Patent Publication Number: US-2009235519-A1

Title: Method for making electrical contact with redundant paths

Description:
BACKGROUND 
     This patent is a divisional patent of U.S. patent application Ser. No. 11/681,583, filed on Mar. 2, 2007, now U.S. Pat. No. ______, Electrical Contact with Redundant Paths, which is hereby incorporated by reference in its entirety. 
     The field of the invention is electrical contacts and electrical switches that use such contacts. 
     Electrical switches have now been in general use for over a hundred years, and have become reliable and commonplace for household and other uses. In applications where higher power, current or voltage is in use, however, some problems of switches have also become commonplace. These problems can include wear and erosion of switch contacts, usually when the switches and contacts are in use for many thousands of operations over an extended period of time. 
     Erosive wear on contacts may occur when a dc switch is being either opened or closed, but is generally most severe when the switch is being opened and the switch contacts draw apart. When the contacts draw apart, the reduction in contact pressure and resulting increase in resistance between the contact surfaces leads to a significant increase in temperature in the zone of contact. Localized melting of the contact material may occur, and an arc may form between the contacts. An arc may cause erosion of the contacts, and eventually lead to pitting, excessive wear, and even mechanical failure. Another possibility for erosive wear occurs when the switch does not reliably hold the contacts in position and “chatter” or high frequency movement between the contacts causes momentary opening and additional arcing. 
     As a result of these problems, switches may be designed to lessen the arcing and chatter that causes erosive wear and failure. Thus, switch contacts may be made from sturdy and reliable designs with high copper content for minimum resistance and maximum reduction of localized heat. Contacts may be alloyed with silver or other metals to minimize the effects of arcing. Contacts may also be spring-loaded, i.e., kept in contact by a spring loaded in compression as part of the contacts circuit. 
     An example of efforts to minimize wear and pitting is disclosed in U.S. Pat. No. 5,221,816. This patent discloses a plunger switch using a bent wiper contact that is held in compression between a common terminal and a normally closed (NC) or a normally open (NO) terminal. The wiper is in the general shape of a U or V that has been bent even further wide open. The outer legs are mounted on or insert molded into a plastic retainer that moves with the plunger. However, in use, these contacts are easily eroded by arcing and fail in service. Without being bound to any particular theory, it is believed that the wipers may be unevenly loaded between the contacts. It is also possible that their relatively thin, two-dimensional nature does not provide enough contact material compared to what is needed to survive erosive wear. 
     In another example, U.S. Pat. No. 7,060,917 discloses a plunger switch that uses a hollow bushing or plate to make or break electrical contact between upper NO contacts and lower NC contacts. Internal springs urge the plunger upward for making contact with the NO contacts while the exterior of the switch is molded to include a hook-engaging portion so that a hook may be used to keep the switch actuated (closed). Engagement of the internal portions of the contacts depends on proper assembly and the alignment and force of the springs. Over time, it appears that this switch also will be subject to uneven wear and arcing as the springs relax and as the plate is deformed. 
     What is needed is a switch with contacts that are resistant to arcing and with sufficient heft and mass to resist nominal pitting or wear resulting from operation of the switch. The contacts should have low resistance to current and voltage and high reliability. 
     SUMMARY 
     Embodiments of the present invention provide such contacts and a switch that uses such contacts. One embodiment is a switch with a plurality of contactors. The switch includes a switch housing, a fixed contactor mounted to the switch housing, the fixed contactor comprising at least one each of NC, NO and common terminals, a plunger slidably mounted to the switch housing, and a movable contactor housing connected to the plunger, the moveable contactor housing further including a plurality of contactors, each contactor having a conductive housing, first and second metallic conductors captured at an angle within the conductive housing, a portion of each of the first and second conductors extending beyond the housing, the first and second conductors joined to a spring having first and second ends, the conductive housing and the contactors forming redundant electrical paths, wherein a position of the plunger determines connections made by the contactors between the terminals. 
     Another embodiment is a switch with a plurality of contactors. The switch includes a switch housing, a fixed contactor mounted to the switch housing, the fixed contactor comprising at least one each NC, NO and common terminals, a plunger slidably mounted to the switch housing, and a movable contactor housing connected to the plunger, the contactor housing further including a plurality of contactors, each contactor having a conductive housing, first and second metallic conductors captured within the conductive housing and joined to first and second ends of a spring inside the conductive housing, the conductors at an angle to a longitudinal axis of the conductive housing and a portion of each of the first and second conductors extending beyond the conductive housing, the conductive housing and the conductors forming redundant electrical paths, wherein a position of the plunger determines connections made by the contactors between the terminals. 
     Another embodiment is an electrical contactor for use in a switch. The electrical conductor includes a conductive housing, a spring with first and second ends within the housing, and it also includes first and second metallic conductors captured within the housing and partly extending beyond the housing, the conductors at an angle to a longitudinal axis of the conductive housing and joined to the first and second ends of the spring, the housing and the conductors forming redundant electrical paths. The spring urges the first and second metallic conductors against the housing. 
     Another embodiment is a method of making an electrical contactor useful in switches. The method includes steps of forming a metallic spring having first and second ends, joining first and second metallic conductors to the first and second ends, the metallic conductors optionally comprising an internal mounting tang. The method also includes inserting the spring and the first and second metallic conductors into a conductive housing; and deforming the housing to retain the spring and the conductors within the housing, wherein the first and second metallic conductors are retained at an angle to a longitudinal axis of the conductive housing and wherein the conductors partly extend beyond the housing. 
     Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         FIG. 1  depicts an exploded view of a contactor with internal conductors at an angle; 
         FIG. 2  depicts a cross-section of the contactor of  FIG. 1 ; 
         FIG. 3  depicts a cross-sectional view of an alternate contact assembly; 
         FIG. 4  depicts a flow chart for a method of assembling a contactor; 
         FIG. 5  depicts an exploded view of a switch incorporating contactors with angled internal conductors; 
         FIG. 6  depicts details of detents and retainers for a two-position plunger switch; 
         FIG. 7  depicts a switch housing and a fixed contacts useful in switches and plunger switches; 
         FIGS. 8-10  depict cross-sectional views of the operation of a contactor with internal angled conductors in a plunger switch; 
         FIGS. 11-12  depict perspective view of contacts useful for switches using contactors with internal conductors at an angle to each other; and 
         FIG. 13  depicts a flow chart for a method of assembly of a switch using contactor embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     The Contactors 
     Contactors with internal conductors at an angle to the conductor help to avoid the adverse effects of arcing and erosion in two ways. The spring loading of the contacts helps to insure even, uniform contact between the contactor and the terminals, thus minimizing contact resistance and subsequent heat build-up. In addition, the angled conductors provide two electrical paths between the common and the normally open and normally closed (NO/NC) portions of the contactor. The first path is that provided by the contacts and the spring. The second path is that between the conductors and the conductive housing. Keeping the contacts always canted or at an angle within the housing insures there are always two points or areas of contact between each of the contact ends and the housing that contains the contact ends. Finally, the bullet-shaped contacts or conductors preferably have working ends that are roughly hemispherical. This shape provides more metal in a stronger shape than the flat shapes previously used. Even if erosive arcing occurs, there is much more metal on the contact that must be worn away before contacts fail. Accordingly, our tests have shown that the contactors with internal conductors at an angle provide much longer life than previous products. 
     Tests have shown that under a 12 Volt, 4 Amp highly inductive dc load, switches with contactors having internal angled contacts as described herein provide a life of more than 100,000 make/break cycles. Contacts described in the prior art failed at 20,000 cycles or less. 
     Contactors with internal conductors at an angle, one embodiment of which is depicted in  FIGS. 1-2 , are useful in a variety of switches and switching applications. This embodiment of a contactor  10  includes two bullet-shaped contacts  12 , each with a void or mounting space  14 . The mounting space  14  is intended for ease of assembly with ends of a spring  16 . The mounting space is on the periphery of the contact, and thus is off the center of a longitudinal axis A of both the contacts and a conductive housing  17 . The radial distance from the center of the contact housing  17  to the ends of the spring is represented by distance C. The overall shape of the contacts is roughly the shape of a rounded bullet, with a generally hemispherical end portion, rather than a more sharply-pointed bullet shape. Without being bound to any particular theory, it is believed that this shape helps to reduce frictional forces when sliding the bullet-shaped contact from one stationary terminal to another, and to maximize localized contact force/pressure once electrical contact is made. The hemispherical shape further helps to prolong the life of the contact by providing physical separation distance from the working, or steady state electrical load carrying surface near the axis and the arcing zone, or first/last-point-of-contact as the contact makes or breaks from the terminal. 
     Contact  12  is preferably made by stamping a copper alloy, including a step of creating a mounting space or void. If the contact is made by stamping, rather than by machining, the metal previously occupying the space will be forced inwards, creating a tang  14   a.  The tang  14   a  will react the force exerted on contact  12 , urging contact  12  away from housing  17 , on both sides of the housing. Housing  17  is preferably brass or other cost-effective, conductive alloy. 
     Because the void and the tang are on only a small portion of the periphery of the contact, the force applied to the contact by the spring will necessarily be off center, i.e., a distance away from longitudinal axis A. This force is applied to both contacts, and thus the force applied to both contacts will be off-center. The force of the contacts applied to one side of the housing is represented by Fc 1 , and on the other side by Fc 2 . The distance between the points of contact on one side, Fc 1 , create a coupling force that is reacted on the opposite side. The longitudinal or axial force exerted by the spring against the crimped ends of the housing is equal to the force required to retain the contacts, and is represented as Ft on either end. Accordingly, the spring will cock or cant the contacts off the longitudinal axis A. Thus, when the contactor  10  is assembled, contacts  12  will be at an angle to housing  17 , as shown in  FIG. 1  by angle D from horizontal. 
     The situation is depicted in  FIG. 2 , a cross-sectional view of the angled contacts and contactor of  FIG. 1 . As shown in  FIG. 2 , compression spring  16  applies an off-axis force to the two bullet-shaped contacts  12 . Two coils of each end of spring  16  are assembled near the tang  14   a  and void or mounting space  14  that were made in forming contacts  12 . The coils may be simply inserted into the spaces if the spaces are sufficiently tight to create a small interference fit with the coils. Alternately, the coils may be permanently assembled to the contacts, or the tangs of the contacts, by a method such as welding, brazing, or soldering. The contacts are thus biased off center, i.e., the contacts are urged off-axis by the spring and thus are canted or cocked within the housing. The outer surface of housing  17  normally is smooth, so that the contact may be press fit into the moveable contacts plunger housing. It may instead be useful to include transverse retainers or other features (not shown) on an exterior of housing  17  in order to better retain the contacts  10  in a housing that holds the contacts. Such retainers will prevent longitudinal (axial) movement of the contact and will also prevent rotation of the contact, thus holding the contact more firmly in place. 
     Accordingly, the spring-loaded contacts will be in electrical contact with the housing in at least two places, as shown by the circled portions B in  FIG. 2 . The intended use of contactor  10  is to provide electrical conductivity from one contact  12  to the other. Accordingly, the angled assembly of the contacts within conductive housing  17  provides at least two conductive paths between the contacts. The first path is from one contact through the spring to the other contact. The second path is from one contact, to and through the housing, to the other contact. Thus, redundant paths are formed through the contacts. 
     The purpose of the contacts is to prevent electrical arcing or wear to the greatest extent possible. It is believed that the force of the spring and the moveable nature of the contacts will urge the faces of the contacts into physical and electrical contact with other devices, such as the terminals discussed below, common, normally open, and normally closed terminals. The common terminal is essentially a continuous bar of copper or other conductor, and electrical contact with the common terminal does not change with the switch and plunger are activated or deactivated. Accordingly, area  12   a,  the central area of contact  10 , and on the left side, is expected to provide the working surface for a contact used against a common terminal. Following the usual terminology, the contact in  FIG. 2  is used in a switch that is pulled up to activate the switch and pushed back down to deactivate. An unactivated switch in the “normal” state will have “normally closed” terminals whose last electrical contact is with the lower contact portion, area  12   c,  and will thus tend to have arcing on this side of the contact when the switch is actuated. An activated switch in the “on” state that is then de-actuated will have its last electrical contact with the upper portion of contact  10 , area  12   b,  and will have arcing or wear in that area. 
     The functional endurance of the angled-bullet contact against the effects of arcing is enhanced by the fact that the working, or steady-state electrical load carrying surface  12   a  is separated by a distance from the arcing zone during contact make or break areas  12   b,    12   c.  Thus, even though erosion occurs in the arcing zones, it will not affect the load-carrying ability of the contact until sufficient erosion or pitting has occurred to reduce or otherwise damage the load carrying zone  12   a.    
     Compression springs, made of stainless steel, as shown in  FIG. 2 , are preferred but are not the only way to cant the contacts off-center. For example, a leaf spring, reliably mounted to the contacts, will also work to insure that contacts  12  are always canted off-center, maintaining electrical contact via two paths. An example of an angle contactor with a leaf spring  13  is depicted in  FIG. 3 , maintaining redundant contact paths through the contactor, and with all other elements of the contactor being the same. While stainless steel is preferred, other conductive and reliable materials, such as alloys of steel, copper or aluminum, may also be used. In a similar manner, cost effective brass, such as alloy C230, is preferred for the housing material, but other reliable, conductive, and cost-effective materials may also be used. C110 copper (alloy 11000) is preferred for the bullet-shaped contacts, but other suitable conductive materials may be used, such as other brass, copper or even aluminum or silver alloys. 
     A canted, off-center electrical contactor, as described above, may be made by many processes. One such process is described in the flowchart of  FIG. 4 . Two contacts, preferably bullet-shaped, are stamped  40  from a conductive alloy as described above. The stamping may include two or more steps, such as a first step to form a contact that is rounded on one end and open on the other, and a second step to create a mounting area and tang. Other stamping or forming processes may be used. The spring that will be assembled to the two contacts is also formed  41 , preferably from a stainless steel alloy. Coiled springs may be wound and shaped on a mandrel or otherwise cold-formed. If leaf springs are used, they may be stamped or otherwise formed, usually from flat (rectangular) stock rather than the round wire typically used for torsion springs. A contact housing is also formed  42  from a conductive alloy, such as brass or copper. The conductive housing for the assembled contacts is preferably formed by stamping. One of the contacts is then inserted  43  into the contact housing. One end of the housing is preferably pre-crimped, i.e., crimped before any other parts are assembled to the housing. The first contact is preferably inserted into the non-pre-crimped end and pushed to the crimped end, which prevents the contact from leaving the housing. As noted, the end-coils of the spring may be inserted with an interference fit into attachment spaces in the contacts. It is preferable to permanently attach the contacts to the spring by brazing, soldering, or welding. Since these are electrical contacts, and they may become warm, welding is preferred. 
     The spring is then inserted  44  into the contact housing and, optionally, conductive grease is added to the contact assembly. An example of a conductive grease is “Silver Filled Conductive Grease,” from SPI Supplies, West Chester, Pa., USA. Another grease that may be used is 789 DM Grease, available from Nye Lubricants, Fairhaven, Mass., USA. The other contact is then inserted  45  into the contact assembly, preferably by joining the other contact firmly to the spring. After insertion, the other end of the housing is also crimped  46  to capture the contacts. After assembly, it is prudent to test  47  the contactor for secure assembly and retention of the contacts within the housing. Finally, a test is conducted  48  to insure the continuity of the electrical circuit through the contactor. As will be readily apparent to those having skill in the art, applying a voltage or current to several areas, and then checking for voltage drops across the possible paths will determine the relative resistance of the paths through the contactor. 
     Switches Using the Contactors 
     The contactors as described above have a great variety of applications, due to their reliable mechanical construction and thus their resulting electrical reliability. The contactors may be used in a great variety of switch applications. One application is a plunger switch in which a single contactor is used to connect a common terminal, sequentially, with a normally-closed contact and a normally-open contact. Such a plunger switch is typically used to operate several electrical devices and thus controls two or three, or even more circuits. 
     A typical plunger switch using contactors with internal conductors at an angle is depicted in  FIG. 5 . The switch  50  includes one or more contactors  51 , the contactors press fit or molded into a moveable contactor housing  52  attached to a hand-operated plunger  56 . The contactors make or break contact between terminals that are molded into a fixed contactor  53  mounted to a switch housing  54 . A clip  55  may be used to retain the plunger in a fixed position with respect to the fixed contactor and housing, as will be described below. 
     Moveable contact housing  52  is preferably made by press-fitting contactors  51  into a suitable non-conductive material, such as a phenolic, nylon, ABS, polypropylene, polycarbonate, or other temperature resistant material. In the embodiment depicted in  FIGS. 5-6 , the moveable contact housing  52  has female snap-fit connections  52   a  for permanent assembly to male snap-fit connections  56   a  of plunger  56 . Moveable contact housing  52  also has two bosses  52   b  for mounting return springs between housing  52  and the upper, inner surface of switch housing  54 . Moveable contact housing  52  is intended for controlled movement of contactors  51  among the fixed contacts within fixed contactor housing  53 . 
     When assembling the switch, fixed contact assembly  53  is preferably sonic welded into switch housing  54 . In  FIG. 7 , this is accomplished with female portions of the sonic weld joint  53   f  on the fixed contact assembly  53  and male portions of the sonic weld joint  54   a  on the housing assembly. Switch housing  54  itself is preferably mounted, by other mounting clips  54   b,  into an instrument panel or control module using the switches. Fixed contact assembly preferably includes a groove  53   d  for sealing against the switch housing. The assembled switch may include an O-ring  53   e  in the groove to prevent ingress of humidity, moisture or fluids. 
     The contacts for the fixed contact assembly include the external portions  71 , shown at the bottom of fixed contactor housing  53  of  FIG. 7 , as well as the internal portions  72 , shown in the upper portion of fixed contactor housing  53 . The external portions typically have apertures  73  for further connections. In this embodiment, the fixed contacts include normally open (NO) contacts  53   a,  normally closed (NC) contacts  53   b,  and common contacts  53   c.  In this embodiment, the switch is in the normal (off) or non-actuated position when the plunger is pushed inwardly. Contactors  51  are normally in contact with the common contacts at all times. When the plunger is pushed inwardly, the switch is in the normal or off, non-actuated position, and the contactors are not in electrical contact with the NO contacts and the contactors are in electrical contact with the NC contacts. When the plunger is pulled out, the switch is deemed actuated or on. Electrical contact is then provided between the common terminals and the NO terminals, and is interrupted between the common terminals and the NC terminals. 
     Switch Operation 
     Operation of the contactors with internal angled conductors and the plunger switches is now described in  FIGS. 8-10 . In  FIG. 8 , the switch is in the off or un-actuated position. Plunger  81  is in a low position, and is in contact with the upper portions  82   a  of the switch housing  82 . Plunger  81  has been assembled to moveable contactor housing  83 , which includes one or more contactors  88 . Switch housing  82  has been assembled to fixed contacts housing  84 . Fixed contacts housing  84  includes at least one common terminal  84 , one NO terminal  86 , and one NC terminal  87 .  FIG. 8  depicts electrical contact between the right side of contactor  88  and common terminal  85 , and also depicts electrical contact between the left side of contactor  88  and NC terminal  87 . Although not apparent from  FIG. 8 , the remaining terminal, NO terminal  86  is not in electrical contact with common terminal  85 . 
     In  FIG. 9 , actuator  81  has been pulled partially upward or out, and the switch is in transition from off or a non-actuated position, to on or an actuated position. In this position, contactor  88  remains in contact with common terminal  85 , but the left-hand side of contactor  88  is no longer in electrical contact with NC terminal  87 , instead contacting a void  89  above NC terminal  87 . In  FIG. 10 , the actuator or plunger  81  has now been fully pulled out or actuated. The right-hand side of angled actuator  88  is still in contact with common terminal  85 . The left-hand side of contactor  88  is now in electrical contact with the upper portion  86   a  of NO terminal  86 . Upper portion  86   a  is nested above NC contact  87 . The horizontal distance from the common terminal  85  to the upper portion of NC contact  87  is equal to the horizontal distance from the common terminal  85  to the NO contact  86 . The NC and NO contacts are displaced vertically to allow for proper connections and operation. The contacts discussed in  FIGS. 8-10  are shown in perspective in  FIG. 11 . NO terminal  86  has a central void  86   b  into which NC contact  87  may nest. This allows the vertical separation of the upper electrical contacts of terminals  86  and  87 . 
       FIG. 12  depicts another switch assembly embodiment. In the embodiment of  FIG. 12 , plunger  91  is assembled to moveable contact housing  93  for switch actuation. Clip  99  may be used to secure the plunger-moveable contact housing assembly with respect to the switch housing (not shown). The plunger  91  includes an lower detent or slot  91   a  whose width is about the same as the width of the wire that makes clip  99 . When the plunger is secured in the non-actuated position, the fit is thus firm. The upper detent or slot  91   b  is significantly wider than the clip wire. If the plunger is secured in the upper slot, for actuation, a user may pull the plunger further upward, thus allowing a contactor to make contact with terminal  98  for momentary operation of a device. 
     The fixed contacts are mounted in the fixed contact housing  94 . Contacts  97  are NO, that is, they are open in the off position but closed as shown in the actuated position. Contact  98  is also NO, but in this embodiment, is also normally closed in the actuated state. Pulling the plunger further upward maintains the closed contact with NO switches  97  while allowing a momentary contact for NO switch  98 . Contacts  95  are common and contacts  96  are NC. Moveable contact housing  93  includes spring bosses  93   a  for springs  92 . 
     In automotive or agricultural applications, this configuration may be useful for momentary activation of a power take off (PTO) device, such as a shaft, the shaft operating an auger, a winch or other power device, which may be operated by an electric clutch. The operator may wish to momentarily rotate the auger or winch, perhaps for an easier hook-up. In other applications, such as for chemical or food processing applications, the operator may wish to jog a pump or other device, perhaps to check its operation or to clear the device. 
     There are many ways of preparing contact and switches for the embodiments herein described. One way of preparing such switches and contacts is disclosed in  FIG. 13 . A first step  101  may be to provide the contactors with internal conductors at an angle as described above, and to clean them, if necessary, before assembly. The contactors are then desirably press fit  102  or otherwise assembled into a moveable contactor housing. 
     After the moveable contactor housing is assembled with the contactors, the housing is assembled  103  to the plunger. The fixed contacts, such as NC, NO, and common terminals, are then prepared, as by cleaning, and are press fit or otherwise assembled  104  into a fixed contact assembly. The fixed contact assembly and plunger are then assembled  105 ,  106  to the switch housing. This completes assembly of the main components of the switch. The switch may then be tested  107  for electrical continuity or leaks, and may be tested  108  for detent positions if detents have been molded into the plunger and a retainer clip is provided. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.