Abstract:
A belt tensioner for a generator belt drive of an internal combustion engine, is provided having: —an annular housing part which is secured on the generator housing so as to enclose the generator shaft, —an annular tensioning part which is mounted on the housing part in a pivotal manner about the generator shaft, —a single tension roller which is mounted on the tensioning part and pretensions the belt, —and a spring which is clamped between the housing part and the tensioning part and which applies a torque to the tensioning part in order to generate the belt tension. The spring should be a bow spring which extends parallel to the belt plane (ε) and circumferentially and completely outside of a pivoting range of the tension roller.

Description:
BACKGROUND 
       [0001]    The invention relates to a belt tensioner for a generator belt drive of an internal combustion engine. The belt tensioner comprises:
       An annular housing part that is mounted on the generator housing so as to enclose the generator shaft,   An annular tensioning part that is supported on the housing part so that it can pivot about the generator shaft,   A single tension pulley that is supported on the tensioning part and pretensions the belt,   And a spring that is clamped between the housing part and the tensioning part and applies a torque to the tensioning part for generating the belt pretension.       
 
         [0006]    Such a belt tensioner is known from EP 2 573 423 A1. The spring is constructed there as a helical spring surrounding the generator shaft. 
       SUMMARY 
       [0007]    The present invention is based on the objective of improving the design of a belt tensioner of the type specified above with respect to its most compact possible construction. 
         [0008]    The solution to achieving the object is given in that the spring is a bow spring that extends parallel to the plane of the belt and circumferentially completely outside of the pivoting range of the tension pulley. This structural design of the tensioner spring mounting—a bow spring is, as is known, always a helical compression spring whose longitudinal extent is not straight, but has a circular arc shape—enables the spring installation space to be limited to a circular arc that is not covered by the operating, pivoting tension pulley. Because the spring installation space does not extend in the pivoting range of the tension pulley relative to the known belt tensioner, the tensioning part and the tension pulley can be arranged with a comparatively small axial state to the generator housing. This then applies also to the belt pulley that runs in the same belt plane as the tension pulley, so that, as a result, an axially especially compact assembly made from the generator and belt tensioner is provided. Because the belt plane also runs at a comparatively small distance to the generator housing, the free length of the generator shaft between the belt pulley and the first bearing point in the generator housing is also accordingly short in support of comparatively small shaft bending. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Additional features of the invention are given from the following description and from the drawings in which an embodiment of a belt tensioner according to the invention is shown. Shown are: 
           [0010]      FIG. 1  the generator of a generator belt drive with the belt tensioner mounted on this drive in a perspective view, 
           [0011]      FIG. 2  the belt tensioner in rear view, 
           [0012]      FIG. 3  the belt tensioner in longitudinal section, 
           [0013]      FIG. 4  the belt tensioner without housing cover in rear view, 
           [0014]      FIG. 5  the detail X from  FIG. 4 , 
           [0015]      FIG. 6  the housing part of the belt tensioner as an individual part in perspective front view, 
           [0016]      FIG. 7  the tensioning part of the belt tensioner as an individual part with plastic lining in perspective rear view, 
           [0017]      FIG. 8  the tensioning part of the belt tensioner as an individual part without plastic lining in perspective front view, 
           [0018]      FIG. 9  the spring cup of the belt tensioner in perspective individual part view, 
           [0019]      FIG. 10  the bow spring of the belt tensioner in perspective individual part view. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]      FIGS. 1 to 3  show a belt tensioner  1  according to the invention in assembly with a generator of a generator belt drive of an internal combustion engine or as a completely assembled individual part. The belt tensioner is mounted on the belt-side end of the generator housing  2  and is here screwed at three fastening points  3  with the generator housing. Because the generator is a conventional generator without a motor mode, i.e., without a starting function of the internal combustion engine, the belt tensioner has only a single tension pulley  4  for pretensioning the belt in its slack section. The ball bearing  5  of the tension pulley is screwed to an annular tensioning part  6  that surrounds the shaft  7  with the belt pulley  8  of the generator and is supported so that it can pivot about the generator shaft and—more precisely—in an annular housing part  9 . The pivoting support of the tensioning part is presently realized concentric to the generator shaft, but in an alternative according to the invention, could also be arranged with an axial offset about the generator shaft (as long as the belt pulley has free room of motion in the radial direction). The inner diameter of the housing part also surrounding the generator shaft is significantly larger than the diameter of the belt pulley, so that the required cooling air feed into the generator is not negatively affected. 
         [0021]    The axial pivot bearing of the tensioning part  6  in the housing part  9  is formed on one side by an end face  10  of the housing part facing the generator and on the other side by an end face  11  of a housing cover  12  that faces away from the generator and is screwed to the housing part on the back side of the belt tensioner  1  at six fastening points  13 . Obviously other known and/or non-detachable fastening types, such as swaging, riveting, are also possible. The radial pivot bearing of the tensioning part in the housing part is formed by the inner annular circumference  14  of the tensioning part and by the outer circumference  15  of a housing-fixed annular collar  16  that extends between the bearing end face  10  of the housing part and the housing cover (see also  FIG. 4 ). With regard to the required bearing properties and the damping of the operating tensioning part movements, the material pairing of the sliding bearing is made from the metallic housing surfaces  10 ,  11  and a plastic lining  17  of the tensioning part made from polyamide with PTFE inserted therein, which covers the two end sides and the inner annular circumference of the tensioning part (see also  FIG. 7 ). The plastic lining clipped on the tensioning part has a two-part design, wherein the first lining part covers the end side of the tensioning part at the bottom in  FIG. 7  and its inner annular circumference with an annular collar and wherein the second lining part covers the end side of the tensioning part at the top in  FIG. 7 . 
         [0022]    The belt tensioner  1  is fixed in the circumferentially pretensioned assembly position of the tension pulley  4  by a removable securing pin  18  that blocks the tensioning part  6  and the housing part  9  in this position relative to each other. Here, the securing pin passes through two aligned holes  19  and  20  in the housing part (see  FIG. 6 ) or in the housing cover  12  (see  FIG. 2 ) and a circumferential recess  21  in the tensioning part (see  FIG. 8 ). 
         [0023]    If necessary, the ingress of foreign particles and spray water into the housing interior can be made more difficult by a (not shown) separate seal between the housing part  9 , the tensioning part  6 , and the housing cover  12 . 
         [0024]    The pretensioning introduced by the tension pulley  4  into the belt is generated by a bow spring  22  that is supported parallel to the plane of the belt ε (epsilon) in a correspondingly arc-shaped space  23  of the housing part  9  at the side of the tensioning part  6  and torque is applied to the tension pulley—in  FIG. 1  in the counterclockwise direction. A clamping direction in the opposite direction is obviously possible with correspondingly adapted tensioner components. The spring holding space  23  is formed by a bulge  24  of the housing part extending in the axial direction toward the tension pulley (i.e., in the axial direction of the shaft and pulley). The axial extent of the bulge is so large that it overlaps with the tension pulley in the axial direction. This can be seen clearly from  FIG. 3 . The axial end face of the bulge is there to the right of the left end face of the tension pulley. The circumferential angle of the bulge is approx. 180° and has a center that runs in the nominal operating position of the belt tensioner  1  essentially diametrically opposite the tension pulley. Its pivoting range is limited by mechanical stops of the tensioning part on the housing part to an angle of approx. 40°, so that the bulge—despite its axial extent overlapping with the tension pulley—has a clear distance to the circumference of the tension pulley also in both end positions of the pivoting range. Because the spring holding space and the bulge limiting this space do not extend circumferentially into the pivoting range of the tension pulley, but instead run completely outside of this range with a circumferential distance, the tension pulley and consequently the entire belt plane are arranged with a comparatively small axial distance to the generator housing  2 . This is shown in  FIG. 3  with reference to the distance measure S between the screw-in plane of the housing part and the belt plane. 
         [0025]      FIGS. 4 and 5  show the assembly state of the belt tensioner  1  in which the bow spring  22  and a spring cup  25  covering the spring holding space  23 , but not the tensioning part  6  and the housing cover  12 , are mounted in the housing part  9 . The spring holding space concentric to the pivoting support of the tensioning part is set back axially relative to the bearing end face  10  of the housing part and limited by two circumferential end walls, wherein the one end wall  26  is used as a spring support for the housing part-side end  27  of the bow spring and the other end wall  28  is used before the assembly of the tensioning part as a circumferential stop for the tensioning part-side spring end  29 . In the embodiment, the bow spring is placed before the assembly of the tensioning part under slight pretensioning between the end walls in the spring holding space. This assembly pretensioning is produced in that the arc angle α (alpha) of the spring holding space is smaller than the arc angle φ (phi) of the non-tensioned bow spring (see  FIG. 10 ) and has the effect that it makes it harder for the bow spring and the spring cup to fall out from the spring holding space before assembly of the tensioning part. 
         [0026]    The arc angle φ of the non-tensioned bow spring  22  is presently approximately 180°. In particular, but nevertheless not only for the case that a larger arc angle is desired but is unfavorable for the winding of the bow spring, two or more bow springs can also be provided in series and a spring holding space  23  can be provided with arc angle α adapted accordingly. Independently, bow springs connected in parallel in the form of a spring package with one outer and one inner bow spring are also possible. With the help of these parameters, the total characteristics of the tensioner spring mounting can be varied within wide limits to the desired torque profile of the tension pulley  4 . 
         [0027]    The housing part  9  shown in  FIG. 6  and the tensioning part  6  shown in  FIGS. 7 and 8  with or without the sliding bearing lining  17  are each made from aluminum castings. Alternatively, sheet-metal formed parts made from one or more parts are possible. The tensioning part is provided with a catch  30  that projects into the spring holding space  23  and is used as a spring support  31  running lateral to the tensioning part for the tensioning part-side spring end  28 . The catch is constructed as a projection that is raised from the ring body of the tensioning part axially to the side of the tension pulley  4 , so that the spring support for the tensioning part-side spring end runs completely next to the annular body and on the same side as the tension pulley. One advantage of this arrangement is that the force pair introduced by the bow spring and the tension pulley into the pivoting support of the tensioning part generates a comparatively small tilting moment. 
         [0028]    The other end wall  28  is provided with a recess  32  that holds the catch  30  during the assembly of the tensioning part  6  in the housing part  9 . The tensioning part assembly is considerably simplified in this way, because during insertion of the catch into the recess, a space-creating displacement of the bow spring  22  contacting the other end wall is not necessary. In the fixed assembled position according to  FIG. 1  and in the nominal operating positions of the tension pulley  4 , the bow spring is clamped between the one end wall  26  on the housing part-side spring end  27  and the catch on the tensioning part-side spring end  29 . 
         [0029]    The bow spring  22  is supported in the radially outward direction on the spring cup  25  that acts against spring oscillations. The spring cup shown in  FIGS. 3 and 9  has an essentially L-shaped cross section that is adapted to approximately half the spring circumference. The spring cup is made from plastic, in particular, with glass-fiber reinforcement, as a PTFE compound, and/or from carbon fiber-reinforced PA46 (polyamide), PPS (polyphenylene sulfide), or PEEK (polyether ether ketone). Depending on the tribological loading of the spring cup by the oscillating bow spring during operation, other wear-resistant materials, coatings, or grease could be used as alternatives in the spring-cup contact. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1  Belt tensioner 
           2  Generator housing 
           3  Fastening point 
           4  Tension pulley 
           5  Ball bearing 
           6  Tensioning part 
           7  Shaft of the generator 
           8  Belt pulley 
           9  Housing part 
           10  End face of the housing part 
           11  End face of the housing cover 
           12  Housing cover 
           13  Fastening point 
           14  Inner ring circumference of the tensioning part 
           15  Outer circumference of the ring collar 
           16  Ring collar 
           17  Plastic lining 
           18  Securing pin 
           19  Hole for the securing pin 
           20  Hole for the securing pin 
           21  Recess for the securing pin 
           22  Bow spring 
           23  Spring holding space 
           24  Bulge of the spring holding space 
           25  Spring cup 
           26  One end wall 
           27  Housing part-side spring end 
           28  Other end wall 
           29  Tensioning part-side spring end 
           30  Catch 
           31  Spring support for the tensioning part-side spring end 
           32  Recess for catch