Abstract:
The present invention relates to a balance shaft unit for mass balancing for an internal combustion engine of a motor vehicle including at least one balance shaft having at least one balance weight and a housing part in which the balance shaft is journaled. A first gear is associated with the balance shaft and meshes with a second gear. An introduction passage is formed in the housing part through which, starting from an assembly opening, at least one of the two gears can be introduced into the interior of the housing part, with the introduction passage extending perpendicular to the longitudinal axis of the balance shaft. The first gear and the second gear are arranged sequentially with respect to the direction of extent of the introduction passage. The housing part surrounds the periphery of the assembly opening in one piece, with the length of the assembly opening at the outer side of the housing part in a normal plane to the longitudinal axis of the balance shaft being smaller than the sum of the diameters of the two gears. A particularly high stability is hereby reached.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit and priority of German Patent Application No. 10 2007 057 338.5, filed Nov. 28, 2007. The entire disclosure of the above application is incorporated herein by reference. 
       FIELD 
       [0002]    The present invention relates to a balance shaft unit for a balance of forces of inertia and/or of moments of inertia for an internal combustion engine of a motor vehicle. 
       BACKGROUND 
       [0003]    This section provides background information related to the present disclosure which is not necessarily prior art. 
         [0004]    Balance shafts have the purpose of balancing the forces of inertia and/or moments of inertia arising in internal combustion engines (so-called mass balancing). With specific engine types, for example with in-line engines having four cylinders, such balance shafts are used pair-wise, with the balance shafts rotating in opposite directions at twice the crankshaft speed. The balance shafts as well as the components supporting them are subject to high load due to the high speed so that the stability of balance shaft units is of great importance. 
         [0005]    Balance shaft units having a housing and such balance shafts or balance shaft pairs are generally known. Known balance shaft units which withstand the high loads and nevertheless reliably compensate the forces of inertia and moments of inertia which occur are, however, undesirably complex and/or expensive in manufacture and/or have a large number of components to be assembled. 
         [0006]    To take account of the increasing pressure on costs and the constantly growing limitation of the available construction space in the area of the engine, attempts are being made to integrate further assemblies into known balance shaft units to operate them together with the balance shafts. Oil pumps are frequently integrated into balance shaft units. Conventional oil pump balance shaft modules, however, have an even larger number of components and are therefore very labor-intensive in assembly. 
       SUMMARY 
       [0007]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0008]    It is the object of the invention to provide a reliable balance shaft unit which satisfies the demands for high stability and simple assembly. 
         [0009]    This object is satisfied by the features of claim  1 . 
         [0010]    The balance shaft unit in accordance with the invention for mass balancing for internal combustion engines includes at least one balance shaft with at least one balance weight and a housing part in which the balance shaft is journaled. A first gear is associated with the balance shaft and meshes with a second gear so that the first gear and the second gear form a drive stage. For example, the first gear can be rotationally fixedly connected to the balance shaft, whereas the second gear is rotationally fixedly connected to a drive shaft of a pump (formation of a transmission stage) or is rotationally fixedly connected to a further balance shaft (formation of a synchronization stage). Additional gears can be provided, for example to realize both a transmission stage between a balance shaft and a pump drive shaft and a synchronization stage between the balance shaft and a further balance shaft. 
         [0011]    The housing part has at least one assembly opening at its outer side. An introduction passage is formed in the housing part through which, starting from the assembly opening, at least one of the two named gears can be introduced into the interior of the housing part. The introduction passage extends perpendicular to the longitudinal axis of the balance sheet, i.e. in the radial direction with respect to the longitudinal axis of the balance shaft, with the first gear and the second gear meshing therewith being arranged sequentially with respect to the direction of extent of the introduction passage. The assembly opening is therefore arranged laterally at the housing part with respect to the longitudinal axis, i.e. it is not located at one of the axial end face sides of the housing part. At least one of the two named gears can thus be introduced into the interior of the housing part in a radial direction for the assembly of the balance shaft unit. 
         [0012]    The housing part surrounds the periphery of the assembly opening in one piece. In other words, the housing part is closed peripherally at its outer side in the environment of the assembly opening and does not have any joint faces at the assembly opening. In addition, the length of the assembly opening at the outer side of the housing part in a normal plane to the longitudinal axis of the balance shaft (i.e. along a direction perpendicular to the longitudinal axis of the balance shaft) is admittedly larger than the diameter of the at least one of the two gears which is introduced into the interior of the housing part through the assembly opening and along the introduction passage for assembly purposes. However, the named length of the assembly opening is smaller than the sum of the diameters of the two named gears (first gear and second gear of the named drive stage). The assembly opening thus has a small extent at the surface of the housing part so that the housing part is particularly stable. A high stability of the balance shaft unit is additionally ensured by the one-part design of the housing part in the region of the assembly opening, which also includes the advantage that the housing part can be made with thin walls and correspondingly light and does not itself have to be assembled in a complex and/or expensive manner through a plurality of assembly steps. 
         [0013]    Provision is made that the named at least one of the two gears is introduced into the interior of the housing part separately from the associated shaft. For this purpose, the assembly opening is arranged at a suitable position of the housing part. The respective gear is introduced into the interior of the housing part through said assembly opening along the introduction passage and is there—that is in the interior of the housing—connected to the balance shaft or to the drive shaft or to the further balance shaft. 
         [0014]    A particular advantage of a separate assembly opening for the gear or gears of the balance shaft unit can be seen in that the assembly opening can be made relatively small—in comparison with a housing opening for the introduction of a complete balance shaft with preassembled gears—so that the housing part is very stable, even if an additional introduction opening is provided for the axial introduction of the balance shaft. 
         [0015]    The manufacturing costs can be reduced by the low number of components. Such a balance shaft unit can moreover be assembled in a few, simple steps, which additionally results in cost savings. A reduction in the number of the required components and of the required assembly steps also improves the process security in the manufacture. 
         [0016]    Advantageous embodiments of the invention are set forth in the dependent claims, in the description and in the drawings. 
         [0017]    In accordance with an embodiment of the balance shaft unit in accordance with the invention, the assembly opening is formed at the outer side of the housing part by a slit—is therefore made lengthways—with the longitudinal extent of the slit extending in a normal plane (that is perpendicular) to the longitudinal axis of the balance shaft. The longitudinal extent of the slit at the surface of the housing part thus corresponds to the arrangement of the respective gear in the interior of the housing part. The slit-like assembly opening is large enough to enable the introduction of the gear or gears of the named drive stage without simultaneously representing an unnecessarily large opening in the wall of the housing part. The term “slit-like” is not to be understood such that the assembly opening necessarily has to have a narrow, rectangular outline. The assembly opening can have any desired geometrical shape—also an irregular one. However, a rectangular outline is preferred. 
         [0018]    The assembly opening can, for example, have an outline which at least corresponds to the outline of the largest of the named gears. All smaller gears can then likewise be introduced into the interior of the housing part through this assembly opening. Depending on the design of the gears, the geometry of the assembly opening can be optimized such that all the gears fit through the assembly opening. Optionally, a plurality of assembly openings can also be provided. 
         [0019]    In accordance with a further preferred embodiment, the width of the assembly opening at the outer side of the housing part corresponds in a direction parallel to the longitudinal axis of the balance shaft (i.e. in a plane including the longitudinal axis of the balance shaft) to the thickness of the gear of the balance shaft unit to be introduced along the introduction passage, with the required clearance being provided to be able to move the respective gear contact-free or at least friction-free through the assembly opening. The assembly opening in this case therefore has a minimally required width, which as an advantageous effect on the stability of the housing part. Provided that a plurality of gears of different drive stages should be introduced into the interior of the housing part through the assembly opening (for example a gear of a transmission stage and a gear of a synchronization stage), the width of the assembly opening can correspond to the thickness of two gears arranged axially adjacent to one another (again while taking account of the required clearance). 
         [0020]    It is furthermore preferred when the named introduction passage extends in the vertical direction in the position of use of the balance shaft unit, with the assembly opening being arranged at the upper side of the housing part and with the housing part being fastened to the lower side of the internal combustion engine. Such a vertical introduction passage can additionally serve the purpose of simultaneously forming a lubrication oil supply for the gears or for further lubrication points of the balance shaft unit. In the position of use of the balance shaft unit, the lubrication oil can namely flow downwardly from the internal combustion engine through the assembly opening and along the named introduction passage to the different lubrication points. 
         [0021]    In accordance with a further preferred embodiment, the housing part is not only made in one piece in the environment of the assembly opening. But the housing part is rather also made in one piece with bearing sections at which the at least one balance shaft is journaled. In other words, the bearing sections (e.g. bearing plates) are made integrally with the housing part. The stability of the housing part is hereby improved even further. 
         [0022]    In accordance with a further development of the balance shaft unit in accordance with the invention, a drive shaft of a pump is furthermore journaled in the housing part, with the named second gear being associated with the drive shaft and being connected, in particular rotationally fixedly connected, to the drive shaft of the pump. The first and the second gears form a transmission stage which drive-operatively connects the balance shaft and the drive shaft of the pump to one another. In other words, a pump is at least partly integrated into the housing part of the balance shaft unit. The pump and the balance shaft are driven together, with the drive-operative coupling being established by the named transmission stage. This coupling of the pump and of the balance shaft allows a compact construction of the balance shaft unit and permits an efficient operation of the named components since separate drive paths do not have to be provided for the balance shaft and the pump. 
         [0023]    An advantageous aspect of such a balance shaft unit provides that the housing part forms a housing of the pump at least in part. This form of integration of the pump into the balance shaft unit enables a further component reduction without the functionality of the balance shaft unit being impaired. 
         [0024]    A particular compact manner of construction results when the pump and the balance shaft are arranged sequentially with respect to the longitudinal axis of the balance shaft, with the transmission stage being arranged between the pump and the balance shaft. The drive shaft of the pump and the balance shaft do not have to be arranged coaxially in this respect. It is only decisive that the transmission stage in this embodiment of the balance shaft unit in accordance with the invention is arranged in a direction of view along the longitudinal extent of the balance shaft unit between the pump and the balance shaft. In special applications, the transmission stage can, however, also be arranged in the region of one of the end face sides of the balance shaft unit so that the pump and the balance shaft are arranged together on a side of the transmission stage. 
         [0025]    The balance shaft can have an input member for the reception of a driving torque from the internal combustion engine so that the pump can be driven by the balance shaft via the transmission stage during the operation of the balance shaft unit. Alternatively to this, the drive shaft of the pump can have an input member for the reception of a driving torque from the internal combustion engine so that the balance shaft can be driven by the drive shaft of the pump via the transmission stage during the operation of the balance shaft unit. 
         [0026]    In other words, the internal combustion engine first drives the balance shaft which in turn drives the pump via the transmission stage in the first of the variants described above. The second variant describes the reversal of the drive order. Independently of the drive order, the transmission stage between the balance shaft and the drive shaft is preferably designed such that the drive shaft of the pump rotates more slowly than the balance shaft on operation of the balance shaft unit. I.e. in the first variant, a rotation of the drive shaft of the pump is speed reduced. In the second variant, a rotation of the balance shaft is speed increased. 
         [0027]    Alternatively or additionally to the formation of the balance shaft unit with a transmission stage for a pump, the balance shaft unit can include a further balance shaft which is journaled in the housing part, with the named second gear to be associated with the further balance shaft and to be connected, in particularly rotationally fixedly connected, to the further balance shaft. The first gear and the second gear in this case form a synchronization stage which connects the balance shaft and the further balance shaft drive-operatively to one another. If both a transmission stage for a pump and a synchronization stage for a further balance shaft are provided, the balance shaft unit preferably includes more than two gears, with one of the aforesaid “second gears” actually being able to be formed by a third gear or a fourth gear. 
         [0028]    It is preferred if at least some of the named gears of the balance shaft unit are rotationally fixedly connected by means of a press fit—in particular a shrink connection—to the associated shaft (e.g. drive shaft or balance shaft). 
         [0029]    In accordance with a particularly compact and robust embodiment of the balance shaft unit in accordance with the invention, the named transmission stage or the named synchronization stage is a spur gear. 
         [0030]    Analog to the explained assembly opening for the introduction of the at least one gear, at least one second assembly opening can be provided through which the named balance weight can be introduced into the interior of the housing part for the assembly of the balance shaft unit. 
         [0031]    The present invention furthermore relates to a method for the assembly of a balance shaft unit in accordance with at least one of the embodiments described above. This method includes the introduction of at least one of the named two gears into the interior of the housing part through the assembly opening and along the introduction passage and the positioning of the respective gear in an assembly position, with afterward a shaft associated with the respective gear being rotationally fixedly connected to the gear inside the housing part. 
         [0032]    The shaft associated with the gear can be introduced axially into the housing part through an associated introduction opening, for example. 
         [0033]    A particularly efficient method for the connection of the associated shaft to the respective gear comprises a shrinking process. Provision is in particular made that the shaft is cooled and/or the gear is heated before the connection of the shaft to the respective gear. 
         [0034]    In addition to the named at least one gear which is introduced into the interior of the housing part through the assembly opening, at least one further gear can be introduced axially adjacent to this gear into the interior of the housing part through the assembly opening and the introduction passage and can be held in an assembly position, with afterward a shaft associated with the further gear being rotationally fixedly connected to the named gear within the housing part. The named gear and the named further gear can hereby be assembled axially adjacent to one another inside the housing part, with the assembly opening having a minimal outline so that the housing part is characterized by a high stability. 
         [0035]    As regards the assembly order, in particular the three following variants are possible for the method in accordance with the invention: 
         [0036]    In accordance with a first variant, the first gear and the second gear can be introduced into the interior of the housing part through the assembly opening and the introduction passage and can simultaneously be held in a respective assembly position, with the balance shaft only afterward being rotationally fixedly connected to the first gear and a shaft associated with the second gear being rotationally fixedly connected to the second gear (parallel assembly). This variant is particularly efficient since the two gears can be connected to the respective shaft simultaneously or at a brief time interval. The introduction and holding of the first gear and of the second gear can take place by means of a respective gripping device, with the one gripping device engaging around the other gripping device, for example. The assembly opening must have a sufficient length for this purpose. 
         [0037]    In accordance with a second variant, the first gear can be introduced into the interior of the housing part through the assembly opening and the introduction passage in a first assembly step and afterward the balance shaft can be rotationally fixedly connected to the first gear. In a second assembly step, the second gear can be introduced into the interior of the housing part through the assembly opening and the introduction passage and afterward a shaft associated with the second gear can be rotationally fixedly connected to the second gear. In this variant, the first assembly step is carried out before or after the second assembly step. The two assembly steps are therefore carried out sequentially (sequential assembly). This simplifies the handling of the gears. The length of the assembly opening can in this case correspond to the diameter of the largest of the gears to be introduced plus the required free space for the gripping device. 
         [0038]    In accordance with a third variant, only one of the two gears is introduced into the interior of the housing part through the assembly opening and the introduction passage, with the other of the two named gears being axially introduced into the interior of the housing part through an associated introduction opening of the housing part. Within the framework of the invention, only one single gear of a drive stage can also therefore be introduced into the interior of the housing part in the radial direction through the assembly opening. 
         [0039]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0040]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0041]      FIG. 1  is a schematic representation of an embodiment of the balance shaft unit in accordance with the invention; 
           [0042]      FIG. 2  is a longitudinal section of a further embodiment of the balance shaft unit in accordance with the invention; 
           [0043]      FIG. 3  is a plan view of the embodiment in accordance with  FIG. 2 ; and 
           [0044]      FIG. 4  is a perspective view of a section through the introduction passage of the embodiment in accordance with  FIG. 2  perpendicular to the axes of rotation of the balance shafts. 
       
    
    
       [0045]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0046]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0047]      FIG. 1  shows a balance shaft unit  10  which is connected to an engine  12  of a motor vehicle. The balance shaft unit  10  includes a balance shaft  14  which is driven by a crankshaft, not shown, of the engine  12  via a drive connection  16  only shown schematically. The balance shaft  14  is provided with a balance weight  18  which is rotationally fixedly connected to the balance shaft  14 . The unbalance of the balance shaft  14  caused by the balance weight  18  counters the free forces of inertia/moments of inertia of the engine  12  to reduce its operating noise and vibrations. The drive connection  16  can, for example, include gears, chains and/or toothed belts. In  FIG. 1 , an input gear  19  rotationally fixedly connected to the balance shaft  14  is shown by way of example. 
         [0048]    The balance shaft  14  is furthermore drive-operatively connected to a drive shaft  20  of a pump  22 . This connection is established by a transmission stage  24 . The transmission stage  24  includes a balance shaft gear  26  which is rotationally fixedly connected to the balance shaft  14  and a drive shaft gear  28  which is rotationally fixedly connected to the drive shaft  20 . 
         [0049]    During the operation of the engine  12 , the balance shaft  14  is thus driven first which in turn drives the pump  22  which is, for example, responsible for the conveying of lubrication oil. In alternative embodiments, the drive of the balance shaft  14  can also take place via a pump  22  driven by the crankshaft, i.e. the drive order is swapped over in this case. 
         [0050]    The balance shaft unit  10  has a housing component  30  which is made in one piece and in which both the balance shaft  14  and the drive shaft  20  are journaled, with details of the journaling not being shown in  FIG. 1 . The housing component  30  also receives a part of the pump  22 , whereby a compact assembly results which satisfies a plurality of functions (mass balancing and lubricant conveying) and which has an efficient drive concept. The one-part design of the housing component  30  has a positive effect on the stability of the balance shaft unit  10  and reduces the steps required for its assembly. 
         [0051]    To simplify the assembly, the housing component  30  has a lateral assembly opening at the outer side in the form of an assembly slit  32  (shown in dashed lines) which continues into the interior of the housing component  30  as an introduction passage  33  (likewise shown in dashed lines) and through which the balance shaft gear  26  and the balance shaft gear  28  can be introduced into the interior of the housing component  30 . The named introduction passage  33  extends perpendicular, i.e. radially, to the axes of rotation  36 ,  38  of the balance shaft  14  or of the drive shaft  20 . The gears  26 ,  28  are arranged sequentially in the housing component  30  with respect to the radial direction of extent of the introduction passage  33 . After the introduction of the gears  26 ,  28  of the transmission stage  24  into the housing component  30 , which is symbolized by an arrow  34  indicating the direction of introduction, the balance shaft  14  and the drive shaft  20  are inserted into the housing component  30  along their respective axes of rotation  36  and  38  through a respective introduction opening  37  and  39  (represented by the arrows  40  and  42  respectively which typify the corresponding assembly movements) until they are in connection with the gears  26  and  28  respectively. The requirement for this is that the gears  26 ,  28  are positioned suitably in the interior of the housing component  30  so that the shafts  14 ,  20  respectively associated with them can be pushed into corresponding bores (not shown) of the gears  26 ,  28 . 
         [0052]    In a similar manner, the balance weight  18  can also be introduced into the interior of the housing component  30  through a lateral assembly window  44  in order there to be connected to the balance shaft  14 . This can take place, for example, by a “pushing through” of the balance shaft  14  through a bore in the balance weight  18 . The balance weight  18  can be rotationally fixedly connected to the balance shaft  14  by means of a shrink connection, by a screw connection, by welding or in another manner. 
         [0053]    An advantageous connection between the gears  26 ,  28  and the shafts  14  or  20  is represented by a press fit which is generated, for example, by thermal shrinking processes. For this purpose, the gears  26 ,  28  are heated for the assembly and/or the shafts  14 ,  20  are cooled so that the shafts  14 ,  20  can be inserted substantially free of resistance into the bores of the gears  26 ,  28 . On the alignment of the temperature level, the shafts  14 ,  20  expand, whereas the diameters of the bores of the gears  26 ,  28  reduce. In other words, the gears  26 ,  28  are “shrunk” onto the shafts  14  and  20 , whereby a very stable and reliable connection of the named components arises. The balance weight  18  can also be connected to the balance shaft  14  in a similar manner. 
         [0054]    In the embodiment of the balance shaft unit  10  in accordance with the invention shown in  FIG. 1 , the transmission stage  24  is arranged between the balance shaft  14  and the pump  22  with respect to the longitudinal extent of the balance shaft unit  10  (or in other words: in a direction parallel to the axes of rotation  36 ,  38  of the shafts  14  or  20 ), with the axes of rotation  36 ,  38  being offset parallel to one another. This geometrical arrangement allows a “slim” design of the balance shaft unit  10 . It is generally also possible to arrange the balance shaft  14  and the pump  22  on a side of the transmission stage  24 , which would result in a more compact configuration. 
         [0055]    The “sequential” arrangement has the result that the assembly slit  32  is arranged laterally at the housing component  30  and not at one of the end face sides of the housing component  30 . Starting from the end face sides, in the assembly of the balance shaft unit  10 , the shafts  14 ,  20  are introduced through corresponding openings. 
         [0056]    The elongate outline of the assembly slit  32  cannot be recognized in  FIG. 1  due to the perspective shown and extends in the embodiment shown in  FIG. 1  at the lower side of the balance shaft unit  10  substantially perpendicular to the axes of rotation  36 ,  38 . In the embodiment shown, a dimensioning of the outline which is marginally larger than the outline of the gear  28  is generally sufficient for the introduction of the gears  26 ,  28 . With different length/width ratios of the gears  26 ,  28 , the assembly slit can have an outline differing from a simple rectangular shape and can be made in L shape, for example. The housing component  30  is in one piece in the environment of the assembly slit. Overall, a high stability of the housing component  30  is achieved by these measures. 
         [0057]    A further advantage of the balance shaft unit  10  in accordance with the invention consists of the fact that the balance shaft unit  10  can be preassembled in a simple manner and can subsequently be fastened to the engine  12 . In addition, a reliable and precisely aligned journaling of the components is automatically ensured by the one-piece structure of the housing component  30  bearing the shafts  14 ,  20 . 
         [0058]      FIGS. 2 to 4  show a more detailed representation of a further embodiment of a balance shaft unit  10  in its position of use. It includes two balance shafts  14 ,  14 ′ which each have a balance weight  18 . The balance shaft  14  (at the top in  FIG. 2 ) has a larger longitudinal extent than the balance shaft  14 ′ (at the bottom in  FIG. 2 ). The former is drive-connected to the engine  12  at its right hand end in  FIG. 2  (cf. input gear  19  in  FIG. 3 ). Despite the different longitudinal extents, the two balance shafts  14 ,  14 ′ produce a vibration and noise damping effect substantially of the same order of magnitude since their balance weights  18  are substantially of the same design. 
         [0059]    The balance shafts  14 ,  14 ′ are journaled in bearings  46  which are inserted into bearing plates  47  which are made in one piece with the housing component  30 . 
         [0060]    The part of the balance shaft unit including the journaling of the balance shafts  14 ,  14 ′ and the balance weights  18  forms a functional unit  48  for mass balancing. A further functional unit  50  includes the pump  22  with its drive shaft  20  which is responsible for the provision of lubricant, for example, for the engine  12 . The design of the pump  22 —for example a vane pump—is of no significance for the present invention and will therefore not be described in detail. It must, however, be noted that the pump  22  does not have a housing of its own. A section of the housing component  30  forms the housing of the pump  22  so that only the drive shaft  20  and the further components of the pump  22  have to be introduced into a correspondingly shaped cut-out in the housing component  30  on the assembly. Subsequently, the end face side of the balance shaft unit  10  at the left in  FIG. 2  is closed by a housing cover  52 . 
         [0061]    In a similar manner as in the embodiment of the balance shaft unit  10  described with reference to  FIG. 1 , the transmission stage  24  is arranged between the functional units  48  and  50 . It includes, in addition to the balance shaft gear  26  and the drive shaft gear  28 , a further two gears  54 ,  54 ′ which form a synchronization stage  55  to establish a drive-operative connection between the balance shafts  14 ,  14 ′. The gear  54  is placed rotationally fixedly onto a hollow shaft section of the balance shaft gear  26  in the embodiment shown and forms a unit together with it. This unit can be preassembled or can only be assembled in the interior of the housing component  30 . It can generally also be made in one piece. Alternatively, the gear  54  of the synchronization stage  55  can be rotationally fixedly connected to the balance shaft  14  separately from the balance shaft gear  26 . 
         [0062]    The gears  54  and  54 ′ of the synchronization stage  55  are designed such that no step-down or step-up of the rotary movement of the balance shaft  14 ′ relative to the rotary movement of the balance shaft  14  takes place since the two balance shafts  14 ,  14 ′ should rotate at the same speed—albeit in opposite rotary directions. The drive-operative connection of the balance shaft  14  and of the drive shaft  20  of the pump  22  via the gears  26 ,  28  (transmission stage  24 ) in contrast includes a speed reduction of the rotary movement of the balance shaft  14  since the pump  22  should be operated at a lower speed than the balance shafts  14 ,  14 ′. 
         [0063]    The assembly slit  32  is arranged in  FIG. 2  at the upper side of the balance shaft unit  10  which adjoins the lower side of the engine in the position of use of the balance shaft unit  10 . The introduction passage  33  extends in a vertical direction into the interior of the housing component  30 . The housing component  30  also surrounds the periphery of the assembly slit  32  in one piece in the embodiment shown in  FIGS. 2 to 4 . At the surface of the housing component  30 , the assembly slit  32  has a substantially rectangular outline whose length approximately corresponds to the diameter of the drive shaft gear  28  (which has the largest diameter of the gears  26 ,  28 ,  54 ,  54 ′) and whose width corresponds to the axial thickness of the balance shaft gear  26  including the named hollow shaft section (said balance shaft gear  26  having the largest thickness of the gears  26 ,  28 ,  54 ,  54 ′). This width at the same time corresponds to the sum of the axial thickness of the drive shaft gear  28  and of the axial thickness of the gear  54 ′. The named length of the outline of the assembly slit  32  is preferably somewhat larger than the diameter of the drive shaft gear  28  so that the drive shaft gear  28  can be held laterally by means of two robot grips or another gripping device for the purpose of alignment during the connection to the drive shaft  20 . This is possible in the embodiment shown since the larger ( 28 ) of the two gears  26 ,  28  is arranged behind the smaller ( 26 ) of the two gears  26 ,  28  with respect to the direction of introduction  34  of the assembly slit  32 . 
         [0064]    The assembly generally takes place in accordance with the procedure described in connection with  FIG. 1 . At least one (or more or all) of the gears  26 ,  28 ,  54 ,  54 ′ is in particular introduced into the interior of the housing component  30  through the assembly opening  32  and along the introduction passage  33  by means of a respective gripping device and is held there in an assembly position, with afterward the shafts  14 ,  14 ′,  20  being introduced axially into the housing component  30  and being rotationally fixedly connected to the respective gear  26 ,  28 ,  54 ,  54 ′ within the housing component  30  on the basis of a shrinking process. It is in this respect not absolutely necessary that all the gears  26 ,  28 ,  54 ,  54 ′ are first introduced into the housing component  30  and the gears are only afterward connected to the shafts ( 14 ,  14 ′,  20  (parallel assembly). A respective one gear can rather also be introduced and be connected to the associated shaft before the next gear is introduced and connected to the associated shaft (sequential assembly). It must also be noted that two gears  28  and  54 ′ or  26  and  54  can be introduced into the housing component  30  simultaneously and can be held there in a respective assembly position so that the four gears  26 ,  28 ,  54 ,  54 ′ can, for example, also be introduced and connected to the associated shaft  14 ,  14 ′,  20  pair-wise. It is furthermore also possible that at least one of the gears (e.g. the drive shaft gear  28 ) is introduced into the housing component  30  in an axial direction through a suitable introduction opening. 
         [0065]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention. 
         [0066]    Reference Numeral List 
         [0067]      10  balance shaft unit 
         [0068]      12  engine 
         [0069]      14 ,  14 ′ balance shaft 
         [0070]      16  drive connection 
         [0071]      18  balance weight 
         [0072]      19  input gear 
         [0073]      20  drive shaft 
         [0074]      22  pump 
         [0075]      24  transmission stage 
         [0076]      26  balance shaft gear 
         [0077]      28  drive shaft gear 
         [0078]      30  housing component 
         [0079]      32  assembly slit 
         [0080]      33  introduction passage 
         [0081]      34  introduction direction 
         [0082]      36  axis of rotation 
         [0083]      37  introduction opening 
         [0084]      38  axis of rotation 
         [0085]      39  introduction opening 
         [0086]      40 ,  42  assembly movement 
         [0087]      44  assembly window 
         [0088]      46  bearing 
         [0089]      47  bearing plate 
         [0090]      48 ,  50  functional unit 
         [0091]      52  housing cover 
         [0092]      54 ,  54 ′ gear 
         [0093]      55  synchronization stage