Patent Publication Number: US-6217292-B1

Title: Variable displacement type refrigerant compressor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a variable displacement type refrigerant compressor. More particularly, the present invention relates to a variable displacement refrigerant compressor accommodating therein a novel displacement control valve, which can be easily and accurately assembled in a compressor housing and can be an inexpensive mechanical element. The present invention further relates to a method of assembling the novel displacement control valve in the compressor housing. 
     2. Description of the Related Art 
     U.S. Pat. No. 4,688,997 discloses a typical variable displacement type refrigerant compressor adapted to be incorporated in a vehicle climate control system. The variable refrigerant compressor has a compressor housing in which a suction chamber, a discharge chamber, a crank chamber, a reciprocating piston mechanism to compress a refrigerant in a cylinder bore, a piston actuating mechanism including a cam plate mounted around a drive shaft to convert the rotation of the drive shaft in the reciprocation of the piston mechanism, and a displacement control valve unit, are mounted. 
     The displacement control valve unit of the refrigerant compressor of U.S. Pat. No. 4,688,997 includes a fluid supply passage extending from the discharge chamber to the crank chamber to supply a refrigerant at a high pressure into the crank chamber, a valve unit arranged in a portion of the fluid supply passage to open and close the portion of the fluid supply passage, a fluid withdrawal passage extending from the crank chamber to the suction chamber to maintain a constant fluid communication between both chambers, and a valve control mechanism to control the opening and closing operation of the valve unit. Namely, the valve control mechanism operates in such a manner that when the pressure in the crank chamber is reduced to below a predetermined reference value, it operates the valve unit to move to its open position where the refrigerant at a high pressure is supplied from the discharge chamber into the crank chamber. Further, when the pressure in the crank chamber goes up beyond the predetermined reference value, the valve control mechanism operates the valve unit to be moved to its closed position where the supply of the refrigerant at a high pressure from the discharge chamber to the crank chamber is stopped. The valve unit and the valve control mechanism constitute an integral displacement control valve unit. 
     When the compressor is operated under a first condition in which the valve unit of the displacement control valve unit is moved to its closing position to stop the supply of the refrigerant gas at a high pressure from the discharge chamber to the crank chamber, the refrigerant gas leaking from a compression chamber in the cylinder bore into the crank chamber, i.e., a blow-by refrigerant is withdrawn continuously from the crank chamber into the suction chamber via the fluid withdrawal passage. Thus, a reduction in the pressure in the crank chamber occurs. Subsequently, when a temperature in the objective area, i.e., a temperature in a vehicle compartment is lowered while reducing a refrigerating load, a suction pressure of the compressor is reduced. When a reduction in the pressure in the crank chamber is lowered to a level below the predetermined reference value, the valve unit of the displacement control valve unit is moved to its opening position. Therefore, a refrigerant at a high pressure is supplied from the discharge chamber into the crank chamber to increase a pressure in the crank chamber. When the pressure in the crank chamber is increased to go up beyond the predetermined value, the valve unit of the displacement control valve unit is moved to its closing position to close the aforementioned portion of the fluid supply passage. Therefore, the pressure increase in the crank chamber is stopped, and the pressure in the crank chamber is held substantially at the predetermined value during the ordinary operation of the compressor. A difference between the pressure in the crank chamber and the suction pressure of the compressor relying on a change in the refrigerating load of the climate control system adjustably changes the reciprocating stroke of the piston mechanism to thereby control the displacement of the compressor. Although the above-mentioned displacement control valve unit is arranged so as to control the opening and closing operation of the valve unit thereof in response to detection of a change in the pressure prevailing in the crank chamber, the controlling operation of the displacement control valve unit may be carried out in a different manner. For example, the displacement control valve unit may be constructed to operate in such a manner that the movement of the valve unit thereof to its open and closed positions is controlled in response to detection of a change in the suction pressure of the refrigerant compressor. Further, the displacement control valve unit may include a valve unit thereof arranged in a portion of the fluid withdrawal passage in the refrigerant compressor so as to control withdrawing of the refrigerant from the crank chamber into the suction chamber. 
     It should be noted that the above-mentioned displacement control valve unit is conventionally assembled in either a cylinder block or a rear housing of a variable displacement type refrigerant compressor. FIG. 4 typically illustrates a conventional control valve unit  20  assembled in a rear housing  3 . The control valve unit  20  is provided with a suction pressure chamber  21  functioning as a pressure sensing chamber, and a discharge pressure chamber  22  arranged axially opposed to the suction pressure chamber  21  and functioning as a valve chamber. The suction pressure chamber  21  is arranged so as to communicate with the suction chamber  3   a  of the refrigerant compressor via a passage  23  formed in the rear housing  3 , and the discharge pressure chamber  22  is arranged so as to communicate with the discharge chamber  3   b  of the refrigerant compressor via a passage  24  formed in the rear housing  3 . Within the suction pressure chamber  21 , a bellows element  26  is centrally arranged so as to axially expand or contract and to define therein an atmospheric chamber  25 . A spring  27  is arranged so as to constantly urge the bellows element  26  toward its extended position, i.e., toward the discharge pressure chamber. 
     The discharge pressure chamber  22  has a valve hole  28  formed at an end thereof confronting the suction pressure chamber  21  so as to communicate with a valve port  29  which is arranged to communicate with the crank chamber  2   a  of the refrigerant compressor via a fluid supply passage  30 . A valve rod  31  connected at its one end to the bellows element  26  extends toward the discharge pressure chamber  22  so that the other end thereof enters into the discharge pressure chamber  22  via the valve port  29  and the valve hole  28 . 
     A valve element  32  is attached to the other end of the above-mentioned valve rod  31  within the discharge pressure chamber  22  so as to oppose to the valve hole  28 . The valve element  32  is thus able to open and close the valve hole  28 , in response to the expanding and contracting movement of the bellows element  26 , and is constantly urged by the spring force of a spring  33  toward the closed position thereof to close the valve hole  28 . Therefore, when the suction pressure introduced into the suction pressure chamber  21  goes down below a predetermined set value, the bellows element  26  is expanded to move the valve rod  31  so that the valve element  32  is moved away from the valve hole  28 . Namely, the valve hole  28  is opened, and accordingly, the refrigerant gas at a discharge pressure (a high pressure) is supplied from the discharge chamber  3   b  into the crank chamber  2   a  via the valve hole  28 , the valve port  29  and the fluid supply passage  30 . 
     Nevertheless, the displacement control valve unit  20  is constructed so that the suction pressure chamber  21  in which the suction pressure Ps is introduced via the passage  23  to be sensed by the accommodated movable bellows element  26 , the discharge pressure chamber  22  having two holes opening toward the fluid supply passage  30  and the discharge chamber  3   b  and accommodating therein the valve element  32  to open and close the valve hole  28 , and the valve rod  31  transmitting the expanding and contracting movement of the bellows element  26  to the valve element  32  to thereby move the valve element  32 , are integrally incorporated in a single member forming a casing member  34  of the displacement control valve unit  20  as shown in FIG.  3  and FIGS. 5A and 5B. Namely, the casing member  34  must be provided as an indispensable element of the displacement control valve unit  20 , and the bellows element  26  must be seated on and welded to a seat  37  ( 37   a  or  37   b ) before the bellows element  26  is accommodated in the suction pressure chamber  21  of the casing member  34 . Further, the casing member  34  of the valve control unit  20  must be provided with a plurality of grooves to contain therein o-rings  35  in order to fixedly disposed in an assembling bore of the rear housing  3  (or the cylinder block  1 ) when the control valve unit  20  is assembled in a compressor body. As a result, a large assembling space is needed to accommodate the control valve unit  20  in the body of the refrigerant compressor. In addition, the assembling of the control valve unit  20  requires delicate assembling operation performed by an operator and thus, causes an increase in the manufacturing cost. 
     Furthermore, since the bellows element  26  functioning as the pressure sensing element is a movable element permitted to expand and contract over a range of only  1  through  2  millimeters, the whole length of the casing member  34 , the position to dispose the bellows element  26  within the suction pressure chamber  21 , the bore depth in the casing member  34  which forms the suction pressure chamber  21  and the discharge pressure chamber  22  must be formed by machining at a very high accuracy to minimize a cumulative error due to addition of dimensional tolerances of the machined portions of the casing member  34 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to solve the described many problems encountered by the conventional control valve unit assembled in the body of a variable displacement type refrigerant compressor. 
     Another object of the present invention is to provide a variable displacement type refrigerant compressor incorporating a novel valve control unit assembled in a simple assembling operation. 
     A further object of the present invention is to provide a valve control unit capable of being produced and assembled in a relatively small mounting space formed in a body of a variable displacement type refrigerant compressor at a low manufacturing cost, and with a high production accuracy. 
     A still further object of the present invention is to provide a method of assembling the above-mentioned novel valve control unit into a body of a variable displacement type refrigerant compressor. 
     In accordance with one aspect of the present invention, there is provided a variable displacement refrigerant compressor including a compressor housing forming an outer framework and provided internally with a compressing mechanism for compressing a refrigerant, a displacement varying mechanism for adjustably varying an amount of the refrigerant compressed and discharged by the compressing mechanism, and a displacement control valve unit adjustably controlling the operation of the displacement varying mechanism due to detecting a change in a pressure acting thereon, 
     wherein the compressor housing is provided with a pressure sensing chamber for receiving a pressure to be detected and a valve chamber forming a fluid passage through which the refrigerant at a high pressure is permitted to flow to be used for changing a compressor displacement, the pressure sensing and valve chambers being bored in the compressor housing, and 
     wherein the displacement control valve unit comprises: a pressure sensing mechanism arranged in the pressure sensing chamber and having a movable element moving in response to detection of a change in a pressure prevailing in the pressure sensing chamber; and 
     a valve element arranged in the valve chamber and operating to open and close a port in the fluid passage according to the movement of the movable element of the pressure sensing mechanism. 
     Preferably, the compressor housing is further provided internally with a plurality of cylinder bores, a suction chamber, a discharge chamber, and a crank chamber, and the refrigerant compressor is further provided with a drive shaft rotatably supported by the compressor housing, a cam plate arranged in the crank chamber to be rotatable together with the drive shaft and permitted to vary an angle of inclination thereof with respect to a reference plane, and a plurality of pistons operatively engaged with the cam plate and reciprocating in the plurality of cylinder bores, the cam plate being arranged to change the angle of inclination thereof, on the basis of a difference between a first pressure prevailing in the crank chamber and a second pressure prevailing in the plurality of cylinder bores to thereby constitute the above-mentioned displacement varying mechanism. 
     Further preferably, the fluid passage is provided as a fluid supply passage extending between the discharge chamber and the crank chamber to supply the refrigerant at a discharge pressure from the discharge chamber to the crank chamber. 
     The compressor housing of the variable displacement type refrigerant compressor preferably may have a first and second ends opposite to one another along a predetermined axis, and the pressure sensing chamber is formed by a first bore bored from the first end while valve chamber is formed by a second bore bored from the second end. Further, a third bore is formed between the first and second bores so as to permit a valve rod to be movably inserted therein so that the valve rod transmits the movement of the pressure sensing mechanism to the valve element. 
     Preferably, the valve chamber formed in the compressor housing is provided with a valve seat formed as a separate element from the compressor housing and fixedly disposed in the valve chamber to cooperate with the valve element. 
     The pressure sensing mechanism preferably includes a bellows element having a flange portion formed at one end thereof and fixedly sandwiched between a shoulder portion formed in the compressor housing and a fixing means cooperating with the shoulder portion. The other end of the bellows element opposite to the above-mentioned end having the flange portion preferably has a reduced diameter projection functioning as a guide portion, and the guide portion is fitted in a guide recess formed in the pressure sensing chamber so that the bellows element stably expands and contracts under the guidance of the guide portion of the bellows element in the guide recess. 
     In accordance with another aspect of the present invention, there is provided a method of assembling a displacement control valve unit in a variable displacement type refrigerant compressor including a compressor housing having internally a compressing mechanism for compressing a refrigerant, and a displacement varying mechanism for adjustably varying an amount of the refrigerant compressed and discharged by the compressing mechanism, 
     wherein the method comprises the steps of: 
     boring, in the compressor housing, a pressure sensing chamber to receive a pressure to be detected, a valve chamber provided to form a fluid passage through which the refrigerant at a high pressure is permitted to flow to be used for changing a compressor displacement, and a valve rod bore extending between the pressure sensing chamber and the valve chamber; 
     inserting a valve unit in the valve chamber at a position operable to open and close a port in the fluid passage; 
     inserting a pressure sensing mechanism in the pressure sensing chamber at a predetermined position where a movable element of the pressure sensing mechanism moving in response to detection of a change in a pressure prevailing in the pressure sensing chamber is operatively connected to the valve unit; and 
     positioning the pressure sensing mechanism at the predetermined position in the pressure sensing chamber. 
     Preferably, the valve unit of the displacement control valve unit includes a valve element movable to open and close a valve port formed in said fluid passage, a valve seat having an opening thereof acting as the valve port in the fluid passage and cooperating with the valve element to open and close the valve port, and a valve rod arranged in the valve rod bore to engage the pressure sensing mechanism with the valve element, and 
     the step of inserting the valve unit comprises: 
     preparing a jig tool provided therein with a recess formed to have a depth corresponding to a predetermined movement stroke of the movable element of the pressure sensing mechanism; 
     positioning the jig tool in the pressure sensing chamber so that the recess of the jig tool is in alignment with and in contact with one end of the valve rod bore, 
     inserting the valve seat into the valve chamber at a position adjacent to a fixed position where the valve seat is press-fitted; 
     inserting the valve rod through the opening of the valve seat into the valve rod bore from the other end of said valve rod bore until one end of said valve rod is positioned adjacent to an end face of the recess of the jig tool 
     placing the valve element in the valve chamber to be kept in contact with the valve seat; and 
     pressing the valve seat and the valve rod, via the valve element, into the valve chamber until the valve rod comes in tight contact with the end face of the jig tool to thereby press-fit the valve seat at the fixed position in the valve chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will be made more apparent from the ensuing description of the preferred embodiments, with reference to the accompanying drawings wherein: 
     FIG. 1 is a cross-sectional view of a displacement control valve unit assembled in a compressor housing, according to an embodiment of the present invention; 
     FIG. 2 is a schematic view for explaining of a method of assembling the displacement control valve unit into the compressor housing according to the present invention; 
     FIG. 3 is a cross-sectional view of a conventional control valve unit incorporated in a variable displacement type refrigerant compressor; 
     FIG. 4 is a longitudinal cross-sectional view of a variable displacement type refrigerant compressor in which the conventional control valve unit is assembled; 
     FIG. 5A is a cross-sectional view of a portion of the conventional control valve unit, illustrating the pressure sensing bellows element welded to a seat member; and 
     FIG. 5B is a cross-sectional view of a portion of the conventional control valve unit, illustrating the pressure sensing bellows element welded to a seat member different from that of FIG.  5 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before starting the description of the preferred embodiment of the present invention, it should be understood that a portion of the variable displacement type refrigerant compressor, shown on the left of the line A—A of FIG. 4, illustrates a constructional portion common to a variable displacement type refrigerant compressor in which a displacement control valve unit according to the present invention is assembled and the compressor incorporates therein the conventional control valve unit  20 . Therefore, the description of the compressor in which a displacement control valve unit  70  according to the present invention is assembled is provided hereinbelow with reference to FIG.  4 . 
     In FIG. 4, the variable displacement type refrigerant compressor is provided with a cylinder block  1 , which forms a part of an outer framework of the compressor. The compressor is further provided with a front housing  2  which is attached to a frontmost end of the cylinder block  1  to define a crank chamber  2   a , and a rear housing  3  attached to a rear most end of the cylinder block  1  to define a suction chamber  3   a  and a discharge chamber  3   b  therein. A drive shaft  4  is rotatably supported by the front housing  2  and the cylinder block  1  and axially extends through the crank chamber  2   a . The cylinder block  1 , the front housing  2 , and the rear housing  3  constitute a compressor housing capable of functioning as an outer framework of the compressor. The compressor housing is made of an aluminum alloy from the viewpoint of light weight and easy machinability. 
     Within the crank chamber  2   a , a rotary support element  5  is fixedly mounted on the drive shaft  4  to have a rearwardly extending support arm  6 . The support arm  6  of the rotary support element  5  is provided with an extended aperture  6   a  in which a pin  7  connected to a rotary drive plate  8  is slidably fitted. The rotary drive plate  8  can function as a rotary cam plate or a rotary swash plate arranged inclinably around the drive shaft  4 . The rotary cam plate  8  and a hinge mechanism formed by the support arm  6  of the rotary support element  5  and the pin  7  connected to the cam plate  8  constitute a displacement varying mechanism for varying a discharge amount of the compressor. 
     A sleeve element  9  is slidably fitted on the drive shaft  4  at a position adjacent to a rear end of the rotary support element  5  and is constantly urged toward the rotary support element  5  by a coil spring  10 . The sleeve element  9  is provided with a pair of laterally projecting pivots  9   a  (only one is shown in FIG. 4) which are engaged in a pair of holes (not shown) formed in the rotary cam plate  8 . Thus, the rotary cam plate  8  is supported by the sleeve element  9  and is able to turn about the pivots  9   a ,  9   a.    
     The rotary cam plate  8  has a rear face and a rear boss portion on which a wobble plate  11  is non-rotatably mounted. A thrust bearing is interposed between the rear face of the rotary cam plate  8  and the wobble plate  11 . The wobble plate  11  has an outer periphery in which a guide recess  11   a  is formed to be engaged with an axial through-bolt  16  inserted from a front side of the front housing  2  and fixed to the cylinder block  1 . Thus, the wobble plate  11  is prevented from rotating together with the rotary cam plate  8 , and is permitted to turn about the above-mentioned pivots  9   a ,  9   a  together with the cam plate  8 . 
     The wobble plate  11  is engaged with a plurality of pistons  13  via respective connecting rods  14  having spherical ends  14   a  and  14   b , respectively. The pistons  13  are slidably fitted in the plurality of axial cylinder bores  12  formed in the cylinder block  1 . Therefore, when the drive shaft  4  is rotated by an external drive power, the rotation of the drive shaft  4  is converted into a wobbling motion of the wobble plate  11  via the rotary cam plate  8 , and the wobbling motion of the wobble plate  11  causes a reciprocating motion of the respective pistons  13  in the corresponding cylinder bores  12 . When the plurality of pistons  13  reciprocate within the cylinder bores  12 , a refrigerant gas is sucked from the suction chamber  3   a  into the cylinder bores  12  to be compressed by the respective pistons  13  within the compression chambers in the respective cylinder bores  12  and is discharged from the cylinder bores  12  into the discharge chamber  3   b . During the refrigerant compressing operation of the compressor, the reciprocating stroke of the respective pistons  13  within the cylinder bores  12  and an angle of inclination of the cam and wobble plates  8  and  11  are adjustably changed by a change in a differential between a first pressure prevailing in the crank chamber  2   a  and a second pressure acting on the working heads of the respective pistons  13 . Thus, the amount of discharge of the compressed refrigerant, i.e., the displacement of the compressor is adjustably changed. The pressure prevailing in the crank chamber  2   a  is controlled by a displacement control valve unit  70 , which will be described with reference to FIGS. 1 and 2. 
     Further, it should be understood that the crank chamber  2   a  constantly communicates with the suction chamber  3   a  via a fluid withdrawal passage (not shown in FIG.  4 ). 
     Referring to FIG. 1, the displacement control valve unit  70  is assembled in a mounting area  40  appropriately defined in either the cylinder block  1  or the rear housing  3 . The mounting area  40  is formed to have a first side  40   a  and a second side  40   b , which are preferably arranged to be axially opposite to one another. The displacement control valve unit  70  is provided with a suction pressure chamber  61  formed as an axial bore drilled from the first side  40   a , and the suction pressure chamber  61  can function as a pressure sensing chamber. The displacement control valve unit  70  is also provided with a discharge pressure chamber  62  which can function as a valve chamber and is formed as a bore coaxial with and axially opposed to the suction pressure chamber  61 . The coaxial suction and discharge pressure sensing chambers  61  and  62  are connected by an axial bore which is also coaxial with the above-mentioned two chambers  61  and  62  and functions as a valve rod bore  41  in which a valve rod  71  is slidably fitted. 
     The suction pressure chamber  61  of the displacement control valve unit  70  includes a large diameter bore  42  having screw threads formed at an upper portion thereof so as to permit a fixing screw  46  is threadedly engaged. The suction chamber  61  is also provided with a coaxial middle diameter bore formed as a bellows-receiving chamber  43  for receiving a bellows element  66  which functions as a pressure-sensing element. The bellows receiving chamber  43  is continuous with the upper large diameter bore  42  and is formed to fluidly communicate with the suction chamber  3   a  via a fluid passage  63 . Thus, a suction pressure Ps prevails in the bellows-receiving chamber  43 , when the valve unit  70  is completely assembled, so as to be sensed by the bellows element  66 . The suction pressure sensing chamber  61  is further provided with a reduced diameter bore functioning as a guide bore  45  in which a cylindrical guide projection  44  formed in a closed end of the bellows element  66  is received and guided during the expanding and contracting movement of the bellows element  66 . 
     The fixing screw  46  is provided with a rear end portion having screw threads formed at an outer circumference thereof, and accordingly, the fixing screw  46  can be threadedly engaged in the screw threads of the large diameter bore  42  of the suction pressure sensing chamber  61 . The fixing screw  46  is provided with a central bore  48  having a partly threaded portion with which a later-described adjusting screw  47  is engaged. The fixing screw  46  is further provided with a tubular spring holder portion  50  which extends coaxially with the central bore  48  and holds therein a coil spring  49 . 
     The bellows element  66  is provided with the aforementioned cylindrical guide projection  44  at one closed end thereof and a flange portion  51  formed at the periphery of the opposite open end thereof. When the bellows element  66  is inserted in the bellows receiving chamber  43 , the guide projection  44  is received in the guide bore  45 . The flange portion  51  of the bellows element  66  is formed to be loosely fitted in the upper large diameter bore  42  and is seated on a shoulder portion  52  formed between the upper large diameter bore  42  and the bellows receiving chamber  43  via an appropriate gasket member  53 . Thus, when the fixing screw  46  is threadedly engaged in the upper large diameter bore  42 , the flange portion  51  is fixed to the shoulder portion  52 . 
     When the fixing screw  46  is engaged in the upper large diameter bore  42 , the tubular spring holder portion  50  is inserted in the interior of the bellows element  66 , and a coil spring  49  is held in the tubular spring holder portion  50  of the fixing screw  46 . An upper end of the coil spring  49  is engaged with a washer member  55  which is engaged with the adjusting screw  47  via a ball member  54 . A lower end of the coil spring  49  is pressed against an inner face of the guide projection  44  of the bellows element  66  so that a spring force is applied to the bellows element  66  in a direction in which the bellows element  66  expands. 
     The discharge pressure chamber  62  formed by boring from the second side  40   b  of the mounting area  40  is provided with an aperture opening toward a fluid passage  64  which extends toward the discharge chamber  3   b  of the rear housing  3 . Thus, the discharge pressure chamber  62  fluidly communicates with the discharge chamber  3   b . The discharge pressure chamber  62  includes a valve operating chamber  56  in which a discharge pressure Pd is introduced from the discharge chamber  3   b  and an innermost valve seat chamber  57  in which a valve seat  76  is inserted and fixed. The valve seat chamber  57  has a port opening toward a fluid passage  65  communicating with the crank chamber  2   a . Thus, a crank pressure Pc is introduced from the crank chamber  2   a  into the valve seat chamber  57 . A valve element  72  is held in the valve operating chamber  56  of the discharge pressure chamber  62  and is constantly urged toward the valve seat  76  by a spring  59  having one end engaged with the valve element  72  and the other end engaged with a ball-like lid member  58 . Thus, the valve element  72  and the valve seat  76  constitute a valve mechanism  77  which controls fluid communication between the discharge chamber  3   b  and the crank chamber  2   a  via a fluid supply passage formed by the fluid passage  64 , the valve operating chamber  56 , the valve seat chamber  57  and the fluid passage  65 . Accordingly, the valve mechanism  77  controls a supply of the refrigerant at a high discharge pressure from the discharge chamber  3   b  to the crank chamber  2   a.    
     The valve seat  76  is a tubular member made of a brass material and having an inner bore permitting a valve rod  71  to extend therethrough. The valve seat  76  is inserted in the valve operating chamber  56  of the discharge pressure chamber  62  and is press-fitted in position in the valve seat chamber  57 . 
     When the valve seat  76  is press-fitted in the valve seat chamber  57 , a specified jig tool  60  shown in FIG. 2 is used. Namely, as shown in FIG. 2, the jig tool  60  is provided with a central recess  60   a  formed at an extreme end thereof to have an axial depth corresponding to a predetermined amount of movement “L” of the bellows element  66 . The jig tool  60  is inserted in the upper large diameter bore  42 , so that the extreme end thereof having the above-mentioned recess  60   a  is pressed against the bottom of the upper large diameter bore  42  and that the central recess  60   a  is in alignment with the valve rod bore  41 . The jig tool  60  is fixed in the upper large diameter bore  42 , due to the threaded engagement between the jig tool  60  and the threaded portion of the bore  42 . Subsequently, the valve rod  71  having the valve element  72  at one end thereof is inserted through the bore of the valve seat  76  and the valve rod bore  41  until the other end of the valve rod  71  comes close to the bottom end face of the recess  60   a  of the jig tool  60 . At this time, the valve element  72  is held to be in contact with an end of the valve seat  76 . 
     Then, the valve element  72  is tapped by an appropriate rod-like tool  80  in a direction shown by an arrow “A” until the other end of the valve rod  71  comes into contact with the bottom end face of the recess  60   a  of the jig tool  60 . When the valve rod  71  comes into contact with the jig tool  60 , the valve seat  76  is assembled and fixed in position in the valve seat chamber  57  of the discharge pressure chamber  62  via the valve element  72 . Namely, the valve element  72  and the valve seat  76  are set in position ready for accurately opening and closing a valve port in the end of the valve seat  76 , in response to a control movement of the pressure sensing means including the bellows element  66 . Finally, the urging spring  59  and the ball-like lid  58  are assembled in the valve operating chamber  56  to press the valve element  72  against the valve seat  76  on the basis of a predetermined spring force of the urging spring  59 . 
     Further, the jig tool  60  is removed from the upper large diameter bore  42  of the pressure sensing chamber  61 , in order to assemble the pressure-sensing mechanism including the bellows element  66 , the coil spring  49 , the fixing screw  46 , and the adjusting screw  47  in the pressure sensing chamber  61 . 
     The operation of the above-described displacement control valve unit  70  of the present embodiment will be described hereinbelow with reference to FIGS. 1 and 4. 
     In the present embodiment, the displacement control valve unit  70  is arranged so as to perform a controlling operation on the basis of detecting the suction pressure Ps of the compressor. Namely, the valve unit  70  controls the crank pressure Pc in the crank chamber  2   a  in response to detection of a change in the suction pressure Ps. More specifically, when the refrigerating load applied from an external refrigerating system to the refrigerant compressor is large to generate a high suction pressure Ps, the high suction pressure Ps is introduced into the suction pressure chamber  61  via the fluid passage  63 . Thus, the bellows element  66  contracts due to the high suction pressure Ps, so that the valve element  72  is pulled via the valve rod  71  to be pressed against the valve seat  76  and closes the valve port of the valve seat  76 . Therefore, a fluid communication between the valve operating chamber  56  and the valve seat chamber  57  is stopped to interrupt a fluid communication between the discharge chamber  3   b  and the crank chamber  2   a . Since the crank chamber  2   a  constantly communicates with the suction chamber  3   a  via the fluid withdrawal passage, the crank pressure Pc in the crank chamber  2   a  is lowered to a level equivalent to the suction pressure Ps. As a result, the rotary cam plate  8  rotating together with the drive shaft  4  is turned about the pivots  9   a ,  9   a  to take a maximum angle of inclination with respect to a reference plane perpendicular to the axis of rotation of the drive shaft  4 . Therefore, the amount of discharge of the compressed refrigerant is kept at a maximum. 
     When the refrigerating load is reduced to generate a low suction pressure Ps, the low suction pressure Ps is introduced into the suction pressure chamber  61  of the displacement control valve unit  70  to permit the bellows element  66  to be expanded. Thus, the valve rod  71  is pressed by the bellows element  66  so as to move the valve element  72  in a direction away from the valve seat  76  against the spring force of the urging spring  59 . Thus, the valve element  72  opens the valve port of the valve seat  76  to establish a fluid communication between the discharge chamber  3   b  and the crank chamber  2   a  via the fluid passages  64  and  65  so that the compressed refrigerant at a high pressure Pd is supplied from the discharge chamber  3   b  to the crank chamber  2   a . Therefore, the crank pressure Pc in the crank chamber  2   a  is increased. Accordingly, the rotating cam plate  8  together with the non-rotatable wobble plate  11  are turned about the pivots  9   a ,  9   a  toward its minimum angle of inclination while the suction pressure Ps is reduced. Consequently, the amount of the compressed refrigerant discharged from the compressor is reduced to the minimum. 
     It should be noted that, in the described displacement control valve unit  70  according to the present invention, the flange portion  51  of the bellows element  66  is fixedly sandwiched via the gasket  53  between the fixing screw  46  and the shoulder portion  52  in the suction pressure chamber  61 . Thus, the bellows receiving chamber  43  forming the suction chamber  61  is completely sealed against the atmosphere. Further, the bellows element  66 , fixed at its flange portion  51  to the shoulder portion  52 , is permitted to expand and contract along a longitudinal direction by the guide of the guide bore  45 , which constantly receives the cylindrical guide projection  44  of the bellows element  66 . As the inner diameter of the bellows receiving chamber  43  is larger than the outermost diameter of the bellows element  66 , the movement of the bellows element  66  can be always stable due to the guidance of the guide bore  45 . The tubular spring holder portion  50  of the fixing screw  46  also contributes to the stable movement of the bellows element  66 . The stable movement of the bellows element  66  is effective for preventing the bellows element  66  from contacting with the wall of the bellows-receiving chamber  43 , and accordingly, the bellows element  66  can be prevented from being frictionally abraded. 
     In the displacement control valve unit  70  of the present embodiment, the valve seat chamber  57  of the discharge pressure chamber  62  is formed in the mounting area  40  (e.g., the rear housing  3 ) of the compressor made of aluminum alloy material. Nevertheless, the valve seat  76  is made of brass material, and is press-fitted in the valve seat chamber  57 . Namely, the material of the valve seat  76  is selected to be hard enough to avoid physical deformation and abrasion due to contacting of the valve seat  76  with the valve element  72  which is also made of hard metallic material. 
     Further, since the important elements of the displacement control valve unit  70 , i.e., the bellows element  66 , the valve rod  71 , the valve element  72 , and the valve seat  76  are all assembled in the mounting area  40  of the compressor body by using the specific jig tool  60 , these elements can be accurately assembled in the compressor body even if the mounting area  40 , i.e., the rear housing  3  or the cylinder block  1  is rather roughly machined. Thus, the control operation of the displacement control valve unit  70  can be accurate. 
     The displacement control valve unit  70  according to the present invention does not have a valve casing, and a part of the compressor body is used as a casing of the valve unit  70 . Further, no o-ring element is incorporated in the valve unit  70 , and accordingly, production and assembly of the displacement control valve unit  70  can be easy and simple to reduce the manufacturing cost of the valve unit  70  per se and of the variable displacement type refrigerant compressor. 
     Although the described embodiment of the displacement control valve unit is assembled in a variable displacement type refrigerant compressor employing a combination of a swash plate-type cam plate and a wobble plate for a displacement varying mechanism, the valve unit may be equivalently used with many different variable displacement type refrigerant compressors such as a variable capacity refrigerant compressor as disclosed in U.S. Pat. No. 5,873,704 to Ota et al., which has a single plate-like cam plate engaged with a plurality of single-headed pistons via shoes, a rotary type variable displacement refrigerant compressor including a vane type refrigerant compressor and a scroll type refrigerant compressor. 
     It should be understood that many and various changes and modifications to the described embodiment of the present invention will occur to a person skilled in the art without departing from the scope and spirit of the present invention as claimed in the accompanying claims.