Laser adjusted set-point of bimetallic thermal disc

Details Laser adjusted set-point of bimetallic thermal disc Laser adjusted set-point of bimetallic thermal disc

2003-04-29

2004-07-13

Vortman, Anatoly (Department: 2835)

Electricity: electrothermally or thermally actuated switches

Electrothermally actuated switches

With bimetallic elements

C337S036000, C337S057000, C337S333000, C337S337000

Reexamination Certificate

active

06762668

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to methods for manufacturing thermally responsive bimetallic members, and in particular to methods for permanently compensating thermal response characteristics of snap-action bimetallic members.
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 switching 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. The bimetallic actuators that drive such switching mechanisms typically exhibit a forceful snapping action between 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, i.e. the bimetallic actuator, 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 forcefully snaps to a second of the two stable states and remains in this second state while the sensed temperature remains above the first lower 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 forcefully 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 thermally responsive bimetallic material is positioned between two opposingly positioned shaping or die members. The shaping members are actuated to engage the blank, thereby forming a bimetallic disc having a configuration that achieves forceful 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 disk-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 and 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 rate of thermal expansion relative to that of the first member
2
. The difference in thermal expansion coefficients between the two conjoined members
2
,
3
is a factor in determining the set-point temperature at which the resulting bimetallic disc actuator
1
operates and in the force F produced by the snap-action. The disk-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 a concave structure having a depth H1, 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
that is stretched into a concave configuration. The set-point operation temperature of the snap-action and the force F applied thereby 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 side
2
to form the central stretched portion
6
, whereby the central portion
6
is stretched to space the inner concave surface thereof to the depth H1 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 H1 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 is also described in above-incorporated U.S. Pat. No. 3,748,888.
In
FIG. 1
, the full line
4
illustrates the bimetallic disc
1
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 rate 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
become distorted by the thermally induced stresses, and the bimetallic disc
1
changes configuration with a slow movement or “creep” action from the upper dotted line configuration
7
to the fill line configuration
4
with the inner concave surface of the central concave portion
6
spaced the depth H1 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 a forceful snap-action downward through the unstretched hoop portion
5
to the second state of stability with the inner concave surface of the central concave portion
6
spaced a distance H2 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 bimetallic disc
1
creeps 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 for

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