Patent Publication Number: US-10781805-B2

Title: Small air compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a small air compressor. More specifically, the present invention relates to a cylinder coupling structure of a small air compressor capable of reducing the size and weight of a compressor by manufacturing a cylinder of a reciprocating piston type compressor suctioning and compressing fluid such as air or a refrigerant gas separately from a block and coupling the cylinder to the block. 
     BACKGROUND ART 
     Compressors are used to produce compressed air or to compress fluid such as refrigerant gas. Compressors are mainly classified into reciprocating piston type compressors which compress air by a reciprocating motion of a piston in a cylinder, and rotary vane type compressors which compress air by rotating a rotator in the cylinder. Rotary vane type air compressors make less noise, but are applied only to large compressors mostly of 20 HP or more because there are difficulties in producing small products. Reciprocating piston type compressors are applied to products in various sizes, mostly of 20 HP or less. 
     Korean Utility Model Registration No. 20-0387141 discloses a reciprocating piston type compressor for compressing air. Korean Patent Registration No. 10-1073763, Korean Utility Model Registration No. 20-0122684 and Korean Patent Laid-Open No. 10-2010-0081807 disclose small reciprocating piston type, i.e., reciprocating, compressors for compressing a refrigerant of a refrigerating apparatus. 
       FIG. 1 a    illustrates a small reciprocating compressor disclosed in Korean Patent Laid-Open No. 10-2010-0081807. Referring to  FIG. 1 a   , conventionally, the typical small reciprocating compressor for compressing a refrigerant gas includes a power unit P generating rotation power inside a case  1  and a compression unit C converting a rotary motion of the power unit P into a reciprocating motion and compressing a refrigerant gas. The power unit P includes a stator  2  elastically supported by springs  2   a  and a rotor  3  rotatably installed inside the stator  2 . 
     The compression unit C includes a block  4  coupled to the stator  2  while integrally having a cylinder unit  4   a  to form a compression space; a crankshaft  5  inserted into a shaft support hole in the block  4  to be supported in the radial direction and axial direction and coupled to the rotor  3  of the power unit P to transfer a rotational force; a connecting rod  6  rotatably coupled to a cam portion of the crankshaft  5  and converting a rotary motion into a linear motion; a piston  7  rotatably coupled to the connecting rod  6  and linearly reciprocating in the cylinder  4   a  to compress a refrigerant; a valve assembly  8  coupled to the front end of the cylinder  4   a  and provided with a suction valve and a discharge valve; a suction muffler  9   a  coupled to the suction side of the valve assembly  8 ; a discharge cover  9   b  coupled to accommodate the discharge side of the valve assembly  8 ; and a discharge muffler  9   c  communicating with the discharge cover  9   b  to attenuate discharge noise of a refrigerant discharged. 
     When power is applied to the power unit P, in the small reciprocating compressor as above, the rotor  3  rotates together with the crankshaft  5  by an interaction force between the stator  2  and the rotor  3 , the connecting rod  6  coupled to the cam portion of the crankshaft  5  performs an orbiting motion, the piston  7  coupled to the connecting rod  6  linearly reciprocates in the cylinder  4   a , compresses a refrigerant suctioned inside the cylinder  4   a  through the suction muffler  9   a  and discharges the refrigerant to the valve cover  9   b , and the refrigerant discharged to the valve cover  9   b  is emitted through the discharge muffler  9   c.    
     In the conventional small reciprocating compressor as illustrated in  FIG. 1 a   , however, the cylinder  4   a  is formed integrally with the block  4 , leading to large size. Thus, there are disadvantages that the conventional small reciprocating compressor needs a large amount of casting or die-casting materials for manufacturing the block  4  and are heavier, resulting in high costs for distribution such as transportation costs. 
     In order to overcome the problems such a conventional small reciprocating piston type compressor has, a small reciprocating compressor was disclosed, as illustrated in  FIG. 1 b   , with a structure in which a cylinder  4   a ′ having a tube-shape body is manufactured separately from a block  4 ′, one end of the cylinder  4   a ′ is brought into contact with the block  4 ′, and a valve cover  9   b ′ and the block  4 ′ are coupled to each other with a press bolt  9   b ′- 1  such that the valve cover  9   b ′ presses the other end of the cylinder  4   a ′ while a valve assembly  8 ′ is coupled to the other end of the cylinder  4   a′.    
     However, in the case of the small reciprocating compressor illustrated in  FIG. 1 b   , the deformation of the cylinder  4   a ′ occurs relatively easily because the cylinder  4   a ′ with a tube-shaped body has a relatively long length, and both ends of the cylinder  4   a ′ having a long length are supported on the block  4 ′ and the valve cover  9   b ′, respectively, and pressed by the fastening of the press bolt  9   b ′- 1 . Further, deformation occurs relatively easily during fastening or operating because the press bolt  9   b ′- 1  for fixing the cylinder  4   a ′ also has a long length, and thus the cylinder  4   a ′ is not precisely coupled to the block  4 ′ but twisted, which is a problem. The deformation of the cylinder  4   a ′ or uneven coupling of the cylinder  4   a ′ as above increases friction of a piston reciprocating therein, and accordingly causes problems of generating noise and significantly degrading durability as well. 
     Meanwhile, the small reciprocating compressor illustrated in  FIG. 1 a    has a structure in which the suction muffler  9   a  and the discharge muffler  9   b  for reducing noise generated from pulsation of air or a refrigerant gas compressed by the reciprocating motion of the piston  7  are manufactured separately from the block  4  and coupled to the valve cover  4  with a tube. Such structure is a factor which makes the structure of the compressor complicated and increases the manufacturing costs. 
     Also, in the case of the small reciprocating compressor illustrated in  FIG. 1 a   , the crankshaft  5  is inserted into the shaft support hole in the block  4  such that both side end portions thereof are shaft-supported by a bearing  5   b  in the axial direction and radial direction. In this regard, much vibration is generated in the crankshaft  5 . The vibration readily damages a ball bearing typically used, and oil feeding is inevitably necessary for reducing vibration and improving durability. Accordingly, the conventional small reciprocating compressor illustrated in  FIG. 1  adopts a structure in which oil in the oil storage unit in the case  1  is pumped by an oil feeder  5   a  and supplied to the bearing  5   b  through an oil path  5   c  formed in the crankshaft  5 . Part of the oil supplied in such a manner is supplied to the cylinder  4   a  to reduce the friction between the piston  7  and the cylinder  4   a.    
     Meanwhile,  FIG. 1 c    illustrates the structure of a piston of another conventional small reciprocating compressor. Conventionally, the compressor has a structure in which a ring insertion groove  7   a ″ is formed around the outer circumference in the upper end portion of a piston  7 ″ to which a connecting rod  6 ″ is rotatably connected, and an O-ring  7   b ″ made of rubber is inserted into the ring insertion groove  7   a ″ to seal a gap between the piston  7 ″ and the inner surface of the cylinder. However, since the conventional piston  7 ″ has a structure in which the O-ring  7   b ″ installed only in the upper end portion is in close contact with the inner surface of the cylinder, left and right vibration of the piston  7 ″ occurs when the piston  7 ″ reciprocates, which is a problem. In addition, since the O-ring  7   b ″ is made of rubber, it has disadvantages of high friction upon contacting the inner surface of the cylinder and degradation in durability. 
     DETAILED DESCRIPTION OF INVENTION 
     Technical Task 
     The present invention was invented by recognizing the aforementioned problems. It is an object of the present invention to provide a small air compressor that can reduce cylinder deformation by forming a supporting end at the side of a cylinder and allowing the supporting end to be supported on a block and also, as a result, precisely retain the coupling of the cylinder and the block upon fastening and operating by reducing the length of a press bolt coupling a valve cover and the block. 
     It is another object of the present invention to provide a small air compressor of simple structure and easy assembling by forming a suction muffler and a discharge muffler integrally with a block. 
     It is another object of the present invention to provide a small air compressor that can reduce the vibration of a crankshaft and improve the durability of a bearing by inserting bushings into the inside of a shaft support hole in a block and installing journal bearings at both ends thereof to support the crankshaft by the journal bearings and also, as a result, allow an oil-free compressor. 
     It is another object of the present invention to provide a small air compressor that can reduce the vibration of a piston by installing sealing rings at both upper and lower ends of the piston. 
     Method for Solving the Technical Task 
     In order to achieve the aforementioned objects, a small air compressor according to the present invention includes a block; a cylinder having a tube-shaped body, coupled to the block; a valve assembly provided with a suction valve and a discharge valve, blocking the front end of the cylinder; a valve cover covering the valve assembly to form a suction space and a discharge space on the top of the valve assembly; at least one press bolt coupling the valve cover and the block such that the cylinder is pressed between the valve cover and the block; a piston reciprocating inside the cylinder; a stator coupled to the block; a rotor positioned to rotate relative to the stator; a crankshaft coupled with the rotor to rotate integrally with the rotor, rotatably shaft-supported on the block; and a connecting rod having both ends connected to the crankshaft and the piston, respectively, to convert a rotary motion of the crankshaft into a linear reciprocating motion of the piston, wherein the block is provided with a supporting end by which the cylinder is pressed to be supported, a latching end is formed on the outer surface of the cylinder, and the press bolt is fastened to the block and the valve cover such that the valve cover presses the front end of the cylinder while the latching end of the cylinder is latched and supported on the supporting end. 
     Also, the small air compressor according to the present invention is characterized in that the block is formed with a cylinder insertion hole penetrating therethrough into which one end portion of the cylinder is inserted, that the supporting end is formed by a step formed in the inner wall of the cylinder insertion hole, and that the cylinder is inserted into the cylinder insertion hole and the latching end is latched and supported on the supporting end. 
     Also, the small air compressor according to the present invention is characterized in that a guide protrusion is formed on any one of the inner surface of the cylinder insertion hole and the outer surface of the cylinder contacting each other, and that a guide groove into which the guide protrusion is inserted while the cylinder is inserted into the cylinder insertion hole is formed on the other one of the inner surface of the cylinder insertion hole and the outer surface of the cylinder. 
     Also, the small air compressor according to the present invention is characterized in that the block is formed integrally with a suction muffler part and a discharge muffler part each having an inlet and an outlet, that a suction connection tube is connected to the outlet of the suction muffler part such that the suction muffler part is connected to the suction space of the valve cover, and that a discharge connection tube is connected to the inlet of the discharge muffler part such that the discharge muffler part is connected to the discharge space of the valve cover. 
     Also, the small air compressor according to the present invention is characterized in that the suction connection tube is formed with an auxiliary suction muffler part. 
     Also, the small air compressor according to the present invention is characterized in that the discharge connection tube is configured to include a pit tube protruding in the direction by which the valve cover presses the cylinder and a pit protruding in the direction of the pit tube, connected to the inlet of the discharge muffler part such that the pit tube is inserted and connected while the valve cover progresses in the direction of pressing the cylinder. 
     Also, the small air compressor according to the present invention is characterized in that the piston is formed with a cut ring insertion end in the front end portion, that an O-ring is inserted into the ring insertion end, and that a fixing ring is inserted into the ring insertion end on the outside thereof to be coupled with the piston. 
     Also, the small air compressor according to the present invention is characterized in that the O-ring is formed of a Teflon material to have a conical shape inclined in the direction toward the outer diameter from the inner diameter and inserted into the ring insertion end such that the outer diameter is oriented in the front end direction of the piston. 
     Also, the small air compressor according to the present invention is characterized in that the fixing ring is coupled to the piston by caulking the portion connected with the piston while being inserted into the ring insertion end. 
     Also, the small air compressor according to the present invention is characterized in that the piston is provided with O-rings in the front end portion and back end portion. 
     Also, the small air compressor according to the present invention is characterized in that the block is provided with a shaft support hole into which the crankshaft is inserted to be shaft-supported, that a tubular journal is inserted into the shaft support hole, that the crankshaft is inserted into the inside of the journal, and that a bushing made of a resin material is inserted into each opening of the journal at both sides. 
     Effect of the Invention 
     By virtue of the aforementioned configuration, the small air compressor according to the present invention has advantages that can reduce cylinder deformation by forming the supporting end at the side of the cylinder and allowing the supporting end to be supported on the block and also, as a result, precisely retain the coupling of the cylinder and the block upon fastening and operating by reducing the length of the press bolt coupling the valve cover and the block. 
     Also, the small air compressor according to the present invention has advantages of simple structure and easy assembling by forming the suction muffler and the discharge muffler integrally with the block. 
     Also, the small air compressor according to the present invention has advantages that can reduce the vibration of the crankshaft and improve the durability of the bearing by inserting bushings into the inside of the shaft support hole in the block and installing journal bearings at both ends thereof to support the crankshaft by the journal bearings and also, as a result, allow an oil-free compressor. 
     Also, the small air compressor according to the present invention has advantages that can reduce the vibration of the piston by installing sealing rings at both upper and lower ends of the piston and also improve wear resistance by forming the sealing ring with a Teflon material, and as a result allow an oil-free compressor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1 a    is a view illustrating a structure of a conventional small reciprocating compressor in which a cylinder is integrally formed in a block. 
         FIG. 1 b    is a view illustrating a coupling structure of a block and a cylinder in a conventional small reciprocating compressor having a structure in which the cylinder manufactured separately is coupled to the block. 
         FIG. 1 c    is a cross-sectional view illustrating the structure of a piston in a conventional small reciprocating compressor. 
         FIG. 2 a    to  FIG. 2 c    are perspective views illustrating a small air compressor to which a cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 3  is a plan view illustrating the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 4  and  FIG. 5  are exploded perspective views illustrating the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 6  is an exploded perspective view illustrating the coupling of a block and a cylinder of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 7  is an exploded perspective view illustrating the coupling of the block, the cylinder, a valve assembly and a valve cover of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 8  is a perspective view illustrating the state of the coupling of a crankshaft, a connecting rod and a piston of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 9  is a perspective view illustrating the valve assembly of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 10  is an exploded perspective view illustrating the valve assembly of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 11  is a cross-sectional view illustrating the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 12  is a cross-sectional view illustrating in detail the coupling of the block and the cylinder of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 13  is a cross-sectional view illustrating another embodiment of the coupling of the block and the crankshaft of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
         FIG. 14 a    to  FIG. 14 c    are perspective, exploded perspective and cross-sectional views respectively illustrating the structure of the piston of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
     
    
    
     Hereinafter, the present invention is described in detail with reference to the accompanying drawings. 
     BEST MODE FOR CARRYING OUT THE INVENTION 
       FIG. 2 a    to  FIG. 2 c    are perspective views illustrating a small air compressor to which a cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 3  is a plan view illustrating the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 4  and  FIG. 5  are exploded perspective views illustrating the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 6  is an exploded perspective view illustrating the coupling of a block and a cylinder of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 7  is an exploded perspective view illustrating the coupling of the block, the cylinder, a valve assembly and a valve cover of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 8  is a perspective view illustrating the state of the coupling of a crankshaft, a connecting rod and a piston of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 9  is a perspective view illustrating the valve assembly of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 10  is an exploded perspective view illustrating the valve assembly of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 11  is a cross-sectional view illustrating the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 12  is a cross-sectional view illustrating in detail the coupling of the block and the cylinder of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied;  FIG. 13  is a cross-sectional view illustrating another embodiment of the coupling of the block and the crankshaft of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied; and  FIG. 14 a    to  FIG. 14 c    are perspective, exploded perspective and cross-sectional views respectively illustrating the structure of the piston of the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied. 
     Referring to the drawings, the small air compressor to which the cylinder coupling structure according to one embodiment of the present invention is applied is configured to include a housing  11 ,  12 ,  13 , a stator  21 , a rotor  25 , a crankshaft  30 , a connecting rod  34 , a block  40 , a cylinder  50 , a piston  60 , a valve assembly  70 , a valve cover  80  and a press bolt  90 . 
     The housing  11 ,  12 ,  13  is a case for accommodating an assembly in which the stator  21 , block  40 , etc. are assembled and protecting the assembly, and is configured with a bottom part  11 , a tubular side-wall part  12  having the top and bottom opened, extending upward from the outer edge of the bottom part  11 , and a cover part  13  for covering the top opening of the side-wall part  12 . The bottom part  11 , the side-wall part  12  and the cover part  13  are integrally coupled while the bottom part  11  and the side-wall part  12  are sequentially disposed vertically to block the top and bottom openings of the cover part  13 , respectively. The housing  11 ,  12 ,  13  is preferably sealed to prevent noise generated upon pumping and prevent the leakage of oil such as a lubricant, etc. to the outside. 
     The stator  21  is to generate a magnetic force for rotating the rotor  25  when electricity is applied, and is fixed to the bottom part  11  of the housing  11 ,  12 ,  13 . The stator  21  is coupled to a fixing plate  22  in an upright position, in order to fix the stator  21 . The fixing plate  22  to which the stator  21  is coupled is supported by four vibration-isolation springs  23  therebeneath and fixed to the bottom part  11  of the housing  11 ,  12 ,  13  by a coupling bolt  24 . The vibration-isolation spring  23  is to absorb vibration generated upon pumping and prevent the transfer of the vibration to the housing  11 ,  12 ,  13 . A member other than the vibration-isolation spring  23 , such as an anti-vibration pad, may be substituted or added for vibration isolation. 
     The stator  21  is in contact with stator coupling pillars  46  protruding downward from a shaft support part  41  in the block  40  and integrally coupled with the block  40 , as described below. 
     The rotor  25  is positioned inside the stator  21  to rotate relative to the stator  21 . The crankshaft  30  is coupled to the rotor  25  to rotate integrally with the rotor  25 . 
     The crankshaft  30  is coupled with the rotor  25  to rotate integrally with the rotor  25  and rotatably shaft-supported on the block  40 . Referring to the drawings, as to the crankshaft  30 , a crank part  32  to which the connecting rod  34  is connected is integrally formed on the top of a shaft part  31 , and an oil feeder  33  for moving a lubricant contained in the bottom part  11  of the housing  11 ,  12 ,  13  to the crankshaft  30  is coupled to the bottom of the shaft part  31 . The lubricant pumped by the oil feeder  33  is supplied to the surface of the crankshaft  30  along an oil path  311  such as a groove or hole formed in the crankshaft  30 . 
     The shaft part  31  in the crankshaft  30  is shaft-supported on a shaft support part  41  in the block  40 . A shaft hole  47  penetrating in the up and down direction is formed in the shaft support part  41  of the block  40 . The shaft part  31  is rotatably inserted into a journal  35  inserted into the shaft hole  47  to be shaft-supported. 
     The crank part  32  in the crankshaft  30  relates to a cam device together with the connecting rod  34  for converting the rotation of the crankshaft  30  into a reciprocating motion of the piston  60 . 
     The connecting rod  34  has both ends connected to the crankshaft  30  and the piston  60 , respectively, to convert a rotary motion of the crankshaft  30  into a linear reciprocating motion of the piston  60 . Referring to the drawings, the connecting rod  34  has a structure that is divided into a rod part  341  connected to the piston  60  with a connecting pin  66  and a journal part  342  connected to the crank part  32  in the crankshaft  30 . The rod part  341  and the journal part  342  divided are connected to each other by a connection pin  343 . Particularly, the connection pin  343  has a position by which the axial direction thereof is twisted perpendicular to the axial direction of the crankshaft  30  such that the rod part  341  and the journal part  342  are rotatable relative to each other around the axis of the connection pin  343 . Thus, a bending force imposed on the connecting rod  34  by the axial displacement of the crankshaft  30  can be absorbed. 
     The block  40  is to shaft-support the crankshaft  30  and to have the cylinder  50  coupled thereto. 
     The present invention is characterized by having a structure in which the cylinder  50  is not formed integrally to the block  40 , but the cylinder  50  is separately formed and coupled to a cylinder coupling part  42  in the block  40 . Referring to the drawings, the block  40  is formed to include a shaft support part  41  of a plate type in the horizontal direction on which the crankshaft  30  is shaft-supported and a cylinder coupling part  42  of a plate type standing vertically with respect to the shaft support part  41 . 
     The shaft support hole  47  is formed in the shaft support part  41  of the block  40 , and the tubular journal  35  is insertedly fixed to the shaft support hole  47 . The crankshaft  30  is inserted into the journal  35  to be shaft-supported. 
     The journal  35  supports smooth rotation of the crankshaft  30  and is made of a material such as bronze having wear resistance such that the crankshaft  30  is supported on the journal  35  in slidingly direct contact thereto.  FIG. 10  and  FIG. 12  are the cross-sectional views illustrating this structure. 
     Meanwhile, according to the present invention, the crankshaft  30  may be formed not to be directly supported on the journal  35  but to be supported by bushings  351 ,  352  made of a resin material coupled to the openings of the journal  35  at both sides, respectively.  FIG. 13  illustrates an embodiment in which the bushings  351 ,  352  made of a resin material such as polyphenylene sulfide (PPS) having excellent thermal resistance and wear resistance are inserted into the openings of the journal  35  at both sides, respectively, and the shaft part  31  in the crankshaft  30  is inserted into the bushings  351 ,  352  to be supported. As illustrated in  FIG. 13 , in the case where the crankshaft  30  is supported by the bushings  351 ,  352  made of a resin material, the feeding of oil is reduced, or an oil-free shaft support structure without the need of oiling itself is secured. In the case of the oil-free shaft support, the present invention may eliminate the configuration on the aforementioned oil feeder  33  or oil path  311 , and accordingly can facilitate reduction of weight and compactness. 
     The present invention has a structure in which the cylinder  50  is manufactured separately from the block  40  and coupled to the cylinder coupling part  42  in the block  40 . A cylinder insertion hole  45  disposed vertically with respect to the shaft support hole  47  is formed in the cylinder coupling part  42 . The cylinder insertion hole  45  is penetratingly formed such that one end portion of the cylinder  50  is inserted into the cylinder coupling part  42 . The cylinder insertion hole  45  forms a step such that the inner diameter on the side where the crankshaft  30  is positioned is smaller, and the diameter on the other side opposite the side where the crankshaft  30  is positioned is greater. Due to the step, a supporting end  451  is formed for a latching end  53  in the cylinder  50  to be supported. The cylinder  50  is inserted into the cylinder insertion hole  45  and the latching end  53  is latched and supported on the supporting end  451  formed by the step formed in the inner wall of the cylinder insertion hole  45 . 
     In the cylinder insertion hole  45 , a guide protrusion  452  corresponding to a guide groove  54  formed in the outer wall of the cylinder  50  for guiding the insertion of the cylinder  50  is formed to be long in the longitudinal direction in the portion having a greater diameter in the inner wall of the cylinder insertion hole  45 . The cylinder  50  moves only in the longitudinal direction and is inserted into the cylinder insertion hole  45  by which the guide protrusion  452  is inserted into the guide groove  54  formed on the outer surface of the cylinder  50  while the cylinder  50  is inserted into the cylinder insertion hole  45 . 
     Meanwhile, the present invention has a structure in which a suction muffler  43  and a discharge muffler  44  for reducing noise generated from pulsation of a fluid upon pumping are integrally formed with the block  40 . Referring to the drawings, the suction muffler  43  and the discharge muffler  44  are formed at both sides of the shaft support part  41  supporting the shaft, respectively. Particularly, the cylinder coupling part  42  is positioned between the suction muffler  43  and the discharge muffler  44 . The end portion of each of the suction muffler  43  and the discharge muffler  44  is coupled to each of both sides of the shaft support part  41  such that the suction muffler  43 , the cylinder coupling part  42 , and then the discharge muffler  44  are interconnected with each other in a “⊏” arrangement shape to form a structure reinforcing the rigidity of the block  40 . The suction muffler  43  is formed with an inlet  431  through which a fluid flows in and an outlet  432  through which a fluid flows out. A suction filter  43   a  is coupled to the inlet  431  in the suction muffler  43  to filter foreign materials contained in air or a refrigerant suctioned. A suction connection tube  93  is connected to the outlet  432  in the suction muffler  43  such that the suction muffler  43  is connected to a suction space  81   a  in the valve cover  80 . Also, the discharge muffler  44  is formed with an inlet  441  through which a fluid flows in and an outlet  442  through which a fluid flows out. A discharge connection tube  94  is connected to the inlet  441  in the discharge muffler  44  such that the discharge muffler  44  is connected to a discharge space  81   b  in the valve cover  80 . A tube connector  44   a  is coupled to the outlet  442  in the discharge muffler  44 . 
     The cylinder  50  is formed to have a circular tube-shaped body to form a space in which fluid such as air or a refrigerant is compressed by the reciprocating motion of the piston  60 . The present invention is characterized in that the cylinder  50  is formed separately from the block  40  and coupled to the block  40 . Particularly, the present invention has a structure in which the latching end  53  is formed in the side portion of the cylinder  50 , and the latching end  53  is latched and supported on the supporting end  451  formed inside the cylinder insertion hole  45  in the block  40 . Referring to the drawings, the cylinder  50  forms a small-diameter part  51  having a smaller outer diameter on the side inserted into the cylinder insertion hole  45  in the block  40  and a great-diameter part  52  having an outer diameter greater than the small-diameter part  51  on the side coupled to the valve assembly  70  and the valve cover  80 , such that a step formed by the small-diameter part  51  and the great-diameter part  52  forms the latching end  53 . The small-diameter part  51  is inserted into the portion having a smaller inner diameter in the cylinder insertion hole  45 , and the great-diameter part  52  is inserted into the portion having a greater inner diameter in the cylinder insertion hole  45  such that the latching end  53  is latched and supported on the supporting end  451  in the cylinder insertion hole  45 . That is, the press bolt  90  fastens the block  40  and the valve cover  80  such that the valve cover  80  presses the front end of the cylinder  50  while the latching end  53  in the cylinder  50  is latched and supported on the supporting end  451 , and thereby the cylinder  50  is coupled to the block  40 . The guide groove  54  is formed on the outer surface of the cylinder  50  for the guide protrusion  452  formed on the inner surface of the cylinder insertion hole  45  to be inserted thereinto such that the cylinder  50  is guided while being inserted into the cylinder insertion hole  45 , and that the cylinder  50  does not rotate when inserted into the cylinder insertion hole  45  and coupled to the block  40 , as disclosed above. Referring to the drawings, the guide groove  54  is formed by cutting the great-diameter part  52  in a certain depth in the longitudinal direction of the cylinder  50  starting from the latching end  53  in the cylinder  50 . 
     Meanwhile, the drawings illustrate an embodiment in which the guide protrusion  452  is formed on the inner surface of the cylinder insertion hole  45 , and the guide groove  54  is formed on the outer surface of the cylinder  50 , but these elements may be formed in the opposite positions. That is, as opposed to the embodiment illustrated in the drawings, the guide protrusion may be formed on the outer surface of the cylinder  50 , and the guide groove may be formed on the inner surface of the cylinder insertion hole  45 . 
     The piston  60  reciprocates inside the cylinder  50  to compress and emit the fluid such as air or a refrigerant suctioned inside the cylinder  50 . The piston  60  is connected to the connecting rod  34  for converting the rotary motion of the crankshaft  30  into a linear motion by the connecting pin  66  and performs a linear reciprocating motion. 
     Meanwhile, the present invention has a structure capable of improving the assemblability and compression sealability of the piston  60 .  FIG. 14 a    to  FIG. 14 c    illustrate in detail the structure of the piston  60 . Referring to  FIG. 14 a    to  FIG. 14 c   , the piston  60  has a structure in which O-rings  63 ,  65  are installed in the front end and back end of a body  61  of tube-shaped body having the front end closed and the back end opened, respectively. Since the O-rings  63 ,  65  are installed in the front end and back end, respectively, as above, and the front end and back end of the piston  60  are closely supported on the inner surface of the cylinder  50 , sealability can be improved, and the vibration of the piston  60  inside the cylinder  50  can be prevented. The present invention has the O-rings  63 ,  65  formed of a Teflon material, not a rubber material, to secure sealability and also secure mechanical properties such as wear resistance, etc. As a result, the feeding of oil to the inner wall of the cylinder  50  can be reduced or removed, and thus an oil-free compressor can be implemented. Particularly, by adopting O-rings  63 ,  65  made of a Teflon material as above, the present invention has a structure to easily couple the O-rings to the piston  60 , in which ring insertion ends  611 ,  612  are cut and formed in the front end portion and back end portion of the body  61  in the piston  60 , respectively, first O-rings  63 ,  65  are inserted into the cut insertion ends  611 ,  612 , respectively, and then fixing rings  62 ,  64  are inserted on the outside thereof and fixed to the piston  60 , and thereby the O-rings  63 ,  65  are coupled to the piston  60 . The fixing rings  62 ,  64  may be press-inserted into the ring insertion ends  611 ,  612  to be coupled with the piston  60 . The fixing rings  62 ,  64  may be coupled to the body  61  in the piston  60  by caulking the portion connected with the body  61  in the piston  60  while being inserted into the ring insertion ends  611 ,  612 , at the same time of press-insertion coupling or separately from press-insertion coupling. 
     Meanwhile, the O-ring  63  coupled to the front end portion of the piston body  61 , among the O-rings  63 ,  65 , mainly seals the gap between the cylinder  50  and the piston  60 . The present invention has a structure in which the O-ring  63  coupled to the front end portion of the body  61  is formed of a Teflon material, as disclosed above, and is formed of a Teflon material to have a conical shape inclined in the direction toward the outer diameter of the O-ring  63  coupled to the front end portion of the body  61  from the inner diameter such that the outer diameter is oriented in the front end direction of the piston  60 , to endure compression pressure.  FIG. 14 c    conceptually illustrates a state in which the O-ring  63  coupled to the front end portion of the body  61  is deformed to be inclined such that the outer rim is oriented to the front end portion of the piston  60 , and thus a donut-shaped plate (in solid line) becomes a conical shape (in dotted line). 
     The valve assembly  70  is provided with a suction valve and a discharge valve and blocks the front end of the cylinder.  FIG. 9 a    and  FIG. 9 b    are perspective and exploded perspective views illustrating in detail the valve assembly  70 . Referring to the drawings, the valve assembly  70  is provided with a valve plate  71  blocking the opening in the front end of the cylinder  50 . A suction inlet  711  connecting the suction space  81   a  formed by the valve cover  80  and the compression space formed inside the cylinder  50  is formed in the valve plate  71 . Also, a discharge outlet  712  connecting the discharge space  81   b  formed by the valve cover  80  and the compression space formed inside the cylinder  50  is formed in the valve plate  71 . A suction valve flip  73  made of an elastic material is coupled to the inside of the valve plate  71  such that the suction inlet  711  is opened only in the direction by which a fluid is suctioned into the compression space in the cylinder  50  from the suction space  81   a . Also, a discharge valve flip  74  made of an elastic material is coupled to the outside of the valve plate  71  such that the discharge outlet  712  is opened only in the direction by which a fluid is discharged to the discharge space  81   b  from the compression space in the cylinder  50 . 
     Meanwhile, in order to prevent excessive opening of the discharge valve flip  74 , a valve stopper  75  is coupled to the outside of the valve plate  71  to be positioned on the top of the discharge valve flip  74 . The valve stopper  75  has a shape corresponding to the discharge valve flip  74  and is coupled to the outside of the valve plate  71  simultaneously with the discharge valve flip  74  by a rivet  76  fastened to a rivet fastener  714  formed in the valve plate  71 . 
     Meanwhile, an emission outlet  713  to which the discharge connection tube  94  connecting the discharge space  81   b  and the discharge muffler  44  is connected is formed in the valve plate  71  such that the compressed fluid discharged to the discharge space  81   b  in the valve cover  80  is emitted to the discharge muffler  44 . 
     The valve assembly  70  coupled as above is disposed to block the opening in the front end of the cylinder  50  and is coupled to the cylinder  50  together with the valve cover  80  by the fastening of the press bolt  90 . In order to seal the portion contacting the cylinder  50 , a cylinder gasket  91  is provided in the edge of the opening in the front end of the cylinder  50 , and a plate gasket  72  is provided in the inner surface of the valve plate  71 . A flip receiving hole  721  is formed in the plate gasket  72  for the suction valve flip  73  to be received. A discharge hole  722  is formed so as not to block the discharge outlet  712  in the valve plate  71 . 
     The valve cover  80  covers the valve assembly to cover the top of the valve assembly  70  and form the suction space  81   a  and the discharge space  81   b  on the top of the valve assembly  70 . A diaphragm  81  is formed inside the valve cover  80  to partition the suction space  81   a  and the discharge space  81   b  and is coupled to the top of the valve plate  71  to cover the top of the valve plate  71  while a cover gasket  92  is interposed therebetween for sealing. The present invention has a structure in which as the valve cover  80  is coupled to the block  40  by the press bolt  90 , the valve cover  80  presses the valve assembly  70  and then the cylinder  50  is pressed and coupled to the block  40 . 
     The press bolt  90  is to couple the cylinder  50 , the valve assembly  70  and the valve cover  80  integrally to the block  40 , as disclosed above. Referring to the drawings, the press bolt  90  performs the coupling by which a bolt head is caught in the valve cover  80  and the front end of the blot is screw-fastened to the block  40 , while the cylinder  50  and the valve assembly  70  are disposed sequentially between the valve cover  80  and the block  40 , such that the latching end  53  in the cylinder  50  is latched on the supporting end  451  in the cylinder insertion hole  45  and the cylinder  50  is pressed to the block  40 . 
     The suction connection tube  93  is a tube body for connecting the suction muffler  43  and the suction space  81   a  in the valve cover  80 . Referring to the drawings, the suction connection tube  93  has one end connected to the outlet  432  in the suction muffler  43  and the other end connected to the inlet formed in the suction space  81   a  in the valve cover  80 . Meanwhile, the present invention is characterized in that an auxiliary muffler part  931  is formed in the suction connection tube  93  to reduce noise generated from suction pulsation of a fluid together with the suction muffler  43 . The auxiliary muffler part  931  is accomplished by an expanded space. 
     The discharge connection tube  94  is a tube body for connecting the discharge muffler  44  and the discharge space  81   b  in the valve cover  80 . Referring to the drawings, the discharge connection tube  94  has one end connected to the inlet  441  in the discharge muffler  44  and the other end connected to the emission outlet  713  formed in the discharge space  81   b  of the valve cover  80 . 
     Meanwhile, the present invention has a structure in which the discharge connection tube  94  is divided into a pit tube  942  and a pit  941  for coupling to be readily assembled while the cylinder  50 , the valve assembly  70  and the valve cover  80  are coupled to the block  40  by the press bolt  90 . Referring to the drawings, the pit tube  942  is connected to the emission outlet  713  in the valve plate  71 , protruding in the progress direction by which the valve cover  80  presses the cylinder. Also, the pit  941  is connected to the inlet  441  in the discharge muffler part  44 , protruding in the direction toward the pit tube  942  such that the pit tube  942  is inserted and connected while the valve cover  80  progresses in the direction of pressing the cylinder  50 . 
     The drawings illustrate a state in which the pit tube  942  is inserted into the pit  941  and coupled thereto, for the sake of convenience. The pit tube  942  is coupled to the valve plate  71  in advance, and the pit tube  942  and the pit  941  are coupled to each other while the distal end of the pit tube  942  is inserted into the opening of the pit  941  connected to the discharge muffler part  44  when assembling the valve cover  80 . 
     The cylinder coupling structure of the small air compressor as disclosed above and illustrated in the drawings is only one embodiment for carrying out the present invention, and should not be construed as limiting the technical idea of the present invention. The protection scope of the present invention is defined only by the matters set forth in the accompanying claims. In addition, embodiments modified and improved without deviating from the gist of the present invention should be construed as falling within the protection scope of the present invention.