Abstract:
A piston type compressor has a housing, a cylinder block and a piston. The cylinder block is fixed to the housing. The piston is accommodated in the cylinder block. A piston ring is provided between the cylinder block and the piston. A sealing coat is made of soft metal, and is provided between the piston ring and the piston.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a compressor. More particularly, the present invention relates to a piston type compressor that provides a piston ring fitted onto a piston. 
     A piston type compressor such as a swash plate type compressor generally includes a cylinder block and suction and discharge chambers so as to sandwich a valve plate assembly, and the cylinder block accommodates a piston. By reciprocation of the pistons, fluid in the suction chamber is sucked into the cylinder block, and the fluid sucked in the cylinder block is compressed and discharged to the discharge chamber. Also, to suck the fluid into the cylinder block and compress and discharge the fluid to the discharge chamber efficiently, sealing performance between the pistons and the cylinder block is important. Japanese Unexamined Patent Publication No. 11-294322 discloses a compressor that provides a coating made of fluoro resin on the outer circumferential surface of the pistons and a piston ring fitted onto the pistons. Thereby, sealing performance between the pistons and the cylinder block is ensured. 
     To achieve higher compression efficiency, sealing performance between the pistons and the piston rings in addition to sealing performance of the pistons and the cylinder block is also required to improve. Alternative refrigerant gas such as carbon dioxide is promoted to be a practical use to deal with environmental problems these days. However, carbon dioxide for using in a compressor as a compressing target requires quite a high compression ratio. Therefore, the above-mentioned requirements for sealing performance have been further increasing these days. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-mentioned problems traceable to a relatively high compression ratio by improving sealing performance between pistons and piston rings. 
     According to the present invention, a piston type compressor has a housing, a cylinder block and a piston. The cylinder block is fixed to the housing. The piston is accommodated in the cylinder block. A piston ring is provided between the cylinder block and the piston. A sealing coat is made of soft metal, and is provided between the piston ring and the piston. 
     In the piston type compressor mentioned above, sealing performance between the piston ring and the piston is improved by the sealing coat made of soft metal. 
     The present invention also provides a method of forming a sealing coat on a surface of a groove on a piston. The method includes forming a coat made of fluoro resin on the outer circumferential surface of the piston, recessing a groove for accommodating a piston ring on the outer circumferential surface of the piston by machining, and immersing the piston in soft metal. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a longitudinal cross-sectional view of a piston type compressor according to an embodiment of the present invention; 
     FIG. 2 is a side view of a piston in FIG. 1; 
     FIG. 3 is an enlarged cross-sectional partial view showing a piston ring fitted onto a piston in FIG. 1; 
     FIG. 4 is a side view of a piston with a plurality of grooves according to another embodiment of the present invention; and 
     FIG. 5 is an enlarged cross-sectional partial view showing a piston ring fitted onto a piston in FIG. 1 according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention, which is applied to a swash plate type variable displacement piston type compressor for compressing refrigerant gas, will now be described with reference to FIGS. 1 through 4. The left side and the right side in FIG. 1 correspond to the front end and the rear end, respectively. 
     As shown in FIG. 1, a bolt  4  screws a front housing  1  to a rear housing  2  via a gasket  3 , thus constructing a housing  5  of a compressor. The rear housing  2  provides a step  6  inside. A retainer plate  7 , a discharge valve plate  8 , a valve plate  9  and a suction valve plate  10  are fitted onto the step  6 . The retainer plate  7  and a rear end wall  11  of the rear housing  2  define a suction chamber  12  and a discharge chamber  13  such that a partition wall  14  separates the suction chamber  12  and the discharge chamber  13  from each other. 
     A cylinder block  15  is fitted onto the suction valve plate  10  in the rear housing  2 . The cylinder block  15  and the front housing  1  rotatably support a drive shaft  16 . The drive shaft  16  protrudes its front end outside the front housing  1 , and connects with a driving source such as an engine and a motor of a vehicle, which is not shown. In the front housing  1 , a lug plate  17  is secured to the drive shaft  16 , and a swash plate  18  engages with the lug plate  17 . The drive shaft  16  extends a through hole, which is formed through the center of the swash plate  18 . A pair of guide pins  19  extending from the swash plate  18  is slidably fitted into a pair of guide holes  20  formed with the lug plate  17 . The swash plate  18  integrally rotates with the drive shaft  16  so that the guide pins  19  engages with the guide holes  20 , and is tiltably supported by the drive shaft  16  so as to slide along the axis of the drive shaft  16 . 
     A plurality of cylinder bores  21  is defined in the cylinder block  15  so as to surround the drive shaft  16 , the cylinder bores  21  each slidably accommodate respective pistons  22 . The pistons  22  each engage with the periphery of the swash plate  18  through a pair of shoes  23 . As the swash plate  18  rotates with the drive shaft  16 , the pistons  22  each reciprocate relative to the axis of the drive shaft  16  in the associated cylinder bores  21  through shoes  23 . Besides, the single cylinder bore  21  and the single piston  22  are shown in FIG.  1 . However, the compressor provides seven cylinder bores  21  and the seven pistons  22  in this embodiment. 
     The discharge chamber  13  communicates with a crank chamber  29 , or a control chamber  29 , which is defined in the front housing  1  via a supply passage  27  and a control valve  28 , and the crank chamber  29  communicates with the suction chamber  12  via a bleed passage  30 . As the control valve  28  opens, refrigerant gas in the discharge chamber  13  flows into the crank chamber  29  via the supply passage  27  and the control valve  28 , thus increasing pressure in the crank chamber  29 . The inclination of the swash plate  18  varies in accordance with the pressure in the crank chamber  29 . As the pressure in the crank chamber  29  increases, the inclination angle relative to the plane perpendicular to the axis of the drive shaft  16  decreases. As the pressure in the crank chamber  29  decreases, the inclination angle increases. Namely, the inclination of the swash plate  18  is varied by adjusting the control valve  28  due to an external control or an internal control. 
     As shown in FIGS. 1 through 3, the outer circumferential surface of the pistons  22  adjacent to a piston head each provide annular grooves  31 . A groove surface  22   a , the cross section of which is rectangular defines the groove  31  on the piston  22 . An annular piston ring  32  occupies the groove  31 . The piston ring  32  is made by shaping a cast iron member, the cross section of which is rectangular, into a ring. Also, the groove surface  22   a  provides soft metal, or a sealing coat  33  made of tin in the present embodiment by nonelectrolytically coating. The thickness of the tin sealing coat  33  is from 2 μm to 3 μm. A process of forming the sealing coat  33  will now be described. In the present embodiment, a coat made of fluoro resin is formed on the outer circumferential surface of the piston  22 , which is made of aluminum. After that, the groove  31  is recessed by machining. The tin sealing coat  33  coats the groove surface  22   a  by immersing the piston  22  with the groove  31  in tin. No tin coats the circumferential surface of the piston  22 , which is coated with fluoro resin. Since the groove  31  is formed by machining, the tin sealing coat  33  coats the groove surface  22   a , which is not coated with fluoro resin. For example, when not the tin sealing coat but a resin sealing coat is formed, the following processes are required: 1) recessing a groove on a piston; 2) coating with resin; and 3) treating the surface of a resin coat. However, when the tin sealing coat is formed, the above-described process 1) recessing a groove on a piston and process 2) coating with tin are required only. Thereby, manufacturing cost is reduced. Also, wettability of the tin sealing coat is higher than that of the resin sealing coat. Therefore, the tin sealing coat is available in performing such higher sealing performance relative to the resin sealing coat without treating the surface of the tin sealing coat. 
     The operation of the piston type compressor constructed above will now be described. Due to motion that the piston  22  moves from a top dead center toward a bottom dead center, refrigerant gas in the suction chamber  12  flows into a suction port  34  of the valve plate  9 , and pushes a suction reed valve of the suction valve plate  10  aside, then flows into the cylinder bore  21 . Due to motion that the piston  22  moves from the bottom dead center toward the top dead center, the refrigerant gas flows into a discharge port  35  of the valve plate  9 , and pushes a discharge reed valve of the discharge valve plate  8  aside, then flows into the discharge chamber  13 . Also, the tin sealing coat  33  performs high wettability with lubricant contained in the refrigerant gas. Thereby, when pressure of refrigerant gas such as carbon dioxide is high, the tin sealing coat  33  raises sealing performance between the piston ring  32  and the piston  22  during reciprocation of the piston  22 , and inhibits the refrigerant gas from leaking therebetween. Therefore, compression efficiency improves, and lubrication is ensured. Also, when roughness of the groove surface  22   a  does not satisfies requirement, high sealing performance is ensured by coating the groove surface  22   a  with the tin sealing coat  33 . 
     The present invention is not limited to the embodiment described above, but may be modified into the following examples. 
     The sealing coat is not limited to the tin sealing coat. For example, other soft metals, which performs high wettability with lubricant such as lead and zinc may be applied. Also, a position coated with the sealing coat, which is made of soft metal, is not limited to the groove surface  22   a . The sealing coat may coat the piston ring  32 . 
     The groove  31  on the piston is not limited to a single groove. As shown in FIG. 4, a plurality of the grooves  31  may be recessed on the piston  22 . 
     The sealing coat may coat parts of the groove surface  22   a , as shown in FIG.  5 . Particularly, the sealing coat resides only on the facing end surfaces of the groove surface  22   a  other than the bottom of the groove surface  22   a.    
     According to the present invention described above, the piston type compressor provides the sealing coat, which is made of soft metal, between the piston ring and the piston. Thereby, sealing performance therebetween improves, and compression efficiency improves. 
     Also, when a sealing coat, which is made of soft metal, is a film coating the surface of a groove on a piston, and even when roughness of the surface of the groove does not satisfies requirement, high sealing performance is ensured. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims.