Patent Publication Number: US-9890775-B2

Title: Discharge valve cover for a linear compressor having a valve spring stopper and discharge pulsation reducing chambers

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority under 35 U.S.C. § 119 to Korean Application No. 10-2014-0091880 filed on Jul. 21, 2014, whose entire disclosure is hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     A linear compressor is disclosed herein. 
     2. Background 
     Cooling systems are systems in which a refrigerant is circulated to generate cool air. In such a cooling system, processes of compressing, condensing, expanding, and evaporating the refrigerant may be repeatedly performed. For this, the cooling system may include a compressor, a condenser, an expansion device, and an evaporator. The cooling system may be installed in a refrigerator or air conditioner, which is a home appliance. 
     In general, compressors are machines that receive power from a power generation device, such as an electric motor or turbine, to compress air, a refrigerant, or various working gases, thereby increasing in pressure. Compressors are being widely used in home appliances or industrial fields. 
     Compressors may be largely classified into reciprocating compressors, in which a compression space into and from which a working gas may be suctioned and discharged, is defined between a piston and a cylinder to allow the piston to be linearly reciprocated in the cylinder, thereby compressing the working gas; rotary compressors, in which a compression space into and from which a working gas is suctioned or discharged, is defined between a roller that eccentrically rotates and a cylinder to allow the roller to eccentrically rotate along an inner wall of the cylinder, thereby compressing the working gas; and scroll compressors, in which a compression space into and from which a working gas is suctioned or discharged, is defined between an orbiting scroll and a fixed scroll to compress the working gas while the orbiting scroll rotates along the fixed scroll. In recent years, a linear compressor which is directly connected to a drive motor, in which a piston is linearly reciprocated, to improve compression efficiency without mechanical losses due to movement conversion and has a simple structure, is being widely developed. 
     The linear compressor may suction and compress a working gas, such as a refrigerant, while a piston is linearly reciprocated in a sealed shell by a linear motor, and then, may discharge the working gas. The linear motor may include a permanent magnet between an inner stator and an outer stator. The permanent magnet may be linearly reciprocated by an electromagnetic force between the permanent magnet and the inner (or outer) stator. As the permanent magnet operates in a state in which the permanent magnet is connected to the piston, the refrigerant may be suctioned and compressed while the piston is linearly reciprocated within the cylinder, and then, may be discharged. 
     The present Applicant has a filed a patent (hereinafter, referred to as a “prior art document”) and then registered the patent with respect to the linear compressor, as Korean Patent No. 10-1307688, filed on Sep. 5, 2013 and entitled “linear compressor”, which is hereby incorporated by reference. The linear compressor according to the prior art document includes a shell that accommodates a plurality of components. A vertical height of the shell may be somewhat high, as illustrated in the prior art document. An oil supply assembly to supply oil between a cylinder and a piston may be disposed within the shell. 
     When the linear compressor is provided in a refrigerator, the linear compressor may be disposed in a machine chamber provided at a rear side of the refrigerator. In recent years, a major concern of customers is increasing an inner storage space of the refrigerator. To increase the inner storage space of the refrigerator, it may be necessary to reduce a volume of the machine room. To reduce the volume of the machine room, it may be important to reduce a size of the linear compressor. 
     However, as the linear compressor disclosed in the prior art document has a relatively large volume, the linear compressor is not applicable to a refrigerator, for which an increased inner storage space is sought. To reduce the size of the linear compressor, it may be necessary to reduce a size of a main component of the compressor. In this case, the compressor may deteriorate performance. 
     To compensate for the deteriorated performance of the compressor, it may be necessary to increase a drive frequency of the compressor. However, the more the drive frequency of the compressor is increased, the more a friction force due to oil circulating in the compressor increases, deteriorate in performance of the compressor. 
     Further, the prior art document discloses a feature in which a discharge valve spring that supports a discharge valve is provided as a coil spring. When the coil spring is applied to the discharge valve spring, the discharge valve may rotate with respect to the coil spring, causing abrasion of the discharge valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described in detail with reference to the following drawings in which like reference numerals refer to like elements, and wherein: 
         FIG. 1  is a cross-sectional view of a linear compressor according to an embodiment; 
         FIG. 2  is a cross-sectional view of a suction muffler according to an embodiment; 
         FIG. 3  is a cross-sectional view of a discharge cover and a discharge valve according to an embodiment; 
         FIG. 4  is an exploded perspective view of a cylinder and a frame according to an embodiment; 
         FIG. 5  is a cross-sectional view illustrating a state in which the cylinder and a piston are coupled to each other according to an embodiment; 
         FIG. 6  is an exploded perspective view of the cylinder according to an embodiment; 
         FIG. 7  is an enlarged cross-sectional view of portion A of  FIG. 5 ; 
         FIG. 8  is a perspective view of a discharge valve assembly coupled to the discharge cover according to an embodiment; 
         FIG. 9  is an exploded perspective view of the discharge cover and the discharge valve assembly of  FIG. 8 ; 
         FIG. 10  is a cross-sectional view of the discharge cover and the discharge valve assembly of  FIG. 8 ; 
         FIG. 11  is a cross-sectional view illustrating a refrigerant flow of the linear compressor according to an embodiment; 
         FIG. 12  is a perspective view of a discharge valve assembly coupled to a discharge cover according to another embodiment; 
         FIG. 13  is an exploded perspective view of the discharge cover and the discharge valve assembly of  FIG. 12 ; 
         FIG. 14  is a cross-sectional view of the discharge cover and the discharge valve assembly of  FIG. 12 ; 
         FIG. 15  is a perspective view of a discharge valve assembly coupled to a discharge cover according to still another embodiment; 
         FIG. 16  is a cross-sectional view illustrating a state in which a valve spring and a stopper are coupled to each other according to an embodiment; and 
         FIG. 17  is a cross-sectional view of a discharge valve assembly coupled to a discharge cover according to still another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments will be described with reference to the accompanying drawings. The embodiments may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, alternate embodiments within the spirit and scope will fully convey the concept to those skilled in the art. 
       FIG. 1  is a cross-sectional view of a linear compressor according to an embodiment. Referring to  FIG. 1 , the linear compressor  100  according to this embodiment may include a shell  101  having an approximately cylindrical shape, a first cover  102  coupled to one or a first side of the shell  101 , and a second cover  103  coupled to the other or a second side of the shell  101 . For example, the linear compressor  100  may be laid out in a horizontal direction. In the linear compressor  100 , the first cover  102  may be coupled to a right or first lateral side of the shell  101 , and the second cover  103  may be coupled to a left or second lateral side of the shell  101 . Each of the first and second covers  102  and  103  may be understood as one component of the shell  101 . 
     The linear compressor  100  may further include a cylinder  120  provided in the shell  101 , a piston  130  linearly reciprocated within the cylinder  120 , and a motor assembly  140  that serves as a linear motor to apply a drive force to the piston  130 . When the motor assembly  140  operates, the piston  130  may be linearly reciprocated at a high rate. 
     The linear compressor  100  according to this embodiment may have a drive frequency of about 100 Hz. The linear compressor  100  further include a suction inlet  104 , through which the refrigerant may be introduced, and a discharge  105 , through which the refrigerant compressed in the cylinder  120  may be discharged. The suction inlet  104  may be coupled to the first cover  102 , and the discharge  105  may be coupled to the second cover  103 . 
     The refrigerant suctioned in through the suction inlet  104  may flow into the piston  130  via a suction muffler  150 . Thus, while the refrigerant passes through the suction muffler  150 , noise may be reduced. The suction muffler  150  may be configured by coupling a first muffler  151  to a second muffler  153 . At least a portion of the suction muffler  150  may be disposed within the piston  130 . 
     The piston  130  may include a piston body  131  having an approximately cylindrical shape, and a piston flange  132  that extends from the piston body  131  in a radial direction. The piston body  131  may be reciprocated within the cylinder  120 , and the piston flange  132  may be reciprocated outside of the cylinder  120 . 
     The piston  130  may be formed of a non magnetic material, such as an aluminum material, such as aluminum or an aluminum alloy. As the piston  130  may be formed of the aluminum material, a magnetic flux generated in the motor assembly  140  may not be transmitted into the piston  130 , and thus, may be prevented from leaking outside of the piston  130 . The piston  130  may be manufactured by a forging process, for example. 
     The cylinder  120  may be formed of a non magnetic material, such as an aluminum material, such as aluminum or an aluminum alloy. Also, the cylinder  120  and the piston  130  may have a same material composition, that is, a same kind and composition. 
     As the cylinder  120  may formed of the aluminum material, a magnetic flux generated in the motor assembly  140  may not be transmitted into the cylinder  120 , and thus, may be prevented from leaking outside of the piston  120 . The cylinder  120  may be manufactured by an extruding rod processing process, for example. 
     Also, as the piston  130  is formed of the same material (aluminum) as the cylinder  120 , the piston  130  may have a same thermal expansion coefficient as the cylinder  120 . When the linear compressor  100  operates, a high-temperature (a temperature of about 100° C.) environment may be created within the shell  100 . Thus, as the piston  130  and the cylinder  120  have the same thermal expansion coefficient, the piston  130  and the cylinder  120  may be thermally deformed by a same degree. As a result, the piston  130  and the cylinder  120  may be thermally deformed with sizes and in directions different from each other to prevent the piston  130  from interfering with the cylinder  120  while the piston  130  moves. 
     The cylinder  120  may accommodate at least a portion of the suction muffler  150  and at least a portion of the piston  130 . The cylinder  120  may have a compression space P, in which the refrigerant may be compressed by the piston  130 . A suction hole  133 , through which the refrigerant may be introduced into the compression space P, may be defined in or at a front portion of the piston  130 , and a suction valve  135  to selectively open the suction hole  133  may be disposed on or at a front side of the suction hole  133 . A coupling hole, to which a predetermined coupling member may be coupled, may be defined in an approximately central portion of the suction valve  135 . 
     A discharge cover  200  that defines a discharge space or discharge passage for the refrigerant discharged from the compression space P, and a discharge valve assembly  220 ,  230 ,  240  coupled to the discharge cover  200  to selectively discharge the refrigerant compressed in the compression space P may be provided at a front side of the compression space P. The discharge valve assembly  220 ,  230 ,  240  may include a discharge valve  220  to introduce the refrigerant into the discharge space of the discharge cover  200  when a pressure within the compression space P is above a predetermined discharge pressure, a valve spring  230  disposed between the discharge valve  220  and the discharge cover  200  to apply an elastic force in an axial direction, and a stopper  240  that restricts deformation of the valve spring  230 . 
     The term “compression space P” may refer to a space defined between the suction valve  135  and the discharge valve  220 . The suction valve  135  may be disposed on or at one or a first side of the compression space P, and the discharge valve  220  maybe disposed on or at the other or a second side of the compression space P, that is, a side opposite of the suction valve  135 . 
     The term “axial direction” may refer to a direction in which the piston  130  is reciprocated, that is, a transverse direction in  FIG. 1 . Also, in the axial direction, a direction from the suction inlet  104  toward the discharge outlet  105 , that is, a direction in which the refrigerant flows may be defined as a “frontward direction”, and a direction opposite to the frontward direction may be defined as a “rearward direction”. On the other hand, the term “radial direction” may refer to as a direction perpendicular to the direction in which the piston  130  is reciprocated, that is, a horizontal direction in  FIG. 1 . 
     The stopper  240  may be seated on the discharge cover  200 , and the valve spring  230  may be seated at a rear side of the stopper  240 . Also, the discharge valve  220  may be coupled to the valve spring  230 , and a rear portion or rear surface of the discharge valve  220  may be supported by a front surface of the cylinder  120 . The valve spring  230  may include a plate spring, for example. 
     While the piston  130  is linearly reciprocated within the cylinder  120 , when the pressure of the compression space P is below the predetermined discharge pressure and a predetermined suction pressure, the suction valve  135  may be opened to suction the refrigerant into the compression space P. On the other hand, when the pressure of the compression space P is above the predetermined suction pressure, the refrigerant in the compression space P may be compressed in a state in which the suction valve  135  is closed. 
     When the pressure of the compression space P is above the predetermined discharge pressure, the valve spring  230  may be deformed to open the discharge valve  220 . The refrigerant may be discharged from the compression space P into the discharge space of the discharge cover  200 . When the discharge of the refrigerant is completed, the valve spring  230  may provide a restoring force to the discharge valve  220  to close the discharge valve  220 . 
     The refrigerant flowing into the discharge space of the discharge cover  200  may be introduced into a bop pipe  165 . The loop pipe  165  may be coupled to the discharge cover  200  to extend to the discharge outlet  105 , thereby guiding the compressed refrigerant in the discharge space into the discharge outlet  105 . For example, the loop pipe  165  may have a shape that is wound in a predetermined direction and extends in a rounded shape. The loop pipe  165  may be coupled to the discharge outlet  105 . 
     The linear compressor  100  may further include a frame  110 . The frame  110  may fix the cylinder  120  and be coupled to the cylinder  120  by a separate coupling member, for example. The frame  110  may surround the cylinder  120 . That is, the cylinder  120  may be accommodated within the frame  110 . Also, the discharge cover  200  may be coupled to a front surface of the frame  110 . 
     At least a portion of the high-pressure gas refrigerant discharged through the opened discharge valve  220  may flow toward an outer circumferential surface of the cylinder  120  through a space at a portion at which the cylinder  120  and the frame  110  are coupled to each other. The refrigerant may be introduced into the cylinder  120  through one or more gas inflows (see reference numeral  122  of  FIG. 7 ) and one or more nozzle (see reference numeral  123  of  FIG. 7 ), which may be defined in the cylinder  120 . The introduced refrigerant may flow into a space defined between the piston  130  and the cylinder  120  to allow an outer circumferential surface of the piston  130  to be spaced apart from an inner circumferential surface of the cylinder  120 . Thus, the introduced refrigerant may serve as a “gas bearing” that reduces friction between the piston  130  and the cylinder  120  while the piston  200  is reciprocated. That is, in this embodiment, a bearing using oil is not applied. 
     The motor assembly  140  may include outer stators  141 ,  143 , and  145  fixed to the frame  110  and disposed to surround the cylinder  120 , an inner stator  148  disposed to be spaced inward from the outer stators  141 ,  143 , and  145 , and a permanent magnet  146  disposed in a space between the outer stators  141 ,  143 , and  145  and the inner stator  148 . The permanent magnet  146  may be linearly reciprocated by a mutual electromagnetic force between the outer stators  141 ,  143 , and  145  and the inner stator  148 . The permanent magnet  146  may be provided as a single magnet having one polarity, or a plurality of magnets having three polarities. 
     The permanent magnet  146  may be coupled to the piston  130  by a connection member  138 , for example. In detail, the connection member  138  may be coupled to the piston flange  132  and be bent to extend toward the permanent magnet  146 . As the permanent magnet  146  is reciprocated, the piston  130  may be reciprocated together with the permanent magnet  146  in the axial direction. 
     The motor assembly  140  may further include a fixing member  147  to fix the permanent magnet  146  to the connection member  138 . The fixing member  147  may be formed of a composition in which a glass fiber or carbon fiber is mixed with a resin. The fixing member  147  may be provided to surround an outside of the permanent magnet  146  to firmly maintain the coupled state between the permanent magnet  146  and the connection member  138 . 
     The outer stators  141 ,  143 , and  145  may include coil winding bodies  143  and  145 , and a stator core  141 . The coil winding bodies  143  and  145  may include a bobbin  143 , and a coil  145  wound in a circumferential direction of the bobbin  143 . The coil  145  may have a polygonal cross-section, for example, a hexagonal cross-section. The stator core  141  may be manufactured by stacking a plurality of laminations in a circumferential direction and be disposed to surround the coil winding bodies  143  and  145 . 
     A stator cover  149  may be disposed on or at one side of the outer stators  141 ,  143 , and  145 . One or a first side of the outer stators  141 ,  143 , and  145  may be supported by the frame  110 , and the other or a second side of the outer stators  141 ,  143 , and  145  may be supported by the stator cover  149 . 
     The inner stator  148  may be fixed to a circumference of the frame  110 . Also, in the inner stator  148 , a plurality of laminations may be stacked in a circumferential direction outside of the frame  110 . 
     The linear compressor  100  may further include a support  137  that supports the piston  130 , and a back cover  170  spring-coupled to the support  137 . The support  137  may be coupled to the piston flange  132  and the connection member  138  by a predetermined coupling member, for example. 
     A suction guide  155  may be coupled to a front portion of the back cover  170 . The suction guide  155  may guide the refrigerant suctioned through the suction inlet  104  to introduce the refrigerant into the suction muffler  150 . 
     The linear compressor  100  may include a plurality of springs  176 , which are adjustable in natural frequency, to allow the piston  130  to perform a resonant motion. The plurality of springs  176  may include a first spring supported between the support  137  and the stator cover  149 , and a second spring supported between the support  137  and the back cover  170 . 
     The linear compressor  100  may further include plate springs  172  and  174 , respectively, disposed on both lateral sides of the shell  101  to allow inner components of the compressor  100  to be supported by the shell  101 . The plate springs  172  and  174  may include a first plate spring  172  coupled to the first cover  102 , and a second plate spring  174  coupled to the second cover  103 . For example, the first plate spring  172  may be fitted into a portion at which the shell  101  and the first cover  102  are coupled to each other, and the second plate spring  174  may be fitted into a portion at which the shell  101  and the second cover  103  are coupled to each other. 
       FIG. 2  is a cross-sectional view illustrating a configuration of a suction muffler according to an embodiment. Referring to  FIG. 2 , the suction muffler  150  according to this embodiment may include the first muffler  151 , the second muffler  153  coupled to the first muffler  151 , and a first filter  310  supported by the first and second mufflers  151  and  153 . 
     A flow space, in which the refrigerant may flow, may be defined in each of the first and second mufflers  151  and  153 . The first muffler  151  may extend from an inside of the suction inlet  104  in a direction of the discharge outlet  105 , and at least a portion of the first muffler  151  may extend to an inside of the suction guide  155 . The second muffler  153  may extend from the first muffler  151  to an inside of the piston body  131 . 
     The first filter  310  may be disposed in the flow space to filter foreign substances. The first filter  310  may be formed of a material having a magnetic property. Thus, foreign substances contained in the refrigerant, in particular, metallic substances, may be easily filtered. For example, the first filter  310  may be formed of stainless steel, for example, and thus, the first filter  310  may have a magnetic property to prevent the first filter  310  from rusting. As another example, the first filter  310  may be coated with a magnetic material, or a magnet may be attached to a surface of the first filter  310 . 
     The first filter  310  may be a mesh-type structure and have an approximately circular plate shape. Each filter hole of the first filter  310  may have a diameter or width less than a predetermined diameter or width. For example, the predetermined size may be about 25 μm. 
     The first muffler  151  and the second muffler  153  may be assembled with each other using a press-fit manner, for example. The first filter  310  may be fitted into a portion at which the first and second mufflers  151  and  153  are coupled to or press-fitted together, and then, may be assembled. 
     For example, a groove may be defined in one of the first muffler  151  or the second muffler  153 , and a protrusion inserted into the groove may be disposed on the other one of the first muffler  151  or the second muffler  153 . The first filter  310  may be supported by the first and second mufflers  151  and  153  in a state in which both sides of the first filter  310  are disposed between the groove and the protrusion. 
     In a state in which the first filter  310  is disposed between the first and second mufflers  151  and  153 , when the first and second mufflers  151  and  153  move in a direction that approach each other and then are coupled to or press-fitted, both sides of the first filter  310  may be inserted and fixed between the groove and the protrusion. 
     As described above, as the first filter  310  is provided on the suction muffler  150 , a foreign substance having a size greater than a predetermined size of the refrigerant suctioned through the suction inlet  104  may be filtered by the first filter  310 . Thus, the first filter  310  may filter the foreign substance from the refrigerant acting as the gas bearing between the piston  130  and the cylinder  120  to prevent the foreign substance from being introduced into the cylinder  120 . Also, as the first filter  310  is firmly fixed to the portion at which the first and second mufflers  151  and  153  are press-fitted, separation of the first filter  310  from the suction muffler  150  may be prevented. 
       FIG. 3  is a cross-sectional view of a discharge cover and a discharge valve according to an embodiment.  FIG. 4  is an exploded perspective view of a cylinder and a frame according to an embodiment. 
     Referring to  FIGS. 3 and 4 , the linear compressor  100  according to this embodiment may include the discharge valve  220  selectively opened to discharge the refrigerant compressed in the compression space P. A rear surface of the discharge valve  220  may be disposed to contact a front portion of the cylinder  120 . In a state in which the rear surface of the discharge valve  220  contacts the front portion of the cylinder  120 , the refrigerant within the compression space P may be compressed. When a pressure in the compression space P is above the predetermined discharge pressure, the rear surface of the predetermined discharge valve  220  may be spaced apart from the front portion of the cylinder  120  to open the discharge valve  220 . Thus, the compressed refrigerant may be discharged through the space. 
     The linear compressor  100  may further include the valve spring  230  coupled to the front portion of the discharge valve  220  to elastically support the discharge valve  220 , and the stopper  240  to restrict deformation of the valve spring  230  to a preset or predetermined degree or less. When the discharge valve  220  is opened, the valve spring  230  may be deformed forward. In this way, the stopper  240  may interfere with the valve spring  230  at a front side of the valve spring  230  to prevent the valve spring  230  from being excessively deformed. 
     The linear compressor  100  may include a plurality of spacers  250  and  260 , respectively, disposed on or at first and second sides of the stopper  240 . The plurality of spacers  250  and  260  may include a first spacer  250  disposed between the valve spring  230  and the stopper  240 , and a second spacer  260  disposed at the front side of the valve spring  230 . 
     The first spacer  250  may space the valve spring  230  from the stopper  240  by a preset or predetermined distance to secure a space in which the valve spring  230  may be deformed. The preset or predetermined distance may be determined by an adjustable thickness of the first spacer  250 . 
     The second spacer  260  may be disposed between the stopper  240  and the discharge cover  200  to stably support the stopper  240  on the discharge cover  220 . Thus, when a repetitive impact occurs between the valve spring  230  and the stopper  240 , damage to the stopper  240  by the discharge cover  200 , in particular, a phenomenon that occurs when the discharge cover  200  has a hardness greater than a hardness of the stopper  240  may be prevented. 
     The linear compressor  100  may include a second filter  320  disposed between the frame  110  and the cylinder  120  to filter a high-pressure gas refrigerant discharged through the discharge valve  220 . The second filter  320  may be disposed on or at a portion of a coupled surface on or at which the frame  110  and the cylinder  120  are coupled to each other. 
     The cylinder  120  may include a cylinder body  121  having an approximately cylindrical shape, and a cylinder flange  125  that extends from the cylinder body  121  in a radial direction. The cylinder body  121  may include a gas inflow  122 , through which the discharged gas refrigerant may be introduced. The gas inflow  122  may be recessed in an approximately circular shape along a circumferential surface of the cylinder body  121 . 
     A plurality of the gas inflow  122  may be provided. The plurality of gas inflows  122  may include gas inflows (see reference numerals  122   a  and  122   b  of  FIG. 6 ) disposed on or at one or a first side with respect to a center or central portion  121   c  of the cylinder body  121  in an axial direction, and a gas inflow (see reference numeral  122   c  of  FIG. 6 ) disposed on or at the other or a second side with respect to the center or central portion  121   c  of the cylinder body  121  in the axial direction. 
     One or more coupling portion  126  coupled to the frame  110  may be disposed on the cylinder flange  125 . Each coupling portion  126  may protrude outward from an outer circumferential surface of the cylinder flange  125 , and be coupled to a cylinder coupling hole  118  of the frame  110  by a predetermined coupling member, for example. 
     The cylinder flange  125  may have a seat surface  127  seated on the frame  110 . The seat surface  127  may be a rear surface of the cylinder flange  125  that extends from the cylinder body  121  in the radial direction. 
     The frame  110  may include a frame body  111  that surrounds the cylinder body  121 , and a cover coupling portion  115  that extends in a radial direction of the frame body  111  and is coupled to the discharge cover  200 . The cover coupling portion  115  may include a plurality of the cover coupling holes  116 , in which the coupling member coupled to the discharge cover  200  may be inserted, and a plurality of the cylinder coupling holes  118 , in which the coupling member coupled to the cylinder flange  125  may be inserted. The cylinder coupling holes  118  may be defined in or at positions recessed somewhat from the cover coupling portion  115 . 
     The frame  110  may have a recess  117  recessed backward from the cover coupling portion  115  to allow the cylinder flange  125  to be inserted therein. That is, the recess  117  may be disposed to surround an outer circumferential surface of the cylinder flange  125 . The recess  117  may have a recessed depth corresponding to a front/rear width of the cylinder flange  125 . 
     A predetermined refrigerant flow space may be defined between an inner circumferential surface of the recess  117  and the outer circumferential surface of the cylinder flange  125 . The high-pressure gas refrigerant discharged from the discharge valve  220  may flow toward the outer circumferential surface of the cylinder body  121  via the refrigerant flow space. The second filter  320  may be disposed in the refrigerant flow space to filter the refrigerant. 
     In detail, a seat having a stepped portion may be disposed on or at a rear end of the recess  117 . The second filter  320  having a ring shape may be seated on the seat. 
     In a state in which the second filter  320  is seated on the seat, when the cylinder  120  is coupled to the frame  110 , the cylinder flange  125  may push the second filter  320  from a front side of the second filter  320 . That is, the second filter  320  may be disposed and fixed between the seat of the frame  110  and the seat surface  127  of the cylinder flange  125 . 
     The second filter  320  may prevent foreign substances in the high-pressure gas refrigerant discharged through the opened discharge valve  220  from being introduced into the gas inflow  122  of the cylinder  120  and absorb oil contained in the refrigerant. For example, the second filter  320  may include a felt formed of polyethylene terephthalate (PET) fiber or an absorbent paper. The PET fiber may have superior heat-resistance and mechanical strength. Also, a foreign substance having a size of about 2 μm or more, which is contained in the refrigerant, may be blocked. 
     The high-pressure gas refrigerant passing through the flow space defined between the inner circumferential surface of the recess  117  and the outer circumferential surface of the cylinder flange  125  may pass through the second filter  320 . In this way, the refrigerant may be filtered by the second filter  320 . 
       FIG. 5  is a cross-sectional view illustrating a state in which the cylinder and a piston are coupled to each other according to an embodiment.  FIG. 6  is an exploded perspective view of the cylinder according to an embodiment.  FIG. 7  is an enlarged cross-sectional view of portion A of  FIG. 5 . 
     Referring to  FIGS. 5 to 7 , the cylinder  120  according to this embodiment may include the cylinder body  121  having an approximately cylindrical shape to form a first body end  121   a  and a second body end  121   b , and the cylinder flange  125  that extends from the second body end  121   b  of the cylinder body  121  in the radial direction. The first body end  121   a  and the second body end  121   b  may form both ends of the cylinder body  121  with respect to the central portion  121   c  of the cylinder body  121  in the axial direction. 
     The cylinder body  121  may include the plurality of gas inflows  122 , through which at least a portion of the high-pressure gas refrigerant discharged through the discharge valve  220  may flow. A third filter  330  as a “filter member” may be disposed in the plurality of gas inflows  122 . 
     Each of the plurality of gas inflows  122  may be recessed from the outer circumferential surface of the cylinder body  121  by a predetermined depth and width. The refrigerant may be introduced into the cylinder body  121  through the plurality of gas inflows  122  and the nozzle  123 . 
     The introduced refrigerant may be disposed between the outer circumferential surface of the piston  130  and the inner circumferential surface of the cylinder  120  to serve as the gas bearing with respect to movement of the piston  130 . That is, the outer circumferential surface of the piston  130  may be maintained in a state in which the outer circumferential surface of the piston  130  is spaced apart from the inner circumferential surface of the cylinder  120  by a pressure of the introduced refrigerant. 
     The plurality of gas inflows  122  may include the first and second gas inflows  122   a  disposed on or at one or the first side with respect to the central portion  121   c  in the axial direction of the cylinder body  121 , and the third gas inflow  122   c  disposed on or at the other or a second side with respect to the central portion  121   c  in the axial direction. 
     The first and second gas inflows  122   a  and  122   b  may be disposed at positions closer to the second body end  121   b  with respect to the central portion  121   c  in the axial direction of the cylinder body  121 , and the third gas inflow  122   c  may be disposed at a position closer to the first body end  121   a  with respect to the central portion  121   c  in the axial direction of the cylinder body  121 . That is, the plurality of gas inflows  122  may be provided in numbers which are not symmetrical to each other with respect to the central portion  121   c  in the axial direction of the cylinder body  121 . 
     Referring to  FIG. 6 , the cylinder  120  may have a relatively high inner pressure at a side of the second body end  121   b , which may be closer to a discharge-side of the compressed refrigerant when compared to that of the first body end  121   a , which may be closer to a suction-side of the refrigerant. Thus, more gas inflows  122  may be provided at the side of the second body end  121   b  to enhance the function of the gas bearing, and relatively less gas inflows  122  may be provided at the side of the first body end  121   a.    
     The cylinder body  121  may further include the nozzle  123  that extends from the plurality of gas inflows  122  toward the inner circumferential surface of the cylinder body  121 . Each nozzle  123  may have a width or size less than a width or size of the gas inflow  122 . 
     A plurality of the nozzle  123  may be provided along the gas inflow  122 , which may extend in a circular shape. The plurality of nozzles  123  may be disposed to be spaced apart from each other. 
     Each nozzle  123  may include an inlet  123   a  connected to the gas inflow  122 , and an outlet  123   b  connected to the inner circumferential surface of the cylinder body  121 . The nozzle  123  may have a predetermined length from the inlet  123   a  to the outlet  123   b.    
     The refrigerant introduced into the gas inflow  122  may be filtered by the third filter  330  to flow into the inlet  123   a  of the nozzle  123  and then flow toward the inner circumferential surface of the cylinder  120  along the nozzle  123 . The refrigerant may be introduced into an inner space of the cylinder  120  through the outlet  123   b.    
     The piston  130  may operate spaced apart from the inner circumferential surface of the cylinder  120 , that is, be lifted from the inner circumferential surface of the cylinder  120  by the pressure of the refrigerant discharged from the outlet  123   b . That is, the pressure of the refrigerant supplied into the cylinder  120  may provide a lifting force or pressure to the piston  130 . 
     A recessed depth and width of each of the plurality of gas inflows  122 , and a length L of the nozzle  123  may be determined to have adequate dimensions in consideration of a rigidity of the cylinder  120 , an amount of third filter  330 , or an intensity in pressure drop of the refrigerant passing through the nozzle  123 . For example, if the recessed depth and width of each of the plurality of gas inflows  122  are very large, or the length of the nozzle  123  is very short, the rigidity of the cylinder  120  may be weak. On the other hand, if the recessed depth and width of each of the plurality of gas inflows  122  are too small, an amount of the third filter  330  provided in the gas inflow  122  may be too small. Also, if the length of the nozzle  123  is too long, a pressure drop of the refrigerant passing through the nozzle  123  may be too large, and it may be difficult to perform the function as the gas bearing. 
     The inlet  123   a  of the nozzle  123  may have a diameter greater than a diameter of the outlet  123   b . In the flow direction of the refrigerant, a flow section area of the nozzle  123  may gradually decrease from the inlet  123   a  to the outlet  123   b.    
     In detail, if the diameter of the nozzle  123  is too small, an amount of refrigerant, which is introduced from the nozzle  123 , of the high-pressure gas refrigerant discharged through the discharge valve  220  may be too large, increasing flow loss in the compressor. On the other hand, if the diameter of the nozzle  123  is too small, the pressure drop in the nozzle  123  may increase, reducing the performance of the gas bearing. 
     Thus, in this embodiment, the inlet  123   a  of the nozzle  123  may have a relatively large diameter to reduce the pressure drop of the refrigerant introduced into the nozzle  123 . In addition, the outlet  123   b  may have a relatively small diameter to control an inflow amount of gas bearing through the nozzle  123  to a predetermined value or less. 
     The third filter  330  may prevent a foreign substance having a predetermined size or more from being introduced into the cylinder  120  and perform a function to absorb oil contained in the refrigerant. The predetermined size may be about 1 μm, for example. 
     The third filter  330  may include a thread wound around the gas inflow  122 . The thread may be formed of a polyethylene terephthalate (PET) material and have a predetermined thickness or diameter. 
     A thickness or diameter of the thread may be determined to have adequate dimensions in consideration of a rigidity of a thread. If the thickness or diameter of the thread is too small, the thread may be easily broken due to a very weak strength thereof. On the other hand, if the thickness or diameter of the thread is too large, a filtering effect with respect to foreign substances may be deteriorated due to a very large pore in the gas inflow  122  when the thread is wound. 
     For example, the thickness or diameter of the thread may be several hundreds μm. The thread may be manufactured by coupling a plurality of strands of a spun thread having several tens μm to each other, for example. 
     The thread may be wound several times, and an end of the thread may be fixed through or by a knot. A number of windings of the thread may be adequately selected in consideration of a pressure drop of the gas refrigerant and the filtering effect with respect to foreign substances. If the number of thread windings is too large, the pressure drop of the gas refrigerant may increase. On the other hand, if the number of thread windings is too small, the filtering effect with respect to the foreign substances may be reduced. 
     Also, a tension force of the wound thread may be adequately controlled in consideration of a strain of the cylinder  120  and fixation of the thread. If the tension force is too large, deformation of the cylinder  120  may occur. On the other hand, if the tension force is too small, the thread may not be well fixed to the gas inflow  122 . 
       FIG. 8  is a perspective view of a discharge valve assembly coupled to the discharge cover according to an embodiment.  FIG. 9  is an exploded perspective view of the discharge cover and the discharge valve assembly of  FIG. 8 .  FIG. 10  is a cross-sectional view of the discharge cover and the discharge valve assembly of  FIG. 8 . 
     Referring to  FIGS. 8 to 10 , the linear compressor  100  according to this embodiment may include the discharge cover  200  coupled to a front portion of the frame  110  to define a discharge passage of the refrigerant discharged from the compression space P. The discharge cover  200  may include a cover body  200   a  that defines a discharge passage of the refrigerant discharged through the discharge valve  220 , a frame coupling portion  201  that extends from the cover body  200   a  in a radial direction and is coupled to the frame  110 , and a pipe connection portion  202  to discharge the refrigerant having passed through the discharge passage of the discharge body  200   a  to outside of the discharge cover  200 . The frame coupling portion  201  may be disposed on or at a rear surface of the discharge cover  200 , and the pipe connection portion  202  may be connected to the loop pipe  165 . 
     The discharge valve assembly may be disposed on the discharge cover  200 . The discharge valve assembly may include the discharge valve  220 , the valve spring  230 , the stopper  240 , the spacer  250 , and the spacer  260 . The cover body  200   a  may include a plurality of steps  203  and  205  stepped forward from the frame coupling portion  201 . The plurality of steps  203  and  205  may include a first step  203  recessed backward from the frame coupling portion  201 , and a second step  205  further recessed from the first step  203  toward a resonance chamber  212 . 
     The cover body  200   a  may further include a step connection portion  203   a  that extends inward from the first step  203  in the radial direction and connected to the second step  205 . That is, in the cover body  200   a , the first step  203  may extend inward in the radial direction, and then, may be further recessed backward to form the second step  205 . 
     The first step  203  may have a discharge hole  204  to guide the refrigerant passing through the discharge passage of the cover body  200   a  into the pipe connection portion  202  to discharge the refrigerant from the discharge cover  200 . The discharge hole  204  may pass through at least a portion of the first step  203 . The refrigerant discharged through the discharge valve  220  may flow into the pipe connection portion  202  via the discharge hole  204 . 
     The cover body  200   a  may further include the resonance chamber  212 , which may be further recessed from the second step  205  to define a space to reduce pulsation of the refrigerant. A plurality of the resonance chamber  212  may be provided. At least a portion of the refrigerant discharged through the discharge valve  220  may flow into the space of the resonance chamber  212 . 
     The cover body  200   a  may further include a seat  210  to partition the plurality of resonance chambers  212  to support the second spacer  260 . The plurality of resonance chambers  212  may be further recessed forward from the seat  210  and be disposed to be spaced apart from each other by the seat  210 . 
     A first guide groove  206  to guide at least a portion of the refrigerant discharged through the discharge valve  220  into the plurality of resonance chambers  212  may be defined in the cover body  200   a  as a “gas passage”. The first guide groove  206  may extend forward from the step connection portion  203   a  toward the second step  205 . At least a portion of each of the step connection portion  203   a  and the second step  205  may be cut to define the first guide groove  206 . 
     A plurality of the first guide groove  206  may be provided to correspond to a number of resonance chambers  212 . The plurality of first guide grooves  206  may be spaced apart from each other. As at least a portion of the refrigerant discharged through the opened discharge valve  220  may be introduced into the plurality of resonance chambers  212  along the first guide groove  206 , pulsation generated when the refrigerant flows while the compressor operates may be reduced. 
     A second guide groove  207  to guide coupling of the stopper  240  may be defined in the cover body  200   a . The second guide groove  207  may guide coupling of a guide protrusion of the stopper  240 . At least a portion of each of the step connection portion  203   a  and the second step  205  may be cut to define the second guide groove  207 . 
     A plurality of the second guide groove  207  may be provided to correspond to a number of guide protrusion  243  of the stopper  240 . The plurality of second guide grooves  207  may be spaced apart from each other. 
     The discharge valve  220  may include a valve body  221  selectively attached to a front surface of the cylinder flange  125  of the cylinder  120 , and a valve recess  223  recessed forward from the valve body  221 . The valve recess  223  may be understood as an “interference prevention groove” to prevent at least a portion of the piston  130  from interfering with the discharge valve  220  while the piston  130  moves forward to compress the refrigerant. At least a portion of the piston  130  may include a coupling member to couple the suction valve  135  to the piston  130 . 
     The discharge valve  220  may further include an insertion protrusion  222  that protrudes forward from the valve body  221  and is coupled to the valve spring  230 . The insertion protrusion  222  may be coupled to an insertion hole  232  defined in the valve spring  230 . 
     Each of the insertion protrusion  222  and the insertion hole  232  may have a noncircular cross-sectional shape. For example, the cross-sectional shape may be a polygonal shape. Thus, when the discharge valve  220  is opened or closed in a state in which the insertion protrusion  222  is inserted into the insertion hole  232 , it may prevent the discharge valve  220  from rotating itself. As a result, it may prevent the discharge valve  220  from behaving unstably. In particular, if the gas bearing instead of the oil bearing is used in the linear compressor as described above, as there may be no lubrication for the discharge valve by oil, abrasion of the discharge valve due to the unstable behavior may be reduced. 
     The valve spring  230  may include a plate spring and have an approximately circular plate shape. In detail, the valve spring  230  may be coupled to a front portion of the discharge valve  220  to allow the discharge valve  220  to elastically move. The valve spring  230  may include a spring body  231  having a plurality of cutouts, and the insertion hole  232  defined in an approximately central portion of the spring body  231  and in which the insertion protrusion  222  of the discharge valve  220  may be inserted. 
     The plurality of cutouts may have a spiral shape. Also, the valve spring  230  may be elastically deformed by the plurality of cutouts. 
     The valve spring  230  may includes a spring recess  233  recessed from an outer circumferential surface of the spring body  231 . The spring recess  233  may guide a position of the guide protrusion  243  of the stopper  240 . 
     The stopper  240  may be disposed on or at a front side of the valve spring  230 . In detail, the stopper  240  may include a stopper body  241  to restrict deformation of the valve spring  230  when the valve spring  230  is deformed. The stopper body  241  may have an approximately circular plate shape. When the valve spring  230  is deformed by a preset or predetermined degree or more, the stopper body  241  may be disposed at a position at which the stopper body  241  interferes with the valve spring  230 . 
     The stopper  240  may further includes a valve avoidance groove  242  recessed forward from the stopper body  241 . The valve avoidance groove  242  may be recessed from an approximately central portion of the stopper body  241  to prevent the stopper body  241  from interfering with the insertion protrusion  222  of the discharge valve  220 . That is, when the insertion protrusion  222  moves forward while the discharge valve  220  is opened, the valve avoidance groove  242  may provide an interference avoidance space so that the stopper body  241  does not interfere with the insertion protrusion  222 . 
     The stopper  240  may further include the guide protrusion  243  that protrudes backward from a rear surface of the stopper body  241  to guide coupling of the discharge cover  200 . When the stopper  240  is coupled to the discharge cover  200 , the guide protrusion  243  may move into the cover body  200   a  along the second guide groove  207 . 
     The guide protrusion  243  may be coupled to the spring recess  233  of the valve spring  230 , and a spacer groove  252  of the first spacer  250 . Thus, the valve spring  230  may be stably coupled to the stopper  240  and the first spacer  250 . For example, the stopper  240  may be press-fitted into and fixed to the second guide groove  207  in a state in which the guide protrusion  243  is coupled to the spring recess  233  and the spacer groove  252 . Thus, the stopper  240  may be stably coupled to the discharge cover  200  without using a separate coupling member. 
     The first spacer  250  may be disposed between the valve spring  230  and the stopper  240  to space the valve  230  from the stopper  240 . In detail, the first spacer  250  may include a spacer body  251  having an approximately ring shape, and a spacer groove  252  recessed from an outer circumferential surface of the spacer body  251  to guide a position of the guide protrusion  243  of the stopper  240 . 
     The second spacer  260  may be seated on the seat  210  of the cover body  200   a  to support the stopper  240 . That is, the second spacer  260  may be disposed between the seat  210  and the stopper  240  to prevent the stopper  240  from directly colliding with the discharge cover  200 . 
       FIG. 11  is a cross-sectional view illustrating a refrigerant flow of the linear compressor according to an embodiment. Referring to  FIG. 11 , a refrigerant flow in the linear compressor according to an embodiment will be described herein below. 
     Referring to  FIG. 11 , the refrigerant may be introduced into the shell  101  through the suction inlet  104  and flow into the suction muffler  150  through the suction guide  155 . The refrigerant may be introduced into the second muffler  153  via the first muffler  151  of the suction muffler  150  to flow into the piston  130 . In this way, suction noise of the refrigerant may be reduced. 
     A foreign substance having a predetermined size (about 25 μm) or more, which is contained in the refrigerant, may be filtered while passing through the first filter  310  provided on the suction muffler  150 . The refrigerant within the piston  130  after passing though the suction muffler  150  may be suctioned into the compression space P through the suction hole  133  when the suction valve  135  is opened. 
     When the refrigerant pressure in the compression space P is above the predetermined discharge pressure, the discharge valve  220  may be opened. Thus, the refrigerant may be discharged into the discharge space of the discharge cover  220  through the opened discharge valve  200 , flow into the discharge outlet  105  through the loop pipe  165  coupled to the discharge cover  200 , and be discharged outside of the compressor  100 . 
     When the discharge valve  220  is opened, the valve spring  230  may be elastically deformed in a forward direction. In this way, the stopper  240  may prevent the valve spring  230  from being deformed by a preset or predetermined degree or more. 
     With this embodiment, when the linear compressor  100  operates at a high frequency, an opening degree of the discharge valve  220 , that is, movement of the discharge valve  220  may increase. Thus, when the discharge valve  220  is closed, an impulse applied to the discharge valve  220  may increase, increasing abrasion of or to the discharge valve. When the gas bearing is applied without using oil, abrasion may increase. 
     Thus, in this embodiment, the discharge valve  220  may be elastically supported by the valve spring  230 , and the stopper  240  may be disposed on or at one side of the valve spring  230  to restrict the opening degree of the discharge valve  220 . At least a portion of the refrigerant within the discharge space of the discharge cover  200  may flow toward the outer circumferential surface of the cylinder body  121  via the space defined between the cylinder  120  and the frame  110 , that is, the inner circumferential surface of the recess  117  of the frame  110  and the outer circumferential surface of the cylinder flange of the cylinder  120 . The refrigerant may pass through the second filter  320  disposed between the seat surface  127  of the cylinder flange  125  and the seat  113  of the frame  110 . In this way, a foreign substance having a predetermined size (about 2 μm) or more may be filtered. Also, oil in the refrigerant may be absorbed onto or into the second filter  320 . 
     The refrigerant passing through the second filter  320  may be introduced into the plurality of gas inflows  122  defined in the outer circumferential surface of the cylinder body  121 . While the refrigerant passes through the third filter  370  provided in the plurality of gas inflows  122 , foreign substances having a predetermined size (about 1 μm) or more, which is contained in the refrigerant, may be filtered, and the oil contained in the refrigerant may be adsorbed. 
     The refrigerant passing through the third filter  330  may be introduced into the cylinder  120  through the nozzle(s)  123  and be disposed between the inner circumferential surface of the cylinder  120  and the outer circumferential surface of the piston  130  to space the piston  130  from the inner circumferential surface of the cylinder  120  (gas bearing). The inlet  123   a  of the nozzle  123  may have a diameter greater than a diameter of the outlet  123   b . Thus, a refrigerant flow section area of the nozzle  123  may gradually decrease with respect to the flow direction of the refrigerant. For example, the inlet  123   a  may have a diameter two times greater than a diameter of the outlet  123   b.    
     As described above, the high-pressure gas refrigerant may be bypassed within the cylinder  120  to serve as the gas bearing with respect to the piston  130 , thereby reducing abrasion between the piston  130  and the cylinder  120 . Also, as oil is not used for the bearing, friction loss due oil may not occur even though the compressor  100  operates at a high rate. 
     Also, as the plurality of filters are provided on or in the passage of the refrigerant flowing in the compressor  100 , foreign substances contained in the refrigerant may be removed. Thus, the refrigerant acting as the gas bearing may be improved in reliability. Thus, the piston  130  or the cylinder  120  may be prevented from being worn by the foreign substances contained in the refrigerant. 
     Further, as the oil contained in the refrigerant may be removed by the plurality of filters, it may prevent friction loss due to oil from occurring. The first, second, and third filters  310 ,  320 , and  330  may be referred to as a “refrigerant filter device” in that the filters  310 ,  320 , and  330  filter the refrigerant that serves as the gas bearing. 
     Hereinafter, a description will be made according to another embodiment. As this embodiment is the same as the previous embodiment except for structures of a discharge cover and a discharge valve assembly, different parts therebetween will be described principally, and descriptions of the same or like parts will be denoted by the same reference numerals as the previous embodiment, and repetitive disclosure has been omitted. 
       FIG. 12  is a perspective view of a discharge valve assembly coupled to a discharge cover according to another embodiment.  FIG. 13  is an exploded perspective view of the discharge cover and the discharge valve assembly of  FIG. 12 .  FIG. 14  is a cross-sectional view of the discharge cover and the discharge valve assembly of  FIG. 12 . 
     Referring to  FIGS. 12 to 14 , a discharge cover  300  according to this embodiment may include a cover body  300   a  that defines a discharge passage of a refrigerant discharged through a discharge valve  325 , and a frame coupling portion  301  that extends backward from the cover body  300   a  and is coupled to frame  110 . Also, although not shown, the discharge cover  300  may include a pipe connection portion  202  similar to that described with respect to the previous embodiment. The pipe connection portion  202  may be connected to loop pipe  165 . 
     A discharge valve assembly may be disposed on the discharge cover  300 . The discharge valve assembly may include the discharge valve  325 , the valve spring  335 , and a stopper  340 . In detail, the cover body  300   a  of the discharge cover  300  may include a step  303  stepped forward from the frame coupling portion  301 . The step  303  may have a discharge hole  304  to discharge the refrigerant outside of the discharge cover  300 . 
     The cover body  300   a  may further include a passage formation portion  305  spaced inward from the step  303  in a radial direction. The passage formation portion  305  may have an approximately cylindrical shape. Also, the passage formation portion  305  may include a resonance chamber  312 . 
     A discharge passage  306 , through which the refrigerant discharged through the discharge valve  325  may flow, may defined between the step  303  and the passage formation portion  305 . The refrigerant of the discharge passage  306  may be discharged outside of the discharge cover  300  through the discharge hole  304 . 
     A seat  310 , on which the stopper  340  may be seated, and a plurality of the resonance chamber  312  partitioned by the seat  310  may be disposed within the passage formation portion  305 . The seat  310  may support a front surface of the stopper  340 , and a coupling groove  314 , in which a coupling protrusion  345  of the stopper  340  may be inserted may be defined in the seat  310 . A plurality of the coupling groove  314  may be provided. 
     Each of the plurality of resonance chambers  312  may be recessed forward from the seat  210  to define a space in which the refrigerant may be received. The plurality of resonance chambers  312  may be defined at positions spaced apart from each other by the seat  310 . The refrigerant discharged through the discharge valve  325  may be introduced into the plurality of resonance chambers  312  through a space defined between the passage formation portion  305  of the discharge cover  300  and the discharge valve assembly. 
     The discharge valve  325  may further include a valve body  321  selectively attached to a front surface of cylinder flange  125  of cylinder  120 , a valve recess  323  recessed forward from the valve body  321 , and an insertion protrusion  322  that protrudes backward from the valve body  321  and is coupled to the valve spring  335 . Descriptions with respect to the discharge valve  325  will be derived from those of the discharge valve  220  described with respect to the previous embodiment. 
     The valve spring  335  may include a plate spring and have an approximately circular plate shape. In detail, the valve spring  335  may include a spring body  331  having a plurality of cutouts, an insertion hole  332  defined in an approximately central portion of the spring body  331  and in which the insertion protrusion  322  of the discharge valve  325  may be inserted, and a spring recess  333  recessed from an outer circumferential surface of the spring body  331 . Descriptions with respect to the valve spring  335  will be derived from those of the valve spring  230  described with respect to the previous embodiment. 
     The stopper  340  may be disposed on or at a front side of the valve spring  335 . In detail, the stopper  340  may include a stopper body  341  to restrict deformation of the valve spring  335  while the valve spring  335  is deformed, a stopper recess  342  recessed forward from the stopper body  341 , and a valve avoidance groove  343  further recessed forward from an approximately central portion of the stopper recess  342 . 
     The stopper body  341  may be seated on or at a rear surface of the valve spring  335 . When the valve spring  335  is deformed by a preset or predetermined degree or more, the stopper recess  342  may be disposed at a position recessed forward from the stopper body  341  to interfere with the valve spring  335 . 
     The valve avoidance groove  343  may prevent the stopper recess  342  from interfering with the insertion protrusion  322  of the discharge valve  325 . That is, the valve avoidance groove  343  may provide an interference avoidance space to prevent interference with the insertion protrusion  322  when the discharge valve  325  is opened. 
     The stopper  340  may further include a guide protrusion  344  that protrudes backward from a rear surface of the stopper body  341  to guide coupling of the valve spring  335 . The guide protrusion  344  may be coupled to the spring recess  333  of the valve spring  335 . 
     The stopper  340  may further include a coupling protrusion  345  that protrudes forward from a front surface of the stopper recess  342 . When the stopper  340  is coupled to the discharge cover  300 , the coupling protrusion  345  may be coupled to the coupling groove  314  of the discharge cover  300 . 
     Thus, as the stopper  340  supports a front portion of the valve spring  335 , an opening degree of the discharge valve  325  may be restricted. As a result, when the discharge valve  325  is closed, an impulse may be reduced. Also, an assembly of the discharge valve  325  and the valve spring  335  may be stably installed on the discharge cover by the stopper  340 . 
       FIG. 15  is a perspective view of a discharge valve assembly coupled to a discharge cover according to still another embodiment.  FIG. 16  is a cross-sectional view illustrating a state in which a valve spring and a stopper are coupled to each other according to an embodiment. 
     Referring to  FIGS. 15 and 16 , a discharge cover  400  according to this embodiment may include a cover body  400   a  that defines a resonance chamber  412 . A coupling groove  414 , in which a coupling protrusion  445  of a stopper  440  may be inserted, may be defined in the cover body  400   a.    
     Descriptions with respect to the resonance chamber  412 , the cover body  400   a , the coupling protrusion  445 , and the coupling groove  414  will be derived from those of the resonance chamber  312 , the cover body  300   a , the coupling protrusion  345 , and the coupling groove  314 , described with respect to the previous embodiment. 
     The discharge valve assembly may include a discharge valve  420 , and a valve spring  430 . The discharge valve  420  may include an insertion protrusion  422 , and a valve recess  423 . Descriptions with respect to the insertion protrusion  422  and the valve recess  423  will be derived from those of the insertion protrusion  322  and the valve recess  323  described with respect to the previous embodiment. 
     The stopper  440  may include a bent portion  447  bent to extend along a circumferential portion of the stopper  440 , and an insertion portion  448  disposed within the bent portion  447  and in which an outer circumferential portion of the valve spring  430  may be inserted. 
     The outer circumferential portion of the valve spring  430  may be inserted inside a circumferential portion of the stopper  440  by the bent portion  447  and the insertion portion  448 . For example, the stopper  440  may be manufactured through insert molding along the circumferential portion of the valve spring  430 . Thus, as the stopper  440  and the valve spring  430  may be integrated with each other, vibration of the valve spring  430  while the compressor operates may be prevented. 
     A through hole  446  to guide the refrigerant so that at least a portion of the refrigerant discharged through the discharge valve  420  may be introduced into the resonance chamber  412  may be defined in the stopper  440 . At least a portion of the stopper  440  may pass through the through hole  446 . As the through hole  446  may be defined in the stopper  440 , the refrigerant may be easily introduced into the resonance chamber  412 . 
       FIG. 17  is a cross-sectional view of a discharge valve assembly coupled to a discharge cover according to yet another embodiment. Referring to  FIG. 17 , a discharge cover  500  according to this embodiment may include a cover body  500   a  that defines a resonance chamber  512 . 
     Descriptions with respect to the resonance chamber  512  and the cover body  500   a  will be derived from those of the resonance chamber  312  and the cover body  300   a  described with respect to the previous embodiment. 
     The discharge valve assembly may include a discharge valve  520 , and a valve spring  530 . The discharge valve  520  may include an insertion protrusion  522 , and a valve recess  523 . Descriptions with respect to the insertion protrusion  522  and the valve recess  523  will be derived from those of the insertion protrusion  322  and the valve recess  323  described with respect to the previous embodiment. 
     The discharge valve assembly according to this embodiment may further include a coupling member  580  to fix the valve spring  530  and the stopper  540 . One or more coupling members  580  may be disposed along a circumferential portion of the valve spring  530  to extend from an upper portion of the valve spring  530  to the stopper  540 . Thus, as the stopper  540  and the valve spring  530  may be firmly fixed by the coupling member  580 , vibration of the valve spring  530  while the compressor operates may be prevented. 
     According to embodiments, the compressor including inner components may decrease in size to reduce a volume of a machine room of a refrigerator and increase an inner storage space of the refrigerant. Also, a drive frequency of the compressor may increase to prevent performance of the inner components from being deteriorated due to the decreasing size thereof. In addition, as the gas bearing is applied between the cylinder and the piston, friction force due to oil may be reduced. 
     Also, the discharge valve to selectively discharge the high-pressure gas compressed in the compression chamber may stably operate. In addition, an impulse occurring while the discharge valve operates may be reduced to reduce abrasion of the discharge valve. As a result, it may prevent foreign substances generated due to abrasion of the discharge valve from having an influence on the gas bearing. 
     Further, the opening degree of the discharge valve may be restricted by the stopper to reduce a time taken to close the discharge valve, thereby improving response for operating the discharge valve. Furthermore, the resonance chamber may be provided in the discharge cover to reduce pulsation of the discharge gas, thereby reducing noise. 
     Additionally, as the plurality of filtering device may be provided in the compressor, it may prevent foreign substances or oil contained in the compression gas (or discharge gas) introduced outside of the piston from being introduced into the nozzle of the cylinder. Therefore, as blocking of the nozzle of the cylinder may be prevented, the gas bearing effect may be effectively performed between the cylinder and the piston, and thus, abrasion of the cylinder and piston may be prevented. 
     Embodiments disclosed herein provide a linear compressor in which abrasion to a discharge valve may be reduced. 
     Embodiments disclosed herein provide a linear compressor that may include a shell including a discharge outlet; a cylinder provided in the shell to define a compression space for a refrigerant; a frame to fix the cylinder to the shell; a piston reciprocated within the cylinder in an axial direction; a discharge valve disposed on or at one side of the cylinder to selectively discharge the refrigerant compressed in the compression space; a discharge cover coupled to the frame, the discharge cover having resonance chambers to reduce pulsation of the refrigerant discharged through the discharge valve; a valve spring disposed on the discharge cover to provide a restoring force to the discharge valve; and a stopper coupled to the valve spring to restrict deformation of the valve spring. The discharge cover may include a cover body having a discharge hole, through which the refrigerant discharged through the discharge valve may be discharged to the outside of the discharge cover, and a guide passage defined in the cover body to guide at least a portion of the refrigerant discharged through the discharge valve into the resonance chambers. 
     The guide passage may include a first guide groove defined by recessing at least a portion of the cover body. The discharge cover may further include a frame coupling part or portion that extends outward from cover body in a radial direction and is coupled to the frame. 
     The cover body may include a first stepped part or step recessed from the frame coupling part, the first stepped part having a first discharge hole, and a second stepped part or step further recessed from the first stepped part toward the resonance chambers. The guide passage may be defined in the second stepped part. 
     The linear compressor may further include a second guide groove defined in the second stepped part to guide coupling of the stopper. The stopper may include a stopper body that supports the valve spring, and a guide protrusion that protrudes from the stopper body to move along the second guide groove. 
     The valve spring may include a plate spring. The valve spring may include a spring body including a plurality of cutoff parts or portions, and an insertion hole defined in the spring body and in which an insertion protrusion of the discharge valve may be coupled. 
     The linear compressor may further include a first spacer disposed between the valve spring and the stopper to space the valve spring from the stopper. The linear compressor may further include a second spacer disposed on the cover body to support the stopper. 
     The cover body may include a seat part or seat, on which the second spacer may be seated. The seat part may partition the plurality of resonance chambers. 
     Embodiments disclosed herein further may provide a linear compressor that may include a shell including a discharge outlet; a cylinder provided in the shell to define a compression space for a refrigerant; a frame to fix the cylinder to the shell; a piston reciprocated within the cylinder in an axial direction; a discharge valve disposed on or at one side of the cylinder to selectively discharge the refrigerant compressed in the compression space; a discharge cover having a resonance chamber to reduce pulsation of the refrigerant discharged through the discharge valve and a discharge hole to guide the discharged refrigerant into the discharge outlet of the shell; a valve spring disposed on the discharge cover to allow the discharge valve to elastically move; and a stopper coupled to the valve spring to restrict an opening degree of the discharge valve. The stopper may be coupled to an inside of the discharge cover. 
     The linear compressor may further include a spacer disposed between the stopper and the discharge cover to support the stopper. A guide groove may be defined in the discharge cover, and the stopper may be press-fitted into and fixed to the guide groove in a state in which the spacer is disposed on the stopper. 
     The discharge cover may include a seat part or seat, on which the stopper may be seated, and a coupling groove recessed from the seat part and in which a coupling protrusion of the stopper may be inserted. 
     The stopper may include an insertion part or portion, in which a circumferential portion of the valve spring may be inserted, and a through part or portion, through which at least a portion of the refrigerant may pass. The through part may guide the refrigerant discharged through the discharge valve into the resonance chamber. 
     The linear compressor may further include a coupling member to couple the stopper to the valve spring. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description. Other features will be apparent from the description and drawings, and from the claims. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.