Patent Publication Number: US-2018038359-A1

Title: Variable-displacement swash plate-type compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a variable displacement swash plate type compressor. 
     BACKGROUND ART 
     For example, Patent Document 1 discloses a fixed displacement swash plate type compressor. The swash plate type compressor includes a first cylinder block, a second cylinder block, a front housing member, and a rear housing member. The first and second cylinder blocks are coupled to each other. The front housing member is coupled to the first cylinder block, and the rear housing member is coupled to the second cylinder block. The housing accommodates a rotary shaft, which is rotationally supported by the housing. One end of the rotary shaft is rotationally supported by the first cylinder block. The other end of the rotary shaft is rotationally supported by the second cylinder block. 
     In the housing, the first cylinder block and the second cylinder block define a swash plate chamber. The swash plate chamber accommodates a swash plate, which rotates when receiving drive force from the rotary shaft. The swash plate is inclined by a fixed inclination angle relative to the direction perpendicular to the axis of the rotary shaft. 
     The first cylinder block has first cylinder bores located about the rotary shaft. Also, the second cylinder block has second cylinder bores located about the rotary shaft. The first cylinder bores and the second cylinder bores extend along the axis of the rotary shaft and are arranged to form pairs. Each pair of the first cylinder bore and the second cylinder bore reciprocally accommodates a double-headed piston. Each double-headed piston is engaged with the peripheral portion of the swash plate with a pair of shoes. When the swash plate rotates together with the rotary shaft, the rotation of the swash plate is converted into linear reciprocation of the double-headed pistons by the shoes. 
     Thrust bearings are each arranged between the rotary shaft and the first cylinder block and between the rotary shaft and the second cylinder block. The thrust bearings are tightly held between the rotary shaft and the first cylinder block and between the rotary shaft and the second cylinder block by fastening force of the housing bolts, which fasten the first cylinder block, the second cylinder block, the front housing member, and the rear housing member together. Accordingly, the rotary shaft is tightly held by the thrust bearings in the axial direction of the rotary shaft, so that the position of the rotary shaft is determined in the axial direction. 
     The swash plate receives compression reaction force due to reciprocation of the double-headed pistons. Accordingly, the swash plate applies thrust to the rotary shaft. At this time, since the position of the rotary shaft is determined in the axial direction and the thrust bearings bear the thrust acting on the rotary shaft, the rotary shaft is restrained from chattering by the applied thrust. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 7-197883 
     SUMMARY OF THE INVENTION 
     Problems that the Invention is to Solve 
     Swash plate type compressors of the above described type include variable displacement compressors, which vary the displacement. This type of compressor is configured to change the inclination angle of the swash plate, thereby causing the double-headed pistons to reciprocate by a stroke corresponding to the swash plate inclination angle. This compressor has, in the swash plate chamber, an actuator for changing the inclination angle of the swash plate. The actuator has a partition body arranged on the rotary shaft, a movable body, which moves in the swash plate chamber along the axis of the rotary shaft, and a control pressure chamber, which is defined by the partition body and the movable body. The movable body is moved along the axis of the rotary shaft by changing the pressure in the control pressure chamber. Also, as the movable body moves along the axis of the rotary shaft, the inclination angle of the swash plate is changed. 
     In this compressor, the compression reaction force applied to the swash plate by the double-headed pistons is increased as the displacement is increased. Accordingly, the thrust transmitted to the rotary shaft from the swash plate is increased. The fastening force in the axial direction generated by the housing bolts needs to be set at a large value so that the thrust transmitted to the rotary shaft can be borne by the thrust bearings. 
     However, the compression reaction force applied to the swash plate from the double-headed pistons is decreased as the displacement is decreased. Accordingly, the thrust transmitted to the rotary shaft from the swash plate is decreased. At this time, if the fastening force in the axial direction generated by the housing bolts is set to be strong, the sliding resistance between the thrust bearings and the rotary shaft is increased. This increases the power loss. 
     Accordingly, it is an objective of the present invention to provide a variable displacement swash plate type compressor that restrains chattering of the rotary shaft caused by thrust acting on the rotary shaft, while reducing power loss. 
     Means for Solving the Problems 
     To achieve the foregoing objective and in accordance with a first aspect of the present invention, a variable displacement swash plate type compressor is provided that includes a housing, a rotary shaft, a thrust bearing, a swash plate chamber, a swash plate, a piston, and an actuator. The housing has a cylinder block, in which a discharge chamber and a plurality of cylinder bores are provided. The rotary shaft is rotationally supported by the housing. The thrust bearing is arranged between the cylinder block, which is arranged along an axis of the rotary shaft, and the rotary shaft. The thrust bearing bears a thrust that acts in an axial direction of the rotary shaft. The swash plate chamber is provided in the housing and draws in refrigerant from outside. The swash plate is accommodated in the swash plate chamber. The swash plate is rotated by receiving a drive force from the rotary shaft and is tiltable relative to a direction perpendicular to the axis of the rotary shaft. The piston is reciprocally received in the cylinder bores. The actuator is arranged in the swash plate chamber and configured to change an inclination angle of the swash plate. The actuator includes a partition body provided on the rotary shaft, a movable body, which is provided in the swash plate chamber and movable along the axis of the rotary shaft, and a control pressure chamber, which is defined by the partition body and the movable body. The movable body is moved by a pressure in the control pressure chamber. As the movable body moves along the axis of the rotary shaft, the inclination angle of the swash plate is changed so that the piston reciprocates by a stroke in accordance with the inclination angle of the swash plate. The rotary shaft receives a load acting toward the thrust bearing. The load is based on a pressure difference between the discharge chamber and the swash plate chamber. 
     With this configuration, when the displacement increases so that the pressure in the discharge chamber increases, the pressure difference between the discharge chamber and the swash plate chamber increases. This increases the load that is applied to the rotary shaft and acts toward the thrust bearing. This presses the rotary shaft against the thrust bearing, thereby fixing the position in the axial direction of the rotary shaft. Thus, even if an increase in the displacement increases the compression reaction force applied to the swash plate from the piston so that the thrust applied to the rotary shaft from the swash plate is increased, the rotary shaft is restrained from chattering due to the applied thrust since the position of the rotary shaft is fixed in the axial direction. In contrast, the compression reaction force applied to the swash plate from the piston is decreased when the displacement is decreased. Accordingly, the thrust transmitted to the rotary shaft from the swash plate is decreased. At this time, since the pressure in the discharge chamber is lowered due to the decrease in the displacement, the pressure difference between the discharge chamber and the swash plate chamber decreases. This reduces the load that is applied to the rotary shaft and acts toward the thrust bearing. Therefore, the sliding resistance between the thrust bearing and the rotary shaft is reduced, which reduces the power loss. From the above, it is possible to restrain chattering of the rotary shaft caused by the thrust acting on the rotary shaft, while reducing the power loss. 
     In the above described variable displacement swash plate type compressor, a spacer is preferably arranged between the cylinder block, which is arranged along the axis of the rotary shaft, and the rotary shaft. The spacer is supported by the rotary shaft while being restricted from rotating and allowed to move along the axis of the rotary shaft. The cylinder block and the spacer preferably define a pressure-acting chamber, which communicates with the discharge chamber. A sealing member is preferably arranged between the spacer and the cylinder block. The sealing member seals off the pressure-acting chamber and the swash plate chamber from each other. 
     With this configuration, since the spacer is restricted from rotating with respect to the rotary shaft, the durability of the sealing member is improved as compared with a case where the spacer rotates integrally with the rotary shaft. Accordingly, the sealing performance between the pressure-acting chamber and the swash plate chamber is improved. 
     In the above described variable displacement swash plate type compressor, a spacer is preferably provided on the rotary shaft to be integrally rotational with the rotary shaft, and the cylinder block and the spacer preferably define a pressure-acting chamber, which communicates with the discharge chamber. A sealing member is preferably arranged between the spacer and the cylinder block. The sealing member seals off the pressure-acting chamber and the swash plate chamber from each other. 
     With this configuration, since the spacer is allowed to rotate integrally with the rotary shaft, there is no need to provide a thrust bearing between the spacer and the rotary shaft, so that the number of components is reduced. This reduces the weight of the variable displacement swash plate type compressor. 
     In the above described variable displacement swash plate type compressor, the spacer preferably has a contact portion, which contacts the cylinder block and is located in a vicinity of the cylinder block that is located in the axial direction of the rotary shaft. 
     With this configuration, when the housing is assembled, the fastening force acting on the housing in the axial direction of the rotary shaft generates a load that acts toward the thrust bearing from the cylinder block via the contact portion. As a result, the rotary shaft is pressed against the thrust bearing, so that the position of the rotary shaft is determined in the axial direction. Therefore, for example, even when the operation of the variable displacement swash plate type compressor is stopped and the rotary shaft is not receiving the load based on the pressure difference between the discharge chamber and the swash plate chamber, the positioning of the rotary shaft in the axial direction is ensured. Therefore, for example, even if the vehicle in which the variable displacement swash plate type compressor is installed vibrates and causes the compressor to vibrate, the rotary shaft is restrained from chattering in the axial direction. 
     In the above described variable displacement swash plate type compressor, the housing preferably includes a pair of cylinder blocks, and the pair of cylinder blocks preferably each have a cylinder bore. The cylinder bores form a pair. The pair of the cylinder bores reciprocally accommodates a double-headed piston, which is the piston. The double-headed piston defines a first compression chamber in one of the pair of the cylinder bores and a second compression chamber in the other one of the pair of the cylinder bores. A link mechanism is arranged between the rotary shaft and the swash plate. The link mechanism allows change of the inclination angle of the swash plate with respect to a direction that is perpendicular to the axis of the rotary shaft. The link mechanism is arranged such that, as the inclination angle of the swash plate is changed, a top dead center position of the double-headed piston in the second compression chamber is displaced by a greater amount than a top dead center position of the double-headed piston in the first compression chamber. A direction of a compression reaction force acting on the swash plate from the double-headed piston in the first compression chamber is the same as a direction of the load applied to the rotary shaft based on the pressure difference between the discharge chamber and the swash plate chamber. 
     When the dead volume of the second compression chamber is increased to a predetermined value due to reduction in the inclination angle of the swash plate, the double-headed piston no longer performs the discharge stroke in the second compression chamber. Then, the compression reaction force applied to the swash plate from the part of the double-headed piston in the first compression chamber exceeds the compression reaction force applied to the swash plate from the part of the double-headed piston in the second compression chamber. At this time, the direction of the compression reaction force acting on the swash plate from the part of the double-headed piston in the first compression chamber is the same as the direction of the load applied to the rotary shaft based on the pressure difference between the discharge chamber and the swash plate chamber. This permits reduction in the load required to press the rotary shaft against the thrust bearing, that is, reduction in the load applied to the rotary shaft based on the pressure difference between the discharge chamber and the swash plate chamber. This efficiently reduces chattering of the rotary shaft caused by the thrust acting on the rotary shaft. 
     In the above described variable displacement swash plate type compressor, an outer diameter of a head of the double-headed piston accommodated in one of the pair of the cylinder bores is preferably larger than an outer diameter of a head of the double-headed piston accommodated in the other cylinder bore of the pair. 
     With this configuration, the compression reaction force applied to the swash plate from the part of the double-headed piston in the first compression chamber is greater than in the case in which the outer diameter of a head of the double-headed piston accommodated in one of the pair of the cylinder bores is the same as or smaller than the outer diameter of the other head of the piston accommodated in the other cylinder bore. This further reduces the load required to press the rotary shaft against the thrust bearing, that is, the load applied to the rotary shaft based on the pressure difference between the discharge chamber and the swash plate chamber. Thus, the chattering of the rotary shaft caused by the thrust acting on the rotary shaft is more efficiently reduced. 
     To achieve the foregoing objective and in accordance with a second aspect of the present invention, a variable displacement swash plate type compressor is provided that includes a housing, a rotary shaft, a thrust bearing, a swash plate chamber, a swash plate, a piston, and an actuator. The housing has a cylinder block, in which a discharge chamber and a plurality of cylinder bores are provided. The rotary shaft is rotationally supported by the housing. The thrust bearing is arranged between the cylinder block, which is arranged along an axis of the rotary shaft, and the rotary shaft. The thrust bearing bears a thrust that acts in an axial direction of the rotary shaft. The swash plate chamber is provided in the housing and draws in refrigerant from outside. The swash plate is accommodated in the swash plate chamber. The swash plate is rotated by receiving a drive force from the rotary shaft and is tiltable relative to a direction perpendicular to the axis of the rotary shaft. The piston is reciprocally received in the cylinder bores. The actuator is arranged in the swash plate chamber and configured to change an inclination angle of the swash plate. The actuator includes a partition body provided on the rotary shaft, a movable body, which is provided in the swash plate chamber and movable along the axis of the rotary shaft, and a control pressure chamber, which is defined by the partition body and the movable body. The movable body is moved by a pressure in the control pressure chamber. As the movable body moves along the axis of the rotary shaft, the inclination angle of the swash plate is changed such that the inclination angle of the swash plate increases when the pressure in the control pressure chamber is increased, and that the inclination angle of the swash plate decreases when the pressure in the control pressure chamber is lowered, thereby causing the piston to reciprocate by a stroke corresponding to the inclination angle of the swash plate. The rotary shaft receives a load acting toward the thrust bearing, the load being based on a pressure difference between the control pressure chamber and the swash plate chamber. 
     With this configuration, when the displacement increases so that the pressure in the control pressure chamber increases, the pressure difference between the control pressure chamber and the swash plate chamber increases. Accordingly, the load that is applied to the rotary shaft and acts toward the thrust bearing increases. This presses the rotary shaft against the thrust bearing, thereby fixing the position in the axial direction of the rotary shaft. Thus, even if an increase in the displacement increases the compression reaction force applied to the swash plate from the piston so that the thrust applied to the rotary shaft from the swash plate is increased, the rotary shaft is restrained from chattering due to the applied thrust since the position of the rotary shaft is fixed in the axial direction. In contrast, the compression reaction force applied to the swash plate from the piston is decreased when the displacement is decreased. Accordingly, the thrust transmitted to the rotary shaft from the swash plate is decreased. At this time, since the pressure in the control pressure chamber is lowered due to the decrease in the displacement, the pressure difference between the control pressure chamber and the swash plate chamber decreases. This reduces the load that is applied to the rotary shaft and acts toward the thrust bearing. Therefore, the sliding resistance between the thrust bearing and the rotary shaft is reduced, which reduces the power loss. From the above, it is possible to restrain chattering of the rotary shaft caused by the thrust acting on the rotary shaft, while reducing the power loss. 
     In the above described variable displacement swash plate type compressor, a spacer is preferably arranged between the cylinder block, which is arranged along the axis of the rotary shaft, and the rotary shaft. The spacer is supported by the rotary shaft while being restricted from rotating and allowed to move along the axis of the rotary shaft. The cylinder block and the spacer preferably define a pressure-acting chamber, which communicates with the control pressure chamber. A sealing member is preferably arranged between the spacer and the cylinder block. The sealing member seals off the pressure-acting chamber and the swash plate chamber from each other. 
     With this configuration, since the spacer is restricted from rotating with respect to the rotary shaft, the durability of the sealing member is improved as compared with a case where the spacer rotates integrally with the rotary shaft. Accordingly, the sealing performance between the pressure-acting chamber and the swash plate chamber is improved. 
     In the above described variable displacement swash plate type compressor, a spacer is preferably provided on the rotary shaft to be integrally rotational with the rotary shaft, and the cylinder block and the spacer preferably define a pressure-acting chamber, which communicates with the control pressure chamber. A sealing member is preferably arranged between the spacer and the cylinder block. The sealing member seals off the pressure-acting chamber and the swash plate chamber from each other. 
     With this configuration, since the spacer is allowed to rotate integrally with the rotary shaft, there is no need to provide a thrust bearing between the spacer and the rotary shaft, so that the number of components is reduced. This reduces the weight of the variable displacement swash plate type compressor. 
     In the above described variable displacement swash plate type compressor, the spacer preferably has a contact portion, which contacts the cylinder block and is located in a vicinity of the cylinder block that is located in the axial direction of the rotary shaft. 
     With this configuration, when the housing is assembled, the fastening force acting on the housing in the axial direction of the rotary shaft generates a load that acts toward the thrust bearing from the cylinder block via the contact portion. As a result, the rotary shaft is pressed against the thrust bearing, so that the position of the rotary shaft is determined in the axial direction. Therefore, for example, even when the operation of the variable displacement swash plate type compressor is stopped and the rotary shaft is not receiving the load based on the pressure difference between the control pressure chamber and the swash plate chamber, the positioning of the rotary shaft in the axial direction is ensured. Therefore, for example, even if the vehicle in which the variable displacement swash plate type compressor is installed vibrates and causes the compressor to vibrate, the rotary shaft is restrained from chattering in the axial direction. 
     In the above described variable displacement swash plate type compressor, the housing preferably includes a pair of cylinder blocks, and the pair of cylinder blocks preferably each have a cylinder bore. The cylinder bores form a pair. The pair of the cylinder bores reciprocally accommodates a double-headed piston, which is the piston. The double-headed piston defines a first compression chamber in one of the pair of the cylinder bores and a second compression chamber in the other one of the pair of the cylinder bores. A link mechanism is arranged between the rotary shaft and the swash plate. The link mechanism allows change of the inclination angle of the swash plate with respect to a direction that is perpendicular to the axis of the rotary shaft. The link mechanism is arranged such that, as the inclination angle of the swash plate is chanced, a top dead center position of the double-headed piston in the second compression chamber is displaced by a greater amount than a top dead center position of the double-headed piston in the first compression chamber. A direction of a compression reaction force acting on the swash plate from the double-headed piston in the first compression chamber is the same as a direction of the load applied to the rotary shaft based on the pressure difference between the control pressure chamber and the swash plate chamber. 
     When the dead volume of the second compression chamber is increased to a predetermined value due to reduction in the inclination angle of the swash plate, the double-headed piston no longer performs the discharge stroke in the second compression chamber. Then, the compression reaction force applied to the swash plate from the part of the double-headed piston in the first compression chamber exceeds the compression reaction force applied to the swash plate from the part of the double-headed piston in the second compression chamber. At this time, the direction of the compression reaction force acting on the swash plate from the part of the double-headed piston in the first compression chamber is the same as the direction of the load applied to the rotary shaft based on the pressure difference between the control pressure chamber and the swash plate chamber. This permits reduction in the load required to press the rotary shaft against the thrust bearing, that is, reduction in the load applied to the rotary shaft based on the pressure difference between the control pressure chamber and the swash plate chamber. This efficiently reduces chattering of the rotary shaft caused by the thrust acting on the rotary shaft. 
     In the above described variable displacement swash plate type compressor, an outer diameter of a head of the double-headed piston accommodated in one of the pair of the cylinder bores is preferably larger than an outer diameter of a head of the double-headed piston accommodated in the other cylinder bore of the pair. 
     With this configuration, the compression reaction force applied to the swash plate from the part of the double-headed piston in the first compression chamber is greater than in the case in which the outer diameter of a head of the double-headed piston accommodated in one of the pair of the cylinder bores is the same as or smaller than the outer diameter of the other head of the piston accommodated in the other cylinder bore. This further reduces the load required to press the rotary shaft against the thrust bearing, that is, the load applied to the rotary shaft based on the pressure difference between the control pressure chamber and the swash plate chamber. Thus, the chattering of the rotary shaft caused by the thrust acting on the rotary shaft is more efficiently reduced. 
     Effects of the Invention 
     The present invention restrains chattering of the rotary shaft caused by the thrust acting on the rotary shaft, while reducing the power loss. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view illustrating a variable displacement swash plate type compressor according to one embodiment. 
         FIG. 2  is an enlarged partial cross-sectional view of the variable displacement swash plate type compressor, illustrating the spacer and the surrounding structure. 
         FIG. 3  is a diagram showing the relationship among the control pressure chamber, the pressure adjusting chamber, the suction chamber, and the discharge chamber. 
         FIG. 4  is a cross-sectional side view of the variable displacement swash plate type compressor when the swash plate is at the minimum inclination angle. 
         FIG. 5  is a partial cross-sectional view illustrating a variable displacement swash plate type compressor according to another embodiment. 
         FIG. 6  is a partial cross-sectional view illustrating a variable displacement swash plate type compressor according to another embodiment. 
         FIG. 7  is a cross-sectional side view illustrating a variable displacement swash plate type compressor according to another embodiment. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     A variable displacement swash plate type compressor  10  according to one embodiment of the present invention will now be described with reference to  FIGS. 1 to 4 . In the following description, the variable displacement swash plate type compressor  10  will simply be referred to as a compressor  10 . The compressor  10  is used in a vehicle air conditioner. The left side and the right side in  FIG. 1  are defined as the front side and the rear side, respectively. 
     As shown in  FIG. 1 , the compressor  10  includes a housing  11 , which has a pair of cylinder blocks, or a first cylinder block  12  and a second cylinder block  13 , which are coupled to each other. The housing  11  further includes a front housing member  14  coupled to the first cylinder block  12  and a rear housing member  15  coupled to the second cylinder block  13 . A first valve-port assembly plate  16  is arranged between the front housing member  14  and the first cylinder block  12 . Further, a second valve-port assembly plate  17  is arranged between the rear housing member  15  and the second cylinder block  13 . 
     A suction chamber  14   a  and a discharge chamber  14   b  are defined between the front housing member  14  and the first valve-port assembly plate  16 . The discharge chamber  14   b  is located radially outward of the suction chamber  14   a . A suction chamber  15   a  and a discharge chamber  15   b  are defined between the rear housing member  15  and the second valve-port assembly plate  17 . A pressure adjusting chamber  15   c  is arranged in the rear housing member  15 . The pressure adjusting chamber  15   c  is arranged at the center of the rear housing member  15 . The suction chamber  15   a  is located radially outward of the pressure adjusting chamber  15   c . The discharge chamber  15   b  is located radially outward of the suction chamber  15   a . The discharge chambers  14   b ,  15   b  are connected to each other through a discharge passage  18 . The discharge passage  18  is connected to an external refrigerant circuit (not shown). The discharge chambers  14   b ,  15   b  are discharge pressure zones. 
     The first valve-port assembly plate  16  has suction ports  16   a , which communicate with the suction chamber  14   a , and discharge ports  16   b , which communicate with the discharge chamber  14   b . The second valve-port assembly plate  17  has suction ports  17   a , which communicate with the suction chamber  15   a , and discharge ports  17   b , which communicate with the discharge chamber  15   b.    
     A rotary shaft  20 , which has an axis L, is rotationally supported in the housing  11 . A cylindrical first supporting member  21  is press fitted to the outer circumferential surface of the front end portion of the rotary shaft  20 . A cylindrical second supporting member  22  is press fitted to the outer circumferential surface of the rear end portion of the rotary shaft  20 . The first and second supporting members  21 ,  22  constitute parts of the rotary shaft  20 . The first supporting member  21 , which constitutes the front end portion of the rotary shaft  20 , extends through a shaft hole  12   h  in the first cylinder block  12 . The second supporting member  22 , which constitutes the rear end portion of the rotary shaft  20 , extends through a shaft hole  13   h  in the second cylinder block  13 . The rear end portion of the second supporting member  22 , that is, the rear end portion of the rotary shaft  20 , is arranged in the pressure adjusting chamber  15   c.    
     A first plain bearing  21   a  is arranged between the first supporting member  21  and the shaft hole  12   h . A second plain bearing  22   a  is arranged between the second supporting member  22  and the shaft hole  13   h . The first supporting member  21  is rotationally supported by the first cylinder block  12  via the first plain bearing  21   a . The second supporting member  22  is rotationally supported by the second cylinder block  13  via the second plain bearing  22   a.    
     A sealing device  20   s  of a lip seal type is located between the front housing member  14  and the rotary shaft  20 . The front end of the rotary shaft  20  is coupled to an external drive source, which is a vehicle engine in this embodiment, through a power transmission mechanism (not shown). In the present embodiment, the power transmission mechanism is a clutchless mechanism formed by a combination of a belt and pulleys and constantly transmits power. 
     In the housing  11 , the first cylinder block  12  and the second cylinder block  13  define a swash plate chamber  24 . The swash plate chamber  24  accommodates a swash plate  23 , which rotates when receiving drive force from the rotary shaft  20  and is tiltable along the axis of the rotary shaft  20 . The swash plate  23  has a through-hole  23   a , through which the rotary shaft  20  extends. The swash plate  23  is assembled to the rotary shaft  20  by inserting the rotary shaft  20  into the through-hole  23   a.    
     The first cylinder block  12  has first cylinder bores  12   a , which extend through the first cylinder block  12  along the axis and are arranged about the rotary shaft  20 .  FIG. 1  shows only one of the first cylinder bores  12   a . Each first cylinder bore  12   a  is connected to the suction chamber  14   a  via the corresponding suction port  16   a  and is connected to the discharge chamber  14   b  via the corresponding discharge port  16   b . The second cylinder block  13  has second cylinder bores  13   a , which extend through the second cylinder block  13  along the axis and are arranged about the rotary shaft  20 .  FIG. 1  shows only one of the second cylinder bores  13   a . Each second cylinder bore  13   a  is connected to the suction chamber  15   a  via the corresponding suction port  17   a  and is connected to the discharge chamber  15   b  via the corresponding discharge port  17   b.    
     The inner diameter of the first cylinder bore  12   a  is larger than that of the second cylinder bore  13   a . The first cylinder bores  12   a  and the second cylinder bores  13   a  are arranged to make front-rear pairs. Each pair of the first cylinder bore  12   a  and the second cylinder bore  13   a  accommodates a double-headed piston  25 , while permitting the piston  25  to reciprocate in the front-rear direction. Specifically, each first cylinder bore  12   a  receives a first head  25   a  of the corresponding double-headed piston  25 , and each second cylinder bore  13   a  receives a second head  25   b  of the corresponding double-headed piston  25 . The outer diameter R 1  of the first head  25   a  is larger than the outer diameter R 2  of the second head  25   b . The compressor  10  of the present embodiment is a double-headed piston swash plate type compressor. 
     Each double-headed piston  25  is engaged with the peripheral portion of the swash plate  23  with two shoes  26 . When the swash plate  23  rotates together with the rotary shaft  20 , the rotation of the swash plate  23  is converted into linear reciprocation of the double-headed pistons  25  by the shoes  26 . Thus, the pairs of the shoes  26  function as a conversion mechanism that reciprocates the double-headed pistons  25  in the pairs of the first cylinder bores  12   a  and the second cylinder bores  13   a  as the swash plate  23  rotates. In each first cylinder bore  12   a , a first compression chamber  19   a  is defined by the double-headed piston  25  and the first valve-port assembly plate  16 . In each second cylinder bore  13   a , a second compression chamber  19   b  is defined by the double-headed piston  25  and the second valve-port assembly plate  17 . 
     The first cylinder block  12  has a first small diameter hole  121   b , which is continuous with the shaft hole  12   h  and has a larger diameter than the shaft hole  12   h . Further, the first cylinder block  12  has a first large diameter hole  122   b , which is continuous with the first small diameter hole  121   b  and has a larger diameter than the first small diameter hole  121   b . The first large diameter hole  122   b  communicates with the swash plate chamber  24  and constitutes a part of the swash plate chamber  24 . The swash plate chamber  24  and the suction chamber  14   a  are connected to each other by a suction passage  12   c , which extends through the first cylinder block  12  and the first valve-port assembly plate  16 . 
     The second cylinder block  13  has a second small diameter hole  131   b , which is continuous with the shaft hole  13   h  and has a larger diameter than the shaft hole  13   h . Further, the second cylinder block  13  has a second large diameter hole  132   b , which is continuous with the second small diameter hole  131   b  and has a larger diameter than the second small diameter hole  131   b . The second large diameter hole  132   b  communicates with the swash plate chamber  24  and constitutes a part of the swash plate chamber  24 . The swash plate chamber  24  and the suction chamber  15   a  are connected to each other by a suction passage  13   c , which extends through the second cylinder block  13  and the second valve-port assembly plate  17 . 
     An inlet  13   s  is provided in the peripheral wall of the second cylinder block  13 . The inlet  13   s  is connected to the external refrigerant circuit. After being drawn into the swash plate chamber  24  from the external refrigerant circuit via the inlet  13   s , refrigerant gas is drawn into the suction chambers  14   a ,  15   a  via the suction passages  12   c ,  13   c . The suction chambers  14   a ,  15   a  and the swash plate chamber  24  are therefore suction pressure zones, and the pressures in the suction chambers  14   a ,  15   a  and the swash plate chamber  24  are substantially equal to each other. 
     An annular first flange  21   f  protrudes from the outer circumferential surface of the first supporting member  21 . The first flange  21   f  is arranged in the first large diameter hole  122   b . A first thrust bearing  27   a  and a spacer  50  are arranged between the first flange  21   f  and the first cylinder block  12 . The first thrust bearing  27   a  and the spacer  50  are arranged such that the axes agree with the axis of the rotary shaft  20 . The first thrust bearing  27   a  is closer to the first flange  21   f  than the spacer  50 . An annular second flange  22   f  protrudes from the outer circumferential surface of the second supporting member  22 . The second flange  22   f  is arranged in the second large diameter hole  132   b . A second thrust bearing  27   b  is arranged between the second flange  22   f  and the second cylinder block  13 . The second thrust bearing  27   b  is arranged such that the axis agrees with the axis of the rotary shaft  20 . The second thrust bearing  27   b  is fitted in the second small diameter hole  131   b . The first thrust bearing  27   a  and the second thrust bearing  27   b  bear the thrust that acts on the rotary shaft  20  in the axial direction. 
     As shown in  FIG. 2 , the spacer  50  has an annular shape and is supported by the rotary shaft  20  while being restricted from rotating. The spacer  50  is fitted in the first small diameter hole  121   b  to be movable in the axial direction of the rotary shaft  20 . An annular contact portion  51 , which contacts the first cylinder block  12 , protrudes from the spacer  50 . The spacer  50  has two end faces arranged in the axial direction of the rotary shaft  20 , and the contact portion  51  is provided on one of the end faces, or an end face  50   a  that is closer to the first cylinder block  12 . The contact portion  51  is located in the vicinity of the inner edge of the spacer  50 . 
     The spacer  50  is arranged in the first small diameter hole  121   b  with the contact portion  51  contacting the first cylinder block  12  and the end face  50   a  of the spacer  50  separated from the first cylinder block  12 . An annular sealing member  52   a  is arranged in the end face  50   a  of the spacer  50  at a position radially outward of the contact portion  51 . The sealing member  52   a  seals the gap between the end face  50   a  and the first cylinder block  12 . A sealing member  52   b  is arranged on the outer circumferential surface of the spacer  50 . The sealing member  52   b  seals the gap between the outer circumferential surface of the spacer  50  and the inner circumferential surface of the first small diameter hole  121   b . Further, a sealing member  52   c  is arranged on the inner circumferential surface of the spacer  50 . The sealing member  52   c  seals the gap between the inner circumferential surface of the spacer  50  and the outer circumferential surface of the first supporting member  21 . 
     The first cylinder block  12  and the spacer  50  define a pressure-acting chamber  55 . Specifically, the pressure-acting chamber  55  is a space defined by the first cylinder block  12 , the spacer  50 , and the sealing members  52   a ,  52   b . The pressure-acting chamber  55  is connected to the discharge chamber  14   b  via a supply passage  55   a . Thus, refrigerant gas is supplied to the pressure-acting chamber  55  from the discharge chamber  14   b  via the supply passage  55   a . The sealing members  52   a ,  52   b ,  52   c  seal off the pressure-acting chamber  55  and the swash plate chamber  24  from each other. The sealing members  52   a ,  52   b ,  52   c  thus prevent the refrigerant gas supplied to the pressure-acting chamber  55  from leaking to the swash plate chamber  24 . 
     As shown in  FIG. 1 , the swash plate chamber  24  accommodates an actuator  30 , which is configured to change the inclination angle of the swash plate  23  with respect to a first direction, which is perpendicular to the axis L of the rotary shaft  20 , that is, with respect to the vertical direction as viewed in  FIG. 1 . The actuator  30  is arranged between the second flange  22   f  and the swash plate  23 . The actuator  30  includes an annular partition body  31 , which is integrally rotational with the rotary shaft  20 . The partition body  31  has a through-hole  31   h , through which the rotary shaft  20  extends. The partition body  31  is integrated with the rotary shaft  20  by press fitting the rotary shaft  20  in the through-hole  31   h.    
     The actuator  30  also has a cylindrical movable body  32 , which has a closed end and is located between the second flange  22   f  and the partition body  31 . The movable body  32  is movable along the axis of the rotary shaft  20  in the swash plate chamber  24 . The movable body  32  is arranged to enter the second large diameter hole  132   b . The movable body  32  includes an annular bottom portion  32   a  and a cylindrical portion  32   b . The bottom portion  32   a  has a through-hole  32   e , through which the rotary shaft  20  extends. The cylindrical portion  32   b  extends along the axis L of the rotary shaft  20  from the outer periphery of the bottom portion  32   a . The movable body  32  is integrally rotational with the rotary shaft  20 . The gap between the inner circumferential surface of the cylindrical portion  32   b  and the outer circumferential surface of the partition body  31  is sealed by a sealing member  33 . The gap between the through-hole  32   e  and the rotary shaft  20  is sealed by a sealing member  34 . The actuator  30  has a control pressure chamber  35  defined by the partition body  31  and the movable body  32 . 
     A restoration spring  28   a  is fixed to the first supporting member  21 . The restoration spring  28   a  extends from the first supporting member  21  toward the swash plate chamber  24 . Also, a tilt reduction spring  28   b  is provided between the partition body  31  and the swash plate  23 . The rear end of the tilt reduction spring  28   b  is fixed to the partition body  31 . The front end of the tilt reduction spring  28   b  is fixed to the swash plate  23 . The tilt reduction spring  28   b  urges the swash plate  23  in a direction for reducing the inclination angle of the swash plate  23 . 
     The rotary shaft  20  has an in-shaft passage  29 , which connects the control pressure chamber  35  and the pressure adjusting chamber  15   c  to each other. The in-shaft passage  29  is constituted by a first in-shaft passage  29   a , which extends along the axis L of the rotary shaft  20 , and a second in-shaft passage  29   b , which communicates with the first in-shaft passage  29   a  and extends in a radial direction of the rotary shaft  20 . The rear end of the first in-shaft passage  29   a  communicates with the pressure adjusting chamber  15   c . The lower end of the second in-shaft passage  29   b  communicates with the front end of the first in-shaft passage  29   a . The upper end of the second in-shaft passage  29   b  opens to the interior of the control pressure chamber  35 . Thus, the control pressure chamber  35  and the pressure adjusting chamber  15   c  are connected to each other by the first in-shaft passage  29   a  and the second in-shaft passage  29   b.    
     As shown in  FIG. 3 , the pressure adjusting chamber  15   c  and the suction chamber  15   a  are connected to each other by a bleed passage  36 . An electromagnetic control valve  36   s , which functions as a control mechanism, is arranged in the bleed passage  36 . The control valve  36   s  is capable of adjusting the opening degree of the bleed passage  36  based on the pressure in the suction chamber  15   a . The control valve  36   s  adjusts the flow rate of the refrigerant flowing through the bleed passage  36  to control the pressure in the pressure adjusting chamber  15   c . The pressure adjusting chamber  15   c  and the discharge chamber  15   b  are connected to each other by a supply passage  37 . The supply passage  37  has an orifice  37   a . The orifice  37   a  limits the flow rate of the refrigerant gas flowing through the supply passage  37 . 
     Refrigerant gas is introduced to the control pressure chamber  35  from the discharge chamber  15   b  via the supply passage  37 , the pressure adjusting chamber  15   c , the first in-shaft passage  29   a , and the second in-shaft passage  29   b . Also, refrigerant gas is discharged from the control pressure chamber  35  to the suction chamber  15   a  via the second in-shaft passage  29   b , the first in-shaft passage  29   a , the pressure adjusting chamber  15   c , and the bleed passage  36 . Accordingly, the pressure in the control pressure chamber  35  is controlled. The pressure difference between the control pressure chamber  35  and the swash plate chamber  24  causes the movable body  32  to move along the axis L of the rotary shaft  20  with respect to the partition body  31 . The refrigerant gas introduced into the control pressure chamber  35  serves as control gas for controlling the movement of the movable body  32 . 
     Referring to  FIG. 1 , in the swash plate chamber  24 , a lug arm  40  is provided between the swash plate  23  and the first flange  21   f . The lug arm  40  serves as a link mechanism that allows change of the inclination angle of the swash plate  23 . The lug arm  40  substantially has an L shape as a whole. A weight portion  40   w  is provided in the rear part of the lug arm  40 . The weight portion  40   w  is passed through a groove  23   b  of the swash plate  23  to be located at a position behind the swash plate  23 . 
     The rear part of the lug arm  40  is coupled to the upper end of the swash plate  23  by a first pin  41 , which extends across the groove  23   b . The rear part of the lug arm  40  is thus supported by the swash plate  23  to be pivotal about a first pivot axis M 1 , which is the axis of the first pin  41 . The front part of the lug arm  40  is coupled to a coupling portion (not shown) of the first supporting member  21  by a columnar second pin  42 . The front part of the lug arm  40  is thus supported by the first supporting member  21  to be pivotal about a second pivot axis M 2 , which is the axis of the second pin  42 . 
     A coupling portion  32   c  is provided at the distal end of the cylindrical portion  32   b  of the movable body  32 . The coupling portion  32   c  protrudes toward the swash plate  23 . A columnar coupling pin  43  is fixed to the coupling portion  32   c . The swash plate  23  has a through-hole  23   h , through which the coupling pin  43  extends. The through-hole  23   h  is located in a part of the swash plate  23  that is radially outward of the through-hole  23   a . That is, the coupling pin  43  couples the coupling portion  32   c  to the lower end of the swash plate  23 . 
     Increase in the opening degree of the control valve  36   s  increases the flow rate of refrigerant gas that is discharged from the control pressure chamber  35  to the suction chamber  15   a  via the second in-shaft passage  29   b , the first in-shaft passage  29   a , the pressure adjusting chamber  15   c , and the bleed passage  36 . This substantially equalizes the pressure in the pressure adjusting chamber  15   c  with the pressure in the suction chamber  15   a  and substantially equalizes the pressure in the control pressure chamber  35  with the pressure in the suction chamber  15   a . This reduces the pressure difference between the control pressure chamber  35  and the swash plate chamber  24 . Thus, the compression reactive force acting on the swash plate  23  from the double-headed pistons  25  causes the swash plate  23  to pull the movable body  32  via the coupling pin  43 . As a result, the bottom portion  32   a  of the movable body  32  approaches the partition body  31 . 
     When the bottom portion  32   a  of the movable body  32  approaches the partition body  31  as shown in  FIG. 4 , the swash plate  23  pivots about the first pivot axis M 1  and the lug arm  40  pivots about the second pivot axis M 2 , so that the lug arm  40  approaches the first flange  21   f . Accordingly, the inclination angle of the swash plate  23  is reduced so that the swash plate  23  contacts the restoration spring  28   a . When the inclination angle of the swash plate  23  is reduced, the stroke of the double-headed pistons  25  is reduced. Accordingly, the displacement is decreased. 
     In the compressor  10  of the present embodiment, each pair of the first cylinder bore  12   a  and the second cylinder bore  13   a  reciprocally accommodates a double-headed piston  25 . In this configuration, as the inclination angle of the swash plate  23  decreases, the dead volume of the second compression chamber  19   b , that is, the gap between the double-headed piston  25  at the top dead center and the second valve-port assembly plate  17  is increased. In contrast, the discharge stroke is executed without significantly increasing the dead volume of the first compression chamber  19   a , that is, the gap between the double-headed piston  25  at the top dead center and the first valve-port assembly plate  16 . Thus, the lug arm  40  is arranged such that, as the inclination angle of the swash plate  23  is changed, the top dead center position of the double-headed piston  25  in each second compression chamber  19   b  is displaced by a greater amount than the top dead center position of the piston  25  in the corresponding first compression chamber  19   a.    
     Thus, when the dead volume of the second compression chamber  19   b  becomes a predetermined volume as the inclination angle of the swash plate  23  is reduced to a predetermined inclination angle, refrigerant gas stops being discharged from the second compression chamber  19   b . Therefore, as the inclination angle of the swash plate  23  is reduced from the predetermined angle to the minimum inclination, the pressure in the second compression chamber  19   b  stops reaching the discharge pressure. This stops discharge and suction of refrigerant gas, and only compression and expansion of refrigerant gas are repeated. 
     Decrease in the opening degree of the control valve  36   s  decreases the flow rate of refrigerant gas that is discharged from the control pressure chamber  35  to the suction chamber  15   a  via the second in-shaft passage  29   b , the first in-shaft passage  29   a , the pressure adjusting chamber  15   c , and the bleed passage  36 . Since refrigerant gas is supplied to the control pressure chamber  35  from the discharge chamber  15   b  via the supply passage  37 , the pressure adjusting chamber  15   c , the first in-shaft passage  29   a , and the second in-shaft passage  29   b , the pressure in the control pressure chamber  35  is substantially equalized with the pressure in the discharge chamber  15   b . This increases the pressure difference between the control pressure chamber  35  and the swash plate chamber  24 . Thus, the movable body  32  pulls the swash plate  23  via the coupling pin  43 . As a result, the bottom portion  32   a  of the movable body  32  is moved away from the partition body  31 . 
     When the bottom portion  32   a  of the movable body  32  is moved away from the partition body  31  as shown in  FIG. 1 , the swash plate  23  is pivoted about the first pivot axis M 1  in a direction opposite to the pivoting direction for decreasing the inclination angle of the swash plate  23 . Also, the lug arm  40  pivots about the second pivot axis M 2  in a direction opposite to the pivoting direction for decreasing the inclination angle of the swash plate  23 . The lug arm  40  thus moves away from the first flange  21   f . This increases the inclination angle of the swash plate  23  and thus increases the stroke of the double-headed pistons  25 . Accordingly, the displacement is increased. 
     Operation of the present embodiment will now be described. 
     When the displacement is increased and the pressure in the discharge chamber  14   b  is raised, the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24  increases. This moves the spacer  50  toward the first thrust bearing  27   a . Accordingly, the spacer  50  pushes the first thrust bearing  27   a , so that the first thrust bearing  27   a  is pressed against the first flange  21   f  by the spacer  50 . As a result, the first thrust bearing  27   a  is tightly held between the spacer  50  and the first flange  21   f . When the first thrust bearing  27   a  is pressed against the first flange  21   f , the rotary shaft  20  is pushed toward the second thrust bearing  27   b . As a result, the second flange  22   f  is pressed against the second thrust bearing  27   b , so that the second thrust bearing  27   b  is tightly held between the second flange  22   f  and the second cylinder block  13 . The rotary shaft  20  thus receives a load that is generated based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24  and acts toward the second thrust bearing  27   b.    
     The rotary shaft  20  is tightly held by the first thrust bearing  27   a  and the second thrust bearing  27   b  with respect to the axial direction. This determines the position of the rotary shaft  20  in the axial direction. Thus, when the displacement increases so that the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is increased, the thrust applied to the rotary shaft  20  from the swash plate  23  is increased. Even in such a case, since the position of the rotary shaft  20  is determined in the axial direction, the rotary shaft  20  is restrained from chattering due to the thrust acting on the rotary shaft  20 . 
     In contrast, when the displacement decreases, the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is decreased, and the thrust transmitted to the rotary shaft  20  from the swash plate  23  is decreased, accordingly. At this time, since the pressure in the discharge chamber  14   b  is lowered due to the decrease in the displacement, the pressure difference between the discharge chamber  14   b  and the swash plate chamber  24  decreases. This reduces the force with which the spacer  50  presses the first thrust bearing  27   a  against the first flange  21   f . As a result, the force with which the second flange  22   f  is pressed against the second thrust bearing  27   b  is reduced. Thus, the load applied to the rotary shaft  20  toward the second thrust bearing  27   b  is reduced. Therefore, the sliding resistance between the first thrust bearing  27   a  and the rotary shaft  20  and the sliding resistance between the second thrust bearing  27   b  and the rotary shaft  20  are both reduced, which reduces the power loss. 
     When the inclination angle of the swash plate  23  is reduced, the dead volume of each second compression chamber  19   b  is increased. When the dead volume of each second compression chamber  19   b  reaches a predetermined value, the double-headed pistons  25  no longer perform the discharge stroke in the second compression chambers  19   b . Then, the compression reaction force applied to the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  exceeds the compression reaction force applied to the swash plate  23  from the parts of the double-headed pistons  25  in the second compression chambers  19   b . At this time, the direction of the compression reaction force acting on the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  is the same as the direction of the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . This permits reduction in the load required to press the rotary shaft  20  against the second thrust bearing  27   b , that is, reduction in the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . 
     The above described embodiment provides the following advantages. 
     (1) The rotary shaft  20  receives a load that is generated based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24  and acts toward the second thrust bearing  27   b . In this configuration, when the displacement is increased and the pressure in the discharge chamber  14   b  is raised, the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24  increases. In this case, the load that is applied to the rotary shaft  20  and acts toward the second thrust bearing  27   b  is increased. This presses the rotary shaft  20  against the second thrust bearing  27   b , thereby fixing the position in the axial direction of the rotary shaft  20 . Thus, when the displacement increases so that the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is increased, the thrust applied to the rotary shaft  20  from the swash plate  23  is increased. Even in such a case, since the position of the rotary shaft  20  is fixed in the axial direction, the rotary shaft  20  is restrained from chattering due to the thrust acting on the rotary shaft  20 . 
     In contrast, when the displacement decreases, the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is decreased, and the thrust transmitted to the rotary shaft  20  from the swash plate  23  is decreased, accordingly. At this time, since the pressure in the pressure-acting chamber  55  is lowered due to the decrease in the displacement, the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24  decreases. Thus, the load applied to the rotary shaft  20  toward the second thrust bearing  27   b  is reduced. This reduces the sliding resistance between the second thrust bearing  27   b  and the rotary shaft  20  and thus reduces the power loss. In this manner, it is possible to restrain the rotary shaft  20  from chattering due to the thrust acting on the rotary shaft  20  while reducing the power loss. 
     (2) The spacer  50  is supported by the rotary shaft  20  while being restricted from rotating and allowed to move in the axial direction of the rotary shaft  20 . Compared to a configuration in which the spacer  50  rotates integrally with the rotary shaft  20 , the durability of the sealing members  52   a ,  52   b  is improved so that the pressure-acting chamber  55  and the swash plate chamber  24  are sealed off from each other in a reliable manner. 
     (3) The contact portion  51  of the spacer  50  contacts the first cylinder block  12 . In this configuration, the fastening force acting on the housing  11  in the axial direction of the rotary shaft  20 , which is generated when the first cylinder block  12 , the second cylinder block  13 , the front housing member  14 , and the rear housing member  15  are assembled, generates a load. The load acts toward the second thrust bearing  27   b  and is applied to the spacer  50  from the first cylinder block  12  via the contact portion  51 . As a result, since the rotary shaft  20  is pressed against the second thrust bearing  27   b , the position of the rotary shaft  20  is determined in the axial direction. Thus, for example, when the compressor  10  is stopped and the rotary shaft  20  receives no load based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 , the position of the rotary shaft  20  in the axial direction is fixed. Therefore, for example, even if the vehicle in which the compressor  10  is installed vibrates and causes the compressor  10  to vibrate, the rotary shaft  20  is restrained from chattering in the axial direction. 
     (4) When the dead volume of each second compression chamber  19   b  is increased to a predetermined value due to reduction in the inclination angle of the swash plate  23 , the double-headed pistons  25  no longer perform the discharge stroke in the second compression chambers  19   b . Then, the compression reaction force applied to the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  exceeds the compression reaction force applied to the swash plate  23  from the parts of the double-headed pistons  25  in the second compression chambers  19   b . At this time, the direction of the compression reaction force acting on the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  is the same as the direction of the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . This permits reduction in the load required to press the rotary shaft  20  against the second thrust bearing  27   b , that is, reduction in the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . This efficiently reduces chattering of the rotary shaft  20  caused by the thrust acting on the rotary shaft  20 . 
     (5) The outer diameter R 1  of the first head  25   a  is larger than the outer diameter R 2  of the second head  25   b . In this configuration, the compression reaction force applied to the swash plate  23  by the parts of the double-headed pistons  25  in the first compression chambers  19   a  is greater than that in the case in which the outer diameter R 1  of the first head  25   a  is equal to the outer diameter R 2  of the second head  25   b  or that in the case in which the outer diameter R 1  of the first head  25   a  is smaller than the outer diameter R 2  of the second head  25   b . This permits further reduction in the load required to press the rotary shaft  20  against the second thrust bearing  27   b , that is, reduction in the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . This further efficiently reduces chattering of the rotary shaft  20  caused by the thrust acting on the rotary shaft  20 . 
     (6) It is now assumed that the direction of the compression reaction force acting on the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  is opposite to the direction of the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . In this case, to press the rotary shaft  20  against the second thrust bearing  27   b  using the load based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 , that load needs to be greater than the compression reaction force applied to the swash plate  23  from the parts of the pistons  25  in the first compression chambers  19   a . Accordingly, the pressure receiving area of the pressure-acting chamber  55  needs to be increased. In the present embodiment, the direction of the compression reaction force acting on the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  is the same as the direction of the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . This reduces the pressure receiving area of the pressure-acting chamber  55 . This allows the size of the spacer  50  and thus the size of the compressor  10  to be reduced. 
     The above described embodiment may be modified as follows. 
     As shown in  FIG. 5 , a spacer  60  that is rotational integrally with the rotary shaft  20  may be employed. The spacer  60  has an annular shape and is press-fitted and fixed to the rotary shaft  20 . A sealing member  61  is arranged in the outer circumferential surface of the spacer  60  to seal the gap between the outer circumferential surface of the spacer  60  and the inner circumferential surface of the first small diameter hole  121   b . The spacer  60  is arranged in the first small diameter hole  121   b  with the end face closer to the first cylinder block  12  separated from the first cylinder block  12 . Also, the first cylinder block  12  and the spacer  50  define a pressure-acting chamber  55 . A sealing member  62  is arranged in the outer circumferential surface of the first supporting member  21  to seal the gap between the shaft hole  12   h  and the outer circumferential surface of the first supporting member  21 . In this configuration, since the spacer  60  is rotational integrally with the rotary shaft  20 , no thrust bearing needs to be arranged between the spacer  60  and the rotary shaft  20 . This reduces the number of components and thus the weight of the compressor  10 . 
     The spacer  60  shown in  FIG. 5  may be integrated with the rotary shaft  20 . 
     As shown in  FIG. 6 , the contact portion  51  may be omitted from the spacer  50 . In this case, the spacer  50  has a flange  50   f  on the outer circumferential surface at a position in the vicinity of the first large diameter hole  122   b . The flange  50   f  contacts an end face  123   b  of the boundary between the first small diameter hole  121   b  and the first large diameter hole  122   b  in the first cylinder block  12 . In this configuration, the spacer  50  is allowed to be arranged in the first small diameter hole  121   b  by bringing the flange  50   f  into contact with the end face  123   b  with the end face  50   a  separated from the first cylinder block  12 . 
     As shown in  FIG. 7 , a pressure-acting chamber  65  may communicate with the control pressure chamber  35 , and the pressure in the pressure-acting chamber  65  may equal to the pressure in the control pressure chamber  35 . Also, the rotary shaft  20  may receive a load that is generated based on the pressure difference between the control pressure chamber  35  and the swash plate chamber  24  and acts toward the second thrust bearing  27   b . In the embodiment shown in  FIG. 7 , the first cylinder block  12 , the second cylinder block  13 , the swash plate  23 , the double-headed pistons  25 , the first thrust bearing  27   a , the second thrust bearing  27   b , the actuator  30 , the lug arm  40 , the spacer  50 , and the like are arranged at reversed positions in relation to the positions shown in  FIGS. 1 to 4  in the axial direction of the rotary shaft  20 . In the embodiment shown in  FIG. 7 , the sealing member  52   a , which is used in the embodiment shown in  FIGS. 1 to 4 , may be omitted. The first cylinder block  12  has a supply passage  65   a , which connects the pressure-acting chamber  65  and the pressure adjusting chamber  15   c . Refrigerant gas is supplied to the pressure-acting chamber  65  from the pressure adjusting chamber  15   c  via the supply passage  65   a . The pressure in the pressure adjusting chamber  15   c  is equal to the pressure in the control pressure chamber  35 . The direction of the compression reaction force acting on the swash plate  23  from the parts of the double-headed pistons  25  in the first compression chambers  19   a  is the same as the direction of the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  65  and the swash plate chamber  24 . 
     When the displacement is increased and the pressure in the control pressure chamber  35  is raised, the pressure difference between the pressure-acting chamber  65  and the swash plate chamber  24  increases. This moves the spacer  50  toward the first thrust bearing  27   a . Accordingly, the spacer  50  pushes the first thrust bearing  27   a , so that the first thrust bearing  27   a  is pressed against the first flange  21   f  by the spacer  50 . As a result, the first thrust bearing  27   a  is tightly held between the spacer  50  and the first flange  21   f . When the first thrust bearing  27   a  is pressed against the first flange  21   f , the rotary shaft  20  is pushed toward the second thrust bearing  27   b . As a result, the second flange  22   f  is pressed against the second thrust bearing  27   b , so that the second thrust bearing  27   b  is tightly held between the second flange  22   f  and the second cylinder block  13 . The rotary shaft  20  thus receives a load that is generated based on the pressure difference between the pressure-acting chamber  65  and the swash plate chamber  24  and acts toward the second thrust bearing  27   b.    
     In this way, the rotary shaft  20  is tightly held by the first thrust bearing  27   a  and the second thrust bearing  27   b  with respect to the axial direction of the rotary shaft  20 . This determines the position of the rotary shaft  20  in the axial direction. Thus, when the displacement increases so that the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is increased, the thrust applied to the rotary shaft  20  from the swash plate  23  is increased. Even in such a case, since the position of the rotary shaft  20  is determined in the axial direction, the rotary shaft  20  is restrained from chattering due to the thrust acting on the rotary shaft  20 . 
     In contrast, the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is decreased when the displacement is decreased. Accordingly, the thrust transmitted to the rotary shaft  20  from the swash plate  23  is decreased. At this time, since the pressure in the control pressure chamber  35  is lowered due to the decrease in the displacement, the pressure difference between the pressure-acting chamber  65  and the swash plate chamber  24  decreases. This reduces the force with which the spacer  50  presses the first thrust bearing  27   a  against the first flange  21   f . As a result, the force with which the second flange  22   f  is pressed against the second thrust bearing  27   b  is also reduced. Thus, the load applied to the rotary shaft  20  toward the second thrust bearing  27   b  is reduced. As a result, the sliding resistance between the first thrust bearing  27   a  and the rotary shaft  20  and the sliding resistance between the second thrust bearing  27   b  and the rotary shaft  20  are both reduced, which reduces the power loss. 
     As the displacement increases, the pressure in the control pressure chamber  35  approaches the pressure in the discharge chamber  15   b . As the displacement decreases, the pressure in the control pressure chamber  35  approaches the pressure in the suction chamber  15   a . When the displacement increases, the load based on the pressure difference between the pressure-acting chamber  65  and the swash plate chamber  24  approaches the load based on the pressure difference between the discharge chamber  15   b  and the swash plate chamber  24 . Thus, the thrust transmitted from the swash plate  23  to the rotary shaft  20  is increased when the displacement increases so that the compression reaction force acting on the swash plate  23  from the double-headed pistons  25  increases. In this case, the rotary shaft  20  receives a load that acts toward the second thrust bearing  27   b . The received load is equivalent to the load that is generated based on the pressure difference between the discharge chamber  15   b  and the swash plate chamber  24 . As described above, when the displacement increases so that the compression reaction force applied to the swash plate  23  from the double-headed pistons  25  is increased, the thrust applied to the rotary shaft  20  from the swash plate  23  is increased. Even in this case, the position of the rotary shaft  20  is fixed in the axial direction. The rotary shaft  20  is thus restrained from chattering due to the thrust acting on the rotary shaft  20 . 
     In contrast, when the displacement decreases, the load based on the pressure difference between the pressure-acting chamber  65  and the swash plate chamber  24  approaches the load based on the pressure difference between the suction chamber  15   a  and the swash plate chamber  24 . Thus, as the displacement decreases, the load applied to the rotary shaft  20  toward the second thrust bearing  27   b  decreases to approach the load based on the pressure difference between the suction chamber  15   a  and the swash plate chamber  24 . Therefore, when the displacement is changed, the load applied to the rotary shaft  20  toward the second thrust bearing  27   b  becomes smaller than the load based on the pressure difference between the discharge chamber  15   b  and the swash plate chamber  24 . This reduces the sliding resistance between the second thrust bearing  27   b  and the rotary shaft  20  and thus reduces the power loss. 
     The embodiment illustrated in  FIG. 7  is basically the same as the embodiment shown in  FIGS. 1 to 4  except that the load based on the pressure difference between the pressure in the control pressure chamber  35  and the swash plate chamber  24  is applied to the rotary shaft  20  toward the second thrust bearing  27   b . Therefore, the embodiment shown in  FIG. 7  achieves the same advantages as the advantages (2) to (6) of the embodiment shown in  FIGS. 1 to 4 . 
     A spacer that is rotational integrally with the rotary shaft  20  as illustrated in  FIG. 5  may be employed in the embodiment illustrated in  FIG. 7 , in which the load based on the pressure difference between the pressure in the control pressure chamber  35  and the swash plate chamber  24  is applied to the rotary shaft  20  toward the second thrust bearing  27   b . With this configuration, since the spacer is allowed to rotate integrally with the rotary shaft  20 , there is no need to provide a thrust bearing between the spacer and the rotary shaft  20 , so that the number of components is reduced. 
     The direction in which the compression reaction force acts on the swash plate  23  from the double-headed pistons  25  in the first compression chambers  19   a  may be opposite to the direction of the load applied to the rotary shaft  20  based on the pressure difference between the pressure-acting chamber  55  and the swash plate chamber  24 . 
     The outer diameter R 1  of the first head  25   a  may be equal to the outer diameter R 2  of the second head  25   b.    
     The outer diameter R 1  of the first head  25   a  is smaller than the outer diameter R 2  of the second head  25   b.    
     The discharge chamber  15   b  may communicate with the pressure-acting chamber  55 . 
     The actuator  30  may be modified to operate such that, when the pressure in the control pressure chamber  35  is substantially equal to the pressure in the suction chamber  15   a , the movable body  32  is moved to increase the inclination angle of the swash plate  23 , and that, when the pressure in the control pressure chamber  35  is substantially equal to the pressure in the discharge chamber  15   b , the movable body  32  is moved to reduce the inclination angle of the swash plate  23 . That is, the actuator  30  may be configured to increase the displacement by lowering the pressure in the control pressure chamber  35 . 
     An electromagnetic control valve may be provided on the supply passage  37 , which connects the pressure adjusting chamber  15   c  and the discharge chamber  15   b  to each other, and an orifice may be provided in the bleed passage, which connects the pressure adjusting chamber  15   c  and the suction chamber  15   a  to each other. 
     The compressor  10  may be a single-headed piston swash plate type compressor, which has single-headed pistons. 
     The compressor  10  may obtain the drive force from an external drive source via a clutch. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
         
           
               10  . . . Variable Displacement Swash Plate Type Compressor;  11  . . . Housing;  12  . . . First Cylinder Block as Cylinder Block;  12   a  . . . First Cylinder Bore as Cylinder Bore;  13  . . . Second Cylinder Block as Cylinder Block;  13   a  . . . Second Cylinder Bore as Cylinder Bore;  14   b ,  15   b  . . . Discharge Chambers;  19   a  . . . First Compression Chamber as One Compression Chamber;  19   b  . . . Second Compression Chamber as The Other Compression Chamber;  20  . . . Rotary Shaft;  23  . . . Swash Plate;  24  . . . Swash Plate Chamber;  25  . . . Double-Headed Piston as Piston;  25   a  . . . First Head as One Head;  25   b  . . . Second Head as The Other Head;  27   b  . . . Second Thrust Bearing as Thrust Bearing;  30  . . . Actuator;  31  . . . Partition Body;  32  . . . Movable Body;  35  . . . Control Pressure Chamber;  40  . . . Lug Arm as Link Mechanism;  50 ,  60  . . . Spacers;  51  . . . Contact Portion;  52   a ,  52   b ,  52   c  . . . Sealing Members;  55 ,  65  . . . Pressure-acting chambers