Abstract:
A plain bearing unit is provided in which a sliding member made of thermoplastics having high recyclability and not containing solid lubricant provides low frictional coefficient and improves wear resistance even under circumstances where oil or grease cannot be used. 
     A rotating, and/or swinging and/or reciprocating plain bearing unit includes a bearing made of resin containing glass and/or a carbon material; a shaft; and a gas supply portion adapted to supply dry gas subjected to dust removal to a bearing sliding portion where the bearing portion and the shaft slide with each other.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to rotating and/or swinging and/or reciprocating bearing unit and more particularly to a bearing unit composed of a resin-made member and a metal member contacting therewith. 
         [0003]    2. Description of the Related Art 
         [0004]    A resin material used as a sliding member is generally lightweight and superior in moldability, self-lubricating ability, etc; therefore, it is used under circumstances where oil or lubricant such as grease or the like cannot be used. A single or raw resin material has problems with load bearing and wear resistance. Therefore, glass fiber or carbon fiber is added to reinforce the resin material and solid lubricant such as graphite, polytetrafluoroethylene (PTFE), molybdenum disulfide or the like is added to increase wear resistance. It is known that such additions provide low friction and low wear compared with non-addition. In recent years, bearings have increasingly been needed and in particular a bearing structure superior in low-friction and wear resistance has been required. 
         [0005]    Sliding members for dry gas described in JP-11-279413-A and JP-4-93397-A are proposed as sliding members used in dry gas circumstances which are circumstances where lubricant cannot be used. 
         [0006]    The sliding member described in JP-11-279413-A is made by mixing a thermoset resin with carbon fiber and graphite powder and hot-press molding it. 
         [0007]    The sliding member described in JP-4-93397-A is made of a mixture of a carbon material such as coke, graphite, carbon black or the like and an organic binder such as coal tar pitch, a thermoset resin or the like. In addition, the sliding member is provided at least close to the sliding surface thereof with a layer embedded with a large number of short circular hard materials made of silicon carbide fiber. 
       SUMMARY OF THE INVENTION 
       [0008]    However, although the methods described in JP-11-279413-A and JP-4-93397-A use a thermoset resin for the sliding member and their bearings are superior in heat resistance, the methods entail a long time molding process and difficulty in recycling. 
         [0009]    Accordingly, it is an object of the present invention to provide a plain bearing unit in which a sliding member made of thermoplastics having high recyclability and not containing solid lubricant provides a low frictional coefficient and improves wear resistance even under circumstances where oil or grease cannot be used. 
         [0010]    To achieve the above object, a plain bearing unit of the present invention includes a bearing portion made of a resin material containing glass and/or a carbon material; a shaft supported by the bearing portion; and a dry gas supply portion adapted to supply dry gas to a bearing sliding portion where the bearing portion and the shaft slide with each other. 
         [0011]    The plain bearing unit of the present invention may have any one of rotating, swinging and reciprocating relationships between a bearing and a shaft. The formation of the bearing portion may be a cylindrical end face type, a block-on-ring type, a pin-disk type, a journal type, or a reciprocating type, or a partial bearing thereof or a complex bearing combining them. 
         [0012]    The resin material of the bearing portion has at least one type of thermoplastics. Examples of the thermoplastics include polyamide, polyphthalamide, polyamide-imide, polyacetal, polyphenylene sulfide, polyetherimide, ultrahigh molecular weight polyethylene, and polyether ether ketone. To reinforce the thermoplastics, single or combined fibrous, spherical, or scale-like glass or carbon is added thereto. 
         [0013]    The bearing may be provided with a through-hole passed through from its outer circumferential surface to a bearing surface and dry gas subjected to dust removal may be supplied via the through-hole to the bearing sliding portion. 
         [0014]    The dry gas supplied to the bearing sliding portion may be gas containing oxygen. An example of such gas is dry air having a temperature of −50° C. or less in terms of dew point. 
         [0015]    The dry gas supplied to the bearing sliding portion may be gas not containing oxygen. Examples of such gas include inert gas such as nitrogen, argon or the like, and hydrogen gas and hydrocarbon gas. 
         [0016]    The present invention can suppresses the growth of wear powder of the resin bearing and accelerates the transfer of the wear powder onto the surface of a counterpart. Thus, the friction and wear of the resin bearing can be reduced to reduce drive energy to various devices. In addition, since oil or solid lubricant is not used, the consumption of resources is suppressed to reduce an environment load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  illustrates an essential portion of a centrifugal compressor according to a sixth embodiment of the present invention. 
           [0018]      FIG. 2  illustrates a journal type testing machine according to a first embodiment of the present invention. 
           [0019]      FIG. 3  illustrates a structure of a bearing portion according to a third embodiment of the present invention. 
           [0020]      FIG. 4  illustrates a block-on-ring type testing machine according to a fourth embodiment of the present invention. 
           [0021]      FIG. 5  illustrates a bearing portion of a centrifugal compressor according to the sixth embodiment of the present invention. 
           [0022]      FIG. 6  illustrates an essential portion of a centrifugal compressor according to a seventh embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. It is to be noted that the present invention is not limited to the embodiments. 
         [0024]    A plain bearing unit as a basic configuration of the present invention will first be described using  FIG. 2 .  FIG. 2  illustrates a testing portion of a journal type testing machine. 
       Experimental Example 1 
     First Embodiment 
       [0025]    The testing portion of the testing machine is configured such that a resin bearing  2  is press fitted into a casing  1  to rotatably carry a shaft  4 . The casing  1  is configured to apply a load to the resin bearing  2 . A rotating-driving motor (not illustrated) is connected to one end of the shaft  4 . 
         [0026]    The testing portion is covered by a protection cover  5 , which is provided with a venting hole  6  at an upper portion as well as with venting holes in the vicinity of the shaft. The resin bearing  2  is made of thermoplastics, polyphenylene sulfide (PPS), containing 30 wt % carbon fiber. The shaft  4  is made of stainless steel (SUS). 
         [0027]    The protection cover  5  is provided with a gas supply tube  3  passed therethrough. A gas flow  7  discharged from the gas supply tube  3  is illustrated. The gas flow  7  moves from the gas supply tube  3  toward a bearing sliding portion between the resin bearing  2  and the shaft  4 . In this case, nitrogen as dry gas was supplied at 20 L/min and relative humidity in the vicinity of the resin bearing  2  was 1.5%, which is a lower limit of a humidity sensor  8  installed in the protection cover  5 . 
         [0028]    As a result, a minimum friction coefficient is 0.08, which significantly drops from 0.25 in damp air. Also wear-depth reduces by about 40% compared in damp air. The wear powder of the resin bearing uniformly transfers onto the SUS shaft as a counterpart. 
       Experimental Example 2 
     Second Embodiment 
       [0029]    A description is given of, as a similar experiment, an example where the resin bearing uses polyether ether ketone (PEEK) containing 30 wt % glass fiber. The others are the same as those of the above-mentioned test. 
         [0030]    The results of the test are as below. Also in this test, the minimum friction coefficient is 0.09, which significantly reduces from 0.3 in dump air. In addition, also wear-depth reduces by about 40%. The wear powder of the resin bearing uniformly transfers onto the SUS shaft as a counterpart. 
       Experimental Example 3 
     Third Embodiment 
       [0031]    A description is given of another gas supply portion adapted to supply gas to a bearing with reference to  FIG. 3 . As illustrated in  FIG. 3 , this gas supply portion has a hole  3   a  which is located on an opposite-load surface of a shaft  4  so as to communicate from a casing  1  to the shaft  4 . In addition, a piping component  9  is secured to the casing  1  and a resin bearing  2  so that the hole  3   a  is shared thereby. The others are similar to those of experimental example 1. In this way, the piping component  9  serves as a lock for the resin bearing  2 . Even if the resin bearing  2  shrinks due to changes in ambient temperature, it is possible to prevent the resin bearing  2  from dropping off from the casing  1  and from being turned together with the shaft  4 . 
       Experimental Example 4 
     Fourth Embodiment 
       [0032]    A test performed for a block-on-ring type is next described with reference to  FIG. 4 . A planar type resin test piece  42  of polyether ether ketone (PEEK) containing 30 wt % carbon fiber is examined which is a resin material used as a bearing portion. This test piece  42  is interposed between a cantilever  41  attached with a weight  40  and a shaft  10  made of stainless steel (SUS). The shaft  10  performs a swing movement of 90 degrees. A testing portion is circumferentially covered by a protection cover  5 , which is provided with an air supply pipe  3 . 
         [0033]    In this case, dry air as dry gas was supplied at 20 L/min. The results of the test are as below. The minimum friction coefficient is 0.15, which reduces by about half compared in damp air. In addition, also wear-depth reduces by about 40%. The wear powder of the resin bearing uniformly transferred onto the swing portion of the SUS shaft. 
       Experimental Example 5 
     Fifth Embodiment 
       [0034]    In the present experimental example, the device of experimental example 4 is used and gas resulting from gasifying liquid nitrogen is supplied in place of dry air to the inside of the protection cover  5  from the air supply pipe  3 . The supply amount of the gas is controlled so that the temperature inside the protection cover  5  may become −100° C. The other conditions are the same as in the above tests. 
         [0035]    The results of this test are as below. The minimum friction coefficient is 0.05. In addition, the wear powder of the resin bearing transferring onto the SUS shaft is most. 
         [0036]    Referring to  FIG. 1 , a description will next be given of an example (a sixth embodiment) in which a plain bearing unit of the present invention is applied to a centrifugal compressor. In the centrifugal compressor  11  of this embodiment, a drive device (not illustrated) rotates an impeller  12  mounted to a main shaft (not illustrated) with the main shaft to compress air passed through a passage  14  provided in a casing  13  and supplied to the impeller  12 . The compressed air is passed through a discharge port (not illustrated) and discharged to the outside. Incidentally, although not illustrated in  FIG. 1 , the main shaft rotating the impeller  12  is provided below the impeller  12  in  FIG. 1 . 
         [0037]    The centrifugal compressor  11  is provided with a plurality of vanes  15  to control a flow rate of gas in a passage adapted to lead the gas to the impeller  12 . 
         [0038]    The vanes  15  are swung by a gear mechanism that includes a first gear  17 , a third gear  19  and a second gear  20 . The first gear  17  is a drive gear secured to a drive shaft  16  connected to a drive mechanism (not illustrated). The third gear  19  is a vane gear secured to a vane shaft  18  connected to the vanes  15 . The second gear  20  is a ring gear interposed between the first and third gears to transmit rotation of the first gear to the third gear. 
         [0039]    The first, third and second gears  17 ,  19 ,  20  are housed in the casing  13 . The first and third gears  17  and  19  are mounted to the drive shaft  16  and the vane shaft  18 , respectively. The drive shaft  16  and the vane shaft  18  are rotatably supported by bearings (plain bearings)  22  and  24 , respectively, secured inside the casing  13 . In addition, the bearings  22  and  24  are made of polyphenylene sulfide (PPS) containing carbon fiber. 
         [0040]    The vane shaft  18  is provided with a seal  25  on the side of the passage  14  to prevent the inflow of gas from the inside of the passage  14 . Although a contact seal as the seal  25  is used in the present embodiment, a non-contact labyrinth seal may be used. 
         [0041]    A gas supply portions  21  are connected to the respective bearings  22  and  24 . Nitrogen with a dew point not higher than −50° C. is supplied to the bearings  22  and  24  through the gas supply portions  21  from a gas supply device  33  at a pressure of 0.02 MPa. The nitrogen is recovered through a communication hole  23  communicating with the outside and a check valve (not illustrated) of the out side into the gas supply device  33  including a cylinder and dehumidification is carried out in the gas supply device  33 . The communication hole  23  is provided at a portion of the casing  13  incorporating the gear mechanism. 
         [0042]    High pressure air or inert gas such as argon subjected to dehumidification and dust removal may be supplied in place of nitrogen. In addition, a gas production device may be used in place of the cylinder. The nitrogen recovered is returned to the gas supply device  33  via a dust removal device  31  and a pump  32 , and circulated and supplied to the bearings  22  and  24 . 
         [0043]    Referring to  FIG. 5  which is an enlarged view illustrating a bearing portion, the bearing  24  has a communicating hole  24   a , which communicates from a bearing surface sliding along the vane shaft  18  to a gas supply hole (a part of the gas supply portion  21 ) on an outer circumferential surface side. Nitrogen is supplied to a sliding portion of the bearing  24  through the communicating hole  24   a . Also, Nitrogen flows along the drive shaft  16 , and flows into a space between the drive shaft  16  and the bearing  22  from the end of the bearing  22 , and is supplied to a sliding portion of the bearing  22 . 
         [0044]    In this way, the centrifugal compressor performs predetermined operation while nitrogen are circulated and supplied to the bearings. The temperature of gas flowing in the passage  14  largely ranges from −160° C. to 60° C. depending on the type of gas. Under various temperature conditions, without using grease or solid lubricant, the torque of the drive shaft  16  can be reduced compared with the case where dry air is not allowed to flow. 
         [0045]    In the sixth embodiment, nitrogen gas is supplied to the bearing portions from the cylinder and thereafter is recovered, circulated and supplied thereto. In a seventh embodiment as illustrated in  FIG. 6 , an excess of supply gas is not recovered but is discharged to the air through a filter (not illustrated) from the communication hole  23  and the check valve. In this case, high-pressure air or the like subjected to dehumidification and dust removal may be used in place of nitrogen so that the torque of the drive shaft  16  can be reduced. As described above, it is effective that the gas supplied to the bearings is subjected to dehumidification as well as to dust removal. 
         [0046]    As a relative example, the same test as that in experimental example 1 or 4 was performed using polyether ether ketone (PEEK) not containing fiber or the like. The results showed the same friction coefficient and wear-depth as those in damp air also in the case where nitrogen was supplied as dry gas. 
         [0047]    As described above, in the case where the bearing made of resin containing carbon fiber or glass having property similar to carbon fiber is used, the conditions of the sliding portion is controlled. Thus, the resin sliding portion in which it is not necessary to supply oil, grease, solid lubricant, etc. thereto can be obtained.