Abstract:
A camshaft may include a tubular outer shaft and an inner shaft arranged coaxially thereto. The inner shaft may be rotatable at least partially with respect to the outer shaft. An annular gap may be disposed between the inner shaft and the outer shaft. The outer shaft may include at least one radially inwardly projecting annular step facing the inner shaft. The at least one annular step may mount the inner shaft with respect to the outer shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. 10 2012 220 652.3, filed Nov. 13, 2012, and International Patent Application No. PCT/EP2013/072546, filed Oct. 29, 2013, both of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a camshaft having a tubular outer shaft and an inner shaft, which is arranged coaxially thereto and can be rotated at least to a limited extent with respect to the outer shaft, according to the preamble of the independent claims. 
       BACKGROUND 
       [0003]    EP 1 963 625 B1 discloses a generic camshaft having an outer shaft and an inner shaft arranged coaxially thereto, the inner shaft being supported directly against a stepless inner lateral face of the outer shaft by means of at least one securing device arranged on an axial end region of the shaft. 
         [0004]    Owing to the necessary freedom of movement between the inner shaft and the outer shaft in an adjustable camshaft and/or owing to a necessary oil duct for a phase adjuster and/or bearing lubrication, a sufficient annular gap of between 0.2 and 2 mm in radial height is usually required between the inner shaft and the outer shaft. Sealing is therefore needed at the end of the two shafts to be able to build up the oil pressure necessary for the phase adjuster between the inner shaft and the outer shaft. With inner shafts known from the prior art, they are usually thickened in the region of a sealing ring, which can be achieved for example by removing material from almost the entire length of the inner shaft adjacent to the annular seal. This is however very expensive and requires an initially unmachined inner shaft with a comparatively large amount of material. 
       SUMMARY 
       [0005]    The present invention is therefore concerned with the problem of specifying an improved embodiment for a camshaft of the generic type, which in particular has a different mounting of an inner shaft in relation to an outer shaft. 
         [0006]    This problem is solved according to the invention by the subject matter of the independent claims. Advantageous embodiments form the subject matter of the dependent claims. 
         [0007]    The present invention is based on the general concept of, in a camshaft known per se having a tubular outer shaft and an inner shaft, which is arranged coaxially thereto and can be rotated at least to a limited extent with respect to the outer shaft, providing at least one annular step, which projects radially inwards and by means of which the inner shaft can be mounted on the outer shaft. In contrast, the inner shaft has a constant outer diameter and does not need further post-machining, for example after drawing of same. The minimum diameter of the inner shaft is thus limited only by the required torsion resistance and the pinning of cams coupled to the inner shaft (pin diameter). It is also of particular advantage in the camshaft according to the invention that the inner shaft does not have to be post-machined, in particular turned, over most of its outer circumferential face as previously customary, as a result of which the inner shaft can be produced comparatively simply and cost-effectively. The outer shaft is partially thickened only in the region of the bearing point and in the region of the sealing point by the radially inwardly projecting annular step, the effort for producing said radial annular step during production of the outer shaft being very low. The annular step can for example be produced by means of a forming process, in particular by upsetting or forming under combined tensile and compressive conditions during drawing of the outer shaft. Depending on the radial thickness of the annular step according to the invention, said step can even hold or mount a sealing ring. 
         [0008]    In a further advantageous embodiment of the solution according to the invention, the inner shaft has an outwardly open groove, in particular a circumferential groove, in which a sealing ring is arranged, which is arranged at the axial height of the annular step when the camshaft is assembled. Additionally or alternatively to the sealing ring arranged in the region of the annular step of the outer shaft, a sealing ring can thus be arranged in an outwardly open groove in the inner shaft, in particular if the inner shaft is formed as a solid profile. If the inner shaft is formed as a tube, the sealing ring is preferably arranged in the inwardly open groove in the outer shaft. The radially inwardly projecting annular step can be arranged on the longitudinal end of the outer shaft, but purely theoretically can also be arranged at a plurality of positions spaced apart axially on the outer shaft. 
         [0009]    In an advantageous development of the solution according to the invention, the at least one annular step has chamfered edges. Such chamfered edges make it easier to insert the inner shaft and thus to assemble the camshaft. Additionally or alternatively, the inner shaft has on at least one end an oblique introduction face, which likewise makes it easier to insert the inner shaft into the outer shaft when assembling the camshaft. 
         [0010]    The annular gap expediently has a radial thickness of 0.2 to 2.0 mm. In order to be able to keep a sufficient oil volume in the annular gap, for example for lubricating sliding bearings or for actuating a phase adjuster, the described annular gap is provided between the inner shaft and the outer shaft. The annular gap is used in particular as an oil guide. 
         [0011]    In general, the camshaft according to the invention can be assembled comparatively simply. To this end, for example, a sealing ring is first placed into an outwardly open groove in the inner shaft, then an assembly sleeve is pushed over the sealing ring. The assembly sleeve causes the sealing ring to be pressed into the groove on the inner shaft side. Then the inner shaft is inserted into the outer shaft and the camshaft is assembled thereby. The assembly sleeve is then withdrawn, as a result of which the sealing ring that is pressed into the groove in the inner shaft bears against an inner lateral face of the outer shaft and thereby seals off the annular gap between the inner shaft and the outer shaft. If, however, the sealing ring is arranged in a groove in the outer shaft in an alternative embodiment, the sealing ring is first placed into the inwardly open groove in the outer shaft and then the inner shaft is inserted into the outer shaft. The oblique introduction face at the front end of the inner shaft, as seen in the insertion direction, can prevent damage to the sealing ring during insertion. Comparatively simple assembly of the camshaft is thus possible in the last-mentioned embodiment. 
         [0012]    Further important features and advantages of the invention can be found in the subclaims, the drawings and the associated description of the figures using the drawings. 
         [0013]    It is self-evident that the above-mentioned features and those still to be explained below can be used not only in the combination given in each case but also in other combinations or alone without departing from the scope of the present invention. 
         [0014]    Preferred exemplary embodiments of the invention are shown in the drawings and are explained in more detail in the description below, the same reference symbols referring to the same or similar or functionally equivalent components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the figures, 
           [0016]      FIG. 1  schematically shows a sectional diagram through a first possible embodiment of a camshaft according to the invention, 
           [0017]      FIG. 2  schematically shows a diagram as in  FIG. 1 , but with a sealing ring mounted on an inner shaft, 
           [0018]      FIG. 3  schematically shows a diagram as in  FIG. 2 , but with a differently formed radial annular step, 
           [0019]      FIG. 4  schematically shows a sectional diagram through a camshaft according to the invention with a phase adjuster arranged at the end, 
           [0020]      FIG. 5A-D  schematically show individual method steps for assembling the camshaft according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    According to  FIGS. 1 to 4 , a camshaft  1  according to the invention has a tubular outer shaft  2  and an inner shaft  3 , which is arranged coaxially thereto and can be rotated at least to a limited extent with respect to the outer shaft  2 . An annular gap  4  is provided at least in some regions between the inner shaft  3  and the outer shaft  2 , to conduct oil for the lubrication of sliding bearings and/or for actuating a phase adjuster  5  (compare  FIG. 4 ). Such a phase adjuster  5  is used in a known manner for the relative rotation of the inner shaft  3  with respect to the outer shaft  2 , the inner shaft  3  being connected in a rotationally fixed manner to first cams  6 , and the outer shaft  2  being connected in a rotationally fixed manner to second cams  7 . The rotationally fixed connection between the inner shaft  3  and the first cams  6  takes place by means of a pin arrangement  8 , which in each case has a pin  9 , which is inserted into the inner shaft  3  and at the same time is connected in a rotationally fixed manner to the first cam  6 . A slot, which extends in the circumferential direction, is provided in the outer shaft  2  at the axial height of the pin  9 , in which slot the pin  9  can be moved during a relative rotation of the inner shaft  3  with respect to the outer shaft  2 . 
         [0022]    In order to be able to seal off the annular gap  4  in the axial direction of the camshaft  1 , the outer shaft  2  has according to the invention at least one radially inwardly projecting annular step  10 , by means of which the inner shaft  3  is mounted and sealed off with respect to the outer shaft  2 . The annular step  10  arranged on an inner lateral face of the outer shaft  2  can be produced comparatively simply and cost-effectively, for example by a forming process, in particular by upsetting or by forming under combined tensile and compressive conditions during drawing of the outer shaft  2 . The annular step  10  offers the great advantage that the inner shaft  3  can be configured with a constant outer diameter and as a result can be produced in a comparatively cost-effective manner. A previously complex and expensive post-machining of the inner shaft  3  can also usually be omitted. 
         [0023]    If  FIGS. 1 and 4  are viewed, it can be seen that the outer shaft  2  has an inwardly open annular groove  11  in the region of the annular step  10 , in which groove a sealing ring  12  is arranged. In contrast to this, the inner shaft  3  according to  FIGS. 2, 3 and 5  has an outwardly open annular groove  11 , in which a sealing ring  12 ′ is arranged, the sealing ring  12 ′ being arranged at the axial height of the annular step  10  when the camshaft  1  is assembled. When the groove  11  is provided in the region of the annular step  10 , no reduction in the cross section of the inner shaft  3  is necessary, so the shaft can be restricted to the minimum required diameter, which is only limited by the necessary torsion resistance and the pin arrangement  8  of the first cams  6  (diameter of the pins  9 ). The sealing ring  12  mounted in the outer shaft  2  also provides advantages in particular during assembly of the camshaft  1 , as is explained below. Since the inner shaft  3  in this case has a much smaller diameter overall, which is limited only by the torsion resistance and the pin arrangement  8 , said shaft can also be made much lighter than inner shafts known previously from the prior art. The inner shaft  3  shown is formed as a solid profile, a hollow inner shaft  3  of course also being conceivable. 
         [0024]    The sealing ring  12 ,  12 ′ can be formed from a plastic, in particular from an elastomer or a polytetrafluoroethylene (PTFE), metallic sealing rings of course also being conceivable. The sealing ring  12 ,  12 ′ can withstand the chemical environment inside the camshaft  1 , as well as the temperatures occurring during operation of an internal combustion engine containing the camshaft  1 , for a long time. The plastics mentioned for the sealing ring  12 ,  12 ′ should be understood as mere examples; other plastics are also conceivable. 
         [0025]    If  FIGS. 1 to 5  are viewed, it can be seen that the at least one annular step  10  has at least one chamfered edge  13 , even two chamfered edges  13 ,  13 ′ depending on the embodiment, which make it easier to insert the inner shaft  3  into the outer shaft  2 . Similarly, an oblique introduction face  14  can be provided on at least one end of the inner shaft  3 , which face makes it easier to introduce the inner shaft  3  into the outer shaft  2  and thus to assemble the camshaft. If the annular step  10  is situated on the axial end region of the outer shaft  2 , as is shown for example in  FIGS. 3 to 5 , the annular step  10  usually only has one chamfered edge  13 . 
         [0026]      FIGS. 5 a  to 5 d    show a possible assembly of the camshaft  1  according to the invention; in the first method step according to  FIG. 5 a   , an assembly sleeve  15  is first aligned coaxially to the inner shaft  3 . In this case the inner shaft  3  bears the sealing ring  12 ′ in its groove  11 ′. In the method step shown in  FIG. 5 b   , the assembly sleeve  15  is then pushed over the sealing ring  12 ′ and thus over the inner shaft  3 , then, in the method step according to  FIG. 5 c   , the inner shaft  3  is inserted into the outer shaft  2  of the camshaft  1 . When the assembly aid  15  is placed over the inner shaft  3 , the assembly aid  15  presses the sealing ring  12 ′ into the groove  11 ′ on the inner shaft side. Once the final position of the inner shaft  3  in the outer shaft  2  is reached, the assembly sleeve  15  is withdrawn according to the method step in  FIG. 5 d   , whereupon the sealing ring  12 ′ rises at least partially out of the groove  11 ′ in the inner shaft  3  and bears against an inner lateral face of the outer shaft  2 , that is, against the inner lateral face of the annular step  10 . 
         [0027]    If, however, the sealing ring  12  is arranged in the groove  11  in the outer shaft  2 , to assemble the camshaft  1 , the sealing ring  12  is first positioned in the groove  11 , whereupon the inner shaft  3  is then inserted into the outer shaft  2 , and, because of the continuous outer diameter of the inner shaft  3 , no damage to the sealing ring  12  occurs even when the inner shaft is inserted into the outer shaft  2 , so in this case an assembly aid  15  can be omitted entirely. 
         [0028]    With the camshaft  1  according to the invention, not only the assembly thereof can be simplified, but also the weight thereof can be much reduced, which is of great advantage in particular in internal combustion engines used in motor vehicles. In addition, the radial annular step  10  is comparatively simple and cost-effective to produce, which constitutes a clear advantage compared with an inner shaft that was previously post-machined in a complex manner over almost its entire axial length.