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

2001-10-11

2003-06-17

Vortman, Anatoly (Department: 2835)

Electricity: electrothermally or thermally actuated switches

Electrothermally actuated switches

With bimetallic elements

C337S379000, C337S036000, C337S057000, C337S333000

Reexamination Certificate

active

06580351

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. Nos. 3,748,888 and 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 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
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 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 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 full line configuration
4
with the inner concave surface of the central concave portion
6
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 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 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 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 form t

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