Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Specified electrical sensor or system
Reexamination Certificate
2001-10-19
2003-11-04
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Specified electrical sensor or system
C073S862381
Reexamination Certificate
active
06640646
ABSTRACT:
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, th
Davis George D.
Jordan Robert F.
Allen Andre
Honeywell International , Inc.
Lefkowitz Edward
Rupnick Charles J.
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