Patent Publication Number: US-10316813-B2

Title: Solenoid drive for a starter for an internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Patent Application No. EP 15202072.2, filed on Dec. 22, 2015, the contents of which are incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a solenoid drive for a starter of an internal combustion engine. The invention also relates to a starter for an internal combustion engine, which starter is equipped with such a solenoid drive. 
     BACKGROUND 
     A starter of this type comprises a support, an electric motor which is arranged on the support and which serves for driving a pinion in rotation, and a solenoid drive which is arranged on the support and which serves for the axial adjustment of the pinion between an engagement position, which is provided for the drive of a gearwheel of the internal combustion engine, and a non-engagement position, which is offset axially with respect to the engagement position. 
     The solenoid drive used here comprises a ferromagnetic housing and a cylindrical coil arrangement which has at least one electric coil, wherein the coil arrangement is arranged in the housing and coaxially surrounds a cylindrical coil interior space. Furthermore, a ferromagnetic plunger stop is provided which is arranged at a first axial end of the coil arrangement in the housing and which has a central region projecting axially into the coil interior space. Finally, a ferromagnetic plunger is provided which, at a second axial end of the coil arrangement, which axial end is opposite the central region of the plunger stop, projects axially into the coil interior space, and which is arranged so as to be adjustable axially bi-directionally relative to the housing between an active position which is proximal with respect to the central region and a passive position which is distal with respect to the central region. The drive coupling between plunger and pinion takes place in such a manner that, in the passive position of the plunger, the pinion is in the non-engagement position while said pinion is transferred into the engagement position thereof by adjustment of the plunger into the active position. 
     For the starting of the internal combustion engine, the solenoid drive is activated so as to transfer the pinion of the starter from the non-engagement position into the engagement position. For this purpose, the plunger is adjusted from the passive position into the active position. In the engagement position, the pinion meshes with a gearwheel of the internal combustion engine, which may be formed for example on a flywheel of a drive train of the internal combustion engine. The electric motor then drives the pinion, which in turn drives said gearwheel, as a result of which a crankshaft of the internal combustion engine is set into rotation in order to start the internal combustion engine. As soon as the internal combustion engine has started and the crankshaft thereof is driven by reciprocating movements of pistons of the internal combustion engine, the solenoid drive is activated such that the pinion is returned again from the engagement position into the non-engagement position. For this purpose, the plunger is adjusted back from the active position into the passive position. In the non-engagement position, the pinion disengages from said gearwheel, that is to say no longer meshes with the latter. 
     In order to be able to adjust the pinion from the non-engagement position into the engagement position and in order to be able to secure the pinion in the engagement position, the coil arrangement has to transmit comparatively large electromagnetic forces to the plunger in order to draw the latter into the coil interior space and hold said plunger therein, for the active position. Since, for the purposes of a failsafe design, the plunger is preferably drawn into the coil interior space counter to the action of a restoring spring, comparatively high magnetic forces are required in particular to hold the plunger static in the active position, and therefore the coil arrangement is supplied with a correspondingly high level of electrical power. 
     The pinion normally has a circumferential toothing with axially extending teeth. Complementary with respect thereto, the gearwheel of the internal combustion engine likewise has a circumferential toothing with axially running teeth. Upon a transfer of the pinion from the non-engagement position into the engagement position, the teeth of the pinion engage in toothed spaces of the gearwheel. However, in many situations, axially leading tooth flanks of the teeth of the pinion do not pass directly into the toothed spaces of the toothing of the gearwheel but strike against axial tooth flanks of the teeth of the gearwheel. In order that the teeth of the pinion nevertheless find the toothed spaces of the gearwheel and can engage therein, the electric motor of the starter may be activated so as to effect a rotation of the pinion as early as during the adjustment of the pinion from the non-engagement position into the engagement position. Said rotation for the threading-in of the pinion into the gearwheel is expediently performed with a considerably reduced torque and/or with a considerably reduced rotational speed in relation to the subsequent starting operation, when the pinion is fully engaged with the gearwheel. 
     For said two-stage starting operation, which may also be referred to as “soft-start”, in the case of a starter of this type an electric series connection of the electric motor and of the solenoid drive is expediently proposed, and therefore, for the reduced driving of the electric motor, the voltage provided for energising the coil arrangement can be used in conjunction with the associated current. The solenoid drive then serves at the same time as a switch for connecting the electric motor to the actual motor current supply. In this respect, the solenoid drive at the same time forms an electromagnetic switch. 
     Owing to the above-described, comparatively high magnetic force with which the plunger is drawn into the coil interior space, the pinion may, by way of the axially leading tooth flanks thereof, collide with the opposite axial tooth flanks of the gearwheel with corresponding intensity, increasing the wear of the toothings of pinion and gearwheel. Furthermore, the toothings may bear against one another via the axial tooth flanks with a comparatively high force, as a result of which a correspondingly high level of friction has to be overcome in order to rotate the pinion relative to the gearwheel such that the toothing of the pinion can mesh with the toothing of the gearwheel. As a result, there is the risk of increased wear here too. 
     A starter of this type is known, for example, from U.S. Pat. No. 8,421,565 B2. To solve the above mentioned problem, in the case of the starter, said document proposes a complex construction of the coil arrangement within the solenoid drive, wherein a retraction coil for pulling the plunger into the coil interior space and a holding coil for holding the plunger that is being pulled into the coil interior space are arranged axially separately from one another. It is also proposed that the plunger be equipped, on the outer circumference thereof, with an encircling annular groove which, in the passive position, is situated radially opposite an edge region circumferentially surrounding a passage opening, through which the plunger passes axially, of an end side wall of a solenoid housing. In this way, in the passive position, there is a radial gap between plunger and edge region. As the plunger is retracted into the coil interior space, the circumferential groove moves into the coil interior space and thereby departs from the above mentioned edge region of the end side wall, such that said edge region is subsequently situated radially opposite a plunger longitudinal section axially adjoining the circumferential groove. As the plunger is retracted, therefore, a radial distance between said edge region and an outer side of the plunger is varied, specifically reduced, as a result of which the density of the magnetic field lines transmitted from said edge region to the plunger when the coil arrangement is switched on, is varied, specifically increased. However, the density of the magnetic field lines correlates with the acting magnetic forces. The circumferential groove formed on the plunger thus yields a reduction in the acting magnetic forces at the start of the retraction movement of the plunger when the pinion is to be transferred from the non-engagement position into the engagement position. Said known measures are, however, relatively cumbersome to realise. Furthermore, the attractive force that pulls the plunger into the coil interior space is reduced only to a comparatively small extent by the annular groove, since said annular groove ultimately merely effects a deflection of the field lines. Also, the annular groove is maintained and, even when the plunger has been retracted into the coil interior space, causes a deflection of the field lines in the plunger, thus reducing the attainable magnetic forces. 
     DE 10 2009 052 938 A1 discloses another solution to this problem. In this document, the solenoid drive, which is referred to as an electromagnetic switch, is equipped with a ferromagnetic bypass device, which, when the coil arrangement is energized, diverts some of the magnetic field lines directly from the plunger into the plunger stop, at least in the passive position of the plunger, such that said field lines do not extend through an air gap formed axially between the plunger and the plunger stop. Since, however, the field lines extending through said air gap are crucial for the magnetic force which drives the plunger into the coil interior space, the force acting on the plunger may be reduced for the beginning of the adjustment movement. With increasing penetration depth of the plunger into the coil interior space, the diversion of the magnetic field lines by the bypass device is reduced, as a result of which the magnetic force driving the plunger increases. It has even been shown that, in the active position, the magnetic holding force which holds the plunger in the active position can be increased with the aid of such a bypass device. The same then holds true for the forces which act on the pinion and drive the pinion from the non-engagement position into the engagement position and optionally hold said pinion therein. In this known configuration a part of the magnetic flux is bypassing the axial gap between plunger and plunger stop by passing directly from the housing via the bypass device to the plunger stop. Therefore, the exact axial position of the bypass device relative to the housing and relative to the plunger stop is essential for the deviating effect. Accordingly, narrow production tolerances have to be used. 
     In the case of the known solenoid drive, the bypass device is formed by a ferromagnetic annular body which is dimensioned and arranged in the coil interior space in such a manner that said annular body extends as far as the second axial end of the coil arrangement and is supported there preferably on the housing and is in contact therewith. 
     SUMMARY 
     The present invention is concerned with the problem of specifying, for a solenoid drive of the type mentioned in the introduction or for a starter equipped therewith, an improved or at least different embodiment which is characterized by a simplified construction and capability of being realised inexpensively. At the same time, the intention is furthermore to ensure reduced wear of the pinion and/or of the gearwheel that interacts therewith. In particular, the intention is to specify an advantageous or alternative way of reducing the acting magnetic forces at the start of the adjustment of the pinion from the non-engagement position into the engagement position. 
     This problem is solved according to the invention by the features of the independent claims. The dependent claims relate to advantageous embodiments. 
     The invention is based, according to a first solution, on the general concept of dimensioning and arranging the bypass device in such a manner that said bypass device is spaced apart axially from both axial face side walls axially limiting a coil receiving chamber in which the coil arrangement is arranged. Therefore, the bypass device does not come into contact with the housing and the plunger stop for the deflection of the magnetic field lines. The invention makes use of the finding that for the purpose of deviating the magnetic field lines the bypass device does not need to come into contact with the housing at the face side wall which is in proximity of the plunger. In the invention a part of the magnetic flux is bypassing the axial gap between plunger and plunger stop by passing directly from the plunger via the bypass device to the plunger stop. The exact axial position of the bypass device relative to said face side wall of the housing is therefore not essential for the deviating effect. Consequently, relatively broad production tolerances can be used. This simplifies the production of the solenoid drive and reduces the production costs. Furthermore, the bypass device can thereby also be of smaller dimensions, as a result of which said bypass device is less expensive. 
     In particular, the dimensioning and arrangement of the bypass device are undertaken in such a manner that a plunger end side facing the central region of the plunger stop is positioned axially within the bypass device in the passive position while said plunger end side is adjusted axially beyond the bypass device in the direction of the central region in the active position. In particular, the plunger end side is then located axially between the plunger stop and the bypass device. Preferably, the separate bypass device and the coil arrangement are arranged in the coil receiving chamber. 
     Preferably, the plunger stop comprises the first face side wall coaxially surrounding the central region, wherein the second face side wall is provided at the housing coaxially surrounding the plunger. This simplifies the manufacture of the solenoid drive. 
     In another advantageous embodiment, the bypass device can be dimensioned in such a manner that said bypass device is at a respective axial distance from both face side walls, which axial distance is at least 20% of an axial length of the coil receiving chamber. The axial length of the coil receiving chamber corresponds here to the axially measured distance between the two face side walls which axially limit the coil receiving chamber. The axial distances are preferably in each case approximately 25% of the axial length of the coil receiving chamber. Accordingly, the bypass device expediently has an axial length of approximately 50% of the axial length of the coil receiving chamber. Furthermore, it can be provided additionally or alternatively that the bypass device is arranged substantially centrally axially in or with respect to the coil receiving chamber. In this case, the axial distances of the bypass device from both face side walls are approximately equal in size. This symmetrical arrangement simplifies the production and the installation of the coil arrangement with the bypass device within the coil receiving chamber. 
     According to another advantageous embodiment, the coil arrangement can have a cylindrical coil carrier onto which the at least one coil of the coil arrangement is wound radially on the outside. The coil carrier can have, radially on the inside, a receiving region in which the bypass device is arranged. By this means, the coil carrier can be used at the same time as support for the bypass device. In particular, it is therefore possible to provide an assembly which can be preassembled outside the housing and can then be uniformly inserted into the housing. 
     According to an advantageous development, the receiving region can be dimensioned in such a manner that said receiving region substantially extends only over the axial length of the bypass device. The bypass device is therefore fitted preferably exactly into the coil carrier. In particular, the plastics coil carrier can be injected onto the bypass device. 
     Alternatively, the receiving region can also be dimensioned in such a manner that said receiving region extends axially as far as one of the axial ends of the coil arrangement, expediently as far as the second axial end. The bypass device can be positioned here axially in the receiving region by means of a positioning ring which extends from the bypass device as far as said axial end of the coil arrangement and which is non-magnetic. In the present context, the term “non-magnetic” is understood as meaning “not magnetic and/or not magnetisable”. A non-magnetic material is accordingly not magnetic and/or not magnetisable. A non-magnetic material is, for example, a plastic. The non-magnetic positioning ring can accordingly be, for example, a plastics component. 
     In an advantageous development, said positioning ring can form, radially on the inside, a cylindrical guide contour on which the plunger is guided in an axially adjustable manner radially on the outside. By this means, the positioning ring obtains a dual function. In particular, a separate guide sleeve for guiding the plunger can be dispensed with. The plunger is in contact with the guide contour of the positioning ring while a radial distance is maintained radially between the bypass device and the plunger. 
     Instead of a positioning ring for positioning the bypass device in the receiving region, it is also possible to realise a latching with brings about an axial fixing of the bypass device when the latter has reached the position provided therefor in the receiving region on the coil carrier. 
     In another embodiment, the bypass device can be arranged radially on the inside of the coil carrier axially between two positioning rings which each extend from the bypass device as far as one of the axial ends of the coil arrangement. Said positioning rings are expediently also non-magnetic, and therefore the magnetic deflecting function is realised only by the bypass device. In this embodiment, the production of the coil carrier is simplified since said coil carrier does not have to have any receiving region on the inside and therefore can be designed without steps. The one positioning ring can be supported axially on the plunger stop while the other positioning ring can be supported axially on the housing. 
     In another embodiment or in another solution according to the invention, which can also be realised independently of the solution described above and accordingly represents an independent solution of the problem mentioned at the beginning, the bypass device can be formed by an integral component of the housing, which component is of cylindrical or sleeve-shaped design and which extends coaxially into the coil interior space at the second axial end of the coil arrangement. In this case, the bypass device is therefore not realised in the form of a separate component, but rather by said cylindrical sleeve section of the housing. This approach reduces the production costs and simplifies the assembly. 
     According to an advantageous development, the coil carrier can have an annular step with which said coil carrier is plugged axially onto the bypass device formed by the sleeve section. In this case, the bypass device can therefore the used as an assembly aid for the coil arrangement. 
     In another embodiment or in another solution according to the invention, which can also be realised independently of the solutions described above and accordingly represents an independent solution of the problem mentioned at the beginning, the bypass device can have at least one winding made from a ferromagnetic wire, or can be formed therefrom. In particular, the bypass device can thereby be integrated particularly simply into the coil arrangement. For example, the winding of the bypass device can be wound onto the coil carrier, onto which the at least one coil of the coil arrangement is also wound. By this means, the coil arrangement with integrated bypass device can be produced particularly inexpensively. 
     In another embodiment or in another solution according to the invention, which can also be realised independently of the solutions described above and accordingly represents an independent solution of the problem mentioned at the beginning, the bypass device can have a plurality of bypass elements which are distributed in the circumferential direction and are made from ferromagnetic material. By means of the use of a plurality of bypass elements distributed in the circumferential direction, instead of an encircling, undivided annular body which is closed in the circumferential direction, the influence of the bypass device on the field lines can be varied. In particular, particularly fine coordination can thereby be realised. The bypass elements can be arranged in an annular support of the bypass device, which simplifies the handling of the bypass device despite there being a plurality of separate bypass elements. It is also conceivable to arrange the individual bypass elements on the coil carrier, either radially on the inside in a corresponding receiving region or radially on the outside in the region of the at least one coil. The bypass elements can directly adjoin one another in the circumferential direction such that said bypass elements together again form a closed ring which is, however, divided or segmented. Alternatively, the individual bypass elements can also be arranged spaced apart from one another in the circumferential direction. 
     In an advantageous embodiment, the plunger can be guided in an axially adjustable manner radially on the outside of a cylindrical guide sleeve which is arranged coaxially on the inside of the coil arrangement and which extends from the first axial end through the coil interior space and beyond the second axial end into a guide region of the housing, through which guide region the plunger passes With the aid of a guide sleeve of this type, precise axial guidance for the plunger can be realised, as a result of which the solenoid drive has increased functional reliability. 
     In another embodiment or in another solution according to the invention, which can also be realised independently of the solutions described above and accordingly represents an independent solution of the problem mentioned at the beginning, the above mentioned guide sleeve can be composed of a ferromagnetic material and the bypass device can be formed by an integral component of the guide sleeve. In this respect, the guide sleeve obtains a dual function since said guide sleeve also serves at the same time as the bypass device. This measure also simplifies the production and reduces the costs. 
     In another embodiment or in another solution according to the invention, which can also be realised independently of the solutions described above and accordingly represents an independent solution of the problem mentioned at the beginning, the bypass device can form, radially in the inside, a cylindrical guide contour on which the plunger is guided in an axially adjustable manner radially on the outside. By this means, the bypass device obtains a dual function. In particular, a separate guide sleeve of the type described above can be dispensed with here. 
     In another embodiment or in another solution according to the invention, which can also be realised independently of the solutions described above and accordingly represents an independent solution of the problem mentioned at the beginning, the bypass device can have, in the coil interior space, a cylindrical, ferromagnetic deflecting body which is supported axially on the central region of the plunger stop via a cylindrical, non-magnetic spacer body. In comparison to a conventional construction, the bypass device is thereby offset radially inward into the coil interior space, as a result of which it is possible in particular to use the coil arrangement unchanged, which simplifies the realisation of the solenoid drive presented here. 
     According to an advantageous development, the deflecting body and the spacer body can be of hollow-cylindrical or annular design and can be arranged in the coil interior space radially on the outside with respect to the plunger. The plunger therefore protrudes into the annular deflecting body and into the annular spacer body during the adjustment from the passive position into the active position. 
     Alternatively thereto, the plunger can be of hollow-cylindrical design at least in an end region facing the central region of the plunger stop and can have a cylindrical plunger wall enclosing a plunger interior space. In this case, the deflecting body and the spacer body can be arranged radially on the inside with respect to said plunger wall. In other words, during the adjustment of the plunger from the passive position into the active position, deflecting body and spacer body protrude axially into the hollow-cylindrical end region of the plunger. This embodiment also leads to a particularly compact construction. 
     In another advantageous development, a restoring spring which drives the plunger into the passive position can be supported on the deflecting body. By this means, the deflecting body serves as an abutment for the restoring spring and thereby has an additional function. 
     The solenoid drive can be equipped with an actuating rod which is connected in terms of drive to the plunger and which is guided axially through the plunger stop. On a side of the plunger stop facing away from the coil interior space, said actuating rod bears an electrically conductive contact plate, with the aid of which, in the active position of the plunger, two electric contacts are connected in an electrically conductive manner to each other for example in order to connect the electric motor of the starter to the main current supply thereof. The contact plate and the contacts therefore form a switch within the solenoid drive, and therefore the entire solenoid drive may also be referred to as an electromagnetic switch. 
     A starter according to the invention for an internal combustion engine comprises a support, an electric motor which is arranged on the support and serves for driving a pinion in rotation, and a solenoid drive of the type described above which is arranged on the support and serves for the axial adjustment of the pinion between an engagement position, which is provided for the drive of a gearwheel of the internal combustion engine, and a non-engagement position, which is offset axially with respect to the engagement position. 
     Further important features and advantages of the invention will emerge from the dependent claims, from the drawings and from the associated description of the figures with reference to the drawings. 
     It is self-evident that the features mentioned above and the features yet to be explained below can be used not only in the respectively stated combination, but also in other combinations or individually, without departing from the scope of the present invention. 
     Preferred exemplary embodiments of the invention are illustrated in the drawings and will be explained in more detail in the description below, wherein the same reference signs relate to identical or similar or functionally identical components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, in each case schematically, 
         FIG. 1  shows a side view with a partial longitudinal section of a starter with a conventional solenoid drive, 
         FIG. 2  shows a side view with half a longitudinal section of a solenoid drive according to the invention in the region of a bypass device, 
         FIGS. 3 to 15  show half longitudinal sections as in  FIG. 2 , but for various other embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     According to  FIG. 1 , a starter  1  which is provided for starting an internal combustion engine  2 , of which only a gearwheel  3  is indicated in  FIG. 1  by dashed lines, comprises a support  4 , an electric motor  5  and a solenoid drive  6 , which serves at the same time as a switch for actuating the electric motor  5 . The gearwheel  3  is incorporated in a suitable manner into a drive train (not shown specifically here) of the internal combustion engine  2  such that said gearwheel is connected in terms of drive to a crankshaft of the internal combustion engine  2  if the internal combustion engine  2  is, as is preferred, a piston engine with a crankshaft. For example, the gearwheel  3  may be formed on a flywheel of the drive train. 
     The support  4  is designed for fastening the starter  1  to the internal combustion engine  2  or to a peripheral of the internal combustion engine  2  which may be located, for example, in a vehicle which is equipped with the internal combustion engine  2 . 
     The electric motor  5  is arranged on the support  4  and serves for driving a pinion  7  in rotation. The pinion  7  serves for driving the gearwheel  3  when the internal combustion engine  2  is intended to be started with the aid of the starter  1 . For this purpose, the pinion  7 , together with a drive shaft  8  on which the pinion  7  is arranged for conjoint rotation therewith, is adjustable bilinearly in an axial direction  9 , which is defined by an axis of rotation  10  of the drive shaft  8  or of the electric motor  5 , between a non-engagement position NES, which is shown in  FIG. 1  by solid lines, and an engagement position ES, which is indicated in  FIG. 1  by dashed lines. In said engagement position ES, the pinion is assigned the reference sign  7 ′. In the engagement position ES, the pinion  7 ′ serves for driving the gearwheel  3  and thus meshes with the latter such that a rotation of the pinion  7 ′ forces a rotation of the gearwheel  3 . In the non-engagement position NES, the pinion  7  is axially offset with respect to the engagement position ES, specifically to such an extent that said pinion does not mesh with the gearwheel  3 . In this respect, the pinion  7  is then arranged axially spaced apart from the gearwheel  3 . 
     The electric motor  5  furthermore has, in the conventional manner, an external stator  11  and an internal rotor  12 , wherein the rotor  12  is connected in terms of drive to the drive shaft  8  via a transmission device  13 . The transmission device  13  may have a clutch, in particular a one-way friction clutch. The transmission device  13  may additionally or alternatively have a gearing  18 , for example a planetary gearing. The stator  11  is accommodated in a stator housing  14  which is fastened to the support  4 . In the situation shown, the support  4  has a base housing  29 , which serves for the fastening of the starter  1  to said peripheral, and an intermediate housing  15 , which is fastened to the base housing  29 . In the example shown, the stator housing  14  is now fastened to said intermediate housing  15 . 
     The drive shaft  8  is mounted by way of a main bearing  16  on the support  4  or on the base housing  29  thereof. A further bearing  17  is provided in the intermediate housing  15 , for the purpose of mounting the drive shaft  8 . 
     The solenoid drive  6  has a solenoid housing  19  which is referred to below in short as housing  19  and which is fastened to the support  4 , specifically to the intermediate housing  15  thereof. The solenoid drive  6  serves for the axial adjustment of the pinion  7 . For this purpose, the solenoid drive  6  has a plunger stop  20  which is static with respect to the support  4 , a plunger  21  which is axially adjustable relative to the plunger stop  20 , and a cylindrical coil arrangement  22 . An axial direction  23  of the axial adjustability of the plunger  21  is defined by a longitudinal central axis  24  of the solenoid drive  6 . The solenoid drive  6  is expediently arranged on the support  4  so as to be parallel and adjacent to the electric motor  5 , such that the longitudinal central axis  24  extends parallel to the axis of rotation  10 . 
     The coil arrangement  22  is arranged on the plunger stop  20  and surrounds a cylindrical coil interior space  25  in a circumferential direction, which is based on the longitudinal central axis  24 . The plunger  21  is coupled by way of a deflecting lever  26  to the drive shaft  8  in such a manner that, for the adjustment of the pinion  7  from the non-engagement position NES into the engagement position ES, the plunger  21  is retracted into the coil interior space  25 . Accordingly, the coil arrangement  22  is in the form of a retraction coil  40  which, when energised, pulls the plunger  21  into the coil interior space  25 . The deflecting lever  26  here effects a reversal of the movement direction, such that the retraction of the plunger  21  toward the top in  FIG. 1  effects a deployment of the pinion  7  toward the bottom in  FIG. 1 . The plunger  21  is therefore adjustable with respect to the plunger stop  20  between an extended passive position PS and a retracted active position AS. In  FIG. 1 , the axial position of a plunger end side  27  facing the plunger stop  20  is indicated by solid lines for the passive position PS while the axial position of the plunger end side  27  is indicated by dashed lines for the active position AS. In the active position AS, the plunger end side  27  preferably comes axially to bear against a stop end side  28  of the plunger stop  20 , which stop end side faces the plunger  21  and therefore forms an axial end stop for the plunger  21 . 
     In addition, the plunger  21  is coupled to an actuating rod  30  which, for this purpose, extends at least partially through the plunger  21 . The actuating rod  30  serves for the axial adjustment of a plate-like contact element  31  which, for its part, serves for the electrical connection of two electric contacts  32 . The electric motor  5  is connected to a main current supply  33  via said electric contacts  32 . In other words, as soon as the contact element  31  electrically connects the two electric contacts  32  to each other, the electric motor  5  can be supplied with a rated electrical power via the main current supply  33  so that the electric motor  5  can output a rated torque at the pinion  7 . In order to realise what is referred to as a “soft-start operation”, provision may be made to connect the electric motor  5  in series with the solenoid drive  6  or with the coil arrangement  22  thereof. The electric motor  5  can therefore be initially supplied with a considerably lower electrical power in order to drive the pinion  7  with a considerably lower torque and/or at a considerably lower rotational speed for as long as said pinion has not yet reached the engagement position ES thereof. 
     The actuating rod  30  is guided coaxially through the plunger stop  20 . Accordingly, the plunger stop  20  is ultimately located axially between the plunger  21  and the contact element  31 . The plunger  21  is assigned at least one restoring spring  34  which, in the example, loops coaxially around the actuating rod  30 . The restoring spring  34  is supported here on one side on the plunger  21  and on the other side on the plunger stop  20 . The restoring spring  34  protrudes here in a cavity  35  formed on the plunger  21 . 
     The actuating rod  30  is also assigned a restoring spring  36  which is supported on one side on the actuating rod  30  and on the other side on a contact housing  37 , on which the electric contacts  32  are located. Furthermore, a pre-tensioning spring  38  can be provided which drives the contact element  31  in the direction of the contacts  32 . Said pre-tensioning spring  38  is supported here on the actuating rod  30 . An axial distance between the contact element  31  and the contacts  32  is discernibly smaller than the entire adjustment travel of the plunger  21  between the passive position PS and the active position AS. The contact element  31  therefore comes into contact with the contacts  32  shortly before reaching the active position AS. On reaching the active position AS, the pre-tensioning spring  38  then brings about a pre-tensioned bearing of the contact element  31  against the contacts  32 . By means of the capacitive effect of coils/windings of the electric motor  5 , the rated torque builds up with a time delay. The coordination is expediently undertaken here in such a manner that the rated torque is present approximately synchronously with the reaching of the active position AS, i.e. also synchronously with the reaching of the engagement position ES. 
     Furthermore, it can be seen that, in the passive position PS, the contact element  31  bears axially against a rear side  39  of the plunger stop  20 , which rear side faces away from the plunger  21 . 
     Since the solenoid drive  6  therefore also serves for the connection of the main current supply  33  of the electric motor  5 , said solenoid drive may also be referred to as an electromagnetic switch. 
     According to  FIGS. 2 to 15 , the solenoid drive  6  comprises the housing  19  produced from a ferromagnetic material, the coil arrangement  22 , the ferromagnetic plunger stop  20  and the ferromagnetic plunger  21 . In the examples shown here, the coil arrangement  22  in each case comprises two coils, specifically a retraction coil  40  for pulling the plunger  21  into the interior of the coil arrangement  22  counter to the plunger stop  20 , and a holding coil  41  for holding the plunger  21  in the active position AS. The coil arrangement  22  is arranged in a coil receiving chamber  72  of the housing  19  and coaxially surrounds the coil interior space  25 . The coil receiving chamber  72  is axially limited by a first face side wall  73  and a second face side wall  74  axially opposing the first face side wall  73 . 
     The plunger stop  20  is arranged at a first axial end  42  of the coil arrangement  22  in the housing  19 . The plunger stop  20  has a central region  43  which projects axially into the coil interior space  25  and has the above mentioned stop end side  28  which can serve as an axial stop for the plunger  21 . The plunger stop  20  is provided with the first face side wall  73  which is ring shaped and coaxially encircling the central region  43 . The second face side wall  74  is provided at the housing  19 . In the depicted examples, the coil arrangement  22  axially abuts with its first axial end  42  to the first face side wall  73 . 
     The plunger  21  projects axially into the coil interior space  25  at a second axial end  44  of the coil arrangement  22 , which second axial end  44  is opposite the central region  43 . In the depicted examples, this second axial end  44  is axially spaced apart from the second face side wall  74 . Thus an axial gap  75  is provided axially between the second axial end  44  and the second face side wall  74 . 
     Furthermore, the plunger  21 , as explained, is arranged so as to be adjustable axially bi-directionally relative to the housing  19  between the active position AS which is proximal with respect to the central region  43  and the passive position PS which is distal with respect to the central region  43 . In the passive position PS an axial air gap  71  is provided within the coil interior space  25  axially between the plunger  21  or the plunger end side  27 , respectively, and the plunger stop  20  or the stop end side  28 , respectively. This axial air gap  71  reduces when the plunger  21  moves from the passive position PS to the active position AS. As explained, in the active position AS, the plunger  21  can be in contact by means of the plunger end side  27  thereof with the stop end side  28  which is located on the central region  43  in the coil interior space  25 . In this case the axial air gap  71  is eliminated in the active position AS. 
     In addition, the solenoid drive  6  shown here is equipped with a ferromagnetic bypass device  45 . The latter is arranged within the coil receiving chamber  72 , coaxially with respect to the coil arrangement  22  and radially within the respective coil  40 ,  41  of the coil arrangement  22 . In a starting region of the adjustment travel of the plunger  21 , which starting region has the passive position PS, the bypass device  45  brings about a deflection of magnetic field lines in such a manner that the deflected magnetic field lines are not guided within the coil interior space  25  through the axial air gap  71  prevailing there between plunger  21  and plunger stop  20 , but rather pass from the plunger  21  via the bypass device  45  directly to the plunger stop  20 . This results in a reduction in the magnetic forces which drive the plunger  21  in the coil interior space  25  in the direction of the plunger stop  20 . With increasing penetration depth of the plunger  21  into the coil arrangement  22 , said deflecting influence of the bypass device  45  decreases. In particular, the field lines run substantially directly within the reduced air gap  71  from the plunger  21  to the plunger stop  20  in an end region of the adjustment travel of the plunger  21 , which end region contains the active position AS. 
     In the embodiments of  FIGS. 2 to 5, 7 to 10 and 13 to 15 , the bypass device  45  is arranged and dimensioned in such a manner that said bypass device  45  is spaced apart axially from both face side walls  73 ,  74  of the coil receiving chamber  72  and also from both axial ends  42 ,  44  of the coil arrangement  22 . According to  FIG. 2 , the bypass device  45  can be at a respective axial distance  46 ,  47  from both face side walls  73 ,  74 , which axial distance is at least 20% of an axial length  48  of the coil receiving chamber  72 . The axial length  48  of the coil receiving chamber  72  is discernibly defined by the axial distance between the two face side walls  73 ,  74 . In the example of  FIG. 2 , the bypass device  45  is arranged approximately centrally axially with respect to the coil receiving chamber  72 . 
     In the examples of  FIGS. 2 to 6 and 9 to 15 , the bypass device  45  is formed in each case by a single cylindrical and preferably annular body. By contrast, in the case of the embodiment shown in  FIG. 7 , the bypass device  45  is formed by a winding  49  made from a ferromagnetic wire. In the case of the embodiment shown in  FIG. 8 , the bypass device  45  is formed with the aid of a plurality of ferromagnetic bypass elements  50  which are arranged distributed in the circumferential direction. The bypass elements  50  can be adjacent to one another in the circumferential direction or preferably arranged spaced apart from one another. 
     In all of the embodiments shown here, the coil arrangement  22  has a cylindrical coil carrier  51  onto which the two coils  40 ,  41  are wound radially on the outside. The holding coil  41  is expediently wound here radially on the outside of the retraction coil  40  and extends in particular over the entire axial length of the retraction coil  40 . The coil carrier  51  is expediently composed of a non-magnetic material. In particular, the coil carrier  51  has a tubular casing (not denoted specifically) which, at the axial ends thereof, has two annular end discs which protrude outward from the casing in the manner of collars and define the axial ends  42 ,  44  of the coil arrangement  22 . The coils  40 ,  41  are arranged radially on the outside of the casing and axially between the end discs. 
     The bypass device  45  can now be arranged radially on the inside of the coil carrier  51 , which is the case in the examples of  FIGS. 2 to 6 and 9 to 13 . In particular, for this purpose, a receiving region  52  which forms a depression on the inner side of the coil carrier  51  can be formed radially on the inside of the coil carrier  51 . The bypass device  45  is inserted in said recessed receiving region  52 . A receiving region  52  of this type can be seen, for example, in the embodiments of  FIGS. 2 to 4 . In the example of  FIG. 2 , the receiving region  52  extends axially only over the axial length of the bypass device  45 . For example, the coil carrier  51  which is produced from a plastic can be sprayed or injection moulded onto the outside of the bypass device  45 . 
     In the examples of  FIGS. 3 and 4 , the receiving region  52  is, by contrast, dimensioned in such a manner that said receiving region extends axially as far as one of the axial ends  42 ,  44 , here as far as the second axial end  44 . In the example of  FIG. 3 , the bypass device  45  is positioned axially in the receiving region  52  with the aid of a positioning ring  54 . The positioning ring  54  is non-magnetic and extends from the bypass device  45  as far as said second axial end  44 . For example, the positioning ring  54  is supported axially on the second face side wall  74  of the housing  19 . In the example of  FIG. 4 , a latching  53  is provided for the axial fixing of the bypass device  45 . An individual latching lug which is latched to an axial end side of the bypass device  45  is shown. A plurality of latching lugs of this type can be arranged distributed in the circumferential direction. It is likewise conceivable to provide a latching contour encircling in the circumferential direction. 
       FIG. 5  shows an embodiment in which the bypass device  45  is positioned axially radially on the inside of the coil carrier  51  with the aid of two positioning rings  54 . For this purpose, the bypass device  45  is arranged axially between the two positioning rings  54 . The respective positioning ring  54  extends axially here from the bypass device  45  as far as one of the axial ends  42 ,  44 . The lower positioning ring  54  in  FIG. 5  is supported here axially on the first face side wall  73  of the plunger stop  20 . The upper positioning ring  54  in  FIG. 5  is supported here axially on the second face side wall  74  of the housing  19 . 
     In the embodiment shown in  FIG. 6 , the bypass device  45  is formed by a sleeve-shaped, cylindrical section  55  of the housing  19 , and therefore the bypass device  45  thus forms an integral component of the housing  19 . At the second axial end  44 , said cylindrical sleeve section  55  extends coaxially into the coil interior space  25  and ends axially spaced apart from the plunger stop  20 . The coil carrier  51  is provided here with an annular step  56  which substantially corresponds to the continuous receiving region  52  of the embodiment shown in  FIG. 3 . In the example of  FIG. 6 , the annular step  56  serves to plug the coil arrangement  22  or the coil carrier  51  axially onto the cylindrical component  55  of the housing  19 . In this embodiment the bypass device  45  or the cylindrical sleeve section  55 , respectively, limits radially the coil receiving chamber  72 . 
     In the examples of  FIGS. 7 and 8 , the bypass device  45  is integrated in the coil arrangement  22 . The bypass device  45  is arranged here on a radial outer side of the coil carrier  51 . In the embodiment shown in  FIG. 7 , the ferromagnetic winding  49  of the bypass device  45  is first of all wound onto the coil carrier  51 , onto which the retraction coil  40  is then wound, followed by the holding coil  41 . 
     In the example of  FIG. 8 , the in particular rod-shaped or circumferential-segment-shaped bypass elements  50  are arranged on the radially outer side of the coil carrier  51  and are fixed there, for example, by the retraction coil  40  being wound on. In principle, according to  FIG. 8 , an unsegmented or undivided annular bypass device  45  may also be arranged on the radially outer side of the coil carrier  51 . For this purpose, a plastics coil carrier  51  can be injected onto said bypass device  45 . It is also conceivable to segment the bypass device  45  in the circumferential direction and to subsequently fit the individual segments onto the coil carrier  51 . 
     According to the examples of  FIGS. 2 to 9 and 14 and 15 , the solenoid drive  6  is expediently provided with a cylindrical guide sleeve  57  which is arranged coaxially on the inside of the coil arrangement  22  and which extends from the first axial end  42  through the coil interior space  25  and beyond the second axial end  44  into a guide region  58  of the housing  19 . The plunger  21  passes through said guide region  58 . The plunger  21  is guided in an axially adjustable manner radially on the outside of said guide sleeve  57 . Said guide sleeve  57  is expediently produced from a non-magnetic material. For example, a low-friction plastic is used. 
     In the embodiment shown in  FIG. 9 , the guide sleeve  57  is by contrast produced from a ferromagnetic material. Furthermore, provision is made here to form the bypass device  45  by an integral component of said guide sleeve  57 . The guide sleeve  57  is discernibly provided with a greater wall thickness in the radial direction in the region of the bypass device  45 , as a result of which the desired deflecting effect for magnetic field lines is produced there. It is also basically conceivable here to spray the plastics coil carrier  51  onto the outer side of the guide sleeve  57 . Furthermore, it is conceivable to segment the guide sleeve  57  or the coil carrier  51  in the circumferential direction. 
     In the embodiments of  FIGS. 10 to 13 , a separate guide sleeve  57  is omitted. In the example of  FIG. 10 , the bypass device  45  is provided radially on the inside with a cylindrical guide contour  59  on which the plunger  21  is guided in an axially adjustable manner radially on the outside. It is basically possible here to guide the plunger  21  radially on the outside directly on the bypass device  45 . However, the bypass device  45  is preferably provided radially on the inside with a low-friction coating  60 , for example made from Teflon. 
     Also in the examples of  FIGS. 11 and 12 , the bypass device  45  is provided radially on the inside with a guide contour  59  of this type which optionally can likewise be realised with the aid of a low-friction coating  60  of this type. While, in the example of  FIG. 10 , the bypass device  45  is spaced apart axially from the two axial ends  42 ,  44 , in the examples of  FIGS. 11 and 12  the bypass device  45  extends in each case as far as the second axial end  44 . In the example of  FIG. 11 , the bypass device  45  is supported axially in the region of the second axial end  44  on the housing  19 . In the example of  FIG. 12 , the bypass device  45  extends axially beyond the second axial end  44  and is supported on the housing  19  in an annular step  61 . 
     In the embodiment shown in  FIG. 13 , similarly as in the embodiment shown in  FIG. 3 , a non-magnetic positioning ring  54  is provided for the axial positioning of the bypass device  45 , said positioning ring, similarly as in  FIG. 12 , being supported purely by way of example in an annular step  61  of the housing  19 . In this embodiment, the positioning ring  54  is provided radially on the inside with a cylindrical guide contour  62  on which the plunger  21  is guided in an axially adjustable manner radially on the outside. A low-friction, tribologically optimised combination of material can be realised by an appropriate selection of material for the positioning ring  54 . 
     In the embodiments of  FIGS. 14 and 15 , the bypass device  45  is arranged in the coil interior space  25 . The bypass device  45  is located here radially within the coil arrangement  22 , radially within the coil carrier  51  and, in the example, also radially within the guide sleeve  57 . Furthermore, the bypass device  45  has a cylindrical and ferromagnetic deflecting body  63  which is supported axially on the central region  43  of the plunger stop  20  via a cylindrical and non-magnetic spacer body  64 . 
     In the example of  FIG. 14 , the deflecting body  63  and the spacer body  64  are positioned bearing radially on the inside of the guide sleeve  57  and also are of hollow-cylindrical or annular design. With regard to an external contour  65  of the plunger  21 , the latter can be arranged with a radial gap with respect to the deflecting body  63  and with respect to the spacer body  64 . Accordingly, in the passive position PS, the plunger  21  protrudes axially into the deflecting body  63 . In the active position AS, the plunger  21  protrudes through the deflecting body  63  axially into the spacer body  64 . In the example of  FIG. 14 , deflecting body  63  and spacer body  64  are therefore located radially on the outside of the plunger  21 . 
     In the embodiment shown in  FIG. 15 , the plunger  21  is of hollow-cylindrical design at least in an end region  66  facing the central region  43  of the plunger stop  20 , and therefore the plunger  21  in said end region  66  has a plunger wall  67  which encloses a plunger interior space  68  in the circumferential direction. That plunger interior space  68  corresponds to the cavity  35  already mentioned further above. Deflecting body  63  and spacer body  64  are now arranged radially on the inside with respect to the plunger wall  67 , but are spaced apart radially therefrom. In the passive position PS, only the deflecting body  63  protrudes axially into the plunger interior space  68 . In the active position AS, the deflecting body  63  and the spacer body  64  protrude axially into the plunger interior space  68 . 
     In the examples of  FIGS. 14 and 15 , the restoring spring  34  which drives the plunger  21  into the passive position PS is supported on the deflecting body  63 . In the two examples, deflecting body  63  and spacer body  64  are of annular design in order at any rate to be able to pass the actuating rod  30  coaxially therethrough. 
     The embodiments shown in  FIGS. 14 and 15  are suitable in a particular way for retrospective integration of the bypass device  45  in an otherwise conventional solenoid drive  6 . In this case, the solenoid drive  6  can be retrofitted or realised in a particularly simple manner. 
     In the examples of  FIGS. 2 to 13 , the central region  43  is provided with a central conical or frustoconical extension  69  which tapers along the longitudinal central axis  24  in the direction of the plunger  21 . The plunger  21  has, on the plunger end side  27  thereof, a conical depression  70  which is complementary with respect to the extension  69  and into which the extension  69  protrudes axially during the transfer into the active position AS.