Abstract:
Axial thrust washers are provided in the axial contact region of the bearing raceway of an unbalance shaft. The axial thrust washers do not extend beyond 360° in the circumferential direction, but extend only over a little more than the unbalance masses which extend in this region. The end sections of the axial thrust washers are bent over so that they are supported on the prominent unbalance masses, and form a funnel-like entry region for the rolling elements or for a cage of the rolling bearing which houses the rolling elements.

Description:
This application claims the priority of DE 10 2008 022 307.7 filed May 6, 2008, which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention refers in general to axial thrust washers for unbalance shafts. 
     During the operation of a piston, a dynamic force is transmitted by means of a connecting rod to the crankshaft. In this way, the oscillating movement of the piston is converted into a rotational movement. As a result of the reciprocating movement of the piston and connecting rod, and also as a result of the irregular transmission behavior of the crankgear, inertia forces occur which are supported in engine bearings and subject adjacent structures to vibration excitations. The inertia forces of the linearly moved parts of the crankgear, that is to say the oscillating masses, can be approximately represented by a formula which is based on a series expansion in which inertia forces of the first and second order are defined. 
     The rotating masses of the crankgear can be compensated by means of counterweights on the crankshaft. 
     Oscillating inertia forces of the first and second order can be avoided or reduced in the case of multi-cylinder engines by means of a skilled arrangement of the cylinders. In the case of in-line engines with fewer than six cylinders and V-engines with fewer than eight cylinders, balance shafts are often used. In order to balance inertia forces of the second order, at least six cylinders are required in the case of the in-line engine, or eight cylinders in the case of the V-engine, or balance shafts upon which corresponding compensating unbalances rotate at double crankshaft speed. 
     Balance shafts, therefore, serve to reduce or to eliminate the free inertia forces of a reciprocating piston engine in order to reduce operating noise and vibrations. The unbalances or eccentric weights which are fastened on the balance shaft counteract the inertia forces which are created by the crankgear. The balance shafts are synchronously driven by the crankshaft by means of toothed wheels, chains, or toothed belts. Depending upon the type of engine construction, one or two balance shafts are used in most cases. 
     The construction of balance shafts is subject to the established principle of providing a mass arrangement between two bearing points, the center of gravity of which arrangement does not lie on the rotational axis of the balance shaft, as a result of which a compensating unbalance is created. Such a balance shaft follows from EP 1 081 410 B1. As illustrated in  FIG. 14  of this document, the largest unbalance radius is greater than the radius of the bearing journal which is directly adjacent to the unbalance masses. Although the bearing journal forms part of a plain bearing, it is also conceivable to support such a balance shaft in the internal combustion engine by means of a rolling bearing, and especially by means of a needle bearing, upon the bearing journal which then serves as a rolling element raceway. Providing, however, no further constructional measures are taken in this case, this can lead to a premature component wear since the bearing journal is axially delimited only on a partial circumference by means of the unbalance masses which radially project there, and a cage which accommodates the rolling elements, or the rolling elements during their rotation, can run against an edge which is created as a result of the unbalance mass which locally projects with regard to the bearing journal. 
     SUMMARY OF THE INVENTION 
     By means of the invention, a possibility is created in the case of an unbalance shaft of avoiding, or at least reducing, the wear of the bearing components. 
     This object is achieved by means of the subject of a respective independent claim. Further preferred embodiments are described in the dependent claims, respectively. 
     With the solution according to the invention, axial thrust washers are generally fastened on the shaft in the axial contact region of the bearing and do not extend over 360° in the circumferential direction but only over little more than the unbalance masses which are prominent in this region. The ends of such ring sections are bent over in such a way that they form a funnel-like entry region for the rolling elements, or for a cage of the rolling bearing which accommodates the rolling elements, and which during operation are supported on the prominent unbalance masses against centrifugal force. 
     According to another embodiment, the axial thrust washers on the entire section, or on a possibly even discontinuous section, are fixed in the axial direction in a groove which is formed in the shaft. 
     In addition to the achieving of the aforementioned object, a reliable positioning of the axial thrust washers both in the radial as well as in the axial direction of the shaft can therefore be ensured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described based on preferred embodiments with reference to the attached drawings. In the drawings: 
         FIG. 1  shows a side view of a bearing point of an unbalance shaft, with axial thrust washers according to a first embodiment of the invention; 
         FIG. 2  shows a partly sectioned side view of a bearing point of an unbalance shaft, with axial thrust washers according a second embodiment of the invention; and 
         FIG. 3  shows a view in the axial direction of an axial thrust washer according to the second embodiment of the invention, along the line III-III in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a side view of a section of an unbalance shaft  10 . The section of the unbalance shaft  10  has a bearing raceway  16  which extends concentrically to the axis X of the unbalance shaft over the entire circumference of the unbalance shaft. Masses  14 , which form the unbalance of the unbalance shaft, are arranged laterally in the axial direction next to the bearing raceway  16 . The unbalance masses  14  are arranged asymmetrically relative to the axis X of the unbalance shaft. In  FIG. 1 , the largest radius of the unbalance masses  14  is arranged below the axis of the unbalance shaft  10 . With the designation  12 , a region is identified which forms a transition between the bearing raceway  16  and the unbalance masses  14 . The transition region  12  for example can be formed in the form of a reinforcing rib. In the radial direction, therefore, the material of the unbalance shaft  10  recedes from the of the unbalance masses  14  in the direction of the transition region  12 . In this way, a shoulder, with an edge  30  which projects with regard to the bearing raceway  16 , is created for the bearing raceway  16  on the transition region between the unbalance mass  14  and the material of the transition region  12 . An unbalance shaft  10  which is formed in such a way for example can be produced via forming processes. 
     Furthermore, two axial thrust washers  20  are shown in  FIG. 1 . Each have a ring section  22  and an end section  24  in each case. The ring section  22  is a segment of a ring. In other words, the ring section  22  extends only over a partial circumference of the unbalance shaft  10 . In the illustrated embodiments, the ring section  22  extends less than 180° about the center axis X, the center axis of the ring section and the circumference of the bearing raceway  16 . In  FIG. 1 , the outwards pointing edge of the axial thrust washer  20  is seen. The axial thrust washer  20  has a smaller extent in the axial direction than in the radial direction. The end section  24  is bent over in the axial direction in such a way that there is a smooth transition in the form of a bend radius with regard to the ring section  22 . As result of this, a funnel-like entry region  28  is formed for the rolling elements, or for a cage of a rolling bearing, which is not shown, which accommodates the rolling elements, so that a stopping of the rolling elements or of the cage against the edge  30  is prevented. 
     As a result of the bending over of the end section  24  of the axial thrust washer  20 , the possibility is also created of the axial thrust washer  20  being supported on one of the unbalance masses  14 , which is are formed laterally to the bearing raceway  16  so that the axial thrust washer  20 , during a rotation of the unbalance shaft  10 , does not lift off the unbalance shaft due to the centrifugal forces. This support also ensures that the rolling elements, which roll on the bearing raceway  16 , or the cage, during contact with the axial thrust washer  20 , do not carry this along in the circumferential direction, i.e. the rolling elements, which roll on the bearing raceway  16 , and the cage which accommodates these, are moving in the upper region (in  FIG. 1 ) without axial guiding and in the lower region enter a channel which is formed by the unbalance masses  14  which are arranged laterally to the bearing raceway  16 . Wear of the hearing components is now reduced by the wall of the unbalance masses  14 , and especially the edges  30 , being covered by the axial thrust washers  20 . 
     During production and installation of the axial thrust washers  20 , it is conceivable that these on the one hand are first bent over at their end sections  24  in order to then be hooked/mounted on the unbalance masses  14 . On the other hand, it is conceivable that an axial thrust washer, which is prefabricated in a flat manner as a ring section, is positioned on the unbalance shaft  10  and the end sections  24  are only then bent over. Depending upon the design of the unbalance shaft  10 , it may come to a preferred sequence of assembly of the bearing components and also of the axial thrust washers  20  during installation. Consequently, an instruction would then also be given with regard to the first or second production/installation variant. 
     In  FIGS. 2 and 3 , a second embodiment according to the invention is shown. With regard to the essential features, this embodiment does not differ from the first embodiment which is described above. Also in this case, the axial thrust washers  20  are formed from ring sections  22  and at least one bent-over end section  24 . This end section also ensures that support of the axial thrust washer  20  on the unbalance mass  14  is possible, so that the axial thrust washer  20  does not become detached as a result of rotation of the unbalance shaft  10 . 
     As an additional feature, the axial thrust washer  20  according to the second embodiment has a projection  26 . This projection is formed radially inwards in a center section on the inside circumference of the axial thrust washer  20 . On account of the projection  26 , use of this axial thrust washer  20  is only possible, if at the same time a groove  18 , which is suitable for accommodating the projection  26 , is formed in the unbalance shaft  10  between the unbalance mass  14  and the bearing raceway  16 . According to  FIGS. 2 and 3 , the groove  18  extends only on a partial circumference of the unbalance shaft  10 . The advantage of the axial thrust washer  20  according to the second embodiment lies in the fact that the axial thrust washer  20  is also secured in the axial direction on account of the engagement of the projection  26  in the groove  18 . 
     Alternatively, it is possible for a plurality of projections  26  to be formed along the inside circumference of the axial thrust washer  20  and to engage in a corresponding groove of the unbalance shaft  10 . Furthermore, the inside circumference of the axial thrust washer  20  can altogether have a smaller radius than the radius of the adjacent bearing raceway  16  so that the axial thrust washer  20  engages in a groove along its entire length, when it is installed. 
     Installation of the axial thrust washer  20  according to the second embodiment can also be carried out via two variants. Firstly, the axial thrust washer  20  can be manufactured with a bent-over end section  24  and then be pushed onto the unbalance shaft  10 . In so doing, it is to be taken into consideration that the at least one bent-over end section  24  must have a certain elasticity so that the axial thrust washer  20  can be positioned completely. For this purpose, the bent-over end section  24  is bent towards the bearing raceway  16  until the projection  26  engages in the groove  18  and as a result the bent-over end section  24  projects over the edge of the unbalance mass  14  and snaps back again into the desired position. 
     According to a second installation variant, the axial thrust washer  20 , which has been prefabricated in a merely flat manner, i.e. the end section  24  has not yet been bent over, can be positioned on the unbalance shaft  10  so that the projection  26  engages in the groove  18 . Only in this position is the at least one end section  24  bent over in such a way that the desired supporting on the unbalance mass  14  is arrived at. Ultimately, it is the aim in the case of the two variants that the axial thrust washer  20  covers only a partial circumference of the unbalance shaft  10 , and that the axial thrust washer  20  is locked or secured in its position both in the radial direction and in the axial direction. 
     It is noted that the embodiments which are described here are only exemplary. A multiplicity of variations are possible, especially with regard to the shape and design of the unbalance shaft  10  and its unbalance mass  14 . Another configuration of the axial thrust washers  20  is also conceivable with regard to their dimensions in the radial and axial directions without deviating from the inventive idea. 
     Axial thrust washers  20  according to the two described embodiments are preferably fitted on an unbalance shaft  10  adjacent to a bearing raceway  16  of the unbalance shaft, under the assumption that the occurring forces/loads as seen from the unbalance shaft  10  always act in the same direction. In case the forces/loads act in a constant direction with regard to for example the casing, similarly configured axial thrust washers  20  can be used in principle, wherein these are then installed on the casing side. 
     As shown in the drawings, the end section  24  can be bent along a line parallel to a radius of the unbalanced shaft  10 . 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of Designations 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 Unbalance shaft 
               
               
                 12 
                 Transition region 
               
               
                 14 
                 Unbalance masses 
               
               
                 16 
                 Bearing raceway 
               
               
                 18 
                 Groove 
               
               
                 20 
                 Axial thrust washer 
               
               
                 22 
                 Ring section 
               
               
                 24 
                 End section 
               
               
                 26 
                 Projection 
               
               
                 28 
                 Funnel-like entry region 
               
               
                 30 
                 Edge