Abstract:
A halved sliding bearing is provided, two of which are paired into a cylindrical shape. The halved sliding bearing includes a steel back metal, and a bearing alloy layer, which serves as a sliding surface, on the inside of the steel back metal. A coat layer of Bi or a Bi-based alloy is formed on the outside back surface of the steel back metal. Preferably, the coat layer consists of 1 to 30 mass % of one or more of Sn, Pb, In, Ag and Cu and the balance being Bi and inevitable impurities.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a halved sliding bearing, two of which are paired into a cylindrical shape and which has a bearing alloy layer, serving as a sliding surface, on the inside of a steel back metal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventionally, for a halved sliding bearing, two of which are paired into a cylindrical shape and which has a bearing alloy layer, serving as a sliding surface, on the inside of a steel back metal, a flash plating layer (also referred to as “coat layer”) has been deposited so as to coat the outer back surface of the steel back metal opposing to the inner peripheral surface on which the bearing alloy layer is formed and side edges thereof or the whole surface of steel back metal, as described e.g. in JP-A-6-74238 (see claim  6 , paragraph [0006]) and JP-A-2002-513890 (see paragraph [0016]). The flash plating layer is deposited to prevent the steel back metal from corrosion and to give the steel back metal a bright and attractive appearance. As the flash plating layer, a plating layer of Sn, Pb or an alloy thereof having a thickness of 0.1 to 10 μm is used. 
         [0003]    The above-described halved sliding bearing  1 ′ is fitted on the inner surface of a bearing housing, for example, comprising a connecting rod  2  connected to a crankshaft of an internal combustion engine and a connecting rod cap  3 , as shown in  FIG. 2 . The bearing housing comprising a connecting rod  2  and of the connecting rod cap  3  is subjected to repeated stress of compression and tension by dynamic load during the internal combustion engine operation. In particular, rigidity of the connecting rod has been lowered since the weight of internal combustion engine is reduced in recent years. In the case where the elastic deformation of the bearing housing increases with the decrease in rigidity, a relative slide occurs between the back surface of the halved sliding bearing  1 ′ and the inner surface of the bearing housing. Furthermore, heat is generated on the inner surface of the halved sliding bearing  1 ′ due to friction caused by sliding with a shaft. If the conventional coat layer of Sn, Pb or an alloy thereof melts accordingly, the coat layer flows under a bearing back surface pressure (radial stress) caused by an interference for fixing the halved sliding bearing  1 ′ to the bearing housing or stress caused by sliding, and it aggregates locally at a low-pressure portion as deformedly shown in  FIGS. 3A and 3B . Since the volume of Sn, Pb or the alloy thereof increases when melt, so that the flow amount thereof increases. Therefore, the volume of an aggregating part increases locally, which swells the sliding surface of the halved sliding bearing  1 ′ to the inner surface side. Therefore, there arises a problem that the halved sliding bearing is liable to come into strong contact with the shaft. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention has been made to solve the above problems, and accordingly an objective thereof is to provide a sliding bearing in which a coat layer formed on the outside back surface of a steel back metal of the halved sliding bearing forms minimized aggregation locally during an internal combustion engine is operated, and therefore a strong contact with a shaft is less liable to occur. 
         [0005]    To achieve the above objective, the present invention provides a halved sliding bearing, two of which are paired into a cylindrical shape and which has a bearing alloy layer, serving as a sliding surface, on the inside of a steel back metal, wherein a coat layer of Bi or a Bi-based alloy is formed on the outside back surface of the steel back metal. 
         [0006]    According to the invention, although Bi or the Bi-based alloy flows due to a back surface pressure or a relative slide between the back surface of the halved sliding bearing and the inner surface of a bearing housing, the volume thereof decreases when melting, so that the flow amount is small. Since the volume of the aggregation is small, a strong contact caused by a direct contact with the shaft due to swelling of the sliding surface of the halved sliding bearing to the inner surface side is less liable to occur. While the thickness of the coat layer is preferably as small as possible in order to reduce the flow amount of the coat layer, the thickness is preferably in a range of 0.1 to 10 μm, further preferably in a range of 0.1 to 5 μm in order to prevent rust of the steel on the back surface of the halved sliding bearing from manufacture of the halved sliding bearing until incorporation in the halved bearing housing. 
         [0007]    The coat layer of the Bi-based alloy preferably consists of 1 to 30 mass % of one or more of Sn, Pb, In, Ag and Cu, and the balance being Bi and inevitable impurities. 
         [0008]    The effect for preventing rust is further improved when  1  to  30  mass% of one or more of Sn, Pb, In and Ag is contained in Bi for alloying, although a coat layer have the effect of preventing rust of steel on the back surface of the halved sliding bearing from manufactured until incorporated in the halved bearing housing. Furthermore, when the Bi-based alloy containing 1 to 30 mass % of one or more of Sn, Pb, In and Ag in Bi melted, the volume thereof decreases, and therefore the flow amount becomes small. Since the volume of the aggregating part is small, a strong contact, which is caused by swelling of the sliding surface of the halved sliding bearing to the inner side to directly contact with the shaft, is less liable to occur. If the content is lower than 1 mass %, further rust preventive effect cannot be obtained, and if the content exceeds 30 mass %, the property that the volume of the Bi-based alloy decreases when melting is diminished since the alloying components of Sn, Pb, In and Ag have the property that their volumes increase when melt. Especially, if the content of Sn, Pb, In and Ag exceeds 50 mass %, the property is nearly lost. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a schematic partial sectional view of a halved sliding bearing according to the present embodiment; 
           [0010]      FIG. 2  is a front view showing the relationship between a halved sliding bearing and a connecting rod; 
           [0011]      FIG. 3A  is schematic front view deformedly showing an aggregating part formed between a back surface of a halved sliding bearing and an inner peripheral surface of a bearing housing; and 
           [0012]      FIG. 3B  is an enlarged view of a portion circled in  FIG. 3A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    An embodiment of the present invention will be described below. A flat plate-shaped multilayer bearing material composed of a steel back metal  6  and a bearing alloy  7  is press-formed to manufacture a halved sliding bearing  1  so that the steel back metal  6  forms an outer peripheral surface like the above-mentioned halved sliding bearing  1 ′ shown in  FIG. 2 . Thereafter, a coat layer  8  of Bi or a Bi-based alloy is formed on the back surface of the steel back metal  6  of the halved sliding bearing  1  by the electroplating process or the like. The method for forming the coat layer  8  of Bi or the Bi-based alloy on the back surface of the halved sliding bearing  1  is not limited to the electroplating process, and it can be formed by any other general coating methods such as a spraying method and a thermal spraying method. Alternatively, to improve bonding strength between the back surface of the steel back metal  6  and the coat layer  8  of Bi or the Bi-based alloy, the coat layer  8  may be formed after a general preliminary treatment such as degreasing or surface roughening, or the coat layer  8  may be formed after an intermediate layer of a metal such as Ag, Cu or an alloy thereof has been formed on the back surface of the steel back metal  6 . The coat layer  8  may be formed only on the outer back surface of steel back metal  6  of the halved sliding bearing  1  excluding the inner sliding surface of the halved sliding bearing  1 . However, in view of the productivity, the coat layer  8  may be formed also on the inner sliding surface at the same time. 
         [0014]    To reduce the flow amount of the coat layer  8 , the thickness of the coat layer  8  is preferably as small as possible. However, the thickness of the coat layer  8  is preferably in a range of 0.1 to 10 μm, further preferably 0.1 to 5 μm in order to prevent rust of the steel on the back surface of the halved sliding bearing  1  from manufacture of the halved sliding bearing  1  until incorporation in a halved bearing housing. The rust preventive effect is further improved by alloying Bi with 1 to 30 mass % of one or more of Sn, Pb, In and Ag. 
         [0015]    Next, the results of a bearing test on the halved sliding bearing  1  manufactured as described above are explained with reference to Tables 1 and 2. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Depth of 
               
               
                   
                   
                   
                 partial wear 
               
               
                   
                   
                 Thickness of 
                 of bearing 
               
               
                   
                 Composition 
                 aggregating 
                 sliding 
               
               
                 No. 
                 of coat layer 
                 part [μm] 
                 surface [μm] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 Bi 
                 1 
                 0 
               
               
                 Example 2 
                 Bi—2 mass % Sn 
                 1 
                 0 
               
               
                 Comparative 
                 Pb 
                 7 
                 5 
               
               
                 example 11 
               
               
                 Comparative 
                 Sn 
                 10 
                 7 
               
               
                 example 12 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Test condition 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Test time 
                 200 hours 
               
               
                   
                 Load 
                 Full load 
               
               
                   
                 Number of revolutions 
                 6500 rpm 
               
               
                   
                 Operating condition 
                 Continuous operation 
               
               
                   
                   
               
             
          
         
       
     
         [0016]    In examples 1 and 2 in Table 1, a multilayer material composed of the steel back metal  6  and the Al-based bearing alloy  7  was press-formed to manufacture the halved sliding bearing  1  having an outer diameter of 48 mm, an inner diameter of 45 mm, and a width of 21 mm so that the steel back metal  6  formed an outer peripheral surface. Next, the coat layer  8  having a composition of Bi or the Bi-based alloy (Bi-2 mass % Sn) given in Table 1 was formed on the back surface of the steel back metal  6  of the halved sliding bearing  1  by the electroplating process so that the thickness of the coat layer  8  was 3 μm. The cross-sectional structure of the halved sliding bearing  1  is shown in  FIG. 1 . 
         [0017]    In comparative examples 11 and 12, the coat layer having a composition of Pb (comparative example 11) or Sn (comparative example  12 ) was formed on the back surface of the halved sliding bearing  1 , which was manufactured under the same conditions as those in examples 1 and 2 by the electroplating process so that the thickness of the coat layer was 3 μm. In examples 1 and 2 and comparative examples 11 and 12, the outer diameter of the halved sliding bearing  1  was made slightly larger than the inner diameter of the bearing housing used for the bearing test by an interference for fixing the sliding bearing. 
         [0018]    Two halved sliding bearings in each of examples 1 and 2 and comparative examples 11 and 12 configured as described above were paired, and inserted into a split-type connecting rod  2  and a connecting rod cap  3  of an internal combustion engine and then fastened by bolts. In this state, the connecting rod  2  and the connecting rod cap  3  were mounted in the internal combustion engine to conduct the bearing test. The bearing test was conducted under the test conditions given in Table 2 by using an inline four-cylinder engine having a displacement of 2000 cc as the internal combustion engine. Since the housing rigidity is thought to be lowest and the relative slippage is the largest in a transverse direction through a center of the bearing, when the direction is taken as 0° in the direction, aggregation is most likely to occur in the vicinity of 45° and 135° (refer to  FIG. 3A ). The thickness of an aggregating part  4  of Bi, the Bi alloy, Sn and Pb in those portions in examples 1 and 2 and comparative examples 11 and 12 and the depth of partial wear of bearing sliding surface caused by the aggregating part  4  were compared and evaluated. The evaluation results are given in Table 1. The aggregating part  4  was cut and the cross-section was photographed by a stereoscopic microscope to measure the thickness of the aggregating part  4  and the depth of partial wear. The thickness of the aggregating part  4  was determined as a numerical value obtained by subtracting the thickness of coat layer  8  before evaluation from the measured value. 
         [0019]    In comparative examples 11 and 12, Sn or Pb melts and flows at a portion in which relative slippage between the inner diameter surface of the connecting rod  2  and the back surface of the halved sliding bearing  1  is increased in the bearing test, so that the local aggregating part  4  is formed on the back surface of the halved sliding bearing  1 . Since the volume of Sn or Pb increases when melting, the flow amount increased, and therefore the local aggregating part  4  became as thick as 7 μm (comparative example 11) or 10 μm (comparative example 12). Furthermore, the aggregating part  4  deforms the sliding surface of the halved sliding bearing  1  as to swell to the bearing inner diameter side, so that a clearance between the sliding surface of the halved sliding bearing  1  and a shaft becomes narrowed, and therefore the sliding surface of the halved sliding bearing  1  comes into direct contact with the shaft. Therefore, the depth of partial wear of the inner surface of bearing became as large as 5 μm (comparative example 11) or 7 μm (comparative example 12). 
         [0020]    On the other hand, in examples 1 and 2, Bi or the Bi-based alloy melts and flows at a portion in which relative slippage between the inner diameter surface of the connecting rod  2  and the back surface of the halved sliding bearing  1  is increased, so that the local aggregating part  4  was formed on the back surface of the halved sliding bearing  1 . However, since the volume of Bi or the Bi-based alloy decreases when melting, the flow amount decreases, and therefore the aggregating part  4  having a thickness as small as about 1 μm was formed in both of examples 1 and 2. This small aggregating part  4  was relaxed by a gap between the inner surface of the halved sliding bearing  1  and the mating shaft, so that the sliding surface of the halved sliding bearing  1  does not come into direct contact with the mating shaft. Therefore, it was found that the depth of wear was 0 μm in both of examples 1 and 2, and partial wear did not occur. Among the Bi-based alloys with which the bearing test was conducted, only example 2 was given in Table 1. However, the present inventors verified that other Bi-based alloys comprising Bi and 1 to 30 mass % of one or more of Sn, Pb, In, Ag and Cu also had a property that the volumes thereof decreased when melting like Bi. 
         [0021]    The present invention is not limited to the halved sliding bearing used for the connecting rod of an internal combustion engine shown in example, and can be applied to a halved sliding bearing used by being incorporated in a split-type bearing housing for any other applications.