Abstract:
Preferred rod/piston assemblies ( 50, 50′, 150 ) comprise unitary rod members ( 52, 52′, 152 ) and compression rings ( 54, 54′, 154 ). In accordance with one preferred embodiment ( 50 ), the unitary rod member ( 52 ) comprises a connecting rod portion ( 60 ), a lower bezel portion ( 62 ) and a knob portion ( 66 ). The lower bezel portion ( 62 ) and the compression ring ( 54 ) define facing surfaces ( 82, 106 ) along which the compression ring ( 54 ) is secured to the unitary rod member ( 52 ). In accordance with a preferred method of manufacture, one or both of the facing surfaces ( 82, 106 ) are chemically treated to promote coupling of the compression ( 54 ) ring to the lower bezel portion ( 62 ). The unitary rod member ( 52 ) and the compression ring ( 54 ) are positioned in the mold or press with the compression ring ( 54 ) fitted over the knob portion ( 66 ) of the unitary rod member. Elevated temperature and pressure are applied to secure the compression ring ( 54 ) to the lower bezel portion ( 62 ). Among the advantages of the preferred structure are minimization the number of parts which must be supplied to form the preferred rod/piston assemblies ( 50, 50′, 150 ) and simplification of the processes for their construction.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to rod/piston assemblies of a type capable of use in non-commercial air compressors and, more particularly, to rod/piston assemblies comprising unitary rod members and to a method for affixing compression rings to such bodies. 
   BACKGROUND OF THE INVENTION 
   One type of conventional oil-free rod/piston assembly comprises an aluminum rod die casting, a preformed polytetrafluoroethylene [“PTFE”] compression ring, an aluminum die cast retaining ring and a metal retaining screw or rivet. The aluminum rod die casting includes a crank bore and a lower bezel connected by an I-section rod. When assembled, the PTFE compression ring is clamped between the retaining ring and the lower bezel to form a piston head. Similar structures are proposed in Droege et al. U.S. Pat. No. 3,961,869 and Droege et al U.S. Pat. No. 3,961,868. 
   Typically, the rod/piston assembly is fitted into a cylinder such that reciprocation of the piston draws air into the cylinder through an intake port and expels the air from the cylinder through an outlet port under pressure. The compression ring presses against the inner surface of the cylinder to provide a pressure-tight seal about the piston head. Conventional PTFE compression rings comprise annuli sufficiently thin to flex against the inner surfaces of the cylinder so as to enhance the seal between the rod/piston structure and the cylinder. 
   The use of flexible PTFE in forming compression rings has numerous advantages, including high strength and low friction against the inner surface of the cylinder. Nevertheless, cast or sintered PTFE generally is too stiff and brittle to flex in the manner of the compression ring described above without further processing steps, such as stretching. One desirable feature of the prior art rod/piston assembly is that the PTFE ring can be cast or sintered and then processed before it is combined with the aluminum rod die casting to form the assembly. 
   One drawback to the prior art rod/piston structure is the relatively large number of pre-formed or pre-cast parts which go into its assembly. The number of pre-formed or pre-cast parts required to assemble the finished structure increases the cost of manufacturing the structure. Likewise, separately fitting the compression ring over the lower bezel of the aluminum rod die casting; fitting the retaining ring over the lower bezel; and fastening the retaining ring to the lower bezel with the retaining screw or rivet adds to the time and cost of assembling the structure. 
   The number and arrangement of the parts also affects the dimensional repeatability of the structure. That is, the overall length of the rod/piston assembly depends on several factors, including cumulative variations in the lengths of the aluminum rod die casting, the PTFE compression ring and the retaining ring; the elasticity of the compression ring; and the torque or force used in tightening the retaining screw or rivet to clamp the parts together. Controlling all of these factors would be difficult and any steps taken to do so would increase further the cost of the finished assembly. 
   Dinkel et al. U.S. Pat. No. 6,200,109 proposes an electric motor/pump assembly including a motor shaft and a sealing element which acts upon the motor shaft radially. The sealing element is made of an elastic material, for example, silicone plastics, which is injected into a groove to seal a gap where the sealing lip of the sealing element abuts on the motor shaft side. 
   Heine et al. U.S. Pat. Nos. 6,139,023; 5,874,170; and 5,756,025, the disclosures of which are incorporated by reference, propose a method for injection molding a seal carrier to a sealing member. The sealing member is modified by admixture or surface treatment with a material compatible with the seal carrier. The modified sealing member then is placed in an injection mold. The mold is closed and plastic material which will form the carrier body is injected into the mold. The sealing member fuses to the plastics of the carrier body as the carrier body cools, thereby providing a durable connection in substance lock. 
   Ebbing U.S. Pat. No. 5,282,412 proposes an angulating piston assembly for a vehicle air conditioning system compressor. The proposed angulating piston assembly is made up of two components: namely, a composite piston and a piston ring subassembly that includes a rigid annular support collar and a stretched piston ring on the collar. A proposed method for making the angulating piston assembly includes the steps of cutting or skiving an elastomeric ring if desired width from an extruded tube of mineral or graphite filled PTFE, stretching the elastomeric ring, sliding the elastomeric ring onto the support collar to form the piston ring subassembly; placing the piston ring subassembly into one half of a mold cavity for forming the molded head of the composite piston; positioning a distal end of a piston rod so as to extend into the same half of the mold cavity; closing the other half of the mold cavity to fully capture the piston ring and the subassembly; and injecting high temperature, high strength glass-reinforced resin into the cavity in order to form the piston head. 
   Preston et al. U.S. Pat. No. 4,986,553 proposes a shaft seal comprising a rigid case ring and a PTFE sealing ring component which is secured to a radial flange of the case ring by being bonded to a molded elastomeric sealing ring component. 
   Various patents, including Kanari et al. U.S. Pat. No. 5,611,260; Holtzberg U.S. Pat. No. 4,432,925; and Dorsch U.S. Pat. No. 4,462,767, propose mechanical techniques for securing PTFE rings to piston heads. 
   Thus, there remains a need in the art for an improved rod/piston assembly comprising relatively few pre-formed or pre-cast parts and an improved method for manufacturing such an assembly requiring relatively few production steps. In particular, there is a need in the art for an improved method which is particularly, though not exclusively, suited for use in manufacturing rod/piston assemblies comprising pre-formed PTFE compression rings and pre-formed or pre-cast metal rod members having repeatable dimensions. 
   SUMMARY OF THE INVENTION 
   These needs and others are addressed by a rod/piston assembly comprising a unitary rod member and a compression ring. The preferred unitary rod member is composed of a rigid material, most preferably a metal such as aluminum or an aluminum alloy. The preferred compression ring is composed of flexible PTFE. 
   The preferred unitary rod member includes a lower bezel portion, a connecting rod portion and a knob portion. The preferred lower bezel portion defines first and second sides. The preferred connecting rod portion terminates along the first side of the lower bezel portion. The preferred knob portion extends from the second side of the lower bezel portion. Most preferably, the knob portion has an outer diameter less than an outer diameter of the lower bezel portion so as to define a shoulder or flange along the second side of the lower bezel portion. The preferred knob portion defines opposed inner and outer sides and a peripheral surface connecting the inner and outer sides. The inner side of the preferred knob portion coincides with the second surface of the lower bezel portion. 
   The preferred compression ring defines a hole fitting about the knob portion of the unitary rod member. Most preferably, the compression ring comprises a thin annulus having an outer diameter greater than the outer diameter the lower bezel portion. 
   In accordance with a first especially preferred embodiment, the outer side of the knob portion defines a free or exposed surface. The compression ring defines a securing surface facing the second surface of the lower bezel portion of the unitary rod member. The compression ring is secured to the unitary rod member along the second surface of the lower bezel portion such that a peripheral portion of the compression ring extends radially beyond the lower bezel portion for contact with an inner surface of a cylinder. Optionally, a plastic filler ring or collar is secured around the peripheral surface of the knob portion to promote engagement between the peripheral portion of the compression ring and an inner surface of a cylinder when the rod/piston assembly is in use, and to control the cylinder volume. 
   In accordance with a first preferred method of manufacture, the compression ring is secured to the unitary rod member by means of a combination of elevated temperature and pressure in a mold or press. In accordance with this method, the second surface of the lower bezel portion, the securing surface of the compression ring, or both, are chemically treated, and a bonding agent is applied, to promote coupling of the compression ring to the lower bezel portion. The unitary rod member and the compression ring are positioned in the mold or press with the compression ring and, optionally, the plastic filler ring, fitted over the knob portion of the unitary rod member. Elevated temperature and pressure are applied to secure the compression ring and, optionally, the plastic filler ring, to the lower bezel portion. 
   In accordance with one preferred option, the unitary rod member and the compression ring are placed in a mold having a recess adjacent the knob portion of the unitary rod member and the compression ring. Resin is injected into the recess and solidified to form the plastic filler ring or collar. 
   In accordance with a second especially preferred embodiment, a preformed plastic filler ring or collar is positioned over the knob portion of the unitary rod member before the compression ring is secured to the unitary rod member. This preferred plastic filler ring includes an inner bead or lip which cooperates to secure the plastic filler ring to the peripheral surface of the knob portion. 
   Thus, in accordance with a second especially preferred method of manufacture, the compression ring and the plastic filler ring are secured to the unitary rod member by means of a combination of elevated temperature and pressure in a mold or press. More specifically, the lower bezel portion of the unitary rod member, the compression ring and the plastic filler ring, or a combination of those parts, are chemically treated and at least partially coated with a bonding agent. The unitary rod member, the compression ring and the plastic filler ring are positioned in the mold or press with the compression ring and the plastic filler ring fitted over the knob portion. Elevated temperature and pressure are applied in the mold or press to secure the compression ring and the plastic filler ring to the unitary rod member. 
   In accordance with a third especially preferred embodiment, the unitary rod member further includes an upper bezel portion extending from the outer side of the knob portion toward a free or exposed surface of the upper bezel portion. The preferred upper bezel portion has an outer diameter greater than that of the outer diameter of the knob portion such that the lower bezel portion, the knob portion and the upper bezel portion cooperate to define a peripheral channel between the upper and lower bezel portions. Most preferably, the hole through the compression ring has an inner diameter at least as large as the outer diameter of the upper bezel portion so as to permit the compression ring to slide over the upper bezel portion during assembly. 
   In accordance with a third preferred method of manufacture, the compression ring is secured to the unitary rod member by means of a polymeric filler or bonding agent, which is injected into the peripheral channel defined in the unitary rod member while the unitary rod member and the compression ring are seated in a mold or press. In accordance with this method, the unitary rod member and the compression ring are positioned in the mold or press with the compression ring fitted over the knob portion of the unitary rod member. Resin is injected into the mold or press and solidified to form the polymeric filler. 
   Thus, the preferred structures minimize the number of parts which must be supplied to form a rod/piston assembly and simplify the processes for their construction. The preferred rod/piston assemblies each comprise no more than three parts: the unitary rod member, the compression ring and, optionally, the preformed plastic filler ring or collar. Each of the preferred rod/piston assemblies is susceptible of assembly by means of a relatively simple pressing or injection molding process. No rivets or threaded fasteners are required to retain the compression rings on the unitary rod members. 
   Furthermore, the preferred structures are adapted for a high level of dimensional repeatability. In each of the preferred rod/piston assemblies, the unitary rod assembly defines a free or exposed surface extending beyond the region where the compression ring is secured to the unitary rod member. Consequently, the overall length of the rod/piston assembly depends solely on the length of the unitary rod member. Since the length of the unitary rod member is the primary factor affecting the overall length of the assembly, the repeatability of the overall length is promoted. 
   Furthermore, it is believed the unitary rod member will be susceptible of casting in a die of relatively simple construction. This will allow an increase in the number of cavities which can be formed in such a die, therefor reducing the cost of parts for the rod/piston assembly. 
   Therefore, it is an object of the invention to provide an improved rod/piston assembly requiring a relatively small number of parts. It is a further object of the invention to provide a relatively simple method for manufacturing the assembly. The invention will be further described in conjunction with the appended drawings and the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of a rod/piston assembly in accordance with the prior art; 
       FIG. 2  is a side elevational view of a first embodiment of a rod/piston assembly in accordance with the present invention; 
       FIG. 3  is a front elevational view of the rod/piston assembly of  FIG. 2 ; 
       FIG. 4  is a side sectional view of the rod/piston assembly of  FIG. 2 , taken along the line  4 — 4  in  FIG. 3 ; 
       FIG. 5  is a plan view of a compression ring for use in the rod/piston assembly of  FIG. 2 ; 
       FIG. 6  is a flow chart illustrating a preferred method for manufacturing the rod/piston assembly of  FIG. 2 ; 
       FIG. 7  is a side elevational view of a second embodiment of a rod/piston assembly in accordance with the invention, partially broken away through a lower bezel portion and a knob portion thereof; 
       FIG. 8  is a flow chart illustrating a preferred method for manufacturing the rod/piston assembly of  FIG. 7 ; 
       FIG. 9  is a schematic view, showing the rod/piston assembly of  FIG. 2  positioned in a cylinder of an air compressor or the like; 
       FIG. 10  is a side elevational view of a third embodiment of a rod/piston assembly in accordance with the present invention; 
       FIG. 11  is a side elevational view of the rod/piston assembly of  FIG. 10 , partially broken away through lower bezel, knob and upper bezel portions thereof; and 
       FIG. 12  is a flow chart illustrating a preferred method for manufacturing the rod/piston assembly of FIG.  10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a prior art rod/piston assembly  10  comprising an aluminum rod die casting  12 , a pre-formed PTFE compression ring  14 , an aluminum die cast retaining ring  16  and a metal retaining screw or rivet  18 . The aluminum rod die casting  12  includes a crank bore  20  and a lower bezel  22  connected by an I-sectioned rod  24 . When assembled, the PTFE compression ring  14  is clamped between the retaining ring  16  and the lower bezel  22  to form a piston head  30 . 
   As noted earlier, the prior art rod/piston assembly  10  is constructed from at least four separate pre-formed or pre-cast parts. The overall length of the rod/piston assembly depends on several factors, including cumulative variations in the dimensions of the aluminum rod die casting  12 , the PTFE compression ring  14  and the retaining ring  16 ; the elasticity of the compression ring  14 ; and the torque or force used in tightening the retaining screw or rivet  18  to clamp the parts together 
   As shown in  FIG. 2 , a first preferred embodiment of a rod/piston assembly  50  in accordance with the present invention comprises a unitary rod member  52 , a compression ring  54  and a polymeric filler or plastic filler ring  56 . The unitary rod member  52  includes a connecting rod portion  60  terminating in a lower bezel portion  62 . The connecting rod portion  60  is I-sectioned over much of its length, except where it defines a crank bore  64  opposite the lower bezel portion  62 . 
   As shown in  FIG. 3 , the unitary rod member  52  ( FIG. 2 ) further includes a knob portion  66  having a peripheral surface  70  and an outer side  72  defining a free or exposed surface. More specifically, as shown in  FIG. 4 , the lower bezel portion  62  has a first side  80  and a second side  82 . The connecting rod portion  60  terminates along the first side  80  of the lower bezel portion  62 . The knob portion  66  extends from the second side  82  of the lower bezel portion  62 . The knob portion  66  has an inner side  84  as well as the outer side  72  and the peripheral surface  70 . The inner side  84  of the knob portion  66  coincides with the second side  82  of the lower bezel portion  62 . 
   The lower bezel portion  62  defines an outer diameter  90  along its second side  82  and the knob portion  66  defines an outer diameter  92  along its inner side  84 . The outer diameter  90  of the lower bezel portion  62  preferably is greater than the outer diameter  92  of the knob portion  66  such that the unitary rod member  52  defines a shoulder or flange  94  where the knob portion  66  extends from the lower bezel portion  62 . 
   As shown in  FIG. 5 , the compression ring  54  preferably comprises a PTFE annulus having a hole  100  defining an inner diameter  102  at least as large as the outer diameter  92  ( FIG. 4 ) of the knob portion  66  (FIG.  4 ). The preferred compression ring  54  also defines an outer diameter  104 . Most preferably, the compression ring  54  defines a securing surface  106  and a peripheral portion  108  spaced radially outwardly from the securing surface  106 . The compression ring  54  is relatively flat and annular in its natural state as seen in  FIGS. 4 and 5 , for example. Upon insertion into cylinder  120  and during use, the peripheral portion  108  is bent upwardly to remain in contact with inner surface  122 . The peripheral portion  108  is of a sufficient radial length to remain in contact throughout the stroke of the rod/piston assembly  50  and thus expands and contracts to conform to the shape needed to provide the seal with the inner surface  122 . For example, as the piston/rod assembly  50  moves, the peripheral portion conforms to fit circular and oval surface configurations depending upon the stroke position. 
   In accordance with a preferred method of manufacture, the compression ring  54  ( FIGS. 4 and 5 ) is secured to the unitary rod member  52  ( FIG. 4 ) by means of a combination of elevated temperature and pressure in a conventional mold or press (not shown). As illustrated in  FIG. 6 , a step  110  includes chemically treating, and applying a bonding agent to, at least a section of the second side  82  ( FIG. 4 ) of the lower bezel portion  62  (FIG.  4 ); at least a section of the securing surface  106  ( FIG. 5 ) of the compression ring  54  (FIGS.  4  and  5 ), or both, to promote coupling of the compression ring  54  ( FIGS. 4 and 5 ) to the lower bezel portion  62  (FIG.  4 ). A step  112  includes positioning the unitary rod member  52  ( FIG. 4 ) and the compression ring  54  ( FIGS. 4 and 5 ) in the mold or press (not shown) with the compression ring  54  ( FIGS. 4 and 5 ) fitted over the knob portion  66  ( FIG. 4 ) of the unitary rod member  52  ( FIG. 4 ) such that the securing surface  106  ( FIG. 5 ) of the compression ring  54  ( FIGS. 4 and 5 ) faces the second side  82  ( FIG. 4 ) of the lower bezel portion  62  (FIG.  4 ). A step  114  includes applying elevated temperature and pressure in the mold or press (not shown) to secure the compression ring  54  ( FIGS. 4 and 5 ) to the lower bezel portion (FIG.  4 ). 
   In accordance with one option, the mold (not shown) includes cavity (not shown) shaped to receive the unitary rod member  52  ( FIG. 4 ) and the compression ring  54  (FIGS.  4  and  5 ). The cavity (not shown) defines a recess (not shown) adjacent the knob portion  66  ( FIG. 4 ) of the unitary rod member ( FIG. 4 ) and the compression ring (FIGS.  4  and  5 ). The preferred recess (not shown) is configured to form the polymeric filler  56  (FIG.  4 ). A step  116  of the preferred method of manufacture includes injecting resin into the recess (not shown) and solidifying the resin to form the polymeric filler  56  (FIG.  4 ). 
   In accordance with another option, illustrated in  FIG. 7 , the assembly  50 ′ includes a polymeric filler or plastic filler ring  56 ′ which is preformed and positioned over a peripheral surface  70 ′ of a knob portion  66 ′ of a unitary rod member  52 ′ before the compression ring  54 ′ is secured to the unitary rod member  52 ′. As shown in  FIG. 7 , the preferred plastic filler ring  56 ′ includes an inner bead or lip  58 ′ which cooperates to secure the plastic filler ring  56 ′ to the peripheral surface  70 ′ of the knob portion  66 ′. The preferred plastic filler ring  56 ′ also includes a fillet  59 ′ adjacent the compression ring  54 ′ for facilitating bending of the compression ring  54 ′. 
   During assembly, the compression ring  54 ′ and the plastic filler ring  56 ′ are secured to the unitary rod member  52 ′ by means of a combination of elevated temperature and pressure in a mold or press (not shown). The method, as illustrated in  FIG. 8 , includes the step  110 ′ of chemically treating, and applying a bonding agent to, one or more of a lower bezel portion  62 ′ ( FIG. 7 ) of the unitary rod member  52 ′ (FIG.  7 ), the compression ring  54 ′ ( FIG. 7 ) and the plastic filler ring  56 ′ (FIG.  7 ); the step  112 ′ of positioning the unitary rod member  52 ′ (FIG.  7 ), the compression ring  54 ′ ( FIG. 7 ) and the plastic filler ring  56 ′ ( FIG. 7 ) in the mold or press (not shown) with the compression ring  54 ′ ( FIG. 7 ) fitted over the knob portion  66 ′ ( FIG. 7 ) of the unitary rod member  52 ′ ( FIG. 7 ) and the plastic filler ring  56 ′ ( FIG. 7 ) fitted over the knob portion  66 ′ ( FIG. 7 ) adjacent the compression ring  54 ′ (FIG.  7 ); and the step  114 ′ of applying elevated temperature and pressure in the mold or press (not shown) to secure the compression ring  54 ′ ( FIG. 7 ) and the plastic filler ring  56 ′ ( FIG. 7 ) to the lower bezel portion  62 ′ (FIG.  7 ). 
   A preferred application for the rod/piston assembly  50  of  FIGS. 2-4  (as well as the assembly  50 ′ of  FIG. 7 ) is in an air compressor (not shown). More specifically, as suggested by  FIG. 9 , the preferred rod/piston assembly  50  is positioned in a cylinder  120  of such compressor (not shown) so that the peripheral portion  108  ( FIG. 5 ) of the compression ring  54  brushes against an inner surface  122  of the cylinder  120 . Most preferably, the inner surface  122  defines an inner diameter slightly less than the outer diameter  104  ( FIG. 5 ) of the compression ring  54  and the compression ring  54  has sufficient flexibility such that the peripheral portion  108  ( FIG. 5 ) of the compression ring  54  engages the inner surface  122  over a finite contact surface so as to define a moveable, air tight seal between the rod/piston assembly  50  and the cylinder  120 . The preferred polymeric filler  56  serves to constrain the flexure of the peripheral portion  108  ( FIG. 5 ) of the compression ring  54  so as to promote engagement of the peripheral portion  108  ( FIG. 5 ) against the inner surface  122  of the cylinder  120  to form this seal and to control the volume of the cylinder. It is believed that the overall design of the preferred rod/piston assembly  50 , including the features highlighted herein, will improve the performance of such air compressors (not shown). 
   As shown in  FIG. 10 , a third preferred embodiment of a rod/piston assembly  150  in accordance with the present invention comprises a unitary rod member  152  and a compression ring  154 . As shown in  FIG. 11 , a plastic filler ring or collar  156  ( FIG. 11 ) secures the compression ring  154  to the unitary rod member  152 . 
   The unitary rod member  152  includes a connecting rod portion  160  terminating in a lower bezel portion  162 . The connecting rod portion  160  defines a crank bore  164 . The unitary rod member  152  further includes a knob portion  166  and an upper bezel portion  168 . The knob portion  166  has a peripheral surface  170  and an outer side  172 . 
   The preferred geometry of the unitary rod member  152  is arranged as follows. The lower bezel portion  162  has a first side  180  and a second side  182 . The connecting rod portion  160  terminates along the first side  180  of the lower bezel portion  162 . The knob portion  166  extends from the second side  182  of the lower bezel portion  162 . The knob portion has an inner side  184  as well as the outer side  172  and the peripheral surface  170 . The inner side  184  of the knob portion  166  coincides with the second side  182  of the lower bezel portion  162 . The upper bezel portion  168  extends outwardly from the outer side  172  of the knob portion  166 . The upper bezel portion  168  defines a free or exposed surface  186  opposite the knob portion  166 . 
   The lower bezel portion  162  defines an outer diameter  190  along its second side  182  and the knob portion  166  defines an outer diameter  192  along its inner side  184 . The outer diameter  190  of the lower bezel portion  162  is greater than the outer diameter  192  of the knob portion  166  such that the unitary rod member  152  defines a peripheral channel  194  aligned with the knob portion  166 . The upper bezel portion  168  defines an outer diameter  196  greater than the outer diameter  192  of the knob portion  166  so as to provide an outer bound for the peripheral channel  194 . 
   In accordance with a third preferred method of manufacture, the compression ring  154  is secured to the unitary rod member  152  by means of the polymeric filler  156 , which is injection molded in the peripheral channel  194  while the unitary rod member  152  and the compression ring  154  are seated in a mold (not shown). As shown in  FIG. 10 , a step  200  of the third preferred method of manufacture includes positioning the unitary rod member  152  ( FIGS. 10 and 11 ) and the compression ring  154  ( FIGS. 10 and 11 ) in a cavity (not shown) of the mold (not shown) with the compression ring  154  ( FIGS. 10 and 11 ) fitted over the knob portion  166  ( FIG. 11 ) of the unitary rod member  152  (FIGS.  10  and  11 ). The preferred compression ring  154  ( FIGS. 10 and 11 ) is a PTFE annulus having a configuration similar to that of the compression ring  54  (FIG.  5 ). Most preferably, the compression ring  154  ( FIGS. 10 and 11 ) has a hole (not shown) defining an inner diameter (not shown) at least as great as the outer diameter  196  ( FIG. 11 ) of the upper bezel portion  168  ( FIG. 11 ) so as to permit the compression ring  154  ( FIGS. 10 and 11 ) to slip easily over the upper bezel portion  168  ( FIG. 11 ) into position adjacent the peripheral channel  194  (FIG.  11 ). 
   As shown in  FIG. 12 , a step  202  of the preferred method of manufacture includes injecting resin (not shown) into the cavity (not shown) of the mold (not shown). The resin (not shown) is solidified to form the polymeric filler  156  ( FIG. 11 ) to secure the compression ring  154  ( FIGS. 10 and 11 ) onto the unitary rod member  152  (FIGS.  10  and  11 ). 
   From the foregoing description, it will be apparent that the preferred rod/piston assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  (FIGS.  10  and  11 ), and the preferred methods of manufacture ( FIGS. 6 ,  8  and  12 ), provide a number of advantages over the prior art. The preferred methods use relatively small numbers of pre-formed or pre-cast parts, namely, just the unitary rod assemblies  52  (FIG.  4 ),  52 ′ (FIG.  7 ),  152  (FIG.  11 ); the compression rings  54  (FIGS.  4  and  5 ),  54 ′ (FIG.  7 );  154  ( FIG. 11 ) and, optionally, preformed plastic filler rings  56 ′ (FIG.  7 ), although it is within the scope of the invention to include additional pre-formed or pre-cast parts for purposes apparent to those of ordinary skill in the art. The preferred methods ( FIGS. 6 ,  8  and  12 ) themselves are relatively simple, although it is again within the scope of the invention to include additional method steps for various purposes apparent to those of ordinary skill. In particular, since the compression rings  54  (FIGS.  4  and  5 ),  54 ′ (FIG.  7 ),  154  ( FIG. 11 ) are pre-formed, they can be cast or sintered and then processed before they are secured to the unitary rod members  52  (FIG.  4 ),  52 ′ (FIG.  7 ),  152  (FIG.  11 ). 
   Preferably, the unitary rod members  52  (FIG.  4 ),  52 ′ (FIG.  7 ), 152  ( FIG. 11 ) are each formed from a single integral die casting of a metal such as aluminum or an aluminum alloy. The preferred compression rings  54  (FIGS.  4  and  5 ),  54 ′ (FIG.  7 ),  154  ( FIG. 11 ) are formed from PTFE annuli having sufficient flexibility to form temporary seals against the inner surfaces  122  ( FIG. 9 ) of cylinders  120  ( FIG. 9 ) in which the rod/piston assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) are placed. Preferably, the polymeric fillers  56  (FIG.  4 ),  56 ′ (FIG.  7 ),  156  ( FIG. 11 ) are formed from castable resins capable of forming polymers resistant to conditions which the rod/piston assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) are likely to encounter during use. The selection of such materials, and suitable methods for forming or casting of the materials to form the aforementioned parts, is within the ordinary skill in the art. 
   It will be apparent from  FIGS. 4 ,  7  and  11  that the overall lengths of the preferred assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) depend solely on the lengths of the unitary rod members  52  (FIG.  4 ),  52 ′ (FIG.  7 ),  152  (FIG.  11 ). This implies that, where the unitary rod members  52  (FIG.  4 ),  52 ′ (FIG.  7 ),  152  ( FIG. 11 ) are formed from integral die castings, the repeatability of the overall lengths of the assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) will depend primarily on the care with which the castings are prepared. As such, the preferred assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) will have improved dimensional repeatability. In addition, it is believed that the preferred unitary rod members  52  (FIG.  4 ),  52 ′ (FIG.  7 ),  152  ( FIG. 11 ) can be shaped in relatively simple dies (not shown), such that a relatively large number of die cavities (not shown) may be formed in a single die (not shown). This will serve to further reduce the cost of manufacture. 
   From the foregoing description, those skilled in the art will appreciate that the present invention is directed toward improved rod/piston assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) having relatively few pre-formed or pre-cast parts and to improved methods ( FIGS. 6 ,  8  and  11 ) for manufacturing such assemblies  50  (FIGS.  2 - 4 ),  50 ′ (FIG.  7 ),  150  ( FIGS. 10 and 11 ) requiring relatively few manufacturing steps. While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.