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REFERENCE TO PRIOR APPLICATION 
   This invention relies for priority on previously filed provisional application No. 60/837,563, filed Aug. 14, 2006 by Kenneth J. Carstensen and entitled “Interconnect Rod for Sucker Rod String”. 

   BACKGROUND OF THE INVENTION 
   A sucker rod string in a petroleum well extends from a power drive at the wellhead, down through the lengths of tubing and concentric casing used in a production system, and connects to the fluid pump at the lowest elevation in the system. The pump receives an accumulating petroleum inflow from the production zone somewhere above it and lifts a column of fluid up through the tubing. The power drive at the wellhead may be a reciprocating beam system, often called a horsehead, and the sucker rod string then reciprocates a downhole plunger pump which lifts fluid upward through the tubing. The power source may alternatively be a rotary drive coupled via the sucker rod string to a longitudinally extended progressing (or “progressive”) cavity pump. 
   At the wellhead, the fluid product raised through the tubing is diverted to a lateral outflow and the flow path is closed off by seals about a special interconnection rod which couples the upper end of the sucker rod string to the horsehead or rotary drive. This interconnection rod, called a polish (or polished) rod, must meet stringent requirements because it must provide a low wear durable seal as well as withstand the tensions and stresses engendered by the well operation. It must bear the weight of the sucker rod string as the string cycles, as well as the mass of the fluid column that is being lifted, and it must also resist transitory forces which occur during the pumping, whether reciprocating or rotary. These forces arise variably from impacts, inertial variations and stresses induced longitudinally or by torsion. Since the polish rod reciprocates through fluid seals at the wellhead, it must function uniformly without leaking or undue friction through the operating life of the polish rod. For these reasons, the polish rod diameter is typically larger in diameter than the sucker rods in the string, and is of a suitable high strength material. To provide affective sealing, the surface of the polish rod is spray metal coated with a hard alloy and ground and polished to provide a long life uniform surface which introduces little wear. 
   Over years of development and use in the oil industry, technical changes have been made to meet constantly increasing artificial lift requirements. Thus a relatively early (pre-1940) specification from the American Petroleum Institute (API) defined a polish rod in which one end had a forged upset area, including a shouldered element having a rod pin machined on it. The other end had no shoulder but terminated in straight threads of a specified API polish rod design. The thread design terminated the straight threads with 3 partial threads along a 9° (nine degree) taper. The shouldered polish rod specification became effectively obsolete while lift requirements were still relatively low because it was felt that there was no structural need for the shoulder. Consequently, both ends of the polish rod have connection threads of the straight API polish rod type without a shoulder. 
   As artificial lift requirements increased because of increases in the depth of wells along with greater demand for petroleum products, new sucker rod designs were developed for greater performance. It became evident that the polish rod thread configuration often became a weak link in the rod string under more demanding conditions. Shock and impact forces encountered with gas or fluid pounding appeared to be the most frequent failure consideration, because these forces excessively stressed the polish rod connection threads as they alone absorbed longitudinal stresses and impact forces on the rod. Such breakdowns are aggravated because, it is well known, movement between threaded members tends to destroy the connection. 
   Polish rod connection failures were evidenced, for both beam pumping and progressive cavity pumping, by “belling out” (radial expansion) of the coupling due to the prior sucker rod design. In beam pumping systems the 9° terminal taper and partial threads allowed the tapered threat sections to go deeper and deeper into the coupling, causing threads to shear or even split the coupling. In rotary systems gas absorbed in the stator elastomer, results in torque increasing, which may be accompanied by torque spikes caused by the ingestion, by the pump of formation solids, with failures like those mentioned above. 
   Applicant has shown that prestressing of connections in sucker rod systems can be used to advantage, as described in Carstensen U.S. Pat. No. 6,942,254 issued Sep. 13, 2005, and Carstensen patent application Ser. No. 09/961,391 filed Jul. 25, 2001. Improvement of the polish rod connection so as to take advantage of these beneficial expedients while providing the needed structural and sealing properties are desirable objectives. 
   SUMMARY OF THE INVENTION 
   An improved polish rod system in accordance with the invention is configured for precise dimensional positioning and structural interaction with a sucker rod system. It provides the structural properties needed and an extremely durable construction for load cycling operation. The improved polish rod integrates a shoulder surface at a position that absorbs stresses both static and dynamic, transmitted along the sucker rod string and in the rod couplings. This improvement enables the connection to take advantage of prestressing, with or without an intermediate torque button. 
   A polish rod in accordance with the invention is externally configured to have the needed principal length and body diameter for the stress on the sucker rod string to which it is to be attached. The polish rod body is machined, ground and polished and spray coated with hardened material in accordance with prior practice, but includes a small diametral reduction for a predetermined length adjacent its lower end that has precisely defined end transitions. A shoulder fitting or sleeve of slightly smaller internal dimension than the reduced rod dimension but of the predetermined length is attached on the polish rod body, after being heated sufficiently to shrunk into place with secure engagement. A corner radius of curvature on the upper end of the shoulder sleeve engages a matched curvature at the corresponding end of the predetermined length of the polish rod, to aid in seating and retention. The sleeve includes a tapered section that diverges from its upper end to a mid-region transition and thereafter is of uniform cross section to its lower end. This end defines a transverse end shoulder in a transverse plane that is precisely aligned longitudinally with the end of a pin neck section in the polish rod. The pin neck provides stress relief and adjoins the terminating straight thread section which is compatible with the female thread in a sucker rod coupling. When the pin end is fully in threaded position in the coupling, the coupling end engages the transverse shoulder surface of the sleeve. 
   With this arrangement, a sucker rod coupling attached to the pin end and abutted against the shoulder surface can be precisely prestressed, in the fashion described in the aforementioned Carstensen patents, to resist the initiation and growth of microcracks and displacements between the threads during repetitive cycling under stress. Alternatively, the combination may include a torque button positioned between the end faces of a the polish rod and an opposing sucker rod end mounted in the coupling. Similar interconnections can be used at both ends of the polish rod, if a direct connection is not used, except that at the upper end the sleeve is inverted (relative to the orientation at the lower end) and the pin end is coupled directly or via an intermediate device to the primary cable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a somewhat idealized partial side and sectional view of a wellhead in which a polish rod in accordance with the invention is attached to a reciprocating pump drive; 
       FIG. 2 , comprising  FIGS. 2A and 2B , is a somewhat idealized, truncated view of a typical downhole petroleum well installation; 
       FIG. 3  is a side-sectional view of a polish rod combination in accordance with the invention, as related to a sucker rod coupling; 
       FIG. 4  is an enlarged side-sectional fragmentary view of the pin end of a polish rod combination in accordance with the invention, and 
       FIG. 5  is a side-section fragmentary view of a polish rod combination including a torque button insert. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 , to which reference is now made, is an idealized general view of a wellhead installation  10  showing a horsehead type of drive  12  mounted on a base  14 . In conventional fashion the beam  16  for the horsehead rocks on a pivot support  18  in a central region of the arm or beam  16 . The head assembly  20  of the drive  12  controls the elevation of the sucker rod in the downhole installation via a primary cable  22  and an interconnecting polish rod. At the opposite end of the beam  16  is a drive motor  26  coupled by a belt  28  to a rotating counter-weight  30  that compensates in part for the mass of the sucker rod system and fluid being lifted. 
   Alternatively, as is well known, the sucker rod system can be driven circumferentially by a rotary drive (not shown) in which event the pumping structure at the downhole location is of the progressive cavity type. Load variations with the progressive cavity drive reflect not only the nominal metal and petroleum masses and inertial loads but also transient variations introduced by solids and gases in the product. 
   The primary cable  22  in this example couples directly to the upper end of a polish rod  36  having a conventional length which extends through a circumferential seal  38  into the vertical tubing  40  supported in part by an encompassing tubing head  42  which contains the polish rod seals. The polish rod  36  is not only about 1.5× the diameter of the sucker rods in the string but has a suitable length, such as 10 ft. to 36 ft. above the tubing head  42 . Below the tubing head  42 , the installation extends downhole in conventional fashion, within the tubing  40  sections that are concentric within a casing  46 . The joined lengths of sucker rod sections  48  are centrally positioned within the tubing  40 . 
   The general downhole construction (as seen in  FIG. 2 ) is well known, but is depicted because it is useful in explaining the forces and stresses introduced in the system. The sucker rod section lengths  48  are interconnected by couplers  50 , and, as seen in the overall view of  FIG. 2 , the horsehead drive cycles the sucker rod system so that a plunger type of pump  52  (in the progressive cavity system this would be a rotary device) is reciprocated to lift fluid accumulated into the fluid column along the tubing. At the lowermost section of the tubular system a column of petroleum product builds along and within the interior of the tubing  40 . Perforations  56  in the casing  46  are at an elevation accessible to the production zone, and above the pump level, so as to maintain the petroleum product flow into the interior annulus between the tubing  40  and casing  46 . As the plunger  52  builds the fluid outflow column, the petroleum product level rises until the outflow line  44  is reached at or near the surface. The downhole installation will conventionally include cement (not shown) between the exterior of the casing  46  and the bore of the well. Further details are not shown in  FIG. 2 , because they are well understood and not relevant to the inventive structure. 
   In the polish rod  36  assembly, shown in greater detail in  FIGS. 3 and 4 , the polish rod  36  diameter is substantially constant throughout its entire uppermost segment. The terms “upper” and “lower” are used in a descriptive way only, to relate to the conventional usage of a polish rod assembly when connected to the upper section of a sucker rod string. For example, at its lower-end a 1.5″ polish rod  36  diameter is reduced along a predetermined length of (approximately 5.832″) long to a smaller diameter section  56 , by machining off about 0.0615 inches from the surface. The upper edge of this length  56  is defined by a small transition ridge  60  having a precise radius of curvature (0.0625″) between the adjoining longitudinal and transverse surfaces. A fitted sleeve  62  seats concentrically on the smaller diameter section  56  of the polish rod  36  and includes an upper corner  63  which matches the curved ridge  60  on the polish rod. The length of this sleeve  62  extends precisely to the start of the stress relief area  66  or neck on the polish rod  36  assembly, and terminates in a shoulder  67  lying in the transverse plane  68 . The shoulder  67  surface on the sleeve  62  the precise geometric reference surface used for the prestressed coupling in accordance with the referenced Carstensen teachings. 
   Along its upper section the fitted sleeve  62  includes a downwardly diverging tapered section  70  which merges smoothly in a mid-region of the sleeve  62  into a constant outer diameter section terminating at the transverse and shoulder  67  surface. On the end portion of the polish rod  36 , the neck or relief area  66  merges to a straight threaded section  74  mating with the internal female threads on the sucker rod coupling  50 . 
   When assembled, as also seen in  FIG. 4 , the coupling  50  receives the pin end of a sucker rod  48 , and when the polish rod  36  and the sucker rod pin end are tightened to a selected prestress level, as per the Carstensen patents referenced above, the connection inhibits the loosening of the threaded connection under cyclic high load operation. This also inhibits the initiation and development of micro-cracks. 
   As seen in  FIG. 5 , a torque disk  86  can be incorporated between the end faces of the pin ends of the sucker rod  48  and the polish rod  36  assembly. The coupling  50  itself again abuts the shoulder  62  at one end and the shoulder on the sucker rod  48  at the other. Prestress can again be applied to assure that incremental movement and displacement between the mating threads does not occur. The sucker rod  48  includes a wrench flat  64  adjacent the transverse end shoulder  6  which engages the coupling  50 . 
   The polish rod assembly, as previously expressed, has a diameter that is greater than the diameter of the sucker rod with which it is being used. Other sizes of sucker rods and polish rods may be used, in accordance with the following chart given as an example only. 
   
     
       
             
             
             
             
             
             
           
         
             
                 
             
             
               Nominal Size of Sucker 
                 
                 
                 
                 
                 
             
             
               Rod 
               ⅝ 
               ¾ 
               ⅞ 
               1 
               1¾ 
             
             
                 
             
           
           
             
               Length Polish Rod Pin 
               1.125 
               1.375 
               1.375 
               1.750 
               2.000 
             
             
               Diameter of Polish Rod 
               0.9362 
               1.0611 
               1.1861 
               1.3235 
               1.3609 
             
             
               Pin Shank 
             
             
                 
             
           
        
       
     
   
   The incorporation of a physically united sleeve  62  on the reduced diameter length of the polish rod involves using a sleeve  62  which is 0.003″ to 0.004″ less in diameter than the outer diameter of the reduced diameter length. The sleeve  62  is heated to between 600° and 750° F. to expand it sufficiently to slip over the dedicated length of the polish rod. When in proper position, which is precisely defined by the curved ridge  60 , the sleeve  62  is in effect physically united with the polish rod  36  on cooling, and withstands the loads that encountered on cyclic operation. 
   With polish rods provided in accordance with the invention, cyclically varying loads, such as are encountered with a modern pumping system, are buffered by coupling to shoulder contact as well as contact at the threaded flanks. Furthermore existing parts inventories of polish rods can be employed after modification at service centers or even on site. 
   Although modifications and variations of devices in accordance with the invention have been described, the invention is not intended to be limited to such but is to apply to all expedients within the scope of the appended claims.

Summary:
A device for coupling a driver system to a downhole string of sucker rods in a petroleum well employs a polish rod construction in which the polish rod is strengthened by an attached sleeve providing a transverse should in a predetermined plane. The sleeve is shrink fit in place and mechanically adhered to withstand the stresses and shocks encountered in long-term cyclic operation. The device provides specific benefits for an improved sucker rod connection which prestresses the sucker rod pin ends and the coupling in a fashion to resist the development of microcracks and fatigue failures.