referring to fig1 to 3 , further explanation will be given over the displacer and seal assembly for stirling cycle machines described in the first aspect of the present invention . the stirling cycle machines are used as a refrigerating machine in the following explanation . the stirling cycle machine shown in fig1 is provided with a displacer 2 , a power piston 9 , a cylinder 5 , either of which is made of aluminum for example , as well as a seal 1 and a retaining ring 3 . the displacer 2 and the power piston 9 are structured so as to reciprocate with each other in a phase relationship along the inner peripheral surfaces 5 a and 5 b of a cylinder bore concentrically formed in the cylinder 5 . the displacer 2 is comprised of a body portion 2 a that is formed in a cylindrical shape , a first and second steps 2 b , 2 c , and a rod 4 concentrically formed with the body portion 2 a respectively . the outer periphery of the body portion 2 a is formed with a diameter that can be inserted into the inner peripheral surface 5 a of the cylinder with a predetermined radial clearance left therebetween . the first step 2 b is integrally disposed on one end of the body portion 2 a , and has a diameter smaller than the outer periphery of the body portion 2 a . a side surface 2 d connecting the outer periphery of the first step 2 b to the outer peripheral end of the body portion 2 a is formed perpendicularly to the center axis . further , the second step 2 c is integrally disposed on one end of the first step 2 b , and has a diameter smaller than the outer peripheral surface of the first step 2 b . the rod 4 is integrally disposed on one end of the second step 2 c , and has a circular cross sectional shape with a diameter smaller than the outer periphery of the second step 2 c . the top end 4 a of the rod 4 passes through a center hole 9 a formed in the center of the power piston 9 , and is radially supported by a bearing seal 10 provided in the cylinder hole 9 a . the radial clearance between the rod 4 and the bearing seal 10 should be small enough to prevent gas leakage between the spaces in front and the rear of the power piston 9 . however , if mass of the displacer 2 is sufficiently small , a simple plastic slide bearing may preferably be employed in which the bearing seal 10 is closely contacted with the rod 4 . for example , the bearing seal 10 using a homopolymer acetal resin processed with polytetrafluoroethylene effectively works for the rod 4 made of anodized aluminum . in order to further reduce the friction force , a gas bearing may be employed for the bearing seal 10 . the end portion 4 a of the rod 4 is supported by the center of a circular spring 21 that is mounted to one end portion of the cylinder 5 by screws 20 . one end portion of the power piston 9 is also supported with a similar circular spring . although the circular spring 21 and the like suitably adjusts a phase difference of the relative reciprocating motions of the power piston 9 and the displacer 2 in an optimum manner , its radial elastic coefficient is small . accordingly , the displacer 2 is radially supported by the bearing seal 10 provided in the outer periphery of the rod 4 and the center of the power piston 9 . the seal 1 shown in fig3 is formed in a ring shape with an injection molded plastic member or teflon3 - coated aluminum for example , having a rectangular cross sectional shape . the outer periphery of the seal 1 is formed in such a shape for the seal to be closely inserted into the inner peripheral surface 5 b of the cylinder 5 . that is , the radial clearance between the outer periphery of the seal 1 and the inner periphery of the cylinder 5 should be small enough to prevent an excessive gas leakage on one hand and just large enough to avoid any excessive friction forces on the other hand . normally , the radial clearance is equal to or less than four times as large as the one required between the outer periphery of the power piston 9 and the inner periphery of the cylinder 5 to the maximum . if the stirling cycle machines is used as an engine , the seal 1 is preferably formed of metal such as stainless , teflon3 - coated aluminum or the like . the inner periphery of the circular hole in the seal 1 is formed with such a diameter for the seal 1 to radially move while inserted into the outer periphery of the first step 2 b . that is , the radial clearance between the inner periphery of the circular hole in the seal 1 and the outer periphery of the first step 2 b is larger than that between the inner peripheral surface 5 a of the cylinder 5 and the outer periphery of the body portion 2 a . accordingly , even if the displacer 2 reciprocates eccentrically enough for the outer periphery of the body portion 2 a to contact with one side of the inner peripheral surface 5 a of the cylinder 5 , the contact between the outer periphery of the seal 1 and the outer periphery of the body portion 2 a is maintained and does not suffer from the eccentric motion of the displacer 2 . further , the width of the seal 1 is formed slightly narrower than the axial length of the first step 2 b . the second step 2 c is provided with a male screw on the outer periphery thereof , and the retaining ring 3 formed of plastic members is screwed into the male screw . one end surface 3 a of the retaining ring 3 is formed perpendicularly to the central axis . the axial position of the retaining ring 3 is defined by a portion where the retaining ring 3 engages with a side surface 2 e that connects the first step 2 b and the second step 2 c . the seal 1 is held between one end surface 3 a of the retaining ring 3 and the side surface 2 d of the first step and is inserted onto the outer periphery of the first step 2 b . as the width of the seal 1 is formed slightly narrower than the axial length of the first step 2 b , the retaining ring 3 does not apply axial compression forces to the seal 2 . referring to fig4 another means for defining the axial position of the retaining ring 3 is presented . reference numerals of relevant portions are differentiated by adding 100 to the reference numerals in fig3 . this means does not have the second step 2 c in fig3 . the axial length of the outer periphery of the first step 102 b in the displacer 102 is longer than the width of the seal 101 . the first step 102 b is provided with a male screw 104 b having an outer diameter smaller than the outer periphery thereof on one end thereof . the retaining ring 103 has a center hole to be closely inserted into the outer periphery of the first step 102 b and a female screw engaging with the male screw 104 b , both of which are formed concentrically with the center axis . further , the axial width of the seal 101 is provided with such a dimension that the axial compression forces are not applied to the seal 101 when the side surface 103 b is in contact with the side surface 102 e of the displacer 102 . next , referring to fig1 to 3 , the function and effect of the seal 1 will be explained while summarizing the operation of the stirling cycle machines used as a refrigerator . the stirling cycle machines is enclosed and sealed with a rear casing 30 and a front casing 31 , respectively formed of a sheet metal in a cylindrical shape , and a front cover 32 . the enclosed volume of the rear casing 30 and front casing 31 is filled with compressed helium , which serves as a working medium . in fig2 when the power piston 9 moves toward the right by the operation of a linear motor 40 , helium gas in a space 11 that is defined by the power piston 9 and the displacer 2 is compressed and the temperature and pressure of the gas increases . the space 11 communicates with a right space 13 defined by the right end surface of the displacer 2 and the front cover 32 through a heat - rejecting exchanger 41 , a regenerator 42 and a heat - absorbing exchanger 43 , all of which are stored in the front casing 31 . accordingly , the pressure of the gas in the space 13 of the displacer 2 simultaneously increases , and each pressure in the left and right spaces 11 , 13 of the displacer becomes roughly equal . on the other hand , the pressure receiving area of the left side of the displacer 2 is smaller than that of the right side thereof by an amount of the cross sectional area of the rod 4 . and the pressure of the sealed gas in the left space 12 of the power piston 9 that is applied to the front end of the rod 4 is lower than that of the compressed gas in the right space 13 . accordingly , due to the small area difference between the pressures respectively applied to the left and the right sides of the displacer 2 , the displacer 2 moves toward the left . by which , the compressed gas in the left space 11 of the displacer transfers to the right space 13 of the displacer 2 through the heat - rejecting exchanger 41 , regenerator 42 and heat - absorbing exchanger 43 . during the transfer , heat of the compressed gas is rejected to the outside by the heat - rejecting exchanger 41 and is absorbed by the heat - absorbing exchanger 42 to be accumulated . when the power piston 9 moves toward the left by the operation of the linear motor 40 , gas in the space 11 that is defined by the right side of the power piston 9 and the left side of the displacer 2 expands . simultaneously , gas in the right space 13 of the displacer 2 that communicates with the space 11 expands to decrease its temperature and pressure . in this process , contrary to the process previously described , as the pressure force in the right side of the displacer 2 becomes lower than that in the left side due to the cross sectional area of the rod 4 , the displacer 2 moves toward the right . accordingly , the gas in the right space 13 of the displacer 2 transfers to the left space 11 through the heat - absorbing exchanger 43 , the regenerator 42 and the heat - rejecting exchanger 41 . during the transfer , the passing gas absorbs heat from the outside in the heat - absorbing exchanger 43 and receives accumulated heat in the regenerator 42 . as explained above , the power piston 9 repeats the compression and expansion processes by the linear motor 40 on one hand , the displacer 2 reciprocates in a phase relationship to the power piston due to the difference of pressure forces applied to the opposite cross sectional areas of the displacer 2 on the other hand . the compressed gas and depressed gas alternately flows through the heat - rejecting exchanger 41 , the regenerator 42 and the heat - absorbing exchanger 43 by the reciprocating motion of the displacer 2 , to discharge heat to the outside in the heat - rejecting exchanger 41 and to absorb heat from the outside in the heat - absorbing exchanger 43 , by which an exterior freezer is workable . during the operation , heat of the gas is stored after the regenerator 42 in the compression process on one hand , the stored heat is recovered into the gas after the expansion process on the other hand . accordingly , coefficient of the operating performance of the refrigerator is increased . as explained above , in the stirling cycle machines , the respective pressures in the left and right spaces 11 , 13 are roughly equal , and the displacer 2 reciprocates due to the difference of pressure force applied to either side of the displacer 2 . under the circumstance , the seal 1 has the following functions and effects . the seal 1 is disposed on the first step 2 b and is axially located between the retaining ring 3 and the side surface 2 d of the first step 2 b . however , as the seal 1 is slightly narrower in width than the axial length of the outer periphery of the first step 2 b , it does not receive axial compression forces . on the other hand , the end surface of the retaining ring 3 and the side surface 2 d of the first step 2 b are disposed perpendicularly to the axial direction . accordingly , when pressures in the right and left sides of the displacer 2 transitionally change , one of the side surfaces of the seal 1 is pushed closely to one of the vertical surfaces due to the pressure difference , by which gas leakage from the side surfaces of the seal can be prevented . further , although the outer periphery of the seal 1 is formed so as to be closely contacted with the inner peripheral surface 5 a of the cylinder 5 , it is not pressed against the inner peripheral surface of the cylinder as the split seal ring . accordingly , when the outer periphery of the seal 1 is formed so as to have a diameter closely contacted with the inner peripheral surface 5 a of the cylinder 5 , friction forces against the cylinder can be significantly small and friction wear can be minimized . on the other hand , as the respective pressures in the front and rear spaces 11 , 13 of the displacer 2 are , as previously explained , substantially equal , gas leakage from the slight clearance between the outer periphery of the seal 1 and the inner periphery 5 a of the cylinder 5 is restricted to the minimum . further , the radial clearance between the inner periphery of the circular hole in the seal 1 and the outer periphery of the first step 2 b is made larger than the estimated maximum radial clearance between the inner peripheral surface 5 a of the cylinder 5 and the outer periphery of the body portion 2 a . on the other hand , the displacer 2 itself is radially supported by the rod 4 and the bearing seal 10 provided in the through hole of the power piston 9 . accordingly , when the clearance between the inner peripheral surface 5 a of the cylinder 5 and the outer periphery of the body portion 2 a is set large to some extent , the outer periphery of the seal 1 is closely contacted with the inner periphery of the cylinder by the radial movement of the seal 1 , even in the case that either the straightness or concentricity between the rod 4 and the body portion 2 a or that of the bearing seal 10 more or less deviates and the outer periphery of the body portion 2 a shifts to one side of the inner peripheral surface of the cylinder 5 . as shown in fig4 the outer periphery of the seal 101 is formed so that it is in axially parallel at the center portion of the seal 1 and so that the radius of the seal 1 gradually decreases from the either edge of the center portion of the seal 1 toward the either side surface thereof . accordingly , wedge - shaped spaces 101 a between the outer periphery of the seal 101 and the inner peripheral surface sa of said cylinder 5 are formed . for the design of the wedge - shaped spaces 101 a , a theoretical computing means is available , which is disclosed under the title : “ gas lubrication of a ringless piston in an internal combustion engine under dynamic loading ” by z . p . mourelatos in asme paper no . 88 - trib - 26 , developed on the theoretical and experimental basis . when the seal 101 is formed in that shape , gas is pressed into the wedge - shaped spaces 101 a at both ends of the outer periphery of the seal 101 during the high - speed reciprocating motion of the displacer 2 . by which , the outer periphery of the seal 101 detaches itself from the inner peripheral surface 5 a of the cylinder 5 in a state like “ floating in the gas ” due to the gas pressure . accordingly , mechanical contacts between the seal 1 and the cylinder 5 are eliminated , and the friction forces decrease to almost zero .