A snap-acting thermally-responsive actuator for an electric switch comprises a pear-shaped dished bimetallic blade having a U-shaped cut-out defining a tongue extending from its root or attached end at the larger end of the pear-shaped blade axially of the blade towards the smaller end of the blade, the tongue extending between outer leg portions of the blade which each have a width which decreases in a direction from the larger towards the smaller end of the blade. When the blade is mounted in a switch with the tip end of its tongue secured to a terminal part of the switch and with a moving switch contact carried on the smaller end of the blade for co-operation with a fixed switch contact secured to another terminal part of the switch, the blade thereby being susceptible to heating by electric current flow through the switch when the switch contacts are closed, the pear-shaped blade configuration coupled with the thinning of the outer legs towards the smaller end of the blade provides improved mechanical switching characteristics and improved self-heating characteristics in the blade leading to improved consistency of operation.

FIELD OF THE INVENTION 
This invention relates to snap-acting thermally-responsive bimetallic 
actuators for use in actuating the switch contacts of thermally responsive 
electric switches, for example, cut-outs, circuit breakers and 
thermostats. 
BACKGROUND OF THE INVENTION 
British Patent Specification No. 1,542,252 contains a discussion of the 
problems associated with the well known bimetallic actuator comprising a 
disc of domed configuration which moves to an oppositely domed 
configuration with a snap action with changes in temperature, such 
problems including drift of the operating temperature at which the disc 
snaps with age, a tendency for stress cracking to occur in the disc, and 
the very small range of useful movement associated with the snap action. 
British Patent Specification No. 1,542,252 further discusses the also well 
known form of actuator described, for example, in British Patent 
Specification No. 657,434, which comprises a rectangular sheet or blade of 
bimetal having a central tongue released from between two outer legs whose 
ends, adjacent the free end of the tongue, are joined by a bridge portion, 
and wherein the blade has imparted thereto a dished configuration such 
that, with changes in temperature, the blade moves between oppositely 
dished configurations with a snap action. As described in British Patent 
Specification No. 657,434, the dished configuration was in the past 
imparted to the blade by virtue of a crimp being formed in the 
aforementioned bridge portion, but more recently the requisite dished 
configuration has been obtained by pressing the blade between two dies. 
The actuator of British Patent Specification No. 657,434 provides a much 
larger range of useful movement than the known disc actuator and is 
capable of being set to an accurately defined operating temperature, but 
has also been found to be susceptible to stress cracking. 
The actuator of British Patent Specification No. 1,542,252 was designed 
particularly to overcome the stress cracking problems of the prior art, 
and in its preferred embodiment comprises a dished circular member of 
sheet bimetal having a curved aperture therein which, similarly to the 
actuator of British Patent Specification No. 657,434, releases a tongue 
from the bimetal member. The aperture has an outer periphery in the form 
of an arc of a circle which merges smoothly with the inner periphery of 
the aperture at rounded ends adjacent the root of the released tongue, 
this configuration having been designed to minimize stresses in the 
actuator when it snaps between its oppositely dished configurations. 
Reference may be made to British Patent Specification No. 1,542,252 for a 
full discussion of the problems associated with the prior art actuators 
and of the design considerations involved in the new actuator of British 
Patent Specification No. 1,542,252. 
While the improved actuator of British Patent Specification No. 1,542,252 
does have some advantageous features, it is unfortunately wasteful in 
material, and rectangular blade actuators of the general form described in 
British Patent Specification No. 657,434 continue to be widely used, 
particularly though not exclusively with the bimetal blade mounted by its 
center leg (i.e., its released tongue). It has been found that when such 
rectangular blade actuators are used as overload current sensing elements 
in circuit breakers, where the blade carries the current to be monitored 
and is responsive to the heating effect of such current flow, several 
problems are experienced. 
The heating pattern in the bimetal blade is uneven and both ends of the 
blade tend to run cool, which gives rise to complex stresses in the 
bimetal. Furthermore, the end of the blade nearest the free or tip end of 
the released tongue, i.e., the end which would conventionally have taken a 
crimp for imparting the dished configuration to the blade, is quite broad 
and can itself snap at a different temperature from the other end of the 
blade nearest the root end of the released tongue, which is the end one 
would normally consider as constituting the snap action part of the 
bimetal; this effect is particularly marked at low temperatures (e.g., 
-40.degree. C.) and, with the bimetal mounted by its central leg, has 
resulted in the larger end of the bimetal nearest the root end of the 
tongue snapping over center and the contacts of the switch nonetheless 
staying closed until the other (contact) end of the bimetal has snapped 
over at a different temperature. 
SUMMARY OF THE INVENTION 
The present invention provides a bimetal blade actuator generally similar 
to the actuator of British Patent Specification No. 657,434, but with the 
blade configured to provide a more consistent and uniform heating pattern 
under current, and with the contact end of the bimetal mechanically 
weakened so that it does not restrain the other end from snapping and 
opening the switch contacts. As will be appreciated from the following, 
the resultant blade has a generally "pear" shaped configuration. 
More particularly stated, the present invention comprises a generally 
pear-shaped bimetallic blade which is stressed so as to be movable with 
temperature change between two oppositely dished configurations, a 
generally U-shaped cut-out in the blade releasing therefrom a tongue 
having a root or attached end adjacent the larger end of the pear-shaped 
blade and a tip or free end adjacent the smaller end, the tongue extending 
between two outer leg portions of the blade which each have a width 
decreasing in a direction from the larger towards the smaller end of the 
blade. The blade is preferably symmetrical on either side of an axis 
bisecting the tongue and, for minimizing the risk of stress fracture, the 
profiles both internal and external of the blade preferably are devoid of 
sharp angles and comprise only curved or curved and straight portions 
merging smoothly with each other.

DETAILED DESCRIPTION 
Referring to FIG. 1, the bimetal blade shown therein is generally 
rectangular and has a central tongue 1 released therefrom, the tongue 1 
extending between leg portions 2 and 3 which are connected adjacent the 
free end of the tongue 1 by a bridge portion 4. When the blade shown in 
FIG. 1 is mounted by its central tongue 1, the bridge portion 4 generally 
mounts the movable contact 5 of a switching contact pair. The blade of 
FIG. 1 is described in a typical application in our British Patent 
Application No. 8031960 (Specification No. 2,061,009 A) which concerns an 
electric switch incorporating such a blade as a snap-action 
thermally-responsive switch actuator, and with a latching member of the 
switch co-operating with an extension tab on the blade (corresponding to 
the tab 6 shown in FIG. 1) to prevent the switch once actuated from 
resetting until the latching action of the latching member is released; 
the tab 6 thus is not an essential part of a bimetal blade of this type 
and serves only a latching function. The blade of FIG. 1 is dished, as is 
well known in the art, and moves with a snap-action between two oppositely 
curved configurations with changes in temperature. 
The bimetal blade of FIG. 1, while exhibiting advantageous characteristics 
as compared to other conventional forms of bimetal blade, nonetheless is 
susceptible to a number of problems which jointly and severally have 
prompted the present invention. For example, whereas bimetallic blades of 
the general type shown in FIG. 1 have performed adequately at higher 
ambient temperatures in a current carrying mode, at low ambient 
temperatures of the order of -30.degree. C. to -40.degree. C., for 
example, the working end of the blade (namely, the end adjacent the root 
of the tongue 1) has been observed to attempt to switch (i.e., to say to 
snap into its oppositely dished configuration) but to be restrained by the 
bridge portion 4. At best this could result in the blade switching at a 
slightly incorrect temperature, but at worst the consequence of this fault 
might be a double action switching of the blade with the working end of 
the blade going overcenter but not affecting the closed condition of the 
contacts until the bridge area of the blade reached its own snap 
temperature. 
Another problem associated with the bimetal blade of FIG. 1 is that of 
uneven heating of the blade by electric current flowing through the blade 
from the tip of the released tongue 1 to the contact 5. It is not possible 
to have an absolutely constant current density throughout the blade, since 
this would require the tongue 1 to be twice as wide as the outer leg 
portions 2 and 3, which experience shows leads to the outer leg portions 
becoming too weak to contribute to the required snap action, so that the 
blade becomes inoperative. The relative thinness of the tongue 1 increases 
the current density therein, so that the tongue becomes the mcst heated 
part of the blade as regards current flow through the blade. To an extent 
this is not disadvantageous, and may even be beneficial, since the excess 
heating of the tongue contributes to the heating of the working part of 
the blade and thus to the responsiveness of the blade, and also causes the 
tongue to flex, so increasing contact pressure right up to the moment the 
blade snaps to its oppositely dished condition. However, the excess 
heating of the tongue has caused a temperature gradient to exist in the 
blade from the tongue 1 through the working portion and the outer leg 
portions 2 and 3 to the bridge portion 4 where heating of the blade has 
been poor, which has led to imprecision in the operation of the blade. 
FIGS. 2 and 3 show pear shaped blades according to the invention wherein 
the bridge portion end of each blade is dimensionally reduced as compared 
with the other end so as to remove stiffness and ensure that it is the 
other end of the blade that dictates the blade switching characteristics. 
This pear shaped configuration has furthermore been found to produce more 
uniform heating of the blade by current flow therethrough. 
It is to be noted that the blade of FIG. 2 is slightly differently shaped 
to the blade of FIG. 3. Variations are possible within the general "pear" 
shape of the blade, and the precise configuration selected for any 
particular application will depend upon the blade characteristics 
required. Broadly speaking, however, each of the blades of FIGS. 2 and 3 
has a larger end 7 which constitutes the working end of the blade and a 
smaller end 8 which constitutes the normally contact-carrying end of the 
blade, a generally U-shaped cut-out 9 defining a tongue 10 between outer 
leg portions 11 which reduce in width from the larger to the smaller ends 
of the blade. As shown, the blades each are generally symmetrical about an 
axis bisecting their respective tongues and are profiled both internally, 
i.e., in the region of the U-shaped cut-out, and externally for avoidance 
of sharp transitions which might constitute centers for the initiation of 
stress fracture. The limbs of the U-shaped cut-out 9 are parallel in the 
embodiments of FIGS. 2 and 3, though the scope of the invention extends 
beyond such a construction. 
It will be seen from FIGS. 2 and 3 that the bridge portion of the 
respective blades, i.e., portion designated 12, is substantially reduced 
dimensionally as compared to the other or working end of the blade, and 
the ends of the outer leg portions 11 where they merge into the bridge 
portion are also thinned down. It has been found that, by so designing the 
blade, the main problems associated with the prior art blade of FIG. 1 are 
largely overcome without loss of the excellent characteristics of that 
blade as regards its degree of operating movement. The combined mechanical 
and electrical effects of the design changes proposed in accordance with 
the present invention thus produce a bimetal blade having substantlally 
improved operating characteristics. 
FIG. 4 shows an exemplary form of thermostatic switch incorporating a 
bimetal blade of the type shown in FIG. 3. A molded plastic body part 40 
has two brass terminal elements 41, 42 molded therein and defining 
mountings for a fixed contact of the switch and for the bimetal blade 43, 
respectively, the blade 43 being mounted by virtue of its tongue being 
spot welded or otherwise secured to a portion of terminal part 42. A 
movable contact is welded to the bridge portion of the blade for 
co-operation with the fixed contact of the switch, the position of the 
contacts being designated 44. In the switch in question, the tab portion 
45 of the blade is not used with a latching mechanism, but instead simply 
co-operates with a back stop provided in the molded body part 40 to limit 
the switch opening movement of the blade and prevent it from being 
overstressed in the course of its snap-action operating movement.