Abstract:
A piston compressor includes single-headed pistons individually received for reciprocal linear motion in cylinder bores, and is arranged to suppress the wear of these pistons with simple machining for wear prevention. The single-headed piston includes a first piston portion disposed in close contact with the cylinder bore, for sucking refrigerant thereinto and compressing the refrigerant, and a second piston portion adjacently formed in a crankcase-side end of the first piston portion and having a diameter smaller than that of the first piston portion, so as to be prevented from being in contact with the cylinder bore, or so as to apply, even if in contact therewith, a small thrust force to the cylinder bore.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a piston compressor for sucking and compressing refrigerant by a reciprocal motion of a single-headed piston.  
           [0003]    2. Related Art  
           [0004]    A conventional piston compressor of the above kind, e.g., a single-headed swash plate type compressor, is typically constructed as described below.  
           [0005]    Specifically, the single-headed swash plate type compressor comprises a plurality of cylinder bores formed in a cylinder block so as to be separated from one another, single-headed pistons individually received for reciprocal motion in the cylinder bores, and a motion conversion mechanism disposed in a crankcase for converting a rotary motion of a driving source into reciprocal motions of the single-headed pistons. The motion conversion mechanism comprises a swash plate mounted to a rotary drive shaft, and a pair of shoes provided at bridge portions of the single-headed pistons, with peripheral portions of the swash plate slidably fitted between the shoes. With rotation of the drive shaft, the swash plate rotates in an inclined state and the single-headed pistons are reciprocated vertically.  
           [0006]    In such a single-headed piston swash plate type compressor, the single-headed piston is in contact with an end portion of the cylinder bore on the side of the crankcase when the piston is linearly reciprocated, so that a concentrated thrust force is applied to the end portion of the cylinder bore. Referring to FIG. 12, such effect will be described. When the single-headed piston  2  starts a compression stroke with the rotation of the swash plate  1 , a force represented by two components, a compression force F 1  and a component F 2  perpendicular thereto, is applied to the single-headed piston  2 , and thus the piston  2  receives a clockwise moment M as shown in FIG. 12. The moment M acts as a concentrated thrust force FM 1  that is applied to a contact portion P 1  between an outer peripheral face of the piston  2  and an inner peripheral face of the cylinder bore  3  on the side of the crankcase. This poses a problem that the inner peripheral face of the cylinder bore  3  and the outer peripheral face of the piston  2  are liable to be worn soon, and the resultant dust can clog piping systems. In recent years, the conversion of fluorine refrigerant into carbon dioxide (CO 2 ) refrigerant has been recommended especially for environment of the earth. An amount of carbon dioxide refrigerant in use may be 1/6 to 1/8 times as much as compared with that of fluorine refrigerant, and accordingly the single-headed piston  2  can be reduced in diameter, so that the diameter of the contact portion between the piston  2  and the cylinder bore  3  may also decrease. On the other hand, an operating pressure increases to about ten times as large as that in the case of fluorine refrigerant, and thus the concentrated thrust force FM 1  extremely increases.  
           [0007]    In this regard, the present inventors investigated wear marks  2   a  in single-headed pistons  2  in single-headed piston swash plate type compressors in which carbon dioxide refrigerant is used. As a result of the investigation, it is found that wear marks  2   a  obliquely extend from proximal portions of single-head pistons  2  to distal ends thereof as shown in FIG. 13A. This indicates that such wear marks  2   a  were formed in compression strokes in view of the fact that a thrust face between the single-headed piston  2  and the cylinder bore  3  moves in the direction shown by arrow A in FIG. 13B in the compression stroke, whereas it moves in the suction stroke in the direction shown by arrow B.  
           [0008]    In the suction stroke, the single-headed piston  2  is inclined to the opposite direction as shown by two-dotted chain line in FIG. 12, so as to be in contact with the inner peripheral face of the cylinder bore  3 , but no wear mark is formed.  
           [0009]    To obviate the aforesaid wear problem in the single-headed piston  2 , the following inventions have been proposed. In the invention disclosed in JP-A-5-302570, an elastic member is provided at an end portion of the cylinder bore on the side of the crankcase, to which portion the concentrated thrust force is applied. With such a construction, the thrust force produced by the single-headed piston is absorbed by the elastic member, whereby the piston is prevented from being worn.  
           [0010]    In another invention disclosed in JP-A-2002-12472, a groove is formed in an outer peripheral face of the single-headed piston so as to extend in the direction along which the piston is slidingly moved. The groove serves to prevent a pressure contact between the single-headed piston and the cylinder bore, thus preventing wear of the piston.  
           [0011]    However, the former piston compressor (the invention disclosed in JP-A-5-302570) requires the provision of the elastic member for every cylinder bore, thus increasing the number of component parts. And mounting of the elastic members is cumbersome.  
           [0012]    In the latter piston compressor (the invention disclosed in JP-A-2002-13472), the single-headed piston must be subject to grooving to vertically form a groove in a cylindrical outer peripheral face of the piston, and such machining is cumbersome.  
         SUMMARY OF THE INVENTION  
         [0013]    An object of the present invention is to provide a piston compressor capable of preventing a single-headed piston from being worn, with simple machining for prevention of wear.  
           [0014]    According to the present invention, there is provided a piston compressor which comprises cylinder bores formed in a cylinder block portion so as to be separated from one another, single-headed pistons individually received for reciprocal motion in the cylinder bores, and a motion conversion mechanism disposed in a crankcase for converting a rotary motion of a driving source into reciprocal motions of the single-headed pistons, wherein each of the single-headed pistons includes a first piston portion disposed in close contact with the corresponding cylinder bore for sucking and compressing refrigerant, and a second piston portion formed adjacent to an axial end portion of the first piston portion on a side of the crankcase and having a diameter smaller than that of the first piston portion.  
           [0015]    According to the present invention, the second piston portion smaller in diameter is prevented from being in contact with the cylinder bore, or alternatively a thrust force applied from the second piston portion to the cylinder bore is made smaller even if the second piston portion is in contact with the cylinder bore. Although both the first and second piston portions can simultaneously be in contact with the cylinder bore, in this case, the thrust force applied from the single-headed piston to the cylinder bore is dispersed to two locations of the cylinder bore, so that the thrust force received at each location is made smaller. The second piston portion is only required to be formed smaller in diameter than the first piston portion, and can easily be fabricated by cutting work.  
           [0016]    In the present invention, the second piston may be formed with an annular oil groove for oil intake in vicinity of the first piston portion.  
           [0017]    With this arrangement, a lubricating oil can be retained in the oil groove, and the retained lubricating oil is supplied to the contact between the single-headed piston and the cylinder bore to reduce a friction resistance therebetween when the concentrated thrust force is applied during the compression stroke, whereby the single-headed piston is suppressed from being worn.  
           [0018]    The first piston portion may have a distal end periphery that is formed into an R shape. In this case, a friction resistance between the distal end periphery of the single-headed piston and the inner face of the cylinder bore is made smaller, thus suppressing the wear of the single-headed piston.  
           [0019]    The first piston portion may be coated at its outer peripheral face with a wear resistant material (such as polytetrafluoroethylene, for example), so as to suppress the wear of the single-headed piston. According to the present invention, the wear of the single-headed piston can positively be prevented even when carbon dioxide refrigerant or hydro carbon refrigerant is used, in which the single-headed piston can produce a higher concentrated thrust force as compared with a case where fluorine refrigerant is used. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a sectional view of a piston compressor;  
         [0021]    [0021]FIG. 2A is a front view of a single-headed piston according to a first embodiment;  
         [0022]    [0022]FIG. 2B is a side view of the piston shown in FIG. 2A;  
         [0023]    [0023]FIG. 3 is a sectional view of an essential part of the first embodiment, in which an initial state in a refrigerant compression stroke is shown;  
         [0024]    [0024]FIG. 4 is a sectional view of the essential part of the first embodiment, in which a state in the course of the refrigerant compression stroke is shown;  
         [0025]    [0025]FIG. 5 is a sectional view of the essential part of the first embodiment, in which a final state of the refrigerant compression stroke is shown;  
         [0026]    [0026]FIG. 6 is a front view of a single-headed piston according to a second embodiment;  
         [0027]    [0027]FIG. 7 is a front view of a single-headed piston according to a third embodiment;  
         [0028]    [0028]FIG. 8 is a sectional view of an essential part of the third embodiment, in which a state in the course of a refrigerant compression stroke is shown;  
         [0029]    [0029]FIG. 9 is an enlarged sectional view of an essential part of the third embodiment, in which a state in the course of a refrigerant compression stroke is shown;  
         [0030]    [0030]FIG. 10A is a front view of a single-headed piston according to a fourth embodiment;  
         [0031]    [0031]FIG. 10B is a side view of the piston shown in FIG. 10A;  
         [0032]    [0032]FIG. 11 is a sectional view of an essential part of the fourth embodiment, in which an initial state of a refrigerant compression stroke is shown;  
         [0033]    [0033]FIG. 12 is a sectional view for explaining a concentrated thrust force that is generated in a conventional single-headed piston; and  
         [0034]    [0034]FIG. 13A is a view of the conventional piston in which a wear track is present; and  
         [0035]    [0035]FIG. 13B is a view showing an operation of the conventional piston. 
     
    
     DETAILED DESCRIPTION  
       [0036]    With reference to FIG. 1, the overall arrangement of a compressor will be explained.  
         [0037]    The compressor  10  is provided with a rear housing  11  and a front housing  12  whose rear housing side is coupled to a cylinder block portion  13  by means of bolts, etc., not shown. The cylinder block portion  13  is formed with a plurality of cylindrical cylinder bores  131  each of which receives a single-headed piston  132  for reciprocal linear motion. The rear housing  11  is formed with a refrigerant suction chamber  111  and a refrigerant discharge chamber  112 . These chambers  111 ,  112  are in communication with the cylinder bore  131  through valves, so that refrigerant is sucked into and compressed in the cylinder bore  131  with a reciprocal motion of the single-headed piston  132 . A rotary shaft  121  extends through the front housing  12  from the cylinder block portion  13  to the outside of the front housing  12 . The rotary shaft  121  is rotatably supported by bearings  133 ,  122  individually affixed to the cylinder block portion  13  and the front housing  12 . The rotary shaft  121  is coupled at its distal end to an electromagnetic clutch  14  having a pulley  141  coupled to a belt of an automotive engine, not shown, so that a torque generated by the engine may be transmitted through the electromagnetic clutch  14  to the rotary shaft  121 .  
         [0038]    The rotary motion transmitted to the rotary shaft  121  is converted into a reciprocal linear motion by means of a motion conversion mechanism  15  which is disposed in a crankcase  123  of the front housing  12 . The motion conversion mechanism  15  is comprised of a rotor  151  fixed to the rotary shaft  121 , a coupling portion  152  coupled to the rotor  151 , a swash plate  153  fixed to the coupling portion  152 , and a pair of shoes  154  interposed between bridge portions  132   a  of the single-headed pistons  132 , wherein peripheral portions of the swash plate  153  are held in the shoes  154 . With rotation of the rotary shaft  121 , the rotor  151  and the coupling portion  152  rotate, and the swash plate  153  also rotates. The swash plate  153  rotates in a state where it is inclined with respect to the rotary shaft  121 , so that the single-headed pistons  132  are linearly reciprocated with a stroke corresponding to the inclination width of the swash plate  153 . In the motion conversion mechanism  15 , the inclination angle of the swash plate  153  varies corresponding to a differential pressure between a pressure in the crankcase  123  and a suction pressure in the cylinder bore  131 , so that the stroke of the single-headed pistons  132  may be changed.  
         [0039]    The aforementioned compressor structure is similar to a conventional one. The compressor  10  of this invention is characterized by a construction of the single-headed pistons  132  which will be described below with reference mainly to FIGS. 2A and 2B.  
         [0040]    Specifically, the single-headed piston  132  comprises a piston body  132   b  extending from a bridge portion  132   a , provided at one axial end of the piston, to another axial end or distal end of the piston  132 . As shown in FIG. 2B, the bridge portion  132   a  is formed into a U-shape in section. In the inside of the bridge portion  132   a , a pair of shoes  154  hold the periphery of the swash plate  153  so as to be slidable as shown in FIG. 1, so that the piston  132  may vertically be reciprocated with a swing motion of the swash plate  153 .  
         [0041]    Referring to FIG. 2B, the piston body  132   b  is constituted by a first piston portion  132   c  formed to have a diameter slightly smaller than the inner diameter of the cylinder bore  131 , and a second piston portion  132   d  formed to have a diameter smaller than that of the first piston portion  132   c . The first piston portion  132   c  serves to suck refrigerant into the cylinder bore  131  and compress the refrigerant in the cylinder bore  131 , by means of a distal end face of the first piston portion  132 . The second piston portion  132   d  is formed to extend from the crankcase-side axial end portion of the first piston portion  132   c  to the bridge portion  132   a . The axial and diameter sizes of the second piston portion  132   d  are determined such that a peripheral side face of the second piston portion  132   d  is not in contact with the crankcase-side end portion  131   a  of the cylinder bore  131  in the refrigerant compression stroke, as shown in FIG. 4. In the present embodiment, the second piston portion  132   d  is formed to have an axial size L 1  that is about 10 mm (which is about half the stroke of the single-headed piston  132 ), and have a diameter size which is, e.g., about 1 mm to 2 mm smaller than that of the first piston portion  132   c.    
         [0042]    The reason why the axial size L 1  of the second piston portion  132   d  is made about half the stroke of the single-headed piston  132  is that, if it is made shorter than the half of the piston stroke, there is a high possibility of the single-headed piston being in contact with the crankcase-side end portion of the cylinder bore to make a wear resistant material liable to be removed and, by extension, make the single-headed piston liable to be worn. On the other hand, if the axial size is made equal to or longer than the half of the piston stroke, there is no substantial possibility of the piston being in contact of the cylinder bore, thus preventing the material removal and piston wear. The axial size may be increased to its upper limit below which a moderate compression function of the single-headed piston can be maintained.  
         [0043]    According to the present embodiment, when the single-headed piston  132  starts the compression stroke with the rotation of the swash plate  153 , the piston is applied with a compression force and another component force perpendicular thereto, so that the single-headed piston  132  receives a moment as in the conventional one, and is brought in an inclined state as shown in FIG. 3.  
         [0044]    At an initial stage of the compression stroke, an outer peripheral face of the first piston portion  132   c  is in press contact with the crankcase-side end portion (i.e., an edge  131   a ) of the cylinder bore  131 , as shown in FIG. 3, and part of the distal end periphery  132   e  of the first piston portion  132   c  is also in press contact with the inner peripheral face of the cylinder bore  131 . In response to concentrated thrust forces being applied from these two parts of the first piston portions, reaction forces FM 1 ′, FM 2 ′ are applied from the cylinder bore  131  to the first piston portion  132   c , which is a cause of wear of the first piston portion  132   c . In particular, wear is caused at the contact (to which the reaction force FM 1 ′ is applied) between the edge  131   a  of the cylinder bore  131  and the outer peripheral face of the first piston portion  132   c.    
         [0045]    With advance of the compression stroke, the first piston portion  132   c  is out of contact with the edge  131   a  of the cylinder bore  131 , and the edge  131   a  is brought to face the outer peripheral face of the second piston portion  132   d . Since the diameter of the second piston portion  132   d  is smaller than that of the first piston portion  132   c , the edge  131   a  is separated from the second piston portion  132   d  as shown in FIG. 4. Thus, the wear of the single-headed piston  132  which would be caused by the contact with the edge  131   a  is prevented from in the course of the compression stroke.  
         [0046]    Although the reaction force FM 1 ′ is applied from the inner face of the cylinder bore  131  to part of the rear end  132   f  of the first piston portion  132   c , the cylinder bore  131  and the first piston portion  132   c  are in face-contact with each other, and thus no so much abrasion force is produced.  
         [0047]    In addition to the contact between the first piston portion  132   c  and the cylinder bore  131 , there can be a contact between the second piston portion  132   d  and the edge  131   a  of the cylinder bore  131 . In this case, the thrust force applied from the single-headed piston  132  to the cylinder bore  131  is dispersed, and therefore, the concentrated thrust force at each contact part is made smaller.  
         [0048]    At the final stage of the compression stroke, the edge  131   a  and the second piston portion  132   d  are completely separated from each other, as shown in FIG. 5.  
         [0049]    According to the present embodiment, therefore, the edge  131   a  is out of contact with the second piston portion  132   d  from in the course of the compression stroke, and both the reaction forces FM 1 ′, FM 2 ′ from the cylinder bore  131  are applied in face contact. Thus, from in the course of the compression stroke, not so much wearing force is applied to the single-headed piston  132 .  
         [0050]    By comparing the single-headed piston  132  of this embodiment shown in FIG. 2 to the conventional piston  2  shown in FIG. 12, it is understood that the wear track  132   a  in the piston  132  is more reduced than the track  2   a  in the piston  2 .  
         [0051]    In the refrigerant compression stroke, the operating pressure increases with advance of refrigerant compression, and accordingly the reaction forces FM 1 ′, FM 2 ′ also increase. The aforementioned arrangement, which suppresses the wear of the single-headed piston  132  from in the course of the compression stroke, serves as effective abrasion suppression means.  
         [0052]    [0052]FIG. 6 shows an essential part, i.e., a single-headed piston, of a piston compressor according to a second embodiment of this invention. The single-headed piston  134  corresponds to one that is obtained by applying a radius to the distal end periphery  132   e  of the first piston portion  132   c  of the first embodiment, i.e., by forming the distal end periphery  132   e  into a rounded shape. As for structural parts common to those of the piston  132  of the first embodiment, they are shown by like numerals and their explanations are omitted herein.  
         [0053]    According to the second embodiment, when the distal end periphery  132   e  of the first piston portion  132   c  is abutted against the inner peripheral face of the cylinder bore  131  in the refrigerant compression stroke, the reaction force FM 2 ′ is applied to the outer face of the first piston portion  132   c , but the reaction force FM 2 ′ is dispersed by means of the rounded shape structure, thus suppressing the wear of the first piston portion  132   c . In the other respects, functions and effects are similar to those of the first embodiment.  
         [0054]    FIGS.  7 - 9  show an essential part, i.e., a single-headed piston, of a piston compressor according to a third embodiment. The single-headed piston  13  corresponds to one that is obtained by forming an oil groove  135   a  in the second piston portion  132   d  of the first embodiment. Structural elements common to the single-headed piston  132  of the first embodiment are shown by like numerals and explanations thereon are omitted.  
         [0055]    As shown in FIG. 7, the oil groove  135   a  is circumferentially formed, e.g., by grooving, in the second piston portion  132   d  in the vicinity of the first piston portion  132   c .  
         [0056]    According to the third embodiment, lubricating oil  16  in the crankcase  123  is adhered to the single-headed piston  135  in a suction stroke, and part of the oil accumulates in the oil groove  135   a . During the refrigerant compression stroke, blowby gas (one-dotted chain line with arrowhead) is sucked into the crankcase  123  as shown in FIGS. 8 and 9, whereas the lubricating oil  16  adhered to the outer face of the single-headed piston  135  is retained as it is. Thus, during the compression stroke, the lubricating oil  16  reduces a frictional resistance between the first piston portion  132   c  and the cylinder bore  131 , whereby the wear of the first piston portion  132   c  is suppressed.  
         [0057]    In the single-headed piston  135  of the third embodiment, the distal end periphery  132   e  may, of course, be formed into a rounded shape as in the second embodiment. In other respects, functions and effects are similar to those of the first or second embodiment.  
         [0058]    [0058]FIGS. 10A and 10B show an essential part, i.e., a single-headed piston of a piston compressor according to a fourth embodiment of this invention. The single-headed piston  136  corresponds to one that is obtained by changing a ratio between axial sizes of the first and second piston portions  132   c ,  132  of the first embodiment. In the fourth embodiment, the first and second piston portions  136   a ,  136   b  are configured that an axial dimension L 2  of the second piston portion  136   b  is set to be longer than that L 1  of the first piston portion  132   a.    
         [0059]    According to the fourth embodiment, the prevention of a contact between the edge  131   a  of the cylinder bore  131  and the first piston portion  136   a  is started earlier by an enlengthened amount of the second piston portion  136   b , i.e., started from an initial stage of the refrigerant compression stroke in FIG. 11. With this arrangement, no wear track is formed in the outer peripheral face of the first piston portion  136   a.    
         [0060]    On the other hand, when compared the inclination angle θ2 of the single-headed piston  136  and that θ1 of the piston of the first embodiment, there is a relation of θ2&gt;θ1, i.e., the inclination angle θ2 of the piston  136  is larger. This is because each of the first piston portions  132   c ,  136   a  serves to restrict the inclination of the piston  132  or  136  associated therewith, and the strength of this inclination restricting function varies in proportion to the axial dimension of the first piston portion  132   c  or  136   a . The single-headed piston  136  has a larger inclination angle by an amount by which the axial dimension of the first piston portion  136   b  of the fourth embodiment is smaller than that of the first piston portion  132  of the first embodiment. As a result, an angle of contact between the first piston portion  136   a  and the cylinder bore  131  becomes large, and hence a friction resistance tends to increase.  
         [0061]    Such increase in friction resistance can be not only a cause to hinder a smooth reciprocal motion of the single-headed piston  136  but also a cause of wear of the piston  136 . Thus, the ratio between the axial dimensions of the first and second piston portions  136   a ,  136   b  is determined by comprehensively taking into account of these factors.  
         [0062]    The single-headed pistons  132 ,  134 ,  135  and  136  that are used in the first to fourth embodiments are constituted by a piston material, such as a ferrous material or aluminum material (including aluminum alloy material) as it is. Alternatively, at least an outer peripheral face of the first piston portion  132   c  or  136   a  may be coated with a wear resistant material (e.g., polytetrafluoroethylene) in uniform film thickness, e.g., of 20 μm to 40 μm, so as to suppress the wear of the piston  132 ,  134 ,  135  or  136 . In the compressor  10  of the embodiment, refrigerant of carbon dioxide or hydro carbon such as propane, butane, etc., other than fleon, ammonium, etc., may be used.