Note: Descriptions are shown in the official language in which they were submitted.
``~ 11398~9
Switch units for electri_ immersion heaters
This invention relates to switch units for electric immersion heaters
for liquid heating vessels, such as kettles, jugs, urns, pans and the like, the
switch units including thermally responsive means sensitive to liquid boiling
within the vessel.
Such immersion heaters are well known and commonly include a switch
unit having thermally responsive means exposed to vapour produced by boiling
liquid and coupled by an overcentre lever mechanism to an electrical switch for
interrupting the supply of power to the heater. An example of such a switch unit
is disclosed and claimed in Taylor British Patent Specification No. 1,470,367
(July 2, 1974~ which is concerned with the difficulty of ensuring an accurate
and reliable operation of the switchl particularly as the geometry of the over-
centre mechanism may be faulty due to inaccurately formed parts, or may distort
during continued use, so that the mechanism becomes unstable or incapable of
operation by the thermally responsive means.
Specification No. 1,470,367 relates generally to a switch unit for an
electric immersion heater for mounting on the wall of a liquid heating vessel,
the switch unit including thermally responsive means sensitive to boiling liquid
on a body of the switch unit in a position for exposure, in use, to vapour
emerging from the vessel, an overcentre spring mechanism coupling said thermally
responsive means to a movable contact of an electrical switch for opening and
closing said switch, said overcentre spring mechanism being movable between a
first position in which said switch is closed and power can be supplied to the
immersion heater, and a second position in which said switch is open for
interrupting the supply of power to the immersion heater, a lever of the over-
centre mechanism being pivotally mounted on said body and a spring of the over-
centre mechanism being mounted between fulcrums provided
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by said lever and an abutment on said body so that the
spring biasses said lever into engagement with the abutment
in the first position of the overcentre mechanism wherein
power can be supplied to the immersion heater.
In the overcentre spring mechanism of the unit
disclosed in Specification 1,470,367, the abutment for the
lever is provided in the closed position of the switch in
order to position the lever close to the position of
unstable equilibrium or dead-centre position and an
accurately determined clearance has to be provided between
the lever and a thermally responsive bimetallic actuator.
This is necessary because of the comparatively limited
force and movement which available actuators such as the
type described in British Patent Specification 657434 were
were capable of developing. The power which a bimetallic
actuator can provide is dependant on the differential, that
is the difference between the temperature at which the
bimetal reverses its curvature with increasing temperature
and the temperature at which the bimetal reverses its
curvature with decreasing temperature. This differential
cannot be too large, as this would prevent the bimetal from
resetting within an acceptable time, for example 10
seconds. Although the resetting of the bimetal is assisted
to a certain extent by the trip lever being manually operated,
it still has to snap through the aforesaid clearance
between the end of the lever and its cold position when
cooling.
Whilst the switch unit disclosed in Speclfication
1 470 367 has been found to operate reliably~nonetheless
the measures necessary for this have resulted in the over-
centre spring mechanism being rather complicated and
expensive.
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Recently there has been developed and marketed a bimetallic actuator
of the kind disclosed in Taylor German Offenlegungsschrift 2,556,062
(December 12, 1975) and Taylor British Patent No. 1,542,252 ~March 12, 1976)
and referred to in the catalogue "Kettle controls and element protection"
available from ~tter Controls Ltd., as used in the Z10 protector.
The actuator comprises a member of sheet bimetal having an aperture
with an outer perimeter and an inner perimeter defining a tongue free at one
end intermediate two lobe portions of the aperture, the inner perimeter and
arcuate portions of the outer perimeter smoothly merging at rounded ends of the
aperture adjacent the tongue root, the tongue being, at least in part, generally
centrally disposed with respect to the mem6er, the member having been deformed
in a die pressing operation to conform in shape to a die of domed configuration,
the domed area being such as to reverse its curvature with a snap action with
change in temperature and the width of the domed area being greatest in the
region of the tongue root.
This actuator is inherently more powerful than the actuator of Taylor
British Specification 657,434 (November 22, 1949~ and it is the object of this
invention to take advantage of the increased power available to simplify and
thus reduce the cost of the overcentre lever mechanism of the switch unit dis-
closed in Specification No, 1,470,367.
Thus, according to the invention, there is provided a switch unit for
an electric immersion heater for mounting on the wall of a liquid heating vessel,
the switch unit including thermally responsive means sensitive to boiling liquid
on a body of the switch unit in a position for exposure, in use, to vapour
emerging from the vessel,
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said thermally responsive means comprisin~
a snap-acting bimetallic actuator comprising a member
of sheet bimetal having an aperture with an outer perimeter
and an inner perimeter defining a tongue free at one end
intermediate two lobe portions of said aperture, said
inner perimeter and arcuate portions of said outer
perimeter smoothly merging at rounded ends of the
aperture adjacent the tongue root, the tongue being, at
least in part, generally centrally disposed with respect
to the member, the member having been deformed in a die
pressing operation to conform in shape to a die of domed
lS configuration, said domed area being such as to reverse
its curvature with a snap action with change in
temperature, and the width of the domed area being
greatest in the region of the tongue root, an
overcentre spring mechanism coupling said bimetallic
actuator to a movable contact of an electrical switch for
opening and closing said switch, said overcentre spring
mechanism being movable between a first position in which
said switch is closed and power can be supplied to the
immersion heater and a second position in which said switch
is open for interrupting the supply of power to the
immersion heater, a lever of the overcentre mechanism being
pi.~-otally mounted on said body and a spring of the over-
centre mechanism being mounted between fulcru~s provided
by said lever and said body so that the spring biasses
said lever into direct or indirect engagement with said
bimetallic actuator in the first position of the overcentre
mechanism.
It will be seen that, according to the invention, the
actuator`and the lever are in contact in the first position
of the overcentre mechanism. This not only avoids the
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need for accurate spacing of the lever and actuator as in
1,470,367, but also has the result that the resetting of
the bimetal when the lever is manually depressed is
assisted by virtue of the fact that the lever presses the
bimetal through its unstable flat position. Therefore,
for the same reset time, the differential of the bimetal can
be increased, thus further increasing the force and
movement of which the bimetallic actuator is capable.
In consequence of the increased power and movement available
from the actuator, it has been found to be possible to make
the overcentre mechanism less "hair-trigger" by increasing
the over dead centre in the first position of the
mechanism.
Furthermore, it has now been found that the actuator
may actually serve as the stop for the lever in the first
position of the overcentre mechanism.There is then a
substantial interference between the actuator and the lever
to assist in resetting the actuator after it has operated.
Thus, pushing the bimetallic actuator right through its
dead centre position after it has operated gives the possibility
of re-setting the bimetal when it has cooled to its operating
temperature with no consideration necessary for the bimetal
differential and auto reset.
In the result, the need to ensure dimensional accuracy
and stability of the overcentre mechanism has been so reduced
that it has been found that it is possible to dispense with
the metal fulcrums, pivot and lever insert provided in the
preferred embodiment of the overcentre mechanism described
in Specification No. 1,470,367 and so to obtain substantial
cost savings.
In a preferred feature of the invention, the switchunit
comprises a second snap-acting thermally responsive
bimetallic actuator comprising a member of sheet bimetal
having an aperture with an outer perimeter and an inner
perimeter defining a tongue free at one end intermediate
two lobe portions of said aperture, said inner perimeter
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and arcuate portions of said outer perimeter smoothly
merging at rounded ends of the aperture adjacent the
tongue root, the tongue being, at least in part, generally
centrally disposed with respect to the member, the member
having been deformed in a die pressing operation to conform
in shape to a die of domed configuration, said domed area
being such as to reverse its curvature with a snap action
with change in temperature, and the width of the domed area
being greatest in the region of the tongue root, mounted
10 on the body of the switch unit so as in use to make direct
thermal contact with a head plate of a heater unit, means
coupling said second actuator to said movable contact and
to said over-centre mechanism whereby operation of said
second actuator to open said switch also moves said
15 overcentre mechanism to said second position thus
maintaining said switch open, the arrangement however,
being such that movement of said overcentre mechanism to
its first position is ineffective to close said switch whilst
said second actuator remains operated.
In a preferred arrangement the -movable contact of
the switch is mounted on a resilient strip, which strip
when displaced by operat~on of said second actuator moves
said overcentre mechanism to said second position, but
when said overcentre mechanism is moved to said first
25 position, said second actuator holds said contacts open in
its hot position.
In a further preferred~feature of the invention, the
body of the switch unit provides a barrier for vapour
between said first actuator and said electrical switch
30 and said lever extends through an ap0rture in said
barrier, the unit comprising a cover member partially
surrounding the position of said lever on the same side of
said barrier as said first actuator so as to reduce
penetration of vapour through said aperture.
In a preferred arrangement, the lever is a bell crank
lever which engages the actuator directly in the first
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position of the overcentre mechanism. Alternatively indirect
engagement is possible as, for example, via a push rod
extending through an aperture in the body of the switch
unit for transmitting the movement of the actuator to the
overcentre mechanism. The push rod engages the actuator in
the first position of the overcentre mechanism thus
affording the advantages set out above.
In the accompanying drawing:
Figure 1 is a side elevation of a preferred
10 embodiment of the switch unit in the switch closed position;
Figure 2 is a side elevation of a drawer member
forming part of the unit shown in Figure l; and
Figure 3 is a plan view of the member shown in
Figure 2.
Referring to Figure 1, there is shown a switch unit
1 for a thermally controlled electric immersion heater of a
kettle, mounted on a kettle wall 2 on one side of an
aperture 3 and an immersion heater unit 4 mounted on the
other side of the aperture 3.
The heater unit 4 comprises an element carried by a
head plate 5, the hot return 6 of the element being brazed
to the head plate 5.
The switch unit 1 comprises a body made of a glass
fibre reinforced plastics material comprising a chassis
25 plate 7. A cover 8 is secured to the chassis plate 7 and
defines therewith a housing. The cover has a recess 9
therein for receiving a mains socket connector 10 for
connecting with terminal pins of the switch unit extending
into the recess 9. The chassis plate 7 further has
30 apertures therein through which pass in use studs mounted
on the head plate 5 of the heater unit, whereby the switch
unit maybe bolted to the head plate. A resilient sealing
ring 11 is arranged in the aperture 3 between a projecting
annular rim 12 formed on the chassis plate 7 and the head
35 plate 5.
- The tongue 13 of a thermally responsive bimetallic
actuator 32 is mounted on a pillar 14 formed on the
chassis plate 7.
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The bimetallic actuator 32 is of the type hereinbefore
described and disclosed in German Offenlegungschrift
2,556,062 and British Patent No. 1,542,252
A second simular actuator 33 is shown in Fig 3.
The tongue is riveted to the pillar 14, and a small
aperture in the tongue is prPssed over a pin projecting
from the end of the pillar 14 to prevent rotation of the
actuator. The actuator is mou~ted so as to be in contact
with the head 5 in the region of the hot return 6 in the
cold position of the actuator. The actuator is such that
at, say 120C, at which temperature the element 4 is
overheating as a result of a switch on dry condition, the
actuator moves with a snap action to its oppositely dished
configuration. As this occurs, a push rod 16 extending
through and slidable in an aperture in the chassis-plate 7
is displaced by actuator 13 which contacts push rod 16-at
its periphary 15 to displace a resilient electrically
conductive strip 17.
The strip 17 forms part of the path of electric
current from the connector 10 to the element 4.
One end of the strip 17 is in contact with a line
terminal pin 18. The pin 18 includes ears 19 and extends
through an aperture in a pillar 20 of chassis plate 7.
An apertured brass plate 44 is arranged on the other side
of the aperture and the pin is secured to the brass plate 44
by deforming a bifurcated end 45. The strip l? is thus
sandwiched between the ears 19 and the pillar 20. A movable
contact 21 is mounted on the strip 17. In the closed
position of the switch, the movable contact 21 engages a
fixed contact 22 mounted on a conductive strip 23. When the
switch unit is assembled with a heater unit on a kettle, the
strip 23 makes contact with a cold lead 24 of the element,
the cold lead 24 passing through an aperture in the chassis
plate 7.
A bell crank lever 25 made of a glass fibre reinforced
plastics material is pivotally mounted on the chassis plate
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by means of a knife edge 26 formed integrally therewith
resting in a pair of notches or bearings27 formed on the
chassis plate 7. A C-spring 28 extends between a notch
29 formed in a pillar 31 extending from the chassis plate 7
and a notch 30 formed on the lever 25.
The second sim~ar thermally responsive bimetallic
actuator 33 is mounted in a drawer member 34 which slides
iII two grooves in the body of the switch unit, a tab 35 on
the r,lember 34 extending over the fulcrum of the lever 25 to
prevent the knife edge 26 being dislod~ed from the notches
27 into which it is sprung b~ the C-spring 28. Normally ~e
tab 35 does not make contact with the lever 25.
The member 34 further comprises a cover portion 36
which is triangular in elevation and substantially surrounds
the upper arm of the lever 25 which passes through an
ape~ure in the chassis plate. In the fully inserted
position of the drawer member 34, a gap remains between the
lowermost corner 42 of cover portion 36 and the chassis plate
7. The actuator 33 is arranged in a position in which, when
the kettle boils, steam impinges thereon after passing through
a vent aperture 37. The steam is prevented from
passing through the aperture in the chassis plate through which
the lever 25 passes by the cover 36. Steam condensing on the
25 actuator 33 and surrounding portions of the switch unit,runs
down inside the cover 36 and remains on the kettle side of the
chassis plate 7, so that water is prevented from entering the
electrics which are all on the other side of the chassis plate.
The end 37 of the lever 25 is in contact with the tongue 43
30 of the bimetallic actuator 33, in the switched on position,
the tongue serving as a stop for the lever. The pillar 31
serves as a stop to prevent excessive displacement of the
lever distorling the actuator 33. When the kettle boils, steam
impinges on the actuator 33 and it reverses its curvature with
35 a snap action, so that the lever passes its position of
unstable equilibrium and is driven to the switched off
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position by the overcentre C-spring 28. In this position,
a stop 38 on the lever 25 engages the resilient strip 17
and separates the contacts 21, 22. A tab on the lever 25
engages a lip on a pillar 41 on the chassis plate 7 to
limit movement of the lever. The lower end of the lever
travels freely within a groove in the pillar 41 which
serves to prevent any substantial sideways movement of the
lever.
A second stop 39 is also provided on the lever 25.
When the contacts 21, 22 are separated by the switch-on
dry actuator 32, the resilient strip 17 engages the stop 39
and trips the lever. Then, if the actuator 32 reverses its
curvature with cooling of the element head 5, the switch
unit remains off. The arrangement is, moreover, "trip
frea", in that when the actuator 32 is holding the contacts
21, 22 apart by means of the push rod 16 acting on the
resilient strip 17, the contacts 21, 22 cannot be closed by
manual pressure on the button 40 on the lever 25, in view
of a small clearance provided between the stop 39 and the
flexible strip 17.
Reference should be made to Specification No. 1,470,
367 for details of the structure which is co~non to the
switch unit disclosed in that Specification and the switch
unit of the present invention.
