Patent Publication Number: US-6664490-B2

Title: Switching mechanism and electric switch using the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switching mechanism and an electric switch, and more particularly to a switching mechanism for a spring-reversal type of electric switch appropriate for use in electric power tools. 
     2. Related Arts 
     Spring-reversal type of electric switches are used in electric power tools for closing and opening their circuits in which heavy current flows. Such spring-reversal type of electric switches give a pleasing click feeling to users at the time of turning on and off, and the quick “on” and “off” switching action is appropriate for closing and opening circuits in which heavy currents flow. Also, spring-reversal type of electric switches can be used commonly for AC and DC. 
     Spring-reversal type of electric switches, however are liable to allow their contacts to bounce at the time of turning on and off. Particularly at the time of turning “on” a rush current flows, thereby making it easy for arcs to appear across confronting contacts as a result of bouncing. Thus, the contacts will be badly worn or deformed and accordingly the life of the electric switches will be shortened. 
     A conventional contact-making mechanism comprises a spring reversal mechanism, a push spring for producing a given strength of pressure and associated movable contacts. The contact-making mechanism, however, is liable to reduce drastically its contact pressure just prior to the reversing action, which is caused by the push spring. If the electric switch should be shaken at the instant of the contact pressure being reduced, arks are liable to appear with the result that the contacts are badly worn or deformed. 
     With a view to solve these problems of spring-reversal type of electric switches, DE19930558A1 proposes an improved contact-making mechanism, which is described below by referring to FIGS. 18 to  25 . 
     The improved spring-reversal type of electric switch  1  comprises a housing  2 , a base  3 , a cover  4 , stationary contacts  5  and associated terminals  6 , a slide  7  and associated movable contacts  8  (see FIG.  19 ), an operating lever  9  for switching operation, a first spring  10  and associated contact detents  11   a  and  11   b  (see FIG.  21 ), a second spring  12 , a slider  13  and compression springs  28  (see FIG.  19 ). 
     As seen from FIG. 18, the housing  2  has four stationary contacts  5  and associated terminals  6  fastened to its bottom, and electric wires are connected to the terminals  6 . 
     The slide  7  has four movable contacts  8  and two compression springs  28  on its lower surface as seen from FIG.  19 . As seen from FIG. 20, the slide  7  is put in the housing  2  with the movable contacts  8  confronting the stationary contacts  5 . 
     A carrier  16  has openings  17  on its opposite end walls (see FIG.  18 ). The carrier  16  contains the second spring  12 , and is movable on the slide  7 . Two stoppers  19   a  and  19   b  and a guide  14  are fastened to the upper surface of the slide  7 . The guide  14  takes the role of guiding the projections  15   a  and  15   b  of the slider  13  for engaging with the second spring  12 . 
     The slide  7  along with the slider  13  can move between the switching “off” position in which the movable contacts  8  are apart from the stationary contacts  5  and the switching “on” position in which the movable contacts  8  are in contact with the stationary contacts  5 . 
     The second spring  12  is a compression spring, which can produce a counter force opposite to the direction in which the slide  7  moves on the way to the switching point, and can produce a force in the direction in which the slide  7  moves when the switching point has been traversed. 
     The first U-shaped spring  10  is a kind of compression spring, and the U-shaped spring  10  has two legs  20   a  and  20   b , each having a ramp  21  projecting outward. The spring constant of the first spring  10  is so determined that the force produced at the switching point of the first spring  10  may be equal to the sum of the two compression springs  28  positioned behind the movable contacts  8 . 
     The contact detents  11   a  and  11   b  are given in the form of ramps  22  projecting inward from the opposite longitudinal sides of the housing  2 . Each ramp  22  is shaped asymmetric. 
     The first spring  10  works in cooperation with the detents  11   a  and  11   b  as follows: when the operating lever  9  is pushed and rotated about its pivot to drive the slide  7  for the switching-on position, the spring  10  is responsive to movement of the slide  7  for storing its resilient energy as a counter reaction until the point of critical compression (switching point) has been reached, at which point of critical compression the resistance to movement of the slide  7  is maximized. Then, the stored energy is suddenly released to jerk the slide  7  to the switching-on position. 
     The slider  13  is operatively connected to the operating lever  9  so that depression of the operating lever  9  may make the slider  13  withdraw, and that release of the operating lever  9  may make the slider  13  advance. The slider  13  has a third spring  24  contained in its chamber  27 , and it has stoppers  18   a ,  18   b  and  25  formed on its front and rear sides respectively. The stoppers  18   a ,  18   b  are formed on the projections  15   a  and  15   b.    
     The projections  15   a  and  15   b  act on the opposite ends of the second spring  12  via the guides  14  of the slide  7 , as seen from FIG.  20 . 
     There is play left between the stoppers  18   a ,  18   b  of the slider  13  and the stoppers  19   a ,  19   b  of the slide  7 , so that the slider  13  when pushed forward may travel the short distance of play before engaging with the second spring  12 . 
     The electric switch  1  turns on and off as follows: first, the electric switch  1  is put in the switching “off”-position as shown in FIG. 22, and then, the operating lever  9  is depressed so that the slider  13  may act on the left end of the second spring  12  via the projection  15   a  to stretch the spring  12 . After reducing the play the stopper  18  mates with the stopper  19   a  with the result that the slide  7  is displaced rightward for the switching “on”-position. 
     The slow displacement continues until the switching point has been reached while overcoming the counter force of the first spring  10  with its opposite legs abutting the detents  11   a ,  11   b . After traversing the switching point the energy stored in the first spring  10  and the second spring  12  are released instantly, thereby jerking the slide  7  rightward to the switching “on” position as shown in FIG.  23 . The movable contacts  8  mate with the stationary contacts  5 , and then, the compression spring  28  is compressed (see FIG.  20 ). 
     If it is desired that the electric switch  1  turn off, the operating lever  9  is released to reset the slider  13  by the third spring  24  (see FIG.  20 ). In resetting the slider  13  the projection  15   b  acts on the right end of the second spring  12 , stretching the second spring  12  after reducing the play. For the while the slide  7  remains still, keeping the movable contacts  8  and stationary contacts  5  mating together. 
     Thereafter the slide  7  moves a very short distance leftward by the force of the first spring  10  abutting the steep inclinations  29   b  of the ramps  22 . The movable contacts  8 , however, are kept still abutting on the stationary contacts  5  as the compression spring  28  is loosened. This position continues until the switching point has been reached (see FIG.  25 ). 
     After the switching point is traversed, the total energy stored in the first spring  10  and the second spring  12  is released to jerk the slide  7  leftward instantly, allowing the movable contacts  8  to leave the stationary contacts  5 . Thus, the electric switch  1  turns “off”, as shown in FIG.  22 . 
     The electric switch  1  uses the compression spring (first spring  10 ) to suppress the bouncing of the movable contacts off the stationary contacts. Specifically the movable contacts are so controlled that they may come to touch the stationary contacts slowly, and that they may leave the stationary contacts quickly. It is, therefore, most likely that the switching “on” and “off” timing varies significantly with the quality of the spring  10  used and with the wearing of the ramps  22  of the detents  11   a  and  11   b . Therefore, electric switches having the same switching characteristics can hardly be reproduced. 
     One object of the present invention is to provide a heavy-current, long-lived AC/DC switching mechanism which is free of bouncing at the time of turning on, and is capable of cutting off the flow of heavy electric current instantly at the time of turning off. 
     SUMMARY OF THE INVENTION 
     A switching mechanism in a spring-reversal type of electric switch comprising: a casing having stationary contacts mounted therein; an actuator having movable contacts to mate with the stationary contacts and springs to push the rear sides of the movable contacts; an operating lever rotatable about its pivot for switching operation; a plunger operatively connected to the operating lever; a rotatable reversal member for driving the actuator; a reversal coiled spring one end of which is connected to the reversal member and the other end of which is connected to the plunger, the reversal coiled spring being responsive to transition across its reversal point for reversing its resilient force in direction, thus making the movable contacts move toward the stationary contacts or leave apart therefrom when depressing or releasing the operating lever, 
     wherein the switching mechanism is so constructed that the actuator is allowed to move a predetermined distance before reaching the reversal point on the way to the switching “on” position, thus reducing the distance to the switching “on” position to travel the remaining distance instantly when the reversal member reverses, thereby making the movable contacts mate with the stationary contacts quickly. The distance to the switching “on” position is reduced to be short enough to cause little or no bouncing even if the movable contacts travel the remaining distance quickly to abut on the stationary contacts. 
     Also, the switching mechanism is so constructed that the actuator is prevented from moving before the reversal point is reached, and that the actuator is released after the reversal point is reached, thereby making the movable contacts leave the stationary contacts quickly. The reversal coiled spring can store a repulsive energy of the quantity large enough to make the movable contacts leave the stationary contacts very quickly when the stored energy is released. Also, advantageously the compressed coiled spring prior to arrival at the reversal point applies a push of good strength to the movable contacts against the stationary contacts, thereby avoiding unstable mechanical and electric contact between the movable and stationary contacts, which would be caused if the contact pressure were decreased between the movable and stationary contacts. 
     The rotatable reversal member has a pinion equipped therewith whereas the actuator has a rack equipped therewith. With this arrangement rotation of the reversal member is converted to the horizontal linear movement. 
     The plunger has a projection formed thereon; the rotatable reversal member has a projection formed thereon. These projections are so arranged that the projection of the plunger is responsive to depression of the operating lever for pushing the projection of the rotatable reversal member, thereby making the reversal member rotate thus to move the actuator and hence, the movable contacts close to the stationary contacts while stressing the reversal coiled spring. 
     The forward end of the plunger has a difference in level via a gentle slope formed on its lower surface. A stopper having a hook formed thereon is biased upward by a stopper spring to keep the stopper abutting on the lower surface of the plunger. The actuator has a projection to be caught by the hook of the stopper. With this arrangement the actuator is locked by allowing the projection of the actuator to be caught by the hook of the stopper. While the stopper follows and climbs the lower surface of the forward end of the plunger the actuator is being unlocked by releasing the projection of the actuator from the hook of the stopper. 
     On the way to the switching “on” position the stopper is raised, and the projection of the actuator climbs the hook of the raised stopper to be caught thereby, when the movable contacts abut on the stationary contacts, together put in locking condition. 
     The operating lever is released toward the switching “off” position to move the plunger, the gentle slope of the forward end of which still holds the hook of the stopper and the projection of the actuator in the locking condition for a while after the reversal point of the reversal spring is traversed. Upon further movement of the operating lever toward the switching “off” position the stopper follows the gentle slope of the forward end of the plunger to be lowered for unlocking and jerking the actuator, thus making the movable contacts leave the stationary contacts quickly. 
     An electric switch according to the present invention comprises: an operating lever rotatable about its pivot; a plunger operatively connected to the operating lever to move linearly in response to rotation of the operating lever; a reversal member operatively connected to the plunger; a pinion fixed to the lower surface of the reversal member; a spring combined with the reversal member, responsive to the linear movement of the plunger for storing its resilient force until a predetermined strength of resilient force has been reached, and for releasing the stored strength of resilient force to rotate the pinion of the reversal member; an actuator having movable contacts and having a rack to meet with the pinion for moving linearly in unison with rotation of the pinion; and a casing having stationary contacts on its opposite sides, whereby the movable contacts and stationary contacts are made to meet with each other in unison with reversal action of the reversal spring. 
     The rotational-and-linear mechanism stores a predetermined strength of driving force, reducing the frictional engagement of associated parts. This has the effect of avoiding the wearing of parts caused by friction, and hence extending the life of the electric switch. 
    
    
     Other objects and advantages of the present invention will be understood from the following description of a spring-reversal type of electric switch according to one preferred embodiment of the present invention, which is shown in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of an electric switch according to one embodiment of the present invention; 
     FIG. 2 is a side view of the electric switch; 
     FIG. 3 is a perspective view of the electric switch; 
     FIG. 4 illustrates, partly in section, the electric switch; 
     FIG. 5 is a similar view as FIG. 4, removing the sidewall of a reversal member; 
     FIG. 6 illustrates, partly in section, a switching mechanism; 
     FIGS. 7 a  and  7   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 8 a  and  8   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 9 a  and  9   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 10 a  and  10   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 11 a  and  11   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 12 a  and  12   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 13 a  and  13   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 14 a  and  14   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 15 a  and  15   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 16 a  and  16   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIGS. 17 a  and  17   b  are longitudinal sections of the electric switch, showing how the electric switch works; 
     FIG. 18 is an exploded view of a conventional electric switch; 
     FIG. 19 is a bottom view of a slide of the conventional electric switch; 
     FIG. 20 is a longitudinal section of the conventional electric switch; 
     FIG. 21 is a plan view of a main part of the conventional electric switch, removing the uppermost layer of a three-layer structure; 
     FIG. 22 is a plan view of the main part of the conventional electric switch, showing the intermediate layer of the three-layer structure; 
     FIG. 23 is a plan view similar to FIG. 22 but with the switch in a different condition; 
     FIG. 24 is another plan view similar to FIG. 22 but with the switch in another different condition; and 
     FIG. 25 is still another plan view similar to FIG. 22 but with the switch in yet another condition. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     A switching mechanism and an electric switch using the same according to one embodiment of the present invention are described below. In the drawings the left sides of the drawings corresponds to the front side of the electric switch and the right sides of the drawings corresponds to the rear side of the electric switch. The electric switch is equipped with the switching mechanism, and therefore, the electric switch is described by describing the switching mechanism only. 
     As seen from FIG. 1, a spring-reversal type of electric switch  50  equipped with a switching mechanism according to the present invention comprises an operating lever  51 , two return springs  52 , a cover  53 , a plunger  54 , a guide plate  55 , upper and lower disks  56   a  and  56   b , a reversal spring  57 , a reversal member  58 , an L-shaped stopper  59 , a stopper spring  60 , an actuator  61 , two terminals  62 , two stationary contacts  63 , four movable contacts  64   a ,  64   b , two movable pieces  65 , two compression springs  66 , two stationary contacts  67 , two terminals  68  and a casing  69 . 
     These parts are assembled as indicated by dot-and-dash lines in FIG. 1 into a spring-reversal type of electric switch  50  as shown in FIGS. 2 to  6 . Referring to these drawings, it is described how these parts are constructed and related operatively with each other, and how these parts work in unison. 
     The operating lever  51  is spring-biased upward. Depression of the operating lever  51  makes the switching mechanism turn on, and release of the operating lever  51  makes the switching mechanism turn off. 
     Specifically the operating lever  51  comprises an upper section curved to be in conformity with the finger, two side sections integrally connected to the upper section and a front section integrally connected to the upper and side sections, opening on its rear and lower sides. The hollow case-like operating lever  51  has two holes  51   a  made on its opposite side sections whereas the cover  53  has two pivots  53   a  projecting from the opposite sides of the rearmost part of the cover  53 . The operating lever  51  can be connected to the cover  53  by fitting the pivots  53   a  in the holes  51   a  of the operating lever  51 . 
     Also, the operating lever  51  has another two holes  51   b  made on its opposite side sections. The pivots  54   e  of the plunger  54  are fitted in the holes  51   b  of the operating lever  51  as later described. In addition, the operating lever  51  has two cocoon-like holes  51   c  made on its opposite side sections. The operating lever  51  has two projections  51   d  projecting from the ceiling of the operating lever, thereby holding the upper ends of the return springs  52  (see FIG.  4 ). The cover  53  has two projections  53   b  standing upright from its floor, thereby holding the lower ends of the return springs  52  (see FIG.  4 ). The return springs  52  bias the cover  53  upward all the time. 
     Referring to FIGS. 2 to  6 , the cover  53  has different functions in its front and rear portions. As shown in FIGS. 4 and 5, the rear portion supports the return springs  52 , and is connected to the rear part of the operating lever  51 . 
     The front portion of the cover  53  covers the casing  69 , enclosing the plunger  54 . The oblique front  53   d  of the cover  53  defines a space allotted to the inclined front  54   d  of the plunger  54 , permitting the inclined front  54   d  of the plunger  54  to move back and forth in the space. 
     The opposite side sections of the cover  53  cover the opposite sides of the casing  69  with the nails  69   a  of the casing  69  snapped in the holes  53   e  of the cover  53 . 
     As seen from FIG. 1, the plunger  54  comprises a stem  54   a , a rear block  54   b  integrally connected to the rear end of the stem  54   a , a rectangular, flattened and inverted “U”-shaped block  54   c , a triangular-pointed front  54   d  integrally connected to the flattened and inverted “U”-shaped block  54   c  and a guide plate  55  fastened to the lower surface of the flattened and inverted “U”-shaped block  54   c . A first projection  54   g  projects downward from the rear end of the flattened and inverted “U”-shaped block  54   c , and a projection  55   a  projects downward from the center of the guide plate  55  (see FIG.  6 ). 
     The rear block  54   b  has pivots  54   e  extending outward from its opposite sides, which are fitted in the pivot holes  51   b  made in the operating lever  51 . Inclination of the operating lever about the pivots  54   e  is transmitted to the rear block  54   b . Reciprocation of the rear block  54   b  is transmitted to the flattened and inverted “U”-shaped block  54   c  via the stem  54   a . 
     The triangular front  54   d  extends from the middle of the flattened and inverted “U”-shaped block  54   c . The upper contour of the triangular front  54   d  is in conformity with the inside of the oblique front of the cover  53 . The lower surface of the triangular front  54   d  is defined by a first horizontal surface  54   d   1 , a first downward-oblique surface  54   d   3  continuous from the rear end of the first horizontal surface, a second horizontal surface  54   d   2  continuous from the rear end of the downward-oblique surface and a second upward oblique surface  54   d   4  continuous from the rear end of the second horizontal surface, reaching the flattened and inverted block  54   c (see FIG.  6 ). 
     As described later, the L-shaped stopper  59  is kept in contact at its top end with the contour of the lower surface of the triangular front  54   d  to control the vertical movement of the L-shaped stopper and the on-and-off timing. 
     The first projection  54   g  of the plunger  54  has the role of moving the projection  58   e  of the reversal member  58 , as described later. The projection  55   a  of the guide plate  55  abuts on the upper disk  56   a , engaging with the upper end of the reversal coiled spring  57 . 
     A packing  54   f  has a center aperture to allow the stem  54   a  to pass therethrough, so that it is fitted in between the cover  53  and the casing  69 , thereby preventing invasion of dust when the plunger  54  moves back and forth. 
     The guide plate  55  is press-fitted in between the opposite legs of the flattened and inverted “U”-shaped block  54   c  of the plunger  54 , and the intermediate projection  55   a  engages with the upper disk  56   a , as described above. 
     The upper disk  56   a  has a concavo-convex surface larger than the diameter of the projection  55   a  of the guide plate  55  (see FIG.  6 ). The projection  55   a  of the guide plate  55  abuts on the concave surface of the upper disk  56   a , thereby permitting the upper disk  56   a  to incline like a spindle. Thus, reciprocation of the plunger  54  can be transmitted from the projection  55   a  to the reversal spring  57  via the upper disc  56   a.    
     The lower disk  56   b  has a concavo-convex surface larger than the diameter of the projection of the reversal member  58 . The round end of the projection of the reversal member  58  abuts on the concave surface of the lower disk  56   b , thereby permitting the lower disk  56   b  to incline like a spindle. 
     The reversal spring  57  is sandwiched between the upper and lower disks  56   a  and  56   b  under a predetermined pressure, and it is responsive to the reciprocation of the plunger  54  for inclining forward and rearward, storing its resilient force. When the reversal spring  57  reaches the reversal point, the stored energy is increased to the maximum. 
     The reversal member  58  comprises a rectangular, upward-curved circular-arc plate  58   a  whose width is somewhat narrower than the inner width of the cover  53 , two side plates  58   b  standing upright from the circular-arc plate  58   a , separated from each other a distance somewhat longer than the diameter of the lower disc  56   b , an elongated pinion  58   c  extending along the outer surface of the circular-arc plate  58   a , patches  58   d  fastened to the upper ends of the side plates  58   b , a rear projection  58   e  integrally connected to the rear end of the pinion  58   c  and a front projection  58   f  integrally connected to the front end of the pinion  58   c.    
     The reversal spring  57  is put in between the opposite side plates  58   b . The pinion  58   c  engages with -the rack  61   a  of the actuator  61  for converting inclination of the operating lever  58  to the linear movement of the actuator  61 , as later described. The patches  58   d  are fitted in the holes made in the upper, inner sides of the cover  53  to provide pivots about which the reversal member  58  can rotate (see FIG.  4 ). The rear projection  58   e  is operatively related with the first projection  54   g  of the plunger  54  as later described. The front projection  58   f  is operatively related with the projection  54   i  of the plunger  54 . 
     The reversal member  58  is pressed by the reversal spring  57  all the time. The pressure is increased to the maximum at the reversal point of the reversal spring  57 . 
     The L-shaped stopper  59  has its vertical leg slidably fitted in the vertical slot, which is provided at the intermediate of the front end of the casing  69 . The vertical leg  59  has a rearward-inclined surface  59   b  defined on its upper end. The L-shaped stopper  59  is kept at its upper end in contact with the lower surface of the front  54   d  of the plunger  54 . 
     The horizontal leg of the L-shaped stopper  59  extends rearward in parallel with the floor of the casing  69 . The horizontal leg of the L-shaped stopper  59  has a rearward-inclined projection formed as a hook  59   a . The hook  59   a  is adapted to be engaged with the projection  61   c  of the actuator  61 . 
     The stopper spring  60  is put in a hole, which is made in the vertical leg of the L-shaped stopper  59 . Thus, the L-shaped stopper  59  is raised upward, so that it may follow the lower surface contour of the front  54   d  of the plunger  54  when moving back and forth. 
     As seen from FIG. 6, when the vertical leg of the stopper  59  is kept at its upper end in contact with the second horizontal surface  54   d   2  of the lower contour of the front  54   d  of the plunger  54 , the stopper  59  is lowered against the stopper spring  60 . As the upper end  59   b  of the vertical leg of the stopper  59  is displaced rearward, it climes the first oblique slope  54   d   3 . While the upper end  59   b  of the vertical leg of the stopper  59  remains in contact with the first horizontal surface  54   d   1  of the front  54   d , the stopper  59  is kept at its raised level. 
     The lengths of the horizontal and oblique surfaces are determined in consideration of the time at which the projection  61   c  of the actuator  61  is caught by the hook  59   a  of the stopper  59 , i.e., at the time of switching “off” or of the movable contacts leaving the stationary contacts. 
     The rack  61   a  engages with the pinion  58   c  of the reversal member  58 ; two box-like guide blocks  61   b  are integrally connected to the opposite sides of the rack  61   a ; two movable contact pieces  65  are fastened to the guide blocks  61   b  on their front sides, each contact piece  65  having upper and lower contacts  64   a  and  64   b  fixed to its front surface; two compression springs  66  push the movable contact pieces  65  forward, each compression spring  66  being fitted in the box-like guide block  61   b ; and two projections  61   c  project downward from the lower surface of the rack  61   a . All of these parts together make up the actuator  61 . 
     The so constructed actuator  61  can be moved back and forth by the reversal member  58 . The actuator  61  moves on an actuator guide, which is laid on the floor of the casing  69 , carrying the movable contacts  64  to attain the on-and-off switching action. Specifically forward movement of the actuator  61  makes the movable contacts  64  touch the stationary contacts  63  and  67  whereas rearward movement of the actuator  61  makes the movable contacts  64  leave the stationary contacts  63  and  67 . 
     The two terminal pieces  62  are fixed to the front, opposite portions of the floor of the hollow casing  69 , and the stationary contacts  63  are fixed to the terminals  62 . These lower stationary contacts  63  confront the lower movable contacts  64   b  of the actuator  61 . 
     On the other hand, two Z-shaped terminal pieces  68  are fixed at their feet to the rear, opposite portions of the floor of the casing  69 , and two stationary contacts  67  are fixed to the bent ends of the raised arms of the Z-shaped terminal pieces  68 , confronting the upper movable contacts  64   a  of the actuator  61 . 
     The casing  69  is like a box having front, rear and opposite sidewalls to define its inner space. Each sidewall is composed of two upright plates, between which the arm of each terminal piece  68  is inserted. 
     The plunger  54 , the reversal member  58  and the actuator  61  together provide a switching mechanism, in which these parts are so linked that the movable contacts  64  may touch the stationary contacts  63 ,  67  slowly, and that the movable contacts  64  may leave the stationary contacts  63 ,  67  quickly. 
     Referring to FIGS. 7 a  to  17   b,  the manner in which the switching mechanism works is described below. Each pair of drawings (i.e. FIGS. 7 a  and  7   b,  FIGS. 8 a  and  8   b,  FIGS. 9 a  and  9   b,  etc.) presents two sectional views illustrating how the movable contacts are displaced with respect to the stationary contacts; and how the reversal member  58  is related with the actuator  61  in operation. 
     Referring to FIGS. 7 a  and  7   b , in the initial position in which the electric switch is not operated, the plunger  54  is energized by the return spring  52  in the direction as indicated by the arrow “A”. The front  54   d  of the plunger  54  abuts against the inner wall of the front of the cover  53 , thus preventing further advance of the plunger  54 . In this position the reversal member  58  is urged counterclockwise by the reversal spring  57 , and therefore, the actuator  61  is energized in the direction as indicated by the arrow “B”, but it cannot move. 
     Referring to FIGS. 8 a  and  8   b , the operating lever  51  is pushed to rotate in the direction as indicated by the arrow “C”, pulling the rear block  54   b  in the direction as indicated by the arrow “D”. Accordingly the flattened and inverted “U”-shaped block  54   c  and the projection  55   a  of the underlying guide plate  55  are pulled in the direction as indicated by the arrow “D”. Then, the projection  55   a  pushes the upper disk  56   a  rearward, beginning compression of the reversal spring  57 , but the reversal member  58  still holds the associated parts as they are. 
     Referring to FIGS. 9 a  and  9   b , the operating lever  51  is rotated further in the direction as indicated by the arrow “C”, and the plunger  54  is moved in the direction as indicated by the arrow “D”, allowing the stopper  59  to follow the lower surface contour of the front  54   d  of the plunger  54 . Further movement of the plunger  54  in the direction “D” makes the projection  54   g  of the plunger  54  abut on the projection  58   e  of the reversal member  58 . The reversal member  58  still holds the associated parts as they are. 
     Referring to FIGS. 10 a  and  10   b , further rotation of the operating lever  51  in the direction as indicated by the arrow “C” pulls the plunger  54  in the direction as indicated by the arrow “D”, thereby making the projection  54   g  of the plunger  54  push the projection  58   e  of the reversal member  58  backward. The reversal member  58  is rotated in the direction as indicated by the arrow “E”, thereby making the rack  61   a  move in the horizontal direction as indicated by the arrow “F” through the agency of the pinion  58   c  of the reversal member  58 . As a result, the distance between the movable contacts  64  and the stationary contacts  63 ,  67  is reduced. As the reversal spring  57  has not reached the reversal point, the reversal member  58  is still prevented from turning toward the opposite side. 
     Further rotation of the operating lever  51  in the direction as indicated by the arrow “C” pulls the plunger  54  in the direction as indicated by the arrow “D” still further (see FIGS. 11 a  and  11   b ), thereby making the projection  55   a  of the guide plate  55  catch and pull the reversal spring  57  by the upper end in the horizontal direction as indicated by the arrow “D”. Then, the reversal spring  57  reaches the reversal point for releasing the energy stored in the reversal spring  57 . 
     At the time of traversing the reversal point the reversal spring  57  extends to apply its resilient force to the reversal member  58 , thereby forcedly rotating the reversal member  58  in the direction as indicated by the arrow “E”. Accordingly the actuator  61  is jerked in the direction as indicated by the arrow “F”, making the forward guide projection  61   c  ride over the oblique surface of the hook  59   a  of the stopper  59  while overcoming the counter force applied by the stopper spring  60 . Then, the actuator  61  moves until the front of the actuator  61  has abutted on the wall of the casing  69 , where the actuator  61  stops. 
     In this position the movable contacts  64  come to touch the stationary contacts  63 ,  67 , making the electric switch turn on. The movable contacts  64  are pushed against the stationary contacts  63 ,  67  by the compression springs  66 , which are contained in the guide blocks  61   b  of the actuator  61 . The distance between the movable contacts and the stationary contacts is reduced to be short enough to prevent the movable contacts from bouncing off the stationary contacts when hitting them. 
     The force applied to the reversal member  58  by the reversal spring  57  is stronger than the force of the compression springs  66 , and therefore, the actuator  61  cannot be moved in the direction opposite to that indicated by the arrow “F” to reduce the pressure appearing between the movable and stationary contacts  64  and  63 ,  67 . 
     Referring to FIGS. 12 a  and  12   b , the operating lever  51  is fully rotated, and then, the reversal member  58  is kept energized in the direction as indicated by the arrow “E”, and the actuator  61  is kept energized in the direction as indicated by the arrow F. The compression springs  66  remain to be compressed. Thus, the movable contacts  64  are pushed against the stationary contacts  63 ,  67  under a predetermined pressure, so that any adverse effect may be caused on the contact-making condition even if the electric switch should be shocked. 
     Referring to FIGS. 13 a  and  13   b , the push given to the operating lever  51  is reduced more or less, the operating lever  51  is moved back by the return spring  52  in the direction as indicated by the arrow “H”, and at the same time, the rear block  54   b  of the plunger  54  is pushed in the direction as indicated by the arrow I. Then, the projection  55   a  of the guide plate  55  pushes the upper disk  56   a  forward, starting compression of the reversal spring  57 . In this position, however, the reversal member  58  remains as it is, while being kept energized in the direction as indicated by the arrow E. 
     Referring to FIGS. 14 a  and  14   b , the operating lever  51  is rotated further in the direction as indicated by the arrow H, moving the plunger  54  in the direction as indicated by the arrow I. As a result the reversal spring  57  comes close to the reversal point. Around the reversal point the reversal member  58  is about to be jerked by the reversal spring  57  and the cooperative compression springs  66  of the actuator  61 , reducing the pressure appearing between the movable contacts  64  and the stationary contacts  63 ,  67 . 
     The actuator  61  cannot be moved backward because the guide projection  61   c  of the actuator  61  is caught by the hook  59   a  of the stopper  59 . Thus, the movable contacts  64  remain to be pushed against the stationary contacts  63 ,  67 . 
     Referring to FIGS. 15 a  and  15   b , further rotation of the operating lever  51  in the direction as indicated by the arrow H brings the reversal spring  57  close to the reversal point for rotating the reversal member  58  in the direction as indicated by the arrow K. As is the case with the position of FIGS. 14 a  and  14   b , the guide projection  61   c  of the actuator  61  is caught by the hook  59   a  of the stopper  59 , thereby preventing the actuator  61  from moving backward. Thus, the electric switch is kept turning on. 
     As the plunger  54  moves in the direction as indicated by the arrow  1 , the hook  59   a  of the horizontal leg of the L-shaped stopper  59  lowers gradually while the vertical leg  59  of the L-shaped stopper  59  following the lower surface contour of the front  54   d  of the plunger  54  overcomes the stopper spring  60 . 
     Referring to FIGS. 16 a  and  16   b , still further rotation of the operating lever  51  in the direction as indicated by the arrow H moves the plunger  54  in the direction as indicated by the arrow I. The hook  59   a  of the stopper member  59  is lowered to release the guide projection  61   c  of the actuator  61  from the hook for unlatching. 
     The reversal spring  57  traverses the reversal point to release the stored energy, thereby making the reversal member rotate instantly in the direction as indicated by the arrow K. Then, the actuator  61  is jerked in the direction as indicated by the arrow J via the pinion-and-rack mechanism, and the movable contacts  64  leave the stationary contacts  63 ,  67  quickly. The electric switch turns off, returning to the initial position as shown in FIGS. 7 a  and  7   b.    
     The electric switch is equipped with a forced contact-separation mechanism, by which the movable contacts  64  can be pulled off from the stationary contacts even if the movable contacts  64  are lightly melted and attached to the stationary contacts  63 ,  67 . 
     Referring to FIGS. 17 a  and  17   b , even if the movable contacts  64  are lightly melted and attached to the stationary contacts  63 ,  67 , the operating lever  51  is rotated in the direction as indicated by the arrow H to move the plunger  54  in the direction as indicated by the arrow I. The lower surface contour of the front  54   d  of the plunger  54  makes the stopper  59  descend to release the guide projection  61   c  of the actuator  61  from the hook  59   a , but the electric switch is kept turning on in spite of the reversal point having been traversed. 
     The plunger  54  is pushed still further by the return spring  52  in the direction as indicated by the arrow I. As a result, the projection  54   i  of the plunger  54  abuts on the projection  58   f  of the reversal member  58  to rotate the reversal member  58  in the direction as indicated by the arrow K. Then, the actuator  61  is moved by the reversal member  58  in the direction as indicated by the arrow J, forcedly separating the movable contacts  64  from the stationary contacts  63 ,  67 . 
     As may be understood from the above, the switching mechanism according to the present invention uses the reversal spring for quickly turning on and off in such a way that the movable contacts may be brought close to the stationary contacts prior to the turning-on, thereby permitting the quick turning-on subsequent to traverse of the reversal point without the bouncing of the movable contacts off from the stationary contacts, and that movement of the movable contacts may be prevented before the reversal spring has stored an increased amount of energy, allowing the quick release of the stored energy to make the movable contacts leave the stationary contacts at a speed high enough to prevent appearance of electric arcs between the movable and stationary contacts, and hence the wearing of the contacts. 
     A coiled spring rather than a spring plate is used as the reversal spring because reversal springs of the same quality are commercially available, thus facilitating reproduction of spring-reversal type of electric switches of the same quality. An AC/DC electric switch suitable for use in electric power tools according to the present invention is guaranteed to be free of bouncing and wearing, and it can have a long-life and is of a high rating.