Abstract:
An electric machine for starting an internal combustion engine, comprising a housing ( 10 ) having at least one sealing device ( 14 ) which seals an interior ( 10 a) of the housing with respect to the surroundings of the housing ( 10 ), and at least one pressure reduction device ( 1 ), wherein the at least one pressure reduction device ( 1 ) is embodied so as to be moveable in a translatory fashion relative to the at least one sealing device ( 14 ) in order to form, in the case of an overpressure in the interior ( 10   a ) of the housing, at least one venting duct via which an at least partial reduction in pressure is ensured.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to an electric machine for starting an internal combustion engine. 
         [0002]    In addition, the invention relates to a method for operating an electric machine for starting an internal combustion engine. 
         [0003]    The invention is based on a system having an electric machine such as a starter or a starter motor. 
         [0004]    The subject matter of the present invention comprises electric machines, in particular starters for vehicles with internal combustion engines which have an electric machine with a commutator. 
         [0005]    Electric machines, in particular starters or generators, having a housing which is sealed, in particular sealed with respect to environmental influences such as the penetration of dirt and/or water, are known from the prior art. These “sealed” starters are used, for example, in mining vehicles or special vehicles. The housing is conventionally sealed by means of various sealing measures such as, for example, by means of O rings. 
         [0006]    If a starter is in its position of rest, for example when the starter is not operational, a starter pinion is held in this position of rest by means of spring forces. If the starter is put into operation and in the process the starter heats up, the air in the housing also expands. Owing to the sealing of the housing, this air cannot escape, with the result that an overpressure comes about in the housing. The overpressure gives rise to a force acting on components arranged in the housing, for example on a shaft, in particular an output shaft, which is partially mounted in the housing and presses the output shaft axially forward, that is to say in the direction of a ring gear of the internal combustion engine. As soon as the force acting on the output shaft owing to the overpressure is larger than the spring force acting in the opposite direction, the overpressure causes the pinion mounted on the output shaft to disengage from the ring gear (similarly to a pneumatic piston). Furthermore, the overpressure has the effect that after the starter is switched off the pinion can no longer be moved back into the position of rest since the spring force which is applied for this purpose is lower than the opposing force of the overpressure. The pinion can then abut against the ring gear or after starting cannot be moved, or cannot be moved sufficiently quickly, out of the ring gear into the position of rest. This leads to increased wear at the pinion/ring gear combination up to the point of destruction of the starter if the pinion cannot leave the ring gear. Increasing the spring force of the engagement spring in order to counteract the force of the overpressure would have an adverse effect on the pull-in behavior of the relay. 
         [0007]    DE 10 2008 002 272 A1 discloses a housing of a starter with a valve for reducing an overpressure in the housing. However, this valve must be attached to the starter in such a way that it is accessible from the outside, as a result of which said valve is subjected to environmental influences. In the case of applications which are susceptible to dirt, the optimum functioning of the valve over its lifetime is not ensured. Furthermore, starters are often surface coated after their attachment to the internal combustion engine. A surface coating layer can block the valve with the result that its function is adversely affected and the valve can, in particular, no longer open. Furthermore, active control of the valve is necessary for the purpose of opening or closing so as to ensure the targeted reduction in pressure. 
       SUMMARY OF THE INVENTION 
       [0008]    The electric machine according to the invention and the method according to the invention for operating an electric machine have the advantage over the prior art that in an electric machine, in particular in a starting device such as a starter, for starting an internal combustion engine, comprising a housing having at least one sealing device which seals an interior of the housing with respect to the surroundings of the housing, and at least one pressure reduction device, that the at least one pressure reduction device is embodied so as to be moveable in a translatory fashion relative to the at least one sealing device in order to form, in the case of an overpressure in the interior of the housing, in particular due to movement, at least one venting duct via which an at least partial reduction in pressure is ensured. The electric machine is advantageously embodied as a starting device such as a starter. An overpressure in the interior of the housing is advantageously at least partially reduced with at least one pressure reduction device. The interior of the housing is preferably sealed with respect to the surroundings in an air-tight and/or water-tight fashion by means of the sealing device. In one embodiment, the sealing device comprises a shaft sealing ring. In a further embodiment, the sealing device additionally comprises a sealing lip. Simple control for a reduction in pressure is implemented by means of the movement-induced reduction in pressure. 
         [0009]    In one embodiment, precisely one pressure reduction device is embodied. Other embodiments provide more than one pressure reduction device. In the case of a reduction in pressure, the pressure reduction device is moved in a preferably at least axially translatory fashion relative to the housing and/or the sealing device. Depending on the movement, the pressure reduction device forms at least one venting duct via which an at least partial reduction in pressure is ensured. In one embodiment, the venting duct is released at least partially by the at least translatory movement of the pressure reduction device. 
         [0010]    In one embodiment there is provision that the at least one pressure reduction device is embodied on a component which projects and can be moved at least partially out of the housing. The pressure reduction device is advantageously formed in the region of the sealing device on the part which projects out and can be moved. The sealing device preferably encloses the component around its circumference in order to ensure a complete and reliable seal. 
         [0011]    In another embodiment there is provision that the at least one pressure reduction device is embodied on a section of the moveable component, which section interacts with the at least one sealing device, in particular in a position of rest between the at least one sealing device and a bearing. The sealing device advantageously seals the housing completely, at least in a position of rest of the component. In the case of a translatory movement of the component, the sealing device of the housing interacts with the pressure reduction device in such a way that in the case of an overpressure a venting duct is generated through which the overpressure can at least partially escape from the interior of the housing. 
         [0012]    In a further embodiment there is provision that the at least one pressure reduction device is embodied on a drive train, in particular on a shaft such as a drive shaft and/or an output shaft, of the electric machine. The shaft is advantageously embodied in such a way that, in addition to the rotational movement, it can move axially and/or radially, and is mounted in a sealed fashion in the housing. In the case of a movement of the shaft in the axial direction owing to an overpressure, the venting duct is periodically released. However, penetration of external influences such as water or air counter to the overpressure is not possible owing to the overpressure. 
         [0013]    Accordingly, the pressure reduction device is preferably embodied in the region of an axial stroke of the shaft relative to the housing, with the result that the pressure reduction device or the venting duct is released when there is an axial movement of the shaft. In the case of a position of rest of the shaft, that is to say when the latter is moved into the housing to a maximum extent, the pressure reduction device is advantageously embodied inside, in particular completely inside the housing, that is to say in a region of the shaft which is arranged in the housing. Considered in the axial direction, the pressure reduction device is located on the side of the housing or of the sealing device facing away from the ring gear, that is to say between the sealing device and a bearing. In the pre-engaged state of the shaft, the venting device is then located in front of the sealing device, that is to say completely outside the housing. 
         [0014]    Soiling of the venting device, for example due to engine oil or the like, is advantageously prevented since the latter is located in the position of rest inside the housing and is therefore protected from environmental influences. Furthermore, automatic cleaning or blowing out of the venting device takes place when the pinion is disengaged or in the case of axial movement of the shaft if dirt has been deposited in the venting device when the engine starts. Correspondingly, the pressure reduction device is advantageously self-cleaning. 
         [0015]    In yet another embodiment there is provision that the at least one pressure reduction device is embodied as a depression in the drive train. The depression is preferably embodied in the shaft. The depression is advantageously integrated into a surface of the shaft. The depression advantageously extends in the axial and/or radial directions. In a further embodiment, a plurality of depressions are provided. In this way, a desired through-flow cross section can be set. 
         [0016]    In yet a further embodiment there is provision that an axial extent of the at least one pressure reduction device is greater than an axial extent of the at least one sealing device of the housing. Owing to the relatively narrow embodiment of the sealing device, the latter in the meantime releases the depression or the venting duct during the relative axial movement for the reduction in pressure. That is to say, the sealing device does not completely cover the pressure reduction device during the movement, with the result that a bypass duct or venting duct is formed under the sealing device and a fluidic connection between the environment and the interior of the housing is implemented. 
         [0017]    In one embodiment there is provision that the at least one pressure reduction device is embodied around part of the circumference of the drive train. According to the invention, the depression extends over only a part of the circumference of the shaft, that is to say not around the entire circumference, for example at most through an angular range of 15° , further preferably at most 10° and most preferably at most 5° , of the circumference of the shaft. A partially circumferential embodiment of the venting device advantageously prevents the situation in which during an axial movement with an associated reduction in pressure by means of the pressure reduction devices the sealing device of the housing does not terminate completely with the shaft or bear against it and therefore a complete reduction in pressure takes place. 
         [0018]    The method according to the invention has the advantage over the prior art that in a method for operating an electric machine, in particular a generator or a starting device such as a starter, for starting an internal combustion engine, comprising a housing having at least one sealing device which seals an interior of the housing with respect to the surroundings of the housing, and at least one pressure reduction device, wherein a change in pressure owing to operation takes place in the interior of the housing, that a movement of the at least one pressure reduction device relative to the at least one sealing device is carried out in order to at least partially reduce an overpressure in the interior of the housing, in particular in a movement-induced fashion. The reduction in pressure does not take place here by means of a movement of a sealing element with respect to an interior but rather as a result of a movement of the pressure reduction device with respect to sealing elements. As a result, the pressure reduction device is made smaller in design and can be integrated into moveable components. The pressure reduction is correspondingly controlled in an automatic fashion. 
         [0019]    In one embodiment of the method there is provision that the movement of the at least one pressure reduction device is carried out automatically, with the result that a reduction in pressure is carried out in an automated fashion. The reduction in overpressure therefore preferably takes place in a movement-induced fashion or as a function of the pressure and in an automated or automatic fashion. An overpressure in the housing brings about an undesired axial movement of the shaft in the direction of the ring gear. The shaft is pressed out of the housing here. According to the invention, during this undesired axial movement the overpressure in the housing is at least partially reduced in an automated fashion. Additional open-loop and/or closed-loop control of the pressure reduction device is not necessary here. During the undesired axial movement, the depression of the pressure reduction device moves along the sealing lip of the shaft sealing ring of the sealing device. In this context, the depression of the pressure reduction device is exposed partially from an interior of the housing as far as the external surroundings, with the result that the overpressure can escape from the housing through the bypass duct which comes about in the process. 
         [0020]    The undesired axial movement of the shaft is advantageously terminated as soon as the overpressure in the interior housing is sufficiently reduced. Through this reduction in pressure, the shaft returns to its initial position. Given a renewed buildup of pressure, for example when the starter heats up, the shaft is pressed out of the housing again and the overpressure is reduced. An axial movement, that is to say a to and fro movement, of the shaft advantageously takes place until the overpressure in the housing is reduced to such an extent that the force acting on the shaft owing to the overpressure is smaller than the spring force acting in the opposite direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the following description, in which: 
           [0022]      FIG. 1  is a cross-sectional view of an electric machine which is embodied as a starting device, 
           [0023]      FIG. 2  is a cross-sectional view of a detail of the starting device in the region of a housing and of an output shaft in a position of rest of the output shaft, 
           [0024]      FIG. 3  is a cross-sectional view of the detail of the starting device according to  FIG. 2  in a pre-engaged position of the output shaft, and 
           [0025]      FIG. 4  is a cross-sectional view of a detail of the starting device in a further embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows a cross-sectional view of an electric machine  100 , embodied as a starting device, of an internal combustion engine having a relay  42 , also embodied as a switching relay or engagement relay. A housing  10  of the starting device comprises a cylindrical housing part  11  and a cover  13  which are connected to one another. The cylindrical housing part  11  is closed off at the rear by the cover  13 , in the center part of which an outwardly directed hub is integrally formed. The housing part  11  forms an interior  10   a  together with the cover  13 . Although the components arranged in the interior  10   a  are not illustrated here, these components are described below for the sake of completeness. In the hub there is a bearing point in which a rear end of an armature shaft of a starter motor, which has an armature, is mounted. A plurality of (permanent) magnets of the starter motor are located radially outside the armature on the wall of the housing part  11 . The front end of the armature shaft is mounted with an end section, reduced in diameter, in a coaxially extending blind hole (not illustrated further) of an output shaft  24 . The other end of the output shaft  24  is mounted in a further cover  25  which closes off the housing part  11 , with a bearing  33  arranged there. In order to seal the interior  10   a  of the housing with the projecting output shaft  24 , the housing  10  has a sealing device  14 . The armature shaft has, near to its end facing the end plate  25 , a toothing (sun gear) into which planetary gears engage which also mesh with an external fixed internal gear wheel of a planetary gear mechanism (=reduction gear). In addition, the electric machine has a freewheel (not denoted here in more detail) which is arranged on a steep lead angle thread  38  of the output shaft  24 . 
         [0027]    A planetary carrier drives the output shaft  24  on whose part projecting out of the housing  10  through the further cover  25  a pinion  35  with an external toothing is arranged. By axially displacing the pinion  35 , the external toothing can be brought into engagement with a ring gear of an internal combustion engine for the purpose of the starting process. This is done with the aid of the (engagement) relay  42  in which, when the current is switched on, a magnet armature  19  pivots, by means of a projection, a lever  44  which pushes the pinion  35  to the left via a link which is located between disks  46 . The lever  44  is embodied as two arms and is arranged so as to be pivotable by means of tappets in a bearing  49  fixed to the housing. The magnet armature  19  is prestressed with an armature restoring spring  39 . Details are only given briefly on this process since it is not essential to the invention. 
         [0028]    A brush plate bears against the cover  13  and is screwed to the cover  13 . The brush plate is embodied in one piece. In particular, brush holders composed of plastic, in which carbon elements are arranged which bear under spring pressure against a commutator, which is arranged on the armature, are attached to said brush plate. The carbon elements are connected by a contact of the engagement relay  42 . 
         [0029]      FIG. 2  shows a cross-sectional view of a detail of the starting device in the region of the housing  10  and of the output shaft  24  in a position of rest of the output shaft  24 . The housing  10  forms an interior  10   a  of the housing which is closed off in an air-tight and water-tight fashion. The output shaft  24  projects into the interior  10   a  of the housing or out of the housing  10 . The output shaft  24  which penetrates the housing  10  is moveably mounted in the interior  10   a  of the housing by means of the bearing  33 . The pinion  35  is arranged on the part of the output shaft  24  projecting out of the interior  10   a  of the housing. The pinion  35  is held prestressed in its position on the output shaft  24  by means of a pinion spring  15 . Furthermore a securing ring  16 , which is arranged around the circumference of the output shaft  24 , prevents an axial movement of the pinion  35  on the output shaft  24 . In order to seal the interior  10   a  of the housing with the projecting output shaft  24 , the housing  10  has the sealing device  14 . The output shaft  24  is enclosed around its entire circumference by the sealing device  14  in the state illustrated. In this embodiment, the sealing device  14  comprises a shaft sealing ring  14   a  with a sealing lip  14   b.  The shaft sealing ring  14   a  is embodied as a prestressed component whose sealing lip  14   b  bears as a rubber seal against the output shaft  24 . The sealing lip  14   b  is pressed against the output shaft  24  by means of the prestress of the shaft sealing ring  14   a,  and in this way the interior  10   a  of the housing is sealed. 
         [0030]    According to the invention, the output shaft  24  has a pressure reduction device  1 . The pressure reduction device  1  serves to at least partially reduce an overpressure in the interior  10   a  of the housing. In the position of rest shown according to  FIG. 2 , the pressure reduction device  1  is arranged completely in the interior  10   a  of the housing, that is to say on the part of the output shaft  24  which is located in the interior  10   a  of the housing. The pressure reduction device  1  is therefore arranged between the sealing device  14  and the bearing  33 . In the embodiment in  FIG. 2 , the pressure reduction device  1  is embodied as a notch-like depression  1   a . In this context, the depression is embodied integrated into the output shaft  24 . The depression  1   a  extends both in the axial direction (see arrow A) and in the radial direction of the output shaft  24 . In this context, the depression  1   a  has a length (extent in the axial direction A) of approximately 1-2 mm. A width of the depression  1   a  (extent perpendicular to the axial direction A) is approximately 0.5 mm, with the result that the depression  1   a  is embodied around part of the circumference of the output shaft  24 . The depression  1   a  extends over an angular range of approximately 5° of the circumference of the output shaft  24 . In this context, the length of the depression la is made larger than the width. The depression la in  FIG. 2  is therefore implemented as a narrow and flat notch or groove. The ends of the depression  1   a  (in the axial direction A) are of flattened and rounded design. A detailed description of the function of the pressure reduction device  1  or of the depression  1   a  follows in  FIG. 3 . 
         [0031]      FIG. 3  is a cross-sectional view of the detail of the starting device according to  FIG. 2  in a pre-engaged position of the output shaft  24 . The embodiment of the starting device in  FIG. 3  corresponds to the embodiment in  FIG. 2 . Identical components are characterized with identical reference symbols. A detailed description of components which have already been described will therefore not be given. In the text which follows, details are given above all on the function of the pressure reduction device  1  or of the depression  1   a . According to the invention, the sealing device  14  interacts with the pressure reduction device  1 . In the position of rest (see  FIG. 2 ) and the pre-engaged position (see  FIG. 3 ) of the output shaft  24 , the housing  10  or the interior  10   a  thereof is completely sealed, there is therefore no reduction of pressure by means of the pressure reduction device  1 . The pressure reduction takes place merely during an undesired axial movement of the output shaft  24 . In this context, the sealing device  14  and the pressure reduction device  1  interact, with the result that an overpressure or air can escape from the housing  10  through the pressure reduction device  1 . In particular, the reduction in pressure takes place in an automated fashion as a function of the respective overpressure prevailing in the housing  10 . 
         [0032]    In the case of heating of the starting device, an overpressure builds up in the interior  10   a  of the housing and cannot escape out of the sealed housing  10  owing to the sealing device  14 . Starting from a starter-specific value of the overpressure, for example between 0.2 and 0.5 bar, the output shaft  24  is pressed undesirably in the axial direction out of the interior  10   a  of the housing, that is to say in the direction of a ring gear, by the overpressure. This occurs, for example, if an overpressure in the interior  10   a  of the housing acting on the output shaft  24  in the axial direction A or a resulting force is greater than a spring force, acting in opposite axial direction A, of the armature restoring spring  39  (see  FIG. 1 ), in particular of an armature restoring spring  39  of a relay  42 . In the case of the undesired axial movement, according to the invention the overpressure in the housing  10  is reduced by means of the pressure reduction device  1 . The air can escape from the housing  10  in a controlled fashion. To be more precise, the air or the overpressure escapes through the depression la which forms a venting duct or bypass duct under the sealing device  14  in the case of an axial movement of the output shaft  24 . For this purpose, the notch-like depression la is made longer considered in the axial direction A than the sealing lip  14   b  of the sealing device  14 . Furthermore, the depression  1   a  or the venting duct which is formed hereby is formed in a section of the output shaft  24  which interacts with the sealing device  14 , that is to say in the region of a relative axial stroke between the output shaft  24  and the sealing device  14 . 
         [0033]    During this axial movement of the output shaft  24 , the depression  1   a  on the output shaft  24  moves relative to the sealing device  14  on the housing  10 . The sealing device  14  is made narrower in the axial direction A than the depression  1   a . As a result, the sealing lip  14   b  of the sealing device  14  does not cover, or never completely covers, the depression  1   a  during the axial movement. If the depression  1   a  is moved through under the sealing device  14  during the axial movement, the depression la forms a venting duct between the interior  10   a  of the housing and the surroundings outside the housing  10 . The venting duct extends between the output shaft  24  and the sealing device  14 . The overpressure is reduced through the venting duct, as a result of air flowing out of the interior  10   a  of the housing. In order to ensure the venting duct is present even during a movement along the sealing device  14 , the axial extent of the depression  1   a  is made larger than an axial extent of the sealing device  14 , with the result that the venting duct is never completely sealed by the sealing device  14 . 
         [0034]    In the pre-engaged position, the output shaft  24  is pushed out or moved out of the housing  10  by the overpressure in the interior  10   a  of the housing, at least to such an extent that the depression  1   a  or the venting duct is partially exposed and air can escape. According to the embodiment in  FIG. 3 , the depression  1   a  is in the pre-engaged position on the output shaft  24 , completely outside the housing  10 . As a result of the flattened shape of the depression  1   a , damage to the sealing device  14  during an axial movement of the output shaft  24  at the edges of the pressure reduction device  1  is prevented. If the overpressure is sufficiently reduced, that is to say as soon as the spring force of the armature restoring spring  39  which acts in the opposite axial direction A is larger than the force acting on the output shaft  24  by means of the overpressure, the output shaft  24  moves back into its position of rest in the axial direction A (see  FIG. 2 ). 
         [0035]    With such a pressure reduction device  1  on the output shaft  24 , the undesired axial movement, that is to say the undesired pre-engagement of the pinion  35  on the output shaft  24  in the direction of the ring gear  40  (see  FIG. 1 ), is reduced or avoided in the case of an overpressure in the interior  10   a  of the housing. The overpressure is reduced in a controlled fashion, while the housing  10  of the starting device continues to be sealed and therefore protected against environmental influences. Furthermore, both reliable engagement and disengagement are possible at high starter temperatures. By means of the pressure reduction device  1 , the undesired axial movement is reduced at least to such an extent that the pinion  35  no longer abuts against the ring gear  40 . The wear of the pinion  35  or of the ring gear  40  is reduced. After the electric machine has been switched off, the pinion  35  can also be moved without difficulty into the position of rest. 
         [0036]      FIG. 4  shows a cross-sectional view of a detail of the starting device in a further embodiment. The starting device in  FIG. 4  corresponds basically to the embodiment in  FIGS. 2 and 3 . The position of rest or home position of the output shaft  24  shown in  FIG. 4  corresponds to the embodiment in  FIG. 2 . Identical components are characterized by identical reference symbols. A detailed description of components which have already been described is therefore not given. However, the embodiments in  FIGS. 2 ,  3  and  4  differ in the embodiment of the depression  1   a  of the pressure reduction device  1  on the output shaft  24 . The arrangement of the depression  1   a  on the output shaft  24  and the function of the pressure reduction device  1  are unchanged. In  FIG. 4 , the depression  1   a  is embodied as a rectangular groove with straight, that is to say non-round, edges. The axial extent of this groove is likewise larger than the axial extent of the sealing device  14  in order to ensure the formation of a venting duct and therefore of the partial reduction of the overpressure.