Patent Publication Number: US-11035415-B2

Title: Universal driveshaft assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/325,475 filed Jan. 11, 2017 (now U.S. Pat. No. 10,408,274, which is a 35 U.S.C. § 371 national stage entry of PCT/US2015/040513, filed Jul. 15, 2015, and entitled “Universal Driveshaft Assembly,” which claims the benefit of U.S. provisional patent application Ser. No. 62/025,322 filed Jul. 16, 2014, and entitled “Universal Driveshaft Assembly,” and U.S. provisional patent application Ser. No. 62/025,326 filed Jul. 16, 2014, and entitled “Universal Driveshaft Assembly,” the contents of each of the foregoing are hereby incorporated herein by reference in their entirety for all purposes. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     The disclosure relates generally to universal joints for transmitting torque between rotating shafts having intersecting but non-coincident rotational axes. More particularly, the disclosure relates to universal joints for driveshafts employed in downhole motors used in the oil and gas drilling operations. 
     In drilling a borehole into an earthen formation, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a drillstring formed from a plurality of pipe joints connected together end-to-end, and then rotate the drill string so that the drill bit progresses downward into the earth to create a borehole along a predetermined trajectory. In addition to pipe joints, the drillstring typically includes heavier tubular members known as drill collars positioned between the pipe joints and the drill bit. The drill collars increase the vertical load applied to the drill bit to enhance its operational effectiveness. Other accessories commonly incorporated into drill strings include stabilizers to assist in maintaining the desired direction of the drilled borehole, and reamers to ensure that the drilled borehole is maintained at a desired gauge (i.e., diameter). In vertical drilling operations, the drillstring and drill bit are typically rotated from the surface with a top dive or rotary table. 
     During the drilling operations, drilling fluid or mud is pumped under pressure down the drill string, out the face of the drill bit into the borehole, and then up the annulus between the drill string and the borehole sidewall to the surface. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall). 
     It has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical boreholes (i.e., “directional drilling”), to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottom hole assemblies” are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration. 
     Directional drilling is typically carried out using a downhole or mud motor incorporated into the bottom hole assembly (BHA) immediately above the drill bit. A typical downhole motor includes several primary components, such as, for example (in order, starting from the top of the motor assembly): (1) a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component); (2) a power section; (3) a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being coupled to the lower end of the rotor of the power section; and (4) a bearing assembly (which includes a mandrel with an upper end coupled to the lower end of the drive shaft, plus a lower end adapted to receive a drill bit). The power section is typically a progressive cavity or positive displacement motor (PD motor). In a PD motor, the rotor comprises a shaft formed with one or more helical vanes or lobes extending along its length, and the stator is formed of an elastomer liner bonded to the inner cylindrical wall of the stator housing. The liner defines helical lobes complementary to that of the rotor lobe or lobes, but numbering one more than the number of rotor lobes. The lower end of the rotor comprises an output shaft, which in turn is coupled to the upper end of a drive shaft that drives the rotation of the drill bit. 
     In drilling operations employing a downhole motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as previously described. However, in route to the drill bit, the pressurized drilling fluid flows through the power section of the downhole motor to generate rotational torque to rotate the drill bit. In particular, high-pressure drilling fluid is forced through the power section, causing the rotor to rotate within the stator, and inducing a pressure drop across the power section (i.e., the drilling fluid pressure being lower at the bottom of the power section). The power delivered to the output shaft is proportional to the product of the volume of fluid passing through the power section multiplied by the pressure drop across the power section (i.e., from fluid inlet to fluid outlet). Accordingly, a higher rate of fluid circulation fluid through the power section results in a higher rotational speed of the rotor within the stator, and correspondingly higher power output. 
     As previously noted, the output shaft is coupled to the upper end of the drive shaft, for transmission of rotational torque to the drill bit. However, the motion of the rotor in a PD motor is eccentric in nature, or “precessional”—i.e., in operation, the lower end of the rotor (i.e., the output end) rotates or orbits about the central longitudinal axis of the stator housing. The output shaft is coupled to the upper end of the drive shaft with a first (or upper) universal joint, thereby allowing rotational torque to be transferred from the rotor to the drive shaft irrespective of the eccentric motion of the rotor or fact that the output shaft and drive shaft are not coaxially aligned. 
     The bearing assembly typically incorporates an elongate tubular mandrel having an upper end coupled to the lower end of the drive shaft by means of a second (or lower) universal joint, and a lower end coupled to the drill bit. The mandrel is encased in a tubular bearing housing that connects to the tubular drive shaft housing above. The mandrel rotates concentrically within the bearing housing. 
     The universal joint assemblies of conventional driveshafts are prone to substantial wear and may fail relatively quickly during operation. In particular, many such conventional driveshafts transfer torque through either point or line contact(s), which disperse a large amount of force over a relatively small surface area, thereby tending to accelerate wear at such contact surfaces. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     Some embodiments disclosed herein are directed to a driveshaft assembly. In an embodiment, the driveshaft assembly includes a driveshaft including a longitudinal shaft axis, a first end, a second end opposite the first end, and a radially outer surface. The first end includes a plurality of recesses extending radially inward from the radially outer surface, the recesses each comprising a planar engagement surface. In addition, the driveshaft assembly includes a first end housing including a longitudinal housing axis, and an axially extending receptacle. The receptacle includes a plurality of planar receptacle surfaces. Further, the driveshaft assembly includes a torque transfer assembly configured to transfer torque between the driveshaft and the first end housing. The torque transfer assembly includes a plurality of torque transfer keys each including a planar key surface and a convex key surface, and an adapter including a plurality of concave adapter surfaces and a plurality of planar adapter surfaces. The planar engagement surface of the each recess engages the planar key surface of one of the torque transfer keys. In addition, the convex key surface of each torque transfer key engages one of the concave adapter surfaces of the adapter. Further, each of the planar adapter surfaces of the adapter engage with one of the planar receptacle surfaces. 
     Other embodiments are directed to a driveshaft assembly. In an embodiment, the driveshaft assembly includes a driveshaft including a longitudinal shaft axis, a first end, a second end opposite the first end, and a radially outer surface. The first end includes a plurality of recesses extending radially inward from the radially outer surface, the recesses each comprising a convex engagement surface. In addition, the driveshaft assembly includes a first end housing including a longitudinal housing axis, and an axially extending receptacle. The receptacle includes a plurality of planar receptacle surfaces. Further, the driveshaft assembly includes a plurality of torque transfer keys configured to transfer torque between the driveshaft and first end housing, each of the torque transfer keys including a planar key surface and a concave key surface. The convex engagement surface of each recess engages the concave key surface of one of the torque transfer keys, and the planar key surface of each torque transfer key engages one of the planar receptacle surfaces. 
     Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly certain of those features and characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily understood by those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the teachings disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of various embodiments, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a schematic partial cross-sectional view of a drilling system including an exemplary embodiment of a driveshaft assembly in accordance with at least some embodiments; 
         FIG. 2  is a partial cross-sectional side view of the driveshaft assembly of  FIG. 1 ; 
         FIG. 3  is an enlarged cross-sectional side view of one of the universal joint assemblies of the driveshaft assembly of  FIG. 1 ; 
         FIG. 4  is a perspective view of the lower end of the driveshaft of  FIG. 1 ; 
         FIG. 5  is a front or axial view of the lower end of driveshaft of  FIG. 1 ; 
         FIG. 6  is another perspective view of the lower end of the driveshaft of  FIG. 1  illustrating the installation of a torque transfer assembly thereon in accordance with at least some embodiments; 
         FIG. 7  is a perspective view of one of the torque transfer keys of the torque transfer assembly of  FIG. 6 ; 
         FIG. 8  is a top view of one of the torque transfer keys of the torque transfer assembly of  FIG. 6 ; 
         FIG. 9  is a perspective view of the adapter of the torque transfer assembly of  FIG. 6 ; 
         FIG. 10  is a top view of the adapter of the torque transfer assembly of  FIG. 6 ; 
         FIG. 11  is an enlarged perspective view of one of the arms of the adapter of the torque transfer assembly of  FIG. 6 ; 
         FIG. 12  is an enlarged top view of one of the arms of the adapter of the torque transfer assembly of  FIG. 6  illustrating the installation of one of the torque transfer keys of  FIG. 6  thereon; 
         FIG. 13  is a perspective view of the end housing of the universal joint assembly of  FIG. 3 ; 
         FIG. 14  is a front or axial view of the end housing of the universal joint assembly of  FIG. 3 ; 
         FIG. 15  is a cross-sectional view of the universal joint assembly taken along section XV-XV of  FIG. 3 ; 
         FIG. 16  is a partial cross-sectional side view of another driveshaft assembly for use within the drill system of  FIG. 1 ; 
         FIG. 17  is an enlarged cross-sectional side view of another universal joint assembly for use in the driveshaft assembly of  FIG. 1  in accordance with at least some embodiments; 
         FIG. 18  is a perspective view of a lower end of the driveshaft of  FIG. 16 ; 
         FIG. 19  is a front or axial view of the lower end of driveshaft of  FIG. 16 ; 
         FIG. 20  is another perspective view of the lower end of the driveshaft of  FIG. 16  illustrating the installation of torque transfer keys thereon in accordance with the at least some embodiments; 
         FIG. 21  is a perspective view of one of the torque transfer keys of  FIG. 20 ; 
         FIG. 22  is a top view of one of the torque transfer keys of  FIG. 20 ; 
         FIG. 23  is a front view of one of the torque transfer keys of  FIG. 20 ; 
         FIG. 24  is a perspective view of the end housing of the universal joint assembly of  FIG. 17 ; 
         FIG. 25  is a front or axial view of the end housing of the universal joint assembly of  FIG. 17 ; and 
         FIG. 26  is a cross-sectional view of the universal joint assembly taken along section XXVI-XXVI in  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following discussion is directed to various exemplary embodiments. However, one of ordinary skill in the art will understand that the examples disclosed herein have broad application, and that the discussion of any exemplary embodiment is meant only to be illustrative of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection that is established via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. Any reference to up or down in the description and the claims is made for purposes of clarity, with “up”, “upper”, “upwardly”, “uphole”, or “upstream” meaning toward the surface of the borehole and with “down”, “lower”, “downwardly”, “downhole”, or “downstream” meaning toward the terminal end of the borehole, regardless of the borehole orientation. 
     Referring now to  FIG. 1 , a system  10  for drilling a borehole  16  in an earthen formation is shown. In this embodiment, system  10  includes a drilling rig  20  disposed at the surface, a drill string  21  extending from rig  20  into borehole  16 , a downhole motor  30 , and a drill bit  90 . Motor  30  forms a part of the bottomhole assembly (“BHA”) and is disposed between the lower end of the drill string  21  and drill bit  90 . Moving downward along the BHA towards bit  90 , motor  30  includes a hydraulic drive or power section  40 , a driveshaft assembly  100  coupled to power section  40 , and a bearing assembly  80  coupled to driveshaft assembly  100 . Bit  90  is coupled to the lower end of bearing assembly  80 . 
     The hydraulic drive section  40  converts pressure exerted by drilling fluid pumped down drill string  21  into rotational torque that is transferred through driveshaft assembly  100  and bearing assembly  80  to drill bit  90 . With force or weight applied to the drill bit  90 , also referred to as weight-on-bit (“\NOB”), the rotating drill bit  90  engages the earthen formation and proceeds to form borehole  16  along a predetermined path toward a target zone. The drilling fluid or mud pumped down the drill string  21  and through motor  30  passes out of the face of drill bit  90  and back up the annulus  18  formed between drill string  21  and the sidewall  19  of borehole  16 . The drilling fluid cools the bit  90 , flushes the cuttings away from the face of bit  90 , and carries the cuttings to the surface. 
     Referring now to  FIG. 2 , driveshaft assembly  100  includes an outer driveshaft housing  110 , a driveshaft  120  rotatably disposed within housing  110 , a first or upper end housing  130  coupled to driveshaft  120 , and a second or lower end housing  140  also coupled to driveshaft  120 . Housing  110  is an elongate, cylindrical tubular member having a central or longitudinal axis  115 , a first or upper end  110   a , and a second or lower end  110   b  opposite upper end  110   a . As is best shown in  FIG. 1 , in this embodiment, housing  110  is coaxially aligned with hydraulic drive section  40  and bearing assembly  80 . In addition, upper end  110   a  of housing  110  is coupled to an outer housing of drive section  40  and lower end  110   b  of housing  110  is coupled to an outer housing of bearing assembly  80 . 
     Referring still to  FIG. 2 , driveshaft  120  has a central or longitudinal axis  125 , a first or upper end  120   a , a second or lower end  120   b  opposite end  120   a , and a generally cylindrical radially outer surface  120   c  extending axially between ends  120   a ,  120   b . As will be described in more detail below, axis  125  of shaft  120  is not coaxially aligned with axis  115  of housing  110 . An annular space  116  is formed between drive shaft housing  110  and driveshaft  120 . During drilling operations, drilling fluid is pumped down drill string  21  and through downhole motor  30  to drill bit  90 . Within driveshaft assembly  100 , drilling fluid flows through annular space  116  from upper end  110   a  to lower end  110   b  in route to bearing assembly  80  and drill bit  90 . 
     Upper end housing  130  has a first or upper end  130   a , a second or lower end  130   b  opposite end  130   a , a connector section  132  extending from upper end  130   a , and a socket section  134  extending from connector section  132  to lower end  130   b . In this embodiment, connector section  132  is a male pin or ping end connector that threadably connects upper end housing  130  to the output shaft of hydraulic drive section  40 . Socket section  134  receives upper end  120   a  of drive shaft  120 . As will be described in more detail below, the coupling between upper end  120   a  and socket section  134  allows driveshaft  120  to pivot about end  120   a  relative to end housing  130  while simultaneously transferring rotational torque and axial thrust loads between end housing  130  and driveshaft  120 . 
     Lower end housing  140  has a first or upper end  140   a , a second or lower end  140   b , a connector section  142  extending from upper end  140   a , and a socket section  144  extending from connector section  142  to the lower end  140   b . In this embodiment, connector section  142  is a female box or box-end connector that threadably connects lower end housing  140  to the mandrel of bearing assembly  80 . Socket section  144  receives lower end  120   b  of driveshaft  120 . As will be described in more detail below, the coupling between lower end  120   b  and socket section  144  allows driveshaft  120  to pivot about end  120   b  relative to end housing  140  while simultaneously transferring rotational torque and axial thrust loads between end housing  140  and driveshaft  120 . 
     In this embodiment, ends  120   a ,  120   b  of driveshaft  120  are structurally identical, and socket sections  134 ,  144  are structurally identical. Therefore, in the description to follow and associated Figures, the details of embodiments of the lower end  120   b , corresponding socket section  144 , and the coupling or connection therebetween are shown and described, it being understood that embodiments of upper end  120   a , corresponding socket section  134 , and the connection therebetween, respectively, may be the same. 
     Referring now to  FIG. 3 , an embodiment of lower end  120   b  of driveshaft  120  and socket section  144  of lower end housing  140  are shown. Socket section  144  has a central or longitudinal axis  145  and includes a receptacle  146  that extends axially from end  140   a  and receives lower end  120   b  of driveshaft  120 . It should be noted that, while axes  125 ,  145  are shown generally aligned in  FIG. 3 , the axis  125  of driveshaft  120  is typically misaligned with axis  145  of socket section  144  due to the pivoting of driveshaft  120  about end  120   b  during operations. 
     Referring briefly to  FIGS. 3, 13, and 14 , receptacle  146  is defined by a radially inner surface  146   c . Moving axially from upper end  140   a , inner surface  146   c  includes an upper generally cylindrical surface  308  extending axially from upper end  140   a , a plurality of circumferentially spaced shoulders  306  extending radially inward from surface  308  (e.g., in this embodiment, there are a total of four shoulders  306 ), a plurality of circumferentially spaced pockets  302  extending axially from shoulders  306 , a generally planar surface  304  extending radially from pockets  302 , and a cylindrical counterbore or recess  320  extending axially from surface  304 . Shoulders  306  and surfaces  304  are planar surfaces disposed in planes oriented perpendicular to axis  145 . In addition, in this embodiment, receptacle  146  includes a total of four pockets  302  spaced uniformly circumferentially about axis  145 , such that each pocket  302  is disposed approximately 90° from each circumferentially adjacent pocket  302 . 
     Referring back now to  FIG. 3 , in this embodiment, a bearing insert  180  is disposed within recess  320 . Insert  180  includes a body  181  coaxially aligned with the axis  145  and having a first or upper end  181   a , and a second or lower end  181   b  opposite the upper end  181   a . In this embodiment, the upper end  181   a  includes a generally upward facing concave spherical bearing surface  182 , and lower end  181   b  comprises a generally planar surface  186  oriented perpendicular to axis  145 . As shown in  FIG. 3 , planar surface  186  of insert  180  is seated within recess  320  such that bearing surface  182  faces axially upward. As is shown in  FIG. 3  lower end  120   b  of shaft  120  includes a cavity  121  extending axially inward from lower end  120   b  and having a concave spherical ball seat or surface  123  that receives a thrust sphere or ball  122 . When lower end  120   b  is mounted within receptacle  146 , upper end  181   a  of body  181  extends into cavity  121  such that concave spherical bearing surface  182  mates with and slidingly engages ball  122 . 
     Referring still to  FIG. 3 , a mounting collar  148  is disposed within the receptacle  146  proximate upper end  140   a . In general mounting collar includes a radially outer surface  148   a , and a radially inner surface  148   b . Collar  148  is threaded into receptacle  146 , via engagement of mating external threads on outer surface  148   a  and internal threads on surface  308 . An annular seal assembly  150  is radially positioned between surfaces  148   a ,  308  to prevent fluid flow therebetween. 
     A flexible closure boot  164  is provided to prevent drilling mud from flowing into receptacle  146  during drilling operations. Closure boot  164  is disposed about driveshaft  120  proximate lower end  120   b  and has a first or upper end  164   a  coupled to driveshaft  120  with a lock ring  160  and a second or lower end  164   b  coupled to end housing  140  with collar  148  and an L-shaped compression ring  166 . Thus, closure boot  164  extends radially between driveshaft  120  and end housing  140 . More specifically, upper end  164   a  of boot  164  is seated in an annular recess on outer surface  120   c  of driveshaft  120 , and a lock ring  160  is disposed on shaft  120  over end  164   a , thereby holding end  164   a  in position between ring  160  and shaft  120  via an interference fit. A snap ring  162  is disposed in a circumferential groove  163  in outer surface  120   c  and axially retains ring  160  on shaft  120 . Lower end  164   b  of boot  164  is similarly held in position through an interference fit. In particular, lower end  164   b  is seated on radially inner surface  148   b  and compressed between collar  148  and compression ring  166  disposed in receptacle  146 . 
     Referring now to  FIGS. 3-5 , lower end  120   b  of driveshaft  120  is shown. In addition to cavity  121  previously described, lower end  120   b  includes a plurality of recesses  124  extending both radially inward from outer surface  120   c  and axially from lower end  120   b . Each recess  124  is at least partially defined by a first planar surface  126 , and a second planar surface  128 . In this embodiment, the surfaces  126 ,  128  of each recess  124  are each perpendicular to one another such that each recess  124  is substantially V-shaped when viewed in cross-section along axis  125  (e.g., as shown in  FIG. 5 ). As will be described in more detail below, during drilling operations, torque is transferred from driveshaft  120  through the surface  126  of each recess  124 , and thus, first planar surface  126  of each recess  124  may be referred to herein as an engagement or torque transfer surface  126 . 
     In this embodiment, lower end  120   b  includes a total of four recesses  124  uniformly circumferentially disposed about axis  125  such that each recess  124  is disposed approximately 90° from each circumferentially adjacent recess  124 . As a result, the planar surfaces  126 ,  128  of each recess  124  are generally parallel to the planar surface  126 ,  128 , respectively, of each radially opposing recess  124  (i.e., the recess  124  disposed 180° from the recess  124  in question) with respect to axis  125 . Moreover, in this embodiment, each of the surfaces  126 ,  128  are parallel to the central axis  125  of driveshaft  120 ; however, in other embodiments, surfaces  126 ,  128  are not parallel to axis  125  and are instead disposed at some non-zero angle thereto. 
     As will be described in more detail below, during rotation of shaft  120  about axis  125 , shaft  120  is free to pivot at lower end  120   b  about a first pivot axis  127  and a second pivot axis  129 . Axes  127 ,  129  are oriented orthogonal to each other and intersect at a center point  119  disposed along axis  125 . Thus, axes  125 ,  127 ,  129  all intersect at center  119 . In addition, axes  127 ,  129  lie in a plane oriented perpendicular or orthogonal to axis  125 . Further, in this embodiment center  119  also corresponds to the center of curvature of concave spherical surface  123  in cavity and the center of thrust ball  122  when ball  122  is installed within cavity  121  as previously described (e.g., see  FIG. 3 ). Thus, sliding engagement between thrust ball  122  and surface  123  of cavity  121  and sliding engagement between ball  122  and surface  182  of bearing insert  180  allows driveshaft  120  to pivot about center  119  during operations. 
     Referring now to  FIGS. 3 and 6 , a torque transfer assembly  185  is disposed about lower end  120   b  of driveshaft  120  within receptacle  146  and transmits torque loads between driveshaft  120  and end housing  140  as driveshaft  120  rotates about axis  125 . In this embodiment, torque transfer assembly  185  generally includes a plurality of torque transfer keys  190  and an adapter  200 . As will be described in more detail below, sliding engagement of the various surfaces of torque transfer assembly  185  (i.e., mating surfaces of keys  190  and adapter  200 ) allow driveshaft  120  to transfer torque to end housing  140  through direct, face-to-face engagement even as driveshaft  120  pivots about axes  127 ,  129  relative to end housing  140  as previously described. 
     Referring now to  FIGS. 7 and 8 , each of the torque transfer keys  190  is generally D-shaped and is disposed on adapter  200 . As is best shown in  FIG. 7 , each key  190  comprises a body  192  having a central axis  195 , a first or top side  192   a , a second or bottom side  192   b  axially opposite the top side  192   a , a first lateral side  192   c , and a second lateral side  192   d  radially opposite the first lateral side  192   c . In this embodiment, the axis  195  passes through the center of mass of key  190  and is parallel to one of the axes  127 ,  129  when driveshaft assembly  100  is fully made up. In addition, in this embodiment, sides  192   a ,  192   b  comprise parallel planar surfaces  193 ,  199 , respectively, oriented perpendicular to axis  195 ; side  192   c  comprises a planar torque transfer surface  194  extending axially between sides  192   a ,  192   b ; and side  192   d  comprises a convex cylindrical surface  196  extending axially between sides  192   a ,  192   b . Surface  196  is concentric about an axis of curvature  197  that is oriented parallel to axis  195  and surface  194 , and radially spaced from axis  195  and surface  194 . Axes  195 ,  197  lie in a plane oriented perpendicular to surface  194 . Further, in this embodiment surfaces  194 ,  196  each intersect chamfered surfaces  198   a ,  198   b , such that surface  194  has a length L 194  extending between surfaces  198   a ,  198   b ; however, it should be appreciated that other embodiments of keys  190  may not include chamfered surfaces  198   a ,  198   b . Still further, surface  196  has a radius R 196  measured radially from axis of curvature  197  to surface  196 . Moreover, as will be described in more detail below, in this embodiment, axis  197  of each key  190  is aligned with one of the pivot axes  127 ,  129  when assembly  100  is fully made up. 
     Referring now to  FIGS. 9-11 , adapter  200  includes a central or longitudinal axis  205  that is aligned with axis  145  of end housing  140  when adapter  200  is installed within receptacle  146  (e.g., as shown in  FIG. 3 ), a plurality of engagement arms  202 , and a central connecting member  204 . Connecting member  204  is formed of a single plate that includes a first or upper side  204   a  and a second or lower side  204   b  opposite upper side  204   a , where sides  204   a ,  204   b  are parallel to one another and are each perpendicular to axis  205 . In addition, member  204  is shaped to correspond with receptacle  146 . Namely, in this embodiment, connecting member  204  includes a plurality of radial extensions  203  that generally correspond in shape and arrangement with pockets  302  of receptacle  146  previously described such that each extension  203  fits within one of the pockets  302  of receptacle  146  when adapter  200  is fully installed therein. Thus, in this embodiment adapter  200  includes a total of four extensions  203  that are uniformly circumferentially spaced about axis  205  such that each extension  203  is disposed approximately 90° from each circumferentially adjacent extension  203 . In addition, one arm  202  extends axially outward from upper side  204   a  on each of the extensions  203 , such that in this embodiment, there are a total of four arms  202  uniformly circumferentially spaced about axis  205  and each arm  202  is disposed approximately 90° from each circumferentially adjacent arm  202 . In addition, a hole or aperture  206  extends axially through member  204 , between sides  204   a ,  204   b  and is coaxially aligned with axis  205 . As will be described in more detail below, hole  206  is sized to allow passage of upper end  181   a  of body  181  of bearing insert  180  therethrough during operations. 
     As is best shown in  FIG. 11 , each arm  202  includes central axis  215  that is parallel to and radially spaced from axis  205  of adapter  200 , a first end  202   a , and a second end  202   b  opposite first end  202   a . In addition, each arm  202  includes a first driveshaft facing side  202   c , a first end housing facing side  202   d  radially opposite first driveshaft facing side  202   c  with respect to axis  215 , a second driveshaft facing side  202   e  extending between sides  202   c ,  202   d , and a second end housing facing side  202   f  also extending between sides  202   c ,  202   d  and radially opposite second driveshaft facing side  202   e  with respect to axis  215 . Each side  202   c ,  202   d ,  202   e ,  202   f  extends axially between ends  202   a ,  202   b . First end housing facing side  202   d  includes an axially extending planar engagement surface  209 , while a cylindrical recess  208  extends radially inward from side  202   c  with respect to axis  215 . Recess  208  is defined by a concave cylindrical surface  212  having an axis of curvature  213  and a planar floor surface  210  extending between side  202   c  and concave cylindrical surface  212 . 
     Referring now to  FIGS. 6 and 12 , during make up of torque transfer assembly  185 , each of the keys  190  are disposed within one of the recesses  208  on arms  202  of adapter  200 . In particular, each key  190  is disposed within one of the recesses  208  such that one of the parallel planar surfaces  193 ,  199  slidingly engages floor surface  210 , and convex cylindrical surface  196  slidingly engages concave cylindrical surface  212 . In addition, as is best shown in  FIG. 12 , when keys  190  are disposed within recesses  208  as described above, the axis of curvature  197  of each surface  196  on each key  190  aligns with and is therefore coincident with the axis of curvature  213  of the respective, corresponding surface  212  in recess  208 . Thus, during operations, each key  190  is allowed to pivot or rotate about the aligned axes  197 ,  213  through sliding engagement of the surfaces  196 ,  212  and sliding engagement of one of the surfaces  193 ,  199  and the surface  210 . In addition, as will be described in more detail below, in this embodiment when assembly  185  is fully installed on lower end  120   b  of driveshaft  120  and lower end  120   b  and assembly  185  are both fully inserted within receptacle  146  of end housing  140  (e.g.,  FIG. 3 ), the aligned axes  197 ,  213  of each of the arm  202  and key  190  pairs are further aligned with one of the pivot axes  127 ,  129 , previously described, such that during operation, each of the keys  190  pivot or rotate about one of the pivot axes  127 ,  129  to further facilitate pivoting of driveshaft  120  about axes  127 ,  129  relative to end housing  140 . 
     Referring again to  FIGS. 13 and 14 , in this embodiment each pocket  302  of receptacle  146  is defined by a first planar surface  310 , a second planar surface  312  parallel to the first planar surface  310 , a third planar surface  314  extending perpendicularly or orthogonal from the first planar surface  310 , and a fourth planar surface  316  extending between surfaces  312 ,  314 . In this embodiment, each of the surfaces  310 ,  312 ,  314 ,  316  extend axially or parallel to axis  145  of end housing  140 ; however, such an arrangement is not required such that in other embodiments, surfaces  310 ,  312 ,  314 ,  316  are disposed at some non-zero angle to axis  145 . As will be described in more detail below, each of the first planar surfaces  310  of pockets  302  engage with mating surfaces in torque transfer assembly  185  (e.g., planar surfaces  209  on arms  202 ) in order to transfer torque between shaft  120  and end housing  140  during rotation of driveshaft  120  about axis  125 . Thus, surfaces  310  may be referred to herein as either engagement or torque transfer surfaces. 
     As is also best shown in  FIG. 14 , pockets  302  are arranged within receptacle  146  such that the first planar engagement surface  310  of each pocket  302  extends to the second planar surface  312  of the immediately circumferentially adjacent pocket  302 . In addition, the first planar engagement surfaces  310  of radially opposing pockets  302  (i.e., pockets  302  that are circumferentially disposed 180° from one another about axis  145 ) are generally parallel to one another. Such a parallel relationship of surfaces  310  ensures that torque transfer between driveshaft  120  and end housing  140  is more evenly distributed. 
     Referring now to  FIGS. 3, 6, and 15 , the assembly of lower end  120   b  of driveshaft  120 , torque transfer assembly  185 , and end housing  140  will now be described. First, as is best shown in  FIG. 6 , torque transfer assembly  185  is made up as previously described above and installed on lower end  120   b  of driveshaft  120  such that each mating pair of keys  190  and arms  202  is disposed within one of the recesses  124  on lower end  120   b . In particular, keys  190  and adapter  200  are installed on lower end  120   b  of driveshaft  120  such that planar surfaces  194  engage with planar surfaces  126  and second driveshaft facing sides  202   e  of each arms  202  oppose planar surfaces  128  within recesses  124 . In addition, when adapter  200  is fully installed on lower end  120   b , hole  206  is generally aligned with cavity  121 . Further, as is shown in  FIG. 6 , when torque transfer assembly  185  is installed on lower end  120   b  in this embodiment, the aligned axes  197 ,  213  of surfaces  196 ,  212 , respectively are further aligned with one of the pivot axes  127 ,  129 . As will be described in more detail below, such alignment with axes  197 ,  213 ,  127 ,  129  allows keys  190  to pivot about one of the axes  127 ,  129  to further facilitate pivoting of driveshaft  120  about axes  127 ,  129  during drilling operations. In this embodiment, either prior or subsequent to installation of torque transfer assembly  185  on lower end  120   b , thrust ball  122  is installed within cavity  121  and is seated on concave spherical bearing surface  123  (e.g., see  FIG. 3 ). 
     As is best shown in  FIGS. 3 and 15 , lower end  120   b  of driveshaft  120 , with torque transfer assembly  185  installed thereon, is then inserted within receptacle  146  on end housing  140  such that lower side  204   b  abuts or engages with planar surface  304 , and upper end  181   a  of body  181  of bearing insert  180  extends through hole  206  and cavity  121  such that concave spherical bearing surface  182  on upper end  181   a  engages thrust ball  122 . Therefore, thrust ball  122  is disposed between and engaged with concave spherical bearing surfaces  123 ,  182  as shown in  FIG. 3 . In addition, as lower end  120   b  of driveshaft  120  and torque transfer assembly  185  are installed within receptacle  146 , each of the surfaces  209  of first end housing facing sides  202   d  on arms  202  engages with one of the engagement surfaces  310  of pockets  302  as shown in  FIG. 15 . 
     Referring still to  FIGS. 3 and 15 , once driveshaft assembly  100  is fully made up, driveshaft  120  is free to pivot relative to lower end housing  140  about center  119 , while rotating about axis  125  in direction  113 . In particular, as shaft  120  rotates about axis  125  in direction  113 , end  120   b  of shaft  120  can pivot about one or both of the axes  127 ,  129  through sliding engagement of thrust ball  122  on surface  123  within cavity  121  and concave spherical bearing surface  182  of insert  180 . Additionally, pivoting of end  120   b  of driveshaft  120  about axes  127 ,  129  is further accommodated by sliding engagement of cylindrical surface  196  of each key  190  and cylindrical surface  212  within recesses  208  on arms  202  of adapter  200  as well as sliding engagement of surfaces  194  on each key  190  and planar surfaces  126  on lower end  120   b  of driveshaft  120 . 
     Moreover, during rotation of shaft  120  about axis  125  in direction  113 , torque is transferred between lower end  120   b  and end housing  140  through torque transfer assembly  185 . In particular, torque is first transferred between lower end  120   b  and keys  190  through engagement of surfaces  126 ,  194 . Thereafter, torque is transferred between keys  190  and adapter  200  through engagement of surfaces  196 ,  212 . Finally, torque is transferred between adapter  200  and end housing  140  through engagement of surfaces  209 ,  310 . Because keys  190  are allowed to pivot about one of the axes  127 ,  129  within recesses  208  on arms  202  of adapter  200  in this embodiment as previously described, keys  190  are able to maintain face-to-face contact between surfaces  194 ,  126  as driveshaft  120  pivots about axes  127 ,  129  simultaneous with rotation about axis  125  in direction  113 . In this embodiment, the coupling between upper end housing  130  and upper end  120   a  of driveshaft  120  is structurally and functionally the same as the coupling between lower end housing  140  and lower end  120   b  of driveshaft described above; however, it should be appreciated that such structural symmetry is not required. In addition, while a specific order of assembly has been described above for lower end  120   b  of driveshaft  120 , it should be appreciated that the specific assembly order may be greatly varied. For example, in some embodiments, the torque transfer assembly  185  may initially be installed within receptacle  146 . Thereafter, in this example, lower end  120   b  is inserted within receptacle  146  and engaged with assembly  185  in the manner previously described, thereby resulting in the arrangement shown in  FIG. 15 . 
     In the manner described, through direct engagement of such mating surfaces (e.g., mating surfaces on keys  190 , adapter  200 , driveshaft  120 , and receptacle  146 ), driveshaft assembly  100  enables the transfer of torque through direct, face-to-face surface contact as opposed to point or line contact. Moreover, for driveshaft assembly  100 , face-to-face surface contact is maintained between mating surfaces (e.g., mating surfaces on driveshaft  120 , torque transfer assembly  185 , and end housing  140 ), even as the driveshaft pivots about orthogonal pivot axes (e.g., pivot axes  127 ,  129 ). Torque transfer through such direct, face-to-face contact of surfaces offers the potential to greatly reduce the rate of wear between the interacting surface and thereby increase the running life of the driveshaft assembly (e.g., assembly  100 ) and other related components. 
     While driveshaft assembly  100  has been described herein to include a driveshaft  120  with structurally identical ends  120   a ,  120   b  as well as structurally identical socket sections  134 ,  144 , it should be appreciated that other embodiments may not include such structural symmetry. Further, while pockets  302  within receptacle  146  have been described as being defined by surfaces  310 ,  312 ,  314 ,  316 , it should be appreciated that the exact size, shape, number, and arrangement of pockets  302  within receptacle  146  may be greatly varied. Thus, embodiments of pockets  302  may assume any suitable shape that presents one or more engagement surfaces for engagement with mating surfaces on torque transfer assembly  185 . Moreover, the specific shape and arrangement shown for pockets  302  in the Figures is merely illustrative of one potential option for the design of pockets  302 , and there is no intent to limit other potential embodiments of pockets  302  to the specific shape shown therein. Similarly, it should also be appreciated that the specific number, shape, arrangement, and surfaces defining recesses  124  on driveshaft  120  may be greatly varied in the same manner, and may assume any suitable shape, arrangement, number, etc., that presents one or more engagement surfaces for engagement with mating surfaces on torque transfer assembly  185 . Still further, while embodiments of driveshaft  120  disclosed herein have included a spherical thrust ball  122 , it should be appreciated that in other embodiments, driveshaft  120  may not include cavity  121  and/or thrust ball  122 . For example, in some embodiments, driveshaft  120  includes a convex spherical bearing surface on end  120   a  and/or end  120   b  in place of thrust ball  122  and/or cavity  121   
     Referring now to  FIG. 16 , another embodiment of driveshaft assembly  400  is shown. Driveshaft assembly  400  is substantially the same as driveshaft assembly  100  previously described, and thus, like numerals are used to indicate like components and the description below will generally focus on the differences between assemblies  100 ,  400 . Specifically, driveshaft assembly  400  includes the outer driveshaft housing  110 , a driveshaft  420  rotatably disposed within housing  110 , a first or upper end housing  430  coupled to driveshaft  420 , and a second or lower end housing  440  also coupled to driveshaft  420 . 
     Referring still to  FIG. 16 , driveshaft  420  has a central or longitudinal axis  425 , a first or upper end  420   a , a second or lower end  420   b  opposite end  420   a , and a generally cylindrical radially outer surface  420   c  extending axially between ends  420   a ,  420   b . As will be described in more detail below, axis  425  of shaft  420  is not coaxially aligned with axis  115  of housing  110 . 
     Upper end housing  430  has a first or upper end  430   a , a second or lower end  430   b  opposite end  430   a , a connector section  432  extending from upper end  430   a , and a socket section  434  extending from connector section  432  to lower end  430   b . In this embodiment, connector section  432  is a male pin or ping end connector that threadably connects upper end housing  430  to the output shaft of hydraulic drive section  40  (see  FIG. 1 ). Socket section  434  receives upper end  420   a  of drive shaft  420 . As will be described in more detail below, the coupling between upper end  420   a  and socket section  434  allows driveshaft  420  to pivot about end  420   a  relative to end housing  430  while simultaneously transferring rotational torque and axial thrust loads between end housing  430  and driveshaft  420 . 
     Lower end housing  440  has a first or upper end  440   a , a second or lower end  440   b , a connector section  442  extending from upper end  440   a , and a socket section  444  extending from connector section  442  to the lower end  440   b . In this embodiment, connector section  442  is a female box or box-end connector that threadably connects lower end housing  440  to the mandrel of bearing assembly  80  (see  FIG. 1 ). Socket section  444  receives lower end  420   b  of driveshaft  420 . As will be described in more detail below, the coupling between lower end  420   b  and socket section  444  allows driveshaft  420  to pivot about end  420   b  relative to end housing  440  while simultaneously transferring rotational torque and axial thrust loads between end housing  440  and driveshaft  420 . 
     In this embodiment, ends  420   a ,  420   b  of driveshaft  420  are structurally identical, and socket sections  434 ,  444  are structurally identical. Therefore, in the description to follow and associated Figures, the details of lower end  420   b , corresponding socket section  444 , and the connection therebetween are shown and described, it being understood that upper end  420   a , corresponding socket section  434 , and the connection therebetween, respectively, are the same. 
     Referring now to  FIG. 17 , lower end  420   b  of driveshaft  420  and socket section  444  of lower end housing  440  are shown. Socket section  444  has a central or longitudinal axis  445  and includes a receptacle  446  that extends axially from end  440   a  and receives lower end  420   b  of driveshaft  420 . While axis  425  of driveshaft  420  and axis  445  are shown generally aligned in  FIG. 17 , it should be noted that the axis  425  of driveshaft  420  is typically misaligned with axis  445  of socket section  444  due to the pivoting of driveshaft  420  about end  420   b  during operations. 
     Referring briefly to  FIGS. 17, 23, and 24 , receptacle  446  is defined by a radially inner surface  446   c . Moving axially from upper end  440   a , inner surface  446   c  includes an upper generally cylindrical surface  608  extending axially from upper end  440   a , a plurality of circumferentially spaced shoulders  606  extending radially inward from surface  608  (e.g., in this embodiment, there are a total of four shoulders  606 ), a plurality of circumferentially spaced pockets  602  extending axially from shoulders  606 , a generally planar surface  604  extending radially from pockets  602 , and a cylindrical counterbore or recess  620  extending axially from surface  604 . Shoulders  606  and surfaces  604  are planar surfaces disposed in planes oriented perpendicular to axis  445 . In addition, in this embodiment, receptacle  446  includes a total of four pockets  602  spaced uniformly circumferentially about axis  445 , such that each pocket  602  is disposed approximately 90° from each circumferentially adjacent pocket  602 . 
     Referring back now to  FIG. 17 , in this embodiment, bearing insert  180 , being the same as previously described above, is disposed within recess  620 . Insert  180  interacts with recess  620  in substantially the same manner as described above for insert  180  and recess  320  (see  FIG. 3 ), and thus, a detailed description of the structure of insert  180  and its interaction with recess  620  is omitted in the interests of brevity. In addition, as is also shown in  FIG. 17 , like lower end  120   b , previously described, lower end  420   b  of shaft  420  includes cavity  121  that receives the thrust ball  122  in the same manner as previously described above. As a result, when lower end  420   b  is mounted within receptacle  446 , upper end  181   a  of body  181  extends into cavity  121  such that concave spherical bearing surface  182  mates with and slidingly engages ball  122 . Further, as shown in  FIG. 17 , mounting collar  148 , closure boot  164 , rings  160 ,  163 ,  166  are couple to lower end  420   b  and/or socket section  444  in substantially the same manner as described above for lower end  120   b  and socket section  144 , respectively. Therefore, a detailed description of these components is also omitted in the interest of brevity. 
     Referring now to  FIGS. 17-19 , lower end  420   b  of driveshaft  420  is shown. In addition to cavity  121  previously described, lower end  420   b  includes a plurality of recesses  424  extending radially inward from outer surface  420   c  and extending axially from lower end  420   b . Each recess  424  is at least partially defined by a convex cylindrical surface  426 , and a second planar surface  428 . As will be described in more detail below, during drilling operations, torque is transferred from driveshaft  420  through the surface  426  of each recess  424 , and thus, convex cylindrical surface  426  of each recess  424  may be referred to herein as either an engagement or torque transfer surface  426 . In addition, lower end  420   b  of driveshaft  420  includes a first pivot axis  427  and a second pivot axis  429 . Axes  427 ,  429  are referred to herein as “pivot” axes because, as described in more detail below, shaft  420  is free to pivot at lower end  420   b  about one or both of axes  427 ,  429  during rotation thereof about central axis  425 . Axes  427 ,  429  are oriented orthogonal to each other and intersect at a center point  419  disposed along axis  425 . Thus, axes  425 ,  427 ,  429  all intersect at center  419 . In addition, axes  427 ,  429  lie in a plane oriented perpendicular or orthogonal to axis  425 . Further, in this embodiment center  419  also corresponds to the center of curvature of concave spherical surface  123  in cavity  121  and the center of thrust ball  122  when ball  122  is installed within cavity  121  as previously described (e.g., see  FIG. 17 ). Thus, sliding engagement between thrust ball  122  and surface  123  of cavity  121  and sliding engagement between ball  122  and surface  182  of bearing insert  180  allows driveshaft  420  to pivot about center  419  during operations. 
     In this embodiment, lower end  420   b  includes a total of four recesses  424  circumferentially spaced uniformly about axis  425 , such that each recess  424  is disposed approximately 90° from each circumferentially adjacent recess  424 . As a result, for each recess  424 , the surface  426  is concentrically disposed about one of the pivot axes  427 ,  429  and the surface  428  is perpendicular to one of the pivot axes  427 ,  429 . Thus, each recess  424  is substantially V-shaped when viewed in cross-section along axis  425  (e.g., as shown in  FIG. 19 ). In addition, surfaces  426  of each pair of radially opposed recesses  424  with respect to axis  425  (i.e., recesses that are disposed 180° from one another about axis  425 ) are each concentrically disposed about the same axis  427  or  429 . Moreover, in this embodiment, each of the surfaces  428  are parallel to the central axis  425  of driveshaft  420 ; however in other embodiments, surfaces  428  are not parallel to axis  425  and are instead disposed at some non-zero angle thereto. 
     Referring now to  FIGS. 17 and 20 , a plurality of torque transfer keys  490  is disposed about lower end  420   b  of driveshaft  420  within receptacle  446  to transmit torque loads between driveshaft  420  and end housing  440  as driveshaft  420  rotates about axis  425 . As will be described in more detail below, sliding engagement of corresponding mating surfaces of torque transfer keys  490 , lower end  420   b , and receptacle  446  allow driveshaft  420  to transfer torque to end housing  440  through direct, face-to-face engagement even as driveshaft  420  pivots about axes  427 ,  429  relative to end housing  440  as previously described. 
     Referring now to  FIGS. 21-23 , each of the torque transfer keys  490  is generally C-shaped and comprises a body  492  having a central axis  495 , a first or top side  492   a , a second or bottom side  492   b  axially opposite the top side  492   a , a first lateral side  492   c , and a second lateral side  492   d  radially opposite the first lateral side  492   c . In this embodiment, the axis  495  passes through the center of mass of key  490  and is parallel to one of the axes  427 ,  429  when driveshaft assembly  400  is fully made up. In addition, in this embodiment, side  492   a  comprises a planar surface  493  that is oriented perpendicular to axis  495  and side  492   b  comprises a convex or outwardly curved surface  499 . In addition, in this embodiment, side  492   c  comprises a planar torque transfer surface  494  extending axially between sides  492   a ,  492   b , and side  492   d  comprises a concave cylindrical torque transfer surface  496  extending axially between sides  492   a ,  492   b . A pair of parallel planar surfaces  491 ,  498  extend between each of the planar surface  494  and the concave cylindrical surface  496  and also extend axially between planar sides  492   a ,  492   b . Surface  496  is concentric about an axis of curvature  497  that is oriented parallel to axis  495  and surface  494 , and radially spaced from axis  495  and surface  494 . Axes  495 ,  497  lie in a plane oriented perpendicular to surface  494 . Further, as will be described in more detail below, in this embodiment, axis  497  of each key  490  is aligned with one of the pivot axes  427 ,  429  when key  490  is installed on lower end  420   b  of driveshaft  420 . Still further, in this embodiment, the transitions between the surfaces  493 ,  499  and each of the surfaces  491 ,  494 ,  496 ,  498  are chamfered in order to allow for proper clearances when assembly  400  is fully made up. Also, in this embodiment, planar surface  498  is larger than planar surface  491  such that keys  490  will substantially conform to the shape of recess  424  during operations; however, it should be appreciated that such an arrangement is not required and in other embodiments surface  491 ,  498  may be the same size or surface  491  may be larger than surface  498 . 
     Referring again to  FIGS. 24 and 25 , in this embodiment each pocket  602  of receptacle  446  is defined by a first planar surface  610 , a second planar surface  612  parallel to the first planar surface  610 , a third planar surface  614  extending perpendicularly relative to both the surfaces  610 ,  612 , a fourth planar surface  616  extending between surfaces  612 ,  614 , and a fifth planar surface  617  extending between surface  610 ,  614 . The transitions between each of the surfaces  610 ,  612 ,  614 ,  616  are radiused in order to increase the manufacturing efficiency of housing  440  as well as to ensure proper clearance of interlocking components during operations. Moreover, in this embodiment, each of the surfaces  610 ,  612 ,  614 ,  616  extend axially or parallel to axis  445  of end housing  440 . As will be described in more detail below, each of the first planar surfaces  610  of pockets  602  engage with mating surfaces on torque transfer keys  490  in order to transfer torque between shaft  420  and end housing  440  during rotation of driveshaft  420  about axis  425 . Thus, surfaces  610  may be referred to herein as either engagement or torque transfer surfaces  310 . 
     As is also best shown in  FIG. 25 , pockets  602  are arranged within receptacle  446  such that the first planar engagement surface  610  of each pocket  602  extends to the second planar surface  612  of the immediately circumferentially adjacent pocket  602 . In addition, the first planar engagement surfaces  610  of radially opposing pockets  602  (i.e., pockets  602  that are circumferentially disposed 180° from one another about axis  445 ) are generally parallel to one another. Such a parallel relationship of surfaces  610  ensures that torque transfer between driveshaft  420  and end housing  440  is more evenly distributed. 
     Referring now to  FIGS. 18, 20, and 21-23 , during make up of driveshaft assembly  400 , each of the keys  490  is disposed within one of the recesses  424  on lower end  420   b . In particular, each key  490  is disposed within one of the recesses  424  such that planar surface  493  slidingly engages planar surface  428 , and concave cylindrical surface  496  slidingly engages convex cylindrical surface  426 . In addition, as is best shown in  FIG. 20 , when keys  490  are disposed within recesses  424  as described above, the axis of curvature  497  of each surface  496  on each key  490  aligns with and is therefore coincident with one of the pivot axes  427 ,  429  of driveshaft  420 . Thus, during operations, each key  490  is allowed to pivot or rotate about one of the pivot axes  427 ,  429  through sliding engagement of the surfaces  426 ,  496  and sliding engagement of surface  493  and surface  428 . This arrangement facilitates the pivoting of driveshaft  420  about axes  427 ,  429  relative to end housing  440 . In addition, in this embodiment, either prior or subsequent to installation of torque transfer keys  490  on lower end  420   b , thrust ball  122  is installed within cavity  121  and is seated on concave spherical bearing surface  123  (e.g., see  FIG. 17 ). 
     Referring now to  FIGS. 17 and 26 , lower end  420   b  of driveshaft  420 , with torque transfer keys  490  installed thereon in the manner described above, is then inserted within receptacle  446  on end housing  440  such that upper end  181   a  of body  181  of bearing insert  180  extends into cavity  121  and concave spherical bearing surface  182  on upper end  181   a  engages thrust ball  122 . In this arrangement, thrust ball  122  is disposed between and engaged with concave spherical bearing surfaces  123 ,  182  as shown in  FIG. 17 . In addition, as lower end  420   b  of driveshaft  420  and keys  490  are installed within receptacle  446 , surfaces  494  on keys  490  slidingly engage with the corresponding engagement surfaces  610  of pockets  602  as shown in  FIG. 26 . 
     Referring still to  FIGS. 17 and 26 , once driveshaft assembly  400  is fully made up, driveshaft  420  is free to pivot relative to lower end housing  440  about center  419 , while rotating about axis  425 . In particular, as shaft  420  rotates about axis  425  in direction  413 , lower end  420   b  can pivot about one or both of the axes  427 ,  429  through sliding engagement of thrust ball  122  on surface  123  within cavity  121  and concave spherical bearing surface  182  of insert  180 . Additionally, pivoting of end  420   b  of driveshaft  420  about axes  427 ,  429  is further accommodated by sliding engagement of cylindrical surface  496  of each key  490  and cylindrical surface  426  of each corresponding recess  424  on lower end  420   b  of driveshaft  420 , as well as sliding engagement of planar surface  493  of each key  490  and planar surface  428  of the corresponding recess  424 . It should be appreciated that in some embodiments, keys  490  move relative to pockets  602  during the rotation and pivoting of driveshaft  420  described above, and thus, the planar surface  494  of each key  490  also sliding engages the planar engagement surface  610  of each corresponding pocket  602  during these operations. 
     Moreover, during rotation of shaft  420  about axis  425  in direction  413 , torque is transferred between lower end  420   b  and end housing  440  through torque transfer keys  490 . In particular, torque is first transferred between lower end  420   b  and keys  490  through engagement of cylindrical surfaces  426 ,  496 . Thereafter, torque is transferred between keys  490  and end housing  440  through engagement of surfaces  494 ,  610 . Because keys  490  are configured to pivot about one of the axes  427 ,  429  relative to recesses  424  on lower end  420   b  of driveshaft  420  in the previously described embodiment, keys  490  are able to maintain face-to-face contact between surfaces  496 ,  426  and surfaces  494 ,  610  as driveshaft  420  pivots about axes  427 ,  429  simultaneous with rotation about axis  425  in direction  413 . In this embodiment, the coupling between upper end housing  430  and upper end  420   a  of driveshaft  420  is structurally and functionally the same as the coupling between lower end housing  440  and lower end  420   b  of driveshaft described above; however, it should be appreciated that such structural symmetry is not required. 
     In the manner described, through direct engagement of such mating surfaces (e.g., mating surfaces on keys  490 , driveshaft  420 , and receptacle  446 ), driveshaft assembly  400  enables the transfer of torque through direct, face-to-face surface contact as opposed to point or line contact. Moreover, for driveshaft assembly  400 , face-to-face surface contact is maintained between mating surfaces (e.g., mating surfaces on driveshaft  420 , torque transfer keys  490 , and end housing  440 ), even as the driveshaft pivots about orthogonal pivot axes (e.g., pivot axes  427 ,  429 ). Torque transfer through such direct, face-to-face contact of surfaces offers the potential to greatly reduce the rate of wear between the interacting surface and thereby increase the running life of the driveshaft assembly  400  and other related components. 
     While driveshaft assembly  400  has been described herein to include a driveshaft  420  with structurally identical ends  420   a ,  420   b  as well as structurally identical socket sections  434 ,  444 , it should be appreciated that other embodiments may not include such structural symmetry. Further, while pockets  602  within receptacle  446  have been described as being defined by surfaces  610 ,  612 ,  614 ,  616 , it should be appreciated that the exact size, shape, number, and arrangement of pockets  602  within receptacle  446  may be greatly varied. Thus, embodiments of pockets  602  may assume any suitable shape that presents one or more engagement surfaces for engagement with mating surfaces on torque transfer keys  490 . Moreover, the specific shape and arrangement shown for pockets  602  in the Figures is merely illustrative of one potential option for the design of pockets  602 , and there is no intent to limit other potential embodiments of pockets  602  to the specific shape shown therein. Similarly, it should also be appreciated that the specific number, shape, arrangement, and surfaces defining recesses  424  on driveshaft  420  may be greatly varied in the same manner, and may assume any suitable shape, arrangement, number, etc., that presents one or more engagement surfaces for engagement with mating surfaces on torque transfer keys  490 . Still further, while embodiments of driveshaft  420  disclosed herein have included a spherical thrust ball  122 , it should be appreciated that in other embodiments, driveshaft  420  may not include cavity and/or thrust ball  122 . For example, in some embodiments, driveshaft  420  includes a convex spherical bearing surface on end  120   a  and/or end  120   b  in place of thrust ball  122  and/or cavity  121   
     While specific embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.