Abstract:
A bearing device having a bearing which rotatably supports a shaft, and a safety bearing ( 1 ) which receives the shaft in the event of a failure of the bearing. The aim of extending the service life of the safety bearing in the bearing device described above is achieved according to the invention in that a bearing race ( 2 ) of the safety bearing ( 1 ) facing the shaft comprises a lateral surface ( 3 ) having a convex contour ( 4 ) facing toward the shaft.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a bearing device. 
       BACKGROUND 
       [0002]    From practical experience, bearing devices are known in which a bearing supports a shaft so that it can rotate, wherein the bearing device has a safety bearing that holds the shaft if the bearing fails, so that the shaft can run down in the safety bearing. The bearing ring of the safety bearing facing the shaft has, during normal operation of the bearing device, a distance to the shaft and is mechanically unloaded. If the safety bearing is constructed as an anti-friction bearing or if the safety bearing comprises at least one anti-friction bearing, the row of these anti-friction bearings is not or only slightly mechanically pre-tensioned by an additional spring means. For example, bearing devices are known with a magnetic bearing and a safety bearing that holds the shaft supported in the magnetic bearing if there is a loss of power. 
         [0003]    If the shaft enters into the safety bearing, high forces and accelerations occur. In particular, if the safety bearing is made from anti-friction bearings and comprises at least one row of anti-friction bodies, there is the risk that the safety bearing will fail on the short term, because the anti-friction bodies are subject to high initial acceleration and slippage occurs due to the lack or only slight mechanical pre-tensioning of the anti-friction bodies. This slippage is associated with damage both to the anti-friction bodies and also of the bearing rings of the safety bearing, so that the safety bearing must be replaced often after only one use. 
         [0004]    In practice, the case often occurs that the shaft acts on the bearing ring at an angle with respect to the rotational axis of this bearing ring of the safety bearing facing the shaft. Likewise the case occurs that the shaft is not only tilted with respect to this rotational axis, but also enters the safety bearing with a bent shape. A reason for this can be the time offset release of the shaft in the bearing or the failure of only one sub-bearing of several sub-bearings of the bearing. Due to the tilting or bending of the shaft, a section of the bearing ring of the safety bearing holding the shaft is subject to a high application of a force, while a different section of this bearing ring is subject to only a small application of force. Especially in a safety bearing formed as a multi-row anti-friction bearing, one row of anti-friction bodies is subject to a strong loading that leads to slippage, while the other row of anti-friction bodies is subject to no loading. If the safety bearing comprises two adjacent anti-friction bearings that contact each other, one of the two anti-friction bearings is loaded strongly and the other only slightly. In both cases, there is strong wear in the safety bearing that might need to be replaced after only one use. 
         [0005]    DE 602 09 752 T2 describes a bearing device with a magnetic bearing and also a safety bearing, wherein the safety bearing comprises two axially adjacent, mutually contacting, mechanically pre-tensioned angular contact ball bearings. The two-piece lateral surface of the two-piece bearing ring of the safety bearing facing the shaft has, close to the end faces, a cylindrical form up to an edge rounding that is typical in anti-friction bearings and has a straight contour pointing parallel to the rotational axis of the two angular contact ball bearings. If the shaft enters into the safety bearing tilted or bent with respect to the rotational axis, the two angular contact ball bearings are loaded unequally, wherein especially strong slippage occurs in one of the two angular contact ball bearings. The safety bearing therefore has only a shortened service life. 
         [0006]    DE 197 29 450 C2 describes a bearing device for supporting a rotor so that it can rotate in a bearing, wherein two anti-friction bearings formed as a single-row ball bearing are provided as a safety bearing. Each bearing ring of the safety bearing facing the rotor shaft has a cylindrical lateral surface up to the typical edge rounding at the transition to the end face with an essentially straight contour running parallel to the rotational axis of the bearing ring. If the shaft enters tilted or bent into one of the two ball bearings, the ball bearing is loaded unequally and its service life is significantly shortened. 
       SUMMARY 
       [0007]    The objective of the invention is to prolong the service life of the safety bearing in the bearing device described in the introduction. 
         [0008]    According to the claim for the safety bearing of the bearing device mentioned in the introduction, this objective is met according to the invention in that a bearing ring of the safety bearing facing the shaft has a lateral surface with a convex contour facing the shaft. 
         [0009]    The convex contour is here provided in addition to an optionally present edge rounding of the bearing ring, wherein the edge rounding has only a slight extent in the axial direction (less than approx. 10%), but the convex contour extends in addition to the edge rounding by more than approx. 20% of the axial extent of the bearing ring. The convexity of the contour is here provided with respect to an imaginary cylindrical reference surface, wherein the cylindrical reference surface is assumed to be rotationally symmetric about the rotational axis of the bearing ring. 
         [0010]    The convex contour offers a defined contact point of the shaft in the bearing ring of the safety bearing facing the shaft, wherein this contact point is adjustable to the respective bearing device. Due to the convexity of the contour, this contact point lies between the two end faces of the bearing ring, for example, centrally between the two end faces of the bearing ring. Here, the resulting force when the shaft enters the bearing is distributed as uniformly as possible over the entire bearing ring and can be better received overall by the safety bearing. As a result, the slippage is reduced and the expected service life of the safety bearing is increased. 
         [0011]    If the bearing ring facing the shaft is formed as a bearing ring of a single-row anti-friction bearing, the resulting force when the shaft enters the bearing acts close to the track of the anti-friction bearing and thus, in particular, in the radial direction, so that axial force components are suppressed. 
         [0012]    In a preferred construction, if the bearing ring facing the shaft is formed as a joint bearing ring of an anti-friction bearing with at least two rows, the force received by the bearing ring is distributed largely uniformly to the at least two rows of the safety bearing. 
         [0013]    Advantageously it is provided that the contour of the bearing ring facing the shaft comprises a cylindrical section that extends over a maximum of approx. 80% of the axial extent of the bearing ring. The cylindrical section here makes possible the support of the shaft falling into the safety bearing on an enlarged surface area. The cylindrical section of the contour is here arranged shifted relative to the shaft with respect to the cylindrical reference surface defined above. 
         [0014]    Advantageously it is provided that the contour of the bearing ring facing the shaft has at least one conical section. With the cylindrical reference surface, the conical section here encloses an angle of less than approx. 15°, in particular, approx. 5°. 
         [0015]    One especially preferred contour is given when a middle third of the axial extent is formed by a cylindrical section and the two sections adjacent to the end faces are formed as conical sections that widen toward the end faces, wherein the two conical sections have an identical axial extent and each section has an angle of approx. 5° with respect to the cylindrical reference surface. 
         [0016]    Advantageously it is provided that the contour of the bearing ring facing the shaft has a curved section. Due to the curve, the resulting force when the shaft enters the safety bearing is distributed better over the bearing ring. 
         [0017]    With respect to the formation of the contour of the curved section of the convex contour facing the shaft, it is advantageously provided that the profile of the curve is given at least in some sections by a polynomial, in particular, by a quadratic function. 
         [0018]    Alternatively—or additionally with respect to a second curved section—it is advantageously provided with reference to the formation of the contour that the profile of the curve is given by a conical section, in particular, by a circular arc segment or a segment of an ellipse. A hyperbolic or parabolic profile of the curve could also be provided depending on for which operating case the safety bearing is to be optimized. 
         [0019]    Advantageously it is provided that the bearing ring facing the shaft is formed mirror symmetric to an axial parting plane. 
         [0020]    As an alternative, it can be advantageously provided that the bearing ring facing the shaft is formed asymmetric to an axial parting plane. Due to the asymmetry, the situation can be counted on that the tilting or bending of the shaft often has a typical angle, so that the forces can be distributed more uniformly on the bearing ring. 
         [0021]    The convex contour of the lateral surface of the bearing ring facing the shaft can be produced by material ablation starting from a cylindrical lateral surface with a straight cross-sectional contour. 
         [0022]    Advantageously it is provided that the contour of the bearing ring facing the shaft is formed by an additional coating. The coating can be applied, for example, with a wet chemical process on an initially cylindrical lateral surface, wherein the thickness of the coating varies in the axial direction so that the convex contour is formed. 
         [0023]    Advantageously it is provided that the contour of the bearing ring facing the shaft is formed by an additional component. The additional component can be, for example, a thin-walled sleeve whose outer cylindrical lateral surface is fixed, for example, by means of an interference fit on the inner, cylindrical lateral surface of the bearing ring, wherein the inner lateral surface of the sleeve has the desired convex contour. Such sleeves can be easily produced as sheet-metal parts. 
         [0024]    Advantageously it is provided that the convex contour facing the shaft extends along the entire axial extent of the bearing ring. Close to the end faces, the convex contour can replace the typical edge rounding or transition into the edge rounding. 
         [0025]    Additional advantages and features are given in the dependent claims and also in the following description of a preferred embodiment. 
         [0026]    The invention will be described and explained in more detail below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  shows, in a section view, a partially cutaway view of an embodiment of a bearing device according to the invention, and 
           [0028]      FIG. 2  shows, in the sub-figures a.) to d.), four modifications of the embodiment shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]      FIG. 1  shows a bearing device comprising a shaft that is not shown in the figure and is supported so that it can rotate in a bearing, namely a magnetic bearing. The bearing device further comprises a safety bearing  1  that is formed as a two-row angular contact ball bearing, wherein the shown safety bearing  1 , optionally together with at least one other safety bearing, holds the shaft if the magnetic bearing fails. 
         [0030]    The safety bearing  1  comprises a bearing ring  2  facing the shaft, namely the inner ring of the anti-friction bearing that has an inner lateral surface  3  that has a convex contour  4  that can be seen in cross section and that faces the shaft. Due to the convex contour  4 , the cross-sectional line is no longer straight as is the case for a cylindrical inner lateral surface, but instead extends toward the shaft. When the bearing device is operating normally and when the shaft is supported in the magnetic bearing, the convex contour  4  has a distance to the lateral surface of the shaft. The shaft then comes in contact with the convex contour  4  when the magnetic bearing fails or is turned off. 
         [0031]    The convex contour  4  comprises a cylindrical section  5  that is arranged centrally between the two end faces  6 ,  7  of the bearing ring  2 . The cross-sectional line is straight in the region of the cylindrical section  5 . The cylindrical section extends over approx. 60% of the entire axial extent of the bearing ring  2 , that is, over approx. 60% of the distance of the two end faces  6 ,  7  measured in the direction of the rotational axis of the bearing ring  2 . In one modification of the illustrated embodiment, the cylindrical section  4  can extend over up to approx. 80% of the entire axial extent of the bearing ring  2 , but not over less than approx. 40% of the axial extent. 
         [0032]    The convex contour  4  further comprises a first conical section  8  and also a second conical section  9 , wherein each of the two conical sections  8 ,  9  is adjacent to the cylindrical section  5  along an edge  10 ,  11  on the inner lateral surface  3 . In the area of the conical sections, the inner diameter of the inner lateral surface increases linearly toward the end faces  6 ,  7 . Both conical sections  8 ,  9  extend together over approx. 35% of the axial extent of the bearing ring  2 . With respect to a cylindrical reference surface that is given, for example, by the cylindrical section  5 , each of the two conical sections  8 ,  9  encloses an angle  12  of approx. 5% as is shown in  FIG. 1  for the first conical section  8 . 
         [0033]    Two edge rounding sections  13 ,  14  on the edges of the lateral surface  3  toward the two end faces  6 ,  7  are not part of the convex contour  4 . With the adjacent conical sections  8 ,  9 , the edge rounding sections  13 ,  14  form edges  15 ,  16 . The edge rounding sections  13 ,  14  extend together over a total of approx. 5% of the total axial extent of the bearing ring  2 , so that the convex contour  4  facing the shaft extends essentially along the entire axial extent of the bearing ring  2 . In one modified embodiment, the edge rounding sections  13 ,  14  can be left out, so that the convex contour, for example, the conical sections  8 ,  9 , are directly adjacent to the end faces  6 ,  7 . 
         [0034]    The bearing ring  2  facing the shaft—and thus the convex contour  4 —is mirror symmetric to an axial parting plane  17 . Furthermore, the bearing ring  2  facing the shaft is formed as a common bearing ring of the two-row angular contact ball bearing. 
         [0035]      FIG. 2  shows, in each of four sub-figures, a modification to the convex contour  4  shown in  FIG. 1  for the inner lateral surface  3  of the bearing ring  2  facing the shaft of the safety bearing  1 . It should be noted that the contour is shown enlarged in the direction toward the rotational axis of the bearing ring, in order to make it easier to see the profile of the contour. 
         [0036]      FIG. 2   a  shows a convex contour  34  that comprises a cylindrical section  35  and two conical sections  38 ,  39 , wherein the first conical section  38  encloses a first angle  42  and the second conical section  39  encloses a second angle  48  with a reference surface formed by the cylindrical section  35 . The angles  42 ,  48  can have different magnitudes and can each equal less than approx. 15°. The conical sections  38 ,  39  transition to the cylindrical section  35  along rounded edges  40 ,  41 . The cylindrical section  35  extends over approx. 45% of the entire axial extent of the bearing ring  32 . The convex contour  34  borders directly on the end faces  36 ,  37  of the bearing ring  2 , so that no edge rounding is provided. 
         [0037]      FIG. 2   b  shows a convex contour  64  that comprises a first conical section  68  and a second conical section  69  bordering this first section along a rounded edge  70 , wherein the conical sections  68 ,  69  border directly on the end faces  66 ,  67  of the bearing ring  62 . 
         [0038]      FIG. 2   c  shows a convex contour  94  that comprises a cylindrical section  95  that is arranged centrally between end faces  96 ,  97  and extends along approx. 80% of the total axial extent of the bearing ring  92 . The convex contour  94  comprises two curved sections  109 ,  110  that each describe a quarter circle and are thus formed as circular arc segments. The curved sections  109 ,  110  transition without an edge into the end faces  96 ,  97  or into the cylindrical section  95 . 
         [0039]      FIG. 2   d  shows a convex contour  124  whose single section is formed by a curved section  139  that extends between the two end faces  106  and  107 . The profile of the curve of the section  139  is defined by a quadratic function along the entire axial extent of the bearing ring  122 . 
         [0040]    Each of the modifications shown in  FIGS. 2   a  to  2   d  provides that the convex contour  34 ,  64 ,  94 ,  124  extends along the entire axial extent of the bearing ring  32 ,  62 ,  92 ,  122 , so that each provides no edge rounding. 
         [0041]    Each of the modifications shown in  FIGS. 2   a  to  2   d  shows convex contours  34 ,  64 ,  94 , and  124  that are formed symmetric to an imaginary axial parting plane of the respective bearing ring  32 ,  62 ,  92 ,  122 . It is understood that contours that are asymmetric to the parting plane can also be provided; for example, starting from the modification shown in  FIG. 2   a , the angles  42  and  48  are different or, starting from the modification shown in  FIG. 2   c , the curvature radii of the curved sections  109  and  110  deviate from each other. Furthermore, starting from the embodiment shown in  FIG. 2   d , the profile of the curve of the curved section  139  cannot be formed by a quadratic polynomial, but also by a polynomial with odd exponents. Furthermore, for forming the asymmetry it can also be provided that one end face borders a cylindrical section and the other end face borders a conical or curved section. In particular, the middle section  35  ( FIG. 2   a ) or  95  ( FIG. 2   c ) or the edge  70  ( FIG. 2   b ) can be shifted toward one of the two end faces. 
         [0042]    The invention was described above with reference to an embodiment in which the shaft was formed as a solid shaft and the bearing ring facing the shaft was the inner ring of a bearing, namely a two-row angular contact ball bearing. It is understood that the bearing ring facing the shaft can also be an outer ring of a bearing if the supported shaft is a hollow shaft. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1  Safety bearing 
           2  Bearing ring 
           3  Lateral surface 
           4  Convex contour 
           5  Cylindrical section 
           6  First end surface 
           7  Second end surface 
           8  First conical section 
           9  Second conical section 
           10  First edge 
           11  Second edge 
           12  Angle 
           13  Edge rounding 
           14  Edge rounding 
           15  Edge 
           16  Edge 
           17  Axial parting plane 
           32  Bearing ring 
           34  Convex contour 
           35  Cylindrical section 
           38  Conical section 
           39  Conical section 
           40  Edge 
           41  Edge 
           42  First angle 
           48  Second angle 
           62  Bearing ring 
           68  First conical section 
           69  Second conical section 
           70  Rounded edge 
           92  Bearing ring 
           94  Convex contour 
           95  Cylindrical section 
           96  End surface 
           97  End surface 
           109  First curved section 
           110  Second curved section
         122  Bearing ring     124  Convex contour     126  End surface     127  End surface     139  Curved section