Note: Descriptions are shown in the official language in which they were submitted.
5~L3S
THERMAL SWITCH AND METHOD OF ASSEMBLY
AND TOOL USED THEREIN
Background of _he Invention
This invention pertains to thermal switches and the
method of manufacture thereof as well as a tool usable in
said method.
In most snap-acting bimetal disc-type thermal
switches, the snap action of the disc is coupled to the
contact mechanism by an insulated coupling pin or plunger,
commonly referred to as the striker pin. This pin is
normally made from vitreous-type material. The length of
this pin must be precisely controlled to properly couple the
snap travel of the disc to the contacts. Incorrect pin
;~ lengths result in improper switch action and either gross
reduction in switching life or susceptibility to
intermittent contact closings during vibration. Normal
manufacturing tolerances do not allow this pin length to be
controlled directly without extraordinarily tight controls
on the several parts that make up the assembly. As a
result, normal practice has been to manufacture the detail
parts to common tolerances, and compensate for the total
accumulation of plus and minus tolerances by u~e of a
striker pin fitted to each specific application. Two common
methods are now used to fit the pin length to each switch.
Both have limitations and advantages. The new method
proposed herein combines the advantayes of both and
~ 25 eliminates their fundamental disadvantages.
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The most co~monly used current procedure utilizes a
free-floating coupling pin, manufactured in incremental
lengths to cover all possible combinations of tolerance
accumulations. Each switch-contact assembly is measured
using specialized gauges, which relate the geometry of each
assembly to a specific pin size. The specified pin length
is selected from available stock and installed in the
switch. Since the design approach does no-t attach the pin
to any support, it is free to rattle and bounce within the
enclosure, where contamination from rubbing surfaces can be
generated. Vibration and shock exposures can also impact
the floating pin on the contact assembly, causing
inadvertent openings or closings of switch contacts.
Fractures of the pin as a result of extreme shock and
vibration levels have also been observed.
The other commonly used procedure for obtaining
correct pin length has been to mechanically attach a pin of
sufficient length to compensate for all combinations of
detail parts to a fixed part of the assembly, and then trim
it to the specific dimension required. This design provides
superior resistance to high vibration and shock levels,
since no "loose" parts are used in the disc-to-armature
geometryO However, the trimming operation does, by its
nature, create debris in the form of chips or grindings
which have the potential for contaminating switch contacts.
Elaborate procedures are sometimes required to be absolutely
sure that switches are thoroughly cleaned.
In grinding of a pin to length, a fIat lower end is
formed which results in harmful abrasive wear of the
actuator disc by repeated contact therewith. Additionally,
the grinding leaves a sharp lower edge with the result that
particles may break off during use to cause contamination.
An improvement over the procedures described above is
shown in the Romerdahl & Knight Patent No. 4,201,967/ owned
by the assignee of this application. The patent shows a
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thermal switch with a striker pin of ceramic material bonded
to a carrier by an adhesive layer of concrolled thickness to
establish the effective length of the striker pin. The
patent also discloses a rnethod of manufacture including a
tool used therein. In this thermal switch, there can be
; problems resulting from direct contact of the striker pin of
ceramic material with the bimetal disc.
Summary of the Invention
A primary feature of the invention is to provide a
thermal switch utilizing the advantages of prior designs
while avoiding the disadvantages thereof. This is
accomplished by utilization of a Eixed, pre-formed striker
pin with a cap adjustably fitted thereon to provide high
vibration and shock resistance and which avoids si~ing or
trimming either before or after assembly. In utilization of
a striker pin which need not be sized or trimmed, the pin is
free of the effects of grinding or other trimming which can
leave frangible cracks, slits or nonuniform sharp edges and,
specifically, the cap provides the striker pin with an end
of a spherical shape. The disclosed thermal switch is an
improvement over that shown in the aforesaid Romerdahl &
Knight patent in that the mounting of a cup-shaped cap onto
the end of the striker pin with a controlled layer of
adhesive is accomplished with simpler tooling requiring a
lower skill level~ Also, there is less potential for tilt
misalignment of the striker pin, a requirement for less
adhesive and reduced likelihood of voids in the adhesive
bond layer.
` ~ In accordance with the foregoing, an object of the
invention is to provide a thermal switch having a movable
contact on a carrier, such as an armature spring
~; positionable under the control of a bimetal actuator with a
striker pin engageable by the actuator for transmitting
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motion of ~he actuator to the carrier and with the striker
pin having a cap fixed thereto by a controlled thickness
layer of adhesive to establish an effective length for the
striker pin.
Another object of the invention is to provide an
installation tool usable in manufacturing a thermal switch
wherein the tool is in the form of a magnetized fixture and
having a c~ntral opening with a bottom wall to hold a
magnetic metal cap in fixed position and extending therefrom
whereby the striker pin can be moved into association with
the cap.
Still another object of the invention is to provide a
method for manufacturing the thermal switch having the
structure set forth in the preceding paragraphs and using
the tool described above wherein the cap is cup-shaped and a
mass of adhesive is placed within the cap while it is held
by the tool and said adhesive is caused to flow out of the
cup and along the striker pin as the striker pin and cap are
brought into final relation to leave a controlled thickness
layer of adhesive which is then set to fix the cap to the
striker pin.
From the foregoing, it will be seen that a thermal
switch is provided without creation of any contamination
within the switch and with a structure and assembly using
the controlled adhesive layer thickness whereby all of the
striker pins can be of one standard, minimal length and each
striker pin is automatically given an effective length by
the controlled thickness layer of adnesive bonding the cap
to an end of the striker pin.
Brief Description of the Drawings
Fig. 1 is a ver~ical central section of the thermal
~ switch;
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Eig. 2 is a view, similar to Fig. 1, showing a step
in assembly of the thermal switch and utilizing the
installation tool; and
Fig. 3 is a fragmentary, vertical section, taken
generally along the line 3-3 in Fig. 2.
Description of the_Preferred Embodiment
The thermal switch has a cylindrical case 10 with a
base 11 with a pair of annular steps or lands 14 and 15
around the interior thereof and spaced above the base. A
header 20 is positioned within the case and spaced from the
base by a ring 21 having its lower edge supported on the
case land 15 and having its upper end interfitting w;th a
peripheral groove 22 on the header 20. The header, by
insulating structure, mounts a pair of spaced-apart
conductive terminal posts 25 and 26 which have their lower
ends extending beneath the header and into the space between
the header and the case base 11. The terminal posts provide
an electrically-conductive path to a fixed contact 30
carried at the lower end of the terminal post 25 and a
movable contact 31 mounted on a carrier 32 in the form of a
planar armature spring which is mounted at an end thereof in
a cantilever fashion to the lower end of the terminal post
26.
As shown in Fig. 17 the contacts 30 and 31 are in
engagement. Upward pivoting of the armature spring 32 moves
the movable contact 31 out of engagement with the fixed
contact 30.
The control of the movable contact 31 is by an
actuator 40 in t~e form of a thermally-responsive bimetal
disc having its peripheral edge resting on the case land
14. As shown in Fig. 1, the disc is upwardly concave, as
permitted by a space between the disc and the case base 11.
In response to a temperature change of a predetermined
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d2gree, the disc ~ill shift to an upwardly convex relation
and, throuyh the intermediary of a striker pin 50 fixed to
the armature spring 32, the latter wi]l be pivoted upwardly
to separate the contacts 30 and 31.
The striker pin 50 is formed of a molded ceramic or
plastic material and has a cup-shaped me.al cap 51 with a
spherical lower end 52 at the free end thereof. The cap
lower end is normally spaced a critical distance from the
actuator disc 40 so that slight movement of the disc will
not effect contact engagement. The striker pin is fixed to
the armature spring by means of an adhesive bond line 53.
As shown in Fig. 3, the cap 51 has an annular wall 54
extending upwardly from the spherical end 52 and which has
an internal diameter greater tlian the output diameter oE the
striker pin 50 to provide a clearance for adhesive flow. By
the process subsequently described, a controllable thickness
layer 55 of adhesive within the cap and b~yond an end 56 of
the striker pin bonds the cap to the striker pin. This
provides a striker pin of the desired effective length and
with the ceramic material of the striker pin out of direct
contact with the bimetal disc 40. Many adhesives are
suitable, including a thermosetting epoxy adhesive.
The thermal switch has components assembled by a
method or process which embodies the use of an installation
tool, shown in Figs. 2 and 3. The installation tool is in
the for~ of a magnetic or magnetized fixture 70 in the form
of a plate having a central opening 71 with a bottom wall 72.
In the tool shown, an electromagnet 75 is associated
with the fixture 70 to create a magnetic field for holding
the cap 51. Alternatively, the cap could be held by a
permanent magnet, gravity, suction, adhesives, or by the
resistance oE the cap to seat on the striker pin due to
hydrostatic pressu~e caused by the resistance of the
adhesive to flow between the cap and the pin or by the
resistance of the cap to seat on the pin due to friction
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between the pin and an expandable cap. The cen~ral opening
71 receives the cap 51 and ~-ith the lower sphe ical end 52
thereof engaging the bottom wall 72.
Tha cap 51 is magnetically held on the tool 70, with a
mass o adhesive therein. The header 20 and spacer rin~ 22
are then moved downwardly to ~oi~e the end 56 of the striker
pin into the cap 51. Sufficient a~hesive has been placed in
the cap to make certain that there will always be an excess
of adhesive and, as the striker pin moves to final-position,
lQ the excess adhesive flows along the striker pin 50 in the
clearance between the striker pin and the cap wall 51 which
is adequate to permit the flow. This results in a
controlled thickness layer of adhesive within the cap and
be~ond the end of the striker pin. The adhesive may then be
set while the parts are held in position. Thereafter, the
parts can be assembled to form the thermal switch as shown
in Fig. 1.
I'here in no need to have striker pins of various
lengths. The only re~uirement is that the siriker pin be
si ZeAd to the short side of the minimum tolerance limits a
minimum thickness OL the controlled thickness layer 55 OL
adhesive.
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