ;200/61.08,163 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Broome; Harold
Attorney, Agent or Firm: O'Brien; Anthony A.
Claims
What is claimed is:
1. A thermal switch comprising
a pair of conductors, one of the pair of conductors having a
surface portion near an edge thereof,
insulative means for supporting the one conductor with the one
conductor surface portion extending in a direction which has a
substantial directional component parallel to an axis,
a rigid member which becomes yieldable at a predetermined
temperature,
a movable electrically conductive contact,
means connected to the contact for engaging the rigid member in a
direction which prevents movement of the contact parallel to the
axis from the one conductor surface portion past the edge,
means for electrically connecting the contact to the other of the
pair of conductors,
biasing means for engaging the contact with the one conductor
surface portion in a direction transverse to the axis to form an
electrical connection between the one conductor and the contact,
and
biasing means for urging the contact toward the edge of the one
conductor in a direction parallel to the axis with a force which
exceeds the sum of the frictional force between the contact and the
conductor and the force to which the rigid member yields at the
predetermined temperature.
2. A thermal switch as claimed in claim 1 wherein the one conductor
surface portion is substantially parallel to the axis.
3. A thermal switch as claimed in claim 1 wherein
the means connected to the contact for engaging the rigid member
includes a head engaging the rigid member,
the biasing means for engaging the contact with the one conductor
surface portion includes a cantilever spring arm extending from the
head in a direction having a substantial directional component
parallel to the axis, and
the contact includes a conductive tip on the spring arm biased by
the spring arm against the one conductor surface portion.
4. A thermal switch as claimed in claim 1 wherein
the one conductor is an insulated conductive wire, and
the one conductor surface portion is a section at the end of the
wire from which the insulation has been stripped.
5. A thermal switch as claimed in claim 4, wherein
the insulative means includes a first portion having a cavity with
a groove formed in the wall of the cavity for receiving the
stripped section of the wire, and
the biasing means for engaging the contact with the stripped
section urges the stripped section into the groove.
6. A thermal switch as claimed in claim 5 wherein
the insulative means includes a second portion having a hole in
alignment with the groove in the first portion of the insulating
means,
the wire extends through the hole and is supported by the second
portion of the insulative means, and
the length of the wire extending from the hole into the cavity has
sufficient rigidity to prevent any substantial bending from the
force of gravity.
7. A thermal switch as claimed in claim 6 wherein
there is included a case for supporting the rigid member in one end
of the case and for supporting the insulating means near another
end of the case,
the means connected to the contact for engaging the rigid member
includes a head engaging the rigid member,
the biasing means for engaging the contact with the stripped
section includes a first cantilever spring arm extending from the
head in a direction having a substantial directional component
parallel to the axis, and
the contact includes a conductive tip on the first spring arm
biased by the spring arm against the stripped section of the
wire.
8. A thermal switch as claimed in claim 7 wherein
the rigid member includes a block of solid material which becomes
fluid at the predetermined temperature, the solid block having a
flat surface perpendicular to the axis;
the biasing means for urging the contact toward the one conductor
edge includes a helical compression spring between the insulative
means and the head; and
the head has a flat surface engaging the flat surface of the solid
block.
9. A thermal switch is claimed in claim 7 wherein the biasing means
for engaging the contact with the stripped section includes
a second cantilever spring arm extending from the head in a
direction having a substantial directional component parallel to
the axis, and
means engaged by the second spring arm for maintaining the first
spring arm in an elastic stressed condition to force the conductive
tip transversely against the stripped section of the wire.
10. A thermal switch comprising
a pair of conductors, one conductor having a surface portion;
insulative means having a groove for receiving and retaining the
one conductor with the one conductor surface portion exposed;
a rigid member which becomes yieldable at a predetermined
temperature;
a movable element including means for engaging the rigid member and
an elastic cantilever arm with a conductive tip biased by the
elastic arm into engagement with the one conductor surface
portion,
means for electrically connecting the conductive tip to the other
conductor, and
biasing means for urging the movable element against the rigid
member such that when rigid member becomes yieldable the movable
element and conductive tip are moved out of engagement with the one
conductor surface portion.
11. A thermal switch as claimed in claim 10 where the conductors
are insulated wires,
the one conductor surface portion is a section at the end of one
wire having the insulation stripped therefrom, and
the insulative means includes a hole aligned with the groove for
receiving the one wire longitudinally and retaining the one wire
laterally.
12. A thermal switch comprising
a pair of conductors each having a longitudinal surface portion
near the edge thereof,
insulative means for supporting the conductors in spaced
relationship with the longitudinal surface portions of the
conductors extending in directions which have substantial
directional components parallel to an axis,
a rigid member which becomes yieldable at a predetermined
temperature,
a movable means for conducting electrical current,
means connected to the movable means for engaging the rigid member
in a direction which prevents movement of the movable means
parallel to the axis from the longitudinal surface portions past
the edges,
means for biasing the movable means transverse to the axis into
engagement against the longitudinal surface portions of both of the
conductors to form an electrical connection between the conductors,
and
means for biasing the movable means toward the edges of the
conductors in a direction parallel to the axis with a force which
exceeds the sum of the frictional force between the contact and the
conductor and the force to which the rigid member yields at the
predetermined temperatures.
13. A thermal switch as claimed in claim 12 wherein
the means connected to the movable means for engaging the rigid
member includes a head engaging the rigid member,
the means for biasing the movable means transverse to the axis
includes a pair of cantilever spring arms extending from the head
in directions which have substantial directional components
parallel to the axis, and
the movable means includes a pair of conductive tips on the
respective ends of the spring arms engaging the conductor surface
portions.
14. A thermal switch as claimed in claim 13 wherein each of the
conductive tips engages both conductor surface portions.
15. A thermal switch as claimed in claim 13 wherein the conductive
tips are between the conductor surface portions and engage
respective surface portions and each other.
16. A thermal switch as claimed in claim 12 wherein
the pair of conductors are insulated wires with the insulation
stripped from sections at ends of the wires to form the
longitudinal surface portions,
the insulative means includes a first portion having a cavity with
a pair of grooves each formed in wall portions of the cavity for
receiving the respective stripped sections,
the means for biasing the movable means transverse to the axis also
bias the stripped sections into the respective grooves.
17. A thermal switch as claimed in claim 16 wherein
the means connected to the movable means includes a head engaging
the rigid member,
the movable means for biasing the movable means transverse to the
axis includes a pair of cantilever spring arms extending from the
head in directions which have substantial directional components
parallel to the axis, and
the movable means includes a pair of conductive tips on the
respective ends of the spring arms engaging the stripped sections
of the wires.
18. A thermal switch as claimed in claim 17 wherein
the pair of grooves are formed in opposite wall portions of the
cavity, and
the conductive tips are wedge shaped and are wedged by the springs
between the stripped sections of the wires.
19. A thermal switch as claimed in claim 17 wherein
the pair of grooves are formed in opposite wall portions of the
cavity, and
the conductive tips are disposed between the stripped sections and
are biased by the spring arms into engagement with the respective
stripped sections and each other.
20. A thermal switch as claimed in claim 19 wherein the conductive
tips engage each other at a fulcrum point and the spring arms bias
the conductive tips in a pivotal relationship about the fulcrum
point against the respective stripped sections.
21. A thermal switch comprising
a pair of conductors,
a tubular case,
a block of rigid material which becomes fluid at a predetermined
temperature secured at one end of the casing,
a movable head with a cross section perpendicular to the axis of
the case slightly smaller than the cross section of the internal
passageway of the case,
means for biasing the head against the block,
means in the other end of the case operated by the movable head for
electrically connecting the pair of conductors until the
predetermined temperature is reached and the head is allowed to be
moved by the biasing means to electrically disconnect the pair of
conductors, and
said tubular case being made of a brittle material which breaks
upon impact rather than bending upon impact.
22. A thermal switch as claimed in claim 21 wherein
the pair of conductors are insulated wires with the insulation
stripped from sections at the ends of the wires,
there is included insulative means for supporting the stripped
sections in spaced relationship longitudinally within the case,
the electrically connecting means includes a pair of cantilever
spring arms extending longitudinally from the head and a conductive
tip on at least one of the spring arms biased by the spring arms
into engagement with the stripped sections to electrically connect
the wires, and
the biasing means includes a helical compression spring between the
head and insulative means.
23. A thermal switch comprising
a tubular case closed at one end;
a block of rigid material which becomes fluid at a predetermined
temperature disposed in the one end of the case;
a pair of insulated wires having the insulation stripped from
sections at the ends of the wires;
insulative means mounted toward the other end in the case having a
central cavity with a pair of parallel grooves formed
longitudinally relative to the case in opposite wall portions of
the cavity for receiving the stripped sections;
a movable conductive element having a head portion engaging the
block, a pair of cantilever spring arms extending longitudinally
from the head portion into the central cavity of the insulative
means, and conductive tip portions on the respective spring arm
portions biased by the spring arm portions transversely relative to
the case against the stripped sections of the wires to form an
electrical connection between the conductors; and
a compression spring between the insulator and the head portion for
moving the movable element and the conductive tip portions from the
central cavity when the solid material becomes fluid.
24. A thermal switch as claimed in claim 23 wherein
the insulative means has a portion with parallel holes extending
longitudinally for receiving and supporting the wires in parallel
spaced relationship with lengths of the wires extending from the
holes into the central cavity, and
the rigidities of the lengths of the wires are selected to prevent
the stripped sections of the wires from being bent by gravitational
forces into engagement.
25. A thermal switch as claimed in claim 23 wherein
the tubular case, the insulative means and the head portion of the
movable element have mating non-circular cross sections
perpendicular to the axis of the tubular case to align the
conducting tip portions relative to the conductors.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to thermal switches and, in particular, to a
switch device containing a circuit connecting element biased
against a member which yields at a specified temperature to open
the circuit connecting element. Cooking ranges, water heaters,
furnaces, and other appliances can be protected against overheating
by such thermal switches.
2. Description of the Prior Art
The prior art, as exemplified by U.S. Pat. Nos. 348,473, 2,997,562,
3,171,924, 3,291,945, and No. 3,519,972, contains many switch
devices with contact elements biased against solid members which
melt at a specified temperature to yield and allow the contact
element to move out of engagement with one or more contacts to open
a circuit. Many prior art thermal switching devices depend on
maintaining contact force by the difference in force of two
springs, one spring urging a movable contact against a stationary
contact, and the other spring urging the movable contact and the
one spring in the opposite direction against the meltable member;
however, this construction doesn't allow the full force change
potential or yield of the meltable element to be used in opening
the circuit. Some prior art thermal switches depend on many small
integral parts to perform a simple switching operation; however,
some essential parts of such devices can be left out during
assembly and the omission is undetectable by practical
nondestructing testing. Also, some prior art thermal switches have
been subject to being rendered inoperable and unsafe by denting or
crushing a cover or case which prevents the movement of an element
biased against a meltable member within the case. Further,
contamination on contacts readily renders some prior art thermal
switches inoperative or highly resistive.
SUMMARY OF THE INVENTION
The invention is summarized in that a thermal switch includes a
pair of conductors, one of the pair of conductors having a surface
portion near an edge thereof, insulative means for supporting the
one conductor with the conductor surface portion extending in a
direction which has a substantial directional component parallel to
an axis, a rigid member which becomes yieldable at a predetermined
temperature, a movable electrically conductive contact, means
connected to the contact for engaging the rigid member in a
direction which prevents movement of the contact parallel to the
axis from the one conductor surface portion past the edge, means
for electrically connecting the contact to the other of the pair of
conductors, biasing means for engaging the contact with the one
conductor surface portion in a direction transverse to the axis to
form an electrical connection between the one conductor and the
contact, and biasing means for urging the contact toward the edge
of the one conductor in a direction parallel to the axis with a
force which exceeds the sum of the frictional force between the
contact and the conductor and the force to which the rigid member
yields at the predetermined temperature.
An object of the invention is to provide a simple thermally
operated switch which is inexpensive and reliable.
Another object is to utilize all the potential force change of a
meltable element to operate the switch mechanism.
It is also an object of the present invention to eliminate
extremely small parts which can be left out of the assembly and
which cannot be detected by practical non-destructive testing.
A further object of the invention is to eliminate the employment of
thermal switch devices which have been rendered unsafe or
inoperative by indentations caused by handling and
installation.
One advantage of the invention is that wires used as connecting
leads within an appliance are used as contacts in the thermal
switch.
Another advantage is that switch resistance and resistance heating
of a rigid member which becomes yieldable at a specified
temperature is kept to a minimum.
Additional features of the invention include the employment of
stripped ends of insulated wires as contact engaging surface
portions; the provision of a movable contact element having a head
portion with a pair of cantilever spring arms extending therefrom
for engaging a pair of conductors; the provision of an inusulated
support with a cavity having a pair of grooves for receiving and
aligning a pair of contact engaging conductors; and the provision
of an outer case which is made from a relatively brittle material
to prevent denting.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation view in cross section of a thermal switch in
accordance with the invention.
FIG. 2 is a side cross section view along lines 2--2 of FIG. 1.
FIG. 3 is a prospective view of a movable contact element of the
thermal switch shown in FIG. 1.
FIG. 4 is a view similar to FIG. 1 but illustrating the movable
contact element in an operated position.
FIG. 5 is an elevation view in cross section of a modified thermal
switch in accordance with the invention.
FIG. 6 is a cross-sectional side view along line 6--6 of FIG.
5.
FIG. 7 is a perspective view of a movable contact element of the
modified thermal switch of FIG. 5.
FIG. 8 is a view similar to FIG. 5 but illustrating the movable
contact element in an operated position.
FIG. 9 is an elevation view in cross section of another modified
thermal switch in accordance with the invention.
FIG. 10 is an elevation view illustrating a step in the manufacture
of the thermal switch of FIG. 9.
FIG. 11 is an elevation view in cross section of still another
modified thermal switch in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As illustrated in FIGS. 1 and 2, the invention is embodied in a
thermal switch which has a case 29 containing an insulator 21
supporting a pair of spaced conductors 22 and 24. A movable contact
element, indicated generally at 26, engages and electrically
connects the conductors 22 and 24. A spring 30 biases the movable
contact element 26 against a rigid member 28 which yields at a
predetermined temperature, as illustrated in FIG. 4, such that the
element 26 is moved out of engagement with the conductors 22 and 24
to break or open a circuit through the conductors 22 and 24.
The case 20 is generally tubular with one end 32 being closed. A
section of the tubular case 20 toward the closed end 32 is reduced
in cross section and has the member 28 disposed therein against the
closed end 32. The case 20 is formed from any suitable material,
such as metal, by any suitable process, such as drawing.
The member 28 is a block of any suitable material or a device which
is relatively rigid until heated to a specified temperature at
which the member 28 becomes yieldable to the force of the spring
30. Suitable electrically nonconductive materials which are solid
but become fluid or melt at specified temperatures are known and
used and are commercially available from a variety of sources.
The insulator 21 is secured within the case 20 agains a shoulder 36
of the case 20 by inwardly deformed detents or tabs 38. The
insulator 34 has a central cavity 40 formed in one end or a forward
portion thereof with longitudinal grooves 42 and 43 in opposite
wall portions of the cavity 40 aligned with respective holes 44 and
45 through a rear portion of the insulator 21. The cavity 40 is
coaxial with the axis of the case 20 while the grooves 42 and 43
and holes 44 and 45 are parallel to the axis of the case 20. The
holes 44 and 45 and the grooves 42 and 43 have cross-sectional
sizes slightly larger than the cross-sectional sizes of the
conductors 22 and 24 such that the conductors can be readily
inserted longitudinally but are laterally retained. The grooves 42
and 43 have depths designed to leave substantial longitudinal
surface portions of the conductors 22 and 24 exposed within the
cavity 40. The insulator 21 is made from any suitable electrically
insulating material, such as pressed steatite.
The conductors 22 and 24 are wires with respective insulative
coatings 48 and 50 and the exposed surface portions are sections of
the wires from which the insulative coatings have been stripped.
The stripped sections extend through the respective holes 44 and 45
into the grooves 42 and 43. The conductors 22 and 24 are
illustrated as being single strand or solid core wires; however,
wires with a multiple strand core can also be employed. The
conductors 22 and 24 are selected to have sufficient rigidity that
lengths of the conductors extending from the holes 44 and 45 to end
edges 46 and 47 in the cavity 40 are prevented from being bent by
gravitational forces into engagement with each other.
A potting command 54 secures the insulated wires and conductors 22
and 24 and seals the open end of case 20. The potting compound 54
is typically a material which is thermal setting and which forms a
good adhesive bond to the insulative coatings 48 and 50, conductors
22 and 24, the insulator 21, and the case 20. One suitable potting
compound is Aremco 540-S-1.
The movable contact element 26, as shown in FIG.. 3, has a head 56
from which extend arms 58 and 60 which are cantilevered; i.e.,
supported only at or near one end. Tips or contacts 62 and 64 of
the arms 58 and 60 having inwardly formed conical wedge portions 66
and 68. As illustrated in FIG. 1, a forward flat surface of the
head 56 engages the face 33 of the member 28 and the arms 58 and 60
extend generally parallel to the axis of the case 20 into the
cavity 40. The helical spring 30 is compressed between the forward
end of the insulator 21 and the rear surface of the head 56. The
head 56 is sufficiently smaller than the interior of the case 20
such that, when heated to the predetermined temperature, fluid
material 28 is allowed to flow freely around the periphery of the
head 56. The forward flat surface of the head 56 is sufficiently
large to distribute the force from the spring 30 over a substantial
portion of the face 33 of the member 28.
The tips 62 and 64 have a width and are spaced such that
longitudinally extending surface portions adjacent opposite lateral
edges of each of the wedge portions 66 and 68 engage the exposed
longitudinal surface portions of both of the conductors 22 and 24.
The wedge portions 66 and 68 are wedged between the conductors 22
and 24 to exert outward forces on the conductors to retain the
conductors within the respective grooves 42 and 43.
At least the arms 58 and 60 of contact element 26 are made from a
material which has sufficient elasticity or spring-like property to
maintain the tips 62 and 64 in engagement with the conductors 22
and 24. At least the tips 62 and 64 of the arms 58 and 60 are made
from an electrically conductive material to form an electrical
connection between the conductors 22 and 24. Conveniently the
contact element 26 is fabricated by stamping from a sheet of
suitable conductive elastic metal or alloy and bending and forming
the arms 58 and 60 and the tips 62 and 64. The tips 62 and 64 can
be plated with contact metals or metals with low coefficients of
friction, such as silver, gold or the like.
The arms 58 and 60 cooperate with each other by being elastically
stressed in opposite pivotal directions about the head 56 to
maintain each other in an elastic stressed condition and to prevent
any tilting of the head 56 relative to the surface 33 of the member
28. The conductors 22 and 24 and the arms 58 and 60 are shown as
being parallel to the axis of the casing 20; however, the
conductors 22 and 24 and the arms 58 and 60 can extend in
directions or at angles which have substantial directional or
vectorial components parallel to the axis and the force of the
spring 30 such that engaging forces between the conductors 22 and
24 and the tips 62 and 64 are transverse to the axis and the force
of the spring 30.
The cross sections of the case 20, the insulator 21 and the head 56
perpendicular to the axis of the case, are non-circular or, as
illustrated in FIGS. 2 and 3, somewhat rectangular or square. The
size and shape of the head 56 relative to the case 20 is designed
to maintain or guide the element 26 during assembly in
substantially a predetermined relative rotative position about the
longitudinal axis of the casing. The size and shape of the
insulator 21 relative to the case 20 is such that the insulator 21
is also held or maintained in predetermined rotative position about
the axis of the case, to assure that the conductors 22 and 24 in
the grooves 42 and 43 of the insulator 21 have a predetermined
relative rotative alignment with respect to the arms 58 and 60 and
the tips 62 and 64 of the element 26.
In assembly of the thermal switch of FIGS. 1-3 the member 28 is
inserted within the tubular case 20 against the end 32 and the
contact element 26 is placed with the head 56 engaging the face 33
of the member 28. The spring 30 and the insulator 21 are assembled
in the case 20 with the spring 30 compressed against the head 56.
The insulator 21, the spring 30, the element 26 and the member 28
are secured in the case 20 by bending the tabs 38 inward. Suitable
lengths of stripped sections 22 and 24 and the ends 46 and 47 of
the wires 48 and 50 are inserted to a desired depth in the holes 44
and 43 and the grooves 42 and 43 either before or after the
assembly of the insulator 21 in the case 20. The mating
relationships of the head 58 and the insulator 21 with the case 20
is selected such that each of the tips 62 and 64 engages both of
the conductors 22 and 24. The sloping surfaces of the conical wedge
portions 66 and 68 allow the conductors 22 and 24 to spread the
tips 62 and 64 as the conductors are inserted, and provide camming
forces between the conductors 22 and 24 and the tips 62 and 64 to
aid in the alignment of the element 26 relative to the conductors
22 and 24. The elastic force of the arms 58 and 60 urge the wedge
portions 66 and 68 of the tips 62 and 64 between the conductors 22
and 24 to guide and hold the conductors in the grooves 42 and 43
during assembly.
After sealing with the potting compound 54, the wires 48 and 50 are
connected as a lead in an applicance in series with the appliance
power source. The thermal switch is mounted in the appliance with
the end 32 disposed in a position to be heated in the event the
appliance becomes defective and overheated.
The operation of the thermal switch in response to an overheated
condition is illustrated in FIG. 4. When the end 32 of the case 20
is exposed to a temperature which exceeds the melting point or
softening point of the member 28, the member 28 becomes fluid
allowing the spring 30 to push the head portion 56 toward the end
32 with fluid material 28 flowing around the periphery of the head
56. Movement of the contact element 26 slides the tips 62 and 64
off the edges 46 and 47 of the conductors 22 and 24, thus breaking
or opening the circuit in which the wires 48 and 50 are connected.
Upon repairing or correcting the cause of the overheating
condition, the thermal switch is replaced by a new thermal
switch.
One advantage of the thermal switch is that the electrical
connection between the lateral edges of the tips 62 and 64 and the
conductors 22 and 24 is made during assembly by a wiping action.
The wiping action tends to eliminate many causes of failure
resulting from dirt or contamination on the tips 62 and 64 or the
conductors 22 and 24.
Another advantage is that the force on the tips 62 and 64 for
engaging the tips with the conductors 22 and 24 is transverse to
the longitudinal axis while the spring 30 biases the contact
element 26 parallel to the longitudinal axis. Thus the spring 30
need only be designed to overcome the sum of the frictional forces
between the tips 62 and 64 and the conductors 22 and 24 and the
viscosity forces or resistance of the material 28 when it becomes
fluid.
Additionally, the entire movement of the head 56 under the force of
the spring 30 is used to withdraw or remove the contacting tips 62
and 64 from the conductors 22 and 24; thus the entire full force
change potential of the yielding member 28 is used in opening the
circuit.
Further, the major conductive paths between the conductors 22 and
24 are through each of the tips 62 and 64 shunting minor conductive
paths through the head 56; thus resistance heating of the head 56
and the member 28 is kept to a minimum.
Still further, the thermal switch utilizes the stripped sections of
wires as contacts, a single interconnecting contact element, a
single spring, an insulator, and a case, all being relatively large
with each part performing such a vital function that omission of a
part during assembly is easily detectable; thus the thermal switch
has the advantages of being less expensive and more reliable than
prior thermal switches which depend upon many small intricate
parts.
A modification of the thermal switch is illustrated in FIGS. 5-8.
The same numerals are used in the modification to illustrate parts
which are substantially similar in structure and/or function to the
similarly identified parts of the embodiment shown in FIGS. 1-4,
and such similarly identified parts for sake of brevity are not
redescribed. The spring arm 58 and 60 of the contact element 26 has
tips 162 and 164 which have outwardly deformed portions 166 and
168. The tips 162 and 164 are designed to be inserted between the
conductors 22 and 24 to engage the outward formed portions 166 and
168 with the exposed surface portions of the conductors 22 and 24.
The elastic property of the spring arms 58 and 60 together with the
shape of the outwardly formed portions 166 and 168 forces the
spring arms 58 and 60 together at a fulcrum point 169 to engage and
form an electrical connection therebetween. The spring arms 58 and
60 urge the tips 162 and 164 to pivot in opposite directions about
fulcrum point 169 into engagement with the conductors 22 and 24 and
maintain the conductors 22 and 24 in the grooves 42 and 43. The
contact at point 169 provides a short conductive path for current
through the conductors 22 and 24 shunting the conductive path
through the head 56.
As illustrated in FIG. 8 the modification operates in a manner
similar to the embodiment of FIGS. 1-4 in that, when the material
28 reaches a temperature where it becomes fluid, the head portion
56 is urged against the end 32 by the force of the spring 30
sliding the tips 162 and 164 off the edges 46 and 47 out of
engagement with the conductors 22 and 24. Only the sum of the
frictional force between the tips 162 and 164 and the conductors 22
and 24 and the viscosity forces of the material 28 when fluid need
be overcome by the spring 30.
Another modification of the thermal switching device is illustrated
in FIG. 9 wherein the same numbers are used to identify parts with
substantially the same structure and/or function. The forward
portion 201 of the insulator including the cavity 40 and the rear
portion 203 of the insulator including the holes 44 and 45 are
formed separately. The conductors 22 and 24 have crimped portions
205 formed at a predetermined distance from the ends 46 and 47 to
engage the bottom of a recess 207 in the rear insulator portion 203
to accurately select the lengths of conductors 22 and 24 which are
positioned within the cavity 40. The conductors 22 and 24 can be
assembled in the rear insulator portion 203 and secured with a
potting compound 209 previous to insertion in the case 20.
In the modification of FIG. 9, the case 20 is made of a fragile
material, such as glass or metal which has been hardened to a point
where it becomes brittle. In securing the insulator portions 201
and 203 within the case 20, a modified procedure illustrated in
FIG. 10 is utilized. The tip of the case 20 containing the
yieldable member 28 is immersed in a bath of cooling liquid.
Thereafter the dimpled portion or detents 38 are formed by
directing an intense source of heat such as a heliarc or plasma arc
against the case 20 to form the detents 38. After sealing with the
potting compound 54 the thermal switch is dipped into a suitable
electrical insulative material, such as epoxy, to form a protective
coating 211.
The use of a hardened or brittle case prevents denting of the case
such as might occur in installation or handling procedures normally
employed in manufacturing steps. In the event the case 20 is
subjected to a force which would dent a nonbrittle case, the
brittle case 20 will break rather than dent thus preventing the
installation of a defective thermal switch which would not work due
to a dent formed near the head 56.
A further modification of the thermal switch is illustrated in FIG.
11 wherein the end 32 is disposed at a position quite remote from
the conductors 22 and 24. The spring arms 58 and 60 are elongated
in the modification of FIG. 11 and are spot welded together at a
point 301 adjacent the tips 62 and 64. Alternatively the tips 62
and 64 can be bifurcations of a rod extending from the head 56. The
case 20 is substantially elongated with an inner tube or spacer 303
inserted between the end of the insulator 201 and the spring 30.
The modification of FIG. 11 illustrates that the contact element 26
can be substantially elongated while maintaining the same
relatively low resistance path path between the conductors 22 and
24 by the tips 62 and 64. The welded portion 301 ensures that the
tips 62 and 64 still have substantial elasticity to insure the
engagement with the conductors 22 and 24.
Since many variations, modifications and changes in detail may be
made to the present embodiments, it is intended that all matter in
the foregoing description and in the accompanying drawings be
interrupted as illustrative and not in a limiting sense.
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