Tripping device for an overload circuit breaker

An overload circuit breaker has a housing and a tripping device supported in the housing and having a helically wound, current-carrying bimetal strip composed of two face-to-face arranged metal components situated, respectively, at the inside and at the outside of the bimetal strip. The bimetal strip has a tripping portion which executes a tripping motion as the bimetal strip undergoes deformation under the effect of heat generated by an excess current flowing through the bimetal strip. The bimetal strip is formed of two electrically serially connected, oppositely wound bimetal helices each having an outer end constituting the opposite ends of the bimetal strip. The bimetal strip is affixed to the housing at both of its opposite ends. Further, the bimetal helices each have an inner end connected to one another by a coupling part. The coupling part constitutes the tripping portion of the bimetal strip.

BACKGROUND OF THE INVENTION 
This invention relates to a tripping device which is incorporated in an 
overload circuit breaker and which has a helically bent bimetal strip 
attached to the housing of the circuit breaker. One metal component of the 
bimetal strip is arranged at the inside of the helix, while the other 
metal component is arranged at the outside thereof. In prior art 
structures one end of the bimetal helix is affixed to the housing, while 
its other, free terminus conventionally constitutes the portion (tripping 
portion) which executes the tripping motion. 
In tripping devices of the above-outlined type the current terminal 
connected with the free end of the bimetal helix is constituted by a 
flexible (braided) wire to ensure that it does not obstruct the free 
mobility of the bimetal helix. A disadvantage of known helically bent 
bimetal strips resides in the fact that the tripping motion of the free 
helix end is, because of the instability of the bimetal helix, 
particularly in the zone of its free end, can be controlled (guided) in a 
localized manner only with difficulty along a predetermined path of 
motion. These difficulties increase as the axial length of the bimetal 
helix is augmented, that is, as the number of turns of the helix 
increases. When an overload circuit breaker is used in relatively 
low-current intensity circuits, the heat generated by the excess current 
as it passes through a planar bimetal strip may not be sufficient for 
tripping. Thus, for increasing the electrical resistance (and thus the 
generated heat) of the bimetal strip, for example, its effective length 
has to be increased within a very limited space. In such instances 
helically wound bimetal strips find advantageous application. An 
appreciable extension of the effective length of the bimetal strip in a 
narrow space, that is, in case of a small helix diameter, however, 
requires a certain minimum number of turns and thus the instability of the 
bimetal strip (which is a direct function of the number of turns) 
increases precisely at the tripping terminus of the helix. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved tripping device of 
the above-outlined type which has a bimetal strip of greater stability in 
the zone of its tripping portion. 
This object and others to become apparent as the specification progresses, 
are accomplished by the invention, according to which; briefly stated, the 
bimetal strip is formed of two electrically serially connected, oppositely 
wound bimetal helices and further, the bimetal strip is secured to the 
housing at both ends, while the tripping portion is constituted by the 
coupling part connecting the two bimetal helices to one another. 
By virtue of the structure according to the invention as defined above, the 
two bimetal helices are arranged parallel to one another as concerns the 
mechanical forces but they are connected in series with regard to their 
electric resistance. The effective length of the bimetal strip determined 
by the required heat output which, in turn, is determined by the electric 
resistance of the bimetal strip, is distributed along a relatively large 
number of turns at a relatively small helix diameter, since the bimetal 
strip is firmly clamped at both ends. As a result, a particularly 
space-saving tripping device is obtained which has a particularly small 
dimension in a direction perpendicular to the axis of the helix. Such a 
structure of narrow construction is of particular significance if one 
considers that the individual components of the tripping mechanism of an 
overload circuit breaker are, as a rule, positioned side-by-side in one 
plane to ensure that if a plurality of overload circuit breakers are 
connected to one another in juxtapositioned planes, the individual 
components take up as little space as possible in the direction of the 
adjacent circuit breakers. 
Accoding to a further feature of the invention, both clamped ends of the 
bimetal strip constitute current terminals. This feature makes it possible 
to entirely dispense with the braided flexible wire terminals which have 
been used heretofore to ensure the free mobility of the bimetal strip but 
which, at the same time have themselves constituted very instable 
components. 
The two bimetal helices may be manufactured separately and can be connected 
to one another, for example, by welding. According to a further feature of 
the invention, however, the bimetal strip is a one-piece component. This 
ensures a particularly advantageous and simple structure of the bimetal 
strip, since the tripping portion of the bimetal strip constituted by the 
coupling part between the two bimetal helices is the bimetal strip itself. 
In accordance with a further feature of the invention, the two bimetal 
helices have the same cross section and the same number of turns. This 
feature results in a symmetrical motion of the two bimetal helices, 
particularly if, according to further features of the invention, the two 
bimetal helices are mirror images of one another with respect to a plane 
of symmetry which extends perpendicularly to the helix axis and medially 
intersects the tripping portion. The path of motion of the tripping 
portion and its orientation are, in this manner, precisely localized as if 
a tripping end of a bimetal helix were positively guided. 
In accordance with a further feature of the invention, both inner ends of 
the bimetal helices are inwardly bent in the direction of the helix axis 
and project beyond the longitudinal helix axis together with the coupling 
part constituting the tripping portion. The surface of the tripping 
portion is oriented approximately radially to the helix axis. In this 
arrangement, when the bimetal strip is heated, the tripping portion 
executes a swinging motion about the longitudinal helix axis. 
According to still another feature of the invention, both bimetal helices 
can be adjusted from the outside by means of an adjusting device radially 
to the helix axis in the direction of the tripping motion of the tripping 
portion. With the aid of this arrangement an adjustment of the bimetal 
strip, that is, a setting of the tripping portion into a desired initial 
position can be effected in a particularly simple manner. By virtue of the 
attachment of the bimetal strip at both ends, the adjusting device effects 
a bending of the longitudinal helix axis in its mid zone between the two 
clamped ends in the direction of the tripping motion of the tripping 
portion. 
According to a further feature of the invention, the adjusting device 
comprises a set screw, whose longitudinal axis extends perpendicularly to 
the helix axis and which engages an insulating member which, in turn, acts 
upon the bimetal helices. Further, the insulating member has, between its 
two ends of engagement, a lug which projects into the intermediate space 
between the bimetal helices. The side portions of the lug are only at a 
very small distance adjacent respective terminal edges of the two bimetal 
helices. This arrangement ensures a certain additional guidance 
particularly of the tripping portion during tripping motion without, at 
the same time, obstructing the bimetal strip in its free mobility 
necessary for executing the tripping motion.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the Figures, the tripping device for a thermal tripping of 
an overload circuit breaker is accommodated in a housing having two 
housing halves 1 (only one shown). The parting plane between the two 
housing halves coincides with the plane of the drawing FIG. 3. The thermal 
tripping device comprises a bimetal strip generally located at 2 whose 
tripping portion 3 is swung outwardly in the direction of the arrow 38 
when the bimetal strip undergoes deformation due to the heat generated by 
an excess current. A tripping pin 6 is longitudinally slidably guided 
between two housing ribs 7 and 8 and engages, at one end, the tripping 
portion 3 of the bimetal strip 2, and contacts, at the other end, a 
compensating bimetal bar 9 connected to a tripping lever 4 which, in turn, 
is swingably supported in the housing by means of a pivot 5. Upon 
heat-caused displacement of the tripping portion 3 of the bimetal strip 2, 
the tripping lever 4 is, via force transmission by means of the tripping 
pin 6 and the compensating bimetal bar 9, swung in a clockwise direction 
as viewed in FIG. 3. This type of force-transmitting mechanism is 
disclosed in U.S. Pat. No. 4,024,487. 
With particular reference to FIG. 1, the bimetal strip 2 is formed of two 
oppositely wound bimetal helices 10 and 11 which, with regard to their 
electric resistances, are serially connected to one another. The coupling 
part 12 between the bimetal helices 10 and 11 constitutes the tripping 
portion 3 of the bimetal strip 2. In the direction of the tripping portion 
3, the bimetal helix 10 has a left-hand course, whereas the bimetal helix 
11 has a right-hand course. The bimetal strip 2 is clamped into the 
housing half 1 at its two outer ends 13 and 14 which, respectively, are 
the outer ends of the helices 10 and 11. The end 14 is welded to a post 15 
of a conductor rail 16. The latter is, in turn, fixedly attached to the 
housing, more precisely to the housing half 1. A terminal clip 17 is 
welded to the lower end of the conductor rail 16. The terminal clip 17 has 
a terminal arm which extends through an opening 18 of the housing half 1 
and is, at its free end 19, connectable to an external current conductor 
(not shown). 
The outer end 13 of the bimetal strip 2 is affixed to a post 20 of a 
contact carrier 21 of a stationary contact 22. Thus, the bimetal strip 2 
is, with its outer ends 13, 14 fixedly clamped in the housing half 1 
while, at the same time, these clamped strip ends serve as current 
terminals. In the embodiment illustrated, the bimetal strip 2 is a one 
piece component between its outer ends 13, 14. It has a rectangular cross 
section and has an outer side 24 oriented parallel to the helix axis 23 
which is common to both bimetal helices 10 and 11. The latter have an 
identical number of turns, such as three, as shown. The bimetal helices 
10, 11 are arranged as mirror images of one another with respect to a 
plane of symmetry 25 which constitutes the sectional plane II--II in FIG. 
1 and which extends perpendicularly to the helix axis 23 and intersects 
the tripping portion 3 in its middle. 
Each bimetal helix 10, 11 is formed of two face-to-face arranged bimetal 
components 39 and 40. The bimetal component 39 which has the property of 
the greater expansion is arranged at the inside of each helix, whereas the 
bimetal component 40 which has the property of smaller expansion is at the 
outside of the helices. 
The two adjacent ends 26, 27 of the respective bimetal helices 10 and 11 
which are oriented towards the helix connection 12 are bent radially 
inwardly in the direction of the helix axis 23, as best seen in FIG. 3. 
The ends 26, 27 intersect the helix axis 23 and, together with their 
coupling part 12 constituting the tripping component 3, project radially 
outwardly from the helix axis 23. The tripping portion 3, however, is 
situated still inside an imaginary envelope (such as a cylinder) 
circumscribable about the bimetal helices 10, 11 for the purpose of saving 
as much space as possible. 
On the terminal clip 17 there is mounted an adjusting device generally 
indicated at 29 which can exert a force radially towards the helix axis 23 
on those turns of the bimetal helices 10 and 11 which adjoin the coupling 
part 12. Thus, by means of the adjusting device 29, from the outside an 
adjusting force may be transmitted to the bimetal strip 2 in the direction 
of motion of the tripping portion 3 (in the direction of the arrow 28), by 
means of which the helix axis 23 of the bimetal strip 2 can be displaced. 
The adjusting device comprises a set screw 30 which is threadedly engaged 
in the terminal clip 17, which, in turn, is affixed to the housing half 1. 
The axis 31 of the set screw 30 is contained in the plane of symmetry 25 
of the two helices 10 and 11. The inner terminus of the set screw 30 
carries a pin 32 which projects into an insulating member 33. The latter, 
in turn, engages end portions 34, 35 of the bimetal helices 10 and 11 
which are situated adjacent the coupling part 12. The insulating member 33 
has a lug 36 which is situated between the two end portions 34, 35 and 
which projects into the intermediate space between the two bimetal helices 
10, 11. The two lateral faces of the lug 36 are located at a very small, 
slit-like clearance from the respective edge faces 37 of the helices 10, 
11. 
By turning the set screw 30 inwardly, the bimetal strip 2 is, between the 
two outer ends 13 and 14, shifted towards the left as viewed in FIG. 3, 
whereupon, as a result, the tripping portion 3 moves towards the left. By 
turning the set screw 30 outwardly, the bimetal strip 2 moves, by virtue 
of the inherent elasticity of the bimetal helices 10, 11, again into its 
initial position which is determined by a linear orientation of the helix 
axis 23. 
It is to be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.