Patent Publication Number: US-2022228658-A1

Title: Electric Drive Unit for a Motor Vehicle

Description:
The invention relates to an electric drive unit for a motor vehicle, having an electric drive, a multi-stage transmission which is connected downstream of the electric drive and is arranged by itself or together with the electric drive in a drive housing which is comprised of a housing shell, the inner side of which annularly surrounds the transmission, and an end wall which is formed integrally with the housing shell and is provided centrally with a transmission opening for the transmission output shaft of the transmission, wherein cooling liquid is conducted through the drive housing in order to cool the components of the transmission. 
     Such an electric drive unit is used as a traction drive of a vehicle. For use, it is part of an electric drive axle for a utility vehicle. In this type of vehicle axle, a multi-stage transmission is connected downstream of the electric drive, which is arranged advantageously in terms of weight in the vicinity of the center of the vehicle. The transmission is arranged by itself or together with the electric drive in a drive housing which is comprised of a substantially cylindrical housing shell and an end wall formed integrally with the housing shell. The inner side of the housing shell of the drive housing annularly surrounds the transmission elements mounted therein, and the end wall of the drive housing is provided centrally with a transmission opening through which the transmission output shaft leads out of the transmission. The transmission output shaft is either simultaneously the drive shaft leading to the respective vehicle wheel, or it is connected for conjugate rotation to a separate drive shaft. The transmission output shaft or the drive shaft rotates in a predominantly tubular axle housing of the electric drive axle. At its end facing the transmission, the axle housing is fastened rigidly to the end side of the drive housing and, at its other end, has a steering knuckle on which the driven vehicle wheel is rotatably mounted. 
     In order to cool the transmission elements, cooling liquid is conducted through the drive housing serving as the transmission housing, wherein this takes place in the form of a circuit in which the cooling liquid flows as far as possible through all the mechanically loaded regions of the transmission, including the rolling bearings which are present, for which purpose the cooling liquid is circulated within the drive housing. 
     The circulation of the cooling liquid within the drive housing runs into limits in the case of mechanically highly loaded transmissions since the transmissions are often of very compact design and there is little spare construction space for accommodating the assemblies required for even better cooling efficiency in the existing housing. 
     The invention is therefore based on the object of improving the cooling efficiency of an electric drive unit, part of which is a drive housing with a liquid-cooled transmission arranged therein, making use specifically of the design-induced characteristics of the electric drive unit. 
     To achieve this object, an annular chamber is proposed in an electric drive unit of the type indicated at the beginning, which annular chamber is arranged around the transmission opening on the outside of the end wall, wherein coolant channels are formed in the drive housing, these coolant channels leading from the inner side of the housing shell into the annular chamber. 
     According to the invention, part of the coolant recirculation is therefore an annular chamber which is, however, not arranged in the interior of the drive housing, but rather is arranged around the transmission opening on the outside of the end wall of the drive housing. In order that cooling liquid enters this annular chamber, coolant channels are formed in the drive housing, these coolant channels leading from the inner side of the housing shell into the annular chamber. 
     The arrangement of the annular chamber on the outside of the end wall of the drive housing is advantageous since the drive housing offers little spare construction space in its interior, and since, due to the design, part of the electric drive axle is in any case an axle housing which is fastened to that end wall on which, according to the invention, the annular chamber is located. 
     Owing to the design-induced characteristic of the electric drive axle, an annular chamber can therefore be realized for circulating the cooling liquid. The annular chamber is delimited on the one hand by the end wall of the drive housing and on the other hand by an annular region which is formed on the axle housing. The annular chamber is therefore not solely formed by the drive housing, but rather by the design-induced combination of drive housing and the axle housing which is fastened thereto on the end side and surrounds the transmission output shaft. 
     A refinement is preferred in which the annular chamber is partially formed, namely by the drive housing, by a conduit in the outer side of the end wall, and in which the axle housing, facing the drive housing, is provided with a flange. The flange of the axle housing is fastened to the end wall, and the annular region which together with the conduit formed in the end wall forms the annular chamber, i.e., delimits or surrounds the latter, is located on the flange. 
     The axle housing is part of the electric drive axle and is provided on its end facing away from the flange with a steering knuckle for the mounting of a vehicle wheel. 
     According to one refinement of the drive unit, a coolant connection piece is arranged on the flange, and a coolant channel leads from the coolant connection piece into the annular chamber in order to fill the latter, for example, with fresh coolant, or in order to continuously pump in coolant in the case of active circulation cooling. 
     A first and a second seal between the flange and the end wall are furthermore preferred. Preferably, the first seal is an annular seal arranged between the annular chamber and the central transmission opening, and the second seal is an annular seal arranged around the annular chamber. Preferably, the annular seals sit in annular grooves which are open toward the outer side of the end wall, wherein the annular grooves are formed in the end wall. 
     According to a further refinement, the two seals are arranged offset axially with respect to each other in the longitudinal direction of the transmission output shaft, wherein the second seal is the seal closer on the drive side. 
     According to a further refinement, further coolant channels are formed in the drive housing, wherein the further coolant channels lead from the annular chamber through the end wall as far as the inner side thereof. Via these further coolant channels, the coolant can therefore leave the annular chamber and can flow back again into the interior of the drive housing. Preferably, the further coolant channels are bores which are arranged with respect to the central axis on a diameter which is smaller than the diameter on which the coolant channels are arranged. 
     It is advantageous in terms of production if the axes of the further coolant channels are oriented parallel to the central axis. 
     With regard to a favorable flow through the annular chamber, it is advantageous if the further coolant channels open on the base, and the coolant channels open in the side wall of the annular chamber which is in the form of a conduit. 
     Furthermore, it is proposed that the flange of the axle housing is fastened to the end wall of the drive housing by means of a plurality of screws arranged on a common pitch circle. The diameter of the pitch circle is preferably larger than the largest diameter of the annular chamber, and preferably larger than the diameter of the second seal. 
     Preferably, the screws engage by way of their threads in threaded blind holes which are formed in the end wall. 
     For a compact design of the drive housing and of the fastening of the axle housing thereto, it is advantageous if the coolant channels lead into the annular chamber on those circumferential portions on which no threaded blind holes are formed in the end wall. 
     Furthermore, it is proposed that the end wall and the flange are oriented axially with respect to each other by centering surfaces which are formed thereon and are in mutual contact. 
     Preferably, the centering surfaces are arranged on a diameter which is smaller than the smallest diameter of the annular chamber and is larger than the diameter of the first seal. 
    
    
     
       Further advantages and details of the electric drive axle for a utility vehicle emerge from the description below of an exemplary embodiment which is reproduced in the drawing. In detail: 
         FIG. 1 : shows, in a perspective illustration, individual parts of an electric drive axle for a utility vehicle, namely a multi-stage transmission in the form of a double planetary gearing, a drive housing, which receives the transmission, in a partial section, and an axle housing which is fastenable on the end side against the drive housing, wherein the aforementioned parts are reproduced separately for reasons of clarity; 
         FIG. 2 : shows an end-side view of the drive housing corresponding to the viewing direction II reproduced in  FIG. 1 ; 
         FIG. 3 : shows a longitudinal section through the one half of the electric drive axle along an offset sectional plane denoted by “III-III” in  FIG. 2 ; 
         FIG. 4 : shows an enlarged detail of  FIG. 3 . 
     
    
    
       FIG. 3  shows, in a partial longitudinal section that is used here as an overview illustration, the one half of an electric drive axle for a motor vehicle. The illustrated half of the drive axle is used for driving a vehicle wheel  2  which is mounted rotatably on a steering knuckle  1  and is illustrated by dashed lines at the left edge in  FIG. 3 . The similarly designed second half of the drive axle is located symmetrically on the other side of the vehicle center plane M plotted in  FIG. 3 , and carries the respective vehicle wheel of the other side of the vehicle. The electric drive axle in its entirety is a rigid axle continuing from the one side of the vehicle to the other. 
     For each side of the vehicle, and therefore for driving the respective vehicle wheel  2 , an electric drive  5 , in particular an electric motor, is provided in the vicinity of the vehicle center plane M. A transmission  6 , here in the form of a two-stage planetary gearing, is in each case connected downstream of each electric motor toward the outside of the vehicle. 
     The two electric drives  5  and two transmissions  6  of the electric drive unit are situated in a drive housing  4  which can be in single-part or multi-part form. In the embodiment described here, the drive housing  4  is comprised of three parts or sections, wherein both electric drives  5  are situated in a common drive housing part, and each of the two transmissions  6  in a further dedicated drive housing part. Alternatively, for each side of the vehicle, the electric drive  5  can be arranged together with its transmission  6  in a common drive housing  4 . 
     The drive housing  4  is therefore at the same time the transmission housing of the drive unit and is comprised of a substantially cylindrical housing shell  11 , the inner side  11   a  of which annularly surrounds the transmission elements of the transmission  6 , and of an end wall  12  formed integrally with the housing shell  11 . The end wall  12  is predominately closed, but leaves a transmission opening  17  free in its center, through which the transmission output shaft  18 , which rotates on the central axis of rotation A, of the transmission  6  leads. 
     The transmission output shaft  18  can be connected for conjoint rotation, preferably via splines, to a drive shaft which leads to the vehicle wheel  2  and drives the latter. Unlike the transmission output shaft  18 , the drive shaft is not illustrated in the drawing. The drive shaft rotates in a tubular axle housing  15  of the drive unit, which axle housing extends on the central axis A. The axle housing  15  is fastened to the end side of the drive housing  4 . 
     At its vehicle-outer end, the axle housing  15  of the drive unit is provided with the steering knuckle  1 . In addition, the brake carrier of the vehicle brake, preferably the brake carrier of a disk brake, is fastened to the axle housing  15 . A longitudinal link and optionally a transverse link of the vehicle axle is likewise fastened to the axle housing  15 . 
     In order to fasten the axle housing  15  to the drive housing  4 , the axle housing  15  is provided at its vehicle-inner end with a radially expanded flange  16 . The flange  16  is screwed rigidly against the end wall  12  of the drive housing  4  by means of a plurality of screws  19 . The screws  19  are supported by their screw heads on the flange  16  and engage by way of their threads in threaded blind holes  20  which are formed in the end wall  12 . 
     The connection of the drive housing parts, i.e., of the drive housing part accommodating the electric drive  5 , on the one hand, and of the drive housing part accommodating the transmission  6 , on the other hand, also takes place by means of screw connections  29 , for which purpose the respective ends of these drive housing parts are designed as flanges which are connected fixedly and preferably liquid-tightly to one another via the screw connections  29 . Where the drive housing  4  is discussed in general below, this means in each case that drive housing part in which the transmission  6  and preferably two-stage planetary gearing is located. 
     The drive housing has not only the function of a housing, but it is also itself part of the planetary gearing by, according to  FIG. 1 , the inner side  11   a  of the housing shell  11  having a helical toothing  17   a , which is arranged around the axis of rotation A, of the first transmission stage, and, axially offset, having a helical toothing  17   b , which is arranged around the axis of rotation A, of the second transmission stage. The orientation or oblique position of the teeth of the helical toothing  17   b  is opposed to the teeth of the first helical toothing  17   a.    
     The planet wheels of the two transmission stages of the planetary gearing mesh with the helical toothings  17   a ,  17   b  formed on the inside of the housing shell  11 . The remaining transmission elements of this planetary gearing are arranged in a transmission carrier  17   c  which is rotatable on the axis A and is inserted in the manner of a preassembled construction unit into the drive housing  4 . 
     The transmission  6  is designed in the manner of a planetary gearing. The planet wheels and ring gears of the planetary gearing have a tooth oblique position producing a scoop effect, which faces away from the end wall  12 , on the coolant and, in the case of a passive sump cooling, producing a scoop effect, which is directed toward the end wall  12 , on the coolant. 
     The central part of the transmission carrier  17   c  is the transmission output shaft  18  which rotates on the axis A, protrudes out of the likewise centrally arranged transmission opening  17  of the drive housing  4  and extends into the axle housing  15 . 
     In the event of an active circulation cooling, the cooling of the transmission  6  and of its sometimes heavily loaded teeth, pinions and bearings takes place by means of the cooling liquid circulated by an external pump. The pump draws the liquid out of the drive housing  4 , conducts it through an external heat exchanger and then conveys it to a connection piece  21  via which the cooling liquid passes over a short distance into the annular chamber  33  and thus back into the circuit. The connection piece  21  is located on the flange  16  of the axle housing  15 . 
     By contrast, in the case of the passive sump lubrication, the liquid is circulated exclusively within the drive housing  4 , including the annular chamber  33 , with an external pump being omitted. 
     Irrespective of the type of circulation, the cooling liquid is preferably a transmission oil, and therefore the liquid both lubricates the transmission elements and also transports away heat. 
     Owing to the design as a planetary gearing, cooling and lubricating liquid is held as a result of centrifugal forces outward against the helical toothings  17   a ,  17   b  by the planet wheels. The cooling liquid therefore accumulates on the inner side  11   a  of the housing shell  11 . The measures described in more detail below serve to transport the liquid thus accumulating on the inner side  11   a  of the housing shell  11  back into more central regions of the transmission  6  and into the region of rolling bearings, for example to the rolling bearings  31  of the transmission output shaft  18 . 
     In order to improve the transport of coolant and the cooling efficiency, an annular chamber  33  which surrounds the central axis of rotation A and the transmission opening  17  in the form of a complete ring and in which coolant can collect for the purpose of further circulation is arranged on the outside of the end wall  12 . At the same time, a plurality of coolant channels  35  are formed in the transition region between the housing shell  11  and the end wall  12  of the drive housing, these coolant channels  35  leading from the inner side  11   a  of the housing shell  11  into the annular chamber  33 . Liquid which has accumulated on the inside of the housing shell  11  and in particular on the helical toothings  17   a ,  17   b  therefore passes, in a first step, through the coolant channels  35  into the annular chamber  33  which therefore forms a collector for cooling liquid. 
     An annular chamber takes up space which is not available in the very compactly configured drive housing  4 . The annular chamber  33  is therefore located on the outside of the end side of the drive housing  4  facing the vehicle wheel  2 . In particular, the annular chamber  33  is not formed solely by the drive housing  4  itself, but rather is formed in another part by the axle housing  15 . This is achieved by the annular chamber  33  being formed in one part by an annular conduit formed in the outer side of the end wall  12  and in another part by an annular region on the end side of the flange  16  that faces the end side  12 . The annular region on the end side of the flange  16  therefore together with the conduit formed in the end wall  12  forms the annular chamber  33  such that not only are components of the transmission involved in the coolant guiding and circulation, but so too are other components of the drive unit, namely the flange  16 , which is in the form of a circular disk, of the axle housing  15 . 
     The circulation of the coolant and the improvement of the cooling efficiency is thereby achieved using specifically the design-induced characteristics of the electric drive unit since the axle housing  15  is also part of this drive unit. 
     In order that the annular chamber  33  can serve as a collector, there is a multiplicity of the aforementioned coolant channels  35 , wherein the coolant channels  35  are preferably arranged distributed uniformly over the circumference of the round drive housing  4 . 
     The cooling liquid which has collected in the annular chamber  33  passes back into the interior of the drive housing  4  via further coolant channels  41 . The further coolant channels  41  are likewise present in a multiplicity and arranged distributed uniformly over the circumference of the round drive housing  4 . They lead from the annular chamber  33  through the end wall  12  and open in each case on the inner side  12   a  of the end wall  12 . 
     In the case of the exemplary embodiment described here, the further coolant channels  41  are in each case through bores which extend parallel to the axis of rotation A of the transmission output shaft  18 . The further coolant channels  41  are arranged with respect to the axis of rotation A on a diameter which is smaller than the diameter on which the coolant channels  35  are arranged. 
     In order to transport the cooling medium which has accumulated in the region of the inner side  11   a  into the annular chamber  33 , the coolant channels  35  run at an inclination with respect to the central axis A of the drive unit. The opening of each coolant channel  35  into the interior of the drive housing  4  is located radially further outward than the opening of each coolant channel  35  into the annular chamber  33 . 
     According to  FIG. 2 , the opening of the coolant channels  35 , on the one hand, and of the further coolant channels  41 , on the other hand, into the annular chamber  33  in the form of a conduit are configured in such a manner that the further coolant channels  41  open at the base  42  of the annular chamber  33  and the coolant channels  35  open in a side wall  43 , which is arranged obliquely with respect to the base  42 , of the annular chamber  33 . In the exemplary embodiment, the side wall  43  of the conduit into which the coolant channels  35  open is arranged obliquely with respect to the base  42  of the conduit, whereas the other side wall is arranged substantially perpendicularly to the base  42  of the conduit. By means of this configuration, a uniform distribution of coolant within the annular chamber  33  is achieved. 
     The annular chamber  33  is sealed radially inward via a first seal  51  and radially outward via a second seal  52  such that liquid cannot escape either to the inside or to the outside. The two seals  51 ,  52  are preferably annular seals arranged around the central axis A. The groove for receiving the respective annular seal is formed in the end side of the end wall  12 . 
     The end wall  12  and the flange  16  are oriented axially with respect to each other by means of centering surfaces  55  which are formed thereon and are in mutual contact. The centering surfaces in the form of a step running in annular form around the central axis A are arranged on a diameter which is smaller than the smallest diameter of the annular chamber  33 . The annular centering surfaces  55  are therefore located radially between the annular chamber  33  and the inner first seal  51 . 
     The screws  19  which fasten the flange  16  to the end wall  12  and are arranged on a standardized pitch circle engage by way of their threads in each case in a threaded blind hole  20  which is formed in the end wall  12 . 
     For an overall compact construction, the threaded blind holes  20 , on the one hand, and the coolant channels  35  are arranged on different circular portions ( FIG. 2 ). For example, a screw  19  or its threaded blind hole  20  can in each case alternate in the circumferential direction with one of the coolant channels  35 . However, the configuration according to  FIG. 2 , in which the number of coolant channels  35  is half as much as the number of screws  19  or threaded blind holes  20  is preferred. 
     According to  FIG. 4 , the further coolant channels  41  are oriented in such a manner that the coolant escaping therefrom passes virtually directly to that rolling bearing  31  which supports the transmission output shaft  18  in the housing shell  11 . This improves the cooling and also the lubrication specifically of this heavily loaded rolling bearing  31 . 
     The radial sealing of the transmission output shaft  18  in relation to the surrounding transmission opening  17  takes place by means of a radial shaft sealing ring  58  arranged between these components. 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  Steering knuckle 
           2  Vehicle wheel 
           4  Drive housing 
           5  Electric drive 
           6  Transmission 
           11  Housing shell 
           11   a  Inner side of housing shell 
           12  End wall 
           12   a  Inner side of end wall 
           15  Axle housing 
           16  Flange 
           17  Transmission opening 
           17   a  Helical toothing 
           17   b  Helical toothing 
           17   c  Transmission carrier 
           18  Transmission output shaft 
           19  Screw 
           20  Threaded blind hole 
           21  Connection piece 
           29  Screw connection 
           31  Rolling bearing 
           33  Annular chamber 
           35  Coolant channel 
           41  Coolant channel 
           42  Base 
           43  Side wall 
           51  First seal, annular seal 
           52  Second seal, annular seal 
           55  Centering surfaces 
           58  Radial shaft seal 
         A Central axis, axis of rotation 
         M Vehicle center plane