Abstract:
For increasing the reliability of contact closure in safety relays, a contact spring ( 10 ) is provided with two contact pieces ( 15, 16 ) which are disposed at the free end of the contact spring ( 10 ), spaced transversely of the longitudinal axis thereof and cooperate with contact pieces ( 17, 18 ) disposed on a common fixed contact. The contact spring ( 10 ) has a zone ( 19 ) in which it is not only flexible but also sufficiently torsional about its longitudinal axis in order to ensure the closure of both contact couples. Disposing the free end of the contact spring ( 10 ) carrying the contact pieces ( 15, 16 ) at an angle with respect to the fixed contact ( 12 ) achieves smooth and low-bounce contact closure and at the same time provides the function of a pre-contact and a main contact.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 09/461,386, filed Dec. 15, 1999, now U.S. Pat. No. 6,300,854. 
    
    
     BACKGROUND OF THE INVENTION 
     For increasing the contact safety of electromagnetic relays, it is known, from German Patent No. 3,224,013, to provide the contact spring with a longitudinal slot to form two flexible ends and to equip each end with a contact piece which cooperates with a corresponding counter contact piece on a common fixed contact. The probability of both contact couples to fail due to contamination by minute glass fibers, moulding burs, or the like is substantially smaller than with single contacts. 
     These “twin” contact springs, which are similarly known from German Published Application No. 1,175,807, German Utility Model No. 9,404,775, and German Patent No. 972,072, however, suffer from the difficulty that the spring arms formed by the longitudinal slot are more prone to breakage than the undivided spring. In such a case, while the relay per se is still operative, the broken spring arm may cause unpredictable short-circuits. A further problem of known twin contact springs resides in the fact that the individual spring arms are much softer than the undivided spring so that, when one contact becomes welded, the corresponding spring arm is not stiff enough to retain the actuator in the closed contact position. These properties prevent the use of known twin contact springs in safety relays. 
     German Patent No. 3,224,468 discloses a contact arrangement in which the contact spring carries two contact pieces each cooperating with a separate fixed contact. In addition to the fact that the total contact resistance of such bridge contacts is twice that of an individual contact couple, the known arrangement increases the safety in contact opening rather than in contact closure. 
     Further known are so-called “crown” contacts in which at least one of two cooperating contact rivets has a raised peripheral portion which, if the two rivets are somewhat offset with respect to each other, form two contact locations. In addition to the fact that these contact locations have very small areas, the raised periphery is relatively quickly worn in use so that the intended double contact feature is rapidly lost. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a contact unit which increases the reliability of contact closure in safety relays, i.e. relays with forcibly guided contacts. 
     To meet this object, a contact unit in accordance with the invention includes a fixed contact providing two contact locations and a contact spring having a longitudinal axis, a fixed end and a free end, and providing two contact locations which are disposed at an undivided portion of the free end, spaced in a direction transverse of the longitudinal axis and cooperating with the contact locations of the fixed contact, a zone of the contact spring between the fixed end and the contact locations providing increased torsional flexibility about the longitudinal axis. 
     The fact that the contact spring has at least one zone which exhibits increased torsional flexibility about its longitudinal axis ensures closure of the contact couples, which are constituted by the contact locations of the contact spring and the counter contact locations of the fixed contact, even when the free end of the contact spring is not parallel to the fixed contact. 
     In contrast to the twin contact spring referred to above, the contact spring of the contact unit according to the invention is undivided throughout its length, so that breakage is less likely to occur and, if it occurs, will cause the relay to fail completely by interruption. 
     Each of the two contact couples may be formed by at least two separate contact pieces provided on the contact spring and the fixed contact. Alternatively, the contact locations of the contact spring and/or those of the fixed contact may be formed at one common contact piece. 
     In a preferred embodiment, the zone of increased torsional flexibility may be formed by reducing the width and/or the thickness of the contact spring. 
     To achieve forcible guidance of the contact spring by the actuator, specifically during opening, an area of the contact spring extending from the location of engagement with the actuator to the contact locations may be stiffer than any other area of the contact spring. Increased stiffness may be achieved by increased thickness or by a deformed portion, preferably by a bead extending along the longitudinal axis throughout the length of the contact spring. 
     In order to use the torsional behavior of the contact spring effectively, it is of advantage for an actuator to have a convex portion for engagement with the contact spring, preferably a pair of convex portions engaging opposite sides of the spring. 
     In another preferred embodiment, the straight line connecting the two contact locations of the contact spring intersects at an acute angle the straight line connecting the contact locations of the fixed contact. An intentional inclination is thus provided between the contact spring and the fixed contact to form a first closing and last opening pre-contact and a last closing and first opening main contact. This arrangement has the advantage of softer contact closure with reduced bouncing. 
     The contact locations of the pre-contact may be made of a less noble contact material, preferably AgSnO, than those of the main contact, which are preferably made of an AuAg alloy. Additionally or alternatively, the contact locations of the pre-contact may be dimensioned larger than those of the main contact. The pre-contact is thereby provided with properties suitable for a load contact, while the main contact has the qualities of a signal contact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a contact unit according to the invention. 
     FIG. 2 is an end view taken in the direction of the arrow II in FIG.  1 . 
     FIG. 3 is a perspective representation of part of an electromagnetic relay incorporating the contact unit in accordance with the invention. 
     FIG. 4 is a view similar to FIG. 1 of the contact unit used in the relay of FIG.  3 . 
     FIG. 5 is an end view showing the actuator of the embodiment of FIG. 3 with one contact spring. 
     FIG. 6 is an enlarged representation of part of FIG.  5 . 
     FIG. 7 shows a modification of the contact arrangement in a representation corresponding to FIG.  2 . 
     FIG. 8 shows a further modification of the contact arrangement. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The contact unit shown in FIGS. 1 and 2 essentially consists of a contact spring  10 , which has one of its ends mounted, such as by riveting, to a carrier  11 , a fixed contact  12  opposite the free end of the contact spring  10 , and an actuator  13  which engages the contact spring  10  and, in the present embodiment, is coupled to a relay armature  14  as indicated in FIG.  2 . 
     The free end of the contact spring  10  opposing the fixed contact  12  is wider than the main part of the spring and carries two contact pieces  15 ,  16 , arranged next to each other (below each other according to the drawing) in a direction transverse of the longitudinal extension of the contact spring  10 . Similarly, the fixed contact  12  is provided with two contact pieces  17 ,  18  which are disposed opposite to, and cooperate with, the respective contact pieces  15 ,  16  of the contact spring  10 . 
     As shown in FIG. 2, the free end of the contact spring  10  extends at an angle with respect to the fixed contact  12  so that the straight line interconnecting the contact pieces  15 ,  16  of the contact spring  10  intersects at an acute angle the straight line interconnecting the contact pieces  17 ,  18  of the fixed contact  12 . This inclined attitude of the free end of the contact spring  10  is achieved by pre-torsioning the contact spring about its longitudinal axis in a torsion zone  19  located between the fixed and free ends of the spring. 
     The actuator  13  is so arranged and shaped that it engages the contact spring  10  close to the free end thereof and is capable of engaging either one of the opposite surfaces of the contact spring  10 . Either zone of engagement  20 ,  21  of the actuator  13  is shaped convexly toward the respective surface of the contact spring  10 . 
     When the armature  14 , upon actuation of the relay, is moved in the direction of the arrow A, it moves the actuator  13  to the right as shown in FIG.  2 . FIG. 2 shows the moment at which the upper contact piece  15  of the contact spring  10  just touches the contact piece  17  of the fixed contact  12 . Upon further movement of the actuator  13  to the right, the front end of the contact spring  10  pivots about the contacting location between the contact pieces  15  and  17 , which pivotal movement is enabled by a sufficient torsional capability of the flexure zone  19 , until the lower contact pieces  16 ,  18  also contact each other. Subsequent further movement of the actuator  13  to the right into the end position of the armature  14  will then deflect the fixed contact  12  to increase the contact force between the two contact couples. 
     During the above-described pivotal motion of the free end of the contact spring  10  carrying the contact pieces  15 ,  16 , the contact spring moves along the convex zone of engagement  20  of the actuator  13 . 
     For opening the relay, the armature  14  is moved in the direction of the arrow B so that now the other zone of engagement  21  of the actuator  13  will engage the opposite surface of the contact spring  10  and cause the contact pieces  15 ,  16  of the contact spring to be lifted off the contact pieces  17 ,  18  of the fixed contact  12 . This will first open the lower contact couple  16 ,  18 , and subsequently the upper contact couple  15 ,  17 . 
     In accordance with the function explained above, the upper contact couple  15 ,  17  forms a pre-contact, and the lower contact couple  16 ,  18  forms a main contact. Since the first closing and last opening pre-contact constitutes a load contact and will wear more rapidly, the contact piece  15 , as shown in FIG. 1, is formed larger than the contact piece  16  belonging to the main or signal contact. Further, the contact pieces  15 ,  17  of the pre-contact are made of less noble material than the contact pieces  16 ,  18  of the main contact. As an example, the contact pieces  15 ,  17  may be of AgSnO, and the contact pieces  16 ,  18  of an AuAg alloy. 
     Instead of pre-torsioning the torsion zone  19 , the inclined attitude of the free end of the contact spring  10  with respect to the fixed contact  12  shown in FIG. 2 may be achieved by inclining the fixed contact  12  or inclining the carrier  11  of the contact spring  10  which, in this case, is planar, in its rest position. 
     The electromagnetic relay shown in part in the perspective view of FIG. 3 (wherein the housing cap has been omitted) includes a base  22  and a leg  23  of a yoke which extends through a coil (not shown). The yoke leg  23  projects from the base  22  and is disposed between the two arms of the relay armature  14  which, in the present embodiment, is generally H-shaped and supported on a bearing stud  23  provided on the base  22  for pivotal movement about a vertical central axis. 
     The actuator  13  coupled to the relay armature  14  is slidably guided in its plane by guide columns  25  formed on the base  22  and, as shown in more detail in FIGS. 4 to  6 , engages the contact spring  10 . The contact pieces  15 ,  16  of the spring cooperate with the contact pieces  17 ,  18  provided on the fixed contact  12 . In FIG. 3, the relay is shown as fitted with two contact springs  10 . Terminal pins  26  of the fixed contacts  12  project downward from the base  22 . 
     In the contact spring  10  shown in more detail in FIG. 4, the torsion zone  19  has been realized by reducing the width of the spring. Alternatively or additionally, the zone  19  of the spring  10  may be reduced in thickness or treated in other ways to increase its torsional flexibility. 
     The zone  19  is situated between the fixed end of the contact spring  10 , which is riveted to the contact carrier  11 , and the zone of engagement of the actuator  13 . The contact spring  10  has its full width within this zone of engagement as well as at both of its ends, the width being again increased at the free end so as to provide sufficient spacing between the contact pieces  15  and  16 . The actuator  13  engages the contact spring  10  in a stiffened zone. 
     As appears from FIG.  5  and the enlarged detail view of FIG. 6, the contact spring  10  extends between the two engagement zones  20 ,  21  of the actuator  13 , of which the zone  20  is convex or crowned, to permit the contact spring  10  to rotate about its longitudinal central axis when pressed, and to ensure contact closure at both contact couples  15 ,  17  and  16 ,  18 . At the opposite engagement zone  21 , which engages the contact spring  10  during opening, a convex or crowned shape is not required. Where the contact spring  10  is biased into the closed position, the region  21  of the actuator  13  engaging the spring should also be crowned. 
     In the embodiment shown in FIG. 7, the two contact locations disposed on the fixed contact  12  are formed as one common contact piece  27 , the contact surface of which is so dimensioned that it can cooperate with the two separate contact pieces  15  and  16  of the contact spring  10 . To enable proper contact closure even when the contact spring  10  is torsioned or the fixed contact  12  extends at an angle, the contact surface of the common contact piece  27  has a crowned shape. 
     Instead of the embodiment shown in FIG. 7, a large common contact piece may be provided on the contact spring  10  and cooperate with two separate contact pieces on the fixed contact  12  (see FIG.  8 ). 
     In a further conceivable alternative, a single continuous contact piece may be provided on both the contact spring  10  and the fixed contact  12 , with at least one of such continuous contact pieces being provided with two projections to produce two spaced contact locations. 
     As further shown in FIG. 7, the contact spring  10  is provided with a bead  28  extending in the direction of its longitudinal central axis to enhance the stiffness of the contact spring  10  along its longitudinal direction within the region between the contact springs  15 ,  16  and the engagement zone of the actuator  13 . 
     Instead of the bead  28 , the stiffness of the contact spring  10  may be achieved by increasing its thickness within the region between the contact pieces  15 ,  16  and the zone of engagement with the actuator  13 . 
     If the bead  28  shown in FIG. 7 is used for stiffening, it may extend throughout the length of the contact spring  10  all the way to its end mounted on the contact carrier  10 . Such a bead  28 , which extends in the direction of the longitudinal central axis and thus in the neutral zone of the contact spring  10 , while resulting in reduced bendability also within the zone  19 , impairs the torsional flexibility in this zone to a small degree only.