Abstract:
First and second sliding parts of a sliding mechanism slide relative to each other. The first sliding part is held in a recess of a metal holding member, which is bounded by an inner wall that meets an open end of the recess along an edge. The first sliding part has a first side surface portion that contacts the inner wall in the recess away from the edge, and a second side surface portion displaced inwardly away from the inner wall so as not to contact the holding member at the edge. By this structure, generation of stress concentration on the sliding part can be avoided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sliding part having a sliding surface of which wear resistance is required, such as a shim used in a valve system of an internal combustion engine of a vehicle, and to a sliding mechanism including the sliding part. 
     2. Description of the Background Art 
     A material having high wear resistance has been used for a mechanical sliding part represented by an engine part of a vehicle, in order to minimize wear. Recently, a hard sliding member comes to be formed of a material such as a super hard alloy, or ceramics having superior wear resistance as compared with the steel material which has been conventionally used as a material for the sliding part. These materials, however, are generally difficult to process and are expensive. Therefore, it is a common practice to form not the entire part but only a sliding portion requiring high wear resistance by using such a material. 
     As a representative example, on an end surface of a valve lifter driving a tappet valve of a valve system in the internal combustion engine, a shim formed of a hard member is positioned, which shim exhibits superior wear resistance. 
     As an example of a sliding mechanism for heavier load, a hard member is used at a tip end of a thrust bolt used for preventing inclination of a ring gear in a reduction mechanism of a vehicle. For example, Japanese Patent Laying-Open No. 8-109956 discloses means having superior durability and allows easy maintenance, for preventing inclination of the ring gear used in a reduction mechanism of a large vehicle such as a bus, a truck, a tractor or the like. 
     Referring to FIG. 10 a , a reduction mechanism  100  contains, in a differential carrier  101 , a propeller shaft  102  including a pinion, and an axle shaft  103  including a differential gear with a bearing (not shown) interposed. In a differential case  104 , a ring gear  105  is secured. Ring gear  105  transmits torque of propeller shaft  102  to axle shaft  103 . In order to prevent deflection of ring gear  105  when the transmitted torque increases, a tip end of a thrust bolt  106  is in contact with a rear surface  107  of ring gear  105 . 
     FIG. 10B is an enlarged view of a portion around thrust bolt  106  and rear surface  107  of ring gear  105 . A boss  108  is provided at a part of differential carrier  101 , thrust bolt  106  is screwed in boss  108 , and thrust bolt  106  is positioned by using a lock nut  109 . At a tip end of thrust bolt  106 , a sliding part  110  is mounted. 
     FIG. 10C shows, in further enlargement, the periphery of sliding part  110 . Sliding part  110  having a sliding surface crowned to have a convex shape and formed of silicon nitride or the like is mounted on a recessed portion  106   a  of thrust bolt  106 . Between the sliding surface of sliding part  110  and rear surface  107  of ring gear  105 , there is generally a clearance of δ. When an excessive torque is transmitted to reduction mechanism  100  and ring gear  105  deflects by more than δ when the vehicle starts or climbs a steep slope, thrust bolt  106  prevents inclination of more than δ. Therefore, during normal running, sliding part  110  does not contact rear surface  107  of ring gear  105 . 
     By the structure of the reduction mechanism, inclination of ring gear  105  by more than δ can be prevented, and therefore abnormal wear of ring gear  105  and the teeth surface of the pinion of propeller shaft  102  or damage to the teeth can be avoided. Further, as the sliding surface of sliding part  110  is crowned to have a convex shape, sliding part  110  is in smooth contact with the rear surface  107  of ring gear  105 , and therefore it is described that a force that would cause damage or displacement of sliding part  110  from the recessed portion  106   a  is hardly generated. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to employ a holding portion formed of metal having such a shape that prevents generation of stress concentration on the sliding part, when the holding portion holds the sliding part formed of ceramics. 
     The above described object can be attained by the sliding part of a sliding mechanism in accordance with the present invention in which one of sliding parts sliding relative to each other is held by a holding portion formed of metal, the sliding part being arranged protruding from a recessed portion provided in the holding portion, and a side surface of the sliding part being apart from an open end of an inner wall of the recessed portion so as not to be in contact with an edge of the open end. 
     As to the manner of holding the sliding part in the holding portion, the sliding part may be inserted loose in the recessed portion of the holding portion, or preferably, held by shrink fit or press fit. 
     If the side surface of the sliding part not in contact with the edge of the open end of inner wall at the recessed portion of the holding portion is formed linear or curved, stress concentration at the contact portion between the holding portion and the sliding part can be relaxed. 
     When the sliding member is formed of ceramics, preferably, the ceramics should be a silicon nitride based ceramics of which bending strength is preferably reinforced to 1000 MPa to 2000 MPa, so that the sliding member can withstand impact load. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross section of a sliding part in accordance with an embodiment of the present invention used in a valve system of an internal combustion engine. 
     FIG. 2 is a cross section of a conventional sliding part seated in a recessed portion of a holding portion of a valve system in a conventional internal combustion engine. 
     FIG. 3 is a cross section of a sliding part having a linearly inclined side surface in accordance with an embodiment of the present invention seated in a recessed portion of a holding portion of a valve system in an internal combustion engine. 
     FIG. 4 is a cross section of a sliding part having curved columnar side surface in accordance with an embodiment of the present invention seated in a recessed portion of a holding portion of a valve system in an internal combustion engine. 
     FIG. 5 is a schematic illustration of a test apparatus for a sliding part, simulating the valve system of an internal combustion engine. 
     FIG. 6 is a cross section of a conventional sliding part seated in a recessed portion of a holding portion of a mechanism for preventing inclination of the ring gear in a conventional reduction mechanism. 
     FIG. 7 is a cross section of a sliding part having a linearly inclined side surface in accordance with an embodiment of the present invention seated in a recessed portion of a holding portion of a mechanism for preventing inclination of the ring gear in the reduction mechanism. 
     FIG. 8 is a cross section of a sliding part having curved columnar side surface in accordance with an embodiment of the present invention seated in a recessed portion of the holding portion of a mechanism for preventing inclination of the ring gear in the reduction mechanism. 
     FIG. 9 is a schematic illustration of a dynamometer as an evaluating apparatus. 
     FIG. 10A is a cross section of an overall reduction mechanism of the prior art, FIG. 10B is a cross section representing positional relation between the thrust bolt and the ring gear, and FIG. 10C is a cross section of a sliding part seated in a recessed portion of the thrust bolt. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment in which the sliding part in accordance with the present invention is used in a valve system of an internal combustion engine of which contact force Q is relatively small, and an embodiment in which the sliding part is used in a mechanism for preventing inclination of the ring gear of which contact surface pressure is relatively high, such as a reduction mechanism of a vehicle, will be described in the following. 
     First Embodiment 
     FIG. 1 is a partial cross section representing behavior of a shim and a cam when the sliding part in accordance with an embodiment of the present invention is used in the valve system of an internal combustion engine. A valve  2  reciprocates along the profile of cam  1  with respect to the combustion chamber (not shown) of the engine, as cam  1  rotates. On an end surface of a valve lifter  4  slidably supported on a cylinder block  3 , a recessed portion  4   a  is provided, in which a shim  5  as the sliding part is seated. Shim  5  is a member for maintaining precision in opening and closing operation of valve  2 , by canceling accumulated error of parts such as cam  1  and valve lifter  4 , by the adjustment of its thickness. In order to facilitate changing of shim  5 , shim  5  is generally inserted loose on the inner wall  4   b  of recessed portion  4   a . An end portion of valve  2  secured by means of a cotter  7  on spring retainer  6  is constantly biased by a spring  8  to be in contact with an inside of valve lifter  4 . 
     FIG. 2 shows a conventional manner of placing shim  5  as the sliding part in the recessed portion  4   a  of valve lifter  4  as the holding portion formed of metal. Referring to FIG. 2, the inner wall  4   b  of the recessed portion  4   a  of valve lifter  4  rises vertically to reach open end  4   c . Side surface  5   a  of shim  5  is also a vertical column. Therefore, when there is generated a tangential force F by sliding friction between cam  1  and shim  5 , there is generated the stress concentration at the side surface  5   a  of shim  5  at the point P on edge  4   d  of open end  4   c  of valve lifter  4 . Even when the inside of edge  4   d  is chamfered, it simply means that the point P moves. When the stress concentration generates on the side surface  5   a  of shim  5 , valve lifter  4  formed of metal is deformed, whereas shim  5  formed of ceramics does not deform, and therefore shim  5  is fragile and prone to chipping. 
     In order to avoid such stress concentration, in the present embodiment, the side surface  5   a  of shim  5  as the sliding part includes a first side surface portion  5   a   1  that is adapted to be in contact with the inner wall  4   b , and a second side surface portion  5   a   2  that extends smoothly from the first side surface portion  5   a   1  and that is adapted not to be in contact with edge  4   d  of open end  4   c  of the recessed portion  4   a , as shown in FIGS. 3 and 4. Accordingly, even when there is generated the tangential force F, the contact point P is below the open end  4   c , and therefore edge  4   d  of inner wall  4   b  of open end  4   c  is not directly brought into contact with side surface  5   a  of shim  5 . Therefore, stress concentration at side surface  5   a  of shim  5  at point P can be relaxed. FIG. 3 represents an embodiment in which side surface  5   a  includes linear side surface portions  5   a   1  and  5   a   2 , and FIG. 4 shows an embodiment in which the side surface is curved, i.e. the side surface portions  5   a   1  and  5   a   2  are each curved to form a continuous overall curve of side surface  5   a.    
     EXAMPLES 
     Evaluation of shapes and effects of commercially available shims  5  formed of steel material and of shims  5  processed as sliding parts having such shapes as shown in FIGS. 2 to  4  using commercially available super hard alloy such as cermet and ceramics such as silicon nitride, alumina and zirconia will be described in the following. 
     FIG. 5 is a schematic illustration of the test apparatus. The test apparatus includes a commercially available 4-cylindered, 1500 cc valve system, having a motor  11  for driving a cam shaft  10  attached thereto, and separately having a pump (not shown) for supplying a lubricating oil. Using the test apparatus, a durability test of shims  5  was performed for 200 hours at a cam shaft rotation speed of 2250 rpm, and the amounts of wear of the shims  5  were measured. The sliding surfaces of shims  5  were finished flat to have the surface roughness of R a =0.2 μm. 
     The dimension of conventional shim  5  shown in FIG. 2 was as follows: D1φ=28 mm, h1=2.5 mm, H1=2.9 to 3.4 mm (optimal value of H1 is selected to maintain precision in opening/closing valve  2 ), and both side surfaces  5   c  were chamfered by 0.2 mm. The dimension of shim  5  in accordance with the embodiment of the present invention was as follows: D1φ=28 mm, h1=2.0 mm, H1=2.9 to 3.4 mm. The dimension of shim  5  in accordance with an embodiment shown in FIG. 4 was similar to that of FIG.  3 . The results of evaluation of respective shims by the test apparatus shown in FIG. 5 are as shown in Table 1. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sample 
                 Shim 5 as Sliding Part 
               
             
          
           
               
                 No. 
                 Material 
                 Shape 
                 Result of Durability Test 
               
               
                   
               
             
          
           
               
                 1 
                 Steel 
                 FIG. 2 
                 Shim Wear; 52 μm 
               
               
                 2 
                 Super Hard Alloy 
                 Same as above 
                 Broken from point P after 
               
               
                   
                   
                   
                 50 hours 
               
               
                 3 
                 Silicon Nitride 
                 Same as above 
                 Broken from point P after 
               
               
                   
                   
                   
                 62 hours 
               
               
                 4 
                 Zirconia 
                 Same as above 
                 Broken from point P after 
               
               
                   
                   
                   
                 73 hours 
               
               
                 5 
                 Alumina 
                 Same as above 
                 Broken from point P after 
               
               
                   
                   
                   
                 54 hours 
               
               
                 6 
                 Steel 
                 FIG. 3 
                 Shim Wear; 50 μm 
               
               
                 7 
                 Super Hard Alloy 
                 Same as above 
                 Shim Wear; 5 μm 
               
               
                 8 
                 Silicon Nitride 
                 Same as above 
                 Shim Wear; 3 μm 
               
               
                 9 
                 Zirconia 
                 Same as above 
                 Shim Wear; 2 μm 
               
               
                 10 
                 Alumina 
                 Same as above 
                 Shim Wear; 8 μm 
               
               
                 11 
                 Steel 
                 FIG. 4 
                 Shim Wear; 48 μm 
               
               
                 12 
                 Super Hard Alloy 
                 Same as above 
                 Shim Wear; 4 μm 
               
               
                 13 
                 Silicon Nitride 
                 Same as above 
                 Shim Wear; 3 μm 
               
               
                 14 
                 Zirconia 
                 Same as above 
                 Shim Wear; 3 μm 
               
               
                 15 
                 Alumina 
                 Same as above 
                 Shim Wear; 7 μm 
               
               
                   
               
             
          
         
       
     
     It can be seen from the results of Table 1 that the shims  5  having such shapes as shown in FIG. 3 (samples 6 to 10) and FIG. 4 (samples 11 to 15) in accordance with the embodiment of the present invention, in which side surfaces of shims  5  are so shaped as not to be in contact with edge  4   d  of open end  4   c , clearly have the advantageous effects. 
     Consider an example in which shim  5  is manufactured using silicon nitride. To silicon nitride powder (Si 3 N 4 ), 5 wt % of Y 2 O 3  and 2 wt % of Al 2 O 3  were added as sintering assisting agents, and the mixture was mixed for 96 hours in a ball mill, in ethanol. After drying, the resulting mixed powder was subjected to CIP (Cold Isostatic Pressing), sintered in a nitride gas atmosphere at a pressure of 2 atmospheres at 1710° C. for 4 hours, and thereafter subjected to HIP (Hot Isostatic Pressing) in a nitrogen gas atmosphere at a pressure of 1000 atmospheres at 1660° C., for 1 hour, whereby the mixed powder was formed to a sintered body. 
     The resulting sintered body had a ratio of 5% and linear density of crystal grains for the length of 50 μm was 153. Here, a ratio can be obtained as peak intensity ratio of diffraction lines ( 102 )+( 210 ) and ( 101 )+( 210 ) of (α-silicon nitride, α′-sialon) and (β-silicon nitride, β′-sialon): α[( 102 )+( 210 )]/{α[( 102 )+( 210 )]+β[( 101 )+( 210 )]}. 
     The sintered body of silicon nitride manufactured in this manner was subjected to 4-point bending strength measurement in compliance with JIS R 1601 “Method of Testing Fine Ceramics Bending Strength”, and the bending strength was 1450 MPa. Commercially available silicon nitride used for samples 3, 8 and 13 of Table 1 had 4-point bending strength of 1050 MPa. 
     The silicon nitride manufactured under the above described condition was formed to the shape of shim  5  shown in FIG. 4, and this shim  5  and a shim  5  formed of commercially available silicon nitride represented by sample  13  of Table 1 were set in the test apparatus of FIG.  5 . When the cycle speed of valve  2  reaches near 3500 cycles per minute, which corresponds to the rotation speed of the cam shaft inducing rattling, the shim  5  formed of commercially available silicon nitride was broken, while the shim  5  formed of silicon nitride manufactured under the above described condition was intact. Accordingly, the 4-point bending strength of the silicon nitride sintered body should preferably be 1000 MPa or higher and more preferably, 1300 MPa to 2000 MPa. Even when the bending strength is reinforced to be higher than 2000 MPa, the effect is not so significant as compared with the increase in the cost of the material powder and the cost of the sintering process. 
     Second Embodiment 
     The sliding part in accordance with an embodiment of the present invention used for a mechanism for preventing inclination of the ring gear in a reduction mechanism of a vehicle will be described in the following. Using commercially available silicon nitride, sliding parts  15  which correspond to sliding part  110  of FIG. 10C were formed to have the shape of the conventional sliding part  15  and the shapes of FIGS. 7 and 8 in accordance with the present embodiment, and shapes and effects of the sliding parts were evaluated. 
     The dimension of conventional sliding part  15  shown in FIG. 6 was as follows: D2φ=21 mm, h2=18 mm, H2=20 mm. The dimension of sliding part  15  in accordance with the embodiment of the present invention shown in FIG. 7 was as follows: D2φ=21 mm, h2=15 mm and H2=20 mm. The dimension of sliding part  15  in accordance with the embodiment shown in FIG. 8 was set similarly to that of FIG.  7 . The sliding parts were prepared by finishing the sliding surfaces to be flat with a flatness of 2 μm by using a diamond grinder having a mean abrasive grain diameter of 8 to 12 μm, and other sliding parts were prepared to have the sliding surfaces finished to have convex crowning shapes of R800 and R1600 with a surface roughness of at most Ra=0.2 μm. The rear surface  107  of ring gears  105  formed of SCM420H was thermally treated to attain Rockwell Hardness of H RC   45  and finished to have a surface roughness of Ra=5 μm. The sliding parts  15  thus prepared were put in a differential case  104  of a truck for the load of 15 t, a lubricating oil in a condition corresponding to accumulated travel of 150,000 km was introduced, and the clearance δ was adjusted to be 0.2 mm, and then a quick start was repeated for 100 times. In this manner, the amounts of wear of rear surfaces  107  of ring gears  105  which were in contact with respective sliding parts  15  were evaluated, and the results are as shown in Table 2. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Sample 
                 Sliding 
                   
                   
               
               
                 No. 
                 Surface 
                 Shape 
                 Test Result 
               
               
                   
               
             
             
               
                 16 
                 Flat 
                 FIG. 6 
                 Broken from point P after 20 quick 
               
               
                   
                   
                   
                 starts 
               
               
                 17 
                 R800 
                 Same as above 
                 Broken from point P after 41 quick 
               
               
                   
                   
                   
                 starts 
               
               
                 18 
                 R1600 
                 Same as above 
                 Broken from point P after 36 quick 
               
               
                   
                   
                   
                 starts 
               
               
                 19 
                 Flat 
                 FIG. 7 
                 Rear Surface Wear Amount; 216 μm 
               
               
                 20 
                 R800 
                 Same as above 
                 Rear Surface Wear Amount; 23 μm 
               
               
                 21 
                 R1600 
                 Same as above 
                 Rear Surface Wear Amount; 19 μm 
               
               
                 22 
                 Flat 
                 FIG. 8 
                 Rear Surface Wear Amount; 226 μm 
               
               
                 23 
                 R800 
                 Same as above 
                 Rear Surface Wear Amount; 25 μm 
               
               
                 24 
                 R1600 
                 Same as above 
                 Rear Surface Wear Amount; 22 μm 
               
               
                   
               
             
          
         
       
     
     As can be seen from the results of Table 2, sliding part  15  having the shapes of FIG. 7 (samples 19 to 21) and FIG. 8 (samples 22 to 24) in accordance with the present embodiment, which have the side surfaces  15   a  including first side surface portions  15   a   1  that are adapted to be in contact with the inner wall  106   b  and second side surface portions  15   a   2  so shaped as not to be in contact with edge  106   d  of open end  106   c  of thrust bolt  106 , clearly have the advantageous effects of their shapes. Further, it can be seen that the sliding surface of sliding part  15  should have convex crowning shape rather than flat shape, to attain improved wear resistance. Further, it can be seen that surface roughness of the sliding surface should be set to at most Ra=0.2 μm. When the sliding part in accordance with the present embodiment is used in a reduction mechanism of a vehicle, there is a clearance of  5  between sliding part  15  and the rear surface  107  of ring gear  105  in normal running, and therefore when sliding part  15  is held loose in the recessed portion  106   a  of thrust bolt  106 , sliding part  15  plays in the clearance δ. In order to avoid unwanted wear, sliding part  15  should be integrally secured on thrust bolt  106  by shrink fit or press fit. 
     The following samples were prepared to evaluate the method of securing and the shapes and effects. Sample 18 of Table 2 corresponding to the conventional shape of sliding part  15  shown in FIG. 6, and sample  21  of Table 2 having the shape of sliding part  15  in accordance with the present embodiment shown in FIG. 7 were fixed on thrust bolt  106  with the margin for shrink fit of 60 μm and the margin for press fit of 20 μm. 
     The samples were subjected to durability test using such a dynamometer as shown in FIG. 9 as an evaluating apparatus. An 8-cylindered, 16750 cc diesel engine  20  is used for the evaluating apparatus. A clutch  21  is provided at a tip end of an output shaft, and torque is transmitted from the output shaft through a coupling  22  to propeller shaft  102  including a pinion. The torque transmitted from propeller shaft  102  to ring gear  105  generates driving force to wheel  23 , through axle shaft  103 . The wheel  23  is attached to a torque generation drum  24 , and load on engine  20  is controlled by a brake drum  25 . When ring gear  105  deflects, contact force Q is measured by a strain gage  26  adhered to a support bolt (not shown). 
     Durability test was repeated for 1500 times while applying a load W corresponding to 15 ton on axle shaft  103  shown in FIG.  9  and connecting/disconnecting clutch  21 . In the sample shown in FIG. 6 which had the shape of the conventional sliding part  15 , a crack and breakage starting from the crack were observed at a portion where side surface  15   a  of sliding part  15  interfered with edge  106   d  of open end  106   c  of thrust bolt  106 . Samples shown in FIG. 7 which had the shape of the sliding part  15  in accordance with the present invention were all intact. 
     After the end of the durability test, the sliding parts in accordance with the present embodiment were subjected to load test while controlling the contact force Q by monitoring strain gauge  26  and varying Hertz&#39;s contact surface pressure between 1100 MPa to 2100 MPa. As a result, it was found that when Hertz&#39;s contact surface pressure exceeded 2000 MPa, there was a breakage of sliding part  15  or a sign of pitching wear at that portion of ring gear  105  which slides over sliding part  15 . Therefore, even for the sliding parts  15  having the shape in accordance with the present embodiment, a condition of use in which Hertz&#39;s contact surface pressure exceeds 2000 MPa is not preferable. 
     Here, “Hertz&#39;s contact surface pressure” in the present embodiment can be quantized by the following equation. 
     
       
         σ=3Q/2πab 
       
     
     where Q: contact force, a: longer radius of contact ellipse and b: shorter radius of contact ellipse. 
     The sliding part in accordance with the present invention has such a shape in that the side surface of the sliding part is not in contact with an inner wall edge of an open end of the holding portion, when the sliding part is secured in a holding portion formed of metal. As a result, stress concentration on the side surface of the sliding part can be relaxed, and premature breakage of the sliding part can be avoided. Thus a sliding part having superior wear resistance is provided. 
     Further, a sliding mechanism is provided in which the sliding surface of the sliding part preferably has a convex crown shape with the surface roughness of at most Ra=0.2 μm, which is preferably used under the optimal condition of use where Hertz&#39;s contact surface pressure is at most 2000 MPa. 
     Although the present invention has been described and illustrated in detail, it is dearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.