Abstract:
This invention provides a swing-type reciprocative compressor capable of maintaining long durability and preventing propagation of heat to the large end portion of a piston, even under high-compression. 
     In the reciprocative compressor having a swing-type piston mechanism, a piston ring is attached to a piston ring groove to seal between a piston and a cylinder. A ring groove is provided separately from the piston ring groove on the outer circumferential side of the piston and on the crankshaft side of the piston ring groove. A guide ring restricted from moving in a radial direction and shaped like a skirt opening toward the crankshaft side is provided in the ring groove.

Description:
CLAIMS OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application serial no. JP2009-127691, filed on May 27, 2009, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to reciprocative compressors and more particularly to a swing-type reciprocative compressor in which a piston swings in a cylinder, assembly is easy and long endurance can be maintained even under high-pressure compression. 
         [0003]    Of compressors for compressing gas, reciprocative compressors have a simple structure and allows for high-compression; therefore, they are used in various fields. 
         [0004]    Reciprocative compressors include a piston type in which a piston is rotatably connected to a connecting rod via a bearing mechanism as illustrated in FIG. 7 of JP-A-2008-297924 and a swing type in which a piston rod is integrally combined with a compression-associated part of the upper portion of a piston as described in JP-A-2006-152960. 
       SUMMARY OF THE INVENTION 
       [0005]    Reciprocative compressors are characterized by being capable of high-compression although they have small-sized and simple mechanisms. Users have increasingly requested the reciprocative compressors to achieve high-performance and high-compression. 
         [0006]    Incidentally, the swing-type reciprocative compressor shown in JP-A-2006-152960 is configured to have a piston ring attached to the upper portion of the piston. Therefore, it has advantages of simplifying assembly and suppressing manufacturing costs. However, when a swing angle is increased along with the rotation of the piston, misalignment occurs between the center of the piston and that of the cylinder. See FIG. 6 of JP-A-2006-152960. 
         [0007]    The piston ring is designed to have such a configuration as to accommodate such misalignment. However, for high-compression, the piston and the inner wall of the cylinder rub together, which poses a major problem of the piston ring “scoring” the cylinder. 
         [0008]    The swing-type reciprocative compressor as described above has a simpler configuration of a compression-associated part of the piston and a less metal portion, compared with the reciprocative compressor having a piston structure as in FIG. 7 of JP-A-2008-297924. This poses a problem in that heat is easily transmitted to the large end portion (the rotary shaft side) of a piston rod portion. 
         [0009]    Such problems are increased particularly on the high-pressure compression side of multi-stage compression. 
         [0010]    The present invention has been made to solve such problems and aims to provide a swing-type reciprocative compressor that can maintain long durability and prevent the propagation of heat to the large end portion of a piston rod portion, even under high-compression. 
         [0011]    A reciprocative compressor of the present invention has a swing-type piston mechanism in which a piston ring is attached to a piston ring groove to seal between a piston and a cylinder. A ring groove is provided separately from the piston ring groove on the outer circumferential side of the piston and on the crankshaft side of the piston ring groove. A guide ring restricted from moving in a radial direction is provided in the ring groove. 
         [0012]    Preferably, the guide ring is shaped like a skirt opening toward the crankshaft side. 
         [0013]    The reciprocative compressor according the configuration of the present invention can deal with further enhanced performance by reducing leak of compressed fluid from the piston ring. In addition, a heat insulating effect by the guide ring can be expected. 
         [0014]    According to the present invention, a swing-type reciprocative compressor capable of maintaining long durability and preventing propagation of heat to a large end portion of a piston rod portion can be provided, even under high-compression. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a cross-sectional view of a reciprocative compressor according to a first embodiment of the present invention. 
           [0016]      FIG. 2  is a cross-sectional view of the reciprocative compressor according to the first embodiment taken along line I-I of  FIG. 1 . 
           [0017]      FIG. 3  is an enlarged view illustrating the vicinity of an upper portion of a piston of the reciprocative compressor according to the first embodiment of the present invention. 
           [0018]      FIGS. 4A ,  4 B and  4 C are views for assistance in explaining a shape of a piston ring  44 . 
           [0019]      FIGS. 5A and 5B  are views for assistance in explaining a shape of a guide ring  43 . 
           [0020]      FIG. 6  is an exploded lateral view of parts in the vicinity of the upper portion of the piston. 
           [0021]      FIG. 7  is an exploded perspective view illustrating a piston rod portion  47  and parts above the piston rod portion  47 . 
           [0022]      FIG. 8A  illustrates a condition where the piston is at top dead center. 
           [0023]      FIG. 8B  illustrates a condition where the piston is at bottom dead center. 
           [0024]      FIG. 8C  illustrates a condition where a swing angle of the piston with respect to the piston is maximized. 
           [0025]      FIGS. 9A and 9B  are views for assistance in explaining the shape of a guide ring according to a first modification of the present embodiment. 
           [0026]      FIG. 10  is an enlarged view illustrating the vicinity of an upper portion of a piston according to a second modification of the present embodiment. 
           [0027]      FIG. 11  is an enlarged view illustrating the vicinity of an upper portion of a piston according to a third modification of the present embodiment. 
           [0028]      FIG. 12  is an enlarged view illustrating the vicinity of an upper portion of a piston according to a fourth modification of the present embodiment. 
           [0029]      FIG. 13  is a cross-sectional view of a reciprocative compressor according to a second embodiment of the present invention. 
           [0030]      FIG. 14  is a cross-sectional view of the reciprocative compressor according to the second embodiment taken along line I-I of  FIG. 13 . 
           [0031]      FIG. 15  is an enlarged view illustrating the vicinity of an upper portion of a piston of the reciprocative compressor according to the second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    Preferred embodiments according to the present invention will hereinafter be described with reference to  FIGS. 1 to 15 . 
       First Embodiment 
       [0033]    A first embodiment according to the present invention will hereinafter be described with reference to  FIGS. 1 to 12 . 
         [0034]    A configuration of a reciprocative compression according to the first embodiment of the present invention is first described with reference to  FIGS. 1 to 7 .  FIG. 1  is a cross-sectional view of the reciprocative compressor according to the first embodiment of the present invention.  FIG. 2  is a cross-sectional view of the reciprocative compressor according to the first embodiment taken along line I-I of  FIG. 1 .  FIG. 3  is an enlarged view illustrating the vicinity of an upper portion of a piston of the reciprocative compressor according to the first embodiment of the present invention.  FIGS. 4A ,  4 B and  4 C are views for assistance in explaining a shape of a piston ring  44 .  FIGS. 5A and 5B  are views for assistance in explaining a shape of a guide ring  43 .  FIG. 6  is an exploded lateral view of parts in the vicinity of the upper portion of the piston.  FIG. 7  is an exploded perspective view illustrating a piston rod portion  47  and parts above the piston rod portion  47 . 
         [0035]    The reciprocative compressor  10  sucks in gas (fluid) and compresses and discharges it. Referring to  FIGS. 1 and 2 , the reciprocative processor  10  has a crankcase  11 , the inside of which serves as a crank chamber  12 . An electric motor  15  is attached to the crankcase  11  as illustrated in  FIG. 1 . The electric motor  15  is composed of a stator  16  and a rotor  17 . The stator  16  is mounted to a stator holder  18 . The rotor  17  is secured to a rotor holding member  21  fitted to a key  20  attached to a key groove  19 . The rotor holding member  21  is secured to a bearing  23  held by a bearing holding portion  22  of the crankcase  11  and to an output shaft  26  supported by a bearing  25  held by a bearing holding portion  24 . 
         [0036]    The output shaft  26  of the electric motor  15  has one end portion projecting into the crank chamber  12 , and a crank member  29  is concentrically secured to this projecting end portion. The crank member  29  and the output shaft  26  of the electric motor  15  constitute a crankshaft  28 . The output shaft  26  is formed with a key groove  31 . The crank member  29  is formed with a fitting hole  32  adapted to receive the output shaft  26  fitted thereinto eccentrically with respect to the outer circumferential portion. In addition, the fitting hole  32  is formed with a key groove  33 . A key  34  is fitted into the key grooves  31 ,  33  to unite the crank member  29  with the output shaft  26 . In this way, the crankcase  11  supports the crankshaft  28  via the bearings  23 ,  25 . 
         [0037]    A balance weight  37  is secured to the output shaft  26  of the electric motor  15  by means of a nut  38  screwed to the output shaft  26  so as to be abutted against the crank member  29  at the intermediate position of the output shaft  26 . A cooling fan  39  is secured to the output shaft  26  at its distal end position. 
         [0038]    A cylindrical cylinder  45  is mounted onto the crankcase  11  on the proximal end side. The cylinder  45  communicates with the inside of the crank chamber  12  on the proximal end side of its inner circumferential surface  46 . In addition, a cylinder head  50  composed of a valve seat plate  48  and a cylinder head body  49  is mounted on the distal end side of the cylinder  45 . 
         [0039]    As illustrated in  FIG. 2 , a suction chamber  51  communicating with the outside and a discharge chamber  52  communicating with the outside are defined in the cylinder head body  49 . 
         [0040]    The valve seat plate  48  is interposed between the cylinder  45  and the cylinder head body  49 . The valve seat plate  48  is formed with a suction hole  57  adapted to allow the suction chamber  51  to communicate with a compression chamber  61  provided on the side of the cylinder  45  and with a discharge hole  58  adapted to allow the discharge chamber  52  to communicate with the compression chamber  61 . A suction valve  59  and a discharge valve  60  which are reed valve are attached to the valve seat plate  48 . The suction valve  59  and the discharge valve  60  each have a proximal end side secured to the valve seat plate  48  with screws or the like and serving as a fixed end and a distal end side serving as a free end. 
         [0041]    A swing-type piston  63  is slidably inserted and fitted into the cylinder  45 . The piston  63  includes a swing member  41  composed of a circular connecting portion  54 , a rod-like piston rod portion  47  and a disk-like receiving portion  40 ; a disk-like ring holding member  42 ; and a disk-like ring holding member  56 . The circular connecting portion  54  is rotatably connected via a bearing  53  to the eccentrically rotating crank member  29  located in the crank chamber  12  and at one end side of the piston  63 . The rod-like piston rod portion  47  is formed integrally with the connecting portion  54  to radially extend into the cylinder  45 . The disk-like receiving portion  40  is formed integrally with the piston rod portion  47  and provided on the side opposite the connecting portion  54  so as to have the center aligned with that of the piston rod portion  47 . The disk-like ring holding member  42  is coaxially screwed to the receiving portion  40  of the swing member  41 . The disk-like ring holding member  56  is fitted to the disk-like ring holding member  42 . The receiving portion  40  of the swing member  41 , the ring holding member  42  and the ring holding member  56  located on the other end side of the piston  63  are connected to each other and are reciprocated while swinging in the cylinder  45  to define the compression chamber  61  between the cylinder head  50  and the piston. Incidentally, the ring holding members  42 ,  56  may be formed into a single piece. 
         [0042]    The ring holding members  42  and  56  are screwed to the disk-like receiving portion  40  to define a circular piston ring groove  64  recessed radially inwardly, therebetween on the outer circumferential side of the piston  63 . Thus, the ring holding members  42  and  56  are formed with a flange portion  66  on the side opposite the piston rod portion  47  (on the side of the compression chamber  61 ) and with a flange portion  67  on the side of the piston rod portion  47  respectively, so that the piston ring groove  64  is defined between the flange portions  66  and  67 . A piston ring  44  is attached to the piston ring groove  64  between both the flange portions  66  and  67  so as to seal between the piston  63  and the cylinder  45 . 
         [0043]    The piston ring  44  is made of a resin material superior in wear resistance and in self-lubricating and formed generally circularly. The piston ring  44  is shaped in general rectangle in cross-section so as to have a radial width uniform along a generally full circle. The piston ring  44  is formed with a closed gap portion in a circumferential direction so that its diameter can be reduced and increased through the closed gap portion while maintaining sealing performance. Additionally, when the piston  63  is at the top dead center position or the bottom dead center position, its inner diameter in the state where the piston ring  44  is in contact with the inner circumferential surface  46  of the cylinder  45  is greater than the minimum diameter of the piston ring groove  64 . Thus, the piston ring  44  can shift in a radial direction with respect to the piston  63 . In addition, the piston ring  44  can turn relative to the piston  63  because of being structured not to restrict the turn. 
         [0044]    A structure of the piston ring  44  is described in detail by use of  FIGS. 4A ,  4 B and  4 C.  FIG. 4A  is a plan view,  FIG. 4B  is a lateral view and  FIG. 4C  is a cross-sectional view taken along line A-A of  FIG. 4A . 
         [0045]    The piston ring  44  whose shape is illustrated in  FIGS. 4A to 4C  is made of a resilient resin material superior in wear resistance and in self-lubricating and generally circularly molded into a single piece. The piston ring  44  includes a generally circular main annular section  88 , a circular base section  89  and a circular base section  90 . The base section  89  is located at one end of the main annular section  88  in the circumferential direction, shifted to one end thereof in the axial direction and formed thinner than the main annular section  88 . The base section  90  is located at the other end of the main annular section  88 , shifted to the other end in the axial direction and formed thinner than the main annular section  88 . Both the base sections  89  and  90  are shifted from each other in the axial direction of the piston ring  44  and overlap each other in the circumferential direction, whereby mating surfaces  89   a  and  90   a  in contact with each other are formed. The total axial length obtained by adding the respective axial lengths of the base sections  89  and  90  together is equal to the axial length of the main annular section  80 . 
         [0046]    These base sections  89 ,  90  constitute the closed gap portion  91 . That is to say, both the base sections  89  and  90  constituting the closed gap portion  91  are circumferentially shifted from each other, thereby allowing for expansion and contraction of the piston ring  44 . The piston ring  44  is formed with a circumferential closed gap  92  between the base section  89  provided on the one end side of the main annular section  88  in the circumferential direction and the other end portion of the main annular section  88  in the natural state. Similarly, a closed gap  93  is defined between the base section  90  provided on the other end portion of the main annular section  88  and the one end portion of the main annular section  88 . When the piston ring  44  is expanded and contracted, these closed gaps  92  and  93  are enlarged and contracted. 
         [0047]    In the present embodiment, the ring holding member  42  is screwed to the disk-like receiving portion  40  to define the circular guide ring groove  65  recessed radially inwardly, on the outer circumferential side of the piston. The generally disk-like guide ring  43  is attached to the guide ring groove  65  so as to secure the ring holding member  42  and the cylinder  45  centrally and coaxially with each other.  FIGS. 5A and 5B  illustrate a shape of the guide ring  43 .  FIG. 5A  illustrates the guide ring  43  as viewed from the side of the piston rod portion  47  and  FIG. 5B  is a cross-sectional view taken along line A-A of  FIG. 5A . The guide ring  43  is formed with a skirt section  71  adapted to increase a contact surface between the guide ring  43  and an inner wall surface  46  of the cylinder  45 . 
         [0048]    Parts in the vicinity of the head of the piston to which the piston ring  44  and the guide ring  43  are attached are disassembled as illustrated in  FIGS. 6 and 7 . Incidentally, a tension ring  44   t  illustrated in  FIG. 7  is fitted into the inside of the piston ring  44  to expand the piston ring  44  outwardly through its expanding force. Thus, the tension ring  44   t  urges the piston ring  44  to adhere tightly to the inner circumferential surface  46  of the cylinder  45 . 
         [0049]    The connecting portion  54  is eccentrically rotated by the rotation of the crank member  29 , and the piston ring  44  and the guide ring  43  held by the ring holding member  42  are slidably guided by the inner circumferential surface  46  of the cylinder  45 . In this way, the piston  63  is reciprocated in the cylinder  45  while the ring holding members  42  and  56  swing in a direction perpendicular to the crankshaft. 
         [0050]    The configuration of the reciprocative compressor  10  according to the embodiment is as described above. The operation of the compressor  10  is next described by use of  FIGS. 8A to 8C  in addition to the previous figures.  FIG. 8A  illustrates a condition where the piston is at the top dead center.  FIG. 8B  illustrates a condition where the piston is at the bottom dead center.  FIG. 8C  illustrates a condition where a swing angle of the piston with respect to the cylinder is maximized. 
         [0051]    When the electric motor  15  is drivingly rotated, the crank member  29  secured to the output shaft  26  thereof performs eccentrically rotating movement. Then, the piston  63  rotatably connected to the crank member  29  via the bearing  53  allows the ring holding members  42  and  56 , the piston ring  44  and the guide ring  43  to reciprocate in the cylinder  45 . In a suction stroke, the ring holding member  56  and the piston ring  44  are moved toward the direction opposite the cylinder head  50  to enlarge the compression chamber  61  and to open the suction valve  59  with the discharge valve  60  remaining closed, introducing gas into the compression chamber  61 . In a subsequent compression stroke, the ring holding member  56  and the piston ring  44  are moved toward the cylinder head  50  to contract the compression chamber  61  and to open the discharge valve  60  with the suction valve  59  remaining closed, discharging the compressed gas from the compression chamber  61  into the discharge chamber  52  in the cylinder head  50 . 
         [0052]    In the operation described above, the ring holding member  56  and the piston ring  44  reciprocate in the cylinder  45  while swinging. 
         [0053]    More specifically, at the bottom dead center where the compression chamber  61  is most enlarged, the piston  63  is coaxial with the cylinder  45  ( FIG. 8B ). From this state, the crank member  29  is rotated counterclockwise to perform the compression stroke to move the ring holding members  42 ,  56 , the piston ring  44  and the guide ring  43  in the direction of contracting the compression chamber  61 . Then, up to the middle between the top dead center and the bottom dead center, the connecting portion  54  is eccentrically rotated while being moved upward. Consequently, in the middle between the top dead center and the bottom dead center, the connecting portion  54  is located closest to the cylinder  45  ( FIG. 8C ). In this case, the ring holding members  42 ,  56  are most tilted with respect to the central axis of the cylinder  45 . 
         [0054]    Subsequently, in the middle of movement toward the top dead center, the ring holding members  42  and  56  generate the maximum downward force F resulting from force based on its own weight and from a centrifugal force based on swing. However, the guide ring  43  restricts the downward movement of the ring holding members  42  and  56 . Therefore, the piston ring groove  64  is maintained in the state where its center is generally coincident with the center of the cylinder  45  so that the piston ring  44  is maintained in the state where its center is generally coincident with the ring holding member  42 . Thereafter, at the top dead center where the compression chamber  61  is most contracted, the piston  63  becomes coaxial with the cylinder  45 , and thus the compression stroke is ended ( FIG. 8A ). 
         [0055]    When the crank member  29  is rotated to perform the suction stroke from the state where the ring holding member  42  is at the top dead center, the piston  63  moves the ring holding members  42  and  56 , the piston ring  44  and the guide ring  43  in the direction of enlarging the compression chamber  61 . Then, up to the middle between the top dead center and the bottom dead center, the connecting portion  54  is eccentrically rotated while being moved downward. Consequently, the connecting portion  54  is located closest to the cylinder side in the middle between the top dead center and the bottom dead center. In this case, the ring holding member  42  is most tilted with respect to the central axis of the cylinder  45 . 
         [0056]    Subsequently, the connecting portion  54  returns to the center as the piston  63  goes toward the dead bottom center. At the bottom dead center where the compression chamber is most enlarged, the piston  63  becomes coaxial with the cylinder  45 , and thus the suction stroke is ended. 
         [0057]    According to the embodiment described above, the guide ring restricts the downward movement of the ring holding members  42  and  56  resulting from the maximum downward force F generated during the compression stroke. Therefore, the piston ring groove  64  is maintained in the state where its center is generally coincident with the center of the cylinder  45 . In this way, the piston ring  44  is constantly located at the center of the ring holding member  42 . Thus, it is possible to prevent falling-off of the piston ring  44  from the ring holding member  42  due to the pressure of compressed air, the pressure of compressed air being generated by the central misalignment between the piston ring  44  and the ring holding member  42 . 
         [0058]    Since the guide ring  43  is attached and screwed to the guide ring groove  65 , the center of the guide ring  43  is coincident with that of the ring holding member  42 . When the cylinder  45  is assembled to the crankcase  11 , the guide ring  43  comes into contact with the cylinder internal wall surface  46  to determine the assembly position of the cylinder  45 . In this way, the center of the cylinder  45  is coincident with that of the ring holding member  42 . Thus, it becomes possible to perform centering between the cylinder  45  and the piston ring  44  attached onto the ring holding member  42 . 
         [0059]    The guide ring  43  can achieve a product breakage measure which can prevent the contact between the ring holding members  42 ,  56  and the cylinder  45  due to worn piston ring  44 . 
         [0060]    Since the guide ring  43  is held between the ring holding member  42  and the receiving portion  40 , it is possible to prevent compression heat generated in the compression chamber  61  from being transmitted from the ring holding member  42  to the piston rod portion  47 . This can lower the temperature of a large end portion of the piston rod portion  47 . Thus, the life of the bearing  53  can be extended. 
         [0061]    Various modifications of the first embodiment according to the present invention are described by use of  FIGS. 9 to 11 .  FIGS. 9A and 9B  are views for assistance in explaining a shape of a guide ring according to a first modification of the first embodiment.  FIG. 10  is an enlarged view illustrating the vicinity of an upper portion of a piston according to a second modification of the first embodiment.  FIG. 11  is an enlarged view illustrating the vicinity of an upper portion of a piston according to a third modification of the first embodiment.  FIG. 12  is an enlarged view illustrating the vicinity of an upper portion of a piston according to a fourth modification of the first embodiment. 
         [0062]    The first modification relates to the shape of a guide ring  43 . Although the guide ring  43  of the first embodiment has the skirt section  71  as illustrated in  FIG. 5 , the guide ring  43  of the first modification is shaped in rectangle in cross-section as illustrated in  FIG. 10  by removing the skirt section  71  from the guide ring  43 . 
         [0063]    The second modification relates to the shape of a ring holding member  4  and of a receiving portion  40 . In the second modification, the receiving portion  40  is formed with a step as illustrated in  FIG. 10 . The ring holding member  42  and the receiving portion  40  are fitted to each other so that the center of the piston rod portion  47  can be allowed to coincide with that of the ring holding member  42 . 
         [0064]    In addition to the configuration of the second modification, the third modification is further provided with a heat-insulating air layer  70  between the ring holding member  42  and the receiving portion  40 . This heat-insulating air layer  70  can prevent heat generated by the compressed air compressed in the compression chamber  61  from being transmitted to the large end portion of the piston rod portion  47 . Thus, the life of the bearing  53  can be extended. 
         [0065]    Referring to  FIG. 12 , a reinforcing plate  95  for supporting the piston ring  44  is provided in a fourth modification. The reinforcing plate  95  can support the piston ring  44  for prevention of its wobbling and also firmly secure the guide ring  43 . 
       Second Embodiment 
       [0066]    A second embodiment of the present invention is hereinafter described by use of  FIGS. 13 to 15 . 
         [0067]    The compression stroke is of one-stage compression in the first embodiment; however, the compression stroke in the second embodiment is of two-stage compression.  FIG. 13  is a cross-sectional view of a reciprocative compressor according to the second embodiment of the present invention.  FIG. 14  is a cross-sectional view of the reciprocative compressor according to the second embodiment taken along line II-II of  FIG. 13 .  FIG. 15  is an enlarged view illustrating the vicinity of an upper portion of a piston of the reciprocative compressor according to the second embodiment of the present invention. 
         [0068]    Referring to  FIG. 13 , a piston  73  having a lip ring  86  is connected to an output shaft  26  of the reciprocative compressor of the present embodiment as well as a piston  63  having a piston ring  44  and a guide ring  43  attached to a piston ring groove  64  and a guide ring groove  65 , respectively, in the follow manner. A key  34  is fitted into a key groove  74  formed on the crank member  75  and into a key groove  31  formed on the output shaft  26 , whereby a crank member  75  is united with the output shaft  26 . 
         [0069]    The swing-type piston  73  is slidably inserted and fitted into a cylinder  76 . The piston  73  includes a swing member  81  composed of a circular connecting portion  78 , a rod-like piston rod portion  79  and a disk-like receiving portion  80 ; and a disk-like ring holding member  82 . The circular connecting portion  78  is rotatably connected via a bearing  77  to the crank member  75  located on one end side of the piston  73  and eccentrically rotated in a crank chamber  12 . The rod-like piston rod portion  79  is formed integrally with the connecting portion  78  to radially extend into the cylinder  76 . The disk-like receiving portion  80  is formed integrally with the piston rod portion  79  on the side opposite the connecting portion  78  so as to have the center aligned with that of the piston rod portion  79 . The disk-like ring holding member  82  is coaxially screwed to the receiving portion  80  of the swing member  81 . The receiving portion  80  of the swing member  81  located on the other end of the piston  73  and the ring holding member  82  are connected to each other and are reciprocated while swinging in the cylinder  76  to define a compression chamber  84  between the cylinder head  83  and the piston  73 . The lip ring  86  is attached to a lip ring groove  85  defined between the ring holding portion  82  and the receiving portion  80 . Incidentally, the operation of the compression stroke is the same as that described in the first embodiment. 
         [0070]    In the present embodiment, primary compression is performed by the piston  73  having the lip ring  86  to compress air. The air thus compressed is delivered via a pipe  87  into the cylinder  45  in which secondary compression is performed by the piston  63  having the piston ring  44  and the guide ring  43 .  FIG. 15  is an enlarged view illustrating an essential portion of the piston  73  having the lip ring  86 . 
         [0071]    According to the present embodiment described above, two-stage compression can be performed using the swing-type piston advantageous in cost for both the primary and secondary compression sides; therefore, air compression can be done effectively. 
         [0072]    A modification of the present embodiment is next described below. 
         [0073]    The two-stage compressor may be configured to use the piston ring  44  in the primary compression part. 
         [0074]    The two-stage compressor may be configured such that both the primary compression and the second compression are performed by means of the piston  63  having both the piston ring  44  and the guide ring  43 . Since the piston ring  44  can compress higher-pressure air than the lip ring  86 , the configuration using the piston ring  44  can compress higher-pressure air in the primary compression although increasing manufacturing cost. Thus, compressor efficiency can be increased to allow for further high-pressurization as the overall compressor.