Abstract:
A floating bearing for a steering gear, having an inner ring for receiving a pinion shaft of the steering gear and having an outer ring which surrounds the inner ring and which serves for the mounting of the floating bearing in a housing of the steering gear, wherein the outer ring and the inner ring form an annular gap between them and wherein the outer ring and the inner ring are connected to one another by means of at least one flexible connecting part such that said outer ring and inner ring are movable relative to one another in at least one radial direction, is characterized in that the outer ring and/or the inner ring form, within the connecting part, at least one projection which projects into the annular gap such that the spacing between the outer ring and inner ring is smaller in the region of the projection than in the remaining section of the annular gap.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a floating bearing for a steering gear. 
     In most motor vehicles, there are installed power steering systems which, during steering maneuvers, generate an assisting torque and thereby reduce the steering torque that has to be imparted to the steering column by the driver. 
     Known power steering systems are based on a steering gear which converts the drive power of a hydraulic or electric drive and transmits said drive power to the steering column. Steering gears of said type are generally in the form of a helical rolling-type gear, and are in particular in the form of a helical wheel gear or worm gear, that is to say said steering gears comprise a gearwheel, which is connected directly or indirectly to the steering column, and a pinion which is driven by the drive via a pinion shaft. 
     In the case of such steering gears, gearing play has proven to be a problem, said gearing play arising owing to component tolerances, different thermal expansion of the gearing elements and wear. In particular during so-called alternating steering, that is to say during directly successive steering maneuvers with alternating steer angle, such gearing play generates undesired noises resulting from alternating abutment of opposite flanks of teeth of the pinion and gearwheel. 
     It is known for said gearing play to be eliminated by virtue of the pinion shaft being mounted so as to be pivotable about an axis running perpendicular to the longitudinal axis of the pinion shaft and at a distance from the toothing engagement of pinion and gearwheel, and being pressed against the gearwheel by means of one or more spring elements. Here, the pivoting capability of the pinion shaft is integrated into one of the two bearing arrangements by which the pinion shaft is mounted at its ends. Said bearing arrangement is also referred to as “fixed bearing”. The bearing arrangement in the region of the other end is then implemented with play (so-called “floating bearing”) in order to permit the deflection caused by the pivoting movement. The fixed bearing is generally provided at the drive side, whereas the floating bearing is provided on the free end of the pinion shaft. The one or more spring elements for pressing the pinion against the gearwheel are generally integrated into the floating bearing. 
     A (generic) floating bearing of said type, and a steering gear comprising a floating bearing of said type, are known for example from DE 10 2007 055 814 A1. In said document, the floating bearing comprises an inner ring, within which a section of a pinion shaft of the steering gear is mounted with the interposition of a rolling bearing, and an outer ring, which is held in a receptacle of a housing of the steering gear. The inner ring and outer ring form an annular gap between them. At one point on the circumference, the inner ring and the outer ring are connected via a connecting part which is in the form of a web and which is formed from an elastic material. The annular gap permits a radial movement of the inner ring and outer ring relative to one another, whereas the connecting part ensures that the inner ring and outer ring are held together, without the desired radial mobility between these being impeded. 
     A disadvantage of the floating bearing known from DE 10 2007 055 814 A1 is the relatively high forces required for a relative displacement of the inner ring with respect to the outer ring as a result of the deformation of the connecting part. 
     Taking this prior art as a starting point, the invention was based on the object of specifying an improved floating bearing for a steering gear. In particular, the improved floating bearing should be distinguished by the fact that only relatively low forces are required for the relative movement of the inner ring with respect to the outer ring. 
     SUMMARY OF THE INVENTION 
     Said object is achieved by means of a floating bearing according to independent patent claim  1 . The coordinate patent claim  9  relates to a steering gear comprising a floating bearing of said type. Advantageous embodiments of the floating bearing according to the invention and of the steering gear according to the invention are the subject of the respective dependent patent claims, and emerge from the following description of the invention. 
     The invention is based on the concept of improving a generic floating bearing, in particular with regard to the forces that must be imparted for the relative movement of inner ring with respect to outer ring, by virtue of the connecting part via which the inner ring is movably connected to the outer being of articulated form, such that the relative movement is based substantially on a rolling or sliding motion of the inner ring and outer ring on one another. The guidance of the relative movement is thus performed by the inner ring and the outer ring themselves. The use of a flexible material, from which the entire connecting part is formed in the case of the floating bearing as per DE 10 2007 055 814 A1, can thus be reduced to a sleeve which, essentially, need merely ensure that the inner ring and outer ring are held together, and which can be distinguished by low deformation forces. 
     This basic concept of the invention is implemented in structural terms in that a generic floating bearing for a steering gear, said floating bearing having an inner ring for (directly or indirectly) receiving a pinion shaft of the steering gear and having an outer ring, which surrounds the inner ring, for the mounting of the floating bearing in a housing of the steering gear, wherein the outer ring and the inner ring form an annular gap between them, and wherein the outer ring and the inner ring are connected to one another by way of at least one flexible connecting part so as to be movable relative to one another in at least one radial direction, is improved in that the outer ring and/or the inner ring form(s), within the connecting part, a (at least one) projection which projects into the annular gap such that the spacing between outer ring and inner ring is smaller in the region of this projection then in the remaining section of the annular gap. 
     Here, the projection may preferably have a (two-dimensionally or three-dimensionally) curved surface profile. 
     In a preferred refinement of the floating bearing according to the invention, it may furthermore be provided that the inner ring and the outer ring each form a (at least one) projection, said projection being situated radially opposite one another. 
     It may preferably be provided that the inner ring and the outer ring are partially or entirely surrounded by a flexible, and preferably elastic, material (for example elastomer) in the region of the projection. The flexible material may in particular form a sleeve which surrounds the inner ring and the outer ring in the region of the projection or projections, which sleeve ensures that the inner ring and outer ring are held together but at the same time impedes the relative movement between these to the least possible extent. 
     A “flexible material” is to be understood to mean a material which can be deformed by forces (considerably) lower than the forces that would cause the inner ring and the outer ring to be correspondingly deformed. In particular, the functional deformability of the “flexible material” is realized under the action of forces which, in the case of generic steering gears, lead to pivoting of the pinion shaft and to the resulting relative movement of inner ring and outer ring. 
     An “elastic material” is to be understood to mean a material whose modulus of elasticity is (considerably) lower than the modulus (moduli) of elasticity of the material(s) of the inner ring and outer ring. In particular, the modulus of elasticity of the “elastic material” is so low that, under the action of forces that cause pivoting of the pinion shaft in the case of generic steering gears, said elastic material leads to the functional deformation and to the relative movement, which is permitted as a result, of the inner ring and outer ring. 
     In a further preferred embodiment of the floating bearing according to the invention, it may be provided that the inner ring (preferably on its inner side) and/or the outer ring (preferably on its outer side) form(s) the at least one depression in which the flexible material is arranged. In this way, it is firstly possible for a positively locking connection to be formed between the inner and/or outer ring and the flexible material itself, which positively locking connection has the effect that the inner ring and outer ring are held together in an improved manner. Furthermore, by means of the arrangement of the flexible material in the depressions, the integration of the floating bearing into the housing of the steering gear can be simplified, in particular if—as is preferably provided—the flexible material does not protrude beyond the edge of the depression(s). 
     It may preferably be provided that the inner ring and the outer ring of the floating bearing according to the invention make contact in the region of the projection or projections, whereby said inner ring and outer ring roll or slide directly on one another during the relative movement. 
     It is however also possible for a (thin) layer of flexible material, which may also be the same material as that which surrounds the inner ring and the outer ring, to be provided in the region of the projection between the inner ring and the outer ring. Here, the thickness of said layer at its thinnest point, which is the point at which the spacing between the inner ring and the outer ring is at its smallest, it is preferably at most 0.5 mm, particularly preferably at most 0.3 mm. Depending on the flexible material, a layer of said type can reduce the friction between the inner and outer rings, which then roll or slide only indirectly on one another. A thin layer of said type can be produced in a simple manner by virtue of the sleeve that surrounds the inner and outer ring being produced by way of an insert molding process with the flexible material, wherein—owing to play between the inner ring and outer ring and/or owing to a deformation of the outer ring and/or inner ring under the pressures that prevail during the injection molding process—the spacing corresponding to the layer thickness is set between the inner ring and outer ring in the region of the one or more projections. 
     A steering gear according to the invention comprises at least one gearwheel, a pinion that meshes with said gearwheel, and a pinion shaft that comprises the pinion, wherein the pinion shaft is mounted, on one side of the pinion, in a fixed bearing which forms for the pinion shaft a pivot axis lying perpendicular to the longitudinal axis of the pinion shaft, and wherein the pinion shaft is mounted, on the other side of the pinion, in a floating bearing according to the invention. 
     In a preferred embodiment of the steering gear according to the invention, the inner ring and/or the outer ring have/has at least one projection which, for the preferably positive positioning of the floating bearing in a housing of the steering gear, engages into a depression, which is preferably of complementary form, of the housing. Here, it is particularly preferably provided that the projection extends in the axial direction of the inner ring or of the outer ring. The production of the depression, which is complementary thereto, in the housing can thus be simplified. 
     The invention will be explained in more detail below on the basis of an exemplary embodiment illustrated in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a steering gear according to the invention in a first cross section; 
         FIG. 2  shows the steering gear from  FIG. 1  in a second cross section; 
         FIG. 3  shows the function of the steering gear of  FIGS. 1 and 2  in a schematic illustration; 
         FIG. 4  shows, in a plan view, the floating bearing used in the steering gear of  FIGS. 1 and 2 ; 
         FIG. 5  shows the floating bearing of  FIG. 4  in a longitudinal section along the section plane V-V in  FIG. 4 ; 
         FIG. 6  shows the floating bearing of  FIGS. 4 and 5  in a radial section along the section plane VI-VI in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  show the main constituent parts of an embodiment of a steering gear according to the invention. Said steering gear comprises a housing  1 , within which there are mounted a gearwheel  2  and a pinion that meshes with the gearwheel  2 . The pinion and a pinion shaft that comprises the pinion are configured integrally in the form of a worm  3 . 
     The gearwheel  2  is fastened fixedly to a steering column  4  of a motor vehicle (not illustrated). 
     The worm  3  has a drive-side end by which it can be connected to the drive output shaft of a drive (for example electric motor) that is not illustrated. In the region of said drive-side end, the worm  3  is mounted in the housing  1  by way of a first bearing arrangement. Said bearing arrangement comprises a fixed bearing  5  which allows the worm  3  to pivot about a pivot axis  6 . 
     Said pivoting causes a deflection of the free end, situated opposite the drive-side end, of the worm  3 , said worm being mounted at said free end by means of a second bearing arrangement in receptacle of the housing  1 . The second bearing arrangement comprises a floating bearing  7  according to the invention, which is designed so as to permit the deflection of the free end of the worm  3 . 
     The bearing arrangements of the worm  4 , both in the region of the fixed bearing  5  and also in the region of the floating bearing  7 , comprise a respective rolling bearing  8 ,  9 , in which the corresponding sections of the worm  3  are mounted substantially without play. The rolling bearings  8 ,  9  themselves are mounted in a respective inner ring  10 ,  11  of the fixed bearing  5  and of the floating tearing  7  respectively. The fixed bearing  5  and floating bearing  7  furthermore comprise a respective outer ring  12 ,  13  surrounding the associated inner ring  10 ,  11 , which outer rings are in turn arranged substantially without play in a respective corresponding receptacle of the housing  1 . The connection between the inner ring  10 ,  11  and the associated outer ring  12 ,  13  is implemented in structural terms such that—in the case of the fixed bearing  5 —said connection permits the pivoting of the worm  3  about the pivot axis  6  and—in the case of the floating bearing  7 —said connection permits the deflection of the free end of the worm  3 . 
     For this purpose, in the case of the fixed bearing  5 , the outer ring  12  is connected to the inner ring  10  via two webs  14  (composed for example of spring steel), wherein the two webs  14  run substantially collinearly and thus form the pivot axis  6  about which the outer ring  12  can be pivoted relative to the inner ring  10 . Here, the webs  14  and thus the pivot axis  6  run not through the center of the fixed bearing  5  but so as to be radially offset relative thereto (cf.  FIG. 3 ). The pivot axis  6  thus does not intersect the longitudinal axis  15  of the worm  3 . 
     Owing to said radial offset of the webs  14  with respect to the centre of the fixed bearing  5 , the pivot axis  6  is relocated into the vicinity of the outer circumference of the pinion of the worm  3 , whereby the generation of reaction moments (M z ), such as arise or would arise owing to the toothing forces (F a1 , F a2 ) acting at the toothing engagement between pinion and gearwheel in conjunction with the spacing between the line of action of the toothing forces (F a1 , F 2 ) and the pivot axis  6 , can be reduced or eliminated. For the most complete possible elimination of the reaction moments (M z =0), it is provided that the pivot axis  6  lies within the tangential plane formed at the contact point between the two pitch circles or rolling circles of gearwheel  2  and pinion (“toothing engagement”  16 ), as is schematically illustrated in  FIG. 3 . 
       FIGS. 4 to 6  show the floating bearing  7  according to the invention of the steering gear on its own. A main function of the floating bearing  7  is that of mounting the worm within the housing  1  in such a manner as to ensure the movement (deflection) of the free end of the worm  3  owing to the pivoting of the latter about the pivot axis  6  formed in the fixed bearing  5 . For this purpose, in unloaded neutral position of the floating bearing  7 , the inner ring  11  is arranged concentrically within the outer ring  13  with a substantially constant spacing. In one section, the inner ring  11  is connected to the outer ring  13  via a radially running connecting part  17 . In the region of the connecting part  17 , and as viewed in radial section (cf.  FIG. 6 ), the inner ring  11  and the outer ring  13  have a respective projection  18  running in curved fashion, which projections project—opposite one another—into the annular gap  19  formed between the inner ring  11  and the outer ring  13 . Here, the tips of the two projections  18  make contact. 
     In the region of the projections  18 , the inner ring  11  and the outer ring  13  are surrounded by an elastomer sleeve  20 . Here, the elastomer sleeve  20  is configured so as not to protrude beyond the outer surfaces of the inner ring  11  and outer ring  13 . For this purpose, it is provided firstly that the axial length of the inner ring  11  and of the outer ring  13  is smaller in the region of the projections  18  than in the remaining sections. In this way, depressions  21  are formed in which the elastic material of the elastomer sleeve  20  is received (cf.  FIG. 5 ). The ring  13  is furthermore provided, in the middle of its axial length, with a radial opening in which the material of the elastomer sleeve likewise arranged (cf.  FIG. 5 ).  FIG. 6  furthermore shows that the inner ring  11 , on its inner side that receives the worm  3 , and the outer ring  13 , on its outer side by which the floating bearing  7  is mounted in the associated receptacle of the housing  1 , each form a further depression  22  which runs in the axial direction and in which the material of the elastomer sleeve  20  is likewise received. Owing to the arrangement of the material of the elastomer sleeve  20  in the depressions  21 ,  22 , it is possible to prevent said material from protruding beyond the correspondingly adjacent regions of inner ring  11  and outer ring  13 , whereby the integration of the floating bearing  7  into the steering gear is simplified. 
     It is also the case in the region of the annular gap  19  that the projections  18  are surrounded on both sides (in the circumferential direction) by elastic material  23 . Said elastic material is preferably the same material as that from which the elastomer sleeve  20  is formed. The elastic material  23  in the region of the annular gap  19  has noise effect, in conjunction with the elastomer sleeve  20 , of holding the inner ring  11  and outer ring  13  together. The elastomer sleeve  20  and the elastic material  23  arranged in the annular gap  19 , can be produced in one working step, for example by (two-component) injection molding. 
     The connecting part  17  forms a pivot joint with a low pivoting moment, which allows the outer ring  13  to pivot relative to the inner ring  11  about a pivot axis lying in the region of the connecting part  17 . During said pivoting movement, the outer ring  13  and the inner ring  11  slide or roll on one another in the sections that make contact, while the elastomer sleeve  20  and the elastic material  23  provided in the annular gap  19  do not significantly impede said relative movement of outer ring  13  and inner ring  11  but at the same time ensure that said outer ring and inner ring are held together. 
     The inner ring  11  of the floating bearing  7  also has a collar  24 . Said collars serves for the axial positioning of the rolling bearing  9 , and thus of the worm  3 , in the housing  1 . 
     Furthermore, the outer ring  13  of the floating bearing  7  also has, at one point on its circumference, a positioning projection  25  that protrudes in an axial direction. Said positioning projection engages into a corresponding recess of the housing  1  (cf.  FIG. 2 ). Positive positioning of the floating bearing  7  in the housing  1  is ensured in this way. Furthermore, the positioning projection prevents rotation of the outer ring  13  in the housing  1 . 
     Provision is made for the floating bearing  7  to be integrated into the housing  1  of the steering gear such that the pivoting of the worm  3  about the pivot axis  6  formed in the fixed bearing  5  leads to a displacement of the inner ring  11  relative to the outer ring  13  in a direction substantially perpendicular to the radial axis  26  extending through the connecting part  17 . By contrast, in the direction perpendicular to said direction, that is to say in the direction of the radial axis  26 , extending through the connecting part  17 , of the floating bearing  7 , there should be as far as possible no possibility for displacement of the outer ring  13  with respect to the inner ring  11 , in order to prevent interlocking of the pinion of the worm  3  on the gearwheel  2  during operation of the steering gear. This is achieved in that the spacing between the inner ring  11  and the outer ring  13  in that section of the floating bearing  7  which situated radially opposite the connecting part is restricted to a small dimension of for example 0.1 mm, and at most 0.5 mm, and preferably at most 0.3 mm. This is achieved by means of a delimiting element  27  which is mounted within the outer ring  13  so as to be displaceable in the radial direction. The delimiting element  27  has a radial width greater than the radial width of the outer ring  13 , wherein, in the ease of a floating bearing  7  that has not yet been installed in the housing  1 , the delimiting element  21  can be displaced so far outward that it protrudes beyond the outer surface of the outer ring  13 . In this way, a relatively large spacing is set between the outer side of the inner ring  11  and the delimiting element  27 . Said spacing is reduced to the desired small dimension during the installation of the floating bearing  7  in the housing  1  because then, as a result of a collision with the housing  1 , the delimiting element  27  can no longer protrude beyond the outer surface of the outer ring  13 , and must consequently be displaced in the direction of the inner ring  11 . 
     The outer ring  13  of the floating bearing  7  furthermore comprises an elastic abutment element  28  which restricts a movement of the inner ring  11  during a pivoting movement or the worm  3  in one direction (upward in  FIG. 1 ). Said elastic abutment element  28 , which may for example be in the form of a metal spring or in the form of a combination of metal spring and elastomer spring, may also be integrated into the inner ring  11 . 
     The steering gear according to the invention realizes compensation of gear play such as arises in particular owing to manufacturing tolerances of the components of the steering gear, owing to different thermal expansions of said components and owing to wear. Said gearing play compensation is achieved by virtue of the worm  3  being mounted in pivotable fashion and being pressed against the gearwheel  2  under spring loading. The spring load is realized—in the same way as the formation of the pivot axis  6 —in particular by the webs  14  of the fixed bearing  5 , which webs are twisted, and thus act functionally as torsional spring bars, as a result of the relative rotation of outer ring  12  with respect to the inner ring  10 . To attain a spring preload sufficient to compensate for the increasing level of wear over the service life of the steering gear, provision is made for the worm  3  to be equipped with a relatively large pivot angle in the newly manufactured state of the steering gear. For this purpose a provision is made for an the worm  3  to be integrated into the housing  1  such that the longitudinal axis  15  of said worm has a slightly smaller spacing to the toothing engagement  16 , or to the line of action of the toothing forces (F a1 , F a2 ), in the region of the fixed bearing  5  than in the region of the floating bearing  7  (cf.  FIG. 1 ). This offset is realized by means of a corresponding offset of the inner ring  10  with respect to the outer ring  12  of the fixed bearing  5 ; these are thus arranged so as not to be exactly concentric. It is made possible in this way for the fixed bearing  5  and the floating bearing  7 , despite the pivot angle provided, be arranged in concentrically arranged receptacles of the housing  1 . The manufacture of the housing  1  can be simplified in this way. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Housing 
           2  Gearwheel 
           3  Worm 
           4  Steering column 
           5  Fixed bearing 
           6  Pivot axis 
           7  Floating bearing 
           8  Rolling bearing 
           9  Rolling bearing 
           10  inner ring of the fixed bearing 
           11  Inner ring of the floating bearing 
           12  Outer ring of the fixed bearing 
           13  Outer ring of the floating bearing 
           14  Web 
           15  Longitudinal axis of the worm 
           16  Toothing engagement 
           17  Connecting part 
           18  Projection 
           19  Annular gap 
           20  Elastomer sleeve 
           21  Depression 
           22  Depression 
           23  Elastic material 
           24  Collar 
           25  Positioning projection