ELECTROMAGNETIC SWITCH

Abstract: In an electromagnetic switch for a starter motor, power produced by friction from repeated opening and closing of the stationary and movable contacts 22, 26 is prevented from entering between the sliding parts of the movable iron core assembly 4, thus ensuring smooth sliding motion of the movable iron core assembly at all times, and the penetration of external water into the spaced defined by the stationary and movable iron cores 9, 4 and a shaft member 20 is effectively prevented. The shaft member is disposed inside the annular stationary iron core for movement in the axial direction. An insulating sleeve 21' is fitted over the shaft member and fixed thereto for movement therewith. The insulating sleeve has a cylindrical outer peripheral surface of a diameter slightly smaller than the inside diameter of the radially innermost surface of the stationary iron core, and the outer peripheral surface of the insulating sleeve has an axial length D larger than the axial distance between the stationary contact 22 and the stationary iron core 9 so that there is maintained a limited constant clearance c between the insulating sleeve and the stationary iron core at all times irrespective of the axial position of the insulating sleeve relative to the stationary iron core, so that the space defined by the stationary and movable iron cores is substantially sealed. The electromagnetic switch includes a cylindrical housing 8 accommodating an electromagnetic coil 12, the housing having one open end closed by a cap 11 to which stationary contact 22 is firmly attached. An annular stationary iron core 9 is disposed in and fixed to the housing. The movable iron core assembly 4 is disposed inside the electromagnetic coil and includes shaft member 20 connected with a movable iron core 16 the axially slidably fitted in the electromagnetic coil in face-to-face relation with the stationary iron core for movement towards and away from the stationary iron core. Movable contact is mounted on one end of the insulating sleeve and thus electrically insulated from the movable shaft, and contacts the stationary contact 22 on the axial movement of the shaft member caused by energization of the electromagnetic coil.

Description: The present invention relates to an electromagnetic switch particularly adapted to be mounted on a starter motor. Description of the Prior Art Fig. 3 is a side elevational view in cross section showing a starter motor equipped with a conventional electromagnetic switch, and Fig. 4 is a side elevational view of the conventional electromagnetic switch illustrated in Fig. 3, showing the upper half thereof in cross section. In these Figures, reference numeral 1 designates a front bracket connected with a casing C in which a starter motor M is accommodated. Mounted on the front bracket 1 is a conventional electromagnetic switch 2 having a movable iron core assembly 4 constituting a movable magnetic pole. The movable magnetic core assembly 4 has a hook 5 operably connected with a shift lever 3 which is pivotally supported by a projected seat 1a on the inner surface of the front bracket 1 and a spacer 7 which is mounted through a grommet 6 on the casing C, the spacer 7 being urged toward the projected seat 1a under the resilient action of the grommet 6. Mounted on the front bracket 1 is a cylindrical housing 8 which is formed at its one end with a radially inwardly opened recess or socket 8a into which a stationary iron core 9 constituting a stationary magnetic pole is fitted and fixed thereto as by caulking together with a cap 11 of a switch cap assembly, generally designated by reference numeral 10. A packing 28 is interposed between an end surface of the cap 11 and the adjacent side surface of the stationary iron core 9. Housed in the cylindrical housing 8 is an electromagnetic coil 12 which is composed of an attraction coil (called P coil) and a support coil (called H coil) and which is wound around a bobbin 13 moulded of a resinuous material. A bushing 14 is mounted on the inner surface of the bobbin 13 and has one end thereof fitted on the radially outer periphery of the annular extension 9a axially extending from and integrally formed with the stationary iron core 9, and the other end thereof fitted in an opening 15 formed in the housing 8 at its front end adjacent the front bracket 1. Slidably fitted in the bushing 14 is a cylindrical-shaped movable iron core 16 having a cylindrical bore 17 into which the hook member 5 is fitted to be axially slidable with a coiled compression spring 18 being disposed under compression between a retainer ring 19 fixedly secured to an end of the movable iron core 16 and a flanged end 5a of the hook member 5 for resiliently urging the hook member 5 against the bottom or inner end of the bore 17. The movable iron core 16 is integrally formed at its rear end (the lefthand end in Fig. 3) with a cylindrical shaft member 20 on which is mounted an annular movable contact 22 through the intermediary of an insulating sleeve 21 of an electrically insulating material, the sleeve 21 being slidably fitted over the shaft member 20 so that the movable contact 22 is electrically insulated from the shaft member 20 by means of the insulating sleeve 21 and is slidable together with the insulating sleeve 21 along the outer periphery of the shaft member 20. The movable contact 22 is urged by a biasing spring 23 toward an annular stationary contact 26 fixedly mounted on the cap 11 in face-to-face relation to the movable contact 22, the biasing spring 23 being formed of a coiled compression spring and disposed between a stepped shoulder on the shaft member 20 and the adjacent end of insulating sleeve 21. The movable contact 22 is also electrically insulated, in addition to the insulating sleeve 21, from the shaft member 20 by means of an insulating washer 24 which is fitted on the shaft member 20 and fixed thereto by means of a stop ring 29. Disposed between the stationary iron core 9 and the movable iron core 16 is a return spring 25 in the form of a coiled compression spring for biasing the movable iron core 26 in the direction away from the stationary iron core 9 so that the movable contact 22 mounted on the shaft member 20 connected with the movable iron core 16 is biased by means of the return spring 25 in the direction away from a stationary contact 26 firmly secured to the cap 11. The stop ring 29 mounted on one end of the shaft member 20 serves to prevent the insulating sleeve 21, the movable contact 22 and the insulating washer 24 from falling out from the shaft member 20. In operation, when the electromagnetic coil 12 including the P coil and the H coil is energized, upon closing of a key switch (not shown), by an external power source (not shown), a magnetic flux is produced which passes through a magnetic circuit constituted by the housing 8, the movable iron core 16, the clearance between the movable and stationary iron cores 16 and 9, and the stationary iron core 9, and the movable iron core 16 is magnetically drawn toward the stationary iron core 9. As a result, the return spring 25 is compressed so that the movable contact 22 is caused thereby to move toward and contact with the stationary iron core 9 to energize the starter motor M. On the other hand, when the movable contact 22 contacts the stationary contact 26, the biasing spring 23 is similarly compressed so as to provide the movable contact 22 with an appropriate contact pressure against the stationary contact 26. Simultaneous with the drawing of the movable iron core assembly 4 toward the stationary contact 26, the P coil of the electromagnetic coil 12 is deenergized and instead the other H coil of the electromatnetic coil 12 acts to hold the entire electromagnetic coil 12 in an energized state. Also, in accordance with the movement of the movable iron core assembly 4 toward the stationary iron core 9, one end (the upper end in Figs. 3 and 4) of the shift lever 3, being engaged with the hook 5 connected with the movable iron core assembly 4 by means of the spring 18, is pulled to pivot around a fulcrum or pivot point at which the shift lever 3 abuts against the spacer 7. Therefore, a pinion gear 30, axially slidably fitted over the rotary shaft 31 of the starter motor M for rotation therewith, is forced by the other end (the lower end in Fig. 3) of the shift lever 3 to displace axially on the rotary shaft 31 of the starter motor M, whereby the pinion gear 30 is brought into engagement with a ring gear 32 operably connected with an engine (not shown) with the result that the engine is turned over to be started. Upon starting of the engine, the key switch (not shown) is opened to deenergize the electromagnetic coil 12 so that the magnetic attraction force acting between the stationary iron core 9 and the movable iron core 16 collapses to permit the movable iron core assembly 4 to return to the initial position as illustrated in Fig. 4 under the action of the return spring 25. With the conventional electromagnetic switch as constructed in the above-described manner, however, the clearance c between the radially inner peripheral surface 21a of the shaft member 20 is opened up upon closing of the stationary and movable contacts 26 and 22 so that powder, produced by the friction of repeated opening and closing of these contacts 26 and 22, is liable to come through the clearance c into the sliding surfaces between the bushing 14 and the movable iron core 16, as a consequence of which the sliding motion of the movable iron core 16 relative to the bushing 14 is impaired and further outside water tends to penetrate through the clearance c into a space or chamber S defined by the stationary and movable iron cores 9 and 16, the bushing 14 and the shaft member 20. German patent specification DE-A-2822164 relates to a magnetic switch comprising fixed and movable contacts. The switch is closed when a winding is energised. This energisation establishes a magnetic circuit which causes a movable armature to move axially within the switch. A shaft connected to the armature passes through an annular stationary core. The shaft is guided within the core by an insulating bushing and collar. The movable contact is mounted on the insulating bushing. European patent specification 58374 and French patent specification 2194030 both relate to magnetic switches similar to the one described in the German patent specification 2822164 except that, during opening and closing of the switch, a constant clearance is not maintained between the core and an insulator which supports the movable contact on the shaft. SUMMARY OF THE INVENTION The present invention is intended to obviate the above-mentioned problems of the prior art, and has for its object the provision of an electromagnetic switch for a starter motor in which powder produced by the friction of repeated opening and closing of the stationary and movable contacts is securely prevented from coming between the sliding portions of the movable iron core assembly, thus ensuring smooth sliding motion of the movable iron core assembly at all times, and in which the penetration of external water into the space defined by the stationary and movable iron cores and the movable shaft member is effectively prevented, thus avoiding various troubles resulting therefrom. According to the present invention, there is provided an electromagnetic switch for a starter motor comprising: a housing made of a magnetic material; an annular stationary iron core disposed in and fixed to said housing; electromagnetic coil disposed within said housing; a movable iron core disposed in said electromagnetic coil and slidable between a contact open and a contact closed position; a stationary contact mounted to said housing; a sleeve mounted on a shaft member connected with said movable iron core and extending within said annular stationary iron core; and a movable contact mounted on said sleeve for contacting with said stationary contact when said movable iron core is in said contact closed position; wherein said housing defines, in cooperation with said movable iron core, a magnetic circuit through which a magnetic flux generated by said electromagnetic coil can pass for electromagnetically driving said movable iron core between said contact open and closed positions; characterised in that a clearance (c) is provided between the sleeve and the stationary core and is substantially constant irrespective of the position of said sleeve relative to said stationary core, said clearance being sufficiently small for substantially sealing a space defined between said movable and stationary iron cores against the entrance of powder produced by the friction of repeated opening and closing of the contacts, and part of said insulating sleeve on which said movable contact is mounted is formed of an electrically insulating material, and the remaining part of said insulating sleeve is formed of a bearing material. The means for maintaining a limited constant clearance between the insulating sleeve and the stationary iron core comprises the cylindrical outer peripheral surface of the insulating sleeve having a diameter slightly smaller than the inside diameter of the radially innermost surface of the stationary iron core. It is preferred that the outer peripheral surface of the insulating sleeve has an axial length larger than the axial distance between the stationary contact and the stationary iron core. Preferably, the bearing material comprises oil-containing sintered metal having self-lubricating properties. The clearance between the insulating sleeve and the stationary iron core is equal to or smaller than 0.5 mm. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of a presently preferred embodiment of the invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partially cutaway side elevational view showing an engine starter motor equipped with an electromagnetic switch constructed in accordance with the present invention; Fig. 2 is a side elevational view of the electromagnetic switch illustrated in Fig. 1, showing the upper half thereof in cross section; Fig. 3 is a partially cutaway side elevational view showing an engine starter motor equipped with a conventional electromagnetic switch; and Fig. 4 is a side elevational view of the electromagnetic switch illustrated in Fig. 3, showing the upper half thereof in cross section. DESCRIPTION OF THE PREFERRED EMBODIMENT Now, the present invention will be described in detail with reference to a presently preferred embodiment thereof as illustrated in Figs. 1 and 2. In these Figures, reference numerals 1 to 32 excluding 21' designate the same component members as those of the conventional starter motor with the electromagnetic switch as illustrated in Figs. 3 and 4. According to the present invention, an insulating sleeve 21' slidably fitted over the shaft member 20 has a cylindrical outer peripheral surface 21'a which has a constant diameter along the axial length thereof and which has an axial length greater than the axial distance D between the stationary contact 26 and the stationary iron core 9. In this case, the outside diameter of the insulating sleeve 21' is determined such that it is slightly smaller than the inside diameter of the radially innermost surface 9b of the stationary iron core 9 so that there is always a very limited clearance c between the outer peripheral surface 21'a of the insulating sleeve 21' and the radially innermost peripheral surface 9b of the stationary iron core 9 irrespective of the axial position of the insulating sleeve 21' relative to the stationary iron core 9. In this connection, it should be noted that according to the experiments conducted by the inventors, it has been found that when the clearance c between the outer peripheral surface 21'a of the insulating sleeve 21' and the radially innermost peripheral surface 9b of the stationary iron core 9 is 1.8 mm, powder, produced by the friction of repeated opening and closing of the stationary and movable contacts 26 and 22, finds its way into the space S defined by the stationary and movable iron cores 9 and 16, the bushing 14 and the shaft member 20 whereby smooth sliding of the movable iron core 16 relative to the bushing 14 is substantially impaired. On the other hand, when the clearance c is 0.5 mm, there is no entry of powder into the space S, thus causing none of the problems such as sliding troubles. Although in the above-described embodiment, the entire insulating sleeve 21' is formed of an electrically insulating material, at least that part of the insulating sleeve 21' which contacts the movable contact 22 has to be formed of an electrically insulating material but the remaining part of the insulating sleeve 21' may be formed of an electrically conductive material. Thus, if the sleeve 21' is formed of a bearing material such as an oil-containing sintered metal having self-lubricating properties, the clearance c can be further reduced. In this case, the outer peripheral surface 21'a of the insulating sleeve 21' may be in sliding contact with the inner peripheral surface 9b of the stationary iron core 9.

Claims: An electromagnetic switch for a starter motor comprising: a housing (8) made of a magnetic material; an annular stationary iron core (9) disposed in and fixed to said housing; an electromagnetic coil (12) disposed within said housing; a movable iron core (16) disposed in said electromagnetic coil and slidable between a contact open and a contact closed position; a stationary contact (26) mounted to said housing; a sleeve (21') mounted on a shaft member (20) connected with said movable iron core (16) and extending within said annular stationary iron core (9); and a movable contact (22) mounted on said sleeve (21') for contacting with said stationary contact (26) when said movable iron core is in said contact closed position; wherein said housing (8) defines, in cooperation with said movable iron core (16), a magnetic circuit through which a magnetic flux generated by said electromagnetic coil (12) can pass for electromagnetically driving said movable iron core between said contact open and closed positions; characterised in that a clearance (c) is provided between the sleeve (21') and the stationary core (9) and is substantially constant irrespective of the position of said sleeve relative to said stationary core, said clearance being sufficiently small for substantially sealing a space defined between said movable and stationary iron cores against the entrance of powder produced by the friction of repeated opening and closing of the contacts (22,26), and part of said insulating sleeve (21') on which said movable contact (22) is mounted is formed of an electrically insulating material, and the remaining part of said insulating sleeve is formed of a bearing material. An electromagnetic switch according to claim 1, wherein the housing (8) is cylindrical, open at one end and closed at its other end; a cap (11) is attached to the open end of said housing; the stationary contact (26) is firmly attached to the inner surface of said cap; a bushing (14) is firmly fitted in said electromagnetic coil; and the movable iron core (16) is an integral part of the shaft member (20). An electromagnetic switch for a starter motor as claimed in claim 1 or claim 2, wherein for maintaining the said constant clearance (c) between said insulating sleeve and said stationary iron core, the outer peripheral surface of said insulating sleeve (12) is cylindrical and has a diameter slightly smaller than the inside diameter of the radially innermost surface of said stationary iron core (9). An electromagnetic switch for a starter motor as claimed in claim 3, wherein the outer peripheral surface of said insulating sleeve (12) has an axial length larger than the axial distance between said stationary contact (26) and said stationary iron core (9). An electromagnetic switch for a starter motor as claimed in any one of the preceding claims, wherein said bearing material comprises oil-containing sintered metal having self-lubricating properties. An electromagnetic switch as claimed in any one of claims 1 to 5, wherein the clearance between said insulating sleeve and said stationary iron core is equal to or smaller than 0.5 mm.

Inventors: GOTOU, TAKEO HIMEJI WORKS OF MITSUBISHI DENKI, ISHIHARA, KOUHEI HIMEJI WORKS OF MITSUBISHI DENKI, KITTAKA, YOSHIAKI HIMEJI DIVISION OF RYODEN, TANAKA, TOSHINORI HIMEJI WORKS OF MITSUBISHI DENKI
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1986
Main classification: H01H51
IPC codes: H01H51, H01H3, H01H50