Abstract:
An electrical switch provides a center contact moving between two outer contacts, at least one of which is mounted on a lever. In the event of a contact weld forming between the center contact and the lever mounted contact, unison motion of the lever mounted contact following the center contact during switching causes a strong shear force tending to break the weld. In one embodiment, a fulcrum wall striking the lever of the lever mounted contact causes increased angulation between the contacts so that unison motion rotates and peels the contacts apart.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 61/506,869 filed Jul. 12, 2011 hereby incorporated by reference in its entirety 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an electrical switch and in particular to an electrical switch for providing a mechanism to break contact welds caused by current surges. 
       BACKGROUND OF THE INVENTION 
       [0003]    Modern appliances such as washing machines and dryers may provide for lid or door switches detecting when the appliance lid or door is open. In many cases, these lid switches serve to help protect the consumer from machinery moving inside the appliance by turning off an appliance motor or otherwise deactivating moving elements of the appliance. Such switches may include a stationary contact disconnecting from a movable contact driven by movement of a switch operator when the lid or door is open. The two contacts may control power to a motor or brake or the like. 
         [0004]    Electrical switches normally provide a snap action mechanism so that the contacts of the switch open and close rapidly (to reduce arcing) independent of the speed of movement of the switch operator. One method of providing a snap action is a so-called “over-center spring” attached to a lever holding the movable contact. When the force applied on the lever by the over-center spring crosses the lever pivot (over-center), the torque on the lever rapidly switches direction causing the switch to “snap” between positions. The switch operator normally applies forces to the over-center spring rather than directly to the contact, so that the operator does not interfere with the snap action. 
         [0005]    Voltage surges in the power line connected to an appliance can cause high currents through the contacts in switches of this type, welding the contacts together. In such cases, the force of the over-center spring on the welded contact may be insufficient to break the weld, with the result that the switch cannot be turned off. One solution is to increase the strength of the over-center spring, but this may be undesirable to the extent that it increases the operating force necessary to activate the switch, for example, preventing ready door closure. Increasing the strength of the over-center spring may also undesirably tax the other components of the switch. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a switch suitable for controlling high amperage loads in an appliance that better resists contact welding by allowing movement of the normally stationary contact. When contact welding occurs, unison movement of the welded contacts on their support arms induces a strong shear force across the face of the contacts tending to break any welds. Proper positioning of the support arms provides a high mechanical advantage in generating this shear force reducing the need to increase the strength of the over-center spring and preventing the weld-breaking force from being dissipated in flexure of the arms. An induced relative angulation of the contacts when moving in unison motion past a certain point (which indicates a continued weld) may optionally provide an additional prying leverage that further helps to separate the contacts. 
         [0007]    Specifically, the present invention provides an electrical switch having a housing holding a first and second contact, the second contact supported at a distal end of a first lever to move under the influence of a switch operator between a first position separated from the first contact and a second position contacting the first contact. The first contact is supported at a distal end of a second lever which is movably supported by the housing to follow the second contact moving from the second position to the first position when the first and second contacts are connected by a welding. Proximal ends of the first and second levers are positioned in the housing to provide a shear force between a first and second contact when the first contact follows the second contact from the second position to the first position by a welding at the second position. 
         [0008]    It is thus a feature of at least one embodiment of the invention to generate strong shear forces between the contacts when a welding occurs to separate the contacts. 
         [0009]    The second lever may be a flexible metal strip affixed to the housing at the proximal end of the second lever and movement from the second position to the first position may flex the metal strip to provide resilient resistance of the second lever to follow the second contact. 
         [0010]    It is thus a feature of at least one embodiment of the invention to provide a simple method of generating shear forces between the contacts by flexibly mounting a normally stationary contact. 
         [0011]    The housing may include a fulcrum wall contacting the second lever at a point between ends of the second lever, when the first contact follows the second contact from the second position to the first position by a welding at the second position, to tip the first contact with respect to the second contact by bending the second lever at the fulcrum wall. 
         [0012]    It is thus a feature of at least one embodiment of the invention to increase a relative angular rotation of the contacts if the weld is not broken by an initial shear force to produce a prying action. 
         [0013]    The first and second contacts may be outwardly convex to contact at a point. 
         [0014]    It is thus an object of the invention to provide a contact surface amenable to a rolling, prying action for separating welded contacts. 
         [0015]    The first and second levers may pivot about effective first and second pivot points and the first and second levers extend from the effective first and second pivot points to be substantially parallel to a plane connecting the first and second pivot points. 
         [0016]    It is thus a feature of at least one embodiment of the invention to maximize the mechanical advantage acting on the over-center spring force to produce a shear force, thereby avoiding the need to increase the strength of the over-center spring and thus the actuation force of the switch. It is a feature of at least one embodiment of the invention to prevent the weld separating forces from being dissipated in flexure of the levers. By generating the shear force along the lever extensions, compressive or tensile forces are primarily generated in lieu of bending forces. 
         [0017]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a perspective view of one embodiment of a switch per the present invention showing a switch housing having a switch operator extending therefrom for actuation of the switch and having electrical conductors for connecting the switch to other elements such as a motor of an appliance; 
           [0019]      FIG. 2  is an elevational cross-section along line  2 - 2  in  FIG. 1  showing a mechanical linkage between the switch operator and an over-center spring mechanism for moving a center contact between two outer flanking contacts each connected to different of the conductors of  FIG. 1 , where the lower flanking contact is mounted to be substantially stationary and the upper flanking contact is mounted on a flexible support arm, the switch being shown in a first “safe” state with the operator released; 
           [0020]      FIG. 3  is a fragmentary detail of  FIG. 2  with the switch in a first position providing a safe state with the center contact contacting the lower contact and disconnected from the upper contact which is flexed downward slightly on its support lever; 
           [0021]      FIG. 4  is a figure similar to that of  FIG. 3  with the switch in a second position providing an active state with the operator activated to move the center contact upward to contact the upper contact, the upper contact flexing upward to substantially center itself on the center contact as shown in a juxtaposed detail; 
           [0022]      FIG. 5  is a figure similar to  FIGS. 3 and 4  showing a release of the operator after a welding of the contacts in the second position of  FIG. 4  showing a unison motion of the upper contact and center contact, the upper contact following the center contact downward creating a strong shear force across the contact interface shown in a juxtaposed detail and further showing a bending of the lever holding the upper contact about a fulcrum point on the housing to create a rotation of the upper contact prying the upper and center contacts apart; 
           [0023]      FIG. 6  an enlarged version of the detail of  FIG. 4  showing a point contact between rounded faces of the upper and center contacts connected by a weld; 
           [0024]      FIG. 7  is an enlarged version of the detail of  FIG. 5  showing a rolling of the two contacts about a new contact point creating a leverage breaking of the weld; 
           [0025]      FIG. 8  is a chart showing a shear separation (S) of the contacts as a function of unison movement (D) of the contacts from the second to first position showing the high degree of mechanical advantage obtained when contact between upper and center contacts and the levers holding the upper and center contacts are substantially aligned along a line between pivot points of the levers in the second position; 
           [0026]      FIG. 9  is a chart showing a relative rolling of the contacts with respect to each other before and after the lever of the upper contact strikes a fulcrum point on the housing such as promotes a peeling apart of the contacts. 
       
    
    
       [0027]    Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Referring now to  FIG. 1 , an electrical switch  10  per the present invention may provide for a housing  12 , for example, of an insulating thermoplastic material. The housing  12  may expose therethrough a pushbutton operator  14  that may be pressed inward toward the housing  12  to activate an internal set of contacts to be described. Conductive leads  16  may extend from the housing  12  to communicate with external electrical circuits, for example motors or actuators of a household appliance (not shown). 
         [0029]    Referring now to  FIG. 2 , the electrical switch  10  may contain an upper contact  18 , a center contact  20 , and lower contact  22  arranged to provide a single pole, double throw electrical switch with the upper contact  18  and lower contact  22  generally flanking the center contact  20 . The center contact  20  may move between the upper contact  18  and lower contact  20  to selectively and alternatively connect to only one of the upper contact  18  and lower contact  22 . 
         [0030]    The center contact  20  may be supported on a relatively rigid conductive lever  24  attached at a knife edge pivot point  26  to a conductive support bracket  28 , the latter communicating with one of the conductive leads  16  and pivot point  26  allowing electrical conduction from the conductive lever  24  to the conductive lead  16 . By pivoting the lever  24  around the pivot point  26 , the lever  24  may be moved upward and downward so that the center contact  20  alternately connects electrically to upper contact  18  and lower contact  22 . 
         [0031]    A helical over-center spring  30  attaches to a center portion of the lever  24  and extends away from the center contact  20  to a support post  32  on the housing  12  to provide a force on the lever  24  tending to engage the lever  24  and support bracket  28  at the pivot point  26 . 
         [0032]    The operator  14 , when pressed inward (into the page depicting  FIG. 2 ), presses against a wedge plate  34  attached at one end of a rocker arm  36  to rotate the rocker arm  36  counterclockwise about a center positioned fulcrum  38 . An opposite end of the rocker arm  36  provides an upwardly extending finger  40  which deflects a center region of the helical over-center spring  30  upward to change its line of action  42  with respect to the pivot point  26 . The line of action  42  represents a force vector asserted on the lever  24  by the helical over-center spring  30 . As will be discussed in greater detail below, when the line of action  42  is above the pivot point  26 , the lever  24  will snap rapidly upward and when the line of action  42  is below the pivot point  26 , lever  24  will snap rapidly downward 
         [0033]    Referring still to  FIG. 2 , the upper contact  18  and lower contact  22  are each generally supported on a cantilevered conductive metal strip to one of the conductive leads  16 . Specifically, the upper contact  18  is supported on a lower distal end of flexible metal lever  46  and the lower contact  22  is supported on an upper distal end of a substantially rigid conductive metal strip  44 . Generally the strip  44  and lever  46  extend from their respective contacts  18  and  22  in the opposite direction as the lever  24 . 
         [0034]    Referring now to  FIGS. 2 and 3 , when the operator  14  is released and the rocker arm  36  rotates to its full clockwise position, the line of action  42  of the helical over-center spring  30  moves below the pivot point  26  and a lower surface of the center contact  20  contacts an upper surface of the lower contact  22  at a first position as pulled together by a torsional vector component of the force along the line of action  42  of the over-center spring  30  the force pulling downward on lever  24 . An upper surface of contact  20  is separated from a lower surface of the upper contact  18  so that a circuit is “made” between contacts  20  and  22  and “broken” between contacts  20  and  18 . 
         [0035]    Referring to  FIGS. 2 and 4 , when the operator  14  is compressed, the rocker arm  36  rotates to a full counterclockwise position pressing upward on the helical over-center spring  30  to move the line of action  42  above the pivot point  26  pulling upward on lever  24  so that an upper surface of contact  22  contacts the lower surface of contact  18  at a second position. Under the force of contact  22 , flexible lever  46  is moved from a fulcrum wall  51  contacting a center region of the lower surface of lever  46  allowing the lever  46  to straighten as it rotates about an effective pivot point  50 . The effective pivot point  50  is displaced slightly upward from the attachment of the lever  46  to the housing  12  as a result of the distributed flexure of the lever  46  and as differs from the focused pivoting of lever  24  about a discrete pivot point  26 . 
         [0036]    The opposed surfaces of the upper contact  18  and center contact  20  are outwardly rounded or convex to contact at a contact point  52  generally along a centerline  54  between pivot point  26  and pivot point  50 . The contact point  52  is approximately centered on centerlines  56  normal to the face of the contacts  18  and  22  and the levers  24  and  46  are roughly parallel to the centerline  54  as will be discussed below. 
         [0037]    Referring now to  FIGS. 2 and 5 , if a contact weld forms at contact point  52  when the switch  10  is in the second position, at the time when the operator  14  is released, upper contact  18  will follow center contact  20  in unison motion (D) as center contact  20  descends under the force of the over-center spring  30 . Specifically, when the operator  14  is released, the rocker arm  36  rotates to the full clockwise position releasing the over-center spring  30  so that the center of action  42  drops below the pivot point  26 . The unison motion of the center contact  20  and the upper contact  18  causes the upper contact  18  to try to slide in a direction perpendicular to the center normals  56  by a shear displacement  60  (S) as a result of the relative stiffness in compression of the lever  46  and  24  and the relative locations of pivot point  26  and pivot point  50 . This creates a strong sheer force at the interface between the upper contact  18  and the lower contact  20  generally along a plane tangential to their contact point  52 . 
         [0038]    Referring momentarily to  FIG. 8 , the amount of shear displacement  60  as a function  66  of unison motion (D) of the upper contact  18  and center contact  20  changes very slowly as the levers  24  and  46  move from a position where the contact point  52  between the upper and lower contacts is centered along the centerline  54  and the levers  24  and  46  are substantially parallel to the centerline  54 . This region of low slope of function  66  provides a highest mechanical advantage in converting unison motion (D) under the force of the over-center spring  30  to a shear displacement  60  (S) thus greatly increasing the force of the shear without the need for a large high force over-center spring  30 . As the contact point  52  is displaced from centerline  54  and the levers  46  and  24  move from being parallel to the centerline  54 , this mechanical advantage is decreased as indicated by a steeper slope of function  66 . 
         [0039]    Referring now to  FIGS. 5 and 6 , as upper contact  18  is pulled downward with center contact  20  as a result of the weld  70 , the lever  46  strikes the fulcrum wall  51  causing it to bend, moving the effective pivot point  50  to a new to pivot point  50 ′ adjacent to the fulcrum wall  51 . The result is an increased tipping or rolling of the upper contact  18  with respect to the center contact  20 . 
         [0040]    Referring momentarily to  FIG. 9 , the movement of the effective pivot point  50  to pivot point  50 ′ decreases an effective radius of the lever  46  increasing a rate of change of relative angulation (A) between the contacts  18  and  20  with unison motion (D). Accordingly, if the initial shear generated between contact  18  and  20  does not release them, the action of the fulcrum wall  51  will tend to rotate the upper contact  18  to further separate the two by a peeling or prying action. 
         [0041]    Referring to  FIGS. 6 and 7 , in this peeling action, a weld  70  formed at contact point  52  between contacts  18  and  20  as shown in  FIG. 6  will be pried apart as the contacts  18  and  20  rotate with respect to each other against a new contact point to  52 ′ which serves as a mechanically advantaged fulcrum point for the separation of the weld  70 . 
         [0042]    Various features of the invention are set forth in the following claims. It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.