Abstract:
The invention relates to an assembly providing a crankshaft having a shaft section that is rotationally supported by a rolling element bearing having an outer ring that is radially divided into two ring halves. To reduce friction and accommodate misalignment, the invention proposes that the bearing which supports the shaft section is a toroidal roller bearing.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a crankshaft assembly comprising one of more shaft sections which are rotationally supported by a rolling element bearing. 
       BACKGROUND 
       [0002]    A crankshaft of a combustion engine, for example, is supported in the engine housing by means of main crankshaft bearings, which are generally sliding bearings. That is, the number of shaft sections by which the crankshaft is supported in the housing has a corresponding number of sliding bearing surfaces in the housing. 
         [0003]    It is also known to support the crankshaft in the engine housing by means of rolling element bearings, such as deep groove ball bearings or cylindrical roller bearings. 
         [0004]    The advantage of using roller bearings instead of sliding bearings for supporting the crankshaft in the engine housing is that the friction during rotation of the crankshaft can be reduced. On the other hand, the application of rolling element bearings can be detrimental, since misalignment of the bearing arrangement and/or offsets in the shaft sections of the crankshaft due to the crankshaft loading can lead to a significant reduction in bearing life. 
         [0005]    Furthermore, when roller bearings are used as main crankshaft bearings, they are prone to noise and vibrations. Due to the geometry of a crankshaft, the bearings have an outer ring consisting of two ring halves, which are split radially. In practice, there is always some radial mismatch at the interface between the two halves, leading to noise and vibration when the interface is over-rolled. 
         [0006]    Thus, it is an object of the present invention to propose a bearing solution for a crankshaft assembly, which has a low friction, which places less strict requirements on correct alignment of a single bearing location relatively to another and which minimizes the generation of noise and vibration. As a result, a long bearing life can be achieved. 
       SUMMARY OF THE INVENTION 
       [0007]    A solution according to the invention is characterized in that the bearing which supports at least one of the shaft sections of the crankshaft, relatively to a housing, is a toroidal roller bearing. 
         [0008]    The toroidal rollers of the bearing have a radius of curvature R with a defined relationship to a pitch circle diameter D of the bearing. Suitably, 1.5D&lt;R&lt;2.5D. In an advantageous embodiment, the radius of curvature R of the toroidal rollers is approximately equal to 2D, which had been found to minimize edge stresses at the edges of the rollers. 
         [0009]    The inner ring of the toroidal roller bearing can be formed directly by the shaft section of the crankshaft. In this case, the shaft section has at least partially a concave surface forming an inner raceway for the toroidal rollers. 
         [0010]    The outer ring of the toroidal roller bearing can be formed directly by a part of the housing, which then has an inner circumferential surface, concave in shape, that forms an outer raceway for the toroidal rollers. 
         [0011]    Alternatively, the outer ring of the toroidal roller bearing can be a separate part mounted in a bore of the housing. 
         [0012]    The outer ring consists of two ring halves which are split in a radial direction, and which may be joined together by means of e.g. screws. At an interface where the two ring halves meet, adjoining edges of the ring halves are provided with a curved profile. According to a further development of the invention, the curved profile has a logarithmic curvature, formed in an electrochemical machining (ECM) process. The logarithmic curvature minimizes edge stresses at the adjoining edges of the ring halves. 
         [0013]    The logarithmic curvature is defined between a start point at the interface and an endpoint on the outer raceway. The start point has a depth relative to the outer raceway that is at least equal to a maximum radial clearance of the bearing. The endpoint has a length relative to the interface, which is at least equal to twice the radial clearance. Consequently, the rollers do not encounter an edge, and noise and vibration are reduced. 
         [0014]    The toroidal roller bearing preferably has a maximum radial clearance of between 0.03 mm and 0.08 mm, specifically preferred between 0.045 mm and 0.065 mm. 
         [0015]    The rollers of the toroidal roller bearing are preferably held by a cage. The cage can consist of at least two split parts. The cage may be made from a metal material such as brass or of a polymer material such PEEK. 
         [0016]    Thus, the invention proposes to support the main bearing of a crankshaft with toroidal roller bearings (known under the trademark CARB from AB SKF) having a split outer ring with logarithmically curved edge profiles at the adjoining edges of the two ring halves. 
         [0017]    By this concept, a cost effective roller bearing is obtained with a high reliability. 
         [0018]    The following advantages are obtained with the proposed solution: 
         [0019]    The friction of the main bearings of the crankshaft assembly is reduced. In the case of a crankshaft of a combustion engine in a vehicle, this helps to reduce the fuel consumption. 
         [0020]    The bearing is able to take a dynamic combustion load; this is specifically relevant in the case of a Diesel engine which delivers high loads. The toroidal bearing is self-aligning, meaning that it runs in the optimized loading zone. Furthermore, crankshaft deflections due to the nature of the loading can be accommodated. 
         [0021]    The toroidal bearing also operates well with a relatively large radial clearance, which allows any radial offset between the two outer ring halves to be accommodated. 
         [0022]    Furthermore, the logarithmically curved profiles minimize edge stresses, as well as reducing noise and vibration. 
         [0023]    Consequently, an assembly according to the invention enables friction and noise reduction in combination with increased bearing life. 
         [0024]    The proposed design is suitable for combustion engines and compressors, and several of the bearings of the crankshaft bearing arrangement can be designed accordingly. 
         [0025]    Not all of the main crankshaft bearings need be executed as toroidal roller bearings, however. In an advantageous embodiment, one of the shaft sections is supported by a cylindrical roller bearing (CRB) or a spherical roller bearings (SRB). The advantage of such a bearing is that it can additionally provide axial thrust bearing functionality. 
         [0026]    Other advantages of the present invention will become apparent from the detailed description and accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The drawings show an embodiment of the invention. 
           [0028]      FIG. 1  is a side cross-sectional view through a part of a crankshaft assembly according to the invention, the crankshaft being rotatably supported in a split housing via a toroidal roller bearing. 
           [0029]      FIG. 2  is a front cross-sectional view of the part of the crankshaft assembly from  FIG. 1 , showing first and second halves of the split housing 
           [0030]      FIG. 3  is an exploded view of a detail from  FIG. 2 , showing an interface where the first and second halves of the housing adjoin. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    In the figures, an example of part of a crankshaft assembly is depicted, suitable for use in a vehicle combustion engine. The crankshaft  10  has several shaft sections  12 , depending on the number of cylinders of the engine. Each shaft section  12  is rotatably supported in a housing  20  by means of a rolling element bearing. According to the invention, at least one of the bearings which supports a shaft section  12  is a toroidal roller bearing  30 , comprising a set of toroidal rollers  31 . The toroidal rollers have a convex outer surface, with a radius of curvature R. 
         [0032]    In the depicted example, an inner ring of the bearing  30  is formed by the shaft section  12  itself. A radially outer surface of the shaft section has a correspondingly curved concave shape, and serves as an inner raceway  32  for the toroidal rollers  31 . The shaft section  12  is axially delimited by radially extending parts, known as crank webs  15 . The webs are depicted in  FIG. 1  in a truncated form and in reality extend much further. To enable assembly of the bearing  30 , an outer ring of the bearing and the housing are split radially into two halves, as best shown in  FIG. 2 . The housing has a first part  21  and a second part  22  that are joined together by means of e.g. bolts  50 . The two parts adjoin at a first interface  25 A and a second interface  25 B 
         [0033]    In this example, the housing  20  also serves as the bearing outer ring. Accordingly, the first and second housing parts  21 ,  22  have a radially inner surface that is convex in shape and which serves an outer raceway for the toroidal rollers  31 . The outer raceway therefore consists of a first portion  33 A and a second portion  33 B. The rollers are held in position by a cage  40 . To permit assembly, the cage is also of split design, whereby the two cages halves may be connected together after assembly, or may remain unconnected. The cage may be made of brass or a polymer material. 
         [0034]    The toroidal roller bearing allows a certain maximal axial displacement, in the present case about 3 mm Also a misalignment can be accepted of up to about 0.5°. The bearing is thus able to accommodate the misalignment which results from the dynamic loading of the crankshaft. 
         [0035]    In order to optimize the load distribution across each roller  31 , to prevent excessive edge loads at the axial extremities, the radius of curvature of the toroidal rollers  31  has a predefined value with reference to a pitch diameter D of the bearing  30 . The pitch diameter is the theoretical median diameter of the bearing, which passes through the center of the rollers  31 . Suitably, the radius of curvature R of the rollers is greater than 1.5 D and less than 2.5D. In the depicted example, R is approximately equal to 2D. 
         [0036]    The toroidal roller bearing  30  is able to operate with a relatively large radial clearance, the radial clearance being the maximum amount by which the bearing inner ring can be displaced in radial direction relative to the outer ring. In the depicted example, the bearing  30  has a radial clearance of approximately 50 microns. A relatively large radial clearance is beneficial for accommodating a radial offset between the first and second sections  33 A,  33 B of the outer raceway. 
         [0037]    A slight radial offset is practically inevitable when the split housing is reassembled. Consequently, there is a risk of noise and vibration as the rollers over-roll the first and second interfaces  25 A,  25 B, as well as a risk of edge stresses. According to the invention, these risks are mitigated by providing a curved profile at the edges of the first and second raceway sections  33 A,  33 B at the corresponding first and second interfaces. 
         [0038]    A detail of the first interface  25 A is shown in  FIG. 3 . As may be seen from this figure, a roller moving in the direction indicated by the arrow  60  encounters a height difference at the first interface  25 A. To prevent an impact with an edge of the second raceway portion  35 B, the edge  35  has a profiled surface defined between a start point  36  and an end point  37 . The start point of the edge profile  35  lies on the plane of the first interface  25 A and has a depth d relative to the second outer raceway portion  33 B that is at least equal to the radial clearance of the bearing. Consequently, the rollers  31  are unloaded at the interface, reducing noise and vibration. The end point  37  of the edge profile surface lies on the second outer raceway portion  35 B, and is situated at a length L from the start point  36  (in circumferential direction), which is at least equal to twice the radial clearance. Between the start and end points, the edge profile  35  has a logarithmic curvature, which minimizes edge stresses and also helps to reduce noise and vibration. The edge profile is created using an electrochemical machining processes, which enables the logarithmic curvature to be realized with precision. Suitably, each edge of the two ring halves is provided with such an edge profile, 
         [0039]    A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims. 
       REFERENCE NUMERALS 
       [0000]    
       
           10  Crankshaft 
           12  Shaft section of crankshaft 
           15  Crank webs 
           20  Housing 
           21  First housing part 
           22  Second housing part 
           25 A First interface between housing parts 
           25 B Second interlace between housing parts 
           30  Toroidal roller bearing 
           31  Toroidal rollers 
           32  Inner raceway 
           33 A First portion of outer raceway 
           33 B Second portion of outer raceway 
           35  Edge profile on outer raceway portion 
           36  Start point of edge profile 
           37  End point of edge profile 
           40  Cage 
           50  Bolts 
           60  Direction of rolling