Patent Publication Number: US-6209444-B1

Title: Piston-operated refrigerant compressor and a method of assembling the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piston-operated refrigerant compressor having at least one compressing piston provided with a piston ring fitted therein and to a method of assembling the piston-operated refrigerant compressor and, particularly, to a method of assembling the pistons in the cylinder bores of the compressor. 
     2. Description of the Related Art 
     In known refrigerant compressors and, particularly, refrigerant compressors accommodated in a vehicle climate control system, carbon dioxide gas (CO 2  gas) is practically used as a refrigerant gas instead of the conventional fluorinated hydrocarbons gas, to prevent environmental problems. When CO 2  gas is used as a refrigerant gas for the compressor of the vehicle climate control system, the gas must be compressed to a relatively high pressure in order to exhibit a refrigerating performance suitable for cooling air in the vehicle compartment. Therefore, it is required that an annular gap between the cylindrical inner wall of each cylinder bore and the outer circumference of each piston is gas-tightly sealed to achieve an effective compression of the CO 2  gas while preventing an increase in an amount of blow-by gas through the gap. Further, a smooth reciprocation of the piston in the cylinder bore must be maintained. Accordingly, the sealing of the annular small gap between the bore wall of the cylinder bore and the outer circumference of the piston must be achieved by a sealing means more effective than the conventional simple sealing method in which the piston and the bore wall of the cylinder bore is maintained in a snug fitting condition without using any particular sealing elements. Thus, adoption of a piston ring for sealing the gap between each piston and the bore wall of the cylinder bore of the refrigerant compressor, which was not adopted in the field of the conventional fluorinated hydrocarbons gas type refrigerant compressors, has been recently reconsidered. 
     When a refrigerant compressor uses reciprocating pistons fitted with piston rings in circumferential grooves therein, the outer diameter of each piston ring in a free and non-compressed condition is formed larger than the inner diameter of the bore wall of the cylinder bore to obtain a good sealing performance. Therefore, when the compressor is assembled and when the respective pistons with the piston rings are fitted in the cylinder bores, the piston rings must be compressed to reduce the outer diameter thereof before the pistons and the piston rings are inserted into the respective cylinder bores of a cylinder block. For example, in the case of a single-headed piston type swash-plate-operated refrigerating compressor, when a piston unit including one set of pistons fitted with piston rings and assembled with a swash plate is inserted into the corresponding cylinder bores of the cylinder block, the ends of the respective cylinder bores opening toward a swash plate chamber of the compressor housing are chamfered to have tapered ends through which the pistons with the piston rings are forcedly inserted into the corresponding cylinder bores. In order to smoothly insert the pistons and the piston rings into the cylinder bores, the bore ends must be chamfered to have a large oblique face, respectively. Nevertheless, when the large oblique faces are formed in the respective bore ends, the length of each cylinder bore to smoothly guide the reciprocation of the pistons is reduced to result in a reduction in a reliable reciprocating operation of the respective pistons. Further, if an inclining angle of the oblique face of each bore end is increased to intentionally enlarge the diameter of the entrance of the bore end, the piston ring fitted in the piston cannot be in a smooth sliding contact with the oblique face of the bore end so that the piston and the piston ring are not smoothly inserted into the corresponding cylinder bore. Namely, assembling of the pistons and the piston rings into the cylinder block cannot be effectively achieved. Therefore, it cannot be said that the provision of the conventional chamfered oblique face at the bore end of each cylinder bore of the cylinder block of a refrigerant compressor is effective for improving the assembling operation of the piston unit into the cylinder bores of the cylinder block. Thus, when the piston unit is inserted into the cylinder bores of the cylinder block, the piston rings fitted in the respective pistons must be manually compressed from the outside to elastically reduce the diameter thereof before the pistons together with the piston rings are urged into the corresponding cylinder bores. Accordingly, it usually takes a long time to assemble the piston unit into the cylinder bores of the cylinder block. Particularly, radially inner portions of the respective cylinder bores arranged radially closer to the central bore of the cylinder block in which a drive shaft is mounted, must be arranged closer to one another in a circumferential direction, so that spacing between the neighboring radially inner portions of the two neighboring cylinder bores is considerably small from the viewpoint of the structural requirement of the cylinder block. Thus, the small spacings between the respective two neighboring inner portions of the cylinder bores cause a difficulty in the assembling of the pistons and the piston rings into the cylinder bores while compressing the piston rings either manually or by the use of a specified assembling jig. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a piston-operated refrigerant compressor provided with reciprocatory pistons fitted with piston rings, and having an internal structure allowing easy assembly of the pistons into corresponding cylinder bores of a cylinder block. 
     Another object of the present invention is to provide a simplified method of assembling reciprocatory pistons fitted with piston rings into corresponding cylinder bores of a cylinder block of a piston-operated refrigerant compressor. 
     In accordance with one aspect of the present invention, there is provided a piston-operated refrigerant compressor including: 
     a cylinder block having a central axis and a plurality of cylinder bores arranged in parallel with one another and equiangularly around the central axis; 
     a drive shaft rotatably supported in a central portion of the cylinder block and supporting thereon a cam plate to be rotatable together with the drive shaft within a crank chamber; and 
     a plurality of pistons fitted in the cylinder bores of the cylinder block to be reciprocated in the cylinder bores in association with the rotation of the cam plate, each of the plurality of pistons being fitted with a piston ring in a circumferential groove formed therein, 
     wherein the cylinder block is centrally provided with an inner end face confronting an interior of the crank chamber and having bore ends of the plurality of cylinder bores lying therein, the inner end face being centrally provided with a boss portion axially projecting into the interior of the crank chamber and defining a circularly extending lip portion of each of the bore ends of the cylinder bores to urge the piston together with the piston ring into the corresponding cylinder bore when assembling the piston and the piston ring into the corresponding cylinder bore. 
     Since the lip portion of each of the bore ends is provided so as to form a lower marginal portion of each bore end which is radially arranged adjacent to the drive shaft supported by the cylinder block and is axially extended with respect to the remaining upper marginal portion of each bore end, when the piston ring and the piston are assembled into the corresponding cylinder bore, the piston ring fitted in the circumferential groove with an non-compressed condition initially comes into contact with the lip portion of the bore end of the corresponding cylinder bore. Accordingly, a compression can be easily applied from the outside to the piston ring while the piston ring is kept in contact with the lip portion, to reduce the diameter of the piston ring. Thus, the diameter-reduced piston ring together with the piston can be smoothly inserted into the corresponding cylinder bore. Namely, the respective lip portions of the bore ends of the plurality of cylinder bores can be used as a sort of assembling tool for assembling the pistons fitted with the piston rings into the respective cylinder bores. 
     Preferably, the boss portion of the cylinder block is formed to have a round outer circumference, the diameter of which is substantially equal to that of a circle passing respective centers of the plurality of cylinder bores. Then, the round boss portion of the cylinder block can be machined easily by the use of a conventional lathe. Further, the circular lip portions of the bore ends formed in the round boss portion can provide a sufficient amount of support for stably guiding the piston rings into the respective cylinder bores during the assembling of the piston rings and the piston into the respective cylinder bores. 
     Preferably, the bore ends of the plurality of cylinder bores, lying in the inner end face of the cylinder block, are provided with a permissible amount of chamfer, respectively. Then, the respective chamfers of the bores ends of the cylinder bores can be very effective for smooth insertion of the pistons and the piston rings into the cylinder bores. 
     In accordance with another aspect of the present invention, there is provided a method of assembling a piston-operated refrigerant compressor including: a cylinder block having a central axis and a plurality of cylinder bores arranged in parallel with one another and equiangularly around the central axis; a drive shaft rotatably supported in a central portion of the cylinder block and supporting thereon a cam plate to be rotatable together with the drive shaft within a crank chamber; and a plurality of pistons fitted in the cylinder bores of the cylinder block to be reciprocated in the cylinder bores due to rotation of the cam plate, each of the plurality of pistons being fitted with a piston ring in a circumferential groove formed therein, respectively, in which the cylinder block is centrally provided with an inner end face confronting an interior of the crank chamber and having bore ends of the plurality of cylinder bores lying therein, the inner end face being centrally provided with a boss portion axially projecting into the interior of the crank chamber and defining a circularly extending lip portion of each of the bore ends of the cylinder bores to allow the piston together with the piston ring to be easily inserted into the cylinder bore, and 
     wherein the method is characterized by a process of assembling the pistons fitted with the piston rings into the respective cylinder bores, the process comprising the steps of: 
     preparing a piston-assembling jig separable into two halves 
     and provided with guide bores, each having the shape of an arcuate bore smaller than a semi-circular bore and having a bore diameter thereof substantially the same as that of each cylinder bore, the guide bores being able to come into registration with radially outer arcuate portions of all of the plurality of cylinder bores of the cylinder block when the piston-assembling jig is brought into contact with the inner end face of the cylinder block; 
     attaching the piston-assembling jig to the inner end face of the cylinder block so that guide bores are in registration with all of the bore ends of the cylinder bores of the cylinder block; 
     inserting the pistons and the piston rings into the respective cylinder bores via the guide bores of the piston-assembling jig while compressing the respective piston rings by the cooperation of the guide bores and the circularly extending lip portions of the bore ends of the cylinder bores to thereby reduce the diameter of the piston rings; and, 
     after inserting all of the pistons fitted with the piston rings into the respective cylinder bores, separating the piston-assembling jig into the two halves to detach the piston-assembling jig from the inner end face of the cylinder block. 
     Preferably, the bore ends of the plurality of cylinder bores, lying in the inner end face of the cylinder block of the compressor, are provided with a permissible amount of a chamfer formed thereat, respectively, and the guide bores of the piston-assembling jig are provided with a chamfered portion sufficient for promoting a smooth reduction in the diameter of the piston rings when the pistons fitted with the piston rings are urged into the respective cylinder bores. 
     It will be understood that, by the use of the above-mentioned piston-assembling jig, all of the plurality of the pistons fitted with the piston rings and assembled on a variable inclination cam plate supported on a drive shaft of a variable capacity piston-operated refrigerant compressor can be assembled into the corresponding cylinder bores of the cylinder block at a single assembling stage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will be made more apparent from the ensuing description of preferred embodiments in conjunction with the accompanying drawings wherein: 
     FIG. 1A is a longitudinal cross-sectional view of a piston-operated refrigerant compressor provided with pistons fitted with piston rings and a fixed-inclination-type swash plate, according to an embodiment of the present invention; 
     FIG. 1B is a longitudinal cross-sectional view of a piston-operated refrigerant compressor provided with pistons fitted with piston rings and a variable-inclination-type swash plate, according to a different embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of a cylinder block assembled in either one of the compressors of FIGS. 1A and 1B, and illustrating a lip portion formed in a central boss portion of an inner end face of the cylinder block; 
     FIG. 3 is a side view of the inner end face of the cylinder block of FIG. 2, illustrating an arrangement of the cylinder bores provided with the lip portions formed in the bore ends thereof; 
     FIG. 4 is a partial cross-sectional view of a part of the cylinder block, illustrating a relationship between one of the cylinder bores and a piston assembling jig attached to the inner end face of the cylinder block when the piston fitted with piston rings are assembled into the cylinder bores from the side of the inner end face of the cylinder block, and; 
     FIG. 5 is a side view of a piston assembling jig, illustrating an arrangement of guide bores, and a dividable construction thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a piston-operated refrigerant compressor formed as a fixed capacity refrigerant compressor is provided with a cylinder block  1  having an inner end face confronting a later-described crank chamber of the compressor and an outer end face to which a later-described valve plate  4  is secured. The inner end face of the cylinder block  1  is closed by a bell-jar like front housing  2  which is hermetically affixed to the inner end face of the cylinder block  1 . The outer end of the cylinder block  1  is hermetically closed by a rear housing  3  via the valve plate  4 . The cylinder block  1  and the front housing  2  define a closed chamber therein, conventionally referred to as a crank chamber  5 , through which an axial drive shaft  6  extends so as to be rotatably supported by the cylinder block  1  and the front housing  2  via front and rear radial bearings  7   a  and  7   b . Namely, the drive shaft  6  can rotate about an axis of rotation which extends through the centers of the front and rear radial bearings  7   a  and  7   b.    
     The cylinder block  1  is provided with a plurality of cylinder bores  8  which are arranged in parallel with one another and substantially equiangularly about the axis of rotation of the drive shaft  6 . The cylinder bores  8  of the cylinder block  1  have an equal bore diameter and an equal axial length, respectively, in which a plurality of single-headed pistons  9  fitted with piston rings  10  made of an iron system material are fitted. The pistons  9  having the piston rings  10  perform a reciprocating motion in the respective cylinder bores  8 , by which a refrigerant gas is sucked from a later-described suction chamber  20 , compressed therein, and is discharged therefrom into a later-described discharge chamber  21 . 
     A circular cam plate, i.e., a swash plate  11  is fixedly mounted on the drive shaft  6  so as to be rotated together with the drive shaft  6  within the crank chamber  5 . The swash plate  11  is sandwiched by the cylinder block  1  and the front housing  2  via a pair of front and rear thrust bearings  12   a  and  12   b . An outer marginal portion of the swash plate  11  is positioned between confronting flat faces of two semi-spherical shoe elements  13 ,  13  received in a pair of support recesses formed in an end of each of the plurality of pistons  9 . Thus, the two shoe elements  13  form one of a plurality of pairs of shoes  13 ,  13  received in the plurality of pistons  9 . 
     The rear housing  3  attached to the outer end face of the cylinder block  1  via the valve plate  4  defines a suction chamber  20  for receiving a refrigerant gas before compression and a discharge chamber  21  for receiving the refrigerant gas after compression. The suction chamber  20  communicates with each of the cylinder bores  8  via each suction port Z 2  formed in a relevant portion of the valve plate  4 , and the discharge chamber  21  also communicates with each of the cylinder bores  8  via each discharge port  23  formed in a relevant portion of the valve plate  4 . Each of the cylinder bores  8  forms a compression chamber between the operating head of the corresponding piston  9  and the face of the valve plate  4  so that the refrigerant gas is compressed within the compression chamber. The suction ports  22  of the valve plate  4  are closed by suction valves (not shown in FIG. 1) held between the outer end face of the cylinder block  1  and the valve plate  4  to be opened when the pistons  9  perform their suction stroke motion within the cylinder bores  8 . The discharge ports  23  of the valve plate  4  are closed by discharge valves (not shown in FIG. 1) held between the valve plate  4  and the end of the rear housing  3  to be opened when the pistons  9  perform their discharge stroke motion within the cylinder bores  8 . The refrigerant compressor according to the embodiment of FIG. 1A is characterized in that the cylinder block  1  has a specified novel construction suitable for permitting the pistons  9  fitted with the piston rings  10  to be inserted into the cylinder bores  8  without any difficulty, and accordingly, the assembly of the entire piston-operated refrigerant compressor can be accomplished with high efficiency. 
     The description of the specified construction of the cylinder block  1  of the compressor of FIG. 1A will be provided later with reference to FIGS. 2 and 3. However, before referring to the specified construction of the cylinder block  1 , the description of a piston-operated refrigerant compressor of a different embodiment will be set forth below with reference to FIG.  1 B. 
     Referring to FIG. 1B, a variable capacity type piston-operated refrigerant compressor, which has a cam plate formed as a variable inclination type swash plate, has a cylinder block  51  having axially opposite ends, i.e., an inner (front) end face and an outer (rear) end face. The inner end face of the cylinder block  51  is closed by a bell-shaped front housing  52  hermetically secured to the cylinder block  51 , and the outer end face of the cylinder block  51  is closed by a rear housing  53  also hermetically secured to the cylinder block  51  via a valve plate  54 . The cylinder block  51  and the front housing  52  define an interior crank chamber  55  located in front of the inner end face of the cylinder block  51 . The crank chamber  55  is formed so as to permit a drive shaft  56  to axially extend therethrough. The drive shaft  56  is rotatably supported by the front housing  52  and the cylinder block  51  via a front radial bearing  7   a  and a rear radial bearing  7   b . A frontmost end of the drive shaft  56  extends toward a front opening of the front housing  52  so as to receive an external drive force from a non-illustrating drive source such as an automobile engine. When driven, the drive shaft  56  rotates about its central axis of rotation to thereby operate the compressor. 
     The cylinder block  51  is provided with a plurality of cylinder bores  58  extending axially from the inner end face to the outer end face. The cylinder bores  58  are arranged equiangularly around the axis of rotation of the drive shaft  56 , and in parallel with one another. The respective cylinder bores  58  receive therein single headed pistons  59  respectively fitted with piston rings  60 , so that the pistons  59  with piston rings  60  are slidable within the cylinder bores  58  of the cylinder block  51 . 
     The drive shaft  56  has a rotor element  69  fixedly mounted thereon at a position adjacent to an inner end wall of the front housing  52  via a thrust bearing  57   c . Thus, the rotor element  69  is rotated together with the drive shaft  56  within the crank chamber  55 . 
     A swash plate element  61  is mounted on the drive shaft  56  at a position spaced rearwardly from the rotor element  69  within the crank chamber  55 . The swash plate element  61  is provided with a substantially central bore  61   b  through which the drive shaft  56  axially extends. 
     The central bore  61   b  of the swash plate  61  has an axially non-linear cylindrical shape and is formed in a bore consisting of a combination of two different bores which are slanted from an axis perpendicular to end faces of the swash plate element  61 . The two slanted bores forming the central bore  61   b  of the swash plate element  61  permit the swash plate  61  to turn about a predetermined axis to thereby change an angle of inclination of the swash plate  61  from a minimum angle of inclination to a maximum angle of inclination. 
     A coil spring  62  is arranged between the rotor element  69  and the swash plate element  61  for constantly rearwardly urging the swash plate element  61 . The swash plate element  61  is provided with outer portion which is engaged with respective pistons  59  via semi-spherical shoes  64 ,  64  having a half-spherical engaging faces fitted in spherical recesses formed in respective pistons  59 , as typically shown by one of the pistons in FIG.  1 . Thus, when the swash plate element  61  is rotated together with the drive shaft  56  via the rotor element  69 , the pistons  59  are reciprocated in the respective cylinder bores  58 . 
     The swash plate element  61  is provided with a bracket  65  shown by a chain line in FIG. 1B, which is formed in a portion thereof on the front side. The bracket  65  in the shape of a projection is provided for forming a part of the hinge unit “K” between the swash plate element  61  and the rotor element  69 . The bracket  65  is provided with an end portion to which an end of a guide pin  66  is secured. The guide pin  66  projects toward the rotor element  69 , and has an outer end in which a spherical portion  66   a  is formed. The spherical portion  66   a  is received in an hole  67   a  of a support arm  67  formed in a portion of the rotor element  69  on the rear side thereof. As shown by a chain line in FIG. 1B, the support arm  67  projects toward the guide pin  66  of the swash plate element  61 , and forms a part cooperating with the bracket  65  and the guide pin  66  in order to constitute the hinge unit “K”. 
     The guide hole  67   a  of the support arm  67  is arranged to be parallel with a plane extending so as to contain therein the line of inclination of the swash plate element  61  and the axis of rotation of the drive shaft  56 . The guide hole  67   a  is bored so as to radially extend toward and to be slanted rearwardly when it approaches toward the axis of rotation of the drive shaft  56 . The guide hole  67   a  of the support arm  67  receiving therein the spherical portion  66   a  of the hinge unit “K” has a center line thereof which is provided so that when the swash plate element  61  changes its angle of inclination under the restrained guide of the hinge unit “K”, the position of the top dead center of the respective pistons  59  operatively engaged with the swash plate element  61  is substantially unchanged. 
     The rear housing  53  is provided therein with a suction chamber  70  for receiving refrigerant gas before compression and a discharge chamber  71  for the compressed refrigerant gas. The suction and discharge chambers  70  and  71  are hermetically separated from one another. The valve plate  54  is provided with suction ports  72  formed therein for providing fluid communication between compression chambers formed in the respective cylinder bores  58  between the valve plate  54  and the operating heads of the respective pistons  59 , and the suction chamber  70 . The valve plate  54  is also provided with discharge ports  73  formed therein for providing fluid communication between the compression chambers in the respective cylinder bores  58  and the discharge chamber  71 . 
     The suction ports  72  of the valve plate  54  are covered by conventional suction valves e.g., suction reed valves which open and close in response to the reciprocation of the pistons  59 , and the discharge ports  73  of the valve plate  54  are covered by conventional discharge valves, e.g., discharge reed valves which open and close in response to the reciprocation of the pistons  59 . The rear housing  53  receives therein a control valve (not shown) for controlling a pressure prevailing in the crank chamber  55 . A typical control valve is disclosed in U.S. Pat. No. 4,729,719 to Kayukawa et al., and is assigned to the same assignee as the present application. 
     The variable inclination type swash plate element  61  is provided with a counter bore  61   b  which is provided to come into contact with a stop ring  63  secured to a rear portion of the drive shaft  56  when the swash plate  61  is moved to the position of the minimum angle of inclination. 
     On the other hand, the position of the maximum angle of inclination of the swash plate element  61  is limited when a contacting area  61   a  of the swash plate element  61  comes into contact with a cooperating contacting area  69   a  formed in the rotor element  69  during the increase in the angle of inclination of the swash plate  61 . 
     When the variable capacity refrigerant compressor having the above-mentioned internal construction is operated by the rotation of the drive shaft  56 , the swash plate  61  connected to the rotor element  69  via the hinge unit “K” is rotated together with the drive shaft  56 . Therefore, the single headed pistons  9  fitted with the piston rings  60  are reciprocated in the respective cylinder bores  58  via the shoes  64 ,  64 . Thus, the refrigerant gas is sucked from the suction chamber  70  into the compression chambers of the respective cylinder bores  58  via the suction ports  72 . The sucked refrigerant gas is compressed within the compression chambers of the respective cylinder bores  58 , and is discharged from the respective cylinder bores  58  into the discharge chamber  71 . The capacity of the compressed refrigerant gas discharged into the discharge chamber  71  is controlled by the control valve which controls the pressure level within the crank chamber  55 . 
     When the pressure prevailing in the crank chamber  5  is increased by the operation of the control valve, the pressure acting on the back of the respective pistons  59  increases. Thus, the stroke of the respective pistons  59  is reduced to reduce an angle of inclination of the swash plate element  61 . Namely, in the hinge unit “K”, the spherical portion  66   a  of the guide pin  66  is rotationally slid down in the guide hole  67   a  of the support arm  67  toward the axis of the drive shaft  56 . Accordingly, the swash plate element  61  is turned about its pivotal axis, and is moved rearwardly by the spring force of the coil spring  62  along the outer circumference of the drive shaft  56 . Namely, the swash plate element  61  is linearly slid on the drive shaft  56 . Therefore, the angle of inclination of the swash plate element  61  is reduced and, accordingly, the capacity of the compressed refrigerant gas discharged from the compression chambers of the respective cylinder bores  58  is reduced. The position of the minimum angle of inclination of the swash plate element  61  is limited when the counter bore  61   b  of the swash plate element  61  comes into contact with the stop ring  63  fixed to the rear portion of the drive shaft  56 . 
     On the other hand, when the compressor operates at a small capacity condition, and when the pressure level in the crank chamber  55  is reduced by the pressure adjusting operation of the control valve, the pressure acting on the back of the respective pistons  59  is decreased to cause an increase in the angle of inclination of the swash plate element  61 . Thus, the spherical portion  66   a  of the guide pin  66  of the hinge unit “K” is rotationally moved up in the guide hole  67   a  of the support arm  67  of the hinge unit “K”. Therefore, the swash plate element  61  is moved forwardly against the spring force of the coil spring  62  while maintaining slide contact of the swash plate element  61  with the outer circumference of the drive shaft  56 . Thus, the angle of inclination of the swash plate element  61  is increased to increase the stroke of the respective pistons  59 . Accordingly, the capacity of the compressor is increased. The position of the maximum angle of inclination is limited by the inclination limiting means, i.e., by the engagement of the contacting area  61   a  of the swash plate element  61  and the rear contacting area  69   a  of the rotor element  69 . 
     The above-described variable capacity piston-operated refrigerant compressor of the embodiment of FIG. 1B is also characterized in that the cylinder block  51  similar to the cylinder block  1  of the embodiment of FIG. 1A has a specified construction suitable for permitting the pistons  59  fitted with the piston rings  60  to be inserted into the cylinder bores  58  by an assembler without any difficulty, and accordingly, the assembly of the entire piston-operated refrigerant compressor can be accomplished at a high efficiency. 
     FIGS. 2 and 3 illustrate commonly the construction of the cylinder block  1  or  51  accommodated in the piston-operated compressor of FIG. 1A or  1 B. 
     The cylinder block  1  or  51  is provided with a plurality of cylinder bores  8  or  58  formed therein to axially extend in parallel with a central axis “CL” of the cylinder block  1  or  51  and arranged substantially equiangularly around the central axis “CL”. The cylinder block  1  or  51  has the inner end face  1   a  confronting the crank chamber  5  or  55  (see FIGS. 1A and 1B) and the outer end face  1   c  closed by the rear housing  3  or  53  via the valve plate  4  or  54  (see FIGS.  1 A and  1 B). The inner end face  1   a  of the cylinder block  1  or  51  is centrally provided with a boss portion  1   b  projecting axially from a plane in which an outer marginal portion of the inner end face  1   a  lies. The boss portion  1   b  of the inner end face  1   a  has its outermost circumference portion formed to have an outer diameter substantially equal to that “RA” of a circle passing the centers of the respective cylinder bores  8  or  58  as shown in FIG.  3 . The outermost circumference of the boss portion  1   b  is formed so as to extend around the central axis of the cylinder block  1  or  51 . 
     The boss portion  1   b  has also a plurality of rim-like portions each being formed as a circularly extending lip portion  80  which surrounds an inward part of the bore end of each cylinder bore  8  or  58  with respect to the central axis “CL” of the cylinder block  8  or  58 . Thus, in the described embodiment, since the cylinder block  1  or  51  has equiangularly arranged six cylinder bores  8  or  58 , the boss portion  1   b  is provided with six lip portions  80 . The respective lip portions  80  of the boss portion  1   b  are preferably formed as an outwardly divergent oblique face, respectively, as will be understood from the illustration of FIGS. 2 and 3. 
     Further, the bore end of each cylinder bore  8  or  58  lying in the inner end face  1   a  of the cylinder block  1  or  51  is chamfered to provide an oblique face portion  81  in the shape of an outwardly diverging portion, shown by thick lines in FIG.  3 . An extent of the oblique face portion  81  formed in the bore end of each cylinder bore  8  or  58  is determined so that each cylinder bore  8  or  58  may have an axial length sufficient for permitting each piston  9  or  59  to reciprocate within the cylinder bore  8  or  58  while conducting suction of an appropriate amount of refrigerant gas, compression of the sucked refrigerant gas, and discharge of the compressed refrigerant gas. It should be understood that, in the described embodiment, the oblique face  81  extends circularly through the entire portion of the bore end of each cylinder bore  8  or  58 . Thus, the oblique portion  81  is formed so as to run through the afore-mentioned lip portion  80  of each cylinder bore  8  or  58  as will be understood form the illustration of FIG.  2 . 
     When the piston  9  or  59  fitted with the piston ring  10  or  60  in the non-compressed condition is inserted into the corresponding cylinder bore  8  or  58  of the cylinder block  1  or  51  from the inner end face  1   a  during the assembling of the piston-operated refrigerant compressor, the piston rings  10  or  60  fitted on the piston  9  or  59  initially come into contact with the semi-circular lip portion  80  of the bore end of the cylinder bore  8  or  58 , so that a circular half portion of the piston ring  10  or  60  is supported by the lip portion  80  during the movement of the piston  9  or  59  with the piston ring  10  or  60  into the cylinder bore  8  or  58 . Thus, it is not required that the assembler manipulates fingers in a narrow region around the inward part of the bore end of each cylinder bore  8  or  58  to reduce the diameter of the piston ring  10  or  60 . Namely, when the assembler applies an appropriate compression to the piston ring  9  or  59  from a portion thereof which is located opposite to the circular half portion of the piston ring  10  or  60  supported by the lip portion  80  of the cylinder block  1  or  51 , the diameter of the piston ring  10  or  60  is easily reduced within an annular groove of the piston  9  or  59 . Then, the diameter-reduced piston ring  10  or  60  fitted on the piston  9  or  59  is brought into contact with the oblique face  81  of the bore end of the cylinder bore  8  or  58 . Therefore, when the piston  9  or  59  is moved further into the cylinder bore  8  or  58 , the piston  9  or  59  together with the piston ring  10  or  60  are smoothly fitted in the corresponding cylinder bore  8  or  58 . Thus, the assembling of the piston  9  or  59  and the piston ring  10  or  60  into the cylinder bore  8  or  58  is accomplished. 
     FIGS. 4 and 5 illustrate a specified method of assembling the piston  9  or  59  fitted with the piston ring  10  or  60  into the cylinder bore  8  or  58  according to the present invention. Namely, in the method, a piston-assembling jig  40  is used for smoothly assembling a plurality of the pistons  9  or  59  and the piston rings  10  or  60  into a plurality of the cylinder bores  8  or  58  at a single assembling stage. 
     The piston-assembling jig  40  is formed as a generally circular disk like tool dividable into two halves along a line P—P as shown in FIG.  5 . The jig  40  includes guide bores  41  which are arranged so as to be in registration with the cylinder bores  8  or  58  of the cylinder block  1  or  51 . Each of the guide bores  41  of the piston-assembling jig  40  is shaped in a partial circular bore having a diameter substantially equal to that of each of the cylinder bores  8  or  58  and an arcuate inner edge portion smaller than a semi-circular inner edge. The arcuate inner edge of each of the guide bores  41  is chamfered to have an oblique face  42  extending inwardly divergently from one of the opposite end faces of the piston-assembling jig  40 , i.e., an end face  40   a  by which the piston-assembling jig  40  is attached to the inner end face  1   a  of the cylinder block  1  or  51 , as best shown in FIG.  4 . The lengths of respective arcuate edge portions of the guide bores  41  are designed so that the two halves of the piston-assembling jig  40  can be easily separated from one another along the line P—P (see FIG. 5) without any mechanical interference with the pistons  9  or  59  even when all of the guide bores  41  hold therein the pistons  9  or  59  to be assembled into the cylinder block  1  or  51 . 
     When the plurality of pistons  9  or  59  (e.g., six pistons) provided with the piston rings  10  or  60  are assembled into the corresponding number of the cylinder bores  8  or  58 , the piston-assembling jig  40  is initially placed on the inner end face  1   a  of the cylinder block  1  or  51  which is disposed at a state where the inner and outer end faces  1   a  and  1   c  of the cylinder block  1  or  51  are positioned up and down. Namely, the end face  40   a  of the piston-assembling jig  40  is brought into contact with the inner end face  1   a  of the cylinder block  1  or  51 . Subsequently, an adjustment is carried out so that respective guide bores  41  of the piston-assembling jig  40  come into accurate registration with the cylinder bores  8  or  58  of the cylinder block  1  or  51 . Thus, the oblique faces  42  of the guide bores  41  of the piston-assembling jig  40  coincide with the corresponding oblique faces  81  formed in the circular lip portions  80  of the cylinder bores  8  or  58 , so that a substantially round oblique face is formed for each of the cylinder bores  8  or  58  as will be understood from FIG.  4 . At this stage, the two halves of the piston-assembling jig  40  should be preferably provided with suitable projections or small pins  43  (refer to FIG. 5) which are engaged in positioning holes  1   d  (refer to FIG. 3) bored in the inner end face  1   a  of the cylinder block  1  or  51  to surely achieve the above-mentioned adjustment of the position of the guide bores  41  of the piston-assembling jig  40  with respect to the cylinder bores  8  or  58  of the cylinder block  1  or  51 . 
     When the adjustment of the guide bores  41  with respect to the cylinder bores  8  or  58  is completed, the plurality of the pistons  9  or  59  fitted with the piston rings  10  and  60  are inserted into the cylinder bores  8  or  58  via the guide bores  41  of the piston-assembling jig  40  by applying a pressure to the respective pistons  9  or  59  to move the pistons deep into the cylinder bores  8  or  58 . At this stage, it should be understood that the pistons  9  or  59  fitted with the piston rings  10  or  60  are preliminarily assembled with a cam plate (a swash plate)  11  or  61  mounted on the drive shaft  6  or  56  to form a piston unit. 
     The non-compressed piston rings  10  or  60  fitted on the pistons  9  or  59  are then gradually guided by the oblique faces  42  of the piston-assembling jig  40  and the oblique face portions  81  formed in the bore ends of the respective cylinder bores  8  or  58  so as to smoothly reduce the diameter of the respective piston rings  10  or  60  in response to the insertion of the pistons  9  or  59  into the cylinder bores  8  or  58 . Therefore, the assembling of the pistons  9  or  59  and the piston rings  10  or  60  into the cylinder bores  8  or  58  of the cylinder block  1  or  51  is accomplished in one single assembling process by the use of the piston-assembling jig  40 . After completion of the assembly of all pistons  9  or  59  and the piston rings  10  or  60  of the piston unit into the corresponding cylinder bores  8  or  58  of the cylinder block  1  or  51 , the two halves of the piston-assembling jigs  40  are separated from one another in a direction perpendicular to the dividing line P—P of the jig  40 . Thus, the piston-assembling jig  40  can be easily detached and removed from the inner end face  1   a  of the cylinder block  1  or  51  without causing any mechanical interference of the jig  40  and the pistons  9  or  59  assembled in the cylinder bores  8  or  58 . 
     From the foregoing description of the assembling method of the pistons  9  or  59  and the piston rings  10  or  60  of the piston unit, it will be easily understood that due to the utilization of the oblique faces  42  of the piston-assembling jig  40  and the oblique face portions  81  formed in the bore ends of the cylinder bores  8  or  58  of the cylinder block  1  or  51 , the piston unit, i.e., the pre-assembly of the pistons  9  ( 59 ), the piston rings  10  ( 60 ) and the cam or swash plate  11  ( 61 ) are easily assembled into the cylinder bores  8  or  58  of the cylinder block  1  or  51  irrespective of whether the piston unit is assembled in the cylinder block  1  of a non-variable capacity piston-operated refrigerant compressor in which axial positions of the respective pistons  9  within the cylinder bores  9  are made different from one another due to the fixed angle of inclination of the cam or swash plate  11  or is assembled in the cylinder block  51  of a variable capacity piston-operated refrigerant compressor in which axial positions of the respective pistons  9  within the cylinder bores  9  are changeable by changing an angle of inclination of the cam plate  61 . 
     In the case of a variable capacity piston-operated refrigerant compressor, it is possible to adjust positions of all pistons  59  of the piston unit so that their piston working ends substantially lie in a plane perpendicular to the axis of the drive shaft  56  on which the cam plate  61  is mounted by changing an angular position of the cam plate  61  before the piston unit is assembled in the cylinder block  51 . Therefore, even if the piston-assembling jig  40  has no oblique faces  42  and even if the bore ends of the respective cylinder bores  58  has no oblique face portions  81 , when the position-adjusted pistons  59  of the piston unit are brought into the bore ends of the corresponding cylinder bores  58  of the cylinder block  51 , and when the two halves of the piston-assembling jig are subsequently fitted around the respective pistons  59  so as to apply a compression to the piston rings  60  and to reduce the diameter of the piston rings  60 , all pistons  59  and the diameter-reduced pistons rings  60  are easily moved into the cylinder bores  58 . Thus, the assembly of the pistons  59  fitted with the piston rings  60  into the cylinder bores  58  of the cylinder block  51  of a variable capacity piston-operated refrigerant assembly can be accomplished by using the piston-assembling jig  40  even if the guide bores  41  of the jig  40  have no oblique faces  42 . 
     It will be understood from the foregoing description of the various embodiments of the present invention that the plurality of pistons fitted with piston rings therein and accommodated in a piston-operated refrigerant compressor irrespective of the capacity being constant and variable, can be easily and in turn effectively assembled in a corresponding number of cylinder bores of a cylinder block by provision of a characteristic boss portion in the inner end face of the cylinder block confronting the crank chamber of the refrigerant compressor and by the use of a specified piston-assembling jig. 
     It should be understood that various changes or modification will occur to a person skilled in the art without departing from the scope and spirit of the invention as claimed in the accompanying claims.