Abstract:
A magnet switch has a coil of a reduced size and a sufficiently large contact and does not have a rod running through the plunger or the fixed iron core. A flange and a joint are fixed at one end of the plunger. The arm of the flange is connected to a holder via an elastic body. Movable contacts are pushed into the holder to be fixed at one end thereof. When the plunger moves, the flange, the holder, and the movable contacts move somewhat simultaneously to work as a switch.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon, claims the benefit of priority of, and incorporates by reference, the contents of Japanese Patent Applications No. 2002-110296 filed Apr. 12, 2002. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a starter motor for starting internal combustion engines, and particularly to an improved magnet switch for use in such a starter motor. 
     2. Description of the Related Art 
     There is a need for a variety of improvements in starter motors to reduce harmful impacts on the environment. For example, engine starter motors should be lighter and more compact, and have greater durability to maintain or withstand frequent engine idling stops (what is called “eco-run”). Japanese Patent Laid-Open Publication No. Hei 9-68142 discloses a technique that reduces damage on the involved gears by ensuring enhanced gear engagement and lowering the current in the switch in order to reduce the size of the switch itself. More specifically, the current in the attraction switch has been reduced by more than 70%, the attraction coil has been downsized significantly, the ON and OFF switching of the main current and engagement with the ring gear of the pinion are performed with a plunger situated through the center of the coil, a rod moves together with the plunger shaft, and a movable contact and a hook are used for starting the engine. Some of the switches move the movable contact and the hook independently of the two separate rods, but because they move somewhat simultaneously while driven by an attraction force, the operation mechanism is common. 
     Although the improved engagement made the switch smaller, the electronic current that intermittently runs in the contact thereof does not change because it is determined by the necessary motor power. Consequently, since the current of the same magnitude runs the switch contact, the current density increases, and the contact is likely to wear at an anomalous rate. Moreover, since a thin rod must hold the contact and the hook (joint) at each end, the weight ratio of the contact and the hook grows, and the rod operation is affected by an increased load from the relatively heavy contact and hook. In addition, the sliding gap between the plunger and the inner circumference of the coil leads to instability in operation and results in such problems. 
     Meanwhile, although the thin rod (its diameter is approximately 2 mm) has to hold a small contact via an insulator washer for electric isolation, the insulator washer of poor mechanical strength often breaks during operation. In general, a contact current of about 700 A runs in the contact. Thus such a break-up of the insulator causes an absence of a safety gap, because the thickness of the insulator is 1 mm or less. Further, when the shock of the pinion engagement reaches the plunger and the contact via the hook, the thin, small rod contact and insulator are likely to break. To solve such problems, the contact may be maintained as large as before. However, this makes the thin rod have a large contact, and its operation becomes unstable. Specifically, the contact chatter by ON and OFF switching produces electric arks and may fuse the contact. Furthermore, as long as the rod exists inside the plunger, the saving of copper used in the coil is limited. 
     Since the plunger has key components (contact and hook) at both ends, both ends of the rod must be disposed outside the switch through the attraction coil thereof. Then the rod must be made of a non-magnetic material to utilize the magnet force of the coil. This non-magnetic material is usually an expensive material such as stainless steel or copper, which results in an expensive switch. Because the plunger must be assembled from the magnetic and non-magnetic parts in the small space in the attraction coil, the production cost becomes higher. If the rod is made of a magnetic material, all the magnetic flux runs in the rod, and the air gap does not have a magnetic flux. Then, no force to decrease the air gap is produced and the switch does not work. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, an object of the present invention is to provide an inexpensive, reliable, compact switch which does not require special materials. The switch will not have a rod running through the plunger and fixed iron core while the contact and the hook are moved, not inside, but outside the attraction coil to reduce the size of the coil and maintain a sufficiently large contact. 
     In the magnet switch according to a first aspect of the present invention, the attraction coil has a plunger alone in its center, and this plunger is allowed to slide toward the side opposite to a fixed iron core. Thus it becomes possible to eliminate the non-magnetic rod from penetrating the fixed iron core, and the entire structure that forms a magnetic circuit can be made of iron. As a result, the inner circumference of the coil can be made small, and the plunger can also be made thin at no additional cost. Because the contact and the hook are formed on a thick plunger, instead of a thin rod, and the sliding gap between the rod and the plunger or fixed iron core becomes unnecessary, the operation of the contact is stable. 
     In the magnet switch according to a second aspect of the invention, the contact for current ON and OFF control is formed at one end of the switch, while the joint that drives the pinion is formed at the other end thereof, and they are connected to each other by a flange formed on the outer periphery of the case. Unlike the conventional thin rod, this flange interferes with nothing, so it can be made thick to be sufficiently strong. Then the flange can be large enough in terms of electrical and mechanical requirements with no need to enlarge the switch including an attraction coil and other parts. 
     In the magnet switch according to a third aspect of the invention, the contact holder is fixed via the flange arm extending from the outer periphery of the case. Thus even a large contact can be fixed with the holder from the outer periphery with a sufficient margin. If the holder is made of an electric insulator, the mechanical strength can be held high and the contact can securely work. 
     In the magnet switch according to a fourth aspect of the invention, the switch itself is firmly fixed in the starter, and the sliding unit such as the joint and the flange are covered with a cover that covers the contact room. Thus there is no need to add a separate dust cover or enlarge the outer diameter of the switch. Specifically, if the switch of the invention is installed in a usual starter, a separate large cover becomes necessary and its outer diameter becomes as large as the conventional switch. However, the structure of the invention is free from such drawbacks and is compact and durable against dust and water. 
     The magnet switch according to a fifth aspect of the invention is highly reliable and can be reduced in size. Engagement control makes the attraction coil current significantly small and permits a reduction in the size of the switch, while reducing production costs. 
     In the magnet switch according to a sixth aspect of the invention, the center of the attraction coil is made of a fixed iron core alone, and any non-magnetic rod penetrating the fixed iron core is not necessary. Additionally, all the parts can be made of iron to form a magnetic circuit. Thus, the inner circumference of the coil can be made thin, and as a result, the attraction coil can be made small and there is no substantial increase in costs. Because the contact and the hook are formed on a thick plunger, instead of a thin rod, and no sliding gap exists between the rod and the plunger or the fixed iron core, the operation of the contact is not affected. Moreover, since the magnetic circuit of the sliding unit of the plunger and case sidewalls is not the inner circumference of the case sidewalls as in a conventional case, but rather the outer circumference, the operation area becomes large, and accordingly the magnetic resistance becomes smaller. Because of this, the coil can be made smaller. 
     In the magnet switch according to a seventh aspect of the invention, the contact for current ON and OFF control is formed at one end of the switch, while the joint that drives the pinion is formed at the other end thereof, and they are connected to each other by a flange formed on the outer periphery of the case. Unlike the conventional thin rod, this flange interferes with nothing, therefore it can be made thick to be sufficiently strong. Then the flange can be large enough in terms of electrical and mechanical requirements with no need to enlarge the switch including an attraction coil and other parts. 
     In the magnet switch according to an eighth aspect of the invention, the contact holder is fixed via the flange arm extending from the outer periphery of the case. Thus even a large contact can be fixed with the holder from the outer periphery with a sufficient margin. If the holder is made of an electric insulator, the mechanical strength can be held high and the contact can securely work. 
     In the magnet switch according to a ninth aspect of the invention, the switch itself is firmly fixed in the starter, and the sliding unit, that is, the joint and the flange, are covered with a cover covering the contact room. Thus there is no need to add a separate dust cover or enlarge the outer diameter of the switch. Specifically, if the switch of the invention is installed in a usual starter, a separate large cover becomes necessary and its outer diameter becomes as large as the conventional switch. However, the structure of the invention is free from such drawbacks, is compact and durable against dust and water. 
     The magnet switch according to a tenth aspect of the invention is highly reliable, small in size, and can be made at a reduced cost. Engagement control makes the attraction coil current significantly small, which permits a reduction of the switch size. 
    
    
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a cross-sectional view of the starter of a first embodiment of the invention; 
     FIG. 2 is a cross-sectional view of the switch of FIG. 1; 
     FIG. 3 is an enlarged diagram of the movable portion of the switch of FIG. 1; 
     FIG. 4 is a cross-sectional view of the switch of a second embodiment of the invention; and 
     FIG. 5 is a partial cross-sectional view of the starter where the switch of FIG. 4 is installed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now the starter switch of the invention will be described with reference to a first embodiment shown in FIGS. 1,  2  and  3 . 
     A switch  50  has an attraction coil  51  that generates electromagnetic force, a plunger  52  that forms a magnetic circuit, a case  55 , a fixed iron core  54 , and an air gap  56  between the plunger  52  and the fixed iron core  54 . The case  55  includes a cylindrical part  55   a  and side walls  55   b ,  55   c , constituting the fixed part of the switch covering the attraction coil  51  together with the fixed iron core  54 . The cylindrical part  55   a , side walls  55   b ,  55   c  of the case  55 , and the fixed iron core  54  may be either separated or integrated, as long as they can form a magnetic circuit (in the figure, the side wall  55   b  and fixed iron core  54  are made in one piece). 
     A first contact  70  is composed of a fixed contact  71  and a movable contact  72 . The movable contact  72  forms part of a first elastic body  73  made of a conductive, mechanically strong material such as phosphor bronze. 
     A second contact  80  is composed of a fixed contact  81  and a movable contact  82 . The first and second contacts  70 ,  80  form a parallel circuit between the battery and the motor. The fixed contact  71  of the first contact  70  made of carbon functions as a resistor in the circuit. The movable contacts  72 ,  82  are provided with contact pressure against the fixed contacts  71 ,  81  respectively, by the first and second elastic bodies  73 ,  83  (two pieces in the figure). In the present embodiment, the elastic bodies  73 ,  83  are installed in the movable unit that moves together with the plunger. However, they may be installed on the fixed contact  71 ,  81  sides or installed in a cross. The first fixed contact  71  is connected to a battery via the holder  62 , while the second fixed contact  81  is connected to the battery (not shown) directly with a terminal  60 . 
     At one end of the plunger  52 , a flange  53  and a joint  53   a  are fixed. An arm  53   b  of the flange  53  is connected to a holder  58  via the second elastic body  83 . At the end of the holder  58 , the movable contacts  72 ,  82  are fixed by an appropriate method such as a press fit by simply pushing the movable contacts  72 ,  82 . Thus when the plunger  52  moves, the flange  53 , holder  58 , movable contacts  72 ,  82  also move almost together to work as a switch as a whole. 
     The joint  53   a  has a hole where an end of a connection means  90  is to be inserted. The other end of the connection means  90  is to restrict the rotation of the pinion  25  via a component  91 . Specifically, when the attraction coil  51  works to pull the plunger  52 , the air gap  56  becomes shorter, and the component  91  contacts, via the connection means, the pinion  25  to restrict its rotation. When the motor is activated under this condition, the pinion  25  moves in the axial direction guided by helical splines  20   a ,  25   a  formed in the outer periphery of the output shaft  20  and the inner circumference of the pinion  25 , respectively, and then engages with the ring gear (not shown) of the engine. The shaft  11  of the motor armature  10  is connected to the output shaft  20  via a gear reduction mechanism  30  and a clutch  27 . 
     A return spring  57  pulls the plunger  52  back to its original position when the attraction coil  51  is deactivated. In the present embodiment, the return spring  57  is inserted in the plunger  52 , which housed in the magnet switch  50 . However, it may be placed anywhere as long as it causes the return of the plunger  52  to its original position. The inner circumference of the attraction coil  51  can be a bearing for the sliding plunger if a sleeve (not shown) made of a thin metal plate (for example, copper) is inserted therein. 
     A plate spring band  95  fastens the switch onto a seat  96 , covering the cylindrical part  55   a  of the case  55  with its elasticity. A cover  98  covers the switch  50  and the contacts  71 ,  81 . 
     Now the operation of the present invention will be described. When the key switch of the vehicle (not shown) is turned ON, the attraction coil  51  exerts an electromagnetic force to the plunger  52  so that it moves against the return spring  57  to shorten the air gap  56 . Then, this motion via the connection means  90  restricts the rotation of the pinion  25 . Next, as the first contact  70  connects to the resistor  91  (the carbon-based fixed contact  71  also serves as a resistor in the embodiment), the motor rotates very slowly. Then, while the output shaft  20  rotates, the rotation of the pinion  25  is restricted, and the pinion  25  moves along the axial direction guided by the splines  20   a ,  25   a , and engages with the ring gear (not shown). 
     When the plunger  52  moves further, the second contact  80  is closed. Since the first and second contacts form a parallel circuit and the first contact  70  has a resistor, the electric current runs dominantly in the second contact circuit and the motor works to activate the engine. Meanwhile, when the engine has been activated and the key switch has been turned OFF, the attraction coil  51  loses electromagnetic force. Then, the elastic force of the return spring  57  pulls the plunger  52  back, and the second contact  80  is opened. Then a current limited by the resistor  91  is provided to the motor. When the plunger  52  is pulled back further, the first contact  70  is also opened. Because the operations of the engaging parts other than the switch are similar to those disclosed in Japanese Patent Laid-Open Publication No. Hei 10-115274, their explanation is not repeated here. 
     Because the present invention has no rod penetrating through the attraction coil in the axial direction, the plunger can be made thin and the coil can be made small. The switch has a substantially cylindrical shape with a bottom, and has a structure where the contact and joint move together with the plunger in one end of the switch. As a result, the rod can be eliminated in the sliding gap, and the mechanical structure becomes stable with less play. Since the contact is fastened, with its outer circumference being fixed by the outer periphery of the switch, the holder may have sufficiently large dimensions and therefore its mechanical strength can be ensured even when the holder is made of a resin. Overall, the switch is compact. Furthermore, the sliding part works also as the switch cover, eliminating additional components. Then, combined with the engaging mechanism of the pinion rotation restricting method, the present invention can provide a compact, reliable starter at a low cost. Although a two-stage operation using two contacts is described in this embodiment, this invention can be applied to one-stage operation type switches having a single contact. 
     (Second Embodiment) 
     Next, a second embodiment will be described below with reference to FIGS. 4 and 5. The fixed iron core  54  has sidewalls  55   b ,  55   c  on its side faces, and in combination with the cylindrical part  55   a  of the case they form a fixed part of the magnetic circuit for the switch. The plunger  52  has a cylindrical shape and a flange  53  at one end and an arm  53   b  at the other end in one piece. The plunger  52  is pulled by the magnetic force of the attraction coil  51  and shortens the air gap  56  so as to close the contact and cause engagement with the pinion. A return spring  57  and an elastic body  83  provide contact pressure. The other parts and their operations are the same as those of the first embodiment, so their explanation is not repeated here. The present invention can thereby provide a reliable, compact switch at a low cost. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.