Abstract:
A ceramic bearing assembly includes a shaft ( 20 ), and a bearing sleeve ( 10 ) rotatably receiving the shaft. The bearing sleeve includes an inside wall. First and second bearing blocks ( 120, 140 ) are formed at the inside wall at first and second ends of the bearing sleeve respectively. The first bearing blocks are arranged in circular fashion, and the second bearing blocks are arranged in circular fashion complementarily offset from the first bearing blocks. Each of the first and second bearing members defines a concave bearing surface ( 122, 142 ), the bearing surfaces cooperatively supporting the shaft therebetween.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to bearings, and more particularly to a sliding ceramic bearing which can be readily produced. 
   2. Prior Art 
   Sliding bearings and rolling bearings are popularly used in applications such as attaching a rotary axle to a machine frame, etc. 
   Common types of rolling bearings include ball bearings, roller bearings and needle bearings, in which rolling members such as balls, rollers, and needles are provided between an inner ring and an outer ring. The friction between the inner ring and the outer ring is known as “rolling friction,” and is generally very small. Therefore, rolling bearings provide good high-speed operating capabilities. However, the roller members between the inner ring and the outer ring are prone to crack or deform under a heavy load. When this happens, the operating precision is dramatically decreased. In addition, the manufacturing costs of roller bearings are very high, especially roller bearings used in small devices such as computer fans. 
   Therefore, in small devices, sliding bearings are often used because of their relatively low manufacturing costs. A typical sliding bearing comprises an annular bearing sleeve having a circular bore, and a cylindrical shaft rotating in the bore. Most bearing sleeves used today are made of a copper-based alloy or stainless steel. The friction between the bearing sleeve and the shaft is known as “sliding friction,” and is generally very large. To reduce the friction between the shaft and the bearing sleeve, a diameter of the bore of the bearing sleeve is configured slightly larger than a diameter of the shaft in order to provide an operating clearance, and an oil film is established in the operating clearance to act as a lubricant. Because of the operating clearance, the shaft is usually not located exactly along a central axis of the bearing sleeve. Instead, the shaft is displaced slightly from the central axis so that it rotates about an axis that is eccentric to the central axis. This leads to unsteady rotation of components mounted on the bearing sleeve. However, if the operating clearance is configured to have a reduced size, the lubricant therein may be forced out. When this happens, the bearing sleeve directly contacts the shaft, and the sliding bearing rapidly wears out. Therefore, sliding bearings usually have short lifetimes. 
   With the development of technology in fields where slide bearings are applied, modem slide bearings are being required to rotate at unprecedented high speeds. The problem of “sliding friction” is becoming commensurately more important. Traditional low abrasion, high hardness materials used for slide bearings are increasingly unable to provide satisfactory high-speed, long-life performance under harsh operating conditions. New materials for bearings are being eagerly sought. It has been found that certain ceramics have high compression strength, high friction resistance, and a small coefficient of friction. Ceramics are now widely considered to be a more serviceable material for slide bearings than traditional materials. Studies have shown that in ceramic slide bearings, it is feasible to reduce the contact area between the bearing sleeve and the shaft in order to reduce the friction therebetween, without diminishing the operating reliability of the slide bearing. 
   Taiwan Patent Publication No. 495118 discloses a sliding bearing made of ceramic material. In order to reduce the contact area between the bearing sleeve and the shaft, either an outer surface of the shaft or an inner surface of the bearing sleeve is configured to be non-cylindrical. When the bearing sleeve receives the shaft therein, at least a portion of the outer surface of the shaft does not contact the inner surface of the bearing sleeve, so that the contact area is reduced. However, the advantages of high compression strengthen and high abrasive resistance of the ceramic material also present novel problems in manufacturing the bearing sleeve, as detailed below. 
   Referring to  FIG. 7 , to attain a high degree of surface smoothness, a bore  4  of a bearing sleeve  1  needs to be ground with a grinding machine  2 . The grinding machine  2  has a grinding bit  3  rotatingly machining the surface of the bearing sleeve  1  that defines the bore  4 . During this process, the grinding bit  3  is subjected to a diametrical force by the bearing sleeve  1 . This causes the grinding bit  3  to bend, especially when the grinding bit  3  is extended far into the bore  4 . As a result, the ground bore  4  is irregular. That is, a diameter of the bore  4  nearest the grinding machine  2 . One means of ameliorating this problem is to perform doubled-ended grinding. Referring to  FIG. 8 , two grinding machines  2  are provided to simultaneously grind the bore  4  at opposite ends thereof. Each grinding bit  3  has to penetrate only halfway into the bore  4 . Accordingly, the grinding bits  3  are subjected to reduced diametrical forces, and the ground bore  4  is more uniform. However, it is generally not possible to completely eliminate irregularity of the bore  4 . In addition, the two ground halves of the bore  4  may not be precisely coaxial, due to inherent manufacturing error. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a ceramic bearing which can be easily produced. 
   To achieve the above-mentioned object, a ceramic bearing assembly in accordance with a preferred embodiment of the present invention comprises a shaft adapted for being mounted to a complementary supporting structure, and a bearing sleeve rotatably receiving the shaft. The bearing sleeve comprises an inside wall surrounding the shaft, and an outer surface adapted for being mounted to a rotatable body. A series of first bearing blocks is formed at the inside wall at a first end of the bearing sleeve, and a series of second bearing blocks is formed at the inside wall at a second end of the bearing sleeve. The first bearing blocks are arranged in circular fashion, and the second bearing blocks are arranged in circular fashion complementarily offset from the first bearing blocks. Each of the first and second bearing blocks defines a concave bearing surface, the bearing surfaces cooperatively supporting the shaft therebetween. 
   Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of the preferred embodiment of the present invention with attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded, isometric view of a ceramic bearing assembly in accordance with the preferred embodiment of the present invention, the ceramic bearing assembly comprising a bearing sleeve and a cylindrical shaft; 
       FIG. 2  is a schematic, cross-sectional view of the bearing sleeve of the ceramic bearing assembly of  FIG. 1 , corresponding to line II-II thereof; 
       FIG. 3  is a schematic, cross-sectional view of the bearing sleeve of the ceramic bearing assembly of  FIG. 1 , corresponding to line III-III thereof; 
       FIG. 4  is a schematic, cross-sectional view of the bearing sleeve of the ceramic bearing assembly of  FIG. 1 , corresponding to line IV-IV thereof; 
       FIG. 5  is a left end elevation of the bearing sleeve of the ceramic bearing assembly of  FIG. 1 ; 
       FIG. 6  is a schematic side elevation of two grinding machines grinding an inside wall of the bearing sleeve of the ceramic bearing assembly of  FIG. 1 ; and 
       FIGS. 7 and 8  are schematic side elevations of respective grinding machines grinding an inside wall of a conventional bearing sleeve. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a ceramic bearing assembly in accordance with the preferred embodiment of the present invention comprises a bearing sleeve  10  made of ceramic material, and a cylindrical shaft  20  fitted in the bearing sleeve  10 . The shaft  20  is adapted to be mounted to a rotatable body (not shown). 
   The bearing sleeve  10  has a cylindrical outer surface. The outer surface is adapted for being mounted in a bore of a complementary supporting structure (not shown). A series of evenly spaced first grooves  12  and a series of evenly spaced second grooves  14  are defined in an inside wall of the bearing sleeve  10 . The first and second grooves  12 ,  14  are each generally parallel to and equidistant from an axis of rotation  18  of the bearing sleeve  10 . The first grooves  12  span from a first end of the bearing sleeve  10  toward a second end of the bearing sleeve  10 . The first grooves  12  are slightly tapered, such that they are narrowest at inmost ends thereof and widest at the first end of the bearing sleeve  10 . The second grooves  14  span from the second end of the bearing sleeve  10  toward the first end of the bearing sleeve  10 . The second grooves  14  are slightly tapered, such that they are narrowest at inmost ends thereof and widest at the second end of the bearing sleeve  10 . The first and second grooves  12 ,  14  are disposed adjacent each other in alternate fashion in a center portion of the bearing sleeve  10  between the first and second ends. 
   Referring also to  FIG. 5 , a series of evenly spaced first bearing blocks  120  is formed at the inside wall of the bearing sleeve  10  at the first end of the bearing sleeve  10 . The first bearing blocks  120  extend from the first end of the bearing sleeve  10  to inmost ends of the second grooves  14  respectively. Thus the first bearing blocks  120  and the first grooves  12  are arranged at the first end of the bearing sleeve  10  in alternate fashion. Each first bearing block  120  is slightly tapered, such that it is widest at an inmost end thereof and narrowest at the first end of the bearing sleeve  10 . Each first bearing block  120  defines a concave first bearing surface  122  thereon. A radius of curvature of the first bearing surface  122  corresponds to the axis  18  of the bearing sleeve  10 . Said radius of curvature is substantially the same as a radius of the shaft  20 . A series of evenly spaced second bearing blocks  140  is formed at the inside wall of the bearing sleeve  10  at the second end of the bearing sleeve  10 . The second bearing blocks  140  extend from the second end of the bearing sleeve  10  to inmost ends of the first grooves  12  respectively. Thus the second bearing blocks  140  and the second grooves  14  are arranged at the second end of the bearing sleeve  10  in alternate fashion. Each second bearing block  140  is slightly tapered, such that it is widest at an inmost end thereof and narrowest at the second end of the bearing sleeve  10 . Each second bearing block  140  defines a concave second bearing surface  142  thereon. A radius of curvature of the second bearing surface  142  corresponds to the axis  18  of the bearing sleeve  10 , and is the same as the radius of curvature of the first bearing surface  122 . 
   Referring to  FIG. 4 , each first groove  12  has an open end at the first end of the bearing sleeve  10 , and an opposite dead end at the corresponding second bearing block  140 . Similarly, each second groove  14  has an open end at the second end of the bearing sleeve  10 , and an opposite dead end at the corresponding first bearing block  120 . It is noted that  FIG. 4  shows only one first groove  12  and its corresponding second bearing block  140 , and only one second groove  14  and its corresponding first bearing block  120 . This construction of the bearing sleeve  10  is advantageously accomplished by injection molding, as described in detail below. However, it should be noted that the configuration of the bearing sleeve  10  may have other alternative forms, and that construction of the bearing sleeve  10  may be accomplished by means other than injection molding. 
   In the preferred embodiment of the present invention, there are three first grooves  12 , three second grooves  14 , three first bearing blocks  120  and three second bearing blocks  140 . In alternative embodiments, other numbers of these components may be adopted according to need. 
   In assembly, the shaft  20  is received in the bearing sleeve  10 . Referring to  FIGS. 2 and 3 , a profile of the shaft  20  is shown in dashed lines. A first end of the shaft  20  is surrounded and supported by the first bearing surfaces  122  of the first bearing blocks  120 , and an opposite second end of the shaft  20  is surrounded and supported by the second bearing surfaces  142  of the second bearing blocks  140 . 
   Generally, the bearing sleeve  10  is produced by three steps. First, a ceramic greenbody is formed, by injection molding a composite comprising ceramic powder dispersed within a thermoplastic polymer. Second, the polymer is burned out, and the resulting porous greenbody is sintered to a dense ceramic body having a same shape. Third and finally, referring to  FIG. 6 , grinding machines  30  are used to grind the first and second bearing surfaces  122 ,  142  until a high degree of surface smoothness is obtained. Preferably, the first and second bearing surfaces  122 ,  142  are ground at a same time by two respective grinding bits  32  of two grinding machines  30 . This not only saves manufacturing time, but also reduces manufacturing error. This is because the bearing sleeve  10  only needs to be fixed on a work table a single time. 
   The grinding process only needs to be applied to the first and second bearing surfaces  122 ,  142  of the first and second bearing blocks  120 ,  140 , with the first and second bearing blocks  120 ,  140  being located at the opposite first and second ends of the bearing sleeve  10 . Therefore, when the first and second bearing surfaces  122 ,  142  are ground by the respective grinding bits  32  of the grinding machines  30 , the grinding bits  32  do not have to penetrate very far into the bearing sleeve  10 . Accordingly, the grinding bits  32  are subjected to reduced diametrical forces produced by the bearing sleeve  10 , and the precision of manufacturing the bearing sleeve  10  is effectively increased. In addition, production of the bearing sleeve  10  is speedier and more efficient, because of the relatively small sizes of the first and second bearing surfaces  122 ,  142  that are ground. 
   In the preferred embodiment of the present invention, the whole of the bearing sleeve  10  is made of ceramic material. In an alternative embodiment, only portions of the bearing blocks  120 ,  140  at the first and second bearing surfaces  122 ,  142  are made of ceramic material. 
   It is understood that the invention may be embodied in other forms without departing from the spirit thereof. The above-described examples and embodiments are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given above.