Abstract:
A standard size, steel oil well sucker rod coupling is machined to decrease its external diameter, producing a constant diameter along its length. Then, the coupling is machined to produce chamfers or bevels around the circumference on the ends, such that the diameter at the radial faces is less than at the body of the coupling. Next, abrasive blast-cleaning of the coupling produces a uniform, angular, anchor profile along the machined areas on the exterior of the coupling. Finally, a specially selected, fusion bonded, resinous coating is applied to the exposed surface. The radial faces may or may not be coated. The unique protective coatings impart abrasion resistance in a pumping oil well and serve as an electrical barrier to preclude electrochemical, corrosive, attack of the mild steel coupling core. Extended service life of the coupling is achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of my previous application Ser. No. 07/113,549, filed Oct. 26, 1987, now U.S. Pat. No. 4,823,456. 
    
    
     RIGHTS TO INVENTIONS UNDER FEDERAL RESEARCH 
     There was no federally sponsored research and development concerning this invention. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to pumping oil wells and more particularly to abrasion and corrosion of sucker rod couplings. Applicant submits that manufacturers and users of oil well sucker rods and sucker rod couplings would be considered as having ordinary skill in this art. 
     (2) Description of the Related Art 
     Oil is removed from the ground using a pump-jack. This equipment is mounted on the surface of the earth above an oil reservoir. The pump-jack is connected to a down-hole pump using a sucker rod string. The sucker rod string includes several sucker rods, each sucker rod connected to the next by a coupling. Through the reciprocating action of the pump jack, oil is pumped from the reservoir to the surface for collection. 
     During the pumping action, the sucker rod string reciprocates in a tubing string. Due to deflection in the tubing string, the sucker rod string wears against the internal diameter of the tubing string. Even though lubricant in the form of crude oil is present in the annulus created between the sucker rod and tubing strings, abrasion occurs to the rods, rod couplings, and tubing. 
     Included in the crude oil are dissolved salts and undissolved minerals. When these come between the rods, rod couplings, and tubing, abrasion occurs. Through time, abrasion will decrease the rod and coupling diameters to a point that they will break under the weight stress upon the sucker rod string. In addition to the damage done to the sucker rod string, the abrasion will also wear away the tubing wall. 
     When a hole develops in the tubing wall, pressure inside the tubing string is lost, and the crude oil is pumped into the annulus created between the tubing and casing. Thus, it cannot be collected. 
     When a rod parts, a coupling breaks, or holes in the tubing occur, the sucker rods and/or tubing must be pulled from the well and inspected. Defective rods, couplings, and tubing must be replaced. The resultant pulling unit costs and down-time are a great expense for the pumping unit operator. 
     At the same time that abrasion is destroying the sucker rods, rod couplings, and tubing; all are being attacked by harsh, down-hole chemicals. Dissolved gasses and salts in the crude oil, in combination with the produced water, will destroy unprotected rods, couplings, and tubing, through corrosion. When sufficient corrosive attack has occurred, the sucker rods and couplings will break under the weight of the sucker rod string. The rods, rod couplings, and tubing must be replaced, due to corrosion. 
     Mild steel sucker rod couplings are the most commonly used couplings today. These couplings are beveled on both ends to preclude gouging of the tubing wall. The sucker rod pin ends are screwed into the coupling, using a specified torque, such that they shoulder up to the coupling face. The coupling is usually threaded throughout its entire length. The diameter of the coupling will vary, dependent upon the diameter of the sucker rod for which the coupling was manufactured. Precise machining of the radial face of the coupling is required to insure uniform contact with the face of the sucker rod pin end, around its circumference. Uneven pressures at this junction will lead to rod pin end failures in service. 
     Because these couplings are made of mild steel, they are subject to corrosive attack by down-hole chemicals. They are also subject to abrasion, when rubbing against the tubing as they reciprocate in the tubing string. Abrasion is magnified because the hardness of these couplings is similar to the mild steel tubing in which they are used. (Metals of similar hardness will abrade one another.) 
     To overcome the abrasion and corrosive attack of the mild steel sucker rod couplings, the oil industry capitalized on the relationship that &#34;metals of dissimilar hardness will polish one another&#34;. Sucker rod couplings with a mild steel core and a stainless steel jacket were developed to take advantage of this relationship. They were first used in pumping wells which were highly deviated and therefore, caused excessive abrasion of the mild steel sucker rods, couplings, and tubing. Special grades of stainless steel were chosen, since stainless steel has greater resistance to corrosive attack, and since it has a greater hardness than mild steel. These rod couplings were prepared by machining away some of the original diameter of the mild steel couplings and depositing a jacket of stainless steel, using an arc plasma spray technique. The final diameter was the same as the original diameter. These stainless steel jacketed couplings had to be chamfered on the ends, to preclude cutting away the mild steel tubing as they reciprocated in the tubing string. The internal thread configuration was not altered. 
     This abrasion resistance technique proved to be beneficial in highly deviated pumping wells. A significant reduction in abrasion was noted. The stainless steel jacket was also impervious to attack by the down-hole chemical environment. In cases where the couplings were not chamfered on the ends, the tubing was quickly cut away, due to the greater hardness of the stainless steel jacket. In addition to this problem, the manufacturing process for these couplings was considerably more expensive than for mild steel couplings. These couplings were also difficult to install and remove. Rod tongs and wrenches could not grip the stainless steel jacketed couplings, because of their surface hardness. Both would slip as torque was applied to install or remove these couplings. 
     Another approach to solving the problems of abrasion and corrosion of sucker rod couplings is described in U.S. Pat. No. 4,602,807 to BOWERS. Using this technique, a sucker rod coupling is first machined to decrease its diameter. Then, one or more grooves are cut into the external diameter of the coupling. Special retaining ridges are also left on the ends of the couplings. Once machined, each coupling is given a jacket of hydrourethane, using an injection molding technique. One set of expensive molds must be made for each coupling diameter. Evidently without the special grooves and ridges, the hydrourethane jacket would slip from its protective position around the coupling. The combination of expensive machining with the expensive molds for the injection molding process, make this an expensive sucker rod coupling. If the hydrourethane jacket is not correctly bonded to the sucker rod coupling surface, corrosive chemicals will spread under the hydrourethane jacket, attacking the metal substrate. This hydrourethane jacket only extends to the radial faces of the coupling. A metal-to-metal seal is formed when the coupling is installed on the sucker rod pin end. 
     In yet another approach, a mild steel coupling is machined to decrease its diameter throughout the center section of the coupling. Ridges are left on both ends to hold the abrasion resistant, injection molded plastic protector in place. Once machined, the plastic is molded into the recessed area of the coupling to have a larger diameter than the original diameter and the retained ridges. The sections of greater diameter provide abrasion resistance where the sucker rod couplings make contact with the inside diameter of the tubing string. If this plastic is not correctly bonded to the coupling, corrosive fluids can spread between the molded plastic jacket and the mild steel coupling body. When installed on a sucker rod, a metal-to-metal seal is formed between the face of the sucker rod pin end and the coupling shoulder. 
     Couplings which have an injection molded, abrasion resistant jacket around them have met with limited success due to the problems already mentioned: Manufacturing expense and lack of a sufficient bond between the plastic jacket and the coupling body to preclude the spread of corrosive chemicals between them. 
     While stainless steel jacketed couplings have provided more abrasion resistance than mild steel couplings, and even though they are more resistant to down-hole chemicals, there are still shortcomings with this style of sucker rod couplings. Problem areas include: Manufacturing costs, and ability to install and remove because of their surface hardness. 
     Fusion bond applied powder plastic coatings have been used for many years, to prevent corrosion of sucker rods. These coatings are applied over abrasive blast-cleaned, mild steel sucker rods, as well as arc plasma sprayed stainless steel jacketed sucker rods. To have utility as a sucker rod coating, the coating resin must withstand the attack by corrosive down-hole chemicals. It must also bond to the sucker rod body and not blister or disbond in-service. The resin system must have a little flexibility, since the sucker rods will bend and flex in-service. Abrasion resistance is also require, to keep the coating from being worn away as the coated sucker rods reciprocate in the tubing string. Resistance to mechanical impact is a concern, since the coated sucker rods will slap against the tubing wall. Excessively high impact resistance, however, is not the primary requirement for a sucker rod coating. To fulfill these requirements, fusion bond coatings of the epoxy and modified-epoxy classes are used. These coating resin classes provide the required protection for sucker rod body. They are applied along the body of the rod, from one pin end face to the other. No coating is deposited on the sucker rod joint faces. Special masks are used to prevent coating deposition at this point, since it will adversely effect the ability of the sucker rod and coupling to form a friction-fit, metal-to-metal seal. Because these coatings resist compression and are hard and smooth, a coupling would unscrew from the sucker rod, if this type of resinous coating were present between the coupling shoulder and the rod face. Coatings of the epoxy and modified-epoxy classes are applied, mostly, in a fusion bond, powder form, (a) via electrostatic spray, or (b) hot flocking. 
     If coating resins with the physical and chemical properties of these sucker rod coatings were used to protect the sucker rod couplings, they would fail, due to their inadequate impact and chip resistance. For this reason, coated sucker rods are used in combination with stainless steel jacketed couplings, or injection molded, plastic jacketed couplings. Rod tongs and wrenches would crack, chip, and disbond coatings of the epoxy or modified-epoxy class, if they were present on the sucker rod couplings. Since the sucker rod couplings have a greater diameter than the sucker rods, most of the forces of down-hole impact are concentrated on the couplings. This concentrated down-hole pounding would destroy coatings of the epoxy or modified-epoxy class, if used to protect the exterior of the sucker rod couplings. 
     The following patents are of record in the patent application of this case: 
     
         ______________________________________Inventor             U.S. Pat. No.______________________________________Stephens et al       319,144Bahr                 1,134,092Gray et al           1,703,232Nathan et al         2,016,905Crake                2,337,711Ferguson             2,646,822Cavallier            2,656,857Holcombe             2,690,934Johnson              3,462,175Beene                3,675,950Sable                3,942,824Carlson              4,205,926Kupcak               4,226,550Bayh, III, et al     4,425,965Garrett              4,436,118Gallagher            4,484,833Takahashi            4,577,664Moore                4,627,490______________________________________ 
    
     Applicant believes the information found in these patents is not as pertinent as the information specifically discussed above. However, applicant believes that the Examiner would be interested in anything of record in the parent application. 
     SUMMARY OF THE INVENTION 
     (1) Progressive Contribution to the Art 
     No attempt has been made, to my knowledge, to qualify other resinous, fusion bond applied powder coatings for the protection of sucker rod couplings. The unique combination of physical and chemical properties, required of the sucker rod coupling coating resin system, preclude the use of nearly all coating resin types. For utility on sucker rod couplings, a coating must be resistant to down-hole attack by corrosive chemicals, at elevated temperatures and pressures. This severely limits the types of coating resins which may be used. The coating must also have a high resistance to mechanical damage. Resins with this physical attribute are common, but they do not have the required chemical resistance for down-hole oil field service. The coating resin must also bond to the coupling substrate, to preclude the spread of corrosive fluids between the protective coating and the coupling body. This further restricts the availability of resin types. In addition to these properties, the resin must also be highly resistant to abrasion. At the same time, it must be soft and be capable of deforming (extruding) without losing a bond, when the coating is subjected to rod tongs and wrenches. Previously, all of these necessary attributes have ruled out the use of fusion bond applied powder coatings on sucker rod couplings. 
     If a coating resin class having all these properties could be found, then an inexpensive method of sucker rod coupling protection would be available. In addition to the abrasion resistance and corrosion protection for the body of the sucker rod coupling, the resin could also be deposited on the radial faces of the sucker rod couplings, to permit a better seal between the coupling and the rod. This, besides overcoming problems associated with imprecise machining of the couplings and rod faces, would lock the coupling in place on the sucker rod. The coupling could not unscrew, down-hole in-service. At the same time, the presence of the coating on all or part of the radial faces of the couplings would preclude the passage of corrosive chemicals into the hollow coupling core. 
     Any diameter of sucker rod coupling can be prepared for abrasion and corrosion resistance, using this new method and apparatus. Machining costs are minimized. Expensive molds for injection molding are not needed. A protective coating is thermally bonded to the sucker rod coupling so it cannot slip out of position in-service. Corrosive fluids cannot get between the coupling and the protective coating. The external protective coating extends along the longitudinal axis, across the bevels on each end, and just beyond the outside edges of the radial faces. Coating on the radial faces will extrude when the coupling is installed, thereby locking the coupling in place. Extending across the entire length of the coupling, the specially selected protective coatings provide more abrasion resistance than would be evidenced with the mild steel coupling, alone. Corrosion resistance would also be provided. 
     Also, in the event the operator preferred to have metal-to-metal contact between the coupling and the rod pin end flange, the resin could be terminated along the edges of the circumferential surface and not extend across the radial faces of the coupling. 
     A coupling is first machined to decrease its diameter. Then, a chamfer or bevel is machined onto both ends of the coupling. After machining, the coupling must be cleaned, thermally or chemically, to remove all traces of oils which are present from the manufacturing operations. Then, the external diameter of the coupling and ends are abrasive blast-cleaned to achieve a suitable, uniform, angular, anchor profile. Before oxidation can occur, and while the coupling surface is free of contaminants, the coupling is placed into an oven and heated to a temperature which will exceed the melting point of the resinous, fusion bond coating which will be applied. Once at this temperature, the coupling is removed from the oven and protective masks are inserted into both ends. Before the coupling has time to cool below the melting point of the resinous powder coating, it is coated with a resinous substance which will bond to the coupling, provide abrasion resistance, and resist attack by down-hole oil field chemicals. 
     Few coating resin types are useful in this method. All must be corrosion resistant in the expected down-hole environment in an oil well. They must also be abrasion and chip resistant, being able to withstand handling abuse and pounding against the tubing wall in-service. The protective coating may be applied by the fluidized bed process, hot flocking, or electrostatic spray. Once coated to the required film thickness, the coated coupling is placed back into an oven, where the resinous coating is allowed to flow to form a uniform film. In cases where a thermoplastic resinous, fusion bond coating is used, a short post-heating time may be required. When thermoset resinous coatings are being used, a longer post-heat time is required to achieve full cure (cross-linking reaction). Regardless of the coating resin type selected, the coating should be present on the radial faces, or extend slightly above the plane of the radial faces, on the ends of the couplings, in order to allow the formation of a locking seal when the coupling is installed on a sucker rod. 
     (2) Objects of this Invention 
     An object of this invention is to provide a sucker rod string in oil wells. 
     Further objects are to provide such a string with couplings which are resistant to corrosion and abrasion. 
     Further objects are to achieve the above with devices that are sturdy, compact durable, lightweight, simple, safe, efficient, versatile, ecologically compatible, energy conserving, and reliable, yet inexpensive and easy to manufacture and maintain. 
     Still further objects are to achieve the above with a product that is easy to store, has a long storage life, is safe, versatile, efficient, stable and reliable, yet is inexpensive and easy to manufacture. 
     The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawing, the different views of which are not scale drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an oil well pumping unit. 
     FIG. 2 is a view of the sucker rod with the sucker rod coupling in half section. 
     FIG. 2A is a partial detail of the shoulder of the sucker rod and the radial face of the coupling with the coupling in section. 
     FIG. 3 is an enlarged portion of a sucker rod coupling in half section showing my new invention. 
     FIG. 4 is a sectional view of a sucker rod coupling with the mask or plug attached thereto being spray coated. 
     FIG. 5 is a schematic representation of the sucker rod coupling submerged in a fluidized bed of resinous coating material. 
     FIG. 6 is a schematic representation of an abrasion test apparatus. 
     FIG. 7 is a sectional view similar to FIG. 2A showing a modified structural form of this invention. 
     FIG. 8 is a half-sectional view similar to FIG. 3 showing the modification of FIG. 7. 
    
    
     As an aid to correlating the terms of the claims to the exemplary drawing, the following catalog of elements and steps is provided: 
     10--pump jack 
     12--pump 
     14--reservoir 
     16--sucker rod string 
     18--tubing string 
     20--casing 
     22--sucker rods 
     24--pin 
     26--coupling or cuff 
     28--wrench flats 
     30--shoulder 
     32--radial face 
     34--bore 
     36--threads 
     38--annulus 
     40--bevel 
     42--cylindrical portion 
     44--distal plug 
     46--eyelet plug 
     48--eyelet 
     50--bed or resinous material 
     52--container 
     54--porous plate 
     56--plenum 
     58--air supply connection 
     60--table 
     62--table mask 
     64--nozzle 
     66--coating 
     70--load 
     72--holding assembly 
     74--drive rod 
     76--tubing samples 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, there may be seen a typical well installation in FIG. 1. Specifically, pump jack 10 is connected to down-hole pump 12 which is located within a petroleum reservoir 14. Sucker rod string 16 extends from the pump jack to the pump. Those skilled in the art will understand that tubing string 18 extends from the surface of the ground and is fluidly connected to the pump 12. Casing 20 is outside the tubing string 13 and extends into the reservoir 14 below the pump 12 The sucker rod string 16 includes sucker rods 22. Each end of the sucker rods will have a pin 24 which is threaded into one end of coupling 26. The sucker rods will have wrench flats 28 adjacent to the pins 24. Between to the pins and the wrench flats, the sucker rod will have shoulder 30 which forms a friction fit against radial face 32 of the coupling 26. The couplings have an inside bore 34 which have threads 36 substantially the entire length. The oil will flow within annulus 38 between the sucker rod string 16 and the tubing string 18. It is known in the art that it is desirable to have as large an annulus 38 as possible and to have obstruction within the annulus at a minimum to permit unimpeded flow of the produced fluid upward through the tubing. Therefore, it is desirable that no portion of coupling 26 be larger than necessary. Also, it will be understood that the outside diameter of the coupling 26 will normally be the largest diameter of any of the elements of the sucker rod string 16. 
     Those having ordinary skill in the art will understand that the description to this point is old and well known before my invention. 
     In order for a coupling according to this invention to be manufactured, a sucker rod coupling for 0.750 inch sucker rods is machined to decrease its diameter by 0.080 inches. Then, a gradual chamfer or bevel 40 is machined onto both ends, such that the diameter at the radial face 32 will be less than that of the sucker rod pin end face or shoulder 30. The coupling 26 is then placed into an oven where it is heated above the incineration point of the oils which might be present from the manufacturing operations, 750° F., in this case. It is maintained at this temperature for sufficient time to burn away all organic substances (2 hours, in this example). Care is taken to insure that the coupling is not heated to a temperature, sufficiently high, to adversely effect the metallurgy of the mild steel. Once thermally cleaned, the coupling is allowed to cool, gradually, to ambient temperatures. 
     Next, the external diameter of the cylindrical portion 42 and the ends of the coupling are abrasive blast-cleaned, to establish a uniform, angular, anchor profile of approximately 0.003 inches, depth. Before any visible oxidation can occur, the coupling 26 is placed into an oven where it is heated to 650° F. Once at this temperature, the coupling is removed from the oven and protective masks or plugs are inserted into both ends. Referring to FIG. 5, distal mask plug 44 is placed into one end of the coupling 26 which is in the form of a cuff. The distal mask or plug 44 fits snugly in the threads at this end and covers all internal surfaces of the bore 34 of the coupling or cuff 26. Eyelet mask or plug 46 is inserted into the other end of the cuff 26 and covers up the threads 36 and inside bore 34 of that end. The eyelet plug has eyelet 48 thereon so that a hook or other means may be attached thereto for handling the hot cuff with the plugs therein. The masks will preclude any coating getting into the internal threaded section of the coupling. 
     Once masked, and while the metal temperature of the coupling still exceeds the melting point of the intended, fusion bond coating, it is immersed into a fluidized bed containing the resinous fusion bond, powder coating (See FIG. 5). As seen, the bed of resinous material 50 is included within a container 52. The bottom of the container has porous plate 54 separating the bed 50 from plenum 56 below it. Air blown into the plenum 56 through air connection 58 will fluidize the bed of finely granulated or powdered material as is well known. A uniform fluidization has been achieved. For this example, a high molecular weight polyamide coating (Nylon-11) is being fluidized. When the hot coupling makes contact with the Nylon powder coating particles in the fluidized bed, the nylon melts onto the surface of the coupling. To insure that a uniform film or coating 66 is established, the coupling is kept in motion, while immersed in the fluidized bed of nylon coating particles. The air supply into the fluidized bed plenum 56 is regulated to achieve a sufficient volume of air, passing through the porous plate 54 at the bottom of the fluidized bed. The upward movement of the airstream keeps the nylon powder particles suspended, and in constant motion. After sufficient immersion time has elapsed (60 seconds in this example) to build the required film or coating thickness (e.g., 0.040 inches), the coupling is removed from the fluidized bed. The protective masks are quickly removed from both ends of the coupling. If the coating film is smooth and has a uniform gloss, it is quenched by immediately dropping into a vessel containing water, which is at, or near, ambient temperature. Cooling in this manner, the Nylon coating 66 will have much greater resistance to chipping and mechanical damage, during handling, and while in-service. If more flow is required, or if the coating surface does not have a uniform gloss, it is placed back into the 650° F. oven, and allowed to remain there until sufficient flow-out is achieved. In this case, the coupling must also be water quenched upon exit from the oven. If sufficient Nylon coating film thickness has not been deposited, the coupling may be reheated to 650° F., and another layer of Nylon coating applied. The second layer will combine with the first. 
     Once the coating operation has been completed, the Nylon coating 66 (see FIGS. 2, 2a, and 3) will extend somewhat across the radial faces of the coupling, or above the plane of the radial faces, to permit a locking seal. It will also extend across the entire length of the coupling body. The exact location of the coating deposition is controlled through the use of coating masks, during the coating operations. The coating film thickness is controlled by the coupling temperature, and the immersion time in the fluidized bed. (Higher temperatures and longer immersion times produce greater coating film thicknesses.) Excess Nylon coating may be machined away from the coupling body or radial faces. Obviously the cylindrical portion 42 of the coupling is an exposed surface; however, the radial face 32 is covered when the coupling 26 is made up against the shoulder 30 of the sucker rod. 
     Referring to FIGS. 7 and 8, there may be seen a modification of this invention. Although it is preferred that the coating material extend over the radial face 32 for the reasons stated, it is also understood that some operators will desire that there be a metal-to-metal contact. FIGS. 7 and 8 disclose an embodiment where the radial face 32 would be bare metal. The radial faces 32 make metal-to-metal contact with sucker rod shoulders 30. Those having ordinary skill in the art would understand that plugs 44, 46, or masks 62 could be made which would include a flange of the exact diameter as the standardized shoulder 30. Therefore, all exposed surfaces of the connector could be coated. By exposed surfaces, it is meant all exterior surfaces such as shown in FIGS. 7 and 8. Particularly, if the radial face 32 of the connector was made to have a lesser diameter than the shoulder 30, the exposed surface would be all exterior surfaces except for the radial face 32. The exposed surface would be the cylindrical surfaces 42 and the bevel 40. 
     At the option of the coupling manufacturer, this procedure may be altered to include the use of a primer. After the couplings have been abrasive-blast-cleaned, and before any oxidation can occur, the couplings may be primed with an adhesion promoting primer, such as Morton-Thiokol NCP-200. This primer, a solvent-based epoxy-phenolic. The primer may be sprayed onto the coupling, or the coupling may be totally immersed in the primer. Once primed, the couplings are placed into the pre-heat oven and the process is continued, as stated before. The use of such a primer will improve the long-term adhesion of the nylon to the rod coupling. 
     Using the same fluidized bed technique, a 0.625 inch sucker rod coupling can be protected with a Teflon powder coating. The coating procedure varies slightly, however. As with Nylon, the coupling is first machined to decrease its diameter. Then it is thermally cleaned to incinerate organic substances which may contaminate the surface to be coated. Next, it is abrasive blast-cleaned, using a flint abrasive, at approximately 90 psi air pressure. Before any oxidation can occur, the coupling is primed with a special primer which will allow the Teflon resin to bond to the coupling. The primer is allowed to dry, while standing in ambient temperature air. The primer seals the freshly cleaned surface, precluding oxidation. Next, the primed coupling is placed into a preheat oven, where it is brought up to the proper coating application temperature (725° F.). Once at this temperature, the coupling is removed from the oven and protective masks or plugs are quickly inserted into both ends of the coupling. The coupling is then immersed into a fluidized bed containing Teflon powder particles. The coupling is moved up and down and/or back and forth under the fluidized bed surface for 60 seconds. Then, it is removed and placed back into an oven, where it is heated at 725° F. for an additional 20 minutes. During this time, the Teflon particles will flow out to form a uniform coating film. After this post-heating cycle, and if sufficient film thickness has been deposited, the masks are removed and the coated coupling is allowed to cool, gradually, to ambient temperatures. Any excess Teflon may be machined away. If insufficient film thickness has been deposited, the coated coupling may be reheated to 725° F. and another layer of Teflon applied. The second layer will bond to the first layer. An additional post-heat for several minutes at 725° F. will be required to allow the second coat to flow out properly. 
     In yet another example of this method, a 0.875 inch mild steel sucker rod coupling can be protected with a polyphenylene sulfide (Ryton) powder coating. Before the coating is applied, the coupling is machined to decrease its diameter and to establish a gradual chamfer or bevel 40 on both ends. Then, the coupling is chemically cleaned to remove all traces of oils which may be present from the manufacturing operations. Next, the coupling is grit-blasted to establish a uniform anchor profile. The coupling is then heated to the proper coating application temperature (750° F.), before oxidation can occur. Once at this temperature, the coupling is removed from the oven and placed onto rotating coating table 60. (See FIG. 4) The protective mask or eyelet plug 46 is placed into the top of the coupling 26. The rotating table platform has table mask or plug 62 for the bottom of the coupling while standing vertically. The plug 62 functions as the distal plug 44. While being rotated about its longitudinal axis, a powdered polyphenylene sulfide coating resin is blown from nozzle 64 onto the surface of the coupling. As the powder particles make contact with the hot coupling, they melt to form a uniform coating film. Once coated, the coupling 26 is removed from the table plug 62 and the eyelet mask 46 is removed and the coated coupling is placed back into the oven for curing. The coated coupling is further heated for 115 minutes at 750° F. to insure that a complete chemical reaction has occurred. After this cure cycle, the couplings maybe quenched in a vessel containing water, which is at, or near, ambient temperature. If excess coating is present, after cooling, it may be machined away. 
     In addition to the fusion bond applied coating resins already mentioned, others can be used under the scope of this disclosure. To qualify for use, however, the coating resin must lend itself to fusion bond application. The coating must be highly chip resistant, abrasion resistant, and must be capable of withstanding attack by harsh, down-hole oil field chemicals. 
     Besides using a conventional, mild steel coupling as the core for this method and apparatus, higher tensile strength steels may be used. Because a small amount of each coupling is machined away before coating, the structural strength of the coupling is diminished, slightly. This can be overcome, using steels of higher tensile strength. Such steels are not preferred for bare sucker rod couplings, since they are more prone to chemical attack, down-hole. Since the protective coatings described in this disclosure act as a shock absorbant, corrosion resistant barrier, this tensile strength/corrosion susceptibility relationship is of no concern These higher tensile strength steels would be protected from corrosion by the coatings described in this disclosure. 
     While examples are given showing fluidized bed and hot flocking coating techniques, it is understood that any coating process may be used under the scope of this method to include electrostatic cold or hot spray, immersion in a fluidized or non-fluidized bed of resinous powder, hot flocking, or any combination of these techniques. Also, immersion coating may be used with Ryton. Further, the spray coating described above may be used with Nylon or Teflon. 
     (1) Useful Coating Resin Types 
     While more coating types may qualify for use, under the scope of this disclosure, the following products have proven to have the necessary physical and chemical properties for use as a sucker rod coupling coating: 
     For Nylon 
     Corvel (R) Black 70-7008, alone, or over NCP-200 Primer 
     Products of Morton Thiokol, Incorporated 
     Reading, Pa. 
     For Ryton 
     Nacote PRO-1 Clear 
     Product of Nacote, Incorporated 
     Bartlesville, Okla. 
     Sometimes called RYTON 
     For Teflon 
     532-5010 Clear Over 850-201 Primer 
     Product of E. I. duPont, de Nemours, Incorporated 
     Wilmington, Del. 
     Sometimes called TEFLON 
     The following physical properties were provided by the manufacturer of these products: 
     
         __________________________________________________________________________    Test    MethodProperty (ASTM)  Units NYLON                       RYTON                            TEFLON__________________________________________________________________________Shore Hardness    D2240   D     75   87   55Tensile Strength    D412    psi.  8500 12000                            4000DielectricStrength D149    Volts/Mil                  1000 400  2000Elongation    D638    %     30   0.7  300Water Absorption    D150    %     1.6  0.07 0.03Specific Gravity    D792    Unity 1.08 2.00 2.15Taber Abrasion    D1044   Grams 0.014                       0.070                            Unknown    CS-10 Wheel    1000 GM. Load    1000 CyclesGlass Transition    D.S.C.  °F.                  109  545  575Impact Resistance    Gardner In.-Lbs.                  &gt;160 &gt;160 &gt;160__________________________________________________________________________ 
    
     (2) Abrasion Resistance Data 
     To compare the relative abrasion resistance of sucker rod couplings, an abrasion tester was constructed. (See FIG. 6) For the test, the coupling 26 is positioned under load 70 of 100 pounds. Holding assembly 72 is connected to reciprocating rod which is driven back and forth across a distance of two inches through the use of a motor and gear box (not shown). The reciprocating frequency is constant, and at a rate of one stroke per second. Below each coupling 26, tubing samples 76 are held in a fixed, horizontal position. When the coupling holding assembly 72 is put into the test position, the couplings 26 are aligned, axially, with the tubing samples 76. Since the connection between the holding assembly and the drive rod is flexible, both couplings receive an equal loading, and their entire axial length rests on the internal diameter of the tubing. 
     Each test was run for 24 hours, or 86,400 strokes. For each test, 2.375 inch diameter tubing and 0.750 inch couplings were used. The tubing was abrasive blast-cleaned to a &#34;white&#34; metal finish, with a uniform, angular, anchor profile of approximately 0.0015 inches, depth. No lubricant was used in the tests to minimize the abrasion. Coupling diameters and tubing wall thicknesses were measured with a micrometer, before and after each test. The following results were evidenced: 
     
         ______________________________________          Average Abrasion Loss          (1/1000 inches)Coupling Description            Coupling Pipe Wall______________________________________Mild Steel       104.0    60.0Spraymetal Jacketed            0.3      5.0Nylon Coated     7.7      0.0Ryton Coated     15.0     0.0Teflon Coated    3.0      0.3*U.S. Pat. No. 4,602,807            8.0      1.8______________________________________ *The Teflon coating was 0.002 inches thick on one end. After the Teflon coating abraded away, the bare metal of the coupling abraded the internal diameter of the tubing. 
    
     The embodiments shown and described above are only exemplary. I do not claim to have invented all the parts, elements or steps described. Various modifications can be made in the construction, material, arrangement, and operation, and still be within the scope of my invention. 
     The restrictive description and drawing of the specific examples above do not point out what an infringement of this patent would be, but are to enable one skilled in the art to make and use the invention. The limits of the invention and the bounds of the patent protection are measured by and defined in the following claims.