Abstract:
A contact arm assembly is provided having an electrical contact for making and breaking an electrical current, a contact arm for supporting the electrical contact, and a bond surface on the contact arm that is conditioned for improving the bond between the electrical contact and contact arm. Also provided is an electrical circuit breaker that utilizes the improved contact arm assembly. The bond surface of the contact arm is provided with pyramid-shaped serrations that serve to more uniformly distribute the electrical current during brazing, provide multiple areas of localized current constriction during brazing, and provide collector pockets for accumulating the molten braze alloy during brazing. The uniform distribution of electrical current during brazing serves to generate a uniform temperature gradient across the braze area for uniform melting of braze alloy. The multiple areas of localized current constriction during brazing serves to temporarily elevate the temperature of the braze joint during brazing by localizing the heat generation proximate the braze alloy, thereby effectively reducing annealing of the contact arm. The collector pockets for accumulating the molten braze alloy during brazing effectively eliminates the overflow of braze alloy onto the edges of the contact and contact arm. A contact arm assembly having uniform melting of braze alloy, reduced annealing of the contact arm, and reduced overflow of braze alloy onto the edges of the contact and contact arm results in an improved bond of contact to contact arm.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a contact arm assembly having an electrical contact for making and breaking an electrical current in an electrical circuit breaker. Contacts and contact arm assemblies are well known in the art of circuit breakers. An example of an electrical contact suitable for circuit breaker applications is described in U.S. Pat. No. 4,162,160 entitled “Electrical Contact material and Method for Making the same.” An example of a method of making an electrical contact material suitable for circuit breaker applications is described in U.S. Pat. No. 4,249,944 entitled “Method of Making Electrical Contact Material.” Examples of contact arm assemblies suitable for circuit breaker applications is described in U.S. Pat. No. 4,999,464 entitled “Molded Case Circuit Breaker Contact and Contact Arm Arrangement”. 
     Contact arm assemblies having electrical contacts for making and breaking an electrical current are not only employed in electrical circuit breakers, but also in other electrical devices, such as rotary double break circuit breakers, contactors, relays, switches, and disconnects. The applications that these electrical devices are used in are vast, and include, but are not limited to, the utility, industrial, commercial, residential, and automotive industries. The primary function of a contact arm assembly is to provide a carrier for an electrical contact that is capable of being actuated in order to separate the contact from a second contact and contact arm arrangement, thereby enabling the making and breaking of an electrical current in an electric circuit. Electrical contacts suitable for the noted applications are typically made of a silver impregnated material, such as, but not limited to; silver-tungsten, silver-tungsten-carbide, silver-nickel, silver-tin oxide, silver-cadmium oxide, silver-graphite, silver-molybdenum, silver-nickel-graphite, and silver-iron. However, the use of copper in place of silver may also be suitable for some lower current applications. The contact must be bonded to the contact arm, which is typically, but not necessarily, a copper alloy, in such a manner that the assembly will not disassemble during operation of the host device. The bonding method that is typically employed is brazing. The process of brazing electrical contacts to contact arms is well know to one skilled in the art and is fully described in Advanced Metallurgy&#39;s article entitled “Brazing Electrical Contacts” by Peter C. Murphy, published by Advanced Metallurgy, Inc., 1028 E. Smithfield Street, McKeesport, Pa. 15135 (July, 1987). 
     To facilitate the brazing process, contacts have been known to be manufactured with serrated detail on the back. The serrated detail on the back of the contact serves to retain the excess silver infiltrant and braze alloy that results during contact manufacturing, thereby providing a silver rich layer and a layer of braze alloy on the back of the contact for brazing. The resulting finished contact is substantially void of any serration pockets on the back since the silver infiltrant and braze alloy have substantially filled them in. Thus, the purpose of the serrated detail on the back of the contact is for contact manufacturing purposes and not for influencing current distribution during brazing. Serrated contacts are described in Advanced Metallurgy&#39;s article entitled “Serrated Backed Contacts” in their publication entitled “Advanced Metallurgy, Inc., Electrical Contacts and Assemblies”, published by Advanced Metallurgy, Inc., 1028 E. Smithfield Street, McKeesport, Pa. 15135 (1987). Various contact manufacturing methods are also described in the aforementioned publication entitled “Advanced Metallurgy, Inc., Electrical Contacts and Assemblies”. 
     In order to accommodate thermal limitations within an electrical device, the cross-sectional areas of the contact, contact arm, and bond area between contact and contact arm, typically increase as the ampacity rating of the contact arm assembly increases. While the cross-sectional areas of the contact and contact arm are readily determined by geometric measurements, the cross-sectional area of the bond surface between contact and contact arm is not so readily determined. Factors such as brazing temperature, brazing time, surface oxidation, brazing electrode geometry variations, and braze alloy geometry variations, can effect the percentage of bond area that is actually brazed, thereby effecting the ability of the brazed joint to withstand adiabatic heating at short circuit, and to withstand shear forces during mechanical opening and closing of the contacts. Thus, it would be beneficial to have an improved method of bonding an electrical contact to a contact carrier and an improved contact arm assembly resulting therefrom. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the present invention, a contact arm assembly and method of making the same are provided having an improved bond between contact and contact arm, thereby enabling the contact arm assembly to withstand increased adiabatic heating and shear forces than would be possible without the improved bond. Also provided is an improved contact arm assembly in accordance with the present invention that also includes nickel metal arranged intermediate a silver-impregnated contact and a copper contact arm, thereby preventing intermixing between the copper and silver when the contact is bonded to the contact arm. Further provided is an electric circuit breaker having an improved contact arm assembly in accordance with the present invention, which enables the circuit breaker to perform according to specification when the contact arm assembly is subjected to increased adiabatic heating and shear forces. An alternative benefit of the present invention is to provide an improved contact arm assembly of a reduced size that is capable of withstanding the same adiabatic heating and shear forces as a contact arm assembly of normal size but with less effective bonding between contact and contact arm. 
     The improved bond between contact and contact arm is accomplished by conditioning the bond surface of the contact arm to produce a serrated finish. While there are many arrangements of serrated finishes that produce satisfactory results, the exemplary embodiment having a plurality pyramid-shaped serrations, or solid geometric saw-like projections, has been s improve the brazed connection between contact and contact arm. The serrated finish on the bond surface of the contact arm serves to more uniformly distribute the electrical current during brazing, provide multiple areas of localized current constriction during brazing, and provide collector pockets for accumulating the molten braze alloy during brazing. A more uniform distribution of electrical current across the contact-to-contact-arm interface during brazing produces a more uniform heat profile throughout the cross-sectional area of the braze alloy, thereby resulting in more uniform melting of the braze alloy. The multiple areas of localized current constriction across the contact-to-contact-arm interface serve to rapidly increase the interface temperature without excessively overheating the contact or contact arm, thereby resulting in rapid melting of the braze alloy while minimizing the degree of annealing experienced by the contact and contact arm. In normal contact-to-contact-arm brazing operations, where annealing of the copper contact arm occurs, the softened copper of the contact arm can result in deformation of the contact arm after the contact arm experiences repeated mechanical on-off impact loads, thereby reducing the term of usability of the contact arm and host device. Minimizing the degree of annealing experienced by the copper contact arm will avoid premature deformation of the contact arm, thereby enhancing the term of usability of the contact arm and host device as compared to a normal contact-to-contact-arm assembly employing a less effective brazing technique. Collector pockets created by the serration pattern provide the molten braze alloy with flow regions, areas defining the valleys of the collector pockets, across the entire bond area, thereby reducing the volume of excess braze flow that is expelled around the outer edge of the bond region. Excessive braze flow that is expelled around the outer edge of the bond region during brazing can weep down to the contact surface and cause undesirable tack welding of the contacts. The presence of collector pockets across the bond area of contact to contact arm significantly reduces the volume of braze alloy that is available to weep down to the contact surface, thereby eliminating the need for post-braze cleaning. 
     Although the bond surface of the silver impregnated contact has serration detail, as described above, the purpose of these serrations is to contain the excess silver infiltrant that results during contact manufacturing, and not to provide an array of current constriction points and collector pockets. Thus, the benefits described above arising from the serration pattern on the bond surface of the contact arm, are not achieved by the silver-filled serrations on the back of the silver impregnated contact. Furthermore, the serration pattern on the bond surface of the contact arm provides an improved contact-to-contact-arm bond with or without the serration detail on the back of the contact. 
     An alternate embodiment of the present invention is to include a layer of nickel between the serrated copper contact arm and the silver impregnated contact, which acts as a barrier to prevent the intermixing of copper and silver. By preventing the intermixing of copper and silver at the bond interface, the resulting bond interface is free of a copper-silver eutectic alloy, which has a melting point lower than that of the copper and the silver. Thus, a contact arm assembly having a serrated bond surface on the copper contact arm and a nickel layer between the copper contact arm and silver impregnated contact, provides a further improved bond by elevating the melt temperature of the bond interface above that of the copper-silver eutectic melt temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical representation of experimental data reflecting the raw data and statistical distribution of effective bond surface area as a percentage of available surface area for a brazed joint according to prior art methods; 
     FIG. 2 is a graphical representation of experimental data reflecting the raw data and statistical distribution of effective bond surface area as a percentage of available surface area for a brazed joint in accordance with the present invention; 
     FIG. 3 is a partial cutaway isometric view of an electrical circuit breaker showing an actuator and containing an electrical contact arm assembly in accordance with the present invention; 
     FIG. 4 is an isometric partial view of the electrical circuit breaker of FIG. 3 with the cover removed to depict the circuit breaker operating mechanism assembly; 
     FIG. 5 is an exploded isometric view of an electrical contact arm and pivot assembly used within the circuit breaker depicted in FIG. 3; 
     FIG. 6 is an enlarged isometric view of the electrical contact arm and pivot assembly depicted in FIG. 5; 
     FIG. 7 is an exploded isometric view of an electrical contact arm assembly showing a solid geometric shaped projection in accordance with the present invention; 
     FIG. 8 is an exploded isometric view of an alternative embodiment of an electrical contact arm and pivot assembly used within the circuit breaker depicted in FIG. 3; 
     FIGS. 9 a-g  are isometric views of alternate embodiments of solid geometric shaped projections as depicted in FIG. 7; and 
     FIG. 10 is an exploded isometric view of an alternate electrical contact arm assembly showing a solid geometric extruded projection in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Contact to Contact Arm Bond Surface Area Generally 
     FIGS. 1 and 2 depict graphical representations of experimental data reflecting the raw data and statistical distribution of effective bond surface area as a percentage of available surface area for a brazed joint according to prior art methods and for a brazed joint in accordance with the present invention, respectively. Brazed joints are typically designed to have a certain effective bond surface area, with an effective bond surface area of greater than 75% being desirable. The effective bond surface area is that part of the total geometric surface area available for brazing that has in fact bonded through alloying. Due to the presence of surface oxides, surface imperfections and varying heat gradients across the bond surface area, variations in the resultant percentage of effective bond surface area can and do occur. To overcome this degree of variability, over-sized brazed joints are employed. However, the present inventors discovered that this degree of variability can more effectively be overcome by employing the serration process of the present invention. As depicted in FIGS. 1 and 2, the present invention significantly improves the percentage of effective bond area resulting from a given available geometric surface area. The experimental data of FIGS. 1 and 2 were generated using a tungsten top electrode of about ⅝-in diameter, a carbon bottom electrode of about 1½-in diameter and typical brazing parameters, such as; about 80-lb electrode clamping force, about 3000-amps rms alternating current (60 hertz), about 3 pulses of about 39 electrical cycles of on time, and water cooling. 
     FIG. 1 depicts the raw data  50  and statistical distribution  52  of effective bond surface for a typical prior art non-serrated brazed assembly that results in a median effective bond surface area of roughly 75%, a minus three-sigma (sigma is representative of “standard deviation”) value of roughly 54%, and a plus three-sigma value of roughly 96%. 
     FIG. 2 depicts the raw data  54  and statistical distribution  56  of effective bond surface for a serrated brazed assembly in accordance with the present invention that results in a median effective bond surface area of roughly 93%, a minus three-sigma value of roughly 78%, and a theoretical plus three-sigma value of roughly 108%. Of course, the plus three-sigma value of roughly 108% is merely a theoretical value since it is the result of a statistical calculation, and does not imply the possibility of producing an actual bond area greater than 100% of the available surface area. As shown in FIG. 2, the raw data  54  as obtained through experimentation does not exceed the 100% threshold. 
     The assemblies of both FIGS. 1 and 2 have the same geometric surface area available for brazing, but significantly different effective bond surface areas. As can be seen, if a minimum effective bond surface area of greater than 75% is desired, the process depicted by FIG. 2 will result in a greater acceptance level within a plus or minus three-sigma range. 
     Circuit Breaker and Contact Arm Assembly Generally 
     Referring to FIG. 3, a current limiting circuit breaker  10  is depicted consisting of a case  11  to which a cover  12  is attached and which further includes an accessory cover  13 . A circuit breaker operating handle  14  extends upward from a slot formed within the circuit breaker cover for manually turning the circuit breaker to its ON and OFF conditions. As described in U.S. Pat. No. 4,757,294, an actuator unit  49  interfaces with an operating mechanism  15  by means of a trip bar  16  to separate the circuit breaker fixed and movable contacts  17 ,  18 , best seen by referring now to FIG.  4 . The operating mechanism acts upon the movable contact arm  19  to drive the movable contact arm to the open position, shown in the circuit breaker  10  depicted in FIG. 4, upon the occurrence of an overcurrent condition of a predetermined magnitude. An arc extinguishing assembly  48  is located in base  10  in each of the three poles, or phases, proximate the stationary and movable contacts  17 ,  18  for controlling and extinguishing an electrical arc that is drawn between the stationary and movable contacts  17 ,  18  during an opening action. The circuit current is sensed by means of current transformers  20 - 22  which connect with the circuit breaker trip unit  46  by means of upstanding pins as indicated at  23 . A molded plastic crossbar arrangement  24 , such as described in U.S. Pat. Nos. 4,733,211 and 4,782,583, insures that the movable contact arms operate in unison when the operating mechanism is articulated. The operating mechanism is held against the bias of a pair of powerful operating springs  25  by means of a latch assembly  26 , such as described in U.S. Pat. Nos. 4,736,174 and 4,789,848. In order to provide the current limiting functions described earlier, the movable contact arms are adapted for independent movement from the crossbar assembly by electrodynamic repulsion acting on the movable contact arm itself. One such example of a current limiting circuit breaker is found within U.S. Pat. No. 4,375,021, which should be reviewed for its teachings of electrodynamic repulsion of a movable contact arm under intense overcurrent conditions through the circuit breaker contacts. 
     When such intense overcurrent conditions occur, it is important that the movable contact arms maintain good electrical contact with the contact arm supports while the movable contacts move away from the fixed contacts. The movable contact assembly  27  shown in FIG. 5 has a pair of shunt plates  28 , arranged on either side of the movable contact arm as well as the parallel braided shunt conductor  29  for providing the necessary electrical contact between movable contact arm  19  and contact arm support  30 . The shunt conductor is welded or brazed to the movable contact arm  19  at one end and is similarly attached to the contact arm support  30  at the opposite end. The movable contact arm includes a central body part through which a through-hole  31  is formed and an extended forward part  32  to the end of which the movable contact  18  is attached by the method to be described below in greater detail. The movable contact arm  19  is positioned within the circuit breaker case by means of a support base  33  which includes integrally-formed upstanding support arms  34 ,  35 . The base  33  is tempered in order for the support arms  34 ,  35  to resiliently capture the movable contact arm  19  in a tight press-fit relation to promote good electrical conduction between the support arms  34 ,  35  and the movable contact arm  19 . A through-hole  36  formed within the support base  33  allows for the electrical connection of the support base  33  with the circuit breaker load strap (not shown). The provision of an elongated slot  37  within the support base  33  intermediate the upstanding support arms  34 ,  35  allows for the flex of the support arms  34 ,  35  when the movable contact arm  19  is inserted. When the movable contact arm  19  is positioned within the support arms  34 ,  35 , the through-hole  31  in the movable contact arm  19  aligns with corresponding through-holes  39  formed within the support arms  34 ,  35 . A pivot pin  40  is next inserted within the through-holes  39  which are slightly oversized to permit rotation of the contact arm  19 , and within through-hole  31  in a press-fit relation. The clearance provided between the through-holes  39  within the support arms  34 ,  35  and the ends of the pivot pin  40  allows the movable contact arm  19  to freely rotate within the support arms  34 ,  35  while maintaining good mechanical and electrical connection between the pivot pin  40  and the movable contact arm  19 . It is important to maintain good electrical contact between the pivot pin  40  and the movable contact arm  19  while the contact arm rotates between its closed and open position in order to deter local ionization and pitting between the contact arm  19  and the pivot pin  40 . The shunt plates  28  which are formed of a conductive material, such as copper or aluminum alloys, are shaped to include bifurcated arms  41 ,  42  extending from an angled base  43 . Openings  44  are formed within the bifurcated ends  41 ,  42  of the shunt plates  28  for supporting the shunt plates  28  on the ends of the pivot pin  40 . A U-shaped contact spring  45  is next positioned over the shunt plates  28  to further promote electrical connection between the shunt plates  28 , support arms  34 ,  35  and the movable contact arm  19 . Upon the occurrence of an intense overcurrent condition, such as a short circuit, the current path between the shunt plates  28  and the pivot pin  40  becomes divided between the bifurcated arms  41 ,  42 . The resulting parallel current path through the bifurcated arms  41 ,  42  electrodynamically drives the bifurcated arms  41 ,  42  against the ends of the pivot pin  40  to maintain good electrical contact under intense short circuit overcurrent conditions. The good electrical conduction between the contact arm  19 , pivot pin  40  and support arms  34 ,  35  insures that no localized arcing and pitting will occur. The shunt plates  28  share the circuit current with the shunt braid conductor  29  such that no pitting occurs between the pivot pin  40 , support arms  34 ,  35  and the movable contact arm  19  even under such intense short circuit conditions. 
     The movable contact arm assembly  27  is depicted in FIG. 6 to show how the shunt plates  28  are forced against the support arms  34 ,  35 , by the bias provided by the U-shaped contact spring  45 . The pivot pin  40  is shown extending through the movable contact arm  19 , the support arms  34 ,  35  and the shunt plates  28 . Also depicted is the shunt braid conductor  29  that cooperates with the shunt plates  28  to provide parallel current paths between the movable contact arm  19  and the support  33  as described earlier. 
     Contact to Contact Arm Bond 
     In accordance with the teachings of the present invention, the movable contact arm  19  is provided with a stippled, or serrated, bond surface  38 , as best seen by referring to FIGS. 5 and 7. An exemplary arrangement of stippling, or serrations, on bond surface  38  is depicted by pyramid-shaped projections  47 , having a base dimension “b” and height dimension “h”. While only one projection  47  is shown, it will be appreciated that the bond surface  38  contains a plurality of projections  47  to create the serrated bond surface  38 . The base “b” and height “h” dimensions are typically between 0.002 inches and 0.200 inches, preferably between 0.005 inches and 0.100 inches, and most preferably between 0.010 inches and 0.030 inches. 
     While the projection  47  is shown to be pyramid-shaped with a base dimension “b” and height dimension “h”, it will be appreciated that any solid geometric shaped projection having the function of discretely distributing the electrical current over the bond area during brazing, providing multiple areas of localized current constriction during brazing, and providing collector pockets for accumulating the molten braze alloy during brazing, will be functionally equivalent to a pyramid-shaped projection shown. For example, FIGS. 9 a-g  depict other shapes or patterns that would be suitable for achieving the functional equivalent of the pyramid-shaped projection. The solid geometric shapes depicted in FIGS. 9 a-g  are known as; hemisphere, spherical cap, right circular cylinder, cylinder of a cross-sectional area, right circular cone, frustum of right circular cone, and rectangular parallelepiped, respectively. 
     Additionally, an extruded solid geometric shaped projection  58 , as shown in FIG. 10, across the bond surface  38  of the contact arm  19  will also provide discrete distribution of the electrical current during brazing, localized current constriction during brazing, and collector pockets for accumulation of molten braze alloy. However, it will be appreciated that an extruded solid geometric shaped projection will not provide as many discrete points of contact as will individual solid geometric shaped projections, and will therefore provide only an incremental improvement over the prior art. The “b” and “h” dimensions shown in FIG. 10 correspond to the “b” and “h” dimensions shown in FIG. 7, and the “w” dimension shown in FIG. 10 corresponds to the width of bond surface  38  on contact arm  19 . 
     Referring now to FIGS. 5-7, a bond layer  18   a  on movable contact  18 , which typically comprises a braze alloy, facilitates bonding of movable contact  18  to contact arm  19 . During brazing of movable contact  18  to contact arm  19 , serrations  47  abut bond layer  18   a,  thereby discretely distributing the electrical current over the bond area, providing multiple areas of localized current constriction, providing collector pockets for accumulating the molten braze, and resulting in a more uniform bond. The reader will appreciate that the number of discrete projections on the bond surface of the contact arm will influence the outcome of the braze process. For example, thousands of projections per square inch over the bond surface will approach the functional equivalence of a planar bond surface, thereby negating the benefit of the projections, and a single projection over the bond surface will negate entirely the benefit of multiple projections. Thus, a reasonable number of projections are needed in order to shift the effective bond surface area from that depicted in FIG. 1 to that depicted in FIG.  2 . Such a reasonable number of projections can be achieved by employing the “b” and “h” dimensions as discussed above. 
     Alternate Embodiment of Contact to Contact Arm Bond 
     In accordance with the further teachings of the present invention, the movable contact arm  19  is first plated with a coating of nickel in order to prevent any silver from transferring from the movable contact  18  to the movable contact arm  19  during the brazing operation. The nickel interface between the copper movable contact arm  19  and the silver impregnated tungsten-carbide contact  18  increases the temperature at which the contact  18  attaches to the contact arm  19  due to the higher melting point of the nickel than that of either silver or copper. The nickel coating thereby prevents the formation of a copper-silver eutectic and thereby substantially increases the temperature at which the contact would loosen and become detached from the movable contact arm. An acid flux is used to provide clean metallic surfaces during the welding or brazing operation. In some high current circuit applications, it is helpful to nickel plate the side of the contact  18  that is welded to the contact arm  19  and thereby promote a nickel to nickel weld. In other circuits, coating the surface of the contact  18  alone is sufficient to deter the transfer of silver out from the tungsten carbide matrix such that the copper movable contact arm  19  is not nickel plated. When the contact arm  19  is nickel plated, it is immersed in either an electroless or electrolytic nickel plating solution in which the nickel is applied to a minimum thickness of 0.1/1000 of an inch. 
     When electrolytic nickel plating solutions such as nickel chloride and nickel sulfamate are employed, electrodeposited nickel coatings having good tensile strength are obtained. Other methods of depositing nickel to selected regions of the contact arm, such as plasma spray and vapor deposition techniques, can be employed in high speed manufacturing processes. 
     In the event that neither the contact  18  nor the contact arm  19  is nickel plated, a thin disc of nickel or an alloy of nickel as indicated at  18   b  in phantom in FIG. 8 is interposed between the silver impregnated tungsten-carbide contact  18  and the copper contact arm  19  to deter the formation of the silver-copper eutectic. 
     The combination of the nickel interface, depicted as  18   b,  and the serrations  47  further enhances the bond of contact  18  to contact arm  19  by elevating the melt temperature of the bond interface above that of the copper-silver eutectic melt temperature. The effective bond surface as depicted in FIG. 2 is representative of a contact arm assembly having a serrated bond surface on the contact arm, regardless of whether there is a nickel interface or not. However, as mentioned earlier the nickel interface produces a brazed joint with a higher melt temperature as compared to a brazed joint without a nickel interface.