Patent Publication Number: US-6664487-B2

Title: Limit switches

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to limit switches used for detection of objects or the like such as safety limit switches which have passed certain safety standard tests including the dielectric strength test, the electrical endurance test and the cable strength test. 
     FIG. 29A shows an example of prior art limit switch S- 1 , comprising a switch unit  190  and a head unit  191 . The switch unit  190  includes a case  192  and a plunger  193  and also contains an inner switch  194  with a mobile switch plunger  195 . The switch plunger  195  supports two mobile members (not shown) and is contained inside a switch plunger holder (not shown) by means of a returning spring (not shown). Mobile contact points (not shown) on the mobile members and fixed points of a fixed terminal (not shown) form a contact point part. The head unit  191  includes a rotary shaft  197  rotatably supported by a head case  196 . A roller lever  199  with a roller  198  is attached to an end part of this rotary shaft  197 , and a cam  200  is provided to the rotary shaft  197 . The limit switch is formed by attaching the head unit  191  to the switch unit  190  such that the plunger  193  is pushed against the cam  200  of the rotary shaft  197 . The head unit  191  is attached to the switch unit  190  by placing the head unit  191  on top of the switch unit  190 , pushing in screws (not shown) into holes formed in the head case  196  of the head unit  191  and engaging these screws in threaded holes (not shown) formed in the case  192  of the switch unit  190 . This method of attachment, however, did not provide enough rigidity. 
     The limit switch is operated by moving a so-called dog (not shown) to cause the roller  198  of the limit switch to contact the dog and to tilt the roller lever  199 . As the roller lever  199  is tilted, the rotary shaft  197  is rotated to push the plunger  193  such that the switch plunger  195  of the inner switch  194  is pressed and the inner switch carries out a switching operation. 
     With a prior art limit switch, if the number of circuits for the inner switch  194  is increased, the total number of components increases and the inner switch  194  becomes too large, causing the limit switch as a whole to become also too large. If each constituent parts is made small so as to prevent the limit switch from becoming too large, however, it becomes difficult to produce a switch which can pass safety tests. 
     For attaching the limit switch to a target structure  201  (such as a wall), as shown in FIG. 29B, a screw  202  is passed through a screw hole  192 A in the case  192  of the switch unit  190  and is screwed into the target structure  201 . With a prior art limit switch, the attachment between the head unit  191  to the switch unit  190  was not sufficiently rigid, as explained above. If an object  203  is unexpectedly inserted or falls into the space between the head unit  191  and the target structure  201  to which the limit switch is to be attached, as shown in FIG. 29B, the head unit  191  may tilt forward as the screw  202  is tightened, causing the limit switch be deformed or otherwise damaged. If the limit switch is provided with an attachment member for attaching it to a target structure (such as shown at  201 ) and if there is a gap provided between the attachment member and the target structure, furthermore, it is necessary to fill the gap with a spacer or the like for preventing deformation and breakage. 
     Another problem with prior art limit switches was that any attempt to improve their sensitivity in response to the market demand had adverse effects on resistance against vibrations, shocks and telegraphing. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a limit switch which need not be increased in size to satisfy the safety requirements of regulations although the total number of circuits is increased. 
     It is another object of the invention to provide a limit switch which is sufficiently strong and hence does not become deformed or damaged at the time of attachment and hence does not require extra components for preventing deformation and breakage or damage easily even if an unwanted object is inserted between the head unit and the target structure on which it is to be attached. 
     A limit switch embodying this invention may be characterized not only as comprising an inner switch having a plurality (say, more than four) of switch element units and an inner case, the inner case containing the inner switch and having covers and a switch case, a partition wall being provided such that independently provided switch areas sandwich the partition wall in between, and the switch element units being distributed individually into these switch areas, and the covers each sealing a corresponding one of the switch areas such that insulation characteristics can be improved. Screening members may be further provided for separating the switch element units when they are contained in the switch areas. 
     Each switch element unit comprises a fixed terminal having two (first and second) fixed contact points, a mobile member having two (first and second) mobile contact points and a switch plunger. The first fixed contact point and the first mobile contact point together form a first contact unit, and the second fixed contact point and the second mobile contact point together form a second contact unit. The switch plunger serves to open and close these contact units. 
     The switch areas include a part for containing the switch plunger (“switch plunger containing part”), parts for attaching the fixed terminals (“fixed terminal attachment parts”) sandwiching the switch plunger containing part from both sides, and lead line areas for keeping lead lines disposed along the fixed terminal installation parts and connected to the fixed terminals. With the switch areas thus structured, the number of switch element units may be increased without increasing the size of the limit switch without adversely affecting the safety characteristics. 
     The fixed terminal attachment parts include a plurality of fixed terminal installation parts for detachably installing the fixed terminals along the direction in which the switch plunger moves. The switch plunger comprises a switch plunger main body having a plurality of mobile members detachably attached and an elastic holder for holding and applying a biasing force on the mobile members. The fixed terminals are detachably set to the fixed terminal installation parts, and the switch plunger is biased by a returning spring when contained movably inside the switch plunger containing part. With a limit switch thus structured, the structure of contact units can be modified without using different components and only by changing the position of the mobile member, making it possible to develop an increased number of variations. Moreover, since common components will be used, it does not incur any extra cost to develop such variations in the structure of the contact units. 
     A limit switch of this invention may comprise plunger means for operating inner switches, including a link plunger for operating switch plungers of a plurality of inner switches. With such a structure, a link plunger as a different component can operate the switch plunger s of a plurality of inner switches and since this does not depend on the shape of the actuator, it becomes easier to develop variations. 
     The main body of the case may be provided with a hole part for the link plunger to pass through and a seal cap holder at this hole part for holding a seal cap. The link plunger has a plunger engaging part. The seal cap comprises a cap main body made of an elastic material with two (first and second) engaging parts at end parts of the cap main body, one of them formed as an O-ring. The link plunger is attached to the seal cap by engaging the first engaging part with the plunger engaging part and the second engaging part with the seal cap holder. Since the seal cap has the function of an O-ring, a more reliable sealing can be effected and the link plunger can follow the motion of the switch plunger more efficiently. 
     The inner case may also include a fastening case such that the cable of lead lines will be sandwiched between the switch case and this fastening case as the lead lines of the cable are connected to the inner switch and the switch case is fastened to the fastening case. With such a structure, the sealing resin (adhesive) can be effectively prevented from flowing into the inner switch. 
     The fastening case may be connected to the lead lines of the cable. The cable may have a sealing member with a sealing ring outside. The fastening case may have a cable-passing hole for passing the cable through. The sealing member is inserted into this cable-passing hole. The fastening case has seal holders and the cable is sealed by pressing the sealing ring with the seal holders. 
     One advantage of connecting the lead lines to the fixed terminal by compression is that solder does not have to be used for the purpose and the production method is gentler to the environment. 
     Compressing means such as protrusions may be provided on the inner surface of the switch cover for a portion of the lead lines. When a plurality of switch element units are disposed separately in the switch areas, each being covered by its own switch cover, the connection of the lead lines can be secured because such compression means is used to apply compressive force. 
     The switch case may be provided with an elastic grounding line holder for holding a grounding line and causing it to contact the main body of the switch. One advantage of this method of attachment is that it is not necessary to drill a hole through a portion of the outer case where the grounding line is to be attached. 
     The invention also teaches to arrange the lead lines symmetrically inside the cable such that the limit switch can be made compact and the arrangement of the lead lines becomes simpler. 
     In order to improve the rigidity, the limit switch of this invention may be characterized as comprising a switch unit, a head unit which operates the switch unit, a protrusion on either of the switch unit and the head unit and an indentation on the other such that the head unit is connected to the switch unit by engaging the protrusion in the indentation. Rigidity improves because the protrusion and the indentation engage each other in a manner of embracing each other. The switch unit comprises a switch main body having shoulders, the protrusion being formed by steps at the shoulders. The head unit has a head case with two legs protruding to form the indentation therebetween. The protrusion has an attachment hole part. A screw member is inserted in this attachment hole part and compresses the legs to the protrusion so as to fasten the switch main body and the head case together. The switch unit contains an inner switch, and the limit switch further comprises a head unit displacing means for moving the head unit in the direction of movement of the switch plunger. The head unit displacing means may include a spacer inserted between the head unit and the switch unit. 
     The head unit may include a head case and a rotary shaft rotatably supported by the head case, the head case displays two or more set zones, the rotary shaft having a zone indicator for selectively indicating one of these set zones. When two or more inner switches with different operating characteristics are included, these set zones can be selectively utilized. Since only one head unit is needed for two or more inner switches, the present invention contributes to the reduction in cost. 
     The limit switch may comprise a lever for rotating the rotary shaft, a lever angle adjuster supporting the lever, and a roulette part formed on a portion of a peripheral part of the lever angle adjuster. The rotary shaft and the lever angle adjuster engage to each other through the roulette part. This lever may be a form lock lever having a pair of holes which are elongated in the direction of the lever and gear parts on the elongated holes. The lever angle adjuster is provided with a pair of lever holders with lever engaging parts. The lever holders are inserted to the elongated holes so as to engage the gear parts with the lever engaging parts so as to connect the form lock lever with the lever angle adjuster. 
     The head unit includes an actuator for operating the switch head. The actuator comprises a roller to be contacted by a mobile member and a lever which supports the roller. The lever tilts when the mobile member contacts the roller. A weight may be provided on the lever at a position opposite to the roller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of a limit switch embodying this invention. 
     FIG. 2 is a partially broken diagonal view of the limit switch of FIG.  1 . 
     FIG. 3 is a front view of an inner switch in the switch unit of the limit switch of FIGS. 1 and 2. 
     FIG. 4 is a diagonal view of the outer case of the limit switch of FIGS. 1 and 2. 
     FIGS. 5A and 5B, together referred to as FIG. 5, are sectional views taken respectively along lines  5 A— 5 A and  5 B— 5 B of FIG.  4 . 
     FIG. 6 is a partially broken diagonal view of the seal cap. 
     FIG. 7 is a diagonal view of the link plunger. 
     FIG. 8 is a front view of the switch case of the switch unit. 
     FIG. 9 is a side view of the switch case. 
     FIG. 10 is a diagonal view of the switch case taken from the upper right-hand side. 
     FIG. 11 is a diagonal view of the switch case taken from the lower left-hand side. 
     FIG. 12 is a diagonal view of the switch cover of the switch unit seen from inside. 
     FIGS. 13A and 13B, together referred to as FIG. 13, are respectively a front view and a plan view of the fastening case of the switch unit. 
     FIGS. 14A and 14B, together referred to as FIG. 14, are respectively a diagonal view of a fixed terminal and a sectional view of the cable. 
     FIGS. 15A,  15 B and  15 C, together referred to as FIG. 15, are respectively a plan view, a front view and a side view of the switch plunger. 
     FIG. 16 is a diagonal view of the head case of the head unit. 
     FIG. 17 is a diagonal view of the rotary shaft of the head unit. 
     FIGS. 18A and 18B, together referred to as FIG. 18 are respectively a view of the outer surface and the inner surface of the lever angle adjusting member of the head unit. 
     FIG. 19 is a diagonal view of the mobile member of the head unit. 
     FIGS. 20A and 20B, together referred to as FIG. 20, are respectively a front view and a side view of the switch plunger after mobile members have been replaced. 
     FIGS. 21A and 21B, together referred to as FIG. 21, are diagrams respectively of two circuits each having a normally open contact point and a normally closed contact and two circuits both having a normally closed contact point. 
     FIG. 22 is a front view of another limit switch according to a second embodiment of the invention. 
     FIG. 23 is a sectional view of the fastening case of the limit switch of FIG.  22 . 
     FIG. 24 is a diagonal view of the fastening case of FIG. 23 for showing its attachment. 
     FIG. 25 is a diagonal view of the seal holder. 
     FIG. 26 is a diagonal view of still another limit switch according to a third embodiment of the invention, separated into parts. 
     FIG. 27A is a diagonal view of still another limit switch embodying this invention, and FIG. 27B is a front view of a form-lock lever formed differently. 
     FIG. 28 is a front view of still another limit switch embodying this invention. 
     FIG. 29A is a vertical sectional view of a prior art limit switch and FIG. 29B is its schematic side view when it is being installed on a target structure. 
    
    
     Throughout herein, corresponding components are indicated by the same symbols or numerals and are not always described repetitiously even where they are components of different limit switches according to different embodiments of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described next by way of an example with reference to FIGS. 1-21. 
     FIGS. 1 and 2 show a limit switch S embodying this invention, comprising a switch unit A and a head unit B. The switch unit A is comprised of an outer case  1  which serves as the main body of the switch, an inner switch  3  and a link plunger mechanism  4 . As shown in FIGS. 4 and 5, the outer case  1  is box-shaped with an upper surface part  1 A, a front surface part  1 B, a back surface part  1 C, a left-hand side surface part  1 D and a right-hand side surface part  1 E, the bottom being open. 
     A head unit attachment part  10  and a plunger attachment part  11  are formed on the upper surface part  1 A of the outer case  1 . The head unit attachment part  10  is formed by cutting off right-hand and left-hand end portions of the upper surface part  1 A at the top of the front and back surface parts  1 B and  1 C of the outer case  1  to form indentations  12 A and  12 B and protrusions  13  sandwiched therebetween. Throughholes  14  in the front-back direction are provided through the protrusion  13 , and screw holes  15  are provided on upper surface parts  13   a  of the protrusion  13 . 
     The plunger attachment part  11  is provided with a circular hole  16  formed at the center of the upper surface part  1 A of the outer case  1  so as to be sandwiched between the two protrusions  13 . This circular hole  16  opens to the center of a ceiling part of the interior space K of the outer case  1 . A seal cap holder  17  is provided inside the hole  16 . 
     A cable opening  18  is provided at the lower right-hand end part of the outer case  1 , and pin-receiving holes  20  are formed on the right-hand and left-hand side end parts near the lower end parts of the front and back surfaces parts  1 B and  1 C of the outer case  1 . 
     As shown in FIG. 2, the link plunger mechanism  4  includes a link plunger  61  and a seal cap  62 . As shown in FIG. 7, the link plunger  61  has a cap engaging part  63  at one end of its rod-shaped main body  61 A. The seal cap  62  is made of an elastic material such as rubber. As shown in FIG. 6, its main body  62 A is shaped like a dome. An upper engagement part  64 A shaped like an O-ring is formed on its upper end and an annular lower engagement part  64 B is formed on its lower end. 
     As shown in FIG. 2, the seal cap  62  is attached to the link plunger  61  by engaging its upper engagement part  64 A with the cap engagement part  63  of the link plunger  61 , and the link plunger  61  is attached to the outer case  1  by engaging the lower engagement part  64 B of the seal cap  62  with the seal cap holder  17  inside the plunger attachment part  11 . 
     The inner switch  3  is contained inside an inner case  2  comprised of a switch case  21 , two switch covers  5  and  6  and a fastening case  7 . As shown in FIGS. 8-11, the switch case  21  has a planar main body  22  with its surface part forming a partition wall part  22 F. Switch areas  25  and  25 - 1  are formed in front of and behind this partition wall part  22 F, sandwiching the partition wall part  22 F in between. A switch plunger containing part  32  for containing a switch plunger (to be described below), attachment parts  32 L- 1  and  32 R- 1  which are on both sides of this switch plunger containing part  32  and serve to detachably attach fixed terminals (to be described below), lead line areas  32 L- 2  and  32 R- 2  which are disposed along these attachment parts  32 L- 1  and  32 R- 1  and serve to dispose lead lines to connected to the fixed terminals, upper and lower switch element containing parts  32 F and  32 G for containing switch element units of the inner switch to be described below and a screening member  32 H for separating the upper and lower switch element containing parts  32 F and  32 G are formed in these switch areas  25  and  25 - 1 . 
     The main body  22  of the switch case  21  has an upper surface  22 A having a cut part  26  at its center and protrusions  35 A formed on the right-hand and left-hand sides. The main body  22  also has a lower surface  22 B at the center of which is a bag-shaped member  22 C protruding downward. A spring receiver  44 A is provided at the bottom of this bag-shaped member  22 C. 
     Left-hand and right-hand vertical ribs  27  and  28  are formed respectively in the switch areas  25  and  25 - 1 , extending parallel to each other. The vertical ribs  27  and  28  are respectively provided with a horizontal rib  29  or  30  extending towards the center of the case main body  22 . The switch plunger containing part  32  is formed with the upper surface part  22 A, a gap  31  between the two horizontal ribs  29  and  30  and the bag-shaped member  22 C. An edge portion of the cut part  26  at the upper surface  22 A serves as a stopper part  26 A. 
     Grooves  33 ,  34  and  35  are formed at upper, middle and lower parts of the pair of ribs  27  and  28 , and engaging parts  36 ,  37  and  38  are formed outside of the case main body  22  at its upper, middle and lower parts, the lower engaging parts  38  being formed in opposite direction to the upper and middle engaging parts  36  and  37 . The upper grooves  33  and the upper engaging parts  36  together form upper installation parts  36 - 1 , the middle grooves  34  and the middle engaging parts  37  together form middle installation parts  36 - 2 , and the lower grooves  35  and the lower engaging parts  38  together form lower installation parts  36 - 3  for the fixed terminals. Of these installation parts  36 - 1 ,  36 - 2  and  36 - 3 , those on the left-hand side and those on the right-hand side are respectively referred to as the left-hand and right-hand attachment parts  32 L- 1  and  32 R- 1 . 
     A tongue-shaped member  39  is formed on the left-hand and right-hand edge parts of the case main body  22  and the aforementioned lead line areas  32 L- 2  and  32 R- 2  are formed between the tongue-shaped member  39  and the engagement parts  36 ,  37  and  38  on both sides. Left-hand and right-hand lead line inlets  40  and  41  are provided on the left-hand and right-hand parts of the lower surface  22 B. The left-hand lead line inlet  40  connects to the left-hand lead line area  32 L- 2  and the right-hand lead line inlet  41  connects to the right-hand lead line area  32 R- 2 . 
     The upper installation parts  36 - 1  form the aforementioned upper switch element containing parts  32 F, and the lower installation parts  36 - 3  (or the middle installation parts  36 - 2 ) form the lower inner switch element containing parts  32 G. The horizontal ribs  29  and  30  form the screening member  32 H for separating the upper and lower switch element containing parts  32 F and  32 G. 
     A grounding line holder  51  is formed on the lower surface  22 B of the case main body  22 , having a protrusion  52  from the lower surface  22 B, a grounding line inlet  53  and an indentation  54  for winding the grounding line around. A semispherical indentation  55  and protrusion  55 A are formed on the right-hand side of the lower surface  22 B. 
     Fixed terminals are intended to be installed on the upper, middle and lower installation parts  36 - 1 ,  36 - 2  and  36 - 3 . Since this example is for a two-element structure (with two circuits), fixed terminals are installed only on the upper and lower installation parts  36 - 1  and  36 - 3 . The fixed terminal  42 L installed on the upper left-hand installation part  36 - 1  and the fixed terminal  44 R installed on the lower right-hand installation part  36 - 3  are of the same structure. As shown in FIG. 14, these two fixed terminals  42 L and  44 R each have a compression contact part  42   b  or  44   b  at the base and a fixed contact part  42   a  or  44   a.  Similarly, the fixed terminal  42 R installed on the upper right-hand installation part  36 - 1  and the fixed terminal  44 L installed on the lower left-hand installation part  36 - 3  are of the same structure, which is the reverse of that of the fixed terminals  42 L and  44 R. 
     Fixed terminals  42 L and  42 R are positioned in the upper switch element containing part  32 F and are engaged with the upper left-hand and right-hand engagement parts  36 , their end parts being inserted into the grooves  33 . Similarly, fixed terminals  44 L and  44 R are positioned in the lower switch element containing part  32 G and are engaged with the lower engagement parts  38 , their end parts being inserted into the grooves  35 . 
     As shown in FIG. 12, the switch covers  5  and  6  are shaped so as to cover the switch area  25 . On the left-hand side and right-hand side of the inner surface of their planar main body  5 A are protrusions  45 ,  46  and  47  serving as compressing means for upper, middle and lower parts. A tongue-shaped member  48  is provided at the center along the upper edge of the main body  5 A and engaging members  49  having a hole  49 A are provided on the left-hand and right-hand sides. Tongue-shaped members  50  are also provided on the left-hand and right-hand edge parts of the main body  5 A, and a protrusion  5 B is provided on the lower edge part of the main body  5 A. 
     As shown in FIGS. 13A and 13B, the fastening case  7  has a rectangular main body  7 A. An engagement plate  57 A having an annular cable holder  57  is formed on the right-hand side of this main body  7 A and a brim-shaped engagement part  58  for a sealing material is provided to this cable holder  57 . A protrusion  60 C is formed at an upper part of the cable holder  57 , and pin-contacting parts  59  on the left-hand and right-hand parts of the outer surface part (lower surface part) of the main body  7 A. An engagement part  60 A and a protrusion  60 B for pressing on the grounding line are provided on the inner surface of the main body  7 A. 
     As shown in FIG. 2, a cap-shaped sealing member  81  made of rubber is provided to a cable  80  which penetrates the annular cable holder  57  of the fastening case  7 , and the sealing member  81  engages with the brim-shaped engagement part  58 . As shown in FIG. 14B, eight lead lines  80 - 1 - 80 - 8  of the cable  80  are divided equally to the left-hand and right-hand sides, there being a grounding line  80 - 9  at the center of the cable  80 . The four lead lines  80 - 1 - 80 - 4  on the left-hand side are led through the left-hand lead line inlet  40 , two in front and two at back, to the left-hand lead line area  32 L- 2  and are connected by compression to the compression contact parts  42   b  and  44   b  of the forward and backward left-hand fixed terminals  42 L and  44 L. Similarly, the four lead lines  80 - 5 - 80 - 8  on the right-hand side are led through the right-hand lead line inlet  41 , two in front and two in back, to the right-hand lead line area  32 R- 2  and are connected by compression to the compression contact parts  42   b  and  44   b  of the forward and backward right-hand fixed terminals  42 R and  44 R. 
     The inner switch  3  is comprised of the fixed terminals  42 L,  42 R,  44 L and  44 R, a switch plunger  66 , mobile members  67  and  69  having mobile contact points  67   a  and  69   a  and being supported by the switch plunger  66  through holding springs  77 A and  77 B serving as elastic holding members and a returning spring  77  serving as an elastic return-biasing member. 
     As shown in FIG. 15, the switch plunger  66  has a main body  66 A having a contact part  78  and a stopper part  79  provided on its upper part. On both side surfaces of the main body  66 A are holes  70  and  71  which are elongated in its axial direction one above the other. Another hole  70 A communicating with the hole  70  is formed on its front surface. Guides  72  and  73  are provided respectively at an upper part and a lower part of the upper hole  70 , and guides  74  and  75  are provided respectively at an upper part and a lower part of the lower hole  71 . A spring container  76  is provided on the lower surface of the main body  66 A. 
     The mobile member  67  is movable upward and downward and provided to the upper guide  72  in the upper hole  70 , being upwardly biased by the holding spring  77 A. The mobile member  69  is movable upward and downward and provided to the lower guide  75  in the lower hole  71 , being downwardly biased by the holding spring  77 B. 
     As shown in FIG. 2, the switch plunger  66 , structured as described above, is disposed inside the containing part  32  of the switch areas  25  and  25 - 1  in a vertically movable manner with its returning spring  77  inserted into the spring container  76 . The returning spring  77  is received by the spring receiver  44 A at the bottom of the bag-shaped member  22 C of the switch plunger containing part  32 . Thus, the switch plunger  66  is upwardly biased by the returning spring  77 , and its contact part  78  at the top protrudes above the cut part  26  of the switch case  21 . The stopper part  79  of the switch plunger  66  contacts the stopper part  26 A on the upper surface part  22 A of the case main body  22 , and the left-hand and right-hand mobile contact points  67   a  of the mobile member  67  contact the fixed contact points  42   a  of the fixed terminals  42 L and  42 R to form a normally closed (NC) contact point part. The left-hand and right-hand mobile contact points  69   a  of the mobile member  69  are separated from the fixed contact points  44   a  of the fixed terminals  44 L and  44 R to form a normally open (NO) contact point part. 
     The aforementioned upper switch element unit of the inner switch  3  is comprised of the left-hand and right-hand fixed terminals  42 L and  42 R, the mobile member  67  and the normally closed (NC) contact part. Likewise, the lower switch element unit of the inner switch  3  is comprised of the left-hand and right-hand fixed terminals  44 L and  44 R, the mobile member  69  and the normally open (NO) contact part. As shown in FIG. 3, these upper and lower switch element units are disposed respectively in the switch areas  25  and  25 - 1  of the switch case  21 . The normally closed and open contact point parts are opened and closed as the switch plunger  66  is forcibly displaced. 
     As the switch covers  5  and  6  are set on the switch case  21 , the switch areas  25  and  25 - 1  are thereby covered. In this situation, the tongue-shaped member  48  at the center along the upper edge of the covers  5  and  6  is inserted into the cut part  26  of the switch case  21 , and the holes  49 A of the left-hand and right-hand engaging members  49  engage detachably with the protrusions  35 A on the switch case  21 . The upper and lower protrusions  45  and  47  on the inner surfaces of the switch covers  5  and  6  compress the lead lines  80 - 1 - 80 - 8  compressed and connected to the compression contact parts  42   b  and  44   b  of the fixed terminals  42  and  44 . The tongue-shaped members  50  on the left-hand and right-hand edge parts of the switch covers  5  and  6  engage with the tongue-shaped members  39  of the switch case  21  to seal in the switch element units of the inner switch  3 . The grounding lines (not shown) of the cable  80  are connected to the grounding line holder  51  of the switch case  21 , inserted into the grounding line inlet  53  and wound around the indentation  54 . 
     The fastening case  7  is on the bottom surface of the switch case  21 , having the bag-shaped member  22 C of the switch case  21  and the protrusions  5 B of the switch covers  5  and  6  covering this bag-shaped member  22 C engaged with the engagement part  60 A of the fastening case  7  and having the protrusion  60 C engaged in a hole (not shown) on the switch case  21 . The aforementioned switch unit A is formed by placing inside the outer case  1  the switch case  21  having the switch covers  5  and  6  and the fastening case  7  attached thereto. 
     Thus, when the switch case  21  with the switch covers  5  and  6  and the fastening case  7  attached thereto is disposed inside the outer case  1 , the contact part  78  on the upper part of the switch plunger  66  contacts the link plunger  61  of the outer case  1  from below. The engagement plate  57 A of fastening case  7  engages with the cable opening  18  at the lower edge part of the right-hand side surface part  1 E of the outer case  1 . A contact surface part  81 A of the end cap  81  contacts the right-hand side surface part  1 E of the outer case  1 . 
     Holder pins  120  are inserted into the left-hand and right-hand pin-receiving holes  20  on the lower edge parts of the front and back surface parts  1 B and  1 C of the outer case so as to connect them. The holder pins  120  contact the pin-contacting parts  59  on the left-hand and right-hand parts of the outer surface part (lower surface part) of the fastening case  7  to keep the switch case  21  inside the outer case  21 . The protrusion  60 B compresses the grounding line  80 - 9  wound around the indentation  54 , causing it to contact the inner surface of the outer case  1  by means of the elastic nature of the grounding line holder  51 . 
     The cable  80 , thus inserted into the cable holder  57  of the fastening case  7 , is sandwiched between the semispherical indentation  55  in the lower surface part  22 B of the case main body  22  and the fastening case  7 , the protrusion  55 A at the indentation  55  compressing the cable  80 . The opening on the lower surface of the outer case  1  is filled with a sealing resin (not shown) serving as an adhesive to make the switch unit A resistant against water. Since the protrusion  55 A compresses the cable  80 , the sealing resin is prevented from flowing into the inner switch  3 . 
     The head unit B is provided with a head case  90 . As shown in FIG. 16, the head case  90  has a main body  90 A with a shaft receiving part  91  formed at its upper part and an engaging part  92  at its lower part. A stopper  93  is formed at an end surface part of the shaft receiving part  91 , and the periphery of the shaft receiving part  91  is provided with an opening  94  communicating with the engaging part  92 . Two set zones Z 1  and Z 2  are prepared on the outer surface of the head case  90  by coloring or by uneven surface characteristic (protrusions and indentations). The engaging part  92  has covering parts  95  on its left-hand and right-hand sides, each having two legs  97  and  98  on both sides of a cut part  96 . Each of these legs  97  and  98  is provided with a cut part  99  and a semicircular screw-receiving part  100  which together form an attachment part U. At both shoulder parts of the covering part  95  is a hole  101  communicating with the cut part  96 . 
     The head case  90  in provided with an actuator F of which the rotary shaft  102  as shown in FIG. 17 is rotatably inserted into the shaft receiving part  91  of the head unit B. The rotary shaft  102  has a semicircular stopper  102 A formed on its back end part, an indented cam part  102 B formed on its peripheral part, a roulette part  103  with many gear teeth formed on its front surface part and also a set zone indicator P. 
     At the front end of the rotary shaft  102  is a lever angle adjuster  104  having, as shown in FIG. 18, an indentation  105  on its inner surface part. Corresponding roulette parts  106  are formed on the peripheral wall parts of this indentation  105 . A mutually opposite pair of lever holders  107  is formed on the outer surface part of the lever angle adjuster  104 . Engagement parts  108  are formed on mutually opposite parts of these lever holders  107  as shown in FIG. 18A. A hole  109  is formed at the center of the lever angle adjuster  104  and protrusions  110 A and  110 B are formed above and below the hole  109 . 
     As shown in FIG. 2, the lever  111  has a screw hole  112  and engagement holes  113 A and  113 B on its base part and supports a roller  114  axially at its upper part. The lever angle adjuster  104  is attached to the front end part of the rotary shaft  102  by engaging the roulette parts  103  and  106  together. The lever  111  is attached to the outer surface part of the lever angle adjuster  104  by inserting the protrusions  110 A and  110 B into the engagement holes  113 A and  113 B. After a screw member  115 - 1  is inserted into the screw hole  112  in the lever  111  and the hole  109  through the lever angle adjuster  104 , it is screwed into a screw hole  103 A through the front end surface part of the rotary shaft  102  and is tightened so as to fasten the lever  111  to the tip of the rotary shaft  102 . A plurality of holes  114 A are formed on the roller  114 . 
     A mobile member  115  is slidably inserted into the opening  94  communicating from the periphery of the shaft receiving part  91  to the engaging part  92 . An upper part of this mobile member  115  is inserted into the cam part  102 B of the rotary shaft  102 . 
     As shown in FIG. 19, this mobile member  115  has a main body  115 A cross-sectionally shaped similarly to the sectional shape of the opening  94 . A protrusion  116 A is formed at the upper end part of this main body  115 A, and a brim-like member  116 B is formed at its lower end part. At the center of the lower end surface is a plunger-receiving opening (not shown). A claw  117  as a hooking device is provided on its side surface. When the mobile member  115  is inserted into the opening  94 , the claw  117  contacts the peripheral surface of the opening  94  by its elastic force in order to prevent it from falling off and rattling. 
     When the head unit B and the switch unit A are assembled, the engaging part  92  of the head unit B is attached to the head unit attachment part  10  at the upper part of the outer case  1  of the switch unit A, that is, the left-hand and right-hand protrusions  13  on the upper part of the outer case  1  are inserted into the cut parts  96  of the covering parts  95  of the engaging part  92 , and the legs  97  and  98  of the left-hand and right-hand covering parts  95  are inserted into the indentations  12 A and  12 B at the upper part of the outer case  1 . Another screw member  123  is inserted into the hole  101  at each shoulder part of the covering part  95  to attach the head unit B to the switch unit A. 
     When the head unit B and the switch unit A are thus assembled, the cut part  99  of an attachment part U of the engaging part  92  and the screw-receiving part  100  are outside of the throughhole  14  of the outer case  1 , and the link plunger  61  attached to the plunger attachment part  11  through the seal cap  62  is inserted into a plunger receiving part on the lower end surface of the mobile member  115 . Thus, the force of the returning spring  77  of the inner switch  3  presses the mobile member  115  upward through the switch plunger  66  and the link plunger  61 , and the protrusion  116 A at the upper end part of the mobile member  115  contacts the cam part  102 B of the rotary shaft  102 . 
     When the limit switch S, thus assembled, is installed on some structure (“target structure”), an attachment screw member N is passed through each protrusion  14  of the outer case  1 , as shown in FIG. 2, and screwed tightly into a corresponding screw hole (not shown) prepared on the target structure. As the screw member N is tightened, the head part N- 1  of the screw member N contacts the semicircular screw-receiving part  100  of the attachment part U, and the back surface of the engaging part  92  contacts the target structure directly (unless a spacer is inserted in between). 
     Rigidity of the limit switch S is thus improved since the engaging part  92  of the head unit B is attached to the head unit attachment part  10  of the outer case  1  of the switch unit A. Since the limit switch S is attached to a target structure through the head unit B, the switch is less likely to become deformed or damaged at the time of its attachment to the target structure even if a foreign object is present and whether the outer case  1  of the switch unit A is made of a metallic or resin material. 
     When the limit switch S is to be operated, a moving body (a “dog”) contacts the roller  114  of the actuator F, causing the lever  111  to tilt and the rotary shaft  102  to rotate. The mobile member  115  in contact with the cam part  102 B of the rotary shaft  102  is thereby pushed downward against the force of the returning spring  77 . Thus, as shown in FIG. 3, the switch plunger  66  is pushed down through the link plunger  61 , causing the mobile contact points  67   a  on the mobile member  67  to be separated from the fixed contact points  42   a  on the fixed terminal  42  while the mobile contact points  69   a  on the mobile member  67  contact the fixed contact points  44   a  of the fixed terminal  44  such that the switched condition changes. According to the embodiment of the invention described above, the switch areas  25  and  25 - 1  are formed in front and at back of the switch case  21 , each containing switch element units of the inner switch  3 . Thus, in the case of a switch structure with four independent circuits as switch element unit, each of the switch areas  25  and  25 - 1  may house two of the circuits such that the limit switch S can be structured compactly with a small height. 
     Since the switch areas  25  and  25 - 1  are provided as explained above, each containing a switch element unit of the inner switch  3 , and each of the switch element unit is covered and sealed by a switch cover  5  or  6 , the switch according to this invention has an improved insulation characteristic. 
     The structure of the contact points can be modified easily according to the present invention without using different components but merely by changing the position of the fixed and mobile members. For example, the fixed terminals  44 L and  44 R may be removed, and fixed terminals  42 L- 1  and  42 R- 1  having the same structures as terminals  42 L and  42 R may be detachably attached to the middle installation parts  36 - 2 . As for the switch plunger  66 , the mobile member  67  is kept in the same way as shown in FIG. 15 but the mobile member  69  may be introduced to the guide  74  and biased upward by means of the holding spring  77 B so as to change its position, as shown in FIG.  20 . If the switch plunger  66 , thus slightly modified, is disposed in the switch plunger containing part  32  of the switch areas  25  and  25 - 1  so as to be vertically movable with the returning spring  77  positioned in the spring container  76 , the change is as shown in FIG. 21, FIG. 21B showing both circuits having normally closed (NC) contact points. Thus, the structure of contact parts can be varied inexpensively in many ways because common components are used. 
     Another advantage of the embodiment described above is that the switch plunger  66  of the inner switch  3  is operated by the link plunger  61  which is another component and hence the switch can be unitized. Since there is no effect on the shape of the actuator, many variations can be easily developed. Since the seal cap  62  has the same function as an O-ring, the switch is better sealed and the link plunger  61  can better follow the switch plunger  66 . Since the grounding line  80 - 9  is connected to the grounding line holder  51  of the switch case  21  and is compressed to the outer case  1  by the elastic property of this grounding line holder  51 , dedicated components such as pins are not required and there is no need for punching holes through the outer case  1  for such components. This makes the entire structure simpler. 
     Since the adhesive can be prevented from flowing toward the inner switch  3  because the cable  80  is sandwiched between the switch case  21  and the fastening case  7 , a separate component such as a rubber piece for preventing the flow, as well as an extra work step for its installation, can be dispensed with. Neither is any extra device necessary to affix the cable  80 , and the cable  80  becomes stronger against being pulled out because the adhesive is hardened with the cable  80  bent by the protrusion  55 A from the switch case  21 . Since the compression contact parts  42   b  and  44   b  are provided on the fixed terminals  42 L,  42 R,  44 L and  44 R and the lead lines  80 - 1 - 80 - 8  are arranged symmetrically inside the cable  80  and compressed by the protrusions  45 ,  46  and  47  formed on the switch covers  5  and  6 , the production process is not only simpler but also environmentally gentler than the prior art method by soldering, and the lines can be arranged efficiently. Since the roulette part  106  of the lever angle adjuster  104  is provided only partially and not entirely around, the production is easier without adversely affecting the accuracy of adjustment. Since the roller  114  is provided with many holes  114 A, it is made lighter and has improved resistance against shock and vibrations. Since less material is needed, the production cost is also reduced. Since the protrusion  116 A is formed on the mobile member  115  where it contacts the cam part  102 B, the action of the plunger can be adjusted at will, while the plunger is being pushed according to a sine curve corresponding to the angle of rotation of the lever according to prior art technologies. 
     FIGS. 22-25 are referenced next to describe another limit switch S 1  according to a second embodiment of the invention characterized as having the cable  80  connected at bottom and being provided with a different fastening case  135 . This fastening case  135  has a rectangular main body  135 A, as shown in FIG. 23, with a closing plate  137  formed on one side and an engagement part  136  and a protrusion (not shown) on the inner surface of the main body  135 A. A quadrangular columnar protrusion  138  is formed on its lower surface. A cable hole  139  is provided from the inner surface of the main body  135 A to the lower surface of this columnar protrusion  138 , its lower end serving as a sealing member receiving part  140 . Pin contacting parts  141  are provided on the left-hand and right-hand sides of the lower surface of the main body  135 A. Pin insertion holes  142  are formed in the front-back direction through the columnar protrusion  138 . 
     The fastening case  135  is attached to the bottom part of the switch case  21  with the switch covers  5  and  6  installed. The bag-shaped member  22 C of the switch case  21  and the protrusions  5 B and  6 B of the switching covers  5  and  6  covering this bag-shaped member  22 C are engaged to the engagement part  136 . The switch case  21  thus having the switch covers  5  and  6  and the fastening case  7  attached to it is contained in the outer case  1 . The cable opening  18  at the lower edge part of the right-hand surface  1 E of the outer case  1  is blocked by the engagement part  136  of the fastening case  135 . The holder pins  120  are inserted into the left-hand and right-hand pin-receiving holes  20  on the front and back surfaces  1 B and  1 C of the outer case  1 , extending therebetween. These holder pins  120  contact the left-hand and right-hand pin contacting parts  141  to support the switch case  21 . The protrusion  138  from the fastening case  135  extends to the exterior of the outer case  1 , and a cap-shaped sealing member  143  penetrated by the cable  80  is inserted into the sealing member receiving part  140 . A sealing ring  143 A is formed on the outer periphery of this sealing member  143 . 
     FIG. 25 shows the structure of a mutually engaging pair of seal holders  145  and  146 . The seal holders  145  and  146  each have bent parts  181  and  182  at both edges, one of the bent parts ( 181 ) having a cut  183  and the other ( 182 ) having a protrusion  184 . Both bent parts  181  and  182  have a pin hole ( 185  or  186 ) and its surface part  180  has an approximately semicircular insertion area  187  prepared. These seal holders  145  and  146  are attached to the protrusion  138  by inserting holding pins  147  from the pin holes  185  and  186  into the pin insertion holes  142  such that the protrusion  184  of one of them will fit into the cut  183  of the other, as shown in FIG.  24 . The sealing member  143  is then sandwiched, as shown in FIG. 23, in the insertion area  187  and the sealing ring  143 A of the sealing member  143  is compressed both in the radial and vertical directions to provide a complete sealing. 
     In the other aspects, the limit switch S 1  according to the second embodiment is the same as the limit switch S according to the first embodiment. 
     The second embodiment is advantageous in that the seal members  145  and  146  hold the cable  80  tightly and since these seal members  145  and  146  are identically shaped, the number of different components for the production can be reduced. 
     As shown in FIGS. 16 and 17, two kinds of set zones Z 1  and Z 2  are formed on the outer surface of the head case and a set zone indicator P is provided to the rotary shaft  102  such that these set zones Z 1  and Z 2  can be selectively used for two kinds of inner switches  3  with different action characteristics. In other words, only one head unit B is required for these two kinds of inner switches with different action characteristics. 
     FIG. 26 shows another limit S- 2  switch according to a third embodiment of the invention, which is the same as the limit switch S according to the first embodiment of the invention described above except that spacers  240  are inserted between the head unit B and the switch unit A to serve as means for changing the position of the head unit B in the direction of displacement of the switch plunger  66  of the inner switch  3 . Each of the spacers  240  comprises a planar main body  240 A of a size nearly equal to that of the upper surface of the protrusion  13  of the outer case  1 , having a hole  241  at its center. 
     When the head unit B is attached to the switch unit A, the spacers  240  are placed on the upper surface part of the protrusion  13  of the outer case  1  and the engagement part  92  of the head unit B is engaged with the head unit attachment part  10  at the upper part of the outer case  1  of the switch unit A by inserting the left-hand and right-hand protrusions  13  into the cut part  96  and the protrusions  97  and  98  into the indentations  12 A and  12 B. The screw members  123  are then inserted through the holes  101  and  241  and screwed into the screw holes  15 . 
     With the spacers  240  thus inserted between the switch unit A and the head unit B, the head unit B can be displaced with respect to the switch unit A in the direction of movement of the switch plunger  66  of the inner switch  3  and to thereby change the action characteristic of the inner switch  3 . It now goes without saying that the relative position of the head unit can be changed by making the head unit B as a different component. 
     FIG. 27A shows still another limit switch S 3  according to a third embodiment of the invention, characterized as having a form-lock lever  230  for its actuator F capable of adjusting its length. As shown in FIG. 27A, this form-lock lever  230  comprises an elongated main body  230 A with a longitudinally elongated opening  231 . On both outer edge parts of the main body  230 A are toothed gear parts  232 , and a roller  233  is axially supported at the tip. 
     With the roulette parts  103  and  106  engaged with each other at an end part of the rotary shaft  102 , as explained above regarding the switch S according to the first embodiment of the invention, the form-lock lever  230  is placed next to the outer surface part of the lever angle adjuster  104 , and the toothed gear parts  232  are engaged with the engagement parts  108  of the lever holders  107 . Thereafter, the screw member  115 - 1  is inserted through the opening  231  of the form-lock lever  230  and the hole  109  of the lever angle adjuster  104  and screwed into the screw hole  103 A at the front end surface part of the rotary shaft  102 . The form-lock lever  230  with the roller  233  is thus attached to the rotary shaft  102  by tightening the screw member  115 - 1 . 
     When the length of the form-lock lever  230  is to be changed, the screw member  115 - 1  is removed and the form-lock lever  230  is separated from the lever angle adjuster  104  and displaced upward or downward. The form-lock lever  230  is thereafter positioned against the lever angle adjuster  104 , its toothed parts  232  are engaged with the engagement parts  108  of the lever holders  107 , and the screw member  115 - 1  is again inserted through the opening  231  of the form-lock lever  230  and the hole  109  of the lever angle adjuster  104  and screwed into the screw hole  103 A at the front end surface part of the rotary shaft  102 . In the other respect, the switch S 3  is structured in the same way as the switch S according to the first embodiment of the invention. 
     As a variation, the form-lock lever  230  may have toothed openings  231 A with the toothed parts  232  and the central opening  231  formed separately, as shown in FIG.  27 B. This design is safer to operate because the toothed gear parts  232  are not facing outward and stronger because the openings  231  and  231 A are separated. 
     FIG. 28 shows still another limit switch S 4  embodying this invention characterized as having a weight G added to the lever  11  of the actuator F on the side opposite to the roller  114  such that the center of gravity of the lever  111  shifts closer to its axis of rotation and hence that the resistance against shock and vibrations will be improved. 
     As should be clear from the disclosure above, limit switches according to this invention are made more rigid and less likely to be damaged at the time of installation and may be assembled without requiring extra components for preventing damage or deformation.