Abstract:
A piston, adapted for use in a swash plate type of air conditioning compressor including a generally cylindrical cylinder block provided with at least one cylinder bore, in which the piston is reciprocally disposed. The piston includes a cylindrical body with a head portion, a bridge portion extending from the body and having a recess, and a pair of shoe pockets formed in opposed walls defined in the recess. Each of the shoe pockets includes an entrance and an apex, and a lower edge portion of the body is positioned at an adjoining portion between the body and the bridge portion extending to a place between the entrance and the apex of the shoe pocket adjacent to the body.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a piston for use in a compressor, and more particularly, to a piston suitable for use in an automotive air conditioning compressor in which there is provided a piston having a construction to minimize a bending moment exerted thereon and a mechanism in response to such piston. 
     BACKGROUND OF THE INVENTION 
     Generally, a piston type compressor for use in an automotive air conditioning system comprises a cylinder block having a plurality of cylinder bores. A plurality of pistons are slidably disposed in the respective cylinder bores and reciprocated by, for example, a swash plate or wobble plate in the cylinder bores. In a variable capacity swash plate type compressor with a mechanism varying an inclination angle of the swash plate, a single-headed piston is generally used. The single-headed piston includes a body with a head, and a support portion for receiving shoes which convert rotation of the swash plate into reciprocation of the pistons. However, a bending moment acts on the pistons due to a component of the force that is exerted normal to the direction of motion of the pistons during operation of the compressor. Accordingly, the bending moment causes the deformation of pistons, and thus, a contact portion between the pistons and the cylinder bores is abraded. 
     In order to clarify the problems occurring in a typical swash plate type compressor with a variable displacement mechanism, description will be made with reference to FIG.  1 . The compressor  1  of this type has a cylinder block  2 , with a plurality of cylinder bores  4 , and with front and rear ends of the cylinder block  2  sealingly closed by front and rear housing portions  6  and  8 , respectively. The cylinder block  2  and the front housing  6  define an air-tight sealed crank chamber  10 . A valve plate  12  is mounted between the rear end of the cylinder block  2  and the rear housing  8 . The rear housing  8  has formed therein inlet and outlet ports  14  and  16  for input and output of a refrigerant gas, a suction chamber  18 , and a discharge chamber  20 . The suction and discharge chambers  18  and  20  are in communication with the respective cylinder bores  4  via suction and discharge valve mechanisms. A drive shaft  22  is centrally arranged to extend through the front housing  6  to the cylinder block  2  and rotatably supported by bearings mounted in the front housing  6  and the cylinder block  2 . The cylinder block  2  and the front and rear housing  6  and  8  are combined by screws  25 . A rotor  26  is mounted on the drive shaft  22  in the crank chamber  10  to be rotatable with the drive shaft  22 , and is supported by a thrust bearing  28  seated on an inner end of the front housing  6 . A spherical sleeve  30 , having an outer spherical surface formed as a support surface, is slidably supported by the drive shaft  22 . A spring  32 , mounted around the drive shaft  22 , is interposed between the rotor  26  and the spherical sleeve  30 , and biases the spherical sleeve  30  toward the rear housing  8 . 
     A swash plate  34  is rotatably supported on the outer surface of the spherical sleeve  30 . The swash plate  34  is connected to the rotor  26  via a hinge mechanism so as to be rotated with the rotor  26 . The hinge mechanism includes a support arm  36  that protrudes axially outwardly from one side surface of the rotor  26 , and an arm  38  that protrudes from one side surface of the swash plate  34  toward the support arm  36  of the rotor  26 . The support arm  36  and the arm  38  overlap each other and are connected to each other by a pin  40 . The pin  40  extends into a pin hole  42  formed through the support arm  36  of the rotor  26  and a rectangular shaped hole  43  formed through the arm  38  of the swash plate  34 . In this manner, the rotor  26  and the swash plate  34  are hinged to each other, and the sliding motion of the pin  40  within the rectangular hole  43  changes the inclination angle of the swash plate  34  so as to change the capacity of the compressor. 
     Pistons  44  are slidably disposed in the respective cylinder bores  4 . Each piston  44  has a body  46  with a head portion which is slidably disposed in the corresponding cylinder bore  4 , and a bridge portion  48  which has formed therein a recess  50 . Semi-spherical shoes  52  are disposed in shoe pockets  54  formed in the bridge portion of the piston  44  and slidably engaged with a peripheral portion of the swash plate  34 . Therefore, the swash plate  34  is rotated together with the rotation of the drive shaft  22 , and the rotation of the swash plate  34  is converted into the reciprocation of the pistons  44 . 
     A cutout portion  56  is formed at a lower front end portion of the piston  44  to prevent contact between a side surface of the swash plate  34  and the body  46  of the piston  44  when the piston is in its bottom dead center position. 
     A control valve means  60  is provided with the compressor to adjust a pressure level in the crank chamber  10 . 
     In the above-described type of compressor, a bending moment generated from various forces acting on the pistons  44  causes a deformation of the pistons  44  and potentially excessive abrasion about a contact portion between the pistons  44  and their corresponding cylinder bores  4 . FIG. 2 illustrates an enlarged partial view of FIG. 1, showing the various forces acting on a piston. During the compression stroke of the piston  44 , the pressure P c  in the crank chamber  10  acts on the forward end of the piston  44  while a compression reaction force P d  acts on the other end of the piston  44 . The pressure P c  in the crank chamber  10  and the compression reaction force P d  act on the swash plate from the piston via the shoes  52  creating an action force on the swash plate  34 , with obviously a reaction force that is equal in magnitude and oppositely directed to the action force. That is, when the piston  44  is in its compression stroke, the force F exerted from the swash plate  34  on the piston  44  acts in a direction that is perpendicular to surfaces of the swash plate  34  at a contact location where the semi-spherical outer surface of the shoe adjacent to the body of the piston  44  comes into contact with the semi-spherical inner surface of the shoe pocket  54 . This location is at an apex of the shoe pocket  54  lying on the central axis O of the piston  44 . If the force F exerted from the swash plate  34  on the piston  44  is decomposed into two components, a horizontal and a vertical component, there will be a horizontal component F x  lying on the central axis O of the piston  44  and a vertical component F y  being perpendicular to the central axis O of the piston  44 . Let “m” be the mass of the piston  44 , “a” the acceleration of piston during the compression stroke, and “A” the surface area against which the pressure acts. Thus, 
     
       
         ΣF x =ma 
       
     
     
       
         ΣF x =AP c −AP d +F x   
       
     
     By combining the above equations, we can write, 
     
       
         F x =ma+A(P d −P c )=ma+(π/4)*d 2 (P d −P c ) 
       
     
     and 
     
       
         F y =F x tan θ=tan θ[ma+(π/4)*d 2 (P d −P c )] 
       
     
     which d is a diameter of piston. 
     The vertical component F y , then, will act on the piston  44  to create a bending moment which is maximum at the lower back edge designated by “p”. As stated above, the cutout portion  56  is provided to prevent the piston  44  from coming into contact with the rear surface of the swash plate  34  when the piston  44  approaches its bottom dead center position. However, the cutout portion  56  creates a horizontal distance x between the operating point of the force F acting on the piston and the location of the reaction force acting on the cutout portion  56  at p. This distance x creates a bending moment which acts on the piston  44 . The maximum bending moment M max  acting on the piston is given by 
     
       
         M max =xF y =xtan θ[ma+(π/4)*d 2 (P d −P c )] 
       
     
     Therefore, due to the bending moment, the piston will tend to cock in its cylinder in a counterclockwise direction with respect to the reaction force-operating point P, creating the possibility of abnormally excessive abrasion on the body of the piston about the reaction force-operating point P and in an edge portion diagonally opposed thereto. 
     SUMMARY OF THE INVENTION 
     In its embodiments, the present invention contemplates a single headed piston, adapted for use in a swash plate type of air conditioning compressor including a generally cylindrical cylinder block provided with at least one cylinder bore, in which the piston is reciprocally disposed. The piston includes a cylindrical body with a head portion. It also includes a bridge portion extending from the body and having a recess and a pair of shoe pockets formed in opposed walls defined in the recess, with each of the shoe pockets including an entrance and an apex. A lower edge portion of the body is positioned at an adjoining portion between the body and the bridge portion extending to a place between the entrance and the apex of the shoe pocket adjacent to the body. 
     The present invention further contemplates a variable capacity swash plate type compressor. The compressor includes a housing having a cylinder block with a plurality of cylinder bores formed therein and enclosing a crank chamber, a suction chamber, and a discharge chamber. A drive shaft is rotatably supported by the housing mechanism. A plurality of single headed pistons are reciprocally disposed in each of the cylinder bores, with each of the pistons having a generally cylindrical body with a head portion, a bridge portion extending from the body and having a recess, and a pair of shoe pockets formed in opposed walls defined in the recess, with each of the shoe pockets including an entrance and an apex, and a lower edge portion of the body positioned at an adjoining portion between the body and the bridge portion extending to a place between the entrance and the apex of the shoe pocket adjacent to the body. A rotor is mounted on and rotationally fixed to the drive shaft so as to rotate together with the drive shaft in the crank chamber, and a hinge mechanism operatively engages the rotor. A swash plate is operatively connected to the rotor via the hinge mechanism and is slidably mounted on the drive shaft. The swash plate includes a side generally facing the cylinder block, with a recess in the side extending circumferentially around the side, radially located to be adjacent to an end of the piston heads. The compressor also includes motion conversion means disposed between the swash plate and the pistons for converting nutational motion of the swash plate into reciprocation of the pistons in the respective cylinder bores. 
     Accordingly, an object of the present invention is to provide a swash plate type compressor with pistons by which the above-mentioned problems can be solved. 
     Another object of the invention is to provide a swash plate type compressor and, more particularly, a variable capacity swash plate type compressor, provided with a piston having a construction to minimize a bending moment by which high durability of the piston and compressor can be accomplished. 
     Still another object of the invention is to provide a swash plate type compressor provided with a mechanism suitable for a piston having a construction to minimize a bending moment. 
     An advantage of the present invention is that the possibility of abnormally excessive abrasion is substantially reduced. 
     Other objects, features, and advantages of the present invention will be understood from the detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross-sectional view of a swash plate type compressor with a variable displacement mechanism according to the prior art; 
     FIG. 2 is an enlarged partial view of FIG. 1 illustrating various forces acting on a piston; 
     FIG. 3 is a longitudinal cross-sectional view of a variable capacity swash plate type compressor with a piston and a mechanism to minimize a bending moment acting on a piston, in accordance with the present invention; and 
     FIG. 4 is an enlarged partial view of FIG. 3 illustrating the configuration of the piston and swash plate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a compressor, for example, a variable capacity swash plate type compressor, having a mechanism for minimizing a bending moment. The variable capacity swash plate type compressor  70  has a cylinder block  72  provided with a plurality of cylinder bores  74 , a front housing  76  and a rear housing  78 . Both front and rear ends of the cylinder block  72  are sealingly closed by the front and rear housings  76  and  78 , respectively. A valve plate  80  is mounted between the cylinder block  72  and the rear housing  78 . The cylinder block  72  and the front housing  76  define a sealed crank chamber  82 . A drive shaft  84  is centrally arranged to extend through the front housing  76  to the cylinder block  72 , and is rotatably supported by radial bearings  86  and  87 . The cylinder block  72  and the front and rear housings  76  and  78  are held together by screws  89 . 
     A rotor  90  is fixedly mounted on the drive shaft  84  within the crank chamber  82  to be rotatable with the drive shaft  84 , and supported by a thrust bearing  92  seated on an inner end of the front housing  16 . A swash plate  94  is rotatably supported on the drive shaft  84 . A spherical sleeve can be mounted between the drive shaft  84  and the swash plate  94 , and in this case, the swash plate  94  is rotatably supported on an outer surface of the shaft. 
     FIG. 3 illustrates the compressor with the swash plate  94  at its maximum inclination angle position. In this position, a spring  98 , which biases the swash plate  94  toward its minimum position, is compressed, and a stop surface  96   a  of a projection  96  comes into contact with the rotor  90  so that a further increase of inclination angle of the swash plate  94  is prevented. On the other hand, when the swash plate  94  is in its minimum angle position (not shown), a further decrease of inclination angle of the swash plate  94  is restricted by a stopper  97  provided on the drive shaft  84 . 
     The swash plate  94  is connected to the rotor  90  via a hinge mechanism to be rotated with the rotor  90 . That is, a support arm  100  protrudes rearwardly from one side surface of the rotor  90 , and an arm  102  protrudes from a front side of the swash plate  94  toward the support arm  100  of the rotor  90 . The support arm  100  and the arm  102  overlap each other and are connected to each other by a pin  104 . The pin  102  extends into a pin hole  106  formed through the support arm  100  of the rotor  90  and a rectangular shaped hole  108  formed through the arm  102  of the swash plate  94 . With this arrangement, the rotor  90  and the swash plate  94  are hinged to each other, and the sliding motion of the pin  104  within the rectangular hole  108  changes an inclination angle of the swash plate  94  so as to change the capacity of the compressor. 
     As best illustrated in FIG. 4, each cylindrical piston  110  has a body  112  with a head and a bridge portion  114 . The bridge portion  114  has a recess  120 , and opposed walls defined in the recess  120  have spherical shoe pockets  124  into which spherical outer surfaces of two semi-spherical shoes  112  are slidably disposed. The inner flat surfaces of the shoes  112  are slidably engaged with side surfaces of the swash plate  94 . With this arrangement, each piston  110  is engaged with the swash plate  94  via the shoes  12  and pockets  124 , and therefore, the nutating motion of the swash plate  94  causes each piston  110  to reciprocate in the cylinder bore  74 . 
     During the compression stroke of the piston  110 , the force F (as illustrated in FIG. 2) exerted on the piston  110  from the swash plate  94  via the shoe adjacent to the body  110  of the piston acts on the piston  110  at a right angle to a rear surface of the swash plate  94  at a contact surface (in case of a line contact) or a contact point (in case of a point contact, both will be referred as a contact position or an apex hereinafter) at which the semi-spherical outer surface of the shoe  122  adjacent to the body  112  comes into contact with the semi-spherical inner surface of the shoe pocket  124 . The force F exerted from the swash plate  94  on the piston  110  can be considered as two components, the horizontal component F x  lying on the central axis O of the piston  110  and the vertical component F y  perpendicular to the central axis O of the piston  110  (again, as is illustrated in FIG.  2 ). The vertical component F y  acts on the piston  110  to create a bending moment. 
     In order to minimize the bending moment, no cutout portion is formed in the body  112  of the piston  110 , as is the case with the prior art. That is, in the construction of the piston in accordance with the present invention, the lower edge P of the body  112  of the piston  110  lies on the line S which passes through the apex Q 2  of the shoe pocket  124  and is perpendicular to the central axis O of the piston  110 . Moreover, the lower edge P of the piston body  112  can be further extended in line with an entrance point Q 1  of the shoe pocket  124  near the piston body  112 , if so desired. Consequently, the maximum bending moment acting on the piston does not occur as is the case with the prior art, shown in equation (3) above. 
     The interference between the swash plate  94  and the piston  110  due to the extension of the piston body  112  is avoided by forming a recess  126  in the rear surface of the swash plate  94 . 
     The rear housing  78  is provided with inlet and outlet ports  130  and  132 , and divided into suction and discharge chambers  134  and  136 . The valve plate  80  has suction and discharge ports  138  and  140 . Each cylinder bore  74  communicates with the suction chamber  134  and the discharge chamber  136  via the suction ports  138  and the discharge ports  140 , respectively. Each suction port  138  is opened and closed by a suction valve  142 , and each discharge port  140  is opened and closed by a discharge valve  144 , in response to the reciprocal movement of the respective pistons  110 . The opening motion of the discharge valve  144  is restricted by a retainer  146 . 
     A control valve means  148  is provided with the compressor  70  for adjusting a pressure level within the crank chamber  82 . 
     In the compressor having the above-described construction, when the drive shaft  84  is rotated, the swash plate  94 , having a certain inclination angle, is also rotated via the hinge mechanism, and thus the nutating motion of the swash plate  94  is converted into the reciprocation of the pistons  110  within the respective cylinder bores  74  via the shoes  122 . This reciprocating motion causes the refrigerant gas to be introduced from the suction chamber  134  of the rear housing  78  into the respective cylinder bores  74  in which the refrigerant gas is compressed by the reciprocating motion of the pistons  110 . The compressed refrigerant gas is discharged from the respective cylinder bores  74  into the discharge chamber  136 . 
     At this time, the capacity of the compressed refrigerant gas discharged from the cylinder bores  74  into the discharge chamber  136  is controlled by the control valve means  148  which changes the pressure level within the crank chamber  82 . Specifically, when the pressure level P sc  in the suction chamber  134  is raised, generally as the result of an increase of the thermal load of an evaporator, the control valve means  148  cuts off the refrigerant gas traveling from the discharge chamber  136  into the crank chamber  82  so that the pressure level P cc  in the crank chamber  82  is lowered. When the pressure level in the crank chamber  82  is lowered, a back pressure acting on the respective pistons  110  is decreased, and therefore, the angle of inclination of the swash plate  94  is increased. Namely, the pin  104  of the hinge means slides downward within the rectangular hole  108 . Accordingly, the swash plate  94  is moved in a forward direction against the force of the spring  98 . Therefore, the angle of inclination of the swash plate  94  is increased, and as a result, the stroke of the respective pistons  110  is increased. 
     On the contrary, when the pressure level P sc  in the suction chamber  134  is lowered with decrease of the thermal load of the evaporator, the control valve means  148  passes the compressed refrigerant gas of the discharge chamber  136  into the crank chamber  82 . As the pressure level P cc  in the crank chamber  82  is raised, a back pressure acting on the respective piston  110  is increased, and therefore, the angle of inclination of the swash plate  94  is decreased. Namely, the pin  104  of the hinge means slides upward within the rectangular hole  108 . Accordingly, the swash plate  94  is moved in a reward direction yielding to the force of the spring  98 . Therefore, the inclination angle of the swash plate  94  is decreased, and as a result, the stroke of the respective pistons  110  is shortened and the discharge capacity is decreased. Whether at a minimum or maximum inclination angle, or anywhere in between, the recess  126  in the swash plate  94  will allow for piston movement without contact between the point P on the piston  110  and the swash plate  94 . 
     In the above described compressor, during the compression stroke of the piston  110 , the pressure P cc  in the crank chamber  82  and the compression reaction pressure P dc  act on the piston  110 . These forces act on the swash plate  94  via the shoes  122  and, in turn, oppositely act on the piston  110  from the swash plate  94  as a reaction force equal in magnitude and oppositely directed. At this time, the maximum bending moment acts on the lower back edge portion P of the piston  110 . However, the lower back edge portion P lies on the same line as the vertical component F y  lies, and thus, the bending moment does not occur on the lower back edge portion P of the piston  110  because the distance x is zero. As a result, deformation and abnormal abrasion of the pistons can be prevented. 
     Although the present invention has been described in connection with the preferred embodiments, the invention is not limited thereto. It will be easily understood by those skilled in the art that variations and modifications can be easily made within the scope of the present invention as defined by the appended claims.