Patent Publication Number: US-6987235-B2

Title: Redundant switch having torsional compliance and arc-absorbant thermal mass

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   NOT APPLICABLE 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   NOT APPLICABLE 
   BACKGROUND OF THE INVENTION 
   The invention relates to switch assemblies, and in particular relates to a reliable contact block with a double break spanner. 
   Electrical switches, such as pushbuttons or rotary switches, and the like, used for the control of industrial equipment, are typically mounted onto a front panel of a cabinet so that the manipulated portion of the switch (termed the “pushbutton operator”) projects out from and is accessible at the front of the cabinet. 
   For a pushbutton switch, a hole of sufficient diameter may be punched in the cabinet to accommodate the threaded portion of the operator The threaded portion is inserted through the hole, and secured to the panel with a threaded retaining nut. The panel is thus sandwiched between the operator and the retaining nut. 
   A latch assembly is mounted on the end of the operator protruding inside the panel and a contact block or a plurality of contact blocks are mounted onto the other side of the latch assembly. The contact blocks are electrically connected to the circuit or circuits that the switch is to control. 
   Contact blocks typically include housings that contain normally open and/or normally closed contacts. A normally open contact may be used, for example, when a user wishes to activate a specified function by actuating the operator, thereby closing the normally open contact. When the operator switch is deactivated, a plunger returns to its normal position, thereby opening the normally open contact and terminating the controlled function. 
   A normally closed contact may be used when a user wishes to stop an ongoing function. One common example of a normally closed contact is an Emergency Stop (E-Stop) function which is activated when the user wishes to immediately terminate the controlled function due, e.g., to a malfunction in the process or the development of a situation that may cause damage to the product line or the operating equipment. In this situation, when the switch operator is actuated, the normally closed contact opens and remains open until the operator is returned to its normal state, thereby closing the normally closed contact and resuming the controlled function. 
   Referring to  FIG. 1 , a conventional switch  20  is illustrated including a spanner  21  that is disposed above a pair of contact plates  24 . Spanner  21  is a double break spanner, meaning that both outer ends  22  engage a contact plate  24  such that the circuit is broken if either outer end becomes disengaged from the corresponding contact plate. In particular, each contact plate is aligned with an outer end  22  of spanner  21 . Spanner  21  and contact plates  24  are of the type that are installed into a contact block (not shown) in the general orientation illustrated. A switch operator of a pushbutton, for instance, may be depressed (in a normally open switch) to bias spanner  21  downwardly along the direction of Arrow A until the outer ends  22  engage the corresponding contact plates  24  to operate a controlled function. A contact  26  is in the form of a conductive nub that protrudes upwardly from each contact plate  24  and towards a corresponding outer end  22  to provide a contact location between the spanner  21  and contact plates  24  when the switch  20  is closed. The pushbutton is released to allow the spanner  21  to translate upwardly away from contact plates  24  under a spring force along the direction of Arrow B to disengage the outer ends  22  from the contact plates  24  when operation of the controlled function is to be discontinued. It has been recognized, however, that the accumulation of a nonconductive mass (such as dirt, dust and the like) may become lodged between the contact  26  and outer ends of spanner  22 , which prevents current from flowing through the closed switch  20 . Electrical conduction between contact plates  24  was thus not reliably established in conventional switch  20 . 
   Referring now to  FIG. 2 , a conventional switch  28  addresses the potential presence of nonconductive masses that could compromise the reliability of switch  20  illustrated in FIG.  1 . In particular, switch  28  includes a spanner  30  having a slot  32  extending longitudinally partially through each outer end  34  to produce a pair of bifurcated fingers  36   a  and  36   b  at each end. Each finger  36  is independently vertically flexible with respect to the spanner  30  and therefore provides a redundant contact that engages a flat contact plate  38 . A contact (not shown) protrudes downwardly from the lower surface of each finger  36  towards the contact plate  38 . Accordingly, if a nonconductive mass were to become lodged between one of the contacts (e.g., of a finger  36   a ) and contact plate  38  to prevent the corresponding finger  36   a  from making electrical contact with the plate, the contact corresponding to the adjacent finger  36   b  would still engage the contact plate  38  to enable current to flow through spanner  30 . Unfortunately, when switch  28  is opened, an electrical arc is often created between the contact plate  38  and the last finger  36  to disconnect from the plate  38 . Because the bifurcated fingers  36  have a reduced mass with respect to the outer end  34 , the fingers tend to melt or otherwise fail in response to the heat produced by the arc. 
   What is therefore needed is a switch usable in a contact block that provides redundancy without compromising the structural integrity of the switch components during use. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, a switch is provided that is of the type that may be installed in a contact block engaging a pushbutton operator via a latch assembly. The switch includes a contact defining a first and second end. The first end is connected to an external device controlled by the switch. A first and second nub extends outwardly from the second end. A laterally extending conductive spanner has a body connected to an outer end that is aligned with the first and second nubs of each second end, respectively. A circuit is formed when the spanner is electrically connected to the second end. The outer ends of the spanner are wider than the central portion so as to render the spanner torsionally compliant. 
   These and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration and not limitation a preferred embodiment of the invention. Such embodiment does not define the scope of the invention and reference must therefore be made to the claims for this purpose. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals correspond to like elements throughout, and in which: 
       FIG. 1  is a perspective view of the spanner portion of a control block constructed in accordance with the conventional techniques; 
       FIG. 2  is a perspective view of another spanner portion of a control block constructed in accordance with conventional techniques; 
       FIG. 3  is a side elevation view of a switch assembly constructed in accordance with the preferred embodiment having a portion of the control block cutaway, wherein the control block is in an open position; 
       FIG. 4  is a perspective view of the spanner portion of the contact block illustrated in  FIG. 3 ; 
       FIG. 5  is a side elevation view similar to  FIG. 3  but with the contact block in a closed position; 
       FIG. 6  is a sectional side elevation view of the contact block taken along line  6 — 6  of  FIG. 3 ; 
       FIG. 7  is a sectional side elevation view of the contact block taken along line  7 — 7  of  FIG. 6 ; 
       FIG. 8  is a sectional side elevation view of the contact block taken along line  8 — 8  of  FIG. 6 , wherein a nonconductive mass lodged between one of the contact locations; 
       FIG. 9  is a sectional side elevation view of a pair of contact blocks vertically stacked to operate in tandem; and 
       FIG. 10  is a sectional side elevation view of a plurality of contact blocks directly connected to a pushbutton to operate in tandem. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 3 and 6 , a contact block  52  is removably connected to a switch operator  54  via a latch assembly  50 . In particular, contact block  52  includes a generally rectangular housing  56  that is connected to a pair of upper flanges  58  that extend upwardly and inwardly from the housing to provide a catch that engages mating flanges  60  extending downwardly from the latch assembly  50 . Latch assembly  50  includes a rotatable collar  61  that removably engages the cylindrical shaft  62  of a switch operator  54  to the latch assembly  50 . One example of such a latch assembly is described in U.S. Pat. No. 6,376,785 entitled “Removable Latch Assembly for an Electrical Switch”, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. A release tab  64  extends outwardly from one of the flanges  58  that is configured to engage the head of a screwdriver, for instance, when it is desired to pull flange  58  out of engagement from flange  60  to disconnect contact block  52  from latch assembly  50 . 
   Switch operator  54  includes a pushbutton  66  located at a head  68  at one end of cylindrical shaft  62 . The pushbutton  66  is attached to a stem  70  that extends axially through the shaft  62  to communicate the action of the pushbutton  66  to a plunger  72  in the contact block  52 . A sheet panel  74 , preferably made of sheet metal, has a hold (not shown) that receives the shaft  62 , such that pushbutton  66  extends from the outer surface of panel  74 , and the contact block  52  extends inwardly from the inner surface of a panel  74 . External threads  76  are formed on the portion of the shaft  62  passing through the hole in panel  74 . The head  68 , remaining on the outside of the panel  74  when the shaft  62  is inserted into the hole, is drawn against the panel by a retaining nut  78 , placed over the shaft inside of the panel, and tightened on the threads  76 . The panel is thus sandwiched between the nut  78  and an inner face of the head  68 . 
   While pushbutton  66  and latch assembly  50  have been described, it should be noted that any suitable apparatus for connecting the switch operator  54  to a contact block  52  may be used such that actuation of the switch operator in turn actuates the contact block plunger. 
   Housing  56  of contact block  52  retains a switch assembly  55  that is in a normally open position. Housing  56  includes opposing front and rear walls  80  that are connected at their outer ends to side walls  82 . Walls  80  and  82  are connected at their lower ends to a base  83 , and are connected at their upper ends to an upper wall  85 . A pair of contact assemblies  84  is formed at each lateral end of walls  80  and are separated by a centrally disposed axially extending column  86  that comprises a void disposed between walls  80  of adjacent contact assemblies  84 . 
   It should be appreciated that the term “axially” is used herein synonymously with “vertical” and defines a direction between contact block  52  and pushbutton  66 . The term “laterally” is used herein to define a direction extending perpendicular to side walls  82 . The term “transverse” is used herein to define a direction extending perpendicular to front and rear walls  80 . These directional terms are used for the purposes of clarity and convenience, however the components of the present invention are not to be construed as limited to these directions. 
   Each contact assembly  84  includes a lower retaining wall  88  that extends upwardly from base  83  parallel to side walls  82  at a distance inwardly of side walls  82 . The upper end  90  of each retaining wall  88  provides a seat for the inner end  92  of an electrically conducting plate  93 . A pair of corresponding lower guide walls  94  extends upwardly from base  83  a distance less than lower retaining wall  88 , and is connected to the adjacent lower retaining wall  88  via a crossbar  96  to ensure structural integrity. 
   A pair of upper retaining walls  98  extends downwardly from upper wall  85  parallel to side walls  82  at a distance inwardly of side walls  82 . The lower end of each upper retaining wall  98  is connected to a mounting wall  100  that extends laterally outwardly to the corresponding side wall  82 . A pair of corresponding upper guide walls  102  extends downwardly from upper wall  85  a distance less than upper retaining walls  98 , and is connected to the adjacent upper retaining wall  98  via a crossbar  104  to ensure structural integrity. 
   An angled wall  107  is connected to the interface  106  of mounting wall  100  and one of the side walls  82 . Wall  107  extends generally upwardly and then generally inwardly and is connected to the upper end of corresponding upper retaining wall  98  to provide structural support for release tab  64 . In particular, the interface  106  provides a hinge that enables the corresponding side wall  82  to flex outwardly in when release tab  64  is engaged. 
   Each mounting wall  100  defines an aperture  108  extending through the wall  100  in a direction perpendicular to the wall  100 . Each electrically conducting plate  93  defines an outer end  110  that extends along the bottom surface of mounting wall  100 . A cylindrical flange  112  extends generally upwardly from outer end  110  and into aperture  108 . Flange  112  defines an internally threaded bore. Flange  112  receives a screw  114  having a middle threaded portion  116 , a lower threaded portion  120  proximal the screw tip, and an upper threaded portion  118  proximal the screw head. 
   A V-shaped conducting electrical connector  122  includes first and second walls  124  joined at an apex whose concave surface faces plate  93 . Apex  124  receives the upper unthreaded portion  118  of screw  114 , which has a smaller diameter than the outer diameter of threads  116 . Flange  112  receives the threaded portion  116 , such that the lower unthreaded portion  120  extends beyond flange  112 . Screw  114  may be rotated clockwise to tighten connector  122  against plate  93 , or counterclockwise to translate connector  122  away from plate  93 . An electrical lead is placed between each connector  122  and plate  93  prior to tightening the respective connector against the plate. Connector  122  is sized too large to fit through a gap  125  disposed between the lower end of side wall  82  and lower retaining wall  88 . Unthreaded portions  120  and  118  are spaced apart a sufficient distance such that, when screw  114  is rotated counterclockwise until threads  116  become disengaged from flange  112 , connector  122  is disposed above gap  125 . The mechanical interference between threads  116  and connector  122  coupled with the interference between connector and gap  125  prevents the screw  114  from being completely removed from contact block  52 . 
   Column  86  is occupied by a housing  130  that carries an electrically conducting laterally extending spanner  126  that, in combination with inner ends  92  of plates  93 , provides a normally open switch  133 . Specifically, referring also to  FIG. 4 , spanner  126  defines lateral outer ends  127  having corresponding lower surfaces  129  that engage the upper surfaces  95  of inner ends  92 . A pair of domed conductive nubs (electrical contacts)  99  and  101  protrude upwardly from ends  92  and are transversely aligned to provide redundant contact points for spanner  126 . Nubs  99  and  101  are preferably formed integrally with ends  92 . Spanner  126  is generally made of copper, however, the lower surfaces  129  of outer ends  127  include a silver coating  131  to increase the electrical contact with nubs  99  and  101 . Silver has been found to conduct electricity sufficiently so as to assist in heat dissipation at the outer ends  127  of the spanner, for example when an arc is present. It should be appreciated, however, that spanner  126  could be made of any suitable conductor, and that the outer ends may be coated with any suitable conductor or, alternatively still, the coating  131  may be eliminated. If coating  131  is present, then outer ends  127  have a greater vertical thickness than the remainder of spanner  126 . Spanner  126  advantageously is torsionally compliant, as is described in more detail below. 
   Plunger  72  extends upwardly from the upper wall  132  of housing  130 . A pair of opposing side walls  134  have corresponding proximal ends  136  that are connected to the transverse outer edges  138  of wall  132  (See also FIG.  10 ). Side walls  134  extend downwardly from upper wall  132  and terminate at distal ends  140 . The distal ends  140  retain a plug  142 , which may be snap-fit between walls  134 . Distal ends  140  of walls  134  extend downwardly a slight distance past plug  142 , and are separated from each other a distance slightly greater than the transverse thickness of base  83  to enable contact blocks  52  to be vertically stacked, as will be described in more detail below. 
   The upper surface  144  of plug  142  provides a seat for spanner  126 . The lateral outer ends of each wall  134  are flared inwardly towards the opposing wall  134  to define flanges  143 . Flanges  143  provide a guide for an upper spring  145  that is disposed in housing  130  such that the upper end  146  of spring  145  rests against the lower surface of upper wall  132 , and the lower end  148  of spring  145  biases spanner  126  against the upper surface  144  of plug  142 . A bore  149  extends axially upwardly through the lower surface  146  of plug  142 . Bore  149  extends towards, but not all the way to, the upper surface  144 . Bore  149  is sized to receive the upper end  150  of a lower spring  152  whose lower end  154  is in contact with base  83  of contact block housing  56 . Lower spring  152  thus biases housing  130  upwardly such that plunger  72  engages the lower end of stem  70  and spanner  126  is disengaged from plates  93  when contact block  52  and operator  54  are initially installed in latch  50 . 
   Referring now also to  FIG. 5 , during operation, electrical leads that form a circuit to control a function of an external device (such as power or a control operation) are connected to contact block  52  via screws  114  and connectors  122 . Housing  130  is then installed in column  86  such that spanner  126  is in a normally open configuration relative to plates  93 . Control block  52  is connected to latch  50  via tabs  58 , and operator  54  is connected to latch  50  in any known manner. When pushbutton  66  is depressed, stem  70  depresses plunger, which translates housing  130  downwardly along the direction of Arrow D against the force of lower spring  152 . 
   Spanner  126 , which is carried by the housing  130 , is thus also biased downwardly until outer ends  127  engage the inner ends  92  of plates  93 . Advantageously, upper spring  145  provides compliance such that housing  130  may continue to be biased downwardly against the force of upper spring  145 , which compresses after spanner  126  engages plates  93 . Spring  145  thus provides a force that biases spanner  126  against plates  93 . The biasing force of spring  145  increases as housing  126  is increasingly depressed. The downward movement of housing  126  is limited by the stroke length of pushbutton  66 , or by interference between the lower surface  146  of plug  142  and base  83 . 
   Referring now to  FIG. 4 , switch  133  is configured to provide a redundant electrical contact, and furthermore to resist failure due to arcing at the interface between outer ends  127  and plates  93 , as experienced in conventional switch assemblies. In particular, spanner  126  includes a central laterally extending beam  156  that defines opposing lateral outer ends  127 . Each lateral end  127  has opposing transverse outer ends  135  and  123  that are vertically aligned with nubs  99  and  101 , respectively. A pair of protrusions  158  extends transversely outwardly from a middle portion  160  of beam  156  to a location proximal walls  134 . Protrusions  158  extend laterally between flanges so as to stabilize the position of spanner  126  and furthermore to provide guides for axial spanner translation in housing  130 . 
   Beam  156  has a width (transverse thickness) at locations  162  between protrusions  158  and outer ends  127  that is less than the width of ends  127 . Ends  127  are thus T-shaped with respect to the beam sections  162 . Ends  127  extend further transversely outwardly than protrusions  158  such that the entire beam  156  has a reduced width with respect to outer ends  127 . The beam structure, along with the fact that beam  156  is made of a flexible material, combine to enable beam  156  to provide torsional compliance during operation. 
   Specifically, referring also to  FIGS. 7 and 8 , a nonconductive mass  161 , such as a piece of dirt, lint, and the like, may become lodged between one of the nubs  99  and transverse outer end  135 . Accordingly, electrical contact is unattainable between spanner  126  and nub  99 . In prior non-torsionally compliant switches, the mass  161  would cause the adjacent transverse outer end  123  to a raised position above, and out of contact with, corresponding nub  101 . In such devices, the switch would be unable to close, and control of the external device would be lost. 
   In accordance with the present invention, however, the portions  162  of spanner  126  have reduced transverse thicknesses relative to the corresponding lateral outer ends  127 . Furthermore, spanner  126  is made of a compliant material and has a reduced axial thickness (within the range of 25 mm). Accordingly, when one transverse outer end  135  is raised with respect to corresponding nub  99 , the force of upper spring  145  acting on the middle portion  160  of spanner  126  is translated to the other transverse outer end  123  so as to bias end  123  against the corresponding nub  101 . Redundant contacts are thus established at each lateral outer end  127  between transverse outer ends  135  and  123 , and nubs  99  and  101 , respectively. Nonconductive mass  161  furthermore does not affect the ability of the opposite outer end  127  of spanner  126  to contact corresponding nubs  99  and  101 . 
   When switch  133  is again opened, one of the transverse outer ends  123  or  135  will, if only for a minute period of time, become disengaged from the corresponding nub prior to the other transverse outer end. For instance, outer end  135  may become disengaged from nub  99  prior to outer end  123  becoming disengaged from nub  101 . An arc may thus form at the interface between the remaining end  123  and nub  101 . Transverse outer ends  135  and  123  are not bifurcated, however, meaning that lateral outer end  127  is a solid member that includes both transverse outer ends. Accordingly, even though an arc may be produced at outer end  123  when the switch  133  is opened, the increased thermal mass of lateral outer end  127  enables spanner  126  to absorb the arc while maintaining its structural integrity. 
   The redundancy of bifurcation in conventional spanners is thus replaced by the redundancy of torsional compliance in accordance with the preferred embodiment of the present invention. The lack of bifurcation allows the total mass of the spanner to participate in the opening and closing of the circuit hence reducing the detrimental thermal effects of the arc. This increases contact life and prevents contact welding. Thus, spanner  126  affords the same contact reliability of a bifurcated spanner while increasing structural reliability in the face of arcing during use. 
   As discussed above, sections  162  have a reduced width compared to the width of outer ends  127 , and further have a reduced width compared to the width of middle portion  160 . The reduced width of sections  162  is achieved by forming a corresponding pair of notches  163  between outer ends  127  and middle portion  160 . Advantageously, notches  163  ensure that heat that accumulates at outer ends  127  thus has a reduced path of conductivity via sections  162 . The middle portion  160  thus does not become heated as rapidly as conventional spanners, thereby further reducing potentially damaging thermal effects on nearby plastic parts. 
   Referring now to  FIG. 9 , an upper contact block  52 A is in communication with a switch operator  54  as described above. In addition, a lower contact block  52 B is connected to the lower end of upper contact block  52 A. Specifically, each contact block housing  56  includes a pair of lower flanges  164  that flare laterally outwardly from the lower end of side walls  82  (see FIG.  5 ). Contact blocks  52  may be vertically stacked by connecting lower flanges  164  to upper flanges  58 . Plunger  72  comprises a pair of fingers  73  (See  FIG. 10 ) that are transversely displaced a greater distance than the transverse thickness of base  83 . The plunger  72  of lower contact block  52 B thus fits over the base  83  of upper contact block  52 A so as to engage the lower end of walls  134  of upper contact block  52 A. The vertically stacked contact blocks  52 A and  52 B act in tandem in response to actuation of a single pushbutton  66  to control multiple external devices, or multiple functions of a single external device. 
   Referring now to  FIG. 10 , a plurality of contact blocks  52 C,  52 D, and  52 E are mounted onto a single latch assembly  50  in a transverse orientation such that front and rear walls  80  of each contact block abut each other. Stem  70  extends transversely so as to engage both fingers  73  of plunger  72  of the middle contact block  52 D along with one of the fingers of the outer contact blocks  52 C and  52 E. Accordingly, when pushbutton  66  is actuated, the plungers  72  of all three contact blocks  52 C- 52 E are depressed. Pushbuttons  52 A- 52 E may individually be normally open as described above, or normally closed as appreciated by one having ordinary skill in the art. 
   The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, as set forth by the appended claims.