Abstract:
A variable displacement compressor includes a swash plate guiding member that rotates integrally with and inclines relative to a drive shaft, a swash plate supported by the guiding member, and a piston engaged with the swash plate via a shoe. A clutch is located between the guiding member and the swash plate. The clutch is switched between an engaged state, where the clutch permits the guiding member and the swash plate to rotate integrally, and a disengaged state, where the clutch permits the guiding member and the swash plate to rotate relative to each other. Therefore, the compressor is capable of switching between a state where improving the displacement controllability is prioritized and a state where reduction of mechanical loss between the swash plate and the shoes is prioritized.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a swash plate type variable displacement compressor, which is capable of changing its displacement.  
         [0002]     For example, in a refrigerant circuit of a vehicle air conditioner, a swash plate type variable displacement compressor is typically used as a refrigerant compressor. The swash plate type variable displacement compressor includes a drive shaft coupled to an output shaft of an engine, a swash plate that is tiltably supported by and rotates integrally with the drive shaft, and pistons each engaged with the swash plate with shoes.  
         [0003]     Since the swash plate type variable displacement compressor has the configuration shown above, when swash plate rotates as the drive shaft rotates, the swash plate slides across the shoes and wobbles back and forth apparently in a direction along the axis of the drive shaft. Wobbling of the swash plate causes the pistons to reciprocate to compress gas. Also, by changing the inclination angle of the swash plate relative to the drive shaft, the stroke of the pistons is varied. Accordingly, the displacement of the swash plate type variable displacement compressor is changed.  
         [0004]     A clutchless type compressor is known as one of the swash plate type variable displacement compressors other than a clutch type compressor, which is another swash plate type variable displacement compressor equipped with a clutch in a power transmission mechanism between its drive shaft and an engine. The clutchless type compressor has no clutch in the power transmission mechanism and permits power of the engine to be constantly transmitted. That is, the drive shaft of the swash plate type variable displacement compressor having no clutch is constantly rotated by the drive shaft of the engine when the engine is running. Therefore, when cooling is not needed, the vehicle air conditioner causes the compressor to perform an “OFF operation”, in which the displacement of the swash plate type variable displacement compressor is minimized. Accordingly, the load on the engine (load for driving the swash plate type variable displacement compressor) is reduced, thereby improving the fuel economy of the engine.  
         [0005]     The swash-plate type variable displacement compressor having no clutch, that is the clutchless type compressor, has shoes that slide on a swash plate. Therefore, mechanical loss due to frictional resistance at the contacting portions of the swash plate and the shoes is great, thereby increasing the load on the engine. Particularly, in the swash plate type variable displacement compressor having no clutch, mechanical loss at the contacting portions of the swash plate and the shoes have been desired to be reduced for the same of decreasing the load on the engine during the OFF operation.  
         [0006]     In response to such demand, improved structures of swash plate type variable displacement compressors have been proposed (for example, see  FIG. 1  of Japanese Laid-Open Patent Publication No 10-196525). In a typical improvement, a swash plate guiding member is coupled to a drive shaft to rotate integrally with and incline relative to the drive shaft. A swash plate is supported by the guiding member with bearings. When the guiding member rotates as the drive shaft rotates, the guiding member slips relative to the swash plate. Accordingly, the rotation speed of the swash plate is lower than that of the guiding member. Accordingly, the relative rotation speed of the swash plate and the shoes is lower than the relative rotation speed of the shoes and the guiding member. This reduces the mechanical loss at the contacting portions of the swash plate and the shoes. As a result, the load of the swash plate type variable displacement compressor acting on the engine is reduced.  
         [0007]     However, since the rotation speed of the swash plate is lower than that of the guiding member in the above described improved swash plate type variable displacement compressor, the swash plate only receives a slight centrifugal force. Centrifugal force of the swash plate acts to cancel the inertial force of the piston, which is generated by reciprocation of the piston. Specifically, while the centrifugal force of the swash plate acts to decrease the inclination angle of the swash plate, the inertial force of the pistons act to increase the inclination angle of the swash plate.  
         [0008]     Therefore, if the swash plate only receives said slight centrifugal force, the inclination angle of the swash plate cannot be smoothly decreased against the inertial force of the pistons. This adversely affects the displacement control of the swash plate type variable displacement compressor. Particularly, when the swash plate type variable displacement compressor is operating at a high speed (high-speed rotation of the drive shaft), that is, when the inertial force of the piston is great, the problem is serious.  
         [0009]     As described above, the prior art has a dilemma. That is, if the displacement control of a swash plate type variable displacement compressor is prioritized, the mechanical loss between the swash plate and the shoes is increased. Conversely, if reduction of mechanical loss generated between the swash plate and the shoes is prioritized, the displacement control of the swash plate type variable displacement compressor is degraded.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, it is an objective of the present invention to provide a swash plate type variable displacement compressor that is capable of switching between a state where improving the displacement controllability is prioritized and a state where reduction of mechanical loss between the swash plate and the shoes is prioritized.  
         [0011]     To achieve the above-mentioned objective, the present invention provides a swash plate type variable displacement compressor. The compressor includes a drive shaft, a swash plate guiding member that is coupled to the drive shaft. The guiding member rotates integrally with and inclines relative to the drive shaft. The compressor further includes a swash plate supported by the guiding member, a shoe, and a piston engaged with the swash plate via the shoe. When the guiding member is rotated by rotation of the drive shaft, the piston reciprocates to compress gas. When an inclination angle of the swash plate relative to the drive shaft is changed as the guiding member inclines, the stroke of the piston is changed to vary the displacement of the compressor. A clutch: is located between the guiding member and the swash plate. The clutch is switched between an engaged state, where the clutch permits the guiding member and the swash plate to rotate integrally, and a disengaged state, where the clutch permits the guiding member and the swash plate to rotate relative to each other.  
         [0012]     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0014]      FIG. 1  is a longitudinal cross-sectional view illustrating a compressor according to a first embodiment of the present invention;  
         [0015]      FIG. 2  is an enlarged cross-sectional view illustrating a portion including a swash plate guiding member of the compressor shown in  FIG. 1 ;  
         [0016]      FIG. 3  is an enlarged cross-sectional view illustrating a portion including the guiding member when the inclination angle is minimized;  
         [0017]      FIG. 4  is a plan view showing the hinge mechanism shown in  FIG. 1  in a disassembled state;  
         [0018]      FIG. 5  is a front view illustrating the axial slide limiting surface shown in  FIG. 1 ;  
         [0019]      FIG. 6  is a front view illustrating an axial slide limiting surface of a compressor according to a second embodiment of the present invention;  
         [0020]      FIG. 7  is a partial cross-sectional view illustrating a portion including a boss of a compressor according to a third embodiment of the present invention;  
         [0021]      FIG. 8  is a partial cross-sectional view illustrating a portion including a boss of a compressor according to a fourth embodiment of the present invention;  
         [0022]      FIG. 9  is a partial cross-sectional view illustrating a portion including a boss of a compressor according to a fifth embodiment of the present invention;  
         [0023]      FIG. 10  is a partial cross-sectional view illustrating a portion including a swash plate guiding member of a compressor according to a sixth embodiment of the present invention;  
         [0024]      FIG. 11  is a partial cross-sectional view illustrating a portion including a swash plate guiding member of a compressor according to a seventh embodiment of the present invention;  
         [0025]      FIG. 12  is a longitudinal cross-sectional view illustrating a compressor according to an eighth embodiment of the present invention;  
         [0026]      FIG. 13  is a partial cross-sectional view illustrating a portion including a swash plate guiding member of a compressor according to a ninth embodiment of the present invention;  
         [0027]      FIG. 14  is a partial cross-sectional view illustrating a portion including a clutch of a compressor according to a tenth embodiment of the present invention;  
         [0028]      FIG. 15  is a plan view illustrating a portion including the clutch shown in  FIG. 14 ;  
         [0029]      FIG. 16  is a partial cross-sectional view illustrating a compressor according to an eleventh embodiment of the present invention;  
         [0030]      FIG. 17  is a partial cross-sectional view illustrating a portion including a swash plate guiding member of a compressor according to a twelfth embodiment of the present invention;  
         [0031]      FIG. 18  is a partial cross-sectional view illustrating the compressor shown in  FIG. 17  when the inclination angle is minimized; and  
         [0032]      FIG. 19  is a partial cross-sectional view illustrating a compressor according to a modified embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     A first embodiment of the present invention will now be described.  
         [0034]      FIG. 1  is a longitudinal cross-sectional view illustrating a swash plate type variable displacement compressor  10  used in a refrigerant circuit of a vehicle air conditioner. As shown in  FIG. 1 , a housing  11  of the compressor  10  includes a cylinder block  12 , a front housing member  13 , and a rear housing member  14 . The front housing member  13  is secured to one end (left end as viewed in the drawing) of the cylinder block  12 . The rear housing member  14  is secured to the other end (right end as viewed in the drawing) of the cylinder block  12  with a valve assembly  15  in between. In  FIG. 1 , the left side is (the side corresponding to the front housing member  13 ) is referred to as the front of the compressor  10 , and the right side (the side corresponding to the rear housing member  14 ) is referred to as the rear of the compressor  10 , as necessary.  
         [0035]     A space defined by the inner wall of the front housing member  13  and the cylinder block  12  forms a crank chamber  16 . One end (front end) of a drive shaft  17  is rotatably supported by the front housing member  13 . The other end (rear end) of the drive shaft  17  is rotatably supported by the cylinder block  12 . The drive shaft  17  extends frontward and rearward in the housing  11  of the compressor  10  and through the crank chamber  16 .  
         [0036]     The drive shaft  17  is connected to a drive source of the vehicle, which is an engine (internal combustion engine) E in this embodiment, through a power transmission mechanism PT. The power transmission mechanism PT is a clutchless mechanism that includes, for example, a belt and a pulley. In other words, the power transmission mechanism PT does not have a clutch mechanism (for example, an electromagnetic clutch) that selectively transmits and disconnects power with external electric control. The power transmission mechanism PT therefore constantly transmits power. Therefore, when the engine E is running, the drive shaft  17  constantly receives power from the engine E and rotates around an axis L.  
         [0037]     A substantially disk-shaped rotor  18  is accommodated in the crank chamber  16 . The rotor  18  is fixed to the drive shaft  17  to rotate integrally with the drive shaft  17 . A swash plate guiding member  19 , which is made of an iron-based metal, is accommodated in the crank chamber  16 . The guiding member  19  is located rearward of the rotor  18 . A boss  19   a  is formed on a center of the rear side of the guiding member  19 . A shaft receiving hole  19   b  is formed in a central portion of the guiding member  19  (including the boss  19   a ). The drive shaft  17  extends through the shaft receiving hole  19   b  and supports the guiding member  19  such that the guiding member  19  slides back and forth along the axis L of the drive shaft  17  and that the inclination angle of the guiding member  19  is variable.  
         [0038]     Specifically, the inclination angle of the guiding member  19  relative to the drive shaft  17  refers to an angle defined by an imaginary plane perpendicular to a central axis M of the guiding member  19  or a central axis M of the boss  19   a , and an imaginary plane perpendicular to the axis L of the drive shaft  17 .  
         [0039]     As shown in  FIGS. 1 and 4 , the rotor  18  is connected to the guiding member  19  with a hinge mechanism  20 . The hinge mechanism  20  is of a “pinless type”. The hinge mechanism  20  has a pair of driving projections  21  protruding from the rear side of the rotor and a pair of driven projections  22  protruding toward the rotor  18  from the front side of the guiding member  19 . The driven projections  22  are located between the driving projections  21 . A cam surface  23  is formed at the proximal portion of each driving projection  21 . The distal end of the corresponding one of the driven projection  22  slidably contacts each cam surface  23 .  
         [0040]     The hinge mechanism  20  thus constructed permits rotation force of the rotor  18  to be transmitted to the guiding member  19  through one of the driving projections  21  and one of the driven projections  22  that contacts the driving projection  21 . In a state where the distal ends of the driven projections  22  contact the cam surfaces  23 , when the distal ends of the driven projections  22  move away from the drive shaft  17 , the inclination angle of the guiding member  19  relative to the axis L of the drive shaft  17  is increased. In contrast, when the distal ends of the driven projections  22  move toward the drive shaft  17 , the inclination angle of the guiding member  19  relative to the axis L of the drive shaft  17  is decreased.  
         [0041]     As shown in  FIGS. 1 and 2 , a disk-shaped swash plate  24  is accommodated in the crank chamber  16 . From the viewpoint of the sliding property with the guiding member  19 , the swash plate  24  is made of a material different from that of the guiding member  19 , for example, of a copper-based metal. A boss receiving hole  24   a  is formed in a center portion of the swash plate  24 . In a section of an inner circumferential surface  41  of the boss receiving hole  24   a  that corresponds to the cylinder block  12  (rear portion), a constant diameter portion  41   a  is formed. In a section of an outer circumferential surface  43  of the boss  19   a  that corresponds to the cylinder block  12  (rear portion), a constant diameter portion  43   a  is formed. The boss receiving hole  24   a  of the swash plate  24  receives the boss  19   a  of the guiding member  19 . The swash plate  24  is supported such that the constant diameter portion  41   a  of the boss receiving hole  24   a  rotates relative to the constant diameter portion  43   a  of the boss  19   a  substantially around the central axis M of the boss  19   a , or around the central axis M of the swash plate  24  (the boss receiving hole  24   a ), and that the constant diameter portion  41   a  slides on the constant diameter portion  43   a  back and forth along a direction of the central axis M.  
         [0042]     A slide limiting surface  19   c  is formed on the guiding member  19  in an area surrounding the proximal portion of the boss  19   a . The limiting surface  19   c  is formed planar and annular shape. The limiting surface  19   c  lies in an imaginary plane perpendicular to the central axis M of the guiding member  19 . Also, a snap ring  25  as a slide limiter is attached to the guiding member  19  at a distal end (rear end) of the boss  19   a , which protrudes from the boss receiving hole  24   a  of the swash plate  24  toward the cylinder block  12 .  
         [0043]     Therefore, the swash plate  24  slides along the central axis M in a range defined by the limiting surface  19   c  and the snap ring  25 . The swash plate  24  is supported by the boss  19   a  and prevented from being coming off the boss  19   a  by the snap ring  25 . The swash plate  24  is inclined together with the guiding member  19  so that the inclination angle with respect to the drive shaft  17  (substantially the same as the inclination angle of the guiding member  19  with respect to the drive shaft  17 ) can be changed.  
         [0044]     As shown in  FIG. 1 , cylinder bores  28  are formed in the cylinder block  12  at constant angular intervals around the axis L of the drive shaft  17 . Each cylinder bore  28  extends through the, cylinder block  12  from the end face corresponding to the front housing member  13  (front end face) to the end face corresponding to the rear housing member  14  (rear end face). A part of each piston  29  (a rear part that is located at the end corresponding to the rear housing member and is referred to as a “head”) is accommodated in the corresponding one of the cylinder bores  28 . Each piston  29  is held by the corresponding cylinder bore  28 , or by the cylinder block  12 , so that the piston  29  approaches and separates from the front end face (end face corresponding to the cylinder block  12 ) of the valve assembly  15 .  
         [0045]     The openings of each cylinder bore  28  are closed by the front end face of the valve assembly  15  and the corresponding piston  29 . In this manner, an enclosed space exists in each cylinder bore, which will be referred to as a compression chamber  30  below. Each compression chamber  30  is an enclosed space defined in the corresponding cylinder bore  28 . The volume of each compression chamber  30  is changed as the corresponding piston  29  slides in the cylinder bore  28 .  
         [0046]     A suction chamber  32  and a discharge chamber  33  are defined by the rear end face of the valve assembly  15  and the inner side (inner surface) of the rear housing member  14  in the housing  11 . The valve assembly  15  has suction ports  34 , which connect the compression chambers  30  with the suction chamber  32  to permit flow of refrigerant gas between the compression chambers  30  and the suction chamber  32 . The valve assembly  15  also has suction valve flaps  35 , each of which corresponds to one of the compression chambers  30  and opens and closes the corresponding suction port  34 . The valve assembly  15  also has discharge ports  36 , which connect the compression chambers  30  with the discharge chamber  33  to permit flow of refrigerant gas between the compression chambers  30  and the discharge chamber  33 . The valve assembly  15  also has discharge valve flaps  37 , each of which corresponds to one of the compression chambers  30  and opens and closes the corresponding discharge port  36 .  
         [0047]     An end of each piston  29  that protrudes into the crank chamber  16  (front end portion, which is opposite to the head) is coupled to a peripheral portion of the swash plate  24  with a pair of semispherical shoes  31 . Rotation of the guiding member  19  is converted into reciprocation of the pistons  29  by the swash plate  24  and the shoes  31 . Reciprocation of the pistons  29  changes the volume of the compression chambers  30 . When each piston  29  moves from the top dead center to the bottom dead center, refrigerant gas in the suction chamber  32  is drawn into the corresponding compression chamber  30  through the suction ports  34  and the opened suction valve flap  35 . Refrigerant gas drawn into each compression chamber  30  is compressed to a predetermined pressure as the piston  29  is moved from the bottom dead center to the top dead center. Then, the refrigerant gas is discharged to the discharge chamber  33  through the corresponding discharge port  36  and the opened discharge valve flap  37 .  
         [0048]     The inclination angle of the guiding member  19  and the swash plate  24  is determined by the equilibrium of various moments, such as the rotational moment caused by centrifugal force generated when the guiding member  19  (or the guiding member  19  and the swash plate  24 ) rotates, the moment caused by reciprocation inertial force of the pistons  29 , and the moment of gas pressure acting on the front and rear end faces of the pistons  29 . The moment caused by gas pressure refers to the moment generated based on the relationship between the inner pressure of the compression chambers  30  and the inner pressure of the crank chamber  16 , which corresponds to a back pressure of the pistons  29 . The gas pressure moment acts to either decrease or increase the inclination angle. A configuration for controlling the inner pressure of the crank chamber  16  will now be described.  
         [0049]     The housing  11  has a bleed passage  38 , a supply passage  39 , and a control valve  40 . The bleed passage  38  connects the crank chamber  16  with the suction chamber  32  so that refrigerant gas flows therebetween. The supply passage  39  connects the discharge chamber  33  with the crank chamber  16  so that refrigerant gas flows therebetween. The control valve  40  is located in the supply passage  39  to adjust the opening degree of the supply passage  39 .  
         [0050]     The control valve  40  has a conventional configuration that permits the opening degree of the supply passage  39  to be externally controlled with electricity. The control valve  40  includes an electromagnetic actuator  40   a  and a valve body  40   b . Electromagnetic force generated by the actuator  40   a  is related to the position of the valve body  40   b . Based on a command of a control computer  60  in accordance with the cooling load and the driving condition of the vehicle, a drive circuit  61  supplies electricity to the electromagnetic actuator  40   a . When the electricity supplied to the electromagnetic actuator  40   a  is increased, the control valve  40  decreases the opening degree of the supply passage  39 . When the electricity supplied to the electromagnetic actuator  40   a  is decreased, the control valve  40  increases the opening degree of the supply passage  39 . When no electricity is supplied to the electromagnetic actuator  40   a , the control valve  40  maximizes the opening degree of the supply passage  39 .  
         [0051]     By adjusting its opening, the control valve  40  controls the ratio between the flow rate of refrigerant gas (highly pressurized gas) drawn to the crank chamber  16  from the discharge chamber  33  through the supply passage  39 , and the flow rate of refrigerant gas supplied to the suction chamber  32  from the crank chamber  16  through the bleed passage  38 , thereby determining the inner pressure of the crank chamber  16 . In accordance with the inner pressure of the crank chamber  16  which is adjusted in this manner, the difference between the inner pressure of the crank chamber  16  and the inner pressure of the compression chambers  30  with the pistons  29  in between is changed. Accordingly, the force applied to the swash plate  24  and the guiding member  19  by the pistons  29  is changed. This, in turn, changes the inclination angle of the guiding member  19  and the swash plate  24 . As a result, the stroke of each piston  29 , that is, the displacement of the compressor  10 , is controlled. Specifically, the displacement of the compressor  10  is controlled in the following manner.  
         [0052]     When the inner pressure of the crank chamber  16  is lowered as the opening degree of the control-valve  40  (the supply passage  39 ) is reduced, the inclination angle of the guiding member  19  and the swash plate  24  is increased. This lengthens the stroke of each piston  29  and the displacement of the compressor  10  is increased, accordingly. In contrast, when the inner pressure of the crank chamber  16  is increased as the opening degree of the control valve  40  is increased, the inclination angle of the guiding member  19  and the swash plate  24  is decreased. This shortens the stroke of each piston  29  and the displacement of the compressor  10  is decreased, accordingly. When the opening degree of the control valve  40  is maximized, the inclination angle of the guiding member  19  and the swash plate  24  is minimized (zero degrees or an minute angle close to zero degrees, for example, an angle in a range between 0° and approximately 3°). In this state, the displacement of the compressor  10  is minimized (see  FIG. 3 ).  
         [0053]     When an occupant commands to turn off the air conditioning (for example, turns an air conditioner switch (not shown) off), or when the vehicle is rapidly accelerated (for example, when an acceleration pedal (not shown) is depressed by an degree greater than a predetermined amount), the control computer  60  commands the drive circuit  61  to stop supplying electricity to the control valve  40 . Therefore, the opening degree of the control valve  40  is maximized and the displacement of the compressor  10  is minimized. That is, the compressor  10  is in an “OFF operation”. Accordingly, the load on the engine E (load for driving the compressor  10 ) is reduced. Thus, for example, the fuel economy of the engine E and the acceleration performance of the vehicle are improved.  
         [0054]     As shown in FIGS.  1  to  3 , a clutch  26  is located between the guiding member  19  and the swash plate  24 . The clutch  26  switches between an engaged state to cause the guiding member  19  and the swash plate  24  to rotate integrally, and a disengaged state to permit the guiding member  19  and the swash plate  24  to rotate relative to each other. The clutch  26  will now be described.  
         [0055]     The clutch  26  is a cone clutch, which is a friction clutch. The clutch  26  includes a driving clutch surface  26   a  and a driven clutch surface  26   b . The driving clutch surface  26   a  is located on an outer circumferential surface  43  of the boss  19   a  of the guiding member  19  and forms part of the outer circumferential surface  43 . The driven clutch surface  26   b  is located on an inner circumferential surface  41  of the boss receiving hole  24   a  of the swash plate  24  and forms part of the inner circumferential surface  41 .  
         [0056]     The driving clutch surface  26   a  is adjacent (connected) to the constant diameter portion  43   a  at a front portion as viewed in a direction along the central axis M of the boss  19   a  (the proximal portion of the boss  19   a ). The driving clutch surface  26   a  is shaped as a cone with a central axis coinciding with the central axis M. Specifically, a section of the driving clutch surface  26   a  that is closer to the constant diameter portion  43   a  (a section corresponding to the distal end of the boss  19   a ) has a smaller diameter. A section of the driving clutch surface  26   a  that corresponds to the proximal portion of the boss  19   a  has a larger diameter. The boundary between the driving clutch surface  26   a  and the constant diameter portion  43   a  is machined to form a concave surface so that the constant diameter portion  43   a  is smoothly connected to the driving clutch surface  26   a . This prevents stress being concentrated on the boundary.  
         [0057]     The driven clutch surface  26   b  is adjacent (connected) to the constant diameter portion  41   a  at a front portion as viewed in a direction along the central axis M of the swash plate  24  (the proximal portion of the boss  19   a ). The driven clutch surface  26   b  is shaped as a cone with a central axis coinciding with the central axis M. Specifically, a section of the driven clutch surface  26   b  that is closer to the constant diameter portion  41   a  (a section corresponding to the distal end of the boss  19   a ) has a smaller diameter. A section of the driven clutch surface  26   b  that corresponds to the proximal portion of the boss  19   a  has a larger diameter. The boundary between the driven clutch surface  26   b  and the constant diameter portion  41   a  is machined to be a convex surface so that the boundary does not contact the outer circumferential surface  43  in its edge.  
         [0058]     To permit the clutch  26  to operate in a favorable manner (for example, to provide the clutch  26  in a narrow space between the boss  19   a  and the boss receiving hole  24   a  while securing a sufficient clutch capacity), the angle α (see  FIG. 2 ) of the driving clutch surface  26   a  and the driven clutch surface  26   b  relative to the central axis M is set preferably in a range between 5° and 20°, more preferably in a range between 10° and 15°.  
         [0059]     When the swash plate  24  slides relative to the guiding member  19  in a direction along the central axis M, the driven clutch surface  26   b  of the clutch  26  approaches or separates from the driving clutch surface  26   a . A state in which the driven clutch surface  26   b  contacts the driving clutch surface  26   a  is the engaged state of the clutch  26 . When the clutch  26  is engaged, rotational force of the guiding member  19  is transmitted to the swash plate  24  substantially at 100%, which causes the swash plate  24  to rotate integrally with the guiding member  19  (see  FIG. 2 ).  
         [0060]     A state in which the driven clutch surface  26   b  is away from the driving clutch surface  26   a  is the disengaged state of the clutch  26 . When the clutch  26  is disengaged, little or no rotational force of the guiding member  19  is transmitted to the swash plate  24 , which permits the swash plate  24  and the guiding member  19  to rotate relative to each other. In other words, the swash plate  24  slips on the guiding member  19  (see  FIG. 3 ).  
         [0061]     That is, the clutch  26  is switched from the disengaged state of  FIG. 3  to the engaged state of  FIG. 2  when the swash plate  24  slides on the guiding member  19  in one direction (towards the limiting surface  19   c ) along the central axis M. Also, the clutch  26  is switched from the engaged state of  FIG. 2  to the disengaged state of  FIG. 3  when the swash plate  24  slides on the guiding member  19  in the other direction (towards the cylinder block  12 ) along the central axis M.  
         [0062]     When the clutch  26  is switched from the disengaged state to the engaged state, the swash plate  24  is slid by compression reaction force CS acting on the swash plate  24  through the pistons  29  as refrigerant gas is compressed. Also, when the clutch  26  is switched from the engaged state to the disengaged state, the swash plate  24  is slid by force (urging force) of the coil springs  27  located between the guiding member  19  and the swash plate  24 . Hereinafter, springs  27  and the structure including the spring  27  will be described.  
         [0063]     As shown in  FIG. 2 , spring holes  46  (only one is shown in  FIG. 2 ) for accommodating the spring  27  are formed in the limiting surface  19   c  of the guiding member  19 . The transverse cross section of the spring hole  46  is substantially circular. The axis of the circle of the hole  46  is parallel to the central axis M of the guiding member  19 . As shown in  FIG. 5 , a portion on the swash plate  24  that corresponds to a top dead center is denoted as TDC, which portion coincides with a center point of the curvature of the shoes  31  corresponding to the piston  29  at the top dead center. An imaginary plate containing the top dead center portion TDC and the central axis M of the swash plate  24  is denoted as an imaginary plane H. One of the spring holes  46  (spring hole  46 A) is located in one of the areas of the swash plate  24  divided by the imaginary plane H (a right area as viewed in the drawing). The other spring hole  46  (spring hole  46 B) is located in the other area (left area as viewed in the drawing). The spring hole  46 A and the spring hole  46 B are symmetrical with respect to the imaginary plane H.  
         [0064]     Specifically, the state of  FIG. 5 , or the state when the limiting surface  19   c  is viewed from the front, is regarded as a dial of a clock (in other words, the central axis M is regarded as the axis of a clock). Sections of lines at intersections of the annular limiting surface  19   c  and the imaginary plane H are referred to as line sections X 1 , X 2 . The line section X 1 , which is located at the side of the top dead center portion TDC of the swash plate  24  (upper side as viewed in the drawing), corresponds to twelve o&#39;clock. The line section X 2 , which is located at a side opposite from the top dead center portion TDC (lower side as viewed in the drawing), corresponds to six o&#39;clock.  
         [0065]     The spring hole  46 A (specifically the center of the opening of the spring hole  46 A) is located in a range from twelve o&#39;clock to three o&#39;clock in the clockwise direction on the limiting surface  19   c  (preferably in a range from one o&#39;clock to two o&#39;clock). Also, the spring hole  46 B (specifically the center of the opening of the spring hole  46 B) is located in a range from nine o&#39;clock to twelve o&#39;clock in the clockwise direction on the limiting surface  19   c  (preferably in a range from ten o&#39;clock to eleven o&#39;clock). That is, the springs  27  accommodated in the spring holes  46 A,  46 B are arranged to apply force in a concentrated and intense manner to an area near the top dead center portion TDC on the swash plate  24  around the central axis M. At the area, the compression reaction force CS acts strongly.  
         [0066]     As shown in  FIG. 2 , a ball  47 , which functions as bearing and a thrust bearing, is held by each of the spring holes  46 A,  46 B. Each ball  47  is urged toward the swash plate  24  by the force of the corresponding spring  27 . Part of each ball  47  projects from the spring hole  46 A,  46 B, or from the limiting surface  19   c , toward the swash plate  24 . Around the opening of the boss receiving hole  24   a  on the front face of the swash plate  24 , a roll groove  48  is formed to correspond to the limiting surface  19   c  of the guiding member  19 . The roll groove  48  is formed annular around the central axis M. The transverse cross section of the roll groove  48  is substantially semicircular.  
         [0067]     As described above, each ball  47 , which is urged by the corresponding spring  27 , contacts the inner surface of the toll groove  48 . That is, each spring  27  applies its force to the swash plate  24  through the corresponding ball  47  and the inner surface of the roll groove  48 . Also, when the guiding member  19  and the swash plate  24  rotate relative to each other in the disengaged state of the clutch  26 , the balls  47  roll on the inner surface of the roll groove  48  along the circumferential direction of the roll groove  48 . That is, the balls  47  are located between the springs  27  and the swash plate  24 , which rotates relative to the guiding member  19  in the disengaged state of the clutch  26 .  
         [0068]     Switching of the clutch  26  will now be described.  
         [0069]     In a state shown in  FIG. 3 , that is, when the compressor  10  is in the OFF operation, the stroke of the pistons  29  is zero or a value close to zero. Therefore, the compression reaction force. CS applied to the swash plate  24  through the pistons  29  is less than a predetermined value. In this state, the force of the springs  27  acts dominantly to determine the position of the swash plate  24  relative to the guiding member  19  (the boss  19   a ) in a direction along the central axis M.  
         [0070]     Therefore, the swash plate  24  is moved by the force of the springs  27  to a position contacting the snap ring  25  against the compression reaction force CS, the clutch  26  is caused to the disengaged state. In this state, the swash plate  24  and the guiding member  19  rotate relative to each other. That is, the swash plate  24  slips on the guiding member  19 , and the rotation speed of the swash plate  24  is lower than the rotation speed of the guiding member  19 . As a result, the mechanical loss between the swash plate  24  and the shoes  31  is further reduced. Accordingly, the load of the compressor  10 , which acts on the engine E, is further reduced.  
         [0071]     Since the load of the compressor  10  on the engine E is further reduced, the fuel economy of the engine E and the acceleration performance of the vehicle are further improved. Also, since the generation of frictional heat at contacting portions of the swash plate  24  and the shoes  31  is suppressed, a lip seal for sealing an end portion of the drive shaft  17  and a bearing supporting the shaft end portion are prevented from being degraded by heat. This improves the reliability of the compressor  10 .  
         [0072]     In the OFF operation state of the compressor  10 , the swash plate  24  is slightly inclined relative to the axis L of the drive shaft  17 , and the pistons  29  are reciprocated with a very small stroke. This causes refrigerant gas to be discharged to the discharge chamber  33  from the compression chambers  30 . The refrigerant gas is then supplied to the crank chamber  16  from the discharge chamber  33  through the supply passage  39  and the control valve  40 . The refrigerant gas in the crank chamber  16  is supplied to the suction chamber  32  through the bleed passage  38 , and then drawn into the compression chambers  30  from the suction chamber  32 . In this manner, when the compressor  10  is in the OFF operation, refrigerant gas circulates within the compressor  10  and passes through the control valve  40 .  
         [0073]     If the opening degree of the control valve  40  is less than the fully opened state (by starting the supply of electricity to the electromagnetic actuator  40   a  of the control valve  40 ), the flow rate of refrigerant gas supplied to the crank chamber  16  is decreased. Accordingly, the compression reaction force CS is relatively increased. Also, when the flow rate of refrigerant gas supplied to the crank chamber  16  decreases, the inclination angle of the swash plate  24  is increased, so that the displacement of the compressor  10  is increased from the minimum displacement (leaves the OFF operation and shifts to “an ON operation”). Therefore, the compression reaction force CS applied to the swash plate  24  is greater than the predetermined value. In this state, the compression reaction force CS acts dominantly to determine the position of the swash plate  24  relative to the guiding member  19  (the boss  19   a ) in a direction along the central axis M.  
         [0074]     Accordingly, the swash plate  24  is moved by the compression reaction force CS to a position where the driven clutch surface  26   b  contacts the driving clutch surface  26   a  against to the force of the springs  27 . This engages the clutch  26 , and the swash plate  24  and the guiding member  19  start rotate integrally. If the opening degree of the control valve  40  is changed in a state where the swash plate  24  and the guiding member  19  rotate integrally, that is, in the on operating state of the compressor  10 , the inclination angle of the guiding member  19  and the swash plate  24  is changed.  
         [0075]     At this time, since centrifugal force acts on the guiding member  19  and the swash plate  24  which rotates integrally with the guiding member  19 , the inertial force due to reciprocation of the pistons  29  is cancelled. This permits the inclination angle of the guiding member  19  and the swash plate  24  to be easily changed. Particularly, when the compressor  10  is operating at a high speed (when the drive shaft  17  is rotating at a high speed), that is, the inertial force of the pistons  29  is great, the advantage of facilitating changes in the inclination angle of the guiding member  19  and the swash plate  24  is remarkable.  
         [0076]     When the control valve  40  is fully opened during the ON operation of the compressor  10  (when the supply of electricity to the electromagnetic actuator  40   a  of the control valve  40  is stopped), the compressor  10  shifts to the OFF operation and the clutch  26  is disengaged.  
         [0077]     The above embodiment provides the following advantages.  
         [0078]     (1) As described above, engaging the clutch  26  improves the displacement control performance of the compressor  10 , and disengaging the clutch  26  reduces the mechanical loss between the swash plate  24  and the shoes  31 . Therefore, when, for example, designing the compressor  10 , the dilemma accompanying the prior art is avoided.  
         [0079]     (2) The boss  19   a  of the guiding member  19  supports the swash plate  24  at the boss receiving hole  24   a . The swash plate  24 , which receives the boss  19   a  at the boss receiving hole  24   a  formed in the center portion of the swash plate  24 , is stably supported by the boss  19   a . This permits the swash plate  24  to smoothly slide in a direction along the central axis M.  
         [0080]     (3) When the clutch  26  is switched from the disengaged state to the engaged state, the swash plate  24  is slid by compression reaction force CS acting on the swash plate  24  through the pistons  29  as refrigerant gas is compressed. Therefore, the clutch  26  is automatically switched from the engaged state to the disengaged state and from the disengaged state to the engaged state according to the operating condition of the compressor  10 . Therefore, no special external control means for directly controlling the clutch  26  is required. This simplifies the structure of the compressor  10 .  
         [0081]     (4) When the clutch  26  is switched from the engaged state to the disengaged state, the swash plate  24  is slid by the force of the springs  27 . Therefore, when in the engaged state, the clutch  26  is reliably switched to the disengaged state by the force of the springs  27 .  
         [0082]     (5) The balls. (bearing, thrust bearings)  47  are located between the guiding member  19  and the swash plate  24 . The balls  47  support the swash plate  24  such that the swash plate  24  is rotatable relative to the guiding member  19 . Therefore, when the guiding member  19  and the swash plate  24  rotate relative to each other, that is, when the clutch  26  is in the disengaged state, the mechanical loss between the guiding member  19  and the swash plate  24  is further reduced.  
         [0083]     (6) The balls  47  (bearing, a thrust bearing) are located between the springs  27  and the swash plate  24 , which rotates in the disengaged state of the clutch  26 . Therefore, when the guiding member  19  and the swash plate  24  rotate relative to each other, that is, when the clutch  26  is in the disengaged state, the mechanical loss between the swash plate  24  and the springs  27  is further reduced. This further reduces the load of the compressor  10  acting on the engine E.  
         [0084]     (7) The roll groove  48 , in which the balls  47  roll, is formed on the swash plate  24 . The roll groove  48  has a semicircular transverse cross-section. Therefore, compared to a case where balls roll on a flat surface of a swash plate, the surface pressure between each ball  47  and a section of the roll groove  48  contacting the ball  47  is small. As a result, the mechanical loss between the swash plate  24  and the springs  27  is further effectively reduced. Accordingly, the load of the compressor  10 , which acts on the engine E, is further reduced.  
         [0085]     (8) The springs  27  are configured to apply force in a concentrated and intense manner to an area near the top dead center portion TDC on the swash plate  24  around the central axis M. That is, in the area of the swash plate  24  near the top dead center portion TDC receives the compression reaction force CS in a concentrated and intense manner. Therefore, when the swash plate  24  slides as the clutch  26  is switched from the engaged state to the disengaged state, the movement of the top dead center portion TDC of the swash plate  24  is hindered by the strong compression reaction force CS compared to other portions. Also, when the swash plate  24  slides as the clutch  26  is switched from the disengaged state to the engaged state, the top dead center portion TDC of the swash plate  24  is excessively moved by the strong compression reaction force CS compared to other portions. That is, since the compression reaction force CS acts on the top dead center portion TDC in a concentrated manner, the inclination of the swash plate  24  relative to the central axis M is facilitated when the swash plate  24  is sliding.  
         [0086]     Therefore, if the force of the springs  27  is controlled to act on the top dead center portion TDC in a concentrated manner against the strong compression reaction force CS as in the above embodiment, the sliding attitude of the swash plate  24  is stabilized (the swash plate  24  is prevented from being inclined relative to the central axis M). This permits the swash plate  24  to smoothly slide, and thus permits the clutch  26  to smoothly and reliably switch the state.  
         [0087]     (9) The clutch  26  is in the engaged state when the driving clutch surface  26   a  formed on the guiding member  19  contacts the driven clutch surface  26   b  formed on the swash plate  24 . Therefore, no medium connecting the driving clutch surface and the driven clutch surface is required (for example, an electromagnetic powder clutch uses powder as the medium). Therefore, the structure of the clutch  26  is simplified and its size is reduced.  
         [0088]     (10) The driving clutch surface  26   a  is formed on the outer circumferential surface  43  of the boss  19   a , and the driven clutch surface  26   b  is formed on the inner circumferential surface  41  of the boss receiving hole  24   a . That is, part of the outer circumferential surface  43  of the boss  19   a  is used as the driving clutch surface  26   a , and part of the inner circumferential surface  41  of the boss receiving hole  24   a  is used as the driven clutch surface  26   b . Thus, for example, when compared to a case where the driving clutch surface  26   a  and the driven clutch surface  26   b  are located outside of the boss receiving hole  24   a , that is, when compared to a case where the driving clutch surface  26   a  and the driven clutch surface  26   b  are independently formed, the structure of the guiding member  19  and the swash plate  24  is simplified and the size is reduced.  
         [0089]     (11) The clutch  26  is a friction clutch. Thus, the shapes of the driving clutch surface  26   a  and the driven clutch surface  26   b  are simple, which simplifies the structure of the clutch  26 . Also, a little sliding of the swash plate  24  relative to the guiding member  19  permits the clutch  26  to switch between the engaged state and the disengaged state. Therefore, the sliding of the swash plate  24  does not adversely affects the efficiency of the compressor  10 . For example, the top clearance (the distance between a piston  29  at the top dead center and the valve assembly  15 ), which degrades the efficiency of the compressor  10  when the top clearance is increased, is not increased. Further, the size of the compressor  10  is reduced.  
         [0090]     During the ON operation of the compressor  10 , adhesion between the swash plate  24  and the shoes  31  increases the torque required for rotating the swash plate  24 . In this case, the clutch  26 , which is a friction clutch, exerts a torque limiting function and slips the driving clutch surface  26   a  and the driven clutch surface  26   b  on each other. Therefore, the swash plate  24  is not forced to rotate when the swash plate  24  receives an excessive torque, so the belt and other members of the power transmission mechanism PT are not damaged. Therefore, the power transmission mechanism PT does not need to have a dedicated torque limiter, which simplifies the structure of the compressor  10 .  
         [0091]     (12) The clutch  26  is a cone clutch. Even if the radial size of the cone clutch is reduced, a large area is secured for the driving clutch surface  26   a  and the driven clutch surface  26   b . Therefore, as in the above embodiment, it is possible to form the driving clutch surface  26   a  on the outer circumferential surface  43  of the boss  19   a  and the driven clutch surface  26   b  on the inner circumferential surface  41  of the boss receiving hole  24   a , and to secure a great capacity of the clutch  26 .  
         [0092]      FIG. 6  illustrates a compressor according to a second embodiment, which is a modification of the first embodiment.  
         [0093]     Hereafter, only the components different from those of the compressor according to the first embodiment are explained. The same, equivalent, or similar components are given the same numbers and detailed explanations are omitted.  
         [0094]     The compressor of the second embodiment is the same as the compressor of the first embodiment except that the compressor of the second embodiment has one spring hole  46 , one spring  27 , and one ball  47 . The spring hole  46  is arranged such that the central axis of the spring hole  46  is located on the imaginary plane H. That is, the spring hole  46  (specifically, the center of the opening of the spring hole  46 ) corresponds to twelve o&#39;clock on the limiting surface  19   c  of the guiding member  19 .  
         [0095]     The spring hole  46  may be misaligned clockwise or counterclockwise from twelve o&#39;clock. Particularly, if the drive shaft  17  rotates clockwise as viewed in  FIG. 6 , the spring hole  46 , or the spring  27  accommodated in the spring hole  46 , is preferably provided in a range from twelve o&#39;clock to three o&#39;clock in the clockwise direction, where the compression reaction force CS strongly acts (more preferably in a range from one o&#39;clock to two o&#39;clock in the clockwise direction). In contrast, if the drive shaft  17  rotates counterclockwise as viewed in  FIG. 6 , the spring hole  46 , or the spring  27  accommodated in the spring hole  46 , is preferably provided in a range between nine o&#39;clock to twelve o&#39;clock in the clockwise direction, where the compression reaction force CS strongly acts (preferably in a range from ten o&#39;clock to eleven o&#39;clock in the clockwise direction).  
         [0096]     The compressor according to the second embodiment has the same advantages (1) to (12) of the compressor according to the first embodiment. In addition, the compressor according to the second embodiment provides the following advantage (13).  
         [0097]     (13) Since the number of each of the spring hole  46 , the spring  27 , and the ball (bearing, thrust bearing)  47  is one, the structure of the compressor  10  is simplified.  
         [0098]      FIG. 7  illustrates a compressor according to a third embodiment, which is a modification of the first embodiment.  FIG. 8  illustrates a compressor according to a fourth embodiment, which is a modification of the first embodiment.  
         [0099]     The compressors according to the third and fourth embodiments are the same as the compressor according to the first embodiment except that a radial bearing  71  is located between the guiding member  19  and the swash plate  24 . The radial bearing  71  supports the swash plate  24  such that the swash plate  24  rotate relative to the guiding member  19  around the central axis M and slides in a direction along the central axis M.  
         [0100]     Particularly, in the compressor according to the third embodiment ( FIG. 7 ), the radial bearing  71  is a linear ball bearing. The linear ball bearing includes an outer ring  72  and a ball holder  73 . The outer ring  72  is fixed to the constant diameter portion  41   a  of the boss receiving hole  29   a , for example, by press fitting. The ball holder  73  is located inside of the outer ring  72  and holds two or more circumferential rows of balls  73   a . Each ball  73   a  in the ball holder  73  contacts the inner circumferential surface of the outer ring  72  and the constant diameter portion  43   a  of the outer circumferential surface  43  of the boss  19   a . Free rolling of the balls  73   a  permits the guiding member  19  and the swash plate  24  to rotate relative to each other and slide in a direction along the central axis M relative to each other.  
         [0101]     In the compressor according to the fourth embodiment ( FIG. 8 ), the radial bearing  71  is a plain bearing. The plain bearing includes an outer cylinder  75  and an inner cylinder  76 . The outer cylinder  75  is fixed to the constant diameter portion  41   a  of the inner circumferential surface  41  of the boss receiving hole  24   a , for example, by press fitting. The inner cylinder  76  is fixed to the constant diameter portion  43   a  of the outer circumferential surface  43  of the boss  19   a , for example, by press fitting. The outer cylinder  75  and the inner cylinder  76  are both made of material having a superior sliding property. Relative rotation and movement in the axial direction of the outer cylinder  75  and the inner cylinder  76 , or relative rotation and relative sliding in a direction along the central axis M of the guiding member  19  and the swash plate  24  are smoothly performed.  
         [0102]     The compressors according to the third and fourth embodiments have the same advantages (1) to (12) of the compressor according to the first embodiment. In addition, the compressors according to the third and fourth embodiments provide the following advantage (14).  
         [0103]     (14) The radial bearing  71  further reduces the mechanical loss between the guiding member  19  and the swash plate  24  when the clutch  26  is in the disengaged state. For example, the load of the compressor that acts on the engine E is further reduced. Further, when the clutch  26  is switched, the swash plate  24  is slid smoothly. Further, when the clutch  26  performs torque limiting or is in the disengaged state, the outer circumferential surface  43  of the boss  19   a  and the inner circumferential surface  41  of the boss receiving hole  24   a  are prevented from being worn.  
         [0104]      FIG. 9  illustrates a compressor according to a fifth embodiment, which is a modification of the first embodiment.  
         [0105]     The compressor of the fifth embodiment is the same as the compressor of the first embodiment except that the compressor of the fifth embodiment has oil supply portion  77 . The oil supply portion  77  supplies lubricant oil (refrigeration oil) to a space between the guiding member  19  and the swash plate  24  when the guiding member  19  and the swash plate  24  rotate relative to each other. The oil supply portion  77  will now be described.  
         [0106]     The oil supply portion  77  has an oil supply hole  78  formed in the guiding member  19  and a reservoir groove  79  formed on the inner circumferential surface  41  of the boss receiving hole  24   a  of the swash plate  24 . The reservoir groove  79  is linearly formed with a curved part, and extends substantially along the central axis M (in a direction frontward and backward). The reservoir groove  79  extends from the constant diameter portion  41   a  to the driven clutch surface  26   b . The oil supply hole  78  has an outlet  78   a  of lubricant oil. The outlet  78   a  opens to (is connected to) the reservoir groove  79  on the outer circumferential surface  43  of the boss  19   a . The oil supply hole  78  has an inlet  78   b  of lubricant oil. The inlet  78   b  opens to (is connected to) the crank chamber  16  at a position that is closer to the drive shaft  17  than the outlet  78   a  on the rear side of the guiding member  19 . Specifically, the inlet  78   b  opens to the crank chamber  16  at a position on the inner circumferential surface of the boss  19   a.    
         [0107]     Therefore, lubricant oil drawn to the inlet  78   b  of the oil supply hole  78  from the crank chamber  16  is moved to the outlet  78   a , which is farther from the drive shaft  17  than the inlet  78   b , by centrifugal force generated as the guiding member  19  rotates. The lubricant oil is then supplied to the reservoir groove  79 . The lubricant oil supplied to the reservoir groove  79  spreads (flows) in the groove  79  substantially along the central axis M. Further, as the guiding member  19  and the swash plate  24  rotate relative to each other, the lubricant oil spreads in a wide area between the outer circumferential surface  43  of the boss  19   a  and the inner circumferential surface  41  of the boss receiving hole  24   a.    
         [0108]     The compressor according to the fifth embodiment has the same advantages (1) to (12) of the compressor according to the first embodiment. In addition, the compressor according to the fifth embodiment provides the following advantages (15) to (19).  
         [0109]     (15) The oil supply portion  77  supplies lubricant oil to a space between the guiding member  19  and the swash plate  24  when the guiding member  19  and the swash plate  24  rotate relative to each other. Therefore, when the guiding member  19  and the swash plate  24  rotate relative to each other, that is, when the clutch  26  is performing torque limiting or in the disengaged state, the mechanical loss between the guiding member  19  and the swash plate  24  is further reduced. Further, for example, the load of the compressor that acts on the engine E is reduced.  
         [0110]     (16) The oil supply portion  77  supplies lubricant oil to a space between the driving clutch surface  26   a  and the driven clutch surface  26   b  when the guiding member  19  and the swash plate  24  rotate relative to each other. Therefore, when the clutch  26  is performing torque limiting or in the disengaged state, the mechanical loss between the driving clutch surface  26   a  and the driven clutch surface  26   b  is further reduced. Particularly, when the clutch  26  is performing torque limiting, wear of the driving clutch surface  26   a  and the driven clutch surface  26   b  is suppressed.  
         [0111]     (17) The oil supply portion  77  supplies lubricant oil to a space between the outer circumferential surface  43  of the boss  19   a  and the inner circumferential surface  41  of the boss receiving hole  24   a  when the guiding member  19  and the swash plate  24  rotate relative to each other. Therefore, when the guiding member  19  and the swash plate  24  rotate relative to each other, that is, when the clutch  26  is performing torque limiting or in the disengaged state, the mechanical loss between the outer circumferential surface  43  of the boss  19   a  and the inner circumferential surface  41  of the boss receiving hole  24   a  is further reduced. Further, the load of the compressor acting on the engine E is reduced. Further, when the clutch  26  is switched, the swash plate  24  is slid smoothly.  
         [0112]     (18) The oil supply portion  77  has the oil supply hole  78 . The oil supply hole  78  uses centrifugal force generated when the guiding member  19  rotates to move lubricant oil from the inlet  78   b  to the outlet  78   a . Since lubricant oil is supplied by using centrifugal force, the structure of the oil supply portion  77  is simplified to have only a passage.  
         [0113]     (19) The oil supply portion  77  has the reservoir groove  79  to which lubricant oil is supplied from the oil supply hole  78 . The reservoir groove  79  is formed on the inner circumferential surface  41  of the boss receiving hole  24   a  to extend substantially along the central axis M. This permits lubricant oil to spread in an wide area between the outer circumferential surface  43  of the boss  19   a  and the inner circumferential surface  41  of the boss receiving hole  24   a . Therefore, mechanical loss between the outer circumferential surface  43  of the boss  19   a  and the inner circumferential surface  41  of the boss receiving hole  24   a  is further effectively reduced.  
         [0114]      FIG. 10  illustrates a compressor according to a sixth embodiment, which is a modification of the first embodiment.  FIG. 11  illustrates a compressor according to a seventh embodiment, which is a modification of the first embodiment.  
         [0115]     The compressors according to the sixth and seventh embodiments are the same as the compressor according to the first embodiment except that the clutch  26 , the driving clutch surface  26   a , and the driven clutch surface  26   b  are located outside of the boss receiving hole  24   a . Therefore, almost the entire inner circumferential surface  41  of the boss receiving hole  24   a  is the constant diameter portion  41   a , and almost the entire outer circumferential surface  43  of the boss  19   a  is the constant diameter portion  43   a.    
         [0116]     In the sixth embodiment ( FIG. 10 ), a cylindrical surface forming projection  80  projects from the rear side of the guiding member  19 . The surface forming projection  80  is located radially outside of the limiting surface  19   c . The inner circumferential surface of the surface forming projection  80  functions as the driving clutch surface  26   a . A driven clutch surface  26   b  is formed in a peripheral portion of the front side of the swash plate  24  that is radially outside of the roll groove  48 .  
         [0117]     In the seventh embodiment ( FIG. 11 ), a cylindrical surface forming projection  81  projects from the front side of the swash plate  24 . The surface forming projection  81  is located radially outside of the roll groove  48 . The inner circumferential surface of the surface forming projection  81  functions as the driven clutch surface  26   b . A driving clutch surface  26   a  is formed in a peripheral portion of the rear side of the guiding member  19  that is radially outside of the limiting surface  19   c.    
         [0118]     The compressors according to the sixth and seventh embodiments have the same advantages (1) to (9), (11), and (12) of the compressor according to the first embodiment. In addition, the compressors according to the sixth and seventh embodiments provide the following advantage (20).  
         [0119]     (20) The driving clutch surface  26   a  and the driven clutch surface  26   b  are located outside of the boss receiving hole  24   a . That is, the driving clutch surface  26   a  is formed separately from the outer circumferential surface  43  of the boss  19   a , and the driven clutch surface  26   b  is formed separately from the inner circumferential surface  41  of the boss receiving hole  24   a.    
         [0120]     Therefore, the structure of the clutch  26  (for example, the shapes and the measurements of the driving clutch surface  26   a  and the driven clutch surface  26   b ) is not significantly affected by the arrangement, the measurements, and the shapes of the boss  19   a  (the outer circumferential surface  43 ) and the boss receiving hole  24   a  (the inner circumferential surface  41 ). This adds to the flexibility of design of the clutch  26 . Accordingly, a wide area can be easily secured for the area of the driving clutch surface  26   a  and the driven clutch surface  26   b . This increases the capacity of the clutch  26 . Increasing the capacity of the clutch  26  reliably suppresses the slipping between the guiding member  19  and the swash plate  24  when the clutch  26  is engaged. This, for example, permits the compressor to maintain a favorable displacement control performance.  
         [0121]     Since the driving clutch surface  26   a  and the driven clutch surface  26   b  are located outside the boss receiving hole  24   a , substantially the entire inner circumferential surface  41  of the boss receiving hole  24   a  is used as the constant diameter portion  41   a  and substantially the entire outer circumferential surface  43  of the boss  19   a  is used as the constant diameter portion  43   a . Therefore, during the OFF operation of the compressor  10 , the swash plate  24  is stably supported by the boss  19   a . Thus, for example, noise and vibration due to chattering of the swash plate  24  and undesirable fluctuations of stroke of the pistons  29  are suppressed. Further, when the clutch  26  is switched, the swash plate  24  is slid smoothly. That is, switching of the clutch  26  is reliably and smoothly performed.  
         [0122]      FIG. 12  illustrates a compressor according to an eighth embodiment, which is a modification of the first embodiment.  
         [0123]     The compressor  10  according to the eight embodiment is the same as the compressor  10  according to the first embodiment except for a first rotation speed sensor (swash plate rotation speed sensor)  85  for detecting the rotation speed of the swash plate  24 , a second rotation speed sensor  86  for detecting the rotation speed of the guiding member  19 , and an indicator  87 , which is, for example, a lamp.  
         [0124]     The first rotation speed sensor  85  includes a magnet  85   a  embedded in the outer edge of the swash plate  24  and a pickup coil  85   b  that is attached to the housing  11  and located in the crank chamber  16 . When the magnet  85   a  passes by the pickup coil  85   b , and a magnetic flux is generated, the pickup coil  85   b  generates pulses corresponding to the rotation speed of the swash plate  24  due to electromagnetic induction, and sends the pulses to the control computer  60 .  
         [0125]     The second rotation speed sensor  86  is connected to a computer (not shown) for controlling, for example, the engine E. The computer uses the second rotation speed sensor  86  to obtain the rotation speed of the engine E (output shaft), in other words, to obtain a physical quantity related to the rotation speed of the guiding member  19 . The computer for controlling the engine E sends information regarding the rotation speed detected by the second rotation speed sensor  86  to the control computer  60 , which is communicatively connected to the engine computer. The control computer  60  obtains the rotation speed of the drive shaft  17 , or the rotation speed of the guiding member  19 , based on the rotation speed of the engine E and a predetermined pulley ratio of the power transmission mechanism PT.  
         [0126]     The control computer  60  judges whether the rotation speed of the swash plate  24  is lower than the rotation speed of the guiding member  19  (for example, whether the rotation speed of the swash plate  24  is equal to or lower than a predetermined value) when the clutch  26  is engaged, that is, when the computer  60  is commanding the drive circuit  61  to supply electricity to the control valve  40 . This process corresponds to a slip judging portion. In the engaged state of the clutch  26 , the rotation speed of the swash plate  24  falls below the rotation speed of the guiding member  19 , that is, the swash plate  24  slips relative to the guiding member  19  (the clutch  26  is unable to transmit power at 100%) when the swash plate  24  and the shoes  31  adhere to each other and the torque required for rotating the swash plate  24  is unduly increased.  
         [0127]     When the outcome of the above judgment is positive, the control computer  60  commands the drive circuit  61  to stop the supply of electricity to the control valve  40 , thereby disengaging the clutch  26 . This process corresponds to clutch controller. This frees the driving clutch surface  26   a  and the driven clutch surface  26   b  from a harsh sliding environment, and thus reduces deterioration due to wearing. Also, the displacement of the compressor  10  is minimized when the clutch  26  is disengaged. In this state, the stroke of the pistons  29  is minimized, and the torque required for wobbling the swash plate  24  is decreased to a low level. Therefore, the load on the guiding member  19  when the swash plate  24  wobbles is reduced.  
         [0128]     When the clutch  26  is disengaged, that is, when the computer  60  is commanding the drive circuit  61  to stop supplying electricity to the control valve  40 , the control computer  60  judges whether the rotation speed of the swash plate  24  is more than a reference rotation speed that is stored in a RAM in advance. This process corresponds to an abnormality judging portion. In the disengaged state of the clutch  26 , the rotation speed of the swash plate  24  surpasses the reference rotation speed if, for example, there is an abnormality in the clutch  26  and the transmission of power is not completely stopped. When the result of the judgment is positive, the control computer  60  stores an occurrence of the abnormality in the RAM (this process corresponds to memory portion) and causes the indicator (lamp)  87 , which has been off, to light up or blink (alternatively, turns off the lighting indicator  87  or causes the lighting indicator  87  to blink or change the lighting color).  
         [0129]     The compressor according to the eighth embodiment has the same advantages (1) to (12) of the compressor according to the first embodiment. In addition, the compressor according to the eighth embodiment provides the following advantages (21) and (22).  
         [0130]     (21). When the clutch  26  starts slipping while being engaged, the computer  60  disengages the clutch  26 . Therefore, the guiding member  19  does not need to rotate the swash plate  24 . This prevents rotation of the guiding member  19 , that is, rotation of the drive shaft  17 , from being locked. Therefore, the power transmission mechanism. PT does not need to have a dedicated torque limiter, which simplifies the structure of the compressor  10 .  
         [0131]     Particularly in this embodiment, the clutch  26  is disengaged when the displacement of the compressor  10  is minimized. Therefore, the torque required for wobbling the swash plate  24  is reduced when the swash plate  24  is adhered to the shoes  31 . This further prevents adhesion between the swash plate  24  and the shoes  31  from affecting the guiding member  19 . Also, since the swash plate  24  is scarcely caused to wobble or not caused to wobble at all, adhesion between the swash plate  24  and the shoes  31  is prevented from developing to a complete locked state.  
         [0132]     (22) When the clutch  26  is disengaged, the control computer  60  stores the state where the rotation speed of the swash plate  24  is more than the reference rotation speed, or where there is an abnormality in the clutch  26 . Therefore, a user or a mechanic can be easily informed of the abnormality by reading out the stored information related to the abnormality from the control computer  60  by using a service computer. Particularly in this embodiment, the indicator  87  shows the occurrence of the abnormality, a user or a mechanic can easily find an abnormality of the compressor  10  without using the service computer. That is, the indicator  87  is regarded as another memory portion other than the RAM incorporated in the control computer  60 . From a different point of view, the indicator  87  is regarded as notifying portion for notifying a user or a mechanic of an abnormality.  
         [0133]      FIG. 13  illustrates a compressor according to a ninth embodiment, which is a modification of the first embodiment.  
         [0134]     The compressor of the ninth embodiment is the same as the compressor of the first embodiment except that the spring holes  46 , the springs  27 , and the balls  47  are omitted. A disc spring  90  is located around the proximal portion of the boss  19   a  and between the limiting surface  19   c  of the guiding member  19  and the front side of the swash plate  24 . The force of the spring  90  acts on the swash plate  24  substantially at equal magnitude around the central axis M.  
         [0135]     The compressor according to the ninth embodiment has the same advantages (1) to (4), and (9) to (12) of the compressor according to the first embodiment.  
         [0136]     If the force at a specific part of the spring  90 , particularly a part corresponding to the top dead center portion TDC, is made stronger than the force at the other portion (for example, by increasing the thickness of the specific part, or the measurement between the guiding member  19  and the swash plate  24 ), the same advantage as the advantage (8) is obtained.  
         [0137]      FIGS. 14 and 15  illustrate a compressor according to a tenth embodiment, which is a modification of the ninth embodiment. Hereafter, only the components different from those of the compressor according to the ninth embodiment are explained. The same, equivalent, or similar components are given the same numbers and detailed explanations are omitted.  
         [0138]     The compressor of the tenth embodiment is the same as the compressor of the ninth embodiment except that the spring  90  is omitted, and that the clutch  26  is changed to a dog clutch from the cone clutch. That is, the limiting surface  19   c  of the guiding member  19  functions as the driving clutch surface  26   a . The driving clutch surface  26   a  has driving projections  92  at a predetermined pitch. The front side of the swash plate  24  that faces the limiting surface  19   c  functions as the driven clutch surface  26   b . The driven clutch surface  26   b  has driven projections  93 , which are arranged at the same pitch as that of the driving projections  92 .  
         [0139]     When the swash plate  24  slides relative to the guiding member  19  in a direction along the central axis M, the driven clutch surface  26   b  is meshed with or separates from the driving clutch surface  26   a . Accordingly, the clutch  26  is switched between the engaged state and the disengaged state. The corners of the driving projections  92  and the driven projections  93  are curved so that there is no edges.  
         [0140]     The compressors according to the tenth embodiment has the same advantages (1) to (3), (9) of the compressor according to the first embodiment, and the advantage (20) of the compressors according to the sixth and seventh embodiments. In addition, the compressor according to the tenth embodiment provides the following advantage (23).  
         [0141]     (23) During the ON operation of the compressor  10 , the clutch  26 , which is a dog clutch, reliably maintains engagement, or connection, between the driving clutch surface  26   a  and the driven clutch surface  26   b  without causing slipping.  
         [0142]      FIG. 16  illustrates a compressor according to an eleventh embodiment, which is a modification of the ninth embodiment.  
         [0143]     The compressor of the ninth embodiment is the same as the compressor of the ninth embodiment except that the hinge mechanism  20  is changed from a pinless type to a pin type. That is, a pair of pin holes  94  are formed in the rotor  18 , and a pair of pins  95  project from the guiding member  19  to be received by the pin holes  94 . When the inclination angle of the guiding member  19  and the swath plate  24  is changed, the pins  95  slide in the pin holes  94 .  
         [0144]     The compressor according to the eleventh embodiment has the same advantages (1) to (4), and (9) to (12) of the compressor according to the first embodiment.  
         [0145]      FIGS. 17 and 18  illustrate a compressor according to a twelfth embodiment, which is a modification of the first embodiment.  FIG. 17  is an enlarged cross-sectional view illustrating a portion including a swash plate guiding member when a clutch is in the engaged state.  FIG. 18  is an enlarged cross-sectional view illustrating a portion including the guiding member when the inclination angle is minimized and the clutch is in a disengaged state.  
         [0146]     The compressor according to twelfth embodiment is different from the compressor according to the first embodiment in that a thrust bearing  96  is located between the swash plate  24  and the snap ring  25 . When the clutch  26  is in the disengaged state, the thrust bearing  96  rotatably supports the swash plate  24  relative to the snap ring  25 . Specifically, a bearing receiving recess  97  is on the rear side of the swash plate  24  around the opening of the boss receiving hole  24   a . The thrust bearing  96  is located in the bearing receiving recess  97 . As shown in  FIG. 17 , when the clutch  26  is in the engaged state, the thrust bearing  96  is separated from the snap ring  25 , and the thrust bearing  96  rotates integrally with the swash plate  24 . As shown in  FIG. 18 , when the clutch  26  is in the disengaged state, the driven clutch surface  26   b  of the swash plate  24  is separated from the driving clutch surface  26   a , and the thrust bearing  96  contacts the snap ring  25 . In this state, since the swash plate  24  rotates relative to the guiding member  19 , the swash plate  24  also rotates relative to the snap ring  25 . The thrust bearing  96  rotates with receiving a load that is equal to the force of the springs  27  acting on the swash plate  24  through the balls  47 .  
         [0147]     The compressor according to the twelfth embodiment has the same advantages (1) to (12) of the compressor according to the first embodiment. In addition, the compressor according to the twelfth embodiment provides the following advantage (24).  
         [0148]     (24) Since the thrust bearing  96  is located between the swash plate  24  and the snap ring  25 , mechanical loss between the swash plate  24  and the snap ring  25  when the clutch  26  is disengaged is further reduced.  
         [0149]     The invention may be embodied in the following forms. The modifications may be combined as long as they do not conflict with each other.  
         [0150]     The clutch  26  may be directly controlled from the outside with dedicated means. That is, for example, the clutch  26  may be replaced by an electromagnetic friction clutch or an electromagnetic powder clutch. Alternatively, a dedicated actuator (for example, an electromagnetic actuator) may be used for sliding the swash plate  24 . In these cases, the clutch  26  can be switched regardless of the displacement of the compressor  10 , that is, the opening degree of the control valve  40 .  
         [0151]     In the first to eighth embodiments, a disc spring or a wave spring may be used as the spring  27 .  
         [0152]     In the first to eighth embodiments, the spring holes  46 , the springs  27 , and the balls  47  may be provided in the swash plate  24 .  
         [0153]     In the first to eighth embodiments, additional sets of the spring holes  46 , the springs  27 , and the balls  47  may be provided in a range from three o&#39;clock to nine o&#39;clock along the clockwise direction.  
         [0154]     In the first to eighth embodiments, the spring holes  46 , the springs  27 , and the balls  47  may be provided only in a range from three o&#39;clock to nine o&#39;clock along the clockwise direction.  
         [0155]     In the first and three to eighth embodiments, the spring hole  46 A and the spring hole  46 B may be located at asymmetrical positions with respect to the imaginary plane H.  
         [0156]     In the first to eighth embodiments, the balls  47  may be omitted.  
         [0157]     In the first to eighth embodiments, the spring holes  46  may be omitted, and projections may be formed on the limiting surface  19   c  to support the springs  27 . In this case, each spring  27  is fitted around a projection. A ball seat for holding the ball  47  is preferably formed on each spring  27 .  
         [0158]     In the first to eighth embodiments, the spring holes  46 , the springs  27 , the balls  47 , and the roll groove  48  may be omitted.  
         [0159]     In the ninth embodiment or the eleventh embodiment, a coil spring or an annular wave spring may be used as the spring  90 .  
         [0160]     In the ninth embodiment or the eleventh embodiment, the spring  90  may be omitted.  
         [0161]     At least one of a surface of the snap ring  25  contacting the swash plate  24  and a surface of the swash plate  24  contacting the snap ring  25  may be coated (for example, with polytetrafluoroethylene) or plated (for example, with tin) to reduce the friction coefficient. In this configuration, mechanical loss between the swash plate  24  and the snap ring  25  when the clutch  26  is disengaged is reduced.  
         [0162]     In the first, second, or fifth to eleventh embodiments, at least one of the constant diameter portion  43   a  of the boss  19   a  and the constant diameter portion  41   a  of the boss receiving hole  24   a  may be coated (for example, with polytetrafluoroethylene) or plated (for example, with tin) to reduce the friction coefficient. In this case, when the guiding member  19  and the swash plate  24  rotate relative to each other, that is, when the clutch  26  is in the disengaged state, the mechanical loss between the constant diameter portion  43   a  of the boss  19   a  and the constant diameter portion  41   a  of the boss receiving hole  24   a  is reduced. Further, when the clutch  26  is switched, the swash plate  24  is slid smoothly.  
         [0163]     In the ninth embodiment or the eleventh embodiment, surface of the guiding member  19  and the swash plate  24  that contact the spring  90  or both side surface of the spring  90  may be coated (for example, with polytetrafluoroethylene) or plated (for example, with tin) to reduce the friction coefficient.  
         [0164]     In the ninth embodiment or the eleventh embodiment, a thrust bearing may be provided in at least one of the space between the guiding member  19  and the spring  90  and the space between the swash plate  24  and the spring  90 .  
         [0165]     In the fifth embodiment, two or more reservoir grooves  79  may be provided.  
         [0166]     In the fifth embodiment, two or more oil supply holes  78  may be provided.  
         [0167]     In the fifth embodiment, the reservoir groove  79  may be formed only on the outer circumferential surface  43  of the boss  19   a . Alternatively, two reservoir grooves  79  may be provided. In this case, one of the grooves  79  is formed on the inner circumferential surface  41  of the boss receiving hole  24   a , and the other groove  79  is formed on the outer circumferential surface  43  of the boss  19   a.    
         [0168]     In the fifth embodiment, the reservoir groove  79  may be provided only in the constant diameter portion  41   a . Alternatively, the reservoir groove  79  may be provided only on the driven clutch surface  26   b.    
         [0169]     In the fifth embodiment, the reservoir groove  79  may be omitted.  
         [0170]     In the fifth embodiment, the configuration of the oil supply portion  77  may be changed to supply lubricant oil to the space between the guiding member  19  and the swash plate  24  without using centrifugal force. For example, a pump that is actuated by rotation of the drive shaft  17  is provided, and by the action of the pump, lubricant oil is supplied to the space between the guiding member  19  and the swash plate  24 .  
         [0171]     In the tenth embodiment, the driving clutch surface  26   a  may formed on a part of the outer circumferential surface  43  of the boss  19   a , and the driven clutch surface  26   b  may be formed on a part of the inner circumferential surface  41  of the boss receiving hole  24   a . This modification has the advantage (10) of the compressor according to the first embodiment. In this case, the heights of the driving projections  92  and the driven projections  93  are preferably low.  
         [0172]     In the tenth embodiment, a spring may be provided between the guiding member  19  and the swash plate  24 , so that, when the clutch  26  is switched from the engaged state to the disengaged state, the force of the spring is used to slide the swash plate  24 .  
         [0173]     In the tenth embodiment, the oil supply portion  77  of the fifth embodiment may be used.  
         [0174]     In the tenth embodiment, the radial bearing of the third or fourth embodiment may be used.  
         [0175]     In the twelfth embodiment, the radial bearing of the third or fourth embodiment may be used as shown in  FIG. 19 .  
         [0176]     The present invention may be applied to a compressor other than the refrigerant compressor. For example, the present invention may be applied to an air compressor.