Patent Publication Number: US-7713040-B2

Title: Rotor shaft sealing method and structure of oil-free rotary compressor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotor shaft sealing method and structure of an oil-free rotary compressor such as a tooth type rotary compressor, whose sealing structure can prevent lubrication oil of the drive mechanism of the rotor from leaking into the compression chamber of the compressor even when the pressure of the compression chamber becomes lower than atmospheric pressure, which occurs under some operation condition of the compressor. 
     2. Description of the Related Art 
     Generally, a tooth type rotary compressor consists of two rotors, a male rotor and a female rotor, each having claw-like teeth, or lobes. The rotors turn in opposite directions without contact to each other to compress gas trapped in the compression pockets formed between the lobes and inner surface of a compressor casing as the rotors rotate. As the rotors do not contact with each other and with the inner surface of the compressor casing, the rotors do not wear and have a long life. Further, lubrication of the rotors is not needed because of non-contact engagement of the rotors, and clean compressed gas not contaminated with lubricant can be obtained. Compression ratio obtained by this type of compressor is relatively low, and required high compression ratio is obtained with high efficiency in many cases by composing a two-stage compressor unit comprised of a lower pressure stage compressor and a higher pressure stage compressor connected in series and driven separately. Working of the tooth type compressor will be explained hereunder referring to  FIG. 6   a  to  FIG. 6   d    
     In  FIG. 6   a , a male rotor  02  having claw-like lobes engages with a female rotor  03  having claw-like lobes with very tight clearances in a compressor housing  01 . Gas g to be compressed is sucked from a suction opening  04  into the compressing chamber as the rotors  02  and  03  rotate in directions indicated by arrows. In  FIG. 6   b , the suction opening  04  is closed by the rotors  02 ,  03 , and the sucked gas g is confined in a pocket surrounding the lobes of the female rotor  03  and in a pocket surrounding the lobes of the male rotor  02 . The rotors convey the gases confined, or trapped in the pockets from the suction side to the pressure side as shown in  FIG. 6   c , where the pockets are communicated and the volume of the sum of the two pockets reduces as the rotors rotate and the gases are compressed until the female rotor  03  uncovers the discharge port  05 . In  FIG. 6   d , the discharge port  05  is uncovered by the female rotor  03  and the compressed gas c between the rotors is discharged through the discharge port  05 . 
     It is necessary requirement for an oil-free rotary compressor such as an oil-free tooth type compressor that lubrication oil for lubricating rotor shaft bearings is prevented from leaking into the compression chamber of the compressor in order to supply clean compressed gas not containing the lubrication oil. Positive pressure is produced in the compression chamber in load operation of the compressor, but when the compressor is operated under no load, pressure in the compression chamber becomes negative, for the upstream side of the suction port of the compressor is shut by a suction closing mechanism. When pressure in the compression chamber becomes negative, intrusion of lubrication oil supplied to the rotor bearing into the compression chamber through the shaft seal may occur. 
     Rotor shaft sealing structure of a screw compressor type supercharger is disclosed in Japanese Laid-Open Utility Model Application No. 3-110138 (hereinafter “JP 3-110138”). The sealing structure is composed such that a lip seal (contact seal) and a non-contact seal are located between rotor shaft bearing and the compression chamber, an airspace is formed between both the seals, a communicating passage is provided to allow the airspace to communicate with outside air, and a check valve is provided in the communicating passage to allow outside air to be sucked into the airspace when negative pressure is produced in the airspace. 
     With the above-described construction, pressure difference between the compression chamber and the airspace is reduced through the non-contact seal having fin-like annular protrusions such as a labyrinth seal. When pressure in the compression chamber is positive, higher than atmospheric pressure, escaping of the positive pressure air in the compression chamber passing through the communicating passage is prevented by the check valve closed by positive pressure in the communicating passage, and when pressure in the compression chamber is negative, the check valve is opened by negative pressure in the communicating passage and outside air is sucked into the air space, thus the airspace serves as a pressure equalizer room. In this way, intrusion of the lubrication oil into the compression chamber is prevented by maintaining the airspace not lower in pressure than that in the bearing part. 
     A rotor shaft sealing structure disclosed in Japanese Laid-Open Patent Application No. 7-317553 (hereinafter “JP 7-317553”) relates also to shaft sealing structure of a screw compressor type supercharger. The shaft sealing structure is composed such that a contact seal (lip seal, for example) for sealing lubrication oil lubricating the rotor shaft bearing and a pressure fluctuation alleviating member (a piston ring movable in axial direction, for example) are located between rotor shaft bearing and the compression chamber, an airspace which serves as a pressure equalizer room is formed between the contact seal and the pressure fluctuation alleviating member, and a communicating passage opened into outside of the compressor. 
     However, with the sealing structure disclosed in the JP 3-110138, in a case where leakage of lubrication oil occurs from the bearing part to the airspace through the lip seal, oil leaked to the airspace is difficult to escape outside because of the presence of the check valve in the communicating passage. When pressure in the compression chamber becomes negative while the leaked lubrication oil is present in the airspace, the lubrication oil residing in the airspace is apt to be ingested into the compression chamber. 
     Further, in a case where the communicating passage is clogged for any reason, the leaked lubrication oil accumulates in the airspace without being allowed to escape outside, and the leaked lubrication oil accumulated in the airspace is easily ingested into the compression chamber when negative pressure is produced in the compression chamber. 
     According to the sealing structure disclosed in the JP 7-317553, the communicating passage for communicating the airspace surrounding the rotor shaft to the outside of the compressor is not provided with a check valve. However, a means for allowing lubrication oil leaked into the airspace to escape outside in a convincing way is also not disclosed in JP 7-317553. Further, a means for allowing lubrication oil accumulated in the airspace when the communicating passage is clogged to escape outside is not disclosed in either JP 3-110138 or JP 7-317553. Further, in the above references, the rotor shaft sealing structure is composed such that atmospheric air can be introduced into the airspace as a pressure equalized room, however, sealing effect will be increased by introducing air pressurized to a pressure higher than atmospheric pressure to the pressure equalized room. 
     SUMMARY OF THE INVENTION 
     The present invention was made in light of the problems of the prior arts, and the object of the invention is to provide a rotor shaft sealing method and structure for an oil-free rotary compressor, with which occurrence of lubrication oil intrusion into the compression chamber of the compressor which is liable to occur when negative pressure is produced in the compression chamber, is prevented, and even if lubrication oil leaks through the bearing side oil seal toward the annular airspace of the shaft sealing part, the leaked lubrication oil is exhausted to the outside of the compressor casing and prevented from intruding into the compression chamber. 
     To attain the object, the present invention proposes a rotor shaft sealing method for an oil-free rotary compressor having a pair of male and female rotors accommodated in a compression chamber formed by a rotor casing, each rotor having a rotor shaft extending from both end faces of the rotor to penetrate both side walls of the rotor casing to be supported by the rotor casing via oil lubricated bearings by both the side walls of the rotor casing, in which 
     a rotor shaft sealing part comprising two shaft seal means is provided to each of rotor shaft bearing parts between the bearing and the compression chamber such that an annular airspace is formed between the shaft seal means, and 
     pressurized air is supplied to the annular airspace of each of the shaft sealing parts, thereby preventing intrusion of lubrication oil into the compression chamber when operating the rotary compressor. 
     The invention proposes as a rotor shaft sealing structure for applying the method a rotor shaft sealing structure of an oil-free rotary compressor having a pair of male and female rotors accommodated in a compression chamber formed by a rotor casing, each rotor having a rotor shaft extending from both end faces of the rotor to penetrate both side walls of the rotor casing to be supported by the rotor casing via oil lubricated bearings by both the side walls of the rotor casing, which includes 
     a rotor shaft sealing part comprising two shaft seal means provided to each of rotor shaft bearing parts between the bearing and the compression chamber such that an annular airspace is formed between the shaft seal means, and 
     a pressurized air supplier for supplying pressurized air to each of the annular airspace. 
     According to the rotor shaft sealing structure of the invention, pressurized air is supplied to the annular airspace formed between the seal means adjacent the oil lubricated bearing and the seal means adjacent the compression chamber. In load operation of the compressor, pressure in the compression chamber is higher than atmospheric pressure and compressed air in the compression chamber may leak slightly toward the annular airspace through the shaft seal means located adjacent the compression chamber. However, as the pressurized air flows through the annular airspace, pressure in the annular airspace is raised and leak of the compressed air to the annular airspace is reduced. The air leaked to the annular airspace flows out through the communicating hole to the outside of the rotor casing together with the pressurized air. Therefore, even if lubrication oil leaks through the oil seal means located adjacent the rotor shaft bearing to the annular airspace, the lubrication oil leaked to the annular airspace is taken away by the pressurized air to the outside of the rotor casing, such that there is no fear that the lubrication oil intrudes into the compression chamber. 
     When the compressor is operated at no load, suction path of the compressor is shut-off and negative pressure is produced in the compressor chamber. Air in the annular airspace may be ingested through the sealing means locates adjacent the compression chamber thereinto. However, pressurized air is supplied to the annular airspace which is communicated to the outside of the rotor casing and maintained at atmospheric pressure, so there is little fear that lubrication oil leaks through the shaft seal means located adjacent the bearing and intrudes into the combustion chamber. 
     As pressurized air is supplied to the annular airspace as mentioned above, the annular airspace is maintained at a pressure higher than atmospheric pressure, and propagation of negative pressure produced in the compression chamber to the bearing side seal means is prevented and lubrication oil in the oil lubricated bearing is prevented from being ingested into the compression chamber of the compressor. The method of the invention is particularly effective when the compressor is operated at no-load at which negative pressure is produced in the compression chamber. 
     In the method, it is preferable that lubrication oil leaked from the bearing to the annular airspace is exhausted to the outside of the rotor casing through a communicating hole which opens at a bottom part of the annular airspace to communicate the annular airspace to the outside of the rotor casing. Even if lubrication oil leaks from the bearing to the annular airspace, it is taken out to the outside of the rotor casing, resulting in that the leaked lubrication oil is prevented from intruding into the compression chamber. 
     As a shaft sealing structure, it is suitable to composed the structure such that at least one communicating hole for communicating each annual airspace to the outside of the rotor casing is provided such that it opens at a bottom part of the annular airspace to communicate the annular airspace to the outside of the rotor casing, and that each of the annular airspaces of the male rotor shaft sealing parts and each of those of the female rotor shaft sealing parts are connected by a between-rotor shaft communication passage respectively so that pressurized air supplied to each annular airspace of one of the rotor shaft sealing parts is supplied to each annular airspace of the other rotor shaft sealing part. 
     As the between-rotor shaft communication passage is provided to connect between the annular airspaces of the male and female rotor shaft sealing parts, even if the communicating hole communicating the annular airspace of the rotor shaft sealing part of one of the rotor shaft bearing part to the outside of the rotor casing is clogged, pressurized air can flow through the communicating hole communicating the annular airspace of the rotor shaft sealing part of the other rotor shaft bearing part to the outside of the rotor casing, and leaked lubrication oil to any of the annular airspaces can be taken away by the pressurized air. 
     By forming pressurized air passages connecting to the between-rotor shaft communication passages respectively in the rotor casing in order to supply pressurized air to the annular airspaces, pressurized air is supplied to the annular airspaces via the passages and between-rotor shaft communication passages. 
     According to the rotor shaft sealing method and structure of the invention, rotor shaft sealing structure of an oil-free rotary compressor is provided with which risk of occurrence of lubrication oil intrusion into the compression chamber of the compressor which is liable to occur when negative pressure is produced in the compression chamber, is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in greater detail with reference to certain preferred embodiments thereof and the accompanying drawings, wherein: 
         FIG. 1  is a longitudinal sectional view of a rotary compressor of which rotor shaft sealing structure of the invention is adopted; 
         FIG. 2  is a partially enlarged section of  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view of the viscoseal part of  FIG. 1 ; 
         FIG. 4  is a sectional view along the line A-A in  FIG. 1 ; 
         FIG. 5  is an example of compression system using compressors to which the rotor shaft sealing structure of the invention is applied; and 
         FIG. 6   a  to  FIG. 6   d  are drawings for explaining the operation of a tooth type rotary compressor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be detailed with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, relative positions and so forth of the constituent parts in the embodiments shall be interpreted as illustrative only not as limitative of the scope of the present invention. 
     An embodiment of the invention will be explained with reference to  FIGS. 1 to 4 .  FIG. 1  is a longitudinal sectional view of a tooth type rotary compressor of which rotor shaft sealing structure of the invention is adopted,  FIG. 2  is a partially enlarged section of  FIG. 1 ,  FIG. 3  is an enlarged sectional view of the viscoseal part of  FIG. 1 , and  FIG. 4  is a sectional view along the line A-A in  FIG. 1 . 
     Referring to  FIG. 1 , a male rotor  2  and a female rotor  3  are accommodated in a compression chamber  9  formed in a rotor casing  1  which is composed of an upper casing member  1   a , a lower casing member  1   b , and an intermediate casing member  1   c . The rotors are center-aligned with dowel pins  11  and connected together by means of bolts  18 . The male rotor  2  and female rotor  3  are respectively fixed to a male rotor shaft  6  and a female rotor shaft  7  supported rotatably by the upper and lower casing members  1   a  and  1   b  via bearings  10  and bearings  10 ′. Reference numerals  14   a  and  15   a  are cover plates for holding bearings  10 ′. 
     A gear  8  is fixed to one end of the male shaft  6 . The gear  8  meshes with a gear  13  fixed to a rotation shaft  12  of an electric motor not shown in the drawing so that the male rotor  2  is driven by the electric motor. Timing gears  14  and  15  are attached to the lower end of the male rotor shaft  6  and the female rotor shafts  7  respectively so that both the rotors are rotated in synchronization in counter directions at the same rotation speed. The timing gears  14  and  15  are covered by a cover  40  bolted by bolts  41  to the lower casing member  1   b , and a drain plug  42  is provided to the bottom of the cover  40 . 
     Another tooth type rotary compressor not shown in the drawing is provided to the right of this tooth type rotary compressor and driven the electric motor via the gear  13 . These two rotary compressors constitute a two-stage compressor unit comprised of a low pressure stage compressor and a high pressure stage compressor connected in series to produce high compression pressure. The two compressors are driven by said single electric motor not shown in the drawing, and the gears  8 ,  13  are located in a driving gear room covered by a gear casing  17  attached to the upper casing member  1   a . Lubrication oil is supplied via an oil supply pipe  16  to the bearings  10 ′ through oil passage not shown in the drawing and then flows out through gaps between the cover plates  14   a ,  15   a  and the timing gears  14 ,  15  to lubricate the teeth of the timing gears. The lubrication oil lubricated the bearings  10 ′ and timing gears  14 ,  15  and fell down to the bottom of the cover  40  is drained through the drain pipe connected to the connector  42  to an oil tank not shown in the drawing. 
     Lubrication oil supplied to lubricate the gears  8  and  12  and fell down to upper surface of the upper casing member  1   a  is also drained to said oil tank through drain path not shown in the drawing. 
     Next, shaft sealing structure of the male and female rotor shafts  6  and  7  will be explained referring to  FIG. 2  showing the sealing structure of the bearing part  10  of the male rotor  6  as a representative of the sealing structure. Sealing structure of the lower bearing parts  10 ′ is similar to that and explanation is omitted. Referring to  FIG. 2 , an inner sleeve  21  is inserted tightly on the male rotor  6  between the bearing  10  and the rotor side end face of the upper casing member  1   a . An outer sleeve  23  is received in a bore of the casing member  1   a  such that the outer surface of the outer sleeve  23  is sealed with O-rings  26  and  27 , and the O-rings also serve to prevent the outer sleeve  23  from rotating by friction force exerting between O-rings and the outer sleeve  23  and the bore of the upper casing member  1   a . A circular groove is formed in the upper casing member  1   a  such that an annular airspace  24  is formed to surround the outer surface of the outer sleeve between the O-rings  26 ,  27 . The outer sleeve  23  has an inner grove  19  which is communicated by radial holes  23   a  of the outer sleeve  23  to the annual airspace  24 . The inner groove  19  and the annular airspace  24  are horizontal when the rotor shafts  6  is vertical, and the bottom face of the annular space  24  is positioned a little lower than the bottom face of the annular groove  19  and the radial holes  23   a  communicate the inner groove  19  to the annular airspace  24  such that lubrication intruded into the inner groove  19  does not accumulate in the inner groove  19  but flows to the annular airspace  24  by gravity. Reference numeral  22  is a snap ring for restricting axial movement of the outer sleeve  23 . 
     A viscoseal zone is formed between the outer surface of the inner sleeve  21  and the inner surface of the outer sleeve  23  along a range indicated by reference numeral  20 . Referring to  FIG. 3 , on the outer surface of the inner sleeve  21  is formed a thread  21   a  in the range  20  and the top face of the thread does not contact with the inner surface of the outer sleeve  23 . Lubrication oil after lubricating the bearing  10  fills the clearance between the thread  21   a  and the inner surface of the outer sleeve  23 . The thread  21   a  is formed such that lubrication oil filled the clearance  21   a  is pressurized by screw pump effect of the thread  21   a  and forced upward (in direction b) by the rotation of the male rotor shaft  6 . This action prevents lubrication oil from intruding into the inner groove  19 . 
     Viscoseal effect can be obtained by forming a female thread on the inner surface of the outer sleeve  23  instead of forming the male thread  21   a  on the outer surface of the inner sleeve  21 . 
     A contact type shaft seal  30  composed of a ring-shaped carbon seal  31  and an outer ring  32  made of metal is provided under the lower end of the outer sleeve  23 . A communication hole  34  descending from the lower end face of the annular airspace  24  to an opening end  33  to communicate the annular airspace  24  to outside is provided in the upper casing member  1   a . The annular airspace  24  is communicated to the inner groove  19  through the radial holes  23   a  of the outer sleeve  23  as mentioned before. The outside opening end  33  of the communication hole  34  is positioned at a position lower than the inner groove  19  so that lubrication oil leaked through the viscoseal zone to the inner groove  19  flows down through radial holes  23   a  and through the communication hole  34  into the gear room enclosed by the gear casing  17  and the upper casing member  1   a.    
     As can be seen in  FIG. 1  and  FIG. 4 , one communication hole  34  to communicate the annular airspace to the outside is provided for each of the annular airspaces  24  of the male and female rotor shaft sides, and further a between-rotor shaft communication passage  35  is provided in the upper casing member  1   a  to communicate the annular airspace  24  of the male rotor side to that of the female rotor side. The rotor shaft sealing structure at the under part of each of the male and female rotor shafts is similar to that of the above mentioned structure as can be seen in  FIG. 1 . 
     A communication hole  37  which is larger in diameter than that of the communication hole  34  is provided to communicate the annular airspace  24  of the female rotor shaft side to the outside such that the communicating hole  37  inclines downward as is the communication hole  34 . Reference numeral  36  indicates the outside opening end of the communication hole  37 . Even if the communication holes  34  are clogged by any cause, lubrication oil intruded into the inner groove  19  can be exhausted to the outside of the upper casing member  1   a  in the driving gear room covered by the gear casing  17 . 
     Next, an example of compression system using tooth type rotary compressors shown in  FIGS. 1˜4  will be explained with reference to  FIG. 5 . Referring to  FIG. 5 , air a to be compressed is taken into the compression system through a filter  41  provided with a silencer  42 . The air a is sucked into a low-pressure stage tooth type compressor  44  through a suction shut-off valve  43  to be compressed to 0.2 MPa for example. The air increased in temperature to about 200° C. by the compression is cooled by an intercooler  45 . 
     The air cooled in the intercooler  45  is deprived of moisture by a moisture separator  50 , then introduced into a high-pressure stage tooth type rotary compressor  46  to 0.7 MPa for example. The compressed air is alleviated in pulsation of pressure in a pulsation damper  47 , then introduced to an aftercooler  48  through a check valve  49 . The air compressed in the high-pressure stage compressor  46  and increased in temperature to about 200° C. is cooled by an aftercooler  48 , deprived of moisture in a moisture separator  51 , then sent to a refrigeration type air drier  52 . The low-pressure stage compressor  44  and the high-pressure stage compressor  46  are tooth type rotary compressors according to the embodiment shown in  FIGS. 1˜4 . 
     The air a is cooled in the refrigeration type air drier  52  by the refrigerant of a refrigerating machine  53 , then moisture in the cooled air is removed in a moisture separator  54 , then supplied via a supply valve  55  to an air tank not shown in the drawing. 
     In a lubricating oil system  60 , lubrication oil in an oil tank  61  is supplied to the low-pressure stage and high-pressure stage compressors  44  and  46  by an oil pump  62  via oil pipe line  63 . Lubrication oil sucked by the oil pump  62  from the oil tank  61  is sent to an oil cooler  64  to be cooled therein and then filtered through an oil filter  65  before supplied to the compressors. A bypass valve  66  is provided to the oil filter  65  to control lubricating oil flow to the compressors. 
     The compression system is usually operated with the supply valve  55  opened. When operating at no load, pressure rise in a delivery pipe to which the supply valve  55  is provided is detected and the shut-off valve  43  is closed based on the detected pressure rise by means of an electromagnetic valve (not shown in the drawing) connected to the shut-off valve  43 . However, if the shut-off valve  43  is completely closed, there occurs abnormal noise, so the shut-off valve  43  is not completely closed but slightly opened so that a slight amount of air can flow through the valve. 
     The slight amount of air passed through the shut-off valve  43  is compressed through the low-pressure stage and high-pressure stage compressors  44  and  46  and returns to the suction shut-off valve  43  via a flow path  56 . The slight amount of air returned to the shut-off valve  43  is usually released from a vent  57 , but in the embodiment, a part or all of the air to be let out from the vent  57  is supplied to the shaft sealing parts of the compressors  44  and  46  through a pressurized air flow path  71 . 
     In load operation, the flow path  56  is shut-off by opening action of the suction shut-off valve  43 . 
     As shown in  FIG. 1 , air passages  74  and  75  are bored in both the casing members respectively for connecting the communication passages  35  to the outside. The pressurized air flow path  71  is connected to the air passages  74  and  75  via branch paths  72  and  73  respectively. The slight amount of air is pressurized usually to 0.1˜0.2 MPa, positive pressure higher than atmospheric pressure. This pressurized air is supplied to the annular airspaces  24  of the rotor shaft sealing parts through the pressurized air flow paths  71 ˜ 73 , air passages  74  and  75  and the between-rotor shaft communication passage  35 . The flow of the pressurized air to the annular spaces  24  can be controlled by providing a flow regulator valve in the pressurized air flow path  72  or  73 . 
     When the compression system is in load operation, pressure in the compression chamber is positive and higher than the pressure in the gear room enclosed by the gear casing  17  and the upper casing member  1   a , and compressed gas may slightly leaks through the contact type shaft seal  30  toward the inner groove  19 . As the viscoseal  20  is provided between the bearing  10  and the inner groove  19 , lubrication oil intruded into the viscoseal zone  20  is forced upward by the rotation of the male rotor shaft  6  as mentioned above and does not leaks into the inner groove  19 . Therefore, ingestion of lubrication oil into the compression chamber  9  does not occur. 
     When the low-pressure stage and high-pressure stage compressors  44  and  46  are in no-load operation, the suction path is shut off by the suction shut-off valve  43 , however in practice slightly opened to allow air to be slightly sucked, for if completely shut off there occurs abnormal noise. Negative pressure is produced in the compression chamber  9  in no-load operation of the compressor. Therefore, there is fear that air is ingested from the inner groove  19  through the contact type shaft seal  30  to the compression chamber  9 , which tends to reduce pressure in the inner groove  19  resulting in decreased oil seal effect of the viscoseal  20 . According to the embodiment, pressurized air is introduced to the annular airspaces  24  from the suction shut-off valve  43  through the pressurized air flow path  71 , bypass paths  72 ,  73 , air passages  74 ,  75  and communication passages  35  in the casing members  1   a ,  1   b , and flows out through the communicating holes  34 ,  34 ′ to the outside of the casing members  1   a ,  1   b . Therefore, if there is leaked lubrication oil in the inner grooves  19  and annular airspaces  24 , it is taken away to the outside of the rotor casing  1  by the pressurized air. 
     Negative pressure propagated from the compression chamber  9  is interrupted by the positive pressure in the inner grooves  19 , not to be propagated to the bearing sides  10 ,  10 ′. 
     Therefore, there is little fear that lubrication oil is ingested into the compression chamber  9 . Thus, positive pressure in the annular spaces  24  serve to interrupt negative pressure produced in the compression chamber when the compressors are operated at no load, and intrusion of lubrication oil into the compression chamber  9  is prevented. 
     Lubrication oil may intrude into the inner groove  19  when operation of the compressor is stopped. The lubrication oil intruded into the inner groove  19  is taken out by the pressurized air through the radial holes  23   a  of the outer sleeve  23 , the annular airspace  24 , and the downward inclining communication hole  34  to the outside of the upper casing member  1   a . As communication hole  34  is also provided for annular airspace  24  of female rotor side and the annular airspace of female rotor side is connected with the communication passage  35 , even when one of the communication hole is clogged by any cause, the lubrication oil can be taken out to the outside of the upper casing member  1   a  through the other communication hole. 
     Shaft sealing structure and its action were explained above concerning those of the upper casing member side rotor shaft sealing part. 
     The rotor shaft sealing parts of the lower casing member side bearing part corresponding to those of the upper casing member side bearing part are designated by reference numerals affixed with ′ mark, and the structure is similar to that of the upper casing member side rotor shaft sealing part except that the communication holes  34 ′ of the lower casing member  1   b  are opened to atmosphere and that the viscoseal is composed to force the lubrication oil intruded into the viscoseal zone downward as the rotor shaft rotates. 
     Action of the shaft sealing structure of the lower casing member side rotor shaft sealing part is similar to that of the upper casing member side rotor shaft sealing part. 
     As the communication holes  34 ′ are opened to atmosphere, there is fear that the communication holes  34 ′ are clogged by dust in atmosphere, and provision of a communication holes  37 ′ larger in diameter is particularly preferable. 
     In the embodiment of the shaft sealing structure, a case the rotary compressor is installed so that the rotor shafts extend vertically is explained. It is applicable when the rotary compressor is installed so that the rotor shafts  6 ,  7  extend horizontally. In this case, it is preferable that the communication hole  34  and  34 ′ are provided only to down side rotor shaft sealing parts of the casing members  1   a  and  1   b  respectively. As the annular airspaces  24  in the casing members  1   a  and  1   b  are connected to those of the upper side rotor shaft sealing parts of the casing members  1   a  and  1   b  by the communicating passages  35  respectively, lubrication oil leaked through the viscoseal zone  20  of each of the upper side rotor shaft sealing parts falls down through each communicating passage  35  to the annular airspace of each of the down side rotor shaft sealing parts and exhausted to outside of the casing member  1   a  in the driving gear room covered by the gear casing  17  and to the outside of the casing member  1   b  to the atmosphere respectively. 
     In the compression system of  FIG. 5 , pressurized air is taken out from the suction shut-off valve  43  when the system is in no-load operation. It is also suitable to provide a separate pressurized air supplier such as an air tank to which pressurized air compressed by the system is supplied. Further, pressurized air may be taken out directly from the pulsation damper  47  or from the air duct connecting the low-pressure stage compressor  44  to the high-pressure stage compressor  46 . In these cases, pressurized air can be supplied to the annular airspaces  24  not only in no-load operation but in load operation of the system, and excellent sealing effect can be expected always in operation of the system. 
     INDUSTRIAL APPLICABILITY 
     According to the invention, rotor shaft sealing structure of an oil-free rotary compressor is provided with which occurrence of lubrication oil intrusion into the compression chamber of the compressor which is liable to occur when negative pressure is produced in the compression chamber, is prevented by providing an annular airspace between the oil lubricated bearing side seal means and compression chamber side seal means and supplying pressurized air to the annular airspace communicated to the outside of the rotor casing. 
     This application is based on, and claims priority to, Japanese Patent Application No: 2007-95583, filed on Mar. 30, 2007. The disclosure of the priority application, in its entirety, including the drawings, claims, and the specification thereof, is incorporated herein by reference.