Abstract:
A disclosed displacer to be inserted into a cylinder to expand a compressed working fluid inside the cylinder by reciprocation of the displacer inside the cylinder includes a cylindrical member; and a regenerative material included inside the cylindrical member, wherein a groove is formed on an outer peripheral surface of the cylindrical member, the outer peripheral surface facing the cylinder, and a sealing film is continuously formed on the outer peripheral surface and the groove over an area where the groove is formed in a longitudinal direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. continuation application filed under 35 USC 111a and 365c of PCT application JP2011/056362 filed Mar. 17, 2011, which claims priority to Application No. 2010-060998 filed in Japan on Mar. 17, 2010. The foregoing applications are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a displacer, a manufacturing method thereof, and a regenerative type refrigerator. More specifically, the present invention relates to a displacer on which surface a groove is formed, a manufacturing method thereof, and a regenerative type refrigerator using the displacer. 
         [0004]    2. Description of the Related Art 
         [0005]    An example of a regenerative type refrigerator including a regenerator in which a regenerative material is accommodated and using a refrigerant gas is a Gifford-McMahon (GM) cycle refrigerator (hereinafter, referred to as a GM refrigerator). An exemplary GM refrigerator has a structure in which a displacer in inserted in a cylinder. 
         [0006]    An expansion chamber is provided on a low temperature side inside the cylinder and a cavity is provided on a high temperature end. A gas passage is provided inside the displacer. A regenerative material fills the inside of the gas passage. The gas passage inside the displacer communicates with the expansion chamber and a cavity on the side of the high temperature end. The displacer is reciprocated along a longitudinal axis direction of the cylinder by a driving mechanism which is formed by, for example, a motor and a scotch yoke mechanism. 
         [0007]    A refrigerant gas supply system is connected to the GM refrigerator. The refrigerant gas supply system supplies a refrigerant gas into the cavity at the high temperature end and recovers the refrigerant gas from the cavity. The supply and recovery of the refrigerant gas are synchronized with reciprocating motion of the displacer. When the refrigerant gas is supplied into the cavity at the high temperature end, the refrigerant gas is introduced into the expansion chamber through the gas passage inside the displacer. The refrigerant gas inside the expansion chamber is recovered by the refrigerant gas supply system via the route for introducing the refrigerant gas. 
         [0008]    When the refrigerant gas expands along with the reciprocating motion of the displacer, the refrigerant gas is cooled to generate cold thermal energy. The refrigerant gas having a cryo temperature absorbs heat from the circumference and cools the regenerative material inside the displacer when the refrigerant gas is recovered from the expansion chamber. After the cold heat is exchanged so as to be transferred from the refrigerant gas to the regenerative material, the heated refrigerant gas is ejected from the cylinder. Further, when the refrigerant gas is introduced into an expansion chamber in a subsequent cycle, the refrigerant gas is cooled by the regenerative material in which the cold heat is accumulated. By repeating the above processes, the low temperature side of the cylinder is maintained to be at a cryo temperature. 
         [0009]    Further, if a gap between the cylinder and the displacer is not sufficiently sealed, there may be a case where the refrigerant gas cannot produce a predetermined refrigeration capacity. In order to prevent the incapability of the refrigerant gas, the Patent Document 1 discloses an example structure in which a helical groove in formed on an outer peripheral surface of the displacer. With this structure, the refrigerant gas intrudes into the gas passage flowing inside the displacer and a gap between the cylinder and the displacer, and is branched into the refrigerant gas flowing along the helical groove. 
         [0010]    Since the refrigerant gas flowing along the helical groove travels a longer route than that of the gas passage along the longitudinal axis of the cylinder, the refrigerant gas can sufficiently exchange the cold heat with the displacer. Therefore, heat loss caused by the refrigerant gas flowing through the gap between the cylinder and the displacer can be reduced to thereby prevent a drop of the refrigeration capacity. 
         [0011]    Further, in order to securely introduce the refrigerant gas into the helical groove, it is necessary to firmly seal the gap between an auger of the displacer and the inner wall of the cylinder. The Patent Document 2 discloses an exemplary structure in which a sealing film made of a resin is formed on an outer peripheral surface of the displacer. 
         [0012]      FIGS. 1A to 1C  illustrate a method of forming a helical groove  138  and a sealing film  139 . When the helical groove  138  and the sealing film  139  are formed in the displacer  103 , a cylindrical member  130  as a base material as illustrated in  FIG. 1A  is prepared and the sealing film  139  is coated on a predetermined outer peripheral portion of the cylindrical member  103  as illustrated in  FIG. 1B  by coating or the like. 
         [0013]    Subsequently, as illustrated in  FIG. 1C , the cylindrical member  130  formed with the sealing film  139  is mounted on a machining apparatus for processing a helical groove such as a lathe turning machine to thereby cut the helical groove  138 .
   [Patent Document 1] Japanese Patent No. 2659684   [Patent Document 2] Japanese Laid-open Patent Publication No. 2001-248929   
 
       SUMMARY OF THE INVENTION 
       [0016]    According to an aspect of the embodiments of the present invention, there is provided a displacer to be inserted into a cylinder to expand a compressed working fluid inside the cylinder by reciprocation of the displacer inside the cylinder, the displacer including a cylindrical member; and a regenerative material included inside the cylindrical member, wherein a groove is formed on an outer peripheral surface of the cylindrical member, the outer peripheral surface facing the cylinder, and a sealing film is continuously formed on the outer peripheral surface and the groove over an area where the groove is formed in a longitudinal direction. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1A  illustrates a cylindrical member before applying an exemplary manufacturing method of manufacturing a displacer; 
           [0018]      FIG. 1B  is a plan view of the cylindrical member provided with a sealing film while applying the exemplary manufacturing method of manufacturing the displacer; 
           [0019]      FIG. 1C  is a plan view of the cylindrical member in which a groove is cut while applying the exemplary manufacturing method of manufacturing the displacer; 
           [0020]      FIG. 2  is a cross-sectional view of a Gifford-McMahon type refrigerator of an embodiment of the present invention; 
           [0021]      FIG. 3  is an exploded perspective view of a rotary valve illustrated in  FIG. 2 ; 
           [0022]      FIG. 4A  is a cross-sectional view of a second stage displacer illustrated in  FIG. 2 ; 
           [0023]      FIG. 4B  is an enlarged view of a circle indicated by a dot chain line in  FIG. 4A ; 
           [0024]      FIG. 5A  is a front view of a cylindrical member before processing for illustrating a manufacturing method of a second stage displacer used in the refrigerator of the embodiment of the present invention; 
           [0025]      FIG. 5B  is a front view of the cylindrical member in which a groove is cut for illustrating the manufacturing method of the second stage displacer used in the refrigerator of the embodiment of the present invention; 
           [0026]      FIG. 5C  is a front view of the cylindrical member where the groove is provided with the a sealing film for illustrating the manufacturing method of the second stage displacer used in the refrigerator of the embodiment of the present invention; 
           [0027]      FIG. 6A  is a cross-sectional view of a second displacer as an modified example; and 
           [0028]      FIG. 6B  is an enlarged view of a circle indicated by a dot chain line in  FIG. 6A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    A helical groove and a sealing film provided in a displacer are important factors in order to reduce heat loss and improve refrigeration capacity in a GM refrigerator. 
         [0030]    Especially, the thickness of a sealing film becomes important in order to firmly seal a gap between the sealing film and an inner wall of a cylinder. If the sealing film is thick on the surface of the displacer, a clearance (a gap) between the sealing film and the inner wall of the cylinder may vary due to a difference of the coefficients of thermal expansion of materials of the sealing film and the cylinder. If this variation of the clearance (the gap) occurs, the refrigerant gas may leak from the clearance (the gap) between the displacer and the cylinder to thereby lower the refrigeration capacity. Therefore, in order to reduce the variation of the clearance (the gap) between the sealing film and the cylinder, it is effective to reduce the film thickness of the sealing film. 
         [0031]    However, if the film is simply made thinner, the strength of the sealing film may be lowered. Therefore, the sealing film coated on the cylindrical member  130  may be peeled off the cylindrical member  130  when the helical groove  138  is cut. As described above, if the sealing film  139  is peeled off the cylindrical member  130 , the refrigerant gas leaks from the portion to thereby lower the refrigeration capacity. 
         [0032]    Accordingly, embodiments of the present invention may provide a novel and useful improved displacer, a manufacturing method thereof and a regenerative type refrigerator. 
         [0033]    More specifically, the embodiments of the present invention may provide a displacer from which a sealing film is prevented from being peeled, a manufacturing method thereof, and a regenerative type refrigerator enabling a stable cooling process by an improved sealing performance between the displacer and a cylinder by preventing the sealing film from peeling off the displacer. 
         [0034]    An embodiment of the present invention is described with reference to figures. 
         [0035]      FIG. 2  is a cross-sectional view of a Gifford-McMahon type refrigerator (hereinafter, referred to as a GM refrigerator) of the embodiment. The GM refrigerator of the embodiment includes a compressor  1  and a cold head  2 . The cold head  2  includes a housing  23  and a cylinder unit  10 . The compressor  1  suctions the refrigerant gas from an intake port  1   a,  compresses the suctioned refrigerant gas, and discharges as a high pressure refrigerant gas from a discharge port  1   b.  The refrigerant gas as a working fluid may be a helium gas. 
         [0036]    The cylinder unit  10  has a two stage structure including a first stage cylinder  10 A and a second stage cylinder  10 B. The second stage cylinder  10 B is narrower than the first stage cylinder  10 A. A first stage displacer  3 A is inserted in the first stage cylinder  10 A, and a second stage displacer  3 B is inserted in the second stage cylinder  10 B so that the first and second stage displacers  3 A and  3 B can reciprocate in axial directions of the first and second stage cylinders  10 A and  10 B, respectively. 
         [0037]    The first stage displacer  3 A and the second stage displacer  3 B are mutually connected by a joint mechanism (not illustrated). A regenerative material  4 A is provided inside the first stage displacer  3 A. A regenerative material  4 B fills the inside of the second stage displacer  3 B. Further, gas passages L 1  to L 4  are formed in the first and second stage displacers  3 A and  3 B in order to make the refrigerant gas pass through the gas passages L 1  to L 4 . 
         [0038]    A first stage expansion chamber  11  is formed on an end portion on the side of the second stage cylinder  10 B inside the first stage cylinder  10 A. A second expansion chamber  12  is formed on an end portion opposite to the side of the first stage cylinder  10 A of the second stage cylinder  10 B. 
         [0039]    An upper chamber  13  and the first stage expansion chamber  11  are connected via the gas passage L 1 , a first stage regenerative material chamber filled with the regenerative material  4 A, and the gas passage L 2 . The first stage expansion chamber  11  and the second stage expansion chamber  12  are connected via the gas passage L 3 , a second stage regenerative material chamber filled with the regenerative material  4 B, and the gas passage L 4 . 
         [0040]    A cooling stage  6  is provided at a position substantially corresponding to the first stage expansion chamber  11  on the outer peripheral surface of the first stage cylinder  10 A. A cooling stage  7  is provided at a position substantially corresponding to the second stage expansion chamber  12  on the outer peripheral surface of the second stage cylinder  10 B. 
         [0041]    A sealing unit  50  is arranged in the vicinity of an end of the outer peripheral surface of the first stage displacer  3 A on the side of the upper chamber  13 . The sealing unit  50  seals a gap between the outer peripheral surface and the inner peripheral surface of the cylinder  10 A. 
         [0042]    The first stage displacer  3 A is connected to an output shaft  22   a  of a scotch yoke  22  forming a transforming mechanism between rotation and reciprocation. The scotch yoke  22  is movably supported in axial directions of the displacers  3 A and  3 B by a pair of slide bearings  17   a  and  17   b  fixed to the housing  23 . Gas tightness is secured in a sliding unit by the slide bearing  17   b  to thereby separate the space inside the housing  23  and the upper chamber  13 . 
         [0043]    A motor  15  is connected to the scotch yoke  22 . The rotation of the motor  15  may be transformed into the reciprocation by a crank  14  and the scotch yoke  22 . The reciprocation is transmitted to the first stage displacer  3 A. Thus, the first stage displacer  3 A reciprocates inside the first stage cylinder  10 A, and the second stage displacer  3 B reciprocates inside the second stage cylinder  10 B. 
         [0044]    When the displacers  3 A and  3 B move upward in  FIG. 2 , the capacity of the upper chamber  13  decreases and the capacities of the first and second expansion chambers  11  and  12  increase. When the displacers  3 A and  3 B move downward in  FIG. 2 , the capacity of the upper chamber  13  increases and the capacities of the first and second expansion chambers  11  and  12  decrease. Along with variations of the capacity of the upper chamber  13  and the capacities of the first and second expansion chambers  11  and  12 , the refrigerant gas moves through the gas passages L 1  to L 4 . 
         [0045]    When the refrigerant gas passes through the regenerative materials  4 A and  4 B filling the displacers  3 A and  3 B, heat is exchanged among the refrigerant gas and the regenerative materials  4 A and  4 B. With this, the regenerative materials  4 A and  4 B are cooled by the refrigerant gas. 
         [0046]    A rotary valve RV is provided between the intake port  1   a  and the discharge port  1   b  of the compressor  1  in a gas passage (a route) of the refrigerant gas. More specifically, the rotary valve RV is arranged among the intake port  1   a,  the discharge port  1   b,  and the upper chamber  13  in the gas passage (the route) of the refrigerant gas. The rotary valve RV has a function of switching the gas passage (the route) of the refrigerant gas. Specifically, the rotary valve RV is provided to switch to a first mode or a second mode. The first mode is to introduce the refrigerant gas discharged from the discharge port  1   b  of the compressor  1  into the upper chamber  13 . The second mode is to introduce the refrigerant gas inside the upper chamber  13  into the intake port  1   a  of the compressor  1 . 
         [0047]    The rotary valve RV includes a valve body  8  and a valve plate  9 . The valve plate  9  may be made of, for example, an aluminum alloy. The valve body  8  may be made of, for example, ethylene tetrafluoride. The valve body  8  and the valve plate  9  include flat surfaces, respectively. The flat surfaces of the valve body  8  and the valve plate  9  mutually contact face to face. A thin film made of a hard material such as diamond-like carbon (DLC) can be formed on at least one of the sliding surfaces of the valve body  8  and the valve plate  9  in order to reduce friction occurring on the sliding surfaces and improve wear resistance. 
         [0048]    The valve plate  9  is supported by a rotational shaft bearing  16  inside the housing  23  so that the valve plate  9  is rotatable. An eccentric pin  14   a  of the crank  14  drives the scotch yoke  22  by the rotation of the crank  14 . When the eccentric pin  14   a  revolves around the rotational shaft, the valve plate  9  is driven to rotate. The valve body  8  is pushed against the valve plate  9  and fixed so as not to rotate. 
         [0049]    A coil spring  20  presses the valve body  8  so that the valve body  8  is not separated from the valve plate  9  when the pressure on an ejection side becomes greater than the pressure on a suction side. The force pressing the valve body  8  to the valve plate  9  is determined not only by the spring force of the coil spring  20  but also by differential pressure acting on the valve body  8  between pressure of the refrigerant gas on the suction side and pressure of the refrigerant gas on the ejection side. 
         [0050]      FIG. 3  is an exploded perspective view of the rotary valve RV. A flat sliding surface  8   a  of the valve body  8  in a cylindrical shape contacts a flat sliding surface  9   a  of the valve plate  9 . Thus, a surface contact between the flat sliding surface  8   a  of the valve body  8  and the flat sliding surface  9   a  of the valve plate  9  occurs. A gas passage  8   b  penetrates the valve body  8  along a central axis of the valve body  8 . Said differently, one end of the gas passage  8   b  opens on the sliding surface  8   a.  The other end of the gas passage  8   b  is connected to the discharge port  1   b  of the compressor  1  illustrated in  FIG. 2 . The gas is supplied from the discharge port  1   b  of the compressor  1  to the gas passage  8   b  of the valve body  8 . 
         [0051]    An arc-like recess  8   c  is formed on the sliding surface  8   a  of the valve body  8  along an arc around the center axis of the valve body  8 . An end of a gas passage  8   d  formed inside the valve body  8  opens on a bottom surface of the arc-like recess  8   c . The other end of the gas passage  8   d  opens on the outer peripheral surface of the valve body  8  and communicates with the upper chamber  13  via a gas passage  21  formed in the housing  23  illustrated in  FIG. 2 . 
         [0052]    A recess  9   d  is formed on the sliding surface  9   a  of the valve plate  9 . The recess  9   d  is elongated on the sliding surface  9   a  along a radius direction from the center axis of the valve plate  9 . When the valve plate  9  rotates, an end portion of the recess  9   d  may partially overlap the arc-like recess  8   c  to cause the gas passage  8   b  and the gas passage  8   d  to mutually communicate via the recess  9   d.    
         [0053]    A gas passage  9   b  arranged parallel to a rotary shaft penetrates the valve plate  9 . The gas passage  9   b  is opened at substantially the same position as that of the arc-like recess  8   c  on the sliding surface  8   a  of the valve body  8 . When the opening of the gas passage  9   b  partially overlaps the arc-like recess  8   c  as a result of the rotation of the valve plate  9 , the gas passage  8   d  communicates with the gas passage  9   b.  The other end of the gas passage  9   b  communicates with the intake port la of the compressor  1  via a cavity inside the housing  23  illustrated in  FIG. 2 . The refrigerant gas is ejected from the gas passage  9   b  of the valve plate  9  to the intake port la of the compressor  1 . 
         [0054]    When the gas passage  8   b  communicates with the gas passage  8   d  via the arc-like recess  8   c,  the refrigerant gas sent from the compressor  1  is sent inside the upper chamber  13  via the rotary valve RV. When the gas passage  8   d  communicates with the gas passage  9   b,  the refrigerant gas inside the upper chamber  13  is recovered by the compressor  1 . Therefore, when the valve plate  9  is rotated, introduction (suction) of the refrigerant gas into the upper chamber  13  and recovery (ejection) of the refrigerant gas from the upper chamber  13  are repeated. 
         [0055]      FIG. 4A  is a partial cross-sectional view of the second stage displacer  3 B.  FIG. 4B  is an enlarged view of the circle indicated by a dot chain line in  FIG. 4A . The base body of the second stage displacer  3 B is a cylindrical member  30 . An upper end and a lower end of the cylindrical member  30  are opened. A lid  31  is inserted in the lower end of the cylindrical member  30  and adhered to the cylindrical member  30 . The cylindrical member  30  is made of stainless steel, and the lid  31  can be made of a phenol resin including fabric. In the cylindrical member  30 , woven metallic wires  32  are provided on the lid  31 , and a felt plug  33  is provided on the woven metallic wires  32 . 
         [0056]    The regenerative material  4 B fills the inside of the second stage displacer  3 B on the felt plug  33 . The regenerative material  4 B may be, for example, small lead spheres or a magnetic regenerative material. The refrigeration capacity can be enhanced by using the regenerative material. A felt plug  34  is arranged on the regenerative material  4 B, and a perforated metal (punching metal)  35  is provided on the felt plug  34 . 
         [0057]    An opening  37  is provided at a vertical position of the woven metallic wire  32  on a side wall of the cylindrical member  30 . A groove is formed at a position above the opening  37  on the outer peripheral surface of the cylindrical member  30 . Within the embodiment, the groove is a helical groove  38 A in a helical shape for connecting the vertical position of the opening  37  to the upper end. The number of the helical groove  38 A may be one. The helical groove  38 A collaborates with the inner surface of the cylinder  10 B to form a helical gas passage. 
         [0058]    Further, the outer diameter of the cylindrical member  30  lower than the opening  37  is slightly smaller than the outer diameter of the cylindrical member  30  upper than the opening  37 . Therefore, a gap is formed between the cylindrical member  30  and the second stage cylinder  10 B at the portion lower than the opening  37 . The gap and the opening  37  form the gas passage L 4  connecting the inside of the cylindrical member  30  and the expansion space  12  illustrated in  FIG. 2  (for convenience, the gas passage L 1  is illustrated so as to vertically penetrate the lid  31 ). 
         [0059]    In the second stage displacer  3 B, if the refrigerant gas flows into the gap between the inner peripheral surface of the cylinder  10 B and the outer peripheral surface of the displacer  3 B, the refrigerant gas flows along the helical groove  38 A. Heat is exchanged between the refrigerant gas and the regenerative material  4 B via the cylindrical member  30 . At this time, by forming the helical groove  38 A on the surface of the cylindrical member  30 , the refrigerant gas flows through the long passage along the helical groove  38 A. Therefore, sufficient heat exchange becomes possible. Thus, the heat exchange is securely performed to thereby prevent the refrigeration capacity from being degraded. Thus, cooling efficiency of the GM refrigerator can be improved. 
         [0060]    Next, the outer peripheral surface of the second stage displacer  3 B installed in the GM refrigerator of the embodiment may be explained. 
         [0061]    As described, the helical groove  38 A is formed on the outer periphery of the second stage displacer  3 B. Within the embodiment, a sealing film  39  is formed at least on a region where the helical groove  38 A is formed on the outer peripheral surface of the cylindrical member  30  in its longitudinal direction. The sealing film coats not only the outer peripheral surface of the cylindrical member  30  but also the inside of the helical groove  38 A. 
         [0062]    The sealing film  39  is provided to enhance the sealing performance of a gap between the second stage displacer  3 B and the inner wall of the second stage cylinder  10 B. Within the embodiment, a fluorine contained resin which has high thermal and mechanical properties and good sliding capability is used as the sealing film  39 . Specifically, Teflon (“Teflon” is a registered trademark) is used as the sealing film  39 . 
         [0063]    As described above, if the sealing film  39  on the surface of the second stage displacer  3 B is thick, variation may occur in the clearance (gap) between the second stage displacer  3 B and the inner wall of the second stage cylinder  10 B due to a difference of the coefficients of thermal expansion of the sealing film  39  and the second stage cylinder  10 B to thereby lower the refrigeration capacity. Within the embodiment, the film thickness of the sealing film is set to be 5 μm or greater and 50 μm or smaller. By thinning the sealing film  39 , it is possible to prevent the variation of the clearance (gap), caused by the difference of the coefficients of thermal expansion between the sealing film  39  and the second stage cylinder  10 B to thereby prevent decrement of the cooling efficiency. 
         [0064]    However, if the film is simply made thin, the strength of the sealing film may be lowered. Therefore, the sealing film coated on the cylindrical member  30  may be peeled off the cylindrical member  30  when the helical groove  38  is mechanically cut. Therefore, within the embodiment, this problem is solved by forming the sealing film  39  after forming the helical groove  38 A. 
         [0065]    Next, referring to  FIGS. 5A to 5C , a method of forming the sealing film  39  on an entire area of the helical groove  38 A of the cylindrical member  30  in its longitudinal direction is described. 
         [0066]    In order to form the cylindrical member  30  of the embodiment, the cylindrical member  30  being the base material of the displacer  3 B illustrated in  FIG. 5A  is prepared. This cylindrical member  30  is made of stainless steel. The cylindrical member  30  has a cylindrical shape inside which a space for accommodating the regenerative material  4 B or the like is formed. 
         [0067]    Within the embodiment, a helical groove cutting process of cutting the helical groove  38 A on the outer peripheral surface of the cylindrical member  30  is performed. The helical groove  38 A may be cut using an ordinary method. The cylindrical member  30  is mounted on machining equipment such as a lathe turning machine to cut the helical groove  38 A. Since the ordinary cutting process can be used to cut the helical groove  38 A, the processing cost does not increase.  FIG. 5B  illustrates the cylindrical member  30  formed with the helical groove  38 A. 
         [0068]    After completing the helical groove cutting process, a sealing film forming process for coating the sealing film  39  on cylindrical member formed with helical groove  38 A is performed. In the sealing film forming process, a fluorine contained resin to be the sealing film  39  is coated on an area including the helical groove  38 A on the outer peripheral surface of the cylindrical member  30  as illustrated in  FIG. 5C . 
         [0069]    A method of coating the sealing film on the cylindrical member  30  is a coating method or a plating method. The film thickness of the sealing film  39  is set to be 5 μm or greater and 50 μm or smaller as described above. However, the film thickness can be easily controlled by managing a time for coating the sealing film  39  or a time for plating the sealing film  39 . Since the sealing film  39  is thinned as described above, it is preferable to use the coating method or the plating method. 
         [0070]    After the helical groove cutting process is completed, the sealing film  39  is coated on not only the outer peripheral surface of the cylindrical member  30  but also the inside of the helical groove  38 A in the sealing film forming process. Therefore, unlike the method of forming the helical groove  138  after coating the sealing film  139  illustrated in  FIGS. 1A to 1C , the sealing film  39  does not peel off the cylindrical member  30  in the manufacturing method of the displacer  3 B of the embodiment. 
         [0071]    Further, in the method of forming the helical groove  138  after coating the sealing film  139  illustrated in  FIGS. 1A to 1C , the sealing film  139  is formed only on the auger of the helical groove  138 , and a portion of the sealing film  139  corresponding to the inside of the helical groove  138  is removed at a time of cutting the helical groove  138 . Within the embodiment, the sealing film  39  is coated entirely on the helical groove  38 A, not only on the auger of the helical groove  38 A but also the inside of the helical groove  38 A. Said differently, the sealing film  39  is not separated by the helical groove  38 A to coat the entire area of the helical groove  38 A of the cylindrical member  30  in its longitudinal direction. Therefore, the sealing film  39  is firmly fixed to the cylindrical member  30  thereby preventing the sealing film from peeling off the cylindrical member  30 . 
         [0072]    As described, in the displacer  3 B of the embodiment, it is possible to prevent the sealing film  39  from peeling off the cylindrical member  30  even if the sealing film  39  is thinned to be 5 μm or greater and 50 μm or smaller. 
         [0073]    Therefore, it is possible to prevent the clearance (the gap) between the sealing film  39  and the inner wall of the second stage cylinder  10  from varying due to the thin sealing film  39 . Thus, the refrigerant gas is prevented from leaking from the clearance (the gap) between the sealing film  39  and the inner wall of the second stage cylinder  10 . Further, it is possible to securely prevent the sealing film  39  from peeling off the cylindrical member  30  to thereby prevent the refrigerant gas from leaking from the peeled portion in the structure illustrated in  FIG. 1C . Thus, it is possible to prevent the refrigerant gas from leaking from the gap between the second stage displacer  3 B and the second stage cylinder  10 B thereby securely preventing the degradation of the refrigeration capacity of the GM refrigerator. 
         [0074]    Within the embodiment, the film thickness of the sealing film is set to be 5 μm or greater and 50 μm or smaller. If the film thickness is smaller than 5 μm, the strength of the sealing film  39  is weakened. Then, the sealing film having the film thickness smaller than 5 μm may peel off the cylindrical member  30  by the reciprocation of the second stage displacer  3 B inside the second stage cylinder  10 B. If the film thickness is greater than 50 μm, the clearance (the gap) between the sealing film  39  and the inner wall of the second stage cylinder  10  may vary. 
         [0075]    Next, another modified embodiment of the present invention is described. 
         [0076]      FIGS. 6A and 6B  illustrate a modified example of the second stage displacer  3 B illustrated in  FIGS. 4A and 4B .  FIG. 6A  is a partial cross-sectional view of a second stage displacer  3 C of the modified example.  FIG. 6B  is an enlarged view of the circle indicated by a dot chain line in  FIG. 6A . Referring to  FIGS. 6A and 6B , the same reference symbols are attached to portions corresponding to those attached to  FIGS. 2 and 4A  and  4 B, and description of the portions is omitted. 
         [0077]    Referring to the second stage displacer  3 B illustrated in  FIGS. 4A and 4B , the one helical groove  38  is formed on the outer periphery of the cylindrical member  30  and extends in a longitudinal direction of the cylindrical member  30 . Within the modified example, plural grooves  38 B (hereinafter, referred to as an annular groove  38 B) are formed as illustrated in  FIGS. 6A and 6B . 
         [0078]    These annular grooves  38 B are independent of each other unlike the one helical groove  38 A. The annular grooves  38 B are arranged mutually in parallel and in a longitudinal direction of the cylindrical member  30 . 
         [0079]    Within the modified example, even though the plural helical grooves  38  are formed in the cylindrical member  30 , it is possible to highly efficiently exchange heat in comparison with a displacer having no groove. Therefore, it is possible to prevent the refrigeration capacity from degrading. 
         [0080]    At this time, a connection groove may be formed between the adjacent annular grooves  38 B to enable the refrigerant gas flowing between the adjacent annular grooves  38 B. With this structure, it is possible to further enhance the efficiency of heat exchange between the refrigerant gas and the second stage displacer  3 C. 
         [0081]    Further, within the modified example, the sealing film  39  is formed in an area where the annular grooves  38 B are formed on the outer peripheral surface of the cylindrical member  30 . The sealing film  39  coats not only the outer peripheral surface of the cylindrical member  30  but also the insides of the annular grooves  38 B. The annular grooves  38 B can be processed by a method similar to that described with reference to  FIGS. 5A to 5C . The difference of the processes is whether the helical groove is formed or the annular grooves are formed. The thickness of the sealing film  39  is 5 μm or greater and 50 μm or smaller in a manner similar to the second stage displacer  3 B. 
         [0082]    Therefore, by using the second stage displacer  3 C of the modified example, the refrigeration capacity of a GM refrigerator can be securely prevented from degrading by using the second stage displacer  3 C of the modified example in a manner similar to the embodiment illustrated in  FIGS. 2 ,  6 A and  6 B. 
         [0083]    Although the Gifford-McMahon (GM) type refrigerator having the two stages is applied to the embodiment and the modified example as described above, the GM type refrigerator may not be limited to the two stage type and may also be a single stage type or a multiple stage type. 
         [0084]    Further, within the embodiment, the helical groove  38 A and the sealing film  39  are provided in the second stage displacer  3 B. However, the helical groove  38 A and the sealing film  39  may also be provided to the first stage displacer  3 A in a structure similar to the second stage displacer  3 B. 
         [0085]    Thus, the refrigerant gas can be prevented from leaking and the refrigeration capacity can be prevented from degrading with the embodiment and the modified example. 
         [0086]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the embodiments and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the embodiments. Although the displacer, the manufacturing method of the displacer and the regenerative type refrigerator have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.