Note: Descriptions are shown in the official language in which they were submitted.
S3204
This invention relates to snap acting thermally
. responsive bimeta].lic actuators for use in actuating the
i switch contacts of thermally responsive electric switches
such as cut-outs,circuit breakers and thermostats.
A well kno~ form of bimetalIic actuator is a bime~allic
j disc of domed configurationwhich moves to an oppositely domed
configuration with a snap action with changes in temperat~lr~
. The simplicity of discs and their ease of manufacture) the
basic operation being pressing a flat disc between steel dies to
1~ l0 a desired domed configuration, has resulted in their wi~7espread
. use, despite serious disadvantages.
One disadvantage part~cularly is that with wide
differential discs ~ery high peripheral stress con-
centrations result ~uring inversion and in consequence the
; 15 operating ~mperature at.which the disc snaps tends to drift
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with age and its life is limi.ted by stress-cracking: thus : .
after about l0,000 sperations the disc may stress-crack, whilst
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; the operating temperature may have drifted by as much as 20C.
. ` ~ The~net~result is that in use such discs do not have accurately
. ~ 20~ determined operating temperatures throughout their life. ~ -
Another disadvantage of discs is that of their very
small ~a~ge of useful:movement with snap-action which is
often~;~of the order oi~the uncertainties in the dimensions of
. ~ other:~related components of a switch ill which the disc may
25 ~ be:~incorlorat~d.
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1053Z04
In view of the widespread use of discs, extensive
research has been made into their characteristics. There
have been various proposals over the past 40 or so years
to increase the movement and to increase the accuracy of
the operating temperature of discs. For example, radial
corrugations have been impressed in a disc, stress-relief
. apertures have been inserted at the centre of a disc and
mechanical arrangements for transmitting and increasing the
effective movement of a disc have been poposed (see for
. example British Patent Specification 1031827). Despite the
large amount of research that has gone into improving the
characteristics of discs, none of the various proposals has ~-
done anything more than mitigate to a certamn extent ~he : ~
disadvantages inherent in bimetal discs. : :
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Another type of snap-acting bimetallic actuator which
is well known and which avoids most of the disadvantages of
discs is that described in British Patent Specification
1 657434. In the preferred form, such a~ actuator comprses
¦ a rectangular sheet or blade of bimetal h ff in~ a central
:.. l tongue released from between two outer lege whose end adja-
~ cent the free end of the tongue are joined by a b~idge
.~: portion. The bridge portion is mechanically crimped to
impart a dished con~iguration to the bimetal blade so that
! it moves with changes in temperaturesbetween oppositely
'l ~ dished congiguration
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1053Z04
with a snap action. Such an actuator has a much larger amount
of movement with snap action than a disc. It is also possible
accuratel~ to set the operating temperature, which is in any
case more stable under repeated use than t~at of a disc.
A disadvantage of such an actuator is that as a
result of the method of deforming the blade by crimping,
it cannot be mass produced to a closely predetermined
- operating temperature. The main disadvantage is that the
life of this actuatorf although greater than that of a disc,
tends to be shortened by cracks developing in maximum stress
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concentration areas at the junction of the central tongue with
the outer legs.
Thus despite extensive research into and extensive
use of various snap-acting bimetallic actuators the need still
exists for a thermally responsive snap-acting bimetallic
actuator whiFh is suitable for mass production, provides an
adequate range of useful movement with snap action, has a
reasonably stable operating temperature over its working life,
and very importantly has a long working life which is not
:1
prematurely ended by stress-cracking.
The present invention provides a snap-acting thermally
rosponsI~e bimetallic actuator comprising a member of sheet bimetal
having an aperture wlth an outer perimeter portion and an inner
perimeter portlon defining a tongue frce at one end, said free
end being close to said outer perimeter portion, said inner
pcrimetcr portlon and~arcuate portions of said outer perimeter
p~rtion smoothly merging at rounded ends of the aperture adjacent
the tongue root, an area of said member surround mg said tongue
and~ln relation to which said tongue, at least in part, is generslly -~
''~,.5~ ~ 30 ; ~ ceM rally disposed baving been deformed in a die pressing operation
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1053Z04
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, the ~idth of the domed area surround-
ing said tongue measured generally radially from the centre of
said domed area being greatest in the region of the tongue root
and the length of said tongue being greater than the width of
the tongue as measured at the mid-point of the length of the
tongue.
; An advantage of an actuator according to the
invention is that for many applications its range of useful
movement with snap-action can be substantially greater than
that of an 'lequivalent disc," by which is meant an actuator
identical to an actuator according to the invention apart
from having no U-shaped aperture and hence no tongue.
A practical measure for many purposes of the useful
movement of an actuator is the movement at the free end of
said tongue with snap action measured from the periphery of
the actuator to one side of the actuator against a nominal
force of SOg wt. This useful movement is that which can
ordinarily be employed in many typical electrical swi*ches.
It has been found that, using the above measure, an actuator
according to the invention can provide between two and three
ti=es the useful =ove=ent of an equivalent disc.
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1053204
It is however the case that the greater the useful move-
ment, th~ greater the differential, i.e. the difference
in operating temperatures for increasing and decreasing
ambient temperature, but this is not important for many
applications.
This increase in useful movement is explica-
ble in ~hat whilst a disc has only a small useful move-
ment, it develops a comparatively large force t~200g wt),
far greater than that required to op~n typical switch con-
tacts, so that although relieving the stresses in the
centre of an ac~uator by providing Said U-shaped aper-
ture substantially rsduces the force developet, this is
unimpor~ant for most purposes, and the resultant freedom
- of movemont of the free end of the tongue results in a
magnified, s~ill useful movement of the actuator~
In view of this increased useful movement, it
is possible to use cheaper types of bimetal sheeting
te.g. Ni-Cr/Ni Pe laminates) rather than the more ex-
pensive types having a high manganese content such as
~: 20 are commonly used in discs.
When designing an actuator according to the
invention for a particular switch application, besites
being necessary to ensure that the tongue can with snap-
action develop a sufficient force and movement to open
: th~ switch contacts, it is also desirable to so dis-
pose and shape the U-sh~ped aperture as to minimise lo-
cal stress cuncentraticns and thus maximise the working
life of the actuator. This consideration results in
certain preferret geometrical shapes of actuator.
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~OS3Z04
Accordingly the invention provides in a preferred
form a snap-acting thermally responsive bimetallic actuator
comprising a circular member of sheet bimetal having an aperture
therein with a circular outer perimeter portion having a diameter
of the order of half that of said member and an inner perimeter
portion defining a tongue free at one end and tapering at an
angle of about 15 towards its free end, said free end being
; close to said outer perimeter portion, said inner and outer
perimeter portions smoothly merging at rounded ends of the
aperture adjacent the tongue root, an area of said member
surrounding said tongue and in relation to which said tongue
at least in part, is generally centrally disposed having
been deformed in a die-pressing operation to conform in
shape to a die of domed configuration and being such as to
reverse its curvature with a snap-action with change in
temperature, and the radial width of the parts of said
member surrounding said tongue progressively reducing
. . . .
from a maximum in the region of the tongue root to a j~
; minimum generally opposite the free end of the tongue,
said radial wid~h in the region of the tongue root being
- more than twice that generally opposite the free end of
the tongue, the length of the tongue being greater than
the width of the tongue as measured at the mid-point of
the length of the tongue.
The design of the region of the tongue root is
important as regards the lifetime of the actuator, since
it is here that the high stress concentratlons are developed.
1 ~ Sharp corners in this region would tend to provide localised
< ~ area of very high stress concentrations; these are avoided
by p~oviding the aperture with rounded ends adjacent the
tongue root.
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~()53ZQ4
Whilst it has been found advantageous to ha~e
the ma~imum radial width of the domed area of the actuator
in the region of the tongue root, it is important not
elsewhere to so reduce the width of the domed area as
to impair the overall mechanical strength of the actuator
to the point where it is incapable of delivering the nec-
essary de~eloped force on inversion. Likewise the tongue
must itself have sufficient mecha~ical strength to deliver
the desired developed force. A tongue of tapered shape
lends itself to this requirement whilst minimising any
tendeney for the tongue itself to invert breadthwise with
snap-ection of the actuator which would tend to reduce the
useful movement of the free end of the tongue. The breadth
of the tongue at its root is desirably of the same order
of magnitude as, preferably rather less than, the width of
the domed area in the region of the tongue root. The
~ongue length is preferably about 1 to 1 1/2 times the
tongue roo~ breadth and the angle of taper of the tongue
is desirably bekween 15 to 30. The shape of the free
end of the tongue is not critical. If desired, the free
~i : end may be bent to change the reference plane of move-
ment of the ~ctuator. The U-shaped slot defining said
tongue is pre~rably symmetrically disposed about a dia-
, meter of the domed area which diameter is also prefera-
bly an axis of symmetry, the longitudinal axis of the tongue
~then coinciding ~ith tho said axis of sy~ otry.
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105320~
Preferably for convenience in manufac~ure the
bimetal member of the actuator according to the invention
is domed over its whole surface, although if desired, some
or all of the tongue may no~ be deformed during the die
pressing operation. However other areas of the bimetal
member may be provided which are left flat and undeformed;
for example flat ears for mounting purposes may be pro-
vided extending from the domed area. Whilst the actuator
is preferably circular for convenience in forming between
dies, it may have other shapes, for example rectangular
or elliptical.
By forming an actuator in accordance with the
invention by a die-pressing operation, it is possible to -
mass produce actuators in a particularly simple and ef-
fective way. It has been found that actuators according
to the invention can be formed in mass-production with
. more accurately determined operating temperaturesJ andwith better stability, such stabili~y in operating temper-
ature being better not only than that of discs but also
than that o bimetallic actutators as described in British
Patent Spocification 657434. m is stability arises in
large measure from the stress relief in said domed area
provided by said U-shaped aperture, and from the method
of formation by die-pressing such an actuator which creates
few undesired and unintentional local stress concentra-
tions. The lack of such stress concentrations also con-
tributes to an accurately determined operating temperature.
Thus an actuator according to ~he present invention can
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lOS3Z~)4
have various advantageous features, ln particular a large
range of useful movement and an accurately predetermined and
stable operating temperature. Depending on the particular
application envisaged, the actuator will be designed to
emphasize one or other of such features. Thus for an
inexpenslve cut-out for high currents, a large range of
useful movement will be required to ensure satisfactorv
~j operation. However a large range of movement involves a high
~s , ~ .
- differential between the operating temperatures of the actuator
? lo for increasing and decreasing temperature and this may be
undesirable for example in some forms of thermostat where
a low differential is required, but an accurately predeter-
I mined and stable operating temperature is necessary. Again,
¦ for example, for circ~it breakers sensitive to an excess
1 15 current in a circuit, neither a large range of useful
movement nor an accurately predetermined operating temperature
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is required, merely an adequate range of movement together
1- with a reasonable consistency and stability in the operating
¦~ temperature so that excess current may reliably be detected.
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In some applications, the actuator may be
loosely nounted; in other applications the actuator
may be welded or rivetted toa mounting boss and may
in addition carry an electrical contact. l~e actuator
may suitably be mounted at the free end of its tongue,
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~ç where the effects on stresses in thè actuator will
t be at a minimum or on the periphery of the actuator
opposite the free end of the tongue.
For actuators arranged to be incorporated in
curren- sensitive switches, an electrical contact
is mounted on the actuator so that current can pass
.,~ .
through the actuator, the actuator being heated
~;~ above its operating temperature as a result of excess
current. A major problem with known current sensitive
actuators is to increase their electrical resistance
to current flow so that they respond to relatively low
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1053~(:)4
currents while ensuring that the actuator is not unduly
mechanically weakened. Thus reducing the bimetal thickness
and the overall dimensions of an actuator will increase
electrical reslstance, but it may unduly weaken the actuator.
This problern is particularly acute since it is common nowadays
to require sensitivity to currents of about 2 amps.
For the production of current sensitivities of such low
order, British Patent 1363541 discloses and claims a switch-
actuating element of the type comprising a sheet or strip .
10 of flexib'.e bimetal.lic material having a tongue released
therefrom between two outer legs, the tongue being connected
to the sheet or strip at one end and being free of the sheet
or strip at the o.ther end, and the sheet or strip being
stressed so that it is caused to buckle in two directions so15 - as to be movable with changes in temperature between two
positions on either side of an intermediate unstable position
with a snap action, wherein the tongue and/or the outer legs of
the element have perforations such as to increase th
electrical resistance to current flow longitudinally
therethrough. The actuator specifically described in Patent
1363541 is of the type described in British Patent : ~ -
Specification 657434 wherein the outer legs are bridged
adjacent the free end of the tongue, and the bridging portion
.". ,
is crimped to draw the endsof the legs together.
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05 32 0 4
It has been found that the actuator according to the
invention isintri~ically more suitable~use in a current
sensitive switch than ~he actuator described in
Specification 6S7434, which in order to provide thc necessary
; 5 mechanical strength and stability has rather wide legs and
tongue and is formed of rather thick ~imetal and hence does
not have substantial electrical resistance. In contrast
the actuator according to the ~nvention, having the stress
concentrations necessary for snap action disposed in the
periphery and tongue root which are inherently strong and
stable regions, may be formed of thin bimetal and of ~mall
dimensions. -
In order to increase the electrical resistance and
hence sensitivity of the actuator according to the invention,
1 15 the periphery may be narrowed, particularly in the regions
opposite the tongue root. Alternatively perforations,following
British Patent1363541, may be formed in such regions. The
tongue may also be narrowed or alternatively and as preferred
; for maintainlng the strength of the tongue, may have an
aperture or apertures, preferably formed as a longitudinal
,~
l ~ slot.
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1053ZO~L
Preferred embodiments of the invention will now be
described with reference to the accompanying drawings wherein:
Figure 1 to 7 are plan views of actuators according
to the invention;
Figure 8 is a table of parameters for the actuators
of Figures 1 to 5; and
Figure 9 is a cross-sectional view of an actuator
according to the invention.
All the actuators shown except that in Figure 5
have a circular perimeter.
Whilst a circular form is preferred for convenience
in mass production, it has been found that that precise shape
of the periphery is not critical to the operation of the
actuator and that, for exampleJ a rectangular shape is equally
good.
Each actuator, apart from that shown in Figure 5, has
a semicircular locating recess 10 in its periphery. Each
actuator has a tongue 12 at least in part centrally disposed ~-of the actuator formed by a generally U-shaped aperture 14 ~ -
with a generally arcuate outer perimeter.
The actuators are formed in a die pressing operation in
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1053204
which a spherical curvature is imparted over the entire
actuator surface. Conventional die pressing operations
may be employed.
The differences between the various actuators sho~
reside in the shape and position of tongue 12 and aperture l~
and these differences give rise to their differing operating
characteristics some of which are lndicated in Figure 8.
Ii The figureslisted in Fig 8 are for selected actuatorsall formed by the same method with an equal spherical radius ;~
of curvature, a diameter of 12 mm and formed from 0.2 mr,
thick bimetal. The blades have been selected as having a -~
break temperature of 100C so that their differential and
useful movement are comparable. Whilst the values given are
representative of the various shapes, they should not necess-
arily be taken as median values for actuators produced in
those particular shapes.
~: :
Considering the values given it will be noted that the
~ ran8e of useful movement is considerably greater than that
; ~ ~ of an equivalent disc, the smallest range being 0.32 mm for
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~; ~ 20 the element of Figure 2, i.e. about 1.8-2.2 times that of
an "equivalent disc" whe.eas the larger:t range, for the
elements of Figures 4 and 5 is 0.45 ~n, about 2.5-3 times
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1053ZQ~
that o~ an "~quîvalent disc".
The differential for the actuators is between 30 and 40C.
The differential for any particular shape is dependent of the
movement of the actuator and hence in order to reduce
slgnificantly the differential, it will usually be necessary
to reduce the movement.
The actuators sho~ ir Figs 1 and 2 have a tongue 12
of constant width, 2.5 mm, with a semicircular end. The
actuator shown in Fig 1 has a relatively long tongue
measuring 4.5 mm from tip to root. Aperture 14 has relatively
large side lobes 16 formed with 1 mm radius rounded ends
adjacent the t~ngue root 18 and a 3.5 mm radius outer perimeter.
The margin 20 is of roughly constant radial width, 2.5 mm,
` although it widens very slightly adjacent the tongue root.
As sho~m in Figure ~ a representative value for the
1 useful movement of the actuator of Figure 1 is 0.36 mm for a
¦ differential of 37C,
I The actuator of Flgure 2 has a tongue 12 measuring 3.5 ~m ~-
from tip to root and an aperture 14 formed with 0.75 mm radlls
~l 20 rounded ends adjacent tongue root 18 and a 3.00 mm radius
outer perimeter. The margin 20 is of roughly constant width,
~ 3.00 mm. .~s shown in Figure ~ a representative value for the
'7 useful movement of the actuator of Figure 2 is 0.32 mm for a
differential of 37C.
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lOS;~2()4
The actuator~ of Figures 1 an~ 2 have constant width
tongues and constant width margins. It has been found that
constant width tongues are not whol]y desirable since whilst
tlle tongue should be wide at ts root for mechanical strength,
substantlal width in its central regions is undesirable
as the tongue tends to invert across its width with snap act.on~
of the actuator, resulting in a smaller movement. Similarly
a constant width margin is not preferred, rather it has been
, ound that a better compromise of the various design fa-tors
involved is achieved by a maximum margin width in the region
~ of the tongue root which width progressively reduces to a
; minimum opposite the free end of the ~ongue.
! Acc~rdingly the actuators of Figures 3 and 4 have been
proposed, with tongues tapering from their roots and their
,l 15 margins continuously decreasing from a maxlmum in the region of
71 the tongue root to a minimum opposit~ the free end of the
tongue.
, The actuator of Figure 3 has a ~ide tongue root, namely
, ; 4 mm. The tongue 12 tapers with a ~0~ taper to a semicircular
1 mm radius free end. The tongue is 4 mm long. The aperture
14 has rounded ends 0.75 mm radius ~t the tongue root and its
outer perimeter is defined by an arc of 3.50 mm radius whose
~t ,,
~¦ centre is displaced 1 mm from the centre of the actuator.
This results in a margin 3 5 mm wide at the tongue and
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decreasing contlnuously to a minirn~lrn of 1. 5 t~n opposite the
free end of the tongue. As show~ in Figure ~ a representative
value of the useful movement of this actuator is 0.34 mm
with a differential of 34C. ~''
In Figure 4, the actnator tongue 12 tapers at 15 from
a tongue root 3.5 mm wide to a rounded free end with a l~m
- radius. The tongue is 4.5 mm long. The aperture 14 has
rounded ends o 0.75 mm radius adjacent the tongue root and the
outer perimeter of the aperturs is defined by an arc of 3,50 mm
radius whose centre is spaced 1 mm from the centre of the
actuator. The margin has a ~idth at the tongue root of
3.5 mm decreasing to a mi,nimum of l,5 mm opposite the free end
of the tongue. From Figure ~5 a representative value of the
useful movement of the actuator is 0.45 mm w;th 38C
differential. It has been found that the actuator of
Figure 4'offers overall minimum stress concentrations as a
result of its flowing contours. The wide margin at the --
tongue root provides ample strength in the snap action movement
and the actuator has proved to have a very stable break ~' -
temperature (2-3C after 10,000 operations) and it has no
abnormally high local stress concentrations where cracking
is likely to occur."
The actuator of Figure 5 has an internal shape of tongue
and aperture exactly the same as that of Figure 4. It llas
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been fo~mcl tllat the characteristics are similar; the 12 mm
squ~re pe-iphery of FLgure 5 not afecting the operat;on of
the actuator to any significant exten~.
~he actua~ors of ~igu~es 6 and ~ are intended ~or use
with current sensitive switches ~contact-breakers) and have a
contact 22 welded to the margin 20 opposite the free end of
tongue 12. In use current flows from the free end of the
tongue, through the tongue root and both parts of
margin 20 to contact 22.
Both actuators of Figures 6 and 7 have the same general
configurati.on as that shown in Figllre 4, but in order to
increase théir electr1cal resistance to current flow, they
are smaller, being 10 mm in diameter, and are formed from
O.1 mm thick bimetal. Both actuators have a longitudina] slot
24 in tongue 12 to increase electrical resistance. The
¦ actuator of Figure 7 has three circular apertures 28 either
side of contact 22 in the narro~est portion of margin 20 and
this serves substantially to increase electrical resistance.
` The actuator of Figure 6, instead of apertures in its margin,
has its margin in the region 26 either side of contact 22
¦ substantially narrowed as compared wi~h the margin in Figure 7
1; whilst still being thick enough to ensure cufficient force is
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produced in the snap action, This also serves substantially
to increase electrical resistance.
Both actua~ors of Figures 6 and 7 are sensitive to
currents as low as 2 amps to perform a snap action movement,
Various other arrangements may be envisaged for
increasing electrical resistance. Thus the tongue may be
' spl;t into two separate tongue portions each extending
Il from the same tongue root.
,~ The electrical contact need not necessarily be positioned
,i 10 on the periphery adjacent the f ee end o~ the tongue but
~, could for example be positioned on the tongue itself or
7 any other convenient position~ In addition more than one
contac4 may be employed.
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