Abstract:
A variable capacity swash plate type compressor  10  incorporates a swash plate  34  to effect movement of at least an associated piston  44  to vary the capacity of the compressor  10 . The structure of the swash plate  34  and piston  44  minimize the bending moment exerted on the piston  44  during operation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a variable capacity swash plate type compressor adapted for use in an air conditioner for a vehicle, and more particularly, to a piston suitable for use in an automotive air conditioning compressor in which the piston includes an associated swash plate to minimize the bending moment exerted thereon. 
     Generally, a piston type compressor for use in an automotive air conditioning system comprises a cylinder block having a plurality of cylinder bores. A plurality of pistons are slidably disposed in the respective cylinder bores and reciprocate by, for example, a swash plate in the cylinder bores. In a variable capacity swash plate type compressor with a mechanism varying an inclination angle of the swash plate, a single-headed piston is generally used. The single-headed piston includes a body with a head, and support portion for receiving shoes which convert rotation of the swash plate into reciprocation of the pistons. However, a bending moment acts on the pistons due to force exerted deflectively on the pistons during operation of the compressor. Accordingly, the bending moment causes the deformation of pistons, and thus, a contact portion between the pistons and the cylinder bores is abraded deflectively. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a swash plate type compressor with pistons by which the problems of the prior art can be solved. 
     Another object of the invention is to provide a swash plate type compressor provided with a piston having a construction to minimize a bending moment by which high durability of the piston and compressor can be accomplished. 
     Still another object of the invention is to provide a swash plate type compressor provided with a mechanism suitable for a piston having a construction to minimize a bending moment. 
     The above as well as other objects of the invention may be typically achieved by producing a variable capacity swash plate type compressor comprising: 
     a cylinder block having a plurality of cylinder bores arranged radially and circumferentially therein; 
     a housing mounted adjacent the cylinder block and cooperating with the cylinder block to define an air-tight sealed crank chamber; 
     a drive shaft rotatably supported by the housing and the cylinder block; 
     a rotor mounted on the drive shaft; 
     a swash plate connected to the rotor and slidably mounted on the drive shaft to thereby change an inclination angle thereof in response to the changes of pressure in the crank chamber; 
     a hinge means disposed between the rotor and the swash plate for changing the inclination angle of the swash plate; 
     a plurality of pistons reciprocatively disposed in each of the cylinder bores, each piston having a cylindrical body with a head, and a bridge portion connected to the body and having a recess and a pair of shoe pockets formed in opposed walls defining the recess, the body of each piston having a lower back edge portion extending to a place between an entrance and an apex of the shoe pocket adjacent to the body, the lower back edge portion being around a portion connected to the bridge portion, whereby contact between the swash plate and the lower back edge portion of the pistons is prevented; 
     a plurality of shoes disposed in the shoe pockets of the recess of each piston to come into contact with the swash plate for converting rotation of the swash plate into reciprocation of the pistons; and 
     a control valve means for adjusting a pressure level in the crank chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the present invention will be understood from the detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings, in which 
     FIG. 1 is a sectional elevational view of a swash plate type compressor with a variable displacement mechanism according to the prior art; 
     FIG. 2 is a fragmentary schematic view of FIG. 1 illustrating various forces acting on a piston; 
     FIG. 3 is a sectional elevational view of a variable capacity swash plate type compressor with a piston and a mechanism to minimize a bending moment acting on a piston according to the present invention; 
     FIG. 4 is a fragmentary schematic view showing elements around the swash plate of FIG. 3 to illustrate the operation of the elements in the compressor; 
     FIG. 5 is a sectional view of a second embodiment of swash plate according to the present invention adapted for use in a variable capacity swash plate type compressor of the type illustrated in FIGS. 3 and 4; 
     FIG. 6 is a perspective view of a cylinder block of the compressor according to a first embodiment of the present invention; and 
     FIG. 7 is a perspective view of a cylinder block of the compressor according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to clarify the problems occurring in a conventional swash plate type compressor with a variable displacement mechanism, description will be made with reference to FIG.  1 . The compressor  1  of this type has a cylinder block  2  with a plurality of cylinder bores  4 , and front and rear ends of the cylinder block  2  are sealingly closed by front and rear housings  6  and  8 . The cylinder block  2  and the front housing  6  define an airtight sealed crank chamber  10 . A valve plate  12  is intervened between the rear end of the cylinder block  2  and the rear housing  8 . The rear housing has formed therein inlet and outlet ports  14  and  16  for input and output of a refrigerant gas, a suction chamber  18 , and a discharge chamber  20 . The suction and discharge chambers  18  and  20  are communicated with the respective cylinder bores  4  via suction and discharge valve mechanisms. A drive shaft  22  is centrally arranged to extend through the front housing  6  to the cylinder block  2  and rotatably supported by bearings  24  mounted in the front housing  6  and the cylinder block  2 . The cylinder block  2  and the front and rear housings  6  and  8  are combined by a long screw  25 . A rotor  26  is mounted on the drive shaft  22  in the crank chamber  10  to be rotatable with the drive shaft  22 , and supported by a thrust bearing  28  seated on an inner end of the front housing  6 . A spherical sleeve  30  having an outer spherical surface formed as a support surface is slidably supported by the drive shaft  22 . A spring  32  mounted around the drive shaft  22  is interposed between the rotor  26  and the spherical sleeve  30 , and pushes the spherical sleeve  30  toward the rear housing  8 . 
     A swash plate  34  is rotatably supported on the outer surface of the spherical sleeve  30 . The swash plate  34  is connected to the rotor  26  via a hinge mechanism so as to be rotated with the rotor  26 . Namely, a support arm  36  protrudes axially outwardly from one side surface of the rotor  26 , and an arm  38  protrudes from one side surface of the swash plate  34  toward the support arm  36  of the rotor  26 . The support arm  36  and the arm  38  overlap each other and are connected to each other by a pin  40 . The pin  40  extends into a pin hole  42  formed through the support arm  36  of the rotor  26  and a rectangular shaped hole  43  formed through the arm  38  of the swash plate  34 . In this manner, the rotor  26  and the swash plate  34  are hinged to each other, and the sliding motion of the pin  40  within the rectangular hole  43  changes an inclination angle of the swash plate  34  so as to change the capacity of the compressor. 
     Pistons  44  are slidably disposed in the respective cylinder bores  4 . Each piston  44  has a body  46  with a head portion which is slidably disposed in the corresponding cylinder bore  4 , and a bridge portion  48  which has formed therein a recess  50 . Semi-spherical shoes  52  are disposed in shoe pockets  54  formed in the bridge portion of the piston  44  and slidably engaged with a peripheral portion of the swash plate  34 . Therefore, the swash plate  34  is rotated together with the rotation of the drive shaft  22 , and the rotation of the swash plate  34  is converted into the reciprocation of the pistons  44 . 
     A cutout portion  56  is formed at a lower left end portion of the piston  44  to prevent a contact between a side surface of the swash plate  34  and the body  46  of the piston  44  when a piston  44  is in its bottom dead center. 
     A control valve means  60  is provided with the compressor to adjust a pressure level in the crank chamber  10 . 
     In the above-described type of compressor, a bending moment among various forces acting on the pistons  44  causes a deformation of the pistons  44  and a partially deflected abnormal abrasion about a contact portion between the pistons  44  and the cylinder bores  4 . 
     FIG. 2 is an enlarged partial view of FIG. 1 to illustrate various forces acting on the pistons. Referring to FIG. 2, during the compression stroke of the piston  44 , the pressure PC in the crank chamber  10  acts on one end of the piston  44  while a compression reaction force Pd acts on the other end of the piston  44 . The pressure PC in the crank chamber  10  and the compression reaction force Pd act on the swash plate from the piston via the shoes  52 , and the action force exerted on the swash plate  34  reversely acts on the piston  44  via the shoes  52  as a reaction force which is equal in magnitude and oppositely directed to the action force. That is, when the piston  44  is in its compression stroke, the force F exerted from the swash plate  34  on the piston  44  acts on the piston  44  at an angle perpendicular to surfaces of the swash plate  34  at a contact position at which the semi-spherical outer surface of the shoe  52  adjacent to the body of the piston  44  comes into contact with the semi-spherical inner surface of the shoe pocket  54 , i.e., at an apex of the shoe pocket  54  lying on the central axis CA of the piston  44 . The force F exerted from the swash plate  34  on the piston  44  is composed of two components, horizontal and vertical components, the horizontal component F x  lying on the central axis CA of the piston  44  and the vertical component F y  being perpendicular to the central axis CA of the piston  44 . Let “m” be the mass of the piston  44 , “a” be the acceleration of the piston  44  during the compression stroke, “A” be the cross sectional area of the piston  44 , “θ” be the angle from horizontal the force F is acting on the piston  44 , and “d” be the diameter of the piston  44 . 
     
       
         Σ F   x   =ma   (1)  
       
     
     
       
         Σ F   x   AP   c   −AP   d   +F   x   (2)  
       
     
     By combining the above equations, we can write, 
     
       
           F   x   =ma+A ( P   d   −P   c )= ma +(π/4)* d   2 ( P   d   −P   c )  
       
     
     
       
         and  
       
     
     
       
           F   y   =F   x  tan θ=tan θ□ ma +(π/4)* d   2 ( P   d   −P   c )□ 
       
     
     The vertical component F y  acts on the piston  44  as a bending moment which is maximized at the lower back edge designated by “P”. Each piston  44  is provided with the cutout portion  56  to prevent a piston  44  from coming into contact with one side surface (front surface) of the swash plate  34  when a piston  44  approaches its bottom dead center during the suction stroke. The cutout portion  56  provides a distance x between an operating point of the force F acting on the piston and an operating point of a reaction force acting on the cutout portion  56 , i.e., the lower back edge of the piston  44 , as shown in FIG. 2, and the distance x causes a bending moment which acts on the piston  44 . The maximum bending moment M max  acting on the piston is given by 
     
       
           M   max   x F   y=   x  tan θ□ ma +(π/4)* d   2 ( P   d   −P   c )□  (3)  
       
     
     Therefore, due to the bending moment, the piston  44  is deformed by the distance x about the bridge portion  48  of the piston  44  in a counterclockwise direction with respect to the reaction force-operating point P, and at the same time, deflected abnormal abrasion also occurs in the body of the piston about the reaction force-operating point P and in an edge portion diagonally opposed thereto. 
     On the other hand, during the suction stroke of the piston  44 ′, the pressure P c ′ in the crank chamber  10 ′ acts on one end of the piston  44 ′ while a suction force P s ′ acts on the other end of the piston  44 ′. The pressure P c ′ in the crank chamber  10 ′ and the suction force P s ′ act on the swash plate from the piston via the shoe  52 ′, and the action force exerted on the swash plate  34 ′ reversely acts on the piston  44 ′ via the shoe  52 ′ as a reaction force which is equal in magnitude and oppositely directed to the action force. That is, when the piston  44 ′ is in its suction stroke, the force F′ exerted from the swash plate  34 ′ on the piston  44 ′ acts on the piston  44 ′ at an angle perpendicular to surfaces of the swash plate  34 ′ at a contact position Q′ at which the semispherical outer surface of the shoe  52 ′ remote from the body  46 ′ of the piston  44 ′ comes into contact with the semi-spherical inner surface of the shoe pocket  54 ′, i.e., at an apex of the shoe pocket  54 ′ lying on the central axis CA′ of the piston  44 ′. The force F′ exerted from the swash plate  34 ′ on the piston  44 ′ is composed of two components, horizontal and vertical, the horizontal component F x ′ lying on the central axis CA′ of the piston  44 ′ and the vertical component F y ′ being perpendicular to the central axis CA′ of the piston  44 ′. Let “m” be the mass of the piston  44 ′, “a” be the acceleration of the piston  44 ′ during the suction stroke, “A” be the cross-sectional area of the piston  44 ′, “θ” be the angle from horizontal the force F′ is acting on the piston  44 ′, and “d” be the diameter of the piston  44 ′. 
     
       
         Σ F   x   ′=−ma   (4)  
       
     
     
       
         Σ F   x   ′=AP   c   ′−AP   s   ′−F   x ′  (5)  
       
     
     By combining the above equations, we can write, 
     
       
           F   x   =A ( P   c   ′−P   s ′)+ ma=ma+A ( P   c   −P   s ′)= ma +(π/4)* d   2 ( P   c   ′−P   s ′)  
       
     
     
       
         and  
       
     
     
       
           F   y   ′=F   x ′ tan θ=tan θ[ ma +(π/4)* d   2 ( P   c   ′−P   s ′)] 
       
     
     The vertical component F y ′ acts on the piston  44 ′ as a bending moment. Let the depth of the piston  44 ′ inserted into the cylinder bore  4 ′ when the piston  44 ′ reaches the maximum suction stroke position be W′, and the length L′ between the contact position, at which the outer surface of the shoe  52 ′ remote from the piston body  46 ′ comes into contact with the inner surface of the corresponding shoe pocket  54 ′, and the rightmost front end of the piston  44 ′. Then, the maximum bending moment M′ max  acts on the piston at a position P′ away by W′ from the front end of the piston  44 ′. We can write this equation as 
     
       
           M′   max =( L′−W ′) F   y ′=( L′−W ′)tan θ[ ma +(π/4)* d   2 ( P   c   ′−P   s ′)].  
       
     
     Since W′ is generally short in an air conditioning compressor, the bending moment acting on the piston during the suction stroke also causes deformation and abnormal abrasion of the piston. 
     FIG. 3 shows a compressor, for example, a variable capacity swash plate type compressor having a mechanism for minimizing a bending moment. As shown in FIG. 3, a variable capacity swash plate type compressor  70  has a cylinder block  72  provided with a plurality of cylinder bores  74 , a front housing  76  and a rear housing  78 . Both front and rear ends of the cylinder block  72  are sealingly closed by the front and rear housings  76  and  78 . A valve plate  80  is intervened between the cylinder block  72  and the rear housing  78 . The cylinder block  72  and the front housing  76  define an air-tight sealed crank chamber  82 . A drive shaft  84  is centrally arranged to extend through the front housing  76  to the cylinder block  72 , and rotatably supported by radial bearings  86  and  87 . The cylinder block  72  and the front and rear housings  76  and  78  are tightly combined by a long screw  89 . 
     A rotor  90  is fixedly mounted on the drive shaft  84  within the crank chamber  82  to be rotatable with the drive shaft  84 , and supported by a thrust bearing  92  seated on an inner end of the front housing  76 . A swash plate  94  is rotatably supported on the drive shaft  84 . If desired, a spherical sleeve (not illustrated) can be intervened between the drive shaft  84  and the swash plate  94 . In this case, the swash plate  94  is rotatably supported on an outer support surface of the rotor  90 . In FIG. 3, the swash plate  94  is in its largest inclination angle position, and at this time a spring  98  is most compressed and a stop surface  96 a of a projection  96  comes into contact with the rotor  90  so that a further increase of inclination angle of the swash plate  94  is restricted by the rotor  90 . On the other hand, a further decrease of inclination angle of the swash plate  94  is restricted by a stopper  97  provided with the drive shaft  84 . 
     The swash plate  94  is connected to the rotor  90  via a hinge mechanism to be rotated with the rotor  90 . That is, a support arm  100  protrudes axially outwardly from one side surface of the rotor  90 , and an arm  102  protrudes from one side surface of the swash plate  94  toward the support arm  100  of the rotor  90 . The support arm  100  and the arm  102  overlap each other and are connected to each other by a pin  104 . The pin  102  extends into a pin hole  106  formed through the support arm  100  of the rotor  90  and a rectangular shaped hole  108  formed through the arm  102  of the swash plate  94 . Support arm  100 , arm  102  and pin  104  constitute a supporting and adjusting means. With this arrangement, the rotor  90  and the swash plate  94  are hinged to each other, and the sliding motion of the pin  104  within the rectangular hole  108  changes an inclination angle of the swash plate  94  so as to change the capacity of the compressor. 
     As best illustrated in FIG. 4, each cylindrical piston  110  has a body  112  with a head and a bridge portion  122 . The bridge portion  122  has a recess  124 , and opposed walls defined in the recess  124  have spherical shoe pockets  126  into which spherical outer surfaces of two semi-spherical flat surfaces of the shoes  128  are slidably disposed. The inner flat surfaces of the shoes  128  are slidably engaged with side surfaces of the peripheral portion of the swash plate  94 . With this arrangement, each piston  110  is engaged with the swash plate  94  via the shoes  128  and pockets  126 , and therefore, the rotation of the swash plate  94  causes each piston  110  to reciprocate in the cylinder bore  74 . 
     During the compression stroke of the piston  110 , the force F exerted on the piston  110  from the swash plate  94  via the shoe  128  adjacent to the body  112  of the piston acts on the piston  110  at a right angle to a front surface of the swash plate  94  at a contact surface (in case of a line contact) or a contact point (in case of a point contact) (both will be referred as a contact position or an apex hereinafter) at which the semi-spherical outer surface of the shoe  128  adjacent to the body  112  comes into contact with the semi-spherical inner surface of the shoe pocket  126 . The force F exerted from the swash plate  94  on the piston  110  is composed of two components, the horizontal component F x  lying on the central axis CA of the piston  110  and the vertical component F y  perpendicular to the central axis CA of the piston  110 . The vertical component F y  acts on the piston  110  as a bending moment. 
     To minimize the bending moment, a cutout portion is not formed in the body  112  of the piston  110 . That is, in the construction of the piston in accordance with the present invention, the lower back edge P of the body  112  of the piston  110  lies on the line S which passes through the apex Q 2  of the shoe pocket  126  and is perpendicular to the central axis CA of the piston  110 . Moreover, the lower back edge P of the piston body  112  is able to be further extended up to an entrance Q 1  of the shoe pocket  126  near the piston body  112 . Therefore, the lower back edge portion is between the entrance Q 1  and apex Q 2  of the shoe pocket  126  near the piston body  112 . As a result, the piston body  112  is compensated by the distance X compared to the piston body of prior art, and thus, the maximum bending moment acting on the piston does not occur from the above equation (3). The lower back portion P extends in a line through apex Q 2  and entrance Q 1  and continues to extend in perpendicular relation proximate to a line B defining an inner surface of the cylinder bore  74 . 
     The interference between the swash plate  94  and the lower back edge portion of the piston body  112  due to compensation for the piston body  112  by the distance X can be solved by changing the shape of the swash plate  94 . For example, as shown in FIGS. 3 to  5 , the swash plate  94  has a depressed portion  130  formed in the side surface thereof confronting the piston body  112 . The depressed portion  130  is positioned axially inward of shoe  128 . The depressed portion  130  may be formed evenly in a central region of the swash plate  94  as shown in FIG. 3 and 4, or only in a region  130 ′ in which the contact interference occurs as shown in FIG.  5 . The depths of the depressed portions  130  and  130 ′ are determined in response to the projection size of a center region of the cylinder block  72  as described hereinafter. Instead of the depressed portion  130 , a thin swash plate or restriction on the smallest inclination angle of the swash plate can be employed to avoid the interference between the swash plate and the piston body. 
     It is advantageous to form a protuberant portion  132  opposed to the depressed portion  130 ′ in response to the formation of the depressed portion  130 ′ for reinforcing the swash plate as shown in FIG.  5 . 
     FIG. 6 shows a cylinder block for use in the compressor of the present invention. As shown in FIG. 6, the cylinder block  72  has an annular projecting portion  73  protruding from an entrance of each cylinder bore  74  as a reference surface B toward the depressed portion  130  of the swash plate  94 . The projecting portion  73  is formed in a central region of the cylinder bore  72  between a central hole  77  for the drive shaft  84  and the cylinder bores  74 . Instead of the annular shape of the projecting portion  73  formed around the cylinder bores  74  for reducing the mass of the compressor, the projecting portion  73  may be formed over the entire central region. 
     FIG. 7 shows another embodiment of the cylinder block in which a circumferential portion of the cylinder block  72  between the outer circumferential surface  88  and the cylinder bores  74  is extended from the cylinder block  72  in response to the projection of the inner projecting portion  73  so as to form an outer projecting portion  79 . With this arrangement, the pistons are stably slid in their cylinder bores during the suction and compression strokes thereof. 
     The projecting portion  73  protrudes by the depth of the depressed portion  130  from the central region. Therefore, the insertion depth W′ of the piston increases in response to the projection of the causes of the bending moment acting on the piston  110  during the suction stroke thereof to be reduced as seen from the equation (6). 
     The rear housing  78  is provided with inlet and outlet ports  134  and  136 , and divided into suction and discharge chambers  138  and  140 . The valve plate  80  has suction and discharge ports  142  and  144 . Each cylinder bore  74  is communicated with the suction chamber  138  and the discharge chamber  140  via the suction ports  142  and the discharge ports  144 . Each suction port  142  is opened and closed by a suction valve  146 , and each discharge port  144  is opened and closed by a discharge valve  148 , in response to the reciprocal movement of the respective pistons  110 . The opening motion of the discharge valve  148  is restricted by a retainer  150 . 
     A control valve means  152  is provided with the compressor  70  for adjusting a pressure level within the crank chamber  82  as shown in FIG.  3 . 
     In the compressor having the above-described construction, when the drive shaft  84  is rotated, the swash plate  94  having a certain inclination angle is also rotated via the hinge mechanism, and thus, the rotation of the swash plate  94  is converted into the reciprocation of the pistons  110  within the respective cylinder bores  74  via the shoes  128 . This reciprocating motion causes the refrigerant gas to be introduced from the suction chamber  138  of the rear housing  78  into the respective cylinder bores  74  in which the refrigerant gas is compressed by the reciprocating motion of the pistons  110 . The compressed refrigerant gas is discharged from the respective cylinder bores  74  into the discharge chamber  140 . 
     At this time, the capacity of the compressed refrigerant gas discharged from the cylinder bores  74  into the discharge chamber  140  is controlled by the control valve means  152  which adjustably changes the pressure level P cc  within the crank chamber  82 . Namely, when the pressure level P sc  in the suction chamber  138  is raised with increase of the thermal load of an evaporator, the control valve means  152  cuts off the refrigerant gas at pressure level P dc  traveling from the discharge chamber  140  into the crank chamber  82  so that the pressure level P cc  in the crank chamber  82  is lowered. When the pressure level P cc  in the crank chamber  82  is lowered, a back pressure acting on the respective pistons  110  is decreased, and therefore, the angle of inclination of the swash plate  94  is increased. Namely, the pin  104  of the hinge means is moved slidably and downwardly within the rectangular hole  108 . Accordingly, the swash plate  94  is moved in a forward direction against the force of the spring  98 . Therefore, the angle of inclination of the swash plate  94  is increased, and as a result, the stroke of the respective pistons  110  is increased. 
     On the contrary, when the pressure level P sc  in the suction chamber  138  is lowered with decrease of the thermal load of the evaporator, the control valve means  152  passes the compressed refrigerant gas at pressure level P dc  of the discharge chamber  140  into the crank chamber  82 . When the pressure level P cc  in the crank chamber  82  is raised, a back pressure acting on the respective piston  110  is increased, and therefore, the angle of inclination of the swash plate  94  is decreased. Namely, the pin  104  of the hinge means is moved slidably and upwardly within the rectangular hole  108 . Accordingly, the swash plate  94  is moved in a rearward direction yielding to the force of the spring  98 . Therefore, the inclination angle of the swash plate  94  is decreased, and as a result, the stroke of the respective pistons  110  is shortened and the discharge capacity is decreased. 
     In the above described compressor, during the compression stroke of the piston  110 , the pressure P cc  in the crank chamber  82  and the compression reaction force act on the piston  110 . These forces act on the swash plate  94  via the shoes  122  and, in turn, reversely act on the piston  110  from the swash plate  94  as a reaction force equal in magnitude and oppositely directed. At this time, the maximum bending moment acts on the lower back edge portion P of the piston  110 . However, the lower back edge portion P lies on the same line as the vertical component F y  lies, and thus, the bending moment does not occurred on the lower back edge portion P of the piston  110  because the distance x is zero. As a result, deformation and abnormal abrasion of the pistons can be prevented. 
     On the other hand, during the suction stroke of the piston  110 ′, the pressure in the crank chamber  82 ′ acts on the piston, and this force acts on the swash plate  94 ′ via the shoe  128 ′ remote from the piston body  112 ′ which, in turn, act on the piston from the swash plate  94 ′ as a reaction force. At this time, the maximum bending moment acts on the piston at a contact surface between the outer surface of the piston  110 ′ and the inner surface of the cylinder bore  74 ′ when the piston  110 ′ is inserted into the corresponding cylinder bore  74 ′ by a certain depth. The central region of the cylinder block  72 ′ is projected in response to the depth of the depressed portion  130 ′ of the swash plate  94 ′. Thus, the insertion depth W′ of the piston  110 ′ into the cylinder bore  74 ′ at the maximum suction stroke is increased so as to reduce the maximum bending moment acting on the piston  110 ′. 
     Although the present invention has been described in connection with the preferred embodiments, the invention is not limited thereto. It will be easily understood by those skilled in the art that variations and modifications can be easily made within the scope of the present invention as defined by the claims.