Abstract:
An electrical contact assembly is disclosed comprising a bracket having at least first and second cavities located on opposite ends of the bracket and an opening extending through the bracket between the first and second cavities. A retention member is arranged to be mounted within the bracket opening with a first portion of the retention member extending outward from the first cavity and a second portion of the retention member extending outward from the second cavity. Each first and second portion terminates in a head end. First and second helix-shaped electrical contacts are positioned about a respective retention member first and second portion and are compressively retained between a respective and associated head end and bracket cavity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electro-mechanical switch contacts and, more specifically, to an electrical contact assembly having helical coil contact structures. 
     2. Discussion of the Related Art 
     Industrial limit switches, such as global limit switches, safety interlocks, safety solenoid interlocks and cable pull limit switches are used in a variety of industrial applications. Limit switches contain switches that are manufactured to suit the particular application. 
     Typically, switches used in these products are designed and manufactured for a particular contact arrangement. Such switches generally contain two or more circuits having one or more normally closed (NC) contacts and/or one or more normally open (NO) contacts. Conventional arrangements typically include one normally open/one normally closed, also called a “single pole” arrangement, or two normally open/two normally closed, also called a “double pole” arrangement. Conventional industrial limit switches often require additional normally closed contacts for added redundancy. Limit switch applications often require at least three positively driven, normally closed contacts along with one normally open monitor circuit. 
     U.S. Pat. No. 6,114,639, assigned to the assignee of the present invention, teaches a configurable switch having a configurable base and a configurable plunger, which together permit the adjustable arrangement of contacts into several electrical switching configurations. Within such a switch, one or more movable contacts are mounted to a plunger that is movable between a first position and a second position. In the first position or the second position, the movable contacts make contact and/or break contact with respective stationary contacts, depending upon the arrangements of the stationary contacts and the movable contacts. The movable contacts shown and typically used in such switch configurations are constructed from conductive material with each including two contact pads spaced at a distance from each other and connected by a contact bracket. The contact pads are each preferably composed of a fine silver disk or other suitable conductive material welded or otherwise attached to the movable contact. The contact bracket of each movable contact is mounted within a respective opening of the movable plunger and biased by a spring, thereby positioning the associated contact pads either apart from a respective stationary contact or engaged to the stationary contact, forming either a make or a break switching arrangement. 
     When such an electro-mechanical switch is used in low energy applications, typically those in the 24 volt, 2-10 milliamp range, other more-conductive materials or contact structures are used in order to enhance continuity and reliability. For example, gold plating may be deposited on a serrated contact, a bifurcated contact can be substituted for the contact pad, or a knife-edge shaped contact using a gold-inlay material substituted for the contact pads. 
     All of these material and structural substitutions, however, over the course of many switching operations, eventually fail in maintaining continuity, causing the low energy switch to fail. For example, gold-plating, deposited typically at a thickness of 0.0001 to 0.0002 inches, tends to wear off over the life of the contact. Serrated contacts attract contaminants from within the switch, which contaminants become trapped as the serrations degrade. Gold-inlay, which is very soft and ductile, also degrades over many mechanical operations of the contacts due to the “hammering” effect imparted by the mechanical closure of the contacts. Gold-plated bifurcated contacts, even though providing a somewhat more reliable contact, due primarily to its plural points of electrical contact, also eventually suffers from wear imparted by the hammering effect and contact bounce. Contact bounce in such switching contacts can lead to equipment malfunctions due to the fast response time of the industrial equipment being controlled. 
     Therefore, reliability of operation is important in such low-energy switching devices since these devices are extensively used in safety relay and safety control applications in industry. A marginally-operating or failed switch can have economic as well as catastrophic consequences to an industrial process if the devices fail to operate correctly. 
     BRIEF SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an improved electrical switch contact. 
     It is also an object of the present invention to provide an electrical switch contact assembly that increases the level of continuity in low-energy switch applications. 
     It is also a further object of the present invention to provide an improved switching contact assembly that minimizes contact bounce. 
     Therefore, there is provided in a first embodiment of the present invention, an electrical contact assembly comprising a bracket having at least a first and a second cavity located on opposite ends of the bracket and an opening extending through the bracket between the first cavity and said second cavity. The electrical contact assembly further includes an electrically conductive retention member mounted within the bracket opening having a first tubular portion extending outward from the first cavity, terminating in a first annular flange. A second tubular portion extends outward from the second cavity, also terminating in a second annular flange. The said first and said second flanges have a diameter greater than their respective and associated retention member first and second portions. A first coiled electrical contact is positioned about and compressively retained on the retention member first portion between the first annular flange and the bracket first cavity. A second coiled electrical contact is also positioned about and compressively retained on the retention member second portion, between the second annular flange and the bracket second cavity. 
     In a second embodiment of the present invention there is provided an electrical contact assembly comprising a bracket having at least a first and a second cavity located on opposite ends of the bracket and an opening extending through the bracket between the said first cavity and said second cavity. An electrically conductive retention member is mounted within the bracket opening and includes a first portion having first and second arms, oriented in a parallel and spaced relationship to the other, extending outward from the first cavity. A second portion, also having first and second arms, oriented in a parallel and spaced relationship to the other, extends outward from the second cavity. Each first and second arm of the first and second portions terminates in a wedge-shaped structure that extends perpendicularly from its associated arm, thereby forming a shoulder member having a width greater than the width of the first and second portions respectively. The electrical contact assembly further includes a first coiled electrical contact positioned about and compressively retained on the retention member first portion, between its associated shoulder member and the bracket first cavity and a second coiled electrical contact positioned about and compressively retained on the retention member second portion, between its associated shoulder member and the bracket second cavity. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Other objects, features, and advantages of the present invention will be apparent from the following description of a preferred embodiment thereof, taken in conjunction with the sheets of drawings, in which: 
     FIG. 1 is a perspective view of a first embodiment of the helical coil switch contact assembly in accordance to the present invention; 
     FIG. 2 is an elevational view of a first embodiment of the helical coil switch contact assembly of the present invention; 
     FIG. 3 is a sectional view taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a perspective view of a second embodiment of the helical coil switch contact assembly in accordance to the present invention; and 
     FIG. 5 is a perspective view of a switch plunger having the movable contacts of the present invention mounted thereon. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1,  2  and  3  show the helical coil switch contact assembly according to one preferred embodiment of the present invention. The assembly is comprised of two barrel-shaped helical coil spring contacts  12  and  12 ′ held in compression about an electrically conductive retention device  15  between first and second head ends  18  and  20  of the retention device  15  and a contact bracket  22 . Each coil spring contact  12  and  12 ′ is composed of a good electrically conductive material. Typically, this material would be copper-based and may also be gold-plated to improve its electrical conductivity. 
     As was previously mentioned, the helical coil switch contact assembly  10  of the present invention may be used to advantage in the configurable switch assembly of U.S. Pat. No. 6,114,639, assigned to the assignee of the present invention and which is incorporated herein by reference. The helical coil switch contact assembly of the present invention can be used in place of the movable contacts shown in the reference. However, it will be well understood by those skilled in the art that the helical coil switch contact assembly  10  of the present invention can also be used in other electrical contact switching applications and is not limited for use with the configurable switching devices shown by U.S. Pat. No. 6,114,639. 
     Each coil of each coil spring contact  12  and  12 ′ provides a contact surface for engagement with an associated stationary contact surface (not shown). As can be seen in FIGS. 1-3, each coil spring contact  12 ,  12 ′ groups together a contact structure having four contact surfaces or contact points that as a group is disposed to mechanically engage an associated stationary contact. It will be appreciated by those skilled in the art that even though the present invention is shown with each coil spring contact  12 ,  12 ′ having four coil turns, any number of individual coil turns may be used to form each coil spring contact  12  and  12 ′ and, therefore, the invention is not limited thereto. 
     Further, each coil spring contact  12 ,  12 ′ is barrel-shaped, that is, the coil turns at the center of each coil spring contact are larger in diameter than the coil turns at each end. This particular form has advantage over a non-barrel or straight coil springs in that it compensates for imperfections and/or lack of flatness with the associated stationary contact (not shown) and also tends to “wipe” across the stationary contact surface upon engagement, thereby minimizing the effects of contact bounce. 
     As can best be seen in FIGS. 2 and 3, each coil spring contact  12 ,  12 ′ is mounted about separate portions of the retention member  15 . Retention member  15  is comprised of a single monolithic pin member having a tubular body  14  that terminates on a first end in an annular head  18  having a diameter greater than the diameter of the tubular body  14 . A second and opposite end of tubular body  14  terminates in a cavity  16  extending from the second end of tubular body  14  longitudinally within its interior. Retention member  15  is composed of a good electrically conductive material such as any copper-based material or the like and may also be gold-plated to improve its electrical conductivity. The retention member  15  may also be made using a standard commercially available rivet fastener of an appropriate dimension that has good electrical conductivity properties. 
     The retention member  15  is held within a molded plastic contact bracket  22 . A through-hole  122  extends through the contact bracket  22  and is arranged to receive therethrough and retain therein, tubular body  14  of retention member  15 . As can be best seen in FIG. 3, coil spring contact  12  is installed on body  14  and held in slight compression between head end  18  and mating surface  124  of contact bracket  22 . Similarly, coil spring contact  12 ′ is installed over body  14  and held in slight compression between head end  20  and a mating surface  125  of the contact bracket  22 . Mating surfaces  124  and  125  are molded at a slight angle with respect to the center plane of each coil spring contact  12 , 12 ′, creating a linear imbalance in each coil spring contact. Over continued actuation of a switch mechanism employing the contact assembly  10  each coil spring contact  12  and  12 ′ will rotate about retention member  15 , thus presenting a new contact surface throughout the life of the contact assembly  10 . 
     The contact bracket  22  further includes cutout portions  30  and  32  formed on the front and back surfaces of the contact bracket  22  respectively and a biasing element retainer cavity  40  formed on the contact bracket  22  top surface. These aforementioned structures are used to advantage in the mounting of the contact assembly  10  within a switch plunger, such as switch plunger  50  in FIG.  5 . Cutouts  30  and  32  are arranged to closely engage with opening  52  of switch plunger  50  and thus restrict unwanted movement of the contact assembly  10  within switch plunger  50 . The contact assembly  10  is mounted within opening  52  preferably with a bias element, such as bias spring  55 , positioned within each opening  52 . Spring  55  urges the contact assembly  10  toward a stop or edge surface of plunger  50  adjacent opening  52 . One end of bias spring  55  is arranged to be set within cavity  40  of the contact bracket  22  with a second and opposite end of biasing spring  55  engaging surface  54  of opening  52 . Each contact assembly  10  is displaceable within the limits allowed by cavity  52  and functions in a manner similar to the movable contacts shown in U.S. Pat. No. 6,114,639. 
     The aforementioned contact assembly  10  of FIGS. 1-3 is assembled by passing coil spring contact  12  over the second end of tubular body  14  to rest against an inside surface of head end  18 . Next, the second end of tubular body  14  is inserted into through-hole  122  of contact bracket  22  until coil spring contact  12  compresses slightly between head end  18  and mating surface  124 . Next coil spring contact  12 ′ is passed over the second end of tubular body  14  until it rests on mating surface  125  of contact bracket  22 . The assembly is completed by cold forming head end  20 . The cold forming is accomplished by the use of a tool (not shown) that includes a bit that is inserted into cavity  16 . Sufficient pressure is applied by the tool to the tubular body cavity  16  to turn aside and slightly outward the peripheral edges of the cavity, thereby permanently forming the structure shown in FIG. 3 as head end  20 . As can be seen in FIG. 3, after the cold forming step, head end  20  forms a generally annular structure having a diameter greater than the tubular body  14  and that slightly compresses coil spring contact  12 ′ between head end  20  and mating surface  125 . 
     Turning now to FIG. 4 of the included drawings, a second preferred embodiment of the helical coil switch contact assembly  10  of the present invention is shown. As will be appreciated by those skilled in the art, the assembly procedure of the helical coil contact assembly  10  of FIG. 1 requires an ordered step process in its assembly. The helical coil switch contact  10  of this second preferred embodiment simplifies its assembly. 
     In this second embodiment of the present invention, the retention member  25  is comprised of a flat, generally rectangular shaped, stamped body  24  that is arranged to be accepted within through-hole  122  of contact bracket  22 . The flat body  24  has a transverse dimension or width that is substantially the same dimension as the diameter of through-hole  122 . As will be appreciated by those skilled in the art, with the flat body width at substantially the same dimension as the diameter of the through-hole  122 , the longitudinal center axis of flat body  24  will be located along and concentric with the center axis of through-hole  122  when the flat body  24  is installed therein. As will also be appreciated, the concentric association between the longitudinal center axis of flat body  24  and the center axis of through-hole  122  is maintained as the flat body  24  is rotated along its longitudinal axis during the operation of the contact assembly  10 . 
     With renewed reference to FIG. 4, the portions of the flat body  24  extending outward from mating surfaces  124  and  125  are formed into two sets of double arms  70  and  80  respectively. Arm-set  70  is comprised of arms  71  and  72  extending from a first end in a spaced and parallel relationship to each other from flat body  24 . A second and opposite end of each arm  71  and  72  terminates in a wedge-shaped head structure  73  and  74  respectively. Coil spring contact  12  is supported by arms  71  and  72  and held in slight compression between mating surface  124  of contact bracket  22  and shoulders  75 ,  76  of head structures  73 ,  74  respectively. Arm-set  80  includes the same structures described for arm-set  70  and supports and holds coil spring contact  12 ′ in the same manner as explained for coil spring contact  12 . 
     The coil spring contact assembly  10  is assembled in this second embodiment by first installing the retention member  25  flat body  24  within through-hole  122  of the contact bracket  22 . Coil spring contact  12  is installed on the retention member  25  by deflecting arms  71  and  72  toward each other, allowing coil spring contact  12  to be passed over head structures  72  and  73  until a first end of the contact  12  engages mating surface  124 . The coil spring contact  12  is compressed toward contact bracket  22  and arms  71  and  72  are allowed to return to their un-deflected positions. When the compression force is released from coil spring contact  12 , the contact attempts to return to its uncompressed state, however, it is held in slight compression by the engagement of the contact  12  second end with shoulders  75  and  76  of head structures  73  and  74 . Coil spring contact  12 ′ is assembled on arm-set  80  in the same manner as previously explained for coil spring contact  12 . The flat body  24  and arm-sets  70  and  80 , forming the retention member  25  of this second embodiment, are constructed as an integral structure from a good electrically conductive material such as any copper-based material or the like and gold-plated to improve its electrical conductivity. 
     The present invention has been described with particular reference to the preferred embodiments thereof. It will be obvious that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.