Patent Publication Number: US-9418797-B2

Title: Electromagnetic switch

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2014-161016 filed Aug. 7, 2014, the description of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an electromagnetic switch for opening and closing electrical contacts in response to respective ON/OFF operation of a solenoid, and especially, is preferably used in an electromagnetic switch mounted on a starter. 
     BACKGROUND 
     When using a starter in cold climates, for example, a surface of a fixed contact provided on an electromagnetic switch might freeze. 
     To be specific, when a power supply terminal of the electromagnetic switch is cooled through a battery cable, a surface temperature of the fixed contact fixed to the power supply terminal is lowered so that water vapor in the air is condensed on a contacting surface and freezes. 
     When the electromagnetic switch is operated under this condition, since an ice layer is formed on the surface of the fixed contact that is the contacting surface of the movable contact, a problem of causing a conductive failure between the contacts may occur. 
     In contrast, as shown in  FIG. 8A , forming a plurality of grooves  110  on a surface of a fixed contact  100  as a conventional technology is disclosed in the Japanese Utility Model Publication No. 54-88563. 
     According to this conventional technology, since a contacting area when the contact is abutting is reduced and a contact pressure per unit area is increased, it becomes possible to crush the ice layer formed on the surface of the fixed contact  100  by an impact force when the contact is abutting. 
     However, in the conventional technology mentioned above (Publication No. &#39;563), since flat surfaces  120  are left between a number of grooves  110  formed on a surface of the fixed contact  100 , that is, between the adjoining grooves  110 , as shown in  FIG. 8B , it is not possible to sufficiently increase the contacting pressure between the contacts. 
     For this reason, in the electromagnetic switch that is used in severe cold environmental conditions or has a structure difficult to discharge humidity, there is a possibility that ice-crushing force is insufficient. 
     In this case, the ON/OFF operation needs to be repeated for several tens of times in order to secure the conduction by crushing the ice on the contacting surface. 
     Further, even if the ice could be crushed, a process of eliminating the crushed ice from the contacting surface is required in order to secure the conduction between the contacts. 
     However, in the conventional technology, since a large number of grooves  110  are formed on the contacting surface, a surface area where the ice adheres increases compared with a flat contacting surface where no grooves  110  are formed, thus there is a possibility that the crushed ice is likely to remain within the grooves  110 . 
     In other words, it is difficult to eliminate the crushed ice from the contacts. 
     SUMMARY 
     An embodiment provides an electromagnetic switch that has a large crushing force for an ice frozen on a surface of a fixed contact, and can easily eliminate the crushed ice from the contacts 
     An electromagnetic switch according to a first aspect includes a solenoid that forms an electromagnet by energization to a coil, a pair of fixed contacts respectively connected to a power supply side and a load side of an electrical circuit via two connecting terminals, and a movable contact that conducts and cuts off between the pair of fixed contacts in response to respective ON/OFF operation of the solenoid. 
     The pair of fixed contacts has a first fixed contact disposed in one side in a radial direction that intersects perpendicular to an axial direction of the solenoid and a second fixed contact disposed in another side in the radial direction. 
     When a direction which intersects perpendicular to the radial direction on a plane of the fixed contact is referred to as a longitudinal direction and another direction which intersects perpendicular to the longitudinal direction is referred to as a lateral direction, the fixed contact has two protrusions extending in the lateral direction on a contacting surface that faces the movable contact. 
     The two protrusions are disposed in the longitudinal direction with a predetermined interval therebetween, and a planar portion recessed relative to apexes of the protrusions is formed between the two protrusions. 
     The movable contact has a first contacting surface facing the first fixed contact and a second contacting surface facing the second fixed contact. 
     A single projection that intersects perpendicular to the two protrusions disposed on the first fixed contact is disposed on the first contacting surface, and another single projection that intersects perpendicular to the two protrusions disposed on the second fixed contact is disposed on the second contacting surface. 
     According to the above configuration, when the movable contact abuts the pair of fixed contacts by the ON operation of the solenoid, the projections disposed on the movable contact and the protrusions disposed on the fixed contacts come to contact with each other at intersections. 
     That is, the first fixed contact and the second fixed contact are contacted at two positions with respect to the movable contact, respectively. 
     In this case, since a contacting area between the movable contact and the fixed contact is decreased and a contacting surface pressure is increased as compared with the conventional technology disclosed in Publication No. &#39;563, a crushing force for the ice frozen on a surface of the contact becomes large. 
     Moreover, since the planar portion is formed between the two protrusions disposed on the fixed contact, it is possible to collect the water that has condensed on the contacting surface of the planar portion, and even if the water freezes, the apexes of the protrusions can be prevented from freezing. 
     In other words, as long as the apexes of the protrusions are exposed from the surface of the ice frozen on the plane portion, it is possible to secure the conduction during the point of contact is abutting. 
     Furthermore, by forming the planar portion between the two protrusions, the surface area where ice adheres can be reduced as compared to the configuration that forms a plurality of grooves on the contacting surface disclosed in Japanese Utility Model Publication No. 54-88563. 
     Thereby, since it is possible to reduce the force with which the ice adheres on the surface of the fixed contact, it becomes easy to eliminate crushed ice from the contacts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  shows a plan view of a pair of fixed contacts and a movable contact in an axial direction according to a first embodiment; 
         FIG. 2  shows a sectional view when the contacts are abutting according to the first embodiment (a sectional view taken along the line II-II of  FIG. 1 ); 
         FIG. 3A  shows a plan view of the fixed contact according to the first embodiment; 
         FIG. 3B  shows a sectional view taken along the line III-III of  FIG. 3A ; 
         FIG. 4A  shows a plan view of the movable contact according to the first embodiment; 
         FIG. 4B  shows a sectional view taken along the line IVb-IVb of  FIG. 4A ; 
         FIG. 4C  shows a sectional view taken along the line IVc-IVc of  FIG. 4A ; 
         FIG. 5  shows a sectional view of an electromagnetic switch according to the first embodiment; 
         FIGS. 6A, 6B and 6C  show sectional views of protrusions of the fixed contacts according to a second embodiment; 
         FIG. 7  shows a sectional view of a protrusion of the fixed contact according to a third embodiment; 
         FIG. 8A  shows a plan view of a fixed contact according to a conventional technology; and 
         FIG. 8B  shows a sectional view taken along the line VIII-VIII of  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments according to the present disclosure will be described with reference to drawings. 
     First Embodiment 
     In the first embodiment, an example where an electromagnetic switch  1  according to the present disclosure is mounted on a starter for starting an engine will be described. 
     Since configurations and functions of the starter are well known, detailed descriptions thereof are omitted, and a structure of the electromagnetic switch  1  according to the present disclosure will be described hereinafter. 
     The electromagnetic switch  1  includes a main point of contact (described below) for turning ON/OFF an electric current to a starter motor (not shown), and a solenoid SL for opening and closing the main point of contact. 
     As shown in  FIG. 5 , the solenoid SL is composed of a metal frame  2  that also serves as a part of a magnetic circuit, a coil  3  accommodated inside the frame  2 , a plunger disposed in an inner periphery of the coil  3  via a cylindrical sleeve  4 , a fixed iron core  6  disposed facing to the plunger  5  in an axial direction, and etc. 
     One end of the coil  3  is connected to a connector terminal (not shown, also referred to as a 50 terminal) and another end of the coil  3  is connected to the ground side through the frame  2 . 
     The connector terminal is connected to a battery via a starter switch or starter relay, for example. 
     The plunger  5  is inserted axially slidable in an inner periphery of the cylindrical sleeve  4 , and is attracted to the magnetized fixed iron core  6  when an electromagnet is formed by energization to the coil  3 . 
     The fixed iron core  6  is disposed in one axial end (shown right in  FIG. 5 ) of the inner periphery of the cylindrical sleeve  4 , and configured integrally with an annular shaped stationary core  7  by being press-fitted into an inner periphery of the stationary core  7 . 
     The stationary core  7  connects between the frame  2  and the fixed iron core  6  magnetically. 
     A return spring  8  that pushes back the plunger  5  to a direction opposite to the fixed core (to the left in  FIG. 5 ) when the attraction force of the electromagnet disappears is provided between the fixed iron core  6  and the plunger  5 . 
     The main point of contact is composed of a pair of fixed contacts  11  connected to a power supply line of the starter motor via two connecting terminals  9 ,  10 , and a movable contact  12  that conducts and cuts off between the pair of fixed contacts  11 . 
     Each of the two connecting terminals  9 ,  10  has a bolt-like shape to which a male screw portion is formed on an outer periphery thereof, and is fixed to a contact cover  13  made of resin through washers  14 ,  15 . 
     A battery cable is connected to one of the connecting terminals  9  protruding axially from the contact cover  13 , and a motor lead is connected to the other one of the connecting terminals  10 . 
     Hereinafter, one of the connecting terminals  9  is referred to as a B terminal bolt  9 , and the other one of the connecting terminals  10  is referred to as an M terminal bolt  10 . 
     The other end side in the axial direction of the contact cover  13  is inserted into the inside of the frame  2 , and is fixed by crimping to an open end of the frame  2  to form a contact chamber  16  to dispose the main point of contact therein. 
     The pair of fixed contacts  11  is composed of a first fixed contact  11   a  and a second fixed contact  11   b.    
     The first fixed contact  11   a  is disposed in one side (upper side in  FIG. 5 ) in a radial direction that intersects perpendicular to the axial direction of the solenoid SL, and is fixed to a pedestal  9   a  of the B terminal bolt  9 . 
     In addition, the second fixed contact  11   b  is disposed in another side in the radial direction, and is fixed to a pedestal  10   a  of the M terminal bolt  10 . 
     The movable contact  12  is supported via a resin washer  18 , which is an insulating material, to an end of a plunger rod  17  that is fixed to the plunger  5 . 
     Further, the movable contact  12  is urged toward a distal end of the plunger rod  17  (to the right in  FIG. 5 ) by a contact pressure spring  19  disposed on an outer periphery of the plunger rod  17 . 
     A stopper washer  20  is fixed to the distal end of the plunger rod  17  by crimping for preventing the movable contact  12  from detaching. 
     The main point of contact becomes an ON condition by the movable contact  12  abutting the pair of fixed contact points  11  by turning on the solenoid SL to electrically connect between both fixed contacts  11 . 
     Further, the main point of contact becomes an OFF condition by the movable contact  12  separating from the pair of fixed contacts  11  by turning off the solenoid SL to electrically disconnect between the both fixed contacts  11 . 
     Next, features of the fixed contact  11  and the movable contact  12  according to the present disclosure will be described. 
     As shown in  FIG. 1 , the fixed contact  11 , a planar shape of a contacting surface that faces the movable contact  12  is formed in a rectangular shape, and two protrusions  21  are formed on the contacting surface. 
     Here, when the radial direction where the first fixed contact  11   a  and the second fixed contact  11   b  are disposed and intersects perpendicular to the axial direction of the solenoid SL (vertical direction in  FIG. 1 ) is referred to as a specific direction, the first fixed contact  11   a  and the second fixed contact  11   b  are disposed so that respective longitudinal direction in the rectangular shape intersects perpendicular to the specific direction, while respective lateral direction becomes parallel to the specific direction. 
     As shown in  FIG. 3A , one each of the protrusion  21  is disposed in one end side (left side in  FIG. 3A ) and another end side (right side in  FIG. 3A ) from a center in the longitudinal direction of the fixed contact  11 , and respective apexes of the protrusions  21  extend in the lateral direction of the fixed contact  11 . 
     The apex of the protrusion  21  is, for example, formed in a sectional shape cut in the longitudinal direction of the fixed contact  11  with a convex surface having a curvature. 
     Further, when defining a side where two protrusions  21  face in the longitudinal direction of the fixed contact  11  as an inside, and a side opposite to the inside as an outside, an inclined surface  21   a  that inclines from the apex of the protrusion  21  to the outside is formed. 
     Specifically, as shown in  FIG. 3B , the inclined surface  21   a  is formed inclining to a tangential direction from an end point of the curvature that forms the convex surface of the apex, and extends toward the pedestal  9   a,    10   a  of the terminal bolt  9 ,  10  where the fixed contact  11  is fixed. 
     An angle of the inclined surface  21   a  relative to the pedestal  9   a,    10   a  is, for example, 45 degrees. 
     Incidentally, a shallow recess for positioning the fixed contact  11  is formed on the pedestal  9   a,    10   a  of the terminal bolt  9 ,  10 . 
     The fixed contact  11  is positioned by fitting the counter-protrusion side thereof into the recess formed on the pedestal  9   a,    10   a,  and is fixed to the pedestal  9   a,    10   a  by means of brazing or the like. 
     Furthermore, a planar portion  22  recessed relative to the apex of the protrusion  21  is formed between the two protrusions  21  disposed in the longitudinal direction with a predetermined interval on the contacting surface of the fixed contacts  11 . 
     Although a height of the apex of the protrusion  21  from the planar portion  22  is about fractions of a millimeter (e.g., 0.32 mm), the height of the apex may be determined appropriately by a balance between the height of the apex and a contact life due to wear of the protrusion  21 . 
     As shown in  FIG. 1 , the movable contact  12  has a first contacting surface  12   a  facing the first fixed contact  11   a , and a second contacting surface  12   b  facing the second fixed contact  11   b.    
     As shown in  FIG. 4A , a single projection  23  is respectively provided on the first contacting surface  12   a  and the second contacting surface  12   b  of the movable contact  12 . 
     The projections  23 , as shown in  FIGS. 4B and 4C , for example, are formed by embossing, and are disposed so as to intersect linearly perpendicular to the two protrusions  21  formed on the fixed contact  11 . 
     That is, as shown in  FIG. 1 , the projection  23  disposed on the first contacting surface  12   a  intersects linearly to the two protrusions  21  dispose on the first fixed contact  11   a , and the projection  23  disposed on the contacting surface  12   b  is intersects linearly to the two protrusions  21  disposed on the second fixed contact  11   b.    
     It is obvious that the length of the projection  23  is longer than the distance between the apexes of the two protrusions  21  disposed on the fixed contact  11 . 
     In addition, the movable contact  12  shown in  FIG. 1  is a plan view seen from an opposite side of the contacting surface, and the recesses formed by embossing the projection  23  are shown in solid lines. 
     Functions and Effects of the First Embodiment 
     1) When the outside air temperature becomes below freezing point in harsh cold climates, for example, water vapor in the air inside the contact chamber  16  is condensed on the surface of the fixed contact  11  (especially, the first fixed contact  11   a  fixed to the B terminal bolt  9 ) and may be frozen in the electromagnetic switch  1  mounted on the starter. 
     In contrast, in the first embodiment, the protrusions  21  are disposed on the fixed contacts  11 , and the projection  23  facing the fixed contacts  11  are disposed on the movable contact  12 . 
     Thus, when the movable contact  12  abuts the pair of fixed contacts  11  by the ON operation of the solenoid SL, the protrusions  21  disposed on the fixed contacts  11  and the projections  23  disposed on the movable contact  12  come to contact with each other at intersections. 
     In other words, since the movable contact  12  does not contact to the fixed contacts  11  in surface-to-surface contact, but only intersections of each other contact as shown in  FIG. 2 , contacting areas between the movable contact  12  and the fixed contacts  11  will be reduced. 
     As a result, since contacting surface pressure between the protrusions  21  and the projections  23  increases so that the force to crush the ice frozen on the surface of the fixed contact  11  also increases, it is possible to secure conduction during the time the points of contact are abutting. 
     2) Since the fixed contact  11  forms the planar portion  22  recessed relative to the apex of the protrusion  21  is formed between the two protrusions  21 , it is possible to collect the water that has condensed on the contacting surface of the planar portion  22 . 
     Thereby, even if moisture collected in the planar portion  22  freezes, the apexes of the protrusions  21  can be prevented from freezing. 
     In other words, as long as the apexes of the protrusions  21  are exposed from the surface of the ice frozen on the plane portion  22 , it is possible to secure the conduction during the point of contact is abutting. 
     3) By forming the planar portion  22  between the two protrusions  21 , the surface area where ice adheres can be reduced as compared to the configuration that forms a plurality of grooves on the contacting surface disclosed in Japanese Utility Model Publication No. 54-88563. 
     Thereby, since it is possible to reduce the force with which the ice adheres on the surface of the fixed contact  11 , it becomes easy to eliminate crushed ice from the contacts. 
     4) Since the protrusion  21  disposed on the fixed contact  11  has the inclined surface  21   a  from the apex to the outside, the moisture condensed on the surface of the fixed contact  11  will not remain on the apex of the protrusion  21 , and it becomes easy for it to flow to the outside of the protrusion  21  along the inclined surface  21   a.    
     In particular, in the first embodiment, since the inclined surface  21   a  extends towards the pedestal  9   a,    10   a  of the terminal bolt  9 ,  10  from the apex of the protrusion  21 , the moisture condensed on the surface of the protrusion  21  can flow to the surface of the pedestal  9   a,    10   a.    
     For this reason, opportunities for condensed water to collect on the surface of the fixed contact  11  become fewer, and as a result, the contacting surface can be suppressed from freezing. 
     Hereinafter, other embodiments of the present disclosure will be described. 
     It should be appreciated that, in the second embodiment and the subsequent embodiments, components identical with or similar to those in the first embodiment are given the same reference numerals, and structures and features thereof will not be described in order to avoid redundant explanation. 
     Second Embodiment 
     Although an example of forming the apex of the protrusion  21  disposed on the fixed contact  11  by the convex surface has been described in the first embodiment, the curvature of the convex surface may be changed accordingly. 
     For example,  FIG. 6A  shows an example of a relatively small curvature, that is, the apex is formed with a large R (radius), while  FIG. 6B  shows an example of a relatively large curvature, that is, the apex is formed with a small R. 
     Alternatively, the apex may not have the convex surface having a curvature, but may have a shape with a small end surface on the apex of the protrusion  21  as shown in  FIG. 6C . 
     Third Embodiment 
     In the third embodiment, as shown in  FIG. 7 , the protrusion  21  disposed on the fixed contact  11  has the inclined surface  21   a  from the apex to the outside, and an end of the inclined surface  21   a  is set to be before the pedestal  9   a,    10   a  of the terminal bolt  9  and  10 . 
     In other words, it is an example where the end of the inclined surface  21   a  does not extend to the pedestal  9   a ,  10   a  of the terminal bolt  9 ,  10 . 
     However, it is desirable that the end of the inclined surface  21   a  is as close to the surface of the pedestal  9   a ,  10   a  as possible. 
     Even in the configuration of the third embodiment, the moisture condensed on the surface of the fixed contact  11  will not remain on the apex of the protrusion  21 , and it becomes easy for it to flow to the outside of the protrusion  21  along the inclined surface  21   a,  thus the contacting surface can be suppressed from freezing. 
     Modification 
     Although the angle of the inclined surface  21   a  formed from the apex of the protrusion  21  to the outside relative to the surface of the pedestal  9   a,    10   a  disposed at the terminal bolt  9 ,  10  is disclosed to be 45 degrees in the first embodiment, it is not limited to 45 degrees, and the angle may be smaller or greater than 45 degrees. 
     That is, it is possible to alter the angle of the inclined surface  21   a  appropriately according to a mounting position of the starter.