Abstract:
A rolling contact bearing includes a shaft having a first radial elevation and a second radial elevation. A first inner ring has a first raceway for the rolling bodies to roll on, a first radial narrowing and a second radial narrowing, the first radial elevation and the first radial narrowing forming a first press fit, and, respectively, the second radial elevation and the second radial narrowing forming a second press fit. The shaft has a third radial elevation and a fourth radial elevation. A second inner ring has a second, radially outer rolling body raceway for rolling bodies to roll on, a third radial narrowing and a fourth radial narrowing, wherein the third radial elevation and the third radial narrowing forming a third press fit and, respectively, the fourth radial elevation and the fourth radial narrowing forming a fourth press fit.

Description:
The invention concerns a rolling bearing particularly a turbocharger rolling bearing, comprising a shaft comprising a first, radial elevation and a second, radial elevation and a first, inner ring which can be fixed to the shaft, said first inner ring comprising at least one first radially outer rolling element raceway for rolling elements to roll on, a first, radial narrowing and a second, radial narrowing, the first, radial elevation and the first, radial narrowing being provided for forming a first press fit, and, respectively, the second, radial elevation and the second, radial narrowing being provided for forming a second press fit. 
     BACKGROUND 
     DE 10 2008 020 067 A1 discloses a turbocharger bearing which comprises a shaft comprising different outer diameters onto which shaft the one-piece inner ring is pushed. Axially on the outer side, the inner ring is seated on the shaft through a press fit, while, centrally, under the inner ring is situated a smaller outer diameter without press fit so that a low frictional force is produced during pressing in. 
     SUMMARY OF THE INVENTION 
     The problem of this construction is that a rolling bearing comprising a two-piece inner ring cannot be adequately centered on a shaft of this type because the inner rings, or the inner ring parts, are seated only on axially outer sides on the shaft. However, at high speeds of rotation, it is exactly an optimal centering that is of vital importance for the life duration of the rolling bearing. 
     It is an object of the present invention to provide a shaft permitting an optimized press-in force for rolling bearings comprising two inner rings, which shaft also enables the required centering. 
     The present invention provides a rolling bearing of the initially described type by the fact that the shaft comprises a third, radial elevation and a fourth, radial elevation, and a second inner ring that can be fixed to the shaft comprises at least one second, radially outer rolling element raceway for the rolling elements to roll on, a third, radial narrowing and a fourth, radial narrowing, the third, radial elevation and the third, radial narrowing being provided for forming a third press fit, and, respectively, the fourth, radial elevation and the fourth, radial narrowing being provided for forming a fourth press fit. 
     By press fit is to be understood a radial interference fit in which the inner diameter of a hollow cylindrical part is substantially equal to or smaller than the associated outer diameter of the pressed-in cylindrical part. In no case may a lash freedom be formed or positioning steps be permitted that allow a non-concentric arrangement of the respect inner ring relative to the shaft. For example, not even a negative lash in the range of less than 5 micrometers, particularly, 1 or 2 micrometers, may be tolerated between the narrowing and the corresponding elevation for achieving a reliable centering. Although this type of fit is occasionally called a transition fit, it is also to be understood as a press fit in the following. The pressed-in cylindrical part in the sense of the present invention is the first, second, third or fourth elevation of the shaft, and the hollow cylindrical part is the first, second, third or fourth radial narrowing of one of the inner rings. 
     The radial narrowings are arranged on the inner side of the inner rings and are configured on these. Therefore, they are narrowings that are situated radially on the inside. 
     Altogether, four press fits are implemented, each inner ring comprising two press fits on which the respective inner ring is supported. 
     It is also possible to provide a temporary press fit during assembly. A temporary press fit is created if two neighboring radial narrowings have the same diameter. In this case, during assembly, that is to say, when the shaft is being inserted into the inner rings, one of the radial elevations must be pushed through the temporary press fit of a radial narrowing to then form a permanent (and thus operable) press fit with a neighboring radial narrowing. 
     According to the invention, the lever action between the two press fits of the respective the inner ring effects the centering of the inner rings on the shaft required for the operation of the rolling bearing. This is possible because one of the two press fits serves as a fulcrum and the other as an actuator of the lever. 
     Advantageously, the first outer diameter of the first, radial elevation is smaller than or equal to the second outer diameter of the second, radial elevation, the second outer diameter of the second, radial elevation is smaller than or equal to the third outer diameter of the third, radial elevation and the third outer diameter of the third, radial elevation is smaller than or equal to the fourth outer diameter of the fourth, radial elevation. The dimension of the outer diameters can be varied depending on the case of use. 
     If temporary press fits are to be avoided, it is purposeful to provide an ascending sequence of outer diameters for the radial elevations that get smaller in insertion direction, i.e. in this case, the first radial elevation, with the smallest outer diameter, is inserted at first. 
     According to the invention, the first and the third press fit form a fulcrum before the second and the fourth press fit, respectively, are formed. During the formation of the second and fourth press fit respectively, the inner ring concerned is oriented such that it is arranged concentric to the shaft. This is accomplished with a low force because the lever arms between the first and the second press fit, as also between the third and the fourth press fit, can be chosen to be long enough to enable any radial positioning still required at the second and, respectively, the fourth press fit. 
     It is further purposeful to center the second inner ring later in time than the first inner ring. During the centering step, the lever arm of the respective inner ring works against the frictional force that is produced on a contact surface to the next-situated component. If the second inner ring were centered at first, frictional forces would be created on both sides of the first inner ring, that is to say, also in direction towards the second inner ring. 
     It is particularly preferred to realize the first press fit as a fulcrum for the first inner ring. Following this, the second press fit of the first inner ring is made in order to arrange the first inner ring concentrically to the shaft through a radial correction at the second press fit. The third press fit then likewise takes over the function of a fulcrum so that it can be used in the then following realization of the fourth press fit as a fulcrum during the radial orientation. 
     Advantageously, the first outer diameter and the second outer diameter are equal to each other. This simplifies the production of the first inner ring. The first inner ring is at first centered i.e. it is brought into as concentric a position as possible to the shaft in that the second press fit is situated on that axial side of the first inner ring on which the shaft is inserted at first. 
     Advantageously, the third outer diameter and the fourth outer diameter are equal to each other. This simplifies the production of the second inner ring in a corresponding manner. 
     In an advantageous form of embodiment, the shaft has a multi-piece configuration in the region of the press fits. For instance, it is not necessary to make the shaft with all its elevations by turning out of one work piece. Instead, it is possible to fix rings on the shaft that form the radial elevations. 
     Preferably, the shaft merges, in the region of the first, second, third and/or fourth radial elevation, beginning from the first, second, third and/or, respectively, fourth outer diameter, in one axial direction, with a linear, concave or convex shape into a further region with a further outer diameter, the further outer diameter being respectively smaller than the first, second, third and fourth outer diameter. This thread-in aid assists in the insertion of the shaft during assembly so that the press-in force required is reduced further. Thus, the outer region of the shaft that carries the respective linear, concave or convex transition separates the elevations from the non-carrying outer regions. An outer region in this case can be, for example, a cylindrical outer surface of the shaft. 
     The simplest manner of configuring the transitions is to make them as linear assembly inclinations which preferably form an angle of between 5 and 30 degrees to the axis of rotation, ideally 15 or 25 degrees. In this way, the axial force of the shaft to be inserted can be converted easily into a small radial force for correction of positions. 
     Preferably, the respective first or second inner ring merges in the region of the first, second, third and/or fourth radial narrowing, beginning from the first, second, third and, respectively, fourth inner diameter, in one axial direction, with a linear, concave or convex shape into a region with a further inner diameter that is respectively larger than the first, second, third and fourth inner diameter. Thus, the respective linear, concave or convex transition separates an outer region that carries the respective inner ring from a non-carrying outer region of the shaft. 
     In the case of these transitions, too, the simplest manner of configuring them is to make them as linear assembly inclinations which preferably form an angle of between 5 and 30 degrees to the axis of rotation, ideally 15 or 25 degrees. 
     Advantageously, a first axial distance of the first press fit from the second press fit is larger than half the axial width of the first inner ring and/or a second axial distance of the third press fit from the fourth press fit is larger than half the axial width of the second inner ring. In this way, the respective lever arm between the press fits is enlarged and the press-in force is reduced still further. 
     Advantageously, the second press fit is arranged on one axial end of the first inner ring and/or the third press fit is arranged on one end of the second inner ring. 
     In one advantageous form of embodiment, the shaft can be inserted into the inner rings, arranged axially relative to each other, till one axial front end of the second inner ring comes to abut against a turbine-side axial stop and thus signalizes the operable position of the shaft within the inner rings. In this way, the installer can be sure that, when the two surfaces contact each other, all the press fits required for centering have been completed and the centering of the two rings has been terminated. 
     Further advantageous embodiments and preferred further developments of the invention can be seen in the description of the figures and/or in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described and explained more closely in the following with reference to the examples of embodiment illustrated in the figures. 
         FIG. 1  shows a turbocharger rolling bearing in a section taken along the shaft, in ready-assembled, operative state, 
         FIG. 2  shows the turbocharger rolling bearing of  FIG. 1 , during a first assembly step, 
         FIG. 3  shows the turbocharger rolling bearing of  FIG. 1 , during a second assembly step, 
         FIG. 4  shows the turbocharger rolling bearing of  FIG. 1 , during a third assembly step, 
         FIG. 5  shows the turbocharger rolling bearing of  FIG. 1 , during a fourth assembly step, 
         FIG. 6  shows the turbocharger rolling bearing of  FIG. 1 , during a fifth assembly step, 
         FIG. 7  shows the turbocharger rolling bearing of  FIG. 1 , during a sixth assembly step, and 
         FIG. 8  shows a schematic press-in force diagram for a conventional turbocharger bearing compared to the turbocharger bearing of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a turbocharger rolling bearing in a section taken along the shaft in an operative state after installation. 
     The fractional part of the shaft  19  which is surrounded by both the inner rings  17 ,  18  merges on a turbine-side into the turbine-side end ET which is either itself a part of the turbocharger turbine or is configured to be connected to this. On the opposite end of the fractional part of the shaft  19 , the fractional part merges with the compressor-side end EK of the fractional part which can be a part of the compressor or be capable of being connected to this. The compressor (not illustrated) has the function of compressing the air sucked in by the piston of an engine. The energy required for this is delivered by a turbine (likewise not illustrated) which is driven in a hot housing by the exhaust gas stream and is transferred via the shaft  19  and also via the fractional part thereof in the turbocharger bearing. 
     The fractional part comprises, in alternating order, from the compressor-side end EK to the turbine-side end ET, non-contacting regions and radial elevations in the following sequence: non contacting region  9 , first radial elevation  5 , non-contacting region  10 , second radial elevation  6 , third radial elevation  7 , non-contacting region  11 , fourth radial elevation  8  and non-contacting region  12 . 
     All the transitions are radially convex in shape and serve as assembly inclinations that form an angle of less than 30 degrees relative to the axis of rotation. This applies both to the transition between the first and the second radial elevation  6 ,  7  and to the transitions between a non-contacting region  9 ,  10 ,  11 ,  12  and the respective neighboring radial elevation  5 ,  6 ,  7 ,  8 . 
     In this way, the first elevation  5  and the radial narrowing  1  form the first operative press fit exactly as also the second elevation  6  and the second narrowing  2  form the second press fit. Both press fits again form the base for the first inner ring  17  and assure its radial centering for an operation at very high speeds of rotation without any detrimental effect on the rolling elements  21  during their rolling motion in their raceways around the inner ring  17  so that they can no longer trigger a disturbing vibration. Analogously, this also applies to the inner ring  18  that is centered through the third and the fourth press fits. 
     The centering of the shaft  19  is realized during installation by the fact that the compressor-side end EK is inserted through the second inner ring  18  and then further through the first inner ring  17 . During this step, it is also possible for the temporary press fits to be formed before the operative position of the shaft  19  in the inner rings  17 ,  18  is reached. The further the first press fit is arranged from the second press fit, respectively, the third press fit from the fourth press fit, the larger is the lever arm during radial centering so that centering can be effected with the lowest possible force. The length of the first lever arm T 1  is smaller than the axial width B 1  of the first inner ring  17 . In most cases, however, a shorter length is required to enable a passing-by of the temporary press fits one after the other during installation, that is to say, to make it possible to pass by as few as possible at the same time. This applies analogously to the length T 2  of the second lever arm between the third and the fourth press fit of the second inner ring  18 . In any case, it is still purposeful to provide at least one lever arm with a length of at least half the width of the respective inner ring  17 ,  18 :
 
 T 1&gt;½* B 1; T 2&gt; V 2* B 2;
 
     In the example of embodiment of  FIG. 1 , the third and the fourth press fits are situated radially further outwards than the first and the second press fits. In particular, the first and the second radial narrowings  1 ,  2  even have the same inner radius and, respectively, the first and the second radial elevations  5 ,  6  have the same outer radius. Analogously, this also applies to the radial narrowings  3 ,  4  and the radial elevations  7 ,  8  but, respectively, with larger same inner radii and larger and larger same outer radii. 
     In this way, only two temporary press fits get formed during assembly of the shaft  19 , viz. when the first radial elevation  5  passes the second radial narrowing  2  and when the third radial elevation  7  passes the radial narrowing  3 . 
     The assembly with the two temporary press fits is illustrated in the following  FIGS. 2 to 7  which show the turbocharger rolling bearing of  FIG. 1  in six different assembly steps. 
       FIGS. 2 to 7  show the turbocharger rolling bearing of  FIG. 1  in the assembly steps one to six. 
     In the first assembly step of  FIG. 2 , the shaft  19  was pushed in between the first radial elevation  5  and the second radial narrowing  2  up to the incipient first temporary press fit  20 . The press-in force is further enhanced through the second temporary press fit  30  that gets formed between the third radial elevation  7  and the fourth radial narrowing, i.e. exists partially simultaneously with the first press fit, as shown in  FIG. 3 . 
     In the following  FIGS. 4 to 7 , further incipient press fits,  40 ,  50 ,  60  and  70  are shown which, however, are intended to become permanent i.e. they are provided for the operation of the rolling bearing as soon as the axial front end surface S of the second inner ring comes to abut against the axial surface A. 
     The length L of the fractional part of the shaft  19  between the two ends EK and ET is equal to the sum of the inner ring widths B 1 , B 2  because the two inner rings  17 ,  18  abut against each other at the axial stop  72 . 
     Alternatively, the press fits can have the same dimensions i.e. the radial narrowings and the radial elevations possess the same inner radii and the same outer radii. In addition, it is also possible that all radial narrowings and also all radial elevations have different inner radii and different outer radii, so that no temporary press fits but only permanent press fits are formed. 
       FIG. 8  shows a schematic press-in force diagram for a conventional turbocharger bearing compared to the turbocharger bearing of  FIG. 1 . 
     The curve G 1  has a linear dependence on the press-in length that is plotted against the length from 0 to L of the fractional part of the shaft  19 . The hatched area under the straight line G 1  corresponds to the press-in energy for a conventional cylindrical shaft which is pressed into a hollow cylindrical interior with axially non-variable respective inner and outer radii. 
     The curve G 2  shows schematically the course of the press-in force as a function of the press-in length, which force rises shortly after L/2, at the first temporary press fit, to F 1 , to then rise further immediately thereafter by reason of the second temporary press fit to F 2 . 
     For instance it can be assumed by approximation that the temporary press fits produce the same press-in force FO. Thus, the following applies:
 
 F 1= F 0; F 2=2* F 0; F 3=3* F 0; F 4=4* F 0;
 
     Further, by idealization, it is assumed for the sake of illustration, that the areas of the press fits approximate zero. A small press fit length results in a low press-in force. In practice, however, these will always form surface contacts which, in the graphical illustration of the curve G 2 , form linearly ascending or, respectively, declining flanks. Idealized, these result in vertical flanks. 
     The curve G 2  is divided into two sections. The smaller one represents the temporary press fits G 20  and G 30  which have to be overcome together with the press-in force FE=F 2 ; in the case of the temporary press fit G 20 , the press-in force FE=F 1  suffices. The hatched area situated under the curve G 2  corresponds to the energy which has to be provided for overcoming the temporary press fits G 20  and G 30 . 
     At the end of the press-in path are situated, at short intervals, the operative, permanent press fits, i.e. their beginnings G 40 , G 50 , G 60  and G 70  that have to be overcome. For this purpose, the press-in force increases stepwise from FE=0 to FE=F 4 . On the whole, the press-in energy of the curve G 2  is clearly lower compared to the press-in energy of the curve G 1  because the integral over the press-in length from 0 to L of G 2  turns out to be clearly higher. 
     The permanent press fit  40  is the first operative press fit and the permanent press fit  50  is the second operative press fit. 
     To summarize, the invention concerns a rolling bearing particularly a turbocharger rolling bearing, comprising a shaft comprising a first, radial elevation and a second, radial elevation and a first inner ring which can be fixed to the shaft, said first inner ring comprising at least one first radially outer rolling element raceway for the rolling elements to roll on, a first, radial narrowing and a second, radial narrowing, the first, radial elevation and the first, radial narrowing being provided for forming a first press fit, and, respectively, the second, radial elevation and the second, radial narrowing being provided for forming a second press fit. The aim of the invention is to provide a rolling bearing with an optimized press-in force for high speeds of rotation which also enables a simple centering of two inner rings. For this purpose, the shaft comprises a third, radial elevation and a fourth, radial elevation, and a second inner ring that can be fixed to the shaft comprises at least one second, radially outer rolling element raceway for the rolling elements to roll on, a third, radial narrowing and a fourth, radial narrowing, the third, radial elevation and the third, radial narrowing being provided for forming a third press fit, and, respectively, the fourth, radial elevation and the fourth, radial narrowing being provided for forming a fourth press fit. 
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 List of reference numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 Axial stop 
                 B1 
                 Axial width 
               
               
                 B2 
                 Axial width 
                 ET 
                 Turbine-side end 
               
               
                 EK 
                 Compressor-side end 
                 G1 
                 First curve 
               
               
                 G2 
                 Second curve 
                 G20 
                 Beginning of temporary 
               
               
                   
                   
                   
                 press fit 
               
               
                 G30 
                 Beginning of temporary press fit 
                 G40 
                 Beginning of permanent 
               
               
                   
                   
                   
                 press fit 
               
               
                 G50 
                 Beginning of permanent press fit 
                 G60 
                 Beginning of temporary 
               
               
                   
                   
                   
                 press fit 
               
               
                 G70 
                 Beginning of temporary press fit 
                 F1 
                 Single press-in force 
               
               
                 F2 
                 Double press-in force 
                 F3 
                 Triple press-in force 
               
               
                 F4 
                 Quadruple press-in force 
                 FE 
                 Press-in force 
               
               
                 L 
                 Length of fractional part of shaft 
                 S 
                 Axial front end 
               
               
                 T1 
                 Axial press fit spacing 
                 T2 
                 Axial press fit spacing 
               
               
                  1 
                 First radial narrowing 
                  2 
                 Second radial narrowing 
               
               
                  3 
                 Third radial narrowing 
                  4 
                 Fourth radial narrowing 
               
               
                  5 
                 First radial elevation 
                  6 
                 Second radial elevation 
               
               
                  7 
                 Third radial elevation 
                  8 
                 Fourth radial elevation 
               
               
                  9 
                 Non-contacting region 
                 10 
                 Non-contacting region 
               
               
                 11 
                 Non-contacting region 
                 12 
                 Non-contacting region 
               
               
                 13 
                 Non-narrowed region 
                 14 
                 Non-narrowed region 
               
               
                 15 
                 Non-narrowed region 
                 16 
                 Non-narrowed region 
               
               
                 17 
                 First inner ring 
                 18 
                 Second inner ring 
               
               
                 19 
                 Shaft 
                 20 
                 Temporary press fit 
               
               
                 21 
                 Rolling element 
                 30 
                 Temporary press fit 
               
               
                 40 
                 Operative press fit 
                 50 
                 Operative press fit 
               
               
                 51 
                 Progressive operative press fit 
                 60 
                 Operative press fit 
               
               
                 70 
                 Operative press fit Operative 
                 71 
                 Progressive operative 
               
               
                   
                 press fit 
                   
                 press fit 
               
               
                 72 
                 Axial stop