Abstract:
Pistons, which are reciprocated by a swash plate of a compressor, have two separate parts joined together. Each piston has a body and a coupler. The coupler is connected to the swash plate. The body is made of thermosetting resin. The body is molded to the coupler. Accordingly, the piston body to be firmly connected to the coupler.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a piston for fluid machines such as compressors that compress refrigerant gas for air-conditioning vehicles. 
     Japanese Unexamined Patent Publication No. 5-99146 describes a compressor piston  112  illustrated in the present specification at FIG. 6, which replicates the figure shown in the abstract of the Japanese reference and adds a leading one (1) digit to the reference numerals for parts described herein. As shown in FIG. 6 of the present specification, the resin piston body  130  is compression-molded to and joined to a metal coupler  120 , to which a piston rod  113  is coupled. Since most of the piston  112  is made of resin, the piston  112  is relatively light. The light piston reduces inertia when the piston  112  reciprocates. As a result, power losses of the compressor are reduced. 
     However, in the publication, the piston body  130  is made of fluororesin such as polytetrafluoroethylene, which is a thermoplastic resin. Since such thermoplastic resin has poor adhesion to metal, the coupler cannot be joined to the piston with desirable strength. 
     In a typical compressor, rotation of a swash plate is converted into piston reciprocation through shoes. Each piston includes a body and a coupler, which are joined. Each piston is coupled to the swash plate through the shoes, which are retained in the coupler to slide freely. 
     In the typical compressor, force is applied to each piston through the shoes and the coupler by the swash plate. This causes frictional resistance between each piston and the wall of the corresponding cylinder bore. Accordingly, a torsional force is applied to the interface between each piston body and coupler. As a result, the metal couplers may be detached from the piston bodies, which are made of thermoplastic resin. This hinders smooth reciprocation of the pistons and damages the seal between the pistons and the cylinder bores. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a piston for fluid machines that allows the piston body to be firmly connected to the coupler. 
     To achieve the above objective, the present invention provides a piston for cooperating with a driving body in a machine. The piston comprises a metal coupler connected to the driving body. A body is made of thermosetting resin. The body is molded to the coupler. 
     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 cross-sectional view of a compressor according to a first embodiment of the present invention; 
     FIG. 2 is a perspective view of a piston in the compressor of FIG. 1; 
     FIG.  3 ( a ) is a side view of one half of an injection mold containing a coupler; 
     FIG.  3 ( b ) is an exploded view of the injection mold of FIG.  3 ( a ); 
     FIG. 4 is a graph showing the proportion of glass fiber (by weight) contained in a piston body in relation to the thermal expansion coefficient; 
     FIG.  5 ( a ) is a side view of an insert in a second embodiment; 
     FIG.  5 ( b ) is a side view of an insert in a third embodiment; 
     FIG.  5 ( c ) is a side view of an insert in a fourth embodiment; and 
     FIG. 6 is a cross-sectional view showing a prior art piston. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A piston for compressors for air-conditioning vehicles according to a first embodiment of the present invention will now be described with reference to FIGS. 1-4. 
     As shown in FIG. 1, a front housing member  11  and a rear housing member  13  are coupled to a cylinder block  12 . A crank chamber  14  is defined between the front housing member and the cylinder block  12 . The front housing member  11 , the cylinder block  12 , and the rear housing member  13  form the compressor housing. 
     A drive shaft  15  passes through the crank chamber  14  and is rotatably supported between the front housing member and the cylinder member. The drive shaft  15  is coupled to an engine (not shown) through a clutch mechanism such as an electromagnetic clutch. The engine serves as an external drive source. Accordingly, the drive shaft  15  rotates when the clutch is connected during the operation of the engine. 
     A swash plate  16  is coupled to the drive shaft  15  to rotate integrally with the drive shaft  15  in the crank chamber  14 . Cylinder bores  12   a  are formed in the cylinder block  12 . The cylinder bores  12   a  are parallel to the axis L of the drive shaft  15  and are equally spaced about the axis L. 
     Single head pistons  17  are respectively accommodated in the corresponding cylinder bores  12   a.  Each piston  17  is coupled to the swash plate  16  through a pair of shoes  18 . Rotation of the drive shaft  15  is converted into reciprocation of each piston  17  through the swash plate  16  and the shoes  18 . Reciprocation of each piston  17  compresses refrigerant gas in the corresponding cylinder bore  12   a.  In the present embodiment, the drive shaft  15 , the swash plate  16 , and the shoes  18  form a driving mechanism. 
     All of the pistons  17  are identical, thus the following description will refer to only one of the pistons  17  for simplicity. 
     As shown in FIGS. 1 and 2, the piston  17  includes a resin body  21  and a metal coupler  22 . The body  21  is joined to the coupler  22 . 
     The coupler  22  is made of metal (Al—Si alloy), which is an aluminum containing 7-13 percent of silicon by weight. The coupler  22  is produced by forging or casting. Using aluminum for the coupler  22  reduces the weight of the piston  17 . Adding silicon reduces friction between the piston  17  and the inner surface of the corresponding cylinder bore  12   a  and between the piston  17  and the shoes  18 . 
     A recess  23  is formed in the proximal end of the coupler  22 . A pair of sockets  23   a  are formed on the opposed inner surfaces of the recess  23 . A pair of shoes  18  are supported in the sockets  23   a  to hold the periphery of the swash plate  16 . Accordingly, the shoes  18  transmit the alternating inclination of the swash plate  16  to the piston  17 , which reciprocates the piston  17  axially (along axis S). 
     An anchor  24  is integrally formed on the coupler  22 . As shown in FIG. 1, the anchor  24  includes a support shaft  24   a  and a flange, or a disc  24   b.  The support shaft  24   a  extends from center of the end surface of the coupler  22  toward the body  21 . The disc  24   b  is supported by the support shaft  24   a.  The diameter of the disk  24   b  is greater than that of the support shaft  24   a.  The body  21  is joined to the coupler  22  and receives the anchor  24 . 
     The coupler  22  of each piston  17  has a partially cylindrical rotation restrictor  23   b.  The curvature of the restrictor&#39;s cylindrical portion is greater than that of each cylinder bore  12   a.  The center of curvature of each rotation restrictor  23   b  is displaced from the center of curvature of the corresponding cylinder bore  12   a.  As each piston  17  reciprocates, the associated rotation restrictor  23   b  slides along the inner surface of the front housing  11  while preventing the piston  17  from rotating about the axis S. 
     The body  21  includes a columnar head  21   a  and a pair of struts  21   b.  The head  21   a  slides along the surface of the corresponding cylinder bore  12   a.  The struts  21   b  extend diagonally from the head  21   a  to the coupler  22 . A trapezoidal hole is formed between the struts  21   b  to make the piston  17  light. 
     FIGS.  3 ( a ) and  3 ( b ) shows an injection mold  31 . A cavity  32  is formed in the mold  31 . The coupler  22  is placed in the rear portion of the cavity  32 . Part of an end surface of the coupler  22  and the anchor  24  are exposed to a front portion of the cavity  32 , which defines the body  21 . A molding material including a heated phenol resin, which is a thermosetting resin, and glass fibers, which serve as reinforcing material, is injected into the cavity  32  for forming the body  21 . Accordingly, the front portion of the cavity  32  is filled with the molding material. The molding material, when solidified, fixes the end surface of the coupler  22  and the anchor  24  to the body  21 . 
     As shown in the graph of FIG. 4, the thermal expansion coefficient of a phenol resin containing a relatively small amount of glass fibers is greater than that (18*10 −6  to 24*10 −6 ) of an aluminum alloy containing 7-13 weight percent of silicon, which forms the coupler  22 . The thermal expansion coefficient of a phenol resin becomes smaller as the proportion of glass fibers contained in the phenol resin increases. Accordingly, adjusting the proportion of glass fibers contained in the phenol resin makes the thermal expansion coefficient of the body  21  substantially equal to that of the metal coupler  22 . That is, the proportion of glass fibers contained in the phenol resin is adjusted within a range of 15-65 weight percent to correspond to aluminum alloy containing 7-13 weight percent of silicon. 
     The illustrated embodiment has the following advantages. 
     A driving force is applied to each body  21  through the shoes  18  and the coupler  22 . This causes frictional resistance between the body  21  and the surface of the cylinder bore  12   a.  Accordingly, a shearing stress which is based on the rotation of the swash plate  16  and reciprocation of the piston  17  is applied to the juncture between the body  21  and the coupler  22 . 
     However, in the present embodiment, thermosetting resin is used to form the body  21 . Thermosetting resin has better adhesion to metal than thermoplastic material does. Accordingly, the coupler  22  is more firmly joined to the body  21  than in the prior art. Adhesion between the body  21  and the coupler  22  can withstand the torsional force. 
     Thermosetting resin is more heat-resistant than thermoplastic resin is. Accordingly, the body  21  is not softened by heat generated by friction between the piston  17  and the surface of the cylinder bore  12   a.  Therefore, firm adhesion between the body  21  and the coupler  22  is maintained. As a result, the piston  17  smoothly slides in the cylinder bore  12   a,  and good seal between the piston  17  and the cylinder bore  12   a  is maintained. 
     Adding reinforcing material hardens the thermosetting resin and increases the durability of the body  21 . 
     Adjusting the proportion of reinforcing material contained in the body  21  alters the thermal expansion coefficient of the body  21  to substantially match that of the coupler  22 . Accordingly, the thermal expansion due to friction heat in the body  21  is substantially equal to that of the coupler  22 . This prevents internal stresses based on a difference in thermal expansion from being generated at the juncture between the body  21  and the coupler  22 . Therefore, the adhesion between the body  21  and the coupler  22  is stable. 
     The resin of the body  21  fills the space between the disc  24   b  and an end surface of the coupler  22 . The disc  24   b  is perpendicular to the axis S of the piston  17 , which prevents axial movement of the body  21  relative to the coupler  22 . Accordingly, if the adhesion between the body  21  and the coupler  22  is weakened, separation of the body  21  from the couple  22  is prevented, which maintains the operation of the compressor. 
     Further embodiments of the present invention will now be described focusing on differences from the first embodiment shown in FIGS. 1-4. 
     FIG.  5 ( a ) shows the anchor  24  according to a second embodiment. Grooves  24   c  are formed in the peripheral surface of the disc  24   b  of the anchor  24  by a knurling tool. The grooves  24   c  may include first grooves that extend axially and second grooves that extend circumferentially. 
     FIG.  5 ( b ) shows the anchor  24  according to a third embodiment. A spiral groove  24   d  centered about the axis S is formed in the peripheral surface of the disc  24   b.    
     FIG.  5 ( c ) shows the anchor  24  according to a fourth embodiment. Projections 24 e  are formed in the peripheral surface of the disc  24   b.  Recesses may be formed instead of the projections  24   e.    
     The disks  24   b  shown in FIGS.  5 ( a )- 5 ( c ) limit rotation of the body  21  relative to the coupler  22 . Accordingly, adhesion between the body  21  and the coupler is more stable. 
     The material for making the body  21  may contain molybdenum disulfide, which serves as a solid lubricant. This reduces friction by friction between the body  21  and the surface of the cylinder bore  12   a.    
     Examples of thermosetting resins that may be used in the molding are an epoxy resin, an unsaturated polyester resin, a polyamidoimido resin, a urea resin, a melamine resin, an alkyd resin, a silicone resin, an urethane resin, and a furan resin. 
     Examples of reinforcing materials other than glass fibers that may be added to the resin are metal fibers, an alumina, carbon fibers, wood powders, an α-cellulose, shell powders, bone powders, and eggshell powders. Combinations of these materials may also be added to the resin material for the body  21 . 
     Molding of the body  21  is not limited to injection molding. The body  21  may be molded by softening a granular or powder resin material in a mold. In this case, the coupler is inserted in the resin material and connected to the body  21 . In other words, the body  21  may be molded by compression molding. 
     The present invention may be applied to a double-headed piston for double-headed piston compressors. In this case, thermosetting resin piston bodies are respectively connected to both end surfaces of a metal coupler. 
     The present invention may further be applied to a piston for wave cam compressors. In this case, a wave cam that serves as a drive plate forms a piston driving portion. 
     The present invention may further be embodied in other fluid machines such as oil pumps and air pumps. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. 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 and equivalence of the appended claims.