Wire assemblies for repetitive, continuous cycle, tensile load conditions, particularly sucker rods for oil wells

Multi-wire steel strand for use under repetitive cyclic load conditions, such as in mechanically pumped wells, is provided with end connecting pieces which are swaged into gripping engagement with the strand in a manner to avoid fatigue failure of the wire at the end connecting pieces, by avoiding abrupt load transitions in the wire. The swaging is such as to gradually and progressively transfer load from the wire strand to the end connecting pieces, preferably by virtue of a gradual and progressive increase of the grip between the wire strand and an end piece, considered in the direction of the terminal end of the strand on which the end piece is mounted, in order to gradually reduce the load in the end portion of the strand and avoid abrupt load transitions therein, which can act as stress risers. A lubricating sleeve insert prevents galling and abrading in the area of relatively slight grip where relative movement may occur, so as to avoid creating stress risers. A particular use is for oil well sucker rods, utilizing high carbon, low-relaxation, steel wire, advantageously covered with fiberglass reinforced plastic while held under tension during curing, although the invention is appropriate for many uses. The strand diameter will be varied appropriately to suit particular conditions.

FIELD OF THE INVENTION 
This invention relates to wire assemblies for use under repetitive cyclic 
tensile load conditions, and more particularly to pump sucker rods for oil 
wells or other mechanically pumped wells. The invention is particularly 
directed to the problem of avoiding fatigue failure of the wire at its 
connections with end pieces, which are used to connect wire sections 
together, or to connect a wire section to other members. In use as a pump 
sucker rod, a number of the assemblies are connected together to form a 
sucker rod string in an oil well, where severe repetitive load conditions 
and corrosion and abrasion conditions exist. 
BACKGROUND AND SUMMARY 
Pump sucker rods in oil wells operate under severe conditions of load, 
abrasion and environment. They are subject to fatigue failure, corrosion, 
wear failure, etc., and these problems have been studied for many years, 
and numerous improved solutions have been proposed to the problem of 
optimum sucker rod structure, function and design. Durability and 
lightness are important features to be achieved. Typical of the approaches 
followed by the prior art are those disclosed in U.S. Pat. No. 4,205,926, 
issued June 3, 1980 (Carlson), and other prior art approaches, patents and 
publications mentioned therein, all of which are incorporated herein by 
reference. 
The instant invention proposes a construction which may be of relatively 
light weight through the use of high carbon steel strand possessing at 
least approximately twice the tensile strength of most currently used 
rods, and in which fatigue failure of the strand is avoided or at least 
minimized. Sucker rods necessarily come in sections formed with end pieces 
or fittings for connection with other sections in assembling a sucker rod 
string of the requisite length. While there are advantages in using wire, 
strand, cable, wire rope, etc. as the major component of a sucker rod 
section, there has been and continues to be a definite problem of fatigue 
failure of the wire or the like in the area of its connection with an end 
piece or fitting. It is believed that this problem of fatigue failure is 
solved by the instant invention. The invention also proposes a 
construction which utilizes multi-wire, high carbon steel strand, which is 
lighter and stronger than materials known to me to be currently used in 
sucker rods, while minimizing problems of corrosion to which all strand is 
susceptible. The use of this lighter and stronger rod results not only in 
reduced pumping power cost, but also permits the use of lighter, less 
costly pumping units for wells of any given depth, thus reducing the 
capital investment. The increased strength also permits use of a lesser 
diameter rod, which facilitates overwrap with a protective material 
without increasing the diameter excessively. Such high carbon strand, 
because of its high strength and modulus of elasticity, also avoids 
problems of stretch which occur in many currently used rods. Finally, the 
invention proposes a construction utilizing a particularly advantageous 
strand, that is, low-relaxation strand, one version of which is well known 
for use in prestressed concrete, but which, to my knowledge, has never 
been used or proposed to be used in oil well sucker rods. Such 
low-relaxation strand has practically no creep, is highly resistant to 
fatigue failure, and, in common with other high carbon steels, is 
resistant to abrasion. While the invention finds particular utility in oil 
well sucker rods, it is not necessarily so limited, since there are 
various other uses for the invention. 
Although the invention itself is as defined in the appended claims, the 
various features, advantages and objects of the invention will be best 
understood from the ensuing description of preferred exemplary 
embodiments, taken in conjunction with the accompanying drawings, which 
form a part of the instant specification and are to be read in conjunction 
therewith.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows an end piece or fitting generally designated 2 mounted on a 
multi-wire strand generally designated 1. The end piece 2, in its 
left-hand portion, has an outer configuration largely the same as a 
conventional sucker rod pin end, although it will be readily understood 
that a conventional box end could have been shown. Specifically, the 
left-hand portion has a conventional externally threaded end 3, a 
circumferential semi-cylindrical flange 4, wrench flat portion 5, and 
tapered enlarged shoulder 6. These parts and portions serve the same 
function as the like parts of a conventional sucker rod pin end. Thus, for 
example, when a sucker rod is suspended in a rack or the like, it may be 
suspended from shoulder 6. The wrench flat portion is for engagement of 
suitable wrenches for turning the sucker rod assembly. Flange 4, among 
other things, limits the inboard travel of a coupling threaded onto the 
threaded end 3, for connecting the end fitting to a corresponding fitting 
of another section, and for sealing the connection. In general, the 
left-hand portion of the fitting is in accordance with American Petroleum 
Institute Standard API-SPEC-11B. 
Apart from the previously described left-hand portion, the end fitting 2 is 
unlike other end fittings for sucker rods or other such structures known 
to me. Fitting 2 includes a passageway 8 bored therein, and opening from 
the right-hand end of the fitting as viewed in FIG. 1. This passageway is 
enlarged as shown at 9 so as to provide clearance for a lubricating sleeve 
insert 10, typically comprising brass shim stock, extending partway into 
the passageway 8, and encompassing a corresponding portion of strand 1. 
Strand 1 is inserted into the full extent of passageway 8, whereas insert 
10 stops at the shoulder formed between passageways 8 and 9. The bored 
passageway is located in an elongate body portion 7 of fitting 2. As 
indicated at 11, the passageway at its right-hand end is further reamed 
slightly so as to provide a slight outward taper. The elongate body 7 is 
tapered gradually outwardly from a point indicated at 12 to a point 
indicated at 13, adjacent shoulder 6. As a typical example, this taper 
might be from an outer diameter of one inch at 12 to an outer diameter of 
one and one-eighth inch at 13. It will be understood that this is in the 
unswaged condition. The purpose of the taper is to permit progressive 
swaging or deformation of elongate body 7 from point 12 to point 13 by 
swaging dies or any convenient swaging or deforming means, such that in a 
typical example, body 7 between points 12 and 13 will be swaged down to a 
uniform outer diameter of one inch. Thus, in a typical example, there will 
be substantially no swaging at point 12, and the degree of swaging will 
progressively increase up to a maximum at point 13, where the outer 
diameter will be swaged perhaps one-eighth inch down to a swaged diameter 
of one inch. The purpose of this is to progressively increase the degree 
of swaging from right to left, and thus progressively increase the grip of 
the strand 1 by the walls of passage 8, 9, and insert 10, such that when 
the assembly is under a repetitive cyclic tensile load, the load will be 
transferred from the strand 1 to the fitting 2 progressively and gradually 
over the strand portion inside the body 7, as opposed to an abrupt 
transition from full load to no load (or a substantially lesser load) in 
the wire strand. This will be explained in more detail subsequently. 
The effects of this progressive swaging or deformation are shown in FIGS. 2 
and 3. Thus, the progressive swaging forms striations 14 in the passageway 
wall, as the material of body 7 is deformed about the exterior wires of 
the strand 1. The striations begin approximately at 15, where they are 
very shallow, and progressively increase in depth approximately to a point 
indicated at 16. The corresponding striations caused in the insert 10 are 
also illustrated, the relatively deep striations being indicated generally 
at 10b, and the relatively unstriated portion of the insert being shown 
generally at 10a. 
Because of the progressively or gradually increasing gripping engagement 
between the fittings and the strand, there is no abrupt load transition in 
the strand, that is, the strand as it passes into the fitting does not 
change abruptly from full load to no load. Rather, the load is transferred 
relatively gradually from the strand to the fitting over an appreciable 
length of the strand, and there is no point of abrupt load change in the 
wire sufficient to act as a stress riser under repetitive or pulsating 
tensile load conditions such as occur in a sucker rod in oil wells. In 
essence, the avoidance of abrupt load transition in the wire of the strand 
by the progressive gripping engagement avoids fatigue failure. However, in 
the area of relatively slight swaging and hence relatively slightly 
gripping engagement, there can be a slight relative movement between the 
strand and te shallow striations of the passageway. This tends to cause 
galling, abrading, etc., which can have a stress rising effect, such that 
failure might occur at the beginning of the striations. To avoid or 
minimize this possibility, the sleeve insert 10 is incorporated to act in 
the nature of a dry lubricant, and thus minimize or avoid any adverse 
affects because of any slight relative movement in the area of relatively 
slight gripping engagement. To achieve its purpose, the insert in general 
should be of a non-galling, non-abrading and non-seizing material, softer 
than the metal of the strand wire or the metal of the elongate body 7. 
Typical materials useful for the insert 10 include brass, lead, tin, 
copper, or alloys of any of these. As a useful example, brass shim stock 
of 0.005 inch thickness has been found to be generally satisfactory. 
In general, in the fabrication of the initial fitting 2, before swaging, 
there advantageously should be a minimum clearance between passageway 8 
and strand 1, and between passageway 9, insert 10 and strand 1, merely 
sufficient to permit easy insertion of the strand end portion fully into 
the passageway 8 of the fitting. For instance, when using one-half inch 
strand, the internal diameter of passageway 8 might range from 0.509 to 
0.516 inches. If brass shim stock 0.005 inches thick is used for insert 
10, passageway 9 may be of substantially 0.526 inches internal diameter. 
Other typical dimensions in the case of one-half inch strand include a 
distance of substantially six inches between points 12 and 13 of elongate 
body 7, a distance of three-fourths inch between point 12 and the 
right-hand end of body 7, a five-eighths inch outer diameter at point 12, 
and a one and one-eighth inches outer diameter at point 13, in the 
unswaged condition, with a one inch outer diameter at 13 after swaging. 
As a rule of thumb, the total wall thickness of elongate body 7 after 
swaging is approximately equal to the strand diameter. Thus, using 
one-half inch strand, the swaged wall thickness would be one-quarter inch. 
The swaging deformation advantageously ranges from substantially 0 at 
point 12 to maximum swaging at point 13. As a further rule of thumb, the 
reduction in outer diameter at the area of maximum swaging may be 
approximately 12 percent to 15 percent of the after-swaging outer 
diameter. However, dimensions, tolerances, degree of swaging, etc. for 
particular operating conditions may be determined by calculation and 
experimentation within the skill of the art. 
While the provision of the taper on elongate body 7 provides an easy manner 
of effecting the progressive swaging using uniform swaging dies, it will 
be understood that a generally corresponding effect could be accomplished 
by using tapered swaging dies or other form of swage on an unswaged 
fitting of substantially uniform outer diameter, the particular device 
used for swaging or deformation being not critical to the invention. 
Furthermore, the progressive increase in swaging need not be uniform, and 
the initially tapered portion may encompass less than the full length of 
body to be swaged. There may be an initial length of increasing swaging, 
followed by a length of uniformly swaged body, in which case the unswaged 
body may have an initial outward taper followed by a portion of generally 
uniform diameter. 
The method of fabrication will be self-evident, it being a simple matter of 
assembling the fitting, the strand and the insert, and then swaging the 
elongate body 7 to the desired degree. 
In a typical example that has performed quite satisfactorily, the strand is 
a seven wire strand of high carbon steel (carbon 0.75 percent to 0.85 
percent), although any carbon steel or alloys thereof could be used 
assuming adequate strength is developed and the end fittings are adapted 
appropriately in accordance with the invention. Wire rope possibly could 
be used. However, high carbon steel, multi-wire strand is much to be 
preferred because of its resistance to stretch and creep. Even more to be 
preferred is low-relaxation strand, which is widely and advantageously 
used in prestressed concrete. Low-relaxation strand is well-known in the 
art, and can be manufactured as disclosed in U.S. Pat. No. 3,398,258 and 
U.S. Pat. No. 3,196,052. Standard specifications for seven-wire 
stress-relieved strand, including low-relaxation strand, suitable for use 
in the present invention, are in accordance with ANSI/ASTM A 416-74. A 
particularly suitable low-relaxation strand is manufactured and sold by 
Florida Wire and Cable Company, Jacksonville, Fla., under the trademark 
"LO-LAX". Such low-relaxation strand has an increase in yield strength of 
approximately five percent over stress-relieved strand, and exhibits 
greatly improved fatigue properties. 
The end fittings advantageously are of 1040 heat treated steel, and are 
formed by drop forging. Depending on the accuracy of the drop-forging, it 
is entirely feasible to swage the forged fitting, but in some instances 
machining of the forged fitting may be necessary before swaging. 
Particularly when used as oil well sucker rods, the assembled sections are 
covered or coated with plastic to minimize corrosion and down-well 
abrasion. Advantageously this is fiberglass reinforced resin or plastic, 
applied to the sections by wrapping while the sections are under tension, 
the tension being maintained until the plastic, typically a thermosetting 
polymer, is cured. Upon release of the tension, the fiberglass reinforced 
plastic will be under compression, thus contributing to the life of the 
sucker rod section. Any unwound part, including couplings between 
sections, may be coated with an epoxy such as diethylene triamine, a 
typical example being Epon 828, produced by Shell Chemical. Since 
fiberglass has very little, if any, affinity for paraffin, such rods are 
very desirable in high paraffin oil wells. 
The abrasion resistance of the fiberglass-resin surface is important. 
Equally as important is the combination of light weight and flexibility of 
the coated rod in combatting abrasion. Thus, few deep oil wells are 
actually straight such that the sucker rod string does not rub against the 
well tubing at bends or curves. Since my improved rod string is light and 
flexible, the abrading force normal to the tubing, caused by weight and 
stiffness, is less than in conventional sucker rods. 
The fiberglass/resin coating also avoids the need for deducting a corrosion 
allowance in calculating rod strength. Additionally, the fiberglass 
winding can be enlarged in diameter at selected points to function as 
tubing wipers, or to serve as stops for rod mounted wipers, which 
frequently slide down on the rod and collect at the couplings. 
The swaging operation may be effected as desired; good results have been 
achieved using an Esco Mark 150 500-ton swaging press. 
The length of the lubricating sleeve insert may be varied as desired. In 
the typical example for which detailed dimensions were given previously, 
an insert length of 3.5 inches was found satisfactory, but this can be 
varied to give optimum performance for given pumping conditions. The 
degree of swaging is also variable, and is not confined to the examples 
given herein. 
From the foregoing, it will be seen that this invention is well adapted to 
attain all of the ends and objects and advantages hereinabove set forth, 
together with other advantages which are obvious and inherent to the 
apparatus. It will be understood that certain features and subcombinations 
are of utility and may be employed without reference to other features and 
combinations. Many possible embodiments may be made of the invention 
without departing from the scope thereof, and hence it is to be understood 
that all matter herein set forth or shown in the accompanying drawings is 
to be interpreted as illustrative and not in a limiting sense.