Patent Publication Number: US-2021189802-A1

Title: Dual rod directional drilling system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16,936,703 filed Jul. 23, 2020, which is a continuation of U.S. application Ser. No. 15/967,965 filed May 1, 2018, which claims the benefit of U.S. Provisional Patent Application Nos. 62/492,818, filed May 1, 2017; 62/530,610, filed Jul. 10, 2017; 62/530,616, filed Jul. 10, 2017; 62/530,642, filed Jul. 10, 2017; 62/566,971, filed Oct. 2, 2017; and 62/567,624 filed Oct. 3, 2017, which applications are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Dual drill rod drilling systems (“dual rod”) for use in directional drilling having an inner rod and an outer rod are known. A typical dual rod drilling system is generally configured to drive into the ground a series of drill rods joined end-to-end to form a drill string. At the end of the drill string is a rotating drilling tool or drill bit. A dual rod drilling system typically includes a first drive mechanism that controls rotation of a drill bit and a second drive mechanism that controls rotation of a steering element. When a straight hole is drilled with a dual rod drilling system, the first and second drive mechanisms are concurrently operated such that both the drill bit and the steering element are rotated as the drill string is thrust into the ground. When a directional change is needed, because the steering element is axially misaligned with the drill string, the drive mechanism that controls the steering element is stopped and the drill string is thrust further into the ground while the drive mechanism that controls the drill bit is rotated. This causes the drill bit to deviate from a straight path and follow the direction dictated by the steering element. 
     Dual rod drilling systems also use drilling fluid that is passed internally within the drill rods for cooling of the drill bit and also for transporting cuttings within the drill hole. Therefore, to ensure proper operation, it is important to reduce obstructions within the drilling fluid flow path. However, this can be difficult due to the unavoidable relative longitudinal offsets between inner and outer drill rods within the drill string. 
     Further, the inner and outer drill rods of each drill rod assembly can have variations in length resulting from manufacturing tolerances. Because of the length variations, drill rod assemblies are designed such that the overall length of interconnected inner drill rods are never longer than the overall length of interconnected outer drill rods. If the interconnected inner drill rods were longer than the outer drill rods, the inner rods would collide while the outer drill rods were being coupled together, causing damage to one or both of the inner and outer drill rods. Accordingly, by design, the length of interconnected inner drill rods is slightly less than the length of interconnected outer drill rods. However, this design requirement results in a situation where certain portions of the drill string, e.g., the inner drill rods, contact the outer drill rods and obstruct the fluid flow path. This results in being able to send less drilling fluid to the drill head and/or possible damage to portions of the drill string. Therefore, improvements in maintaining an open drilling fluid flow path are needed. 
     To drive the drill bit with the first drive mechanism, flexible and/or bent drive shafts have been used in order to allow steering and still facilitate torque transfer. Other designs have used a coupling (sometimes referred to as a “transmission”) so as to allow misalignment between a straight drill bit shaft and a straight drive shaft. However, such a coupling, or transmission, has traditionally included several components and required separate lubrication and isolation from the drilling fluid, thus complicating manufacture and maintenance. Therefore, improvements to the drill head of a dual rod drilling system are needed. 
     To drive the rotation of the drill string, a gearbox having a plurality of motors has traditionally been used. The gearbox can include a gear arrangement that transfers power from the plurality of motors to the inner and out drill rods of the dual rod drilling system. Drilling fluid has also been traditionally introduced at the gearbox to the drill string; however, isolating the drilling fluid from the internal components of the gearbox can be difficult. Further, should a malfunction occur and drilling fluid be introduced to the interior of the gearbox, due to the internal positioning of the gearbox components, it is difficult for an operator to realize this before the components of the gearbox are damaged. Therefore, improvements to the gearbox of a dual rod drilling system are needed. 
     SUMMARY 
     The present disclosure relates generally to a dual rod horizontal directional drilling system. In one possible configuration, and by non-limiting example, the horizontal directional drilling system includes a drill head that has a spherical hexagonal end having torque transmitting features and radial load bearing features. In another possible configuration, and by non-limiting example, the horizontal directional drilling system includes a drill string arrangement that includes at least one inner rod and at least one coupling that are together configured to provide an unobstructed fluid flow path within the drill string. In another possible configuration, and by non-limiting example, the horizontal directional drilling system includes a gearbox that includes a drilling fluid inlet at the rear of the gear box and a fluid weep indicator at the front of the gear box. 
     In one aspect of the present disclosure, a coupling system for a dual rod drilling system is disclosed. The coupling system includes a coupler that comprises an inner bore that has a non-circular profile and a longitudinal axis. The coupler also includes a cross aperture that has an axis that is perpendicular to the longitudinal axis, and the cross aperture has a first width. The coupling system includes an inner member that comprises a torque-carrying section that has a non-circular profile adapted to mate with the non-circular profile of the inner bore of the coupler. The inner member also includes a non-torque carrying portion that has a cross-sectional width that is smaller than a cross-sectional width of the inner bore of the coupler. The inner member also includes a groove positioned between the torque-carrying section and the non-torque carrying section. The groove has a width at least equal to the first width of the cross aperture of the coupler. The inner member also includes a pin positioned within the cross aperture of the coupler and within the groove of the inner member to secure the inner member at least partially within the coupler. 
     In another aspect of the present disclosure, a coupler for a drill rod is disclosed. The coupler includes a main body that has an inner bore. The inner bore has a non-circular profile and a longitudinal axis. The coupler includes a cross aperture disposed in the main body. The cross aperture has an axis that is nonintersecting with the longitudinal axis of the main body. The coupler also includes a sleeve that is positioned around an exterior surface of the main body. The sleeve has at least one drilling fluid flow passage. 
     In another aspect of the present disclosure, a drill rod is disclosed. The drill rod includes a torque-carrying section that has a non-circular profile. The torque-carrying section has a first cross-sectional width. The drill rod also includes a non-torque carrying portion that has a second cross-sectional width. The second cross-sectional width is less than the first cross-sectional width of the torque-carrying section. The drill rod also includes a groove positioned between the torque-carrying section and the non-torque carrying section. 
     In another aspect of the present disclosure, a drill rod assembly is disclosed. The drill rod assembly includes an outer drill rod assembly that has a shoulder and an inner drill rod assembly positioned at least partially inside the outer drill rod assembly. The inner drill rod assembly includes a sleeve. The sleeve is movable relative to the inner drill rod assembly upon receipt of a force exceeding a predetermined amount from the shoulder. The force is generally parallel with a longitudinal axis of the inner drill rod assembly. 
     In another aspect of the present disclosure, a drill rod assembly is disclosed. The drill rod assembly includes an outer drill rod assembly that includes a first shoulder at a first end and a second shoulder at a second end. The drill rod assembly includes an inner drill rod assembly that is positioned at least partially inside the outer drill rod assembly. The inner drill rod assembly includes a first and a second flow element, each positioned at first and second ends of the inner drill rod assembly, respectively. The first and second ends of the inner drill rod assembly correspond with the first and second ends of the outer drill rod assembly. The first and second flow elements each include at least one fluid flow passage, wherein fluid flow is permitted within an annular fluid flow passage defined between the inner and outer drill rod assemblies when either the first flow element is in contact with the first shoulder or the second flow element is in contact with the second shoulder. 
     A variety of additional aspects will be set forth in the description that follows. The aspects can relate to individual features and to combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad inventive concepts upon which the embodiments disclosed herein are based. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are illustrative of particular embodiments of the present disclosure and therefore do not limit the scope of the present disclosure. The drawings are not to scale and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present disclosure will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
         FIG. 1  illustrates a schematic side view of a drilling machine and a drill string, according to one embodiment of the present disclosure. 
         FIG. 2  illustrates a perspective view of a drilling machine, according to one embodiment of the present disclosure. 
         FIG. 3  illustrates another perspective view of the drilling machine of  FIG. 2 . 
         FIG. 4  illustrates a perspective view of a drilling rod assembly, according to one embodiment of the present disclosure. 
         FIG. 5  illustrates a side cross-sectional view of the drilling rod assembly of  FIG. 4 . 
         FIG. 5 a    illustrates a side cross-sectional view of a coupled pair of drilling rod assemblies of  FIG. 4 . 
         FIG. 6  illustrates a perspective view of an inner drill rod, inner drill rod coupling, and flow collar, according to one embodiment of the present disclosure. 
         FIG. 7  illustrates a side view of an uphole end of the inner drill rod of  FIG. 6 . 
         FIG. 8  illustrates an end view of a downhole end of the inner drill rod, inner drill rod coupling, and flow collar of  FIG. 6 . 
         FIG. 9  illustrates a side cross-sectional view of the inner drill rod, inner drill rod coupling, and flow collar of  FIG. 8  along line  9 - 9 . 
         FIG. 10  illustrates a cross-sectional view of the inner drill rod and inner drill rod coupling of  FIG. 9  along line  10 - 10 . 
         FIG. 11  illustrates a cross-sectional view of the inner drill rod and inner drill rod coupling of  FIG. 9  along line  11 - 11 . 
         FIG. 12  illustrates a cross-sectional view of the inner drill rod and inner drill rod coupling of  FIG. 9  along line  12 - 12 . 
         FIG. 13  illustrates a perspective view of an inner drill rod coupling, according to one embodiment of the present disclosure. 
         FIG. 14  illustrates another perspective view of the inner drill rod coupling of  FIG. 13 . 
         FIG. 15  illustrates a side view of the inner drill rod coupling of  FIG. 13 . 
         FIG. 16  illustrates an uphole end view of the inner drill rod coupling of  FIG. 13 . 
         FIG. 17  illustrates a downhole end view of the inner drill rod coupling of  FIG. 13 . 
         FIG. 18  illustrates a cross-sectional view of the inner drill rod coupling of  FIG. 15  along line  18 - 18 . 
         FIG. 18 a    illustrates a perspective view of an inner drill rod coupling, according to one embodiment of the present disclosure. 
         FIG. 18 b    illustrates a side view of the inner drill rod coupling of  FIG. 18   a.    
         FIG. 19  illustrates a perspective view of a flow collar, according to one embodiment of the present disclosure. 
         FIG. 20  illustrates another perspective view of the flow collar of  FIG. 19 . 
         FIG. 21  illustrates a side view of the flow collar of  FIG. 19 . 
         FIG. 22  illustrates a side cross-sectional view of a drill head, according to one embodiment of the present disclosure. 
         FIG. 23  illustrates a side cross-sectional view of an outer assembly of the drill head of  FIG. 22 . 
         FIG. 24  illustrates a side cross-sectional view of an inner assembly of the drill head of  FIG. 22 . 
         FIG. 25  illustrates an exploded side view of the inner assembly of the drill head of  FIG. 22 . 
         FIG. 26  illustrates a perspective view of a drill bit shaft, according to one embodiment of the present disclosure. 
         FIG. 27  illustrates a side view of the drill bit shaft of  FIG. 26 . 
         FIG. 28  illustrates a cross-sectional view of the drill bit shaft of  FIG. 27  along line  28 - 28 . 
         FIG. 29  illustrates a perspective view of a drive coupling, according to one embodiment of the present disclosure. 
         FIG. 30  illustrates a side view of the drive coupling of  FIG. 29 . 
         FIG. 31  illustrates a cross-sectional view of the drive coupling of  FIG. 30  along line  31 - 31 . 
         FIG. 32  illustrates a downhole end view of the drive coupling of  FIG. 29 . 
         FIG. 33  illustrates a cross-sectional view of the drive coupling of  FIG. 29  along line  33 - 33 . 
         FIG. 34  illustrates an uphole end view of the drive coupling of  FIG. 29 . 
         FIG. 35  illustrates a perspective view of a drive shaft, according to one embodiment of the present disclosure. 
         FIG. 36  illustrates a zoomed-in perspective view of a downhole end of the drive shaft of  FIG. 35 . 
         FIG. 37  illustrates a side view of the drive shaft of  FIG. 35 . 
         FIG. 38  illustrates a cross-sectional view of the drive shaft of  FIG. 37  along line  38 - 38 . 
         FIG. 39  illustrates a cross-sectional view of the drive shaft of  FIG. 37  along line  39 - 39 . 
         FIG. 40  illustrates a cross-sectional view of the drive shaft of  FIG. 37  along line  40 - 40 . 
         FIG. 41  illustrates a cross-sectional view of the drive shaft of  FIG. 37  along line  41 - 41 . 
         FIG. 42  illustrates a cross-sectional view of the drive shaft of  FIG. 37  along line  42 - 42 . 
         FIG. 43  illustrates a zoomed-in cross-sectional side view of an uphole end of the drive shaft of  FIG. 42 . 
         FIG. 44  illustrates a zoomed-in cross-sectional side view of the downhole end of the drive shaft of  FIG. 42 . 
         FIG. 45  illustrates a zoomed-in cross-sectional side view of a drive coupling and drive shaft of the inner assembly of  FIG. 24 . 
         FIG. 46  illustrates a zoomed-in cross-sectional view of the drive coupling and drive shaft of  FIG. 45  along line  46 - 46 . 
         FIG. 47  illustrates a side cross-sectional view of a drill head, according to one embodiment of the present disclosure. 
         FIG. 48  illustrates a zoomed-in cross-sectional side view of a drive coupling and drive shaft, according to one embodiment of the present disclosure. 
         FIG. 49  illustrates a side cross-sectional view of a drill head, according to one embodiment of the present disclosure. 
         FIG. 50  illustrates a perspective view of the drive coupling of  FIG. 48 . 
         FIG. 51  illustrates a side view of the drive coupling of  FIG. 48 . 
         FIG. 52  illustrates a cross-sectional view of the drive coupling of  FIG. 48  along line  52 - 52 . 
         FIG. 53  illustrates an uphole end view of the drive coupling of  FIG. 48 . 
         FIG. 54  illustrates a perspective view of a drive coupling, according to one embodiment of the present disclosure. 
         FIG. 55  illustrates a side view of the drive coupling of  FIG. 54 . 
         FIG. 56  illustrates a cross-sectional view of the drive coupling of  FIG. 54  along line  56 - 56 . 
         FIG. 57  illustrates an uphole end view of the drive coupling of  FIG. 54 . 
         FIG. 58  illustrates a perspective view of a drive coupling, according to one embodiment of the present disclosure. 
         FIG. 59  illustrates a side view of the drive coupling of  FIG. 58 . 
         FIG. 60  illustrates a cross-sectional view of the drive coupling of  FIG. 58  along line  60 - 60 . 
         FIG. 61  illustrates an uphole end view of the drive coupling of  FIG. 58 . 
         FIG. 62  illustrates a longitudinal cross-sectional view of an end casing with a balancing feature, according to one embodiment of the present disclosure. 
         FIG. 63  illustrates a perspective view of a gearbox including a sub saver, according to one embodiment of the present disclosure. 
         FIG. 64  illustrates another perspective view of the sub saver of  FIG. 63 . 
         FIG. 65  illustrates another perspective view of the sub saver of  FIG. 63 . 
         FIG. 66  illustrates a side cross-sectional view of the sub saver of  FIG. 63 . 
         FIG. 67  illustrates a perspective view of an inner assembly of a sub saver, according to one embodiment of the present disclosure. 
         FIG. 68  illustrates an exploded view of the inner assembly of  FIG. 67 . 
         FIG. 69  illustrates a side view of the inner assembly of  FIG. 67 . 
         FIG. 70  illustrates a cross-sectional view of the inner assembly of  FIG. 69  along line  70 - 70 . 
         FIG. 71  illustrates a cross-sectional view of the inner assembly of  FIG. 69  along line  71 - 71 . 
         FIG. 72  illustrates a cross-sectional view of the inner assembly of  FIG. 69  along line  72 - 72 . 
         FIG. 73  illustrates a cross-sectional view of the inner assembly of  FIG. 69  along line  73 - 73 . 
         FIG. 74  illustrates a cross-sectional view of the inner assembly of  FIG. 69  along line  74 - 74 . 
         FIG. 75  illustrates a side cross-sectional view of a sub saver, according to one embodiment of the present disclosure. 
         FIG. 76  illustrates an exploded view of the sub saver of  FIG. 75 . 
         FIG. 77  illustrates a perspective view of a gearbox, according to one embodiment of the present disclosure. 
         FIG. 78  illustrates a side view of the gearbox of  FIG. 77 . 
         FIG. 79  illustrates a front view of the gearbox of  FIG. 77 . 
         FIG. 80  illustrates a side cross-sectional view of the gearbox of  FIG. 79  along line  80 - 80 . 
         FIG. 81  illustrates a zoomed-in cross-sectional side view of the gearbox of  FIG. 80 . 
         FIG. 82  illustrates a side view of the gearbox of  FIG. 77  with an outer drill rod drive chuck decoupled. 
         FIG. 83  illustrates a side cross-sectional view of the outer drill rod drive chuck of  FIG. 82  along line  83 - 83 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the appended claims. 
       FIGS. 1-3  show a dual rod drilling system  100 . The dual rod drilling system  100  includes a drill string  102  that is directed into the ground  101  by a drilling machine  104 . An example drill string  102  is shown in  FIG. 1 . 
     The drilling machine  104  includes a prime mover  122  (e.g., a diesel engine), gearbox  124 , a rack  126 , and a break out mechanism  128  (e.g., a vise system). Optionally, the drilling machine  104  can include a drill rod storage box  130 , an operator&#39;s station  132 , and a set of tracks or wheels  134 . 
     The drill string  102  consists of individual sections of drill rod assemblies  106  that are connected to the drilling machine  104  at an uphole end  108  and a drill head  110  at a downhole end  112 . Each drill rod assembly  106  includes a downhole end  109  and an uphole end  111 . The drill rod assemblies  106  are strung together end-to-end to form the drill string  102 , which can extend significant distances in some drilling applications. 
     Each drill rod assembly  106  includes an outer tubular drill rod  114  having external threads on one end and internal threads on the opposite end. In some examples, the drill rod assembly  106 , and the associated drilling machine  100 , is configured so that, when the drill string  102  is constructed, the external threads of the outer drill rod  114  are positioned at the uphole end  111  of the drill rod assembly  106  and the internal threads of the outer drill rod  114  are positioned at the downhole end  111  of the drill rod assembly  106 . 
     Each drill rod assembly  106  further includes a smaller, inner drill rod  116 . The inner drill rod  116  fits inside the tubular outer drill rod  114 . The inner drill rod  116  of each drill rod assembly is interconnected to the adjacent inner drill rods by an inner rod coupling  118 . In some examples, each inner rod coupling  118  is affixed to each inner drill rod  116  at the uphole end  111  of each drill rod assembly  106  (shown in  FIG. 5 ). 
     During a drilling operation, the drilling machine  104  individually removes drill rod assemblies  106  from the drill rod storage box  130  and moves each drill rod assembly  106  onto the rack  126 . Once positioned on the rack  126 , both the break out mechanism  128  and the gearbox  124  engage the drill rod assembly  106  and couple the drill rod assembly with an immediately preceding downhole drill rod assembly  106 . Once coupled, the gearbox  124  is configured to travel longitudinally on the rack  126  toward the break out mechanism  128 , while simultaneously rotating one or both of the outer and inner drill rods  114 ,  116  of the drill rod assembly  106 . When the gearbox  124  reaches the break out mechanism  128  at the end of the rack  126 , the gearbox  124  is de-coupled from the drill rod assembly  106 , and thereby the drill string  102 , and retracts up the rack  126  so that another drill rod assembly  106  can be added to the drill string  102 . This process is repeated until the drilling operation is complete, and then reversed during a pullback operation in which the drilling machine  104  removes the drill rod assemblies  106  from the ground  101 . 
     The dual rod drilling system  100  is operable to execute a plurality of software instructions that, when executed by the controller  550 , cause the system  100  to implement the methods and otherwise operate and have functionality as described herein. In some examples, the controller  550  is in communication the prime mover  122 , gearbox  124 , rack  126 , break out mechanism  128 , operator&#39;s station  132  and/or other components of the system  100 . The controller  550  may comprise a device commonly referred to as a microprocessor, central processing unit (CPU), digital signal processor (DSP), or other similar device, and may be embodied as a standalone unit or as a device shared with components of the system  100 . The controller  550  may include memory for storing software instructions, or the system  100  may further comprise a separate memory device for storing the software instructions that is electrically connected to the controller  550  for the bi-directional communication of the instructions, data, and signals therebetween. In some examples, the controller  550  waits to receive signals from the operator&#39;s station  132  before communicating with and operating the components of the drilling machine  104 . In other examples, the controller  550  can operate autonomously, without receiving signals from the operator&#39;s station  132 , to communicate with and control the operation of the components of the drilling machine  104 . 
     The operator&#39;s station  132  can be mounted to the drilling machine  104  to allow an operator to control the operation of the drilling machine  104 . In some examples, the operator&#39;s station  132  includes a plurality of controls  552  with which the operator can interact to control the components of the drilling machine  104 . In some examples, the controls  552  include joysticks, knobs, buttons, and the like. In some examples, the controls  552  can be in communication with the controller  550 . In some examples, as the user interacts with the controls  552 , the controls  552  generate a signal that is sent to the controller  550  that can indicate operations the user would like the drilling machine  104  to perform. Such operations can include, but not be limited to, separate rotation of the inner and outer drill rods  116  via the gearbox  124 , movement of the gearbox  124  via the rack  126  on the drilling machine  104 , and operation of the break out mechanism  128 . In some examples, the controls  552  and controller  550  are an open loop system and there does not exist any feedback between the drilling machine  104 &#39;s actual operation and the controller  550  and controls  552 . In other examples, the controls  552  and controller  550  are a closed loop system and there exists feedback between the drilling machine  104 &#39;s operation and the controller  550  and controls  552 . In such a closed loop system, a plurality of sensors can be used to monitor the performance of the components of the drilling machine  104 . 
       FIG. 4  shows a perspective view of a single drill rod assembly  106 , and  FIG. 5  shows a longitudinal cross-section of a drill rod assembly  106 . The drill string  102 , and each drill rod assembly  106 , defines a fluid flow path  103  that extends along the lengths of the drill rod assemblies  106 . In some examples, the drill string  102  can have multiple fluid flow paths such as an annular fluid flow  105  path disposed between the inner and outer drill rods  116 ,  114  and an inner rod fluid flow path  107  disposed within the inner drill rod  116 . In operation, fluid is pumped into the drill rod assembly  106  and travels to the drill head  110  for cooling, transporting cuttings, lubricating, and drill hole stabilizing. As will be described herein, drilling fluid can be provided to the drill string  102  at the gearbox  124 . 
     In some examples, the inner rod coupling  118  and a flow collar  119  are flow elements that are configured to allow fluid flow within the fluid flow path  103  through each of the inner rod coupling  118  and the flow collar  119 . The flow collar  119  is secured around the inner drill rod  116  at the downhole end  109  of the drill rod assembly  106  at an opposite end from the inner rod coupling  118 . In some examples, the inner rod coupling  118  and the flow collar  119  help to retain the inner drill rod  116  within the outer drill rod  114  by interfacing with an uphole shoulder  117   a  and a downhole shoulder  117   b  of the outer drill rod  114 , respectively. The inner rod coupling  118  and the flow collar  119  are configured to allow fluid flow along the fluid flow path  103  no matter the relative position of the inner drill rod  116  and the outer drill rod  114  of each drill rod assembly  106 . The inner rod coupling  118  and the flow collar  119  are configured to allow fluid flow along the fluid flow path  103  while the flow collar  119  and/or the inner rod coupling  118  are interfacing (e.g., contacting) with the uphole shoulder  117   a  and/or the downhole shoulder  117   b  of the outer drill rod  114 . Fluid flow through the flow collar  119  and the inner rod coupling  118  is represented in  FIG. 5  with arrows F. In some examples, the flow collar  119  and/or the inner rod coupling  118  interface with the uphole shoulder  117   a  and/or the downhole shoulder  117   b  of the outer drill rod  114  with continuous annular surfaces. 
       FIG. 5 a    shows two drill rod assemblies  106   a ,  106   b  coupled to one another. The outer drill rods  114   a ,  114   b  are shown coupled to one another, and the inner drill rods  116   a ,  116   b  are shown coupled to one another via the inner rod coupling  118 . Further, the uphole drill rod assembly  106   b  is shown to be coupled, but not attached to, the inner rod coupling  118 , adjacent the flow collar  119 . Fluid flow is permitted from the uphole drill rod assembly annular flow path  105   a , through and around the flow collar  119 , through and around the inner rod coupling  118 , and into the downhole drill rod assembly annular flow path  105   b . Therefore, as shown, even when the inner rod coupling  118  is contacting the uphole shoulder  117   a  of the outer drill rod  114   a  of the downhole drill rod assembly  106   a  and the flow collar  119  is contacting the downhole shoulder  117   b  of the outer drill rod  114   b  of the uphole drill rod assembly  106   b , annular flow between the two drill rod assemblies  106   a ,  106   b  is permitted. 
       FIG. 6  shows a perspective view of an inner drill rod  116  with an inner rod coupling  118  installed on the uphole end  111  and a flow collar  119  installed on the downhole end  109 . The inner drill rod  116  includes features that allow each inner drill rod  116  to be coupled with additional similar inner rods and/or drilling tools. 
       FIG. 7  shows a side view of the uphole end  111  of the inner drill rod  116  without the inner rod coupling  118  installed. The uphole end  111  of the inner drill rod  116  includes a torque-carrying section  121 , a groove  123 , and a non-torque-carrying section  125 . 
     The torque-carrying section  121  is configured to mate with the inner rod coupling  118  so that torque can be transferred through the inner rod coupling  118  and to the inner drill rod  116 . In some examples, the torque carrying section  121  can have a polygonal cross-section. In some examples, the torque-carrying section  121  has a hexagonal cross-section. The torque-carrying section  121  can be of any cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming (e.g., lobes, flat faces, curved faces, etc.). The torque-carrying section  121  has a maximum width of W 1 . 
     The groove  123  is configured to receive a fastening device (shown in  FIG. 9 ) to secure the inner rod coupling  118  to the inner drill rod  116 . In some embodiments, the groove  123  is configured to receive a pair of fastening devices such as pins, bolts, or other like devices. In some examples, the groove  123  can have a width G that is greater than the width of the fastening device. 
     The non-torque-carrying section  125  is configured to be positioned within the inner rod coupling  118  so that it does not bear any torque forces from the inner rod coupling  118 . The non-torque-carrying section  125  has a maximum width of W 2 . W 2  is less than the width W 1  of the torque-carrying section  121 . In some examples, the non-torque-carrying section  125  has a circular cross-section. 
     The uphole end  111  of the inner drill rod  116  is described herein as an example and it is considered within the scope of the present disclosure that other drilling components in the dual rod drilling system  100  may have a similar construction to the uphole end  111  of the inner drill rod  116  described herein. For example, such components can include, but are not limited to, a sub saver, as discussed with respect to  FIGS. 48-61  herein, and the drill head  110 , as discussed with respect to  FIGS. 22-47  herein. 
       FIG. 8  shows an end view of the inner drill rod  116 , and  FIG. 9  shows a longitudinal cross-section of the inner drill rod  116 , inner rod coupling  118 , and flow collar  119  along line  9 - 9  in  FIG. 8 .  FIG. 8  shows both the downhole end  109  and the uphole end  111  of the inner drill rod  116 . Further,  FIG. 8  depicts break lines to represent the middle of the inner drill rod  116 . 
     At the downhole end  109 , the flow collar  119  is secured around the inner drill rod  116 . In some examples, the flow collar is configured to be welded onto the inner drill rod  116 . In other examples, the flow collar  119  is press fit and secured around the downhole end of the inner drill rod  116 . In other examples, the flow collar  119  is attached to the inner drill rod  116  via a fastener (not shown). In other examples still, the flow collar  119  is attached loosely to the downhole end  109 . 
     Similar to  FIG. 5 ,  FIG. 8  also depicts arrows F that travel through the flow collar  119  to depict fluid flow. As will be discussed with respect to  FIGS. 19-21 , the flow collar  119  includes at least one peripheral fluid passage  127  positioned within the annular fluid flow passage  103  between the inner drill rod  116  and the outer drill rod  114  so as to allow generally axial fluid flow within the annular fluid flow passage  107 . 
     At the uphole end  111  of the inner drill rod  116 , the inner rod coupling  118  is secured to the inner drill rod  116  by a pair of pins  129 . The pins  129  are configured to pass through the inner rod coupling  118  and through the groove  123  in the inner drill rod  116 . Due to the size of the groove  123 , the inner drill rod  116  is captured in an axial direction within the inner rod coupling  118 . In some examples, the groove  123  can have a width G that allows for limited axial movement between the inner drill rod  116  and inner rod coupling  118 . In some examples, a single pin  129  can be utilized with the inner rod coupling  118 . 
     The inner rod coupling  118  includes a longitudinal axis  131 , an inner bore  133 , at least one cross aperture  135 , and a flow sleeve  137 . The inner bore  133  has a non-circular profile that is configured to mate with the torque-carrying section  121  of the uphole end  111  of the inner drill rod  116 . The inner bore  133  can also have a profile that is configured to mate with a downhole end torque-carrying section  139  of the inner drill rod  116  so that it can couple two like inner drill rods  116 . The torque-carrying section  139  can be of any cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming (e.g., lobes, flat faces, curved faces, etc.). The inner bore  133  is configured to interface with the inner drill rod  116  to transfer torque between successive inner drill rods  116 . 
     The cross aperture  135  is configured to receive and hold the pin(s)  129 . In some examples, the inner rod coupling  118  includes a plurality of cross apertures  135 . 
     The flow sleeve  137  of the inner rod coupling  118  is configured to allow fluid flow therethrough so as to allow generally axial fluid flow within the annular fluid flow passage  105 , similar to the peripheral fluid passage  127  of the flow collar  119 . Further, the flow sleeve  137  is configured to interface with the outer drill rod  114  so as to aid in retaining the inner drill rod  116  within the outer drill rod  114 . In some examples, the flow sleeve  137  can have an outer diameter that is larger than the inner diameter of the outer drill rod  114 . 
       FIG. 10  shows a cross-section of the inner drill rod  116  and the inner rod coupling  118  taken along line  10 - 10  in  FIG. 9 . As shown, the non-torque-carrying section  125  of the inner drill rod  116  does not make contact with the inner bore  133  of the inner rod coupling  118 . Further, in the depicted example, the flow sleeve  137  of the inner rod coupling  118  includes a plurality of flow sleeve fluid passages  147  that are positioned around the periphery of the inner rod coupling  118 . In some examples, the flow sleeve  137  can include a single flow sleeve fluid passage  147 . 
       FIG. 11  shows a cross-section of the inner drill rod  116  and the inner rod coupling  118  taken along line  11 - 11  in  FIG. 9 . The pins  129  are positioned in the groove  123  of the inner drill rod  116  and also within the cross apertures  135  of the inner rod coupling  118 . In some examples, the cross apertures  135  of the inner rod coupling  118  are positioned at opposite sides of the inner rod coupling  118 . 
       FIG. 12  shows a cross-section of the inner drill rod  116  and the inner rod coupling  118  taken along line  12 - 12  in  FIG. 9 . The torque-carrying section  121  of the inner drill rod  116  is mated with the inner bore  133  of the inner rod coupling  118 . In some examples, the inner bore  133  can have a hexagonal cross-section that matches the cross-section of the torque-carrying section  121 . 
       FIGS. 13 and 14  show perspective views of the inner rod coupling  118 .  FIG. 15  shows a side view of the inner rod coupling  118 .  FIGS. 16 and 17  show the ends of the inner rod coupling  118 . 
     The inner rod coupling  118  includes a downhole end  149  and an uphole end  151 . The downhole end  149  is configured to be secured to the inner drill rod  116  via pins  129  (as shown in  FIG. 9 ). Further, the inner bore  133  of the inner rod coupling  118  has a consistent cross-section along the length of the inner coupling. 
     The flow sleeve  137  of the inner rod coupling  118  can include a flow sleeve main body  153  and a ring  155 . In some examples, the ring  155  includes a larger outer diameter than the flow sleeve main body  153 . In some examples, the flow sleeve main body  153  can be press fit around a main body  159  of the inner rod coupling  118  while the ring  155  remains spaced away from the main body  159  of the inner rod coupling  118 . Further, as noted above, the flow sleeve  137  includes a plurality of flow sleeve fluid passages  147  that allow for axial fluid flow from the downhole end  149  to the uphole end  151  of the inner rod coupling  118 . In some examples, the flow sleeve fluid passages  147  are radial apertures disposed around the periphery of the flow sleeve  137  in both the ring  155  and the flow sleeve main body  153 . The flow sleeve fluid passages  147  allow fluid to flow around the flow sleeve main body  153 , through the flow sleeve fluid passages  147 , and between the ring  155  and main body  159  of the inner rod coupling  118 . In some examples, the flow sleeve fluid passages  147  are generally perpendicular to the longitudinal axis  131  of the inner rod coupling  118 . In some examples, the flow sleeve  137  can include flow sleeve fluid passages  147  of varying sizes. 
     In some examples, the flow sleeve  137  includes an outer rod interfacing surface  163  on the ring  155 . The outer rod interfacing surface  163  is generally perpendicular to the longitudinal axis  131  of the inner rod coupling  118 . The outer rod interfacing surface  163  is configured to periodically contact the outer drill rod  114  of the drill rod assembly  106  of which the inner rod coupling  118  is a part. Specifically, the outer rod interfacing surface  163  is configured to contact the uphole end shoulder  117   b  of the outer drill rod  114 , as shown in  FIG. 5 . In some examples, the outer rod interfacing surface  163  is a continuous annular surface that extends around the entire perimeter of the flow sleeve  137  that surrounds the main body  159  of the inner rod coupling  118 . The outer rod interfacing surface  163  aids in retaining the inner drill rod  116  within the outer drill rod  114 . Once the outer rod interfacing surface  163  interfaces with the outer drill rod  114 , the inner drill rod  116  cannot move further toward the downhole end  109  of the drill rod assembly  106 . Further, the flow sleeve fluid passages  147  of the flow sleeve  137  are longitudinally offset from the outer rod interfacing surface  163 . In some examples, such a longitudinal offset prevents the flow sleeve fluid passages  147  from becoming blocked when the outer rod interfacing surface  163  contacts the outer drill rod  114 . 
     In some examples, the flow sleeve  137  can be configured to be forced off of, and removed from, the main body  159  by the uphole end shoulder  117   b  of the outer drill rod  114  during a malfunction during a drilling operation. This can be advantageous because the integrity of the inner rod coupling  118  can be maintained during a malfunction. The flow sleeve  137  acts similar to a fuse, failing by being removed from the inner rod coupling  118  during a malfunction, but saving the inner rod coupling  118  from damage at the same time. 
       FIG. 18  shows a cross-section of the inner rod coupling  118  taken along line  18 - 18  in  FIG. 15 . The cross apertures  135  are disposed in the main body  159  having axes  171  so as to not intersect the longitudinal axis  131  of the inner rod coupling  118 . By positioning the cross apertures  135  through the main body  159  and not intersecting the longitudinal axis, the pins  129  are positioned at sides of the inner bore  133  so as to only interface with the groove  123  of the inner drill rod  116  and not obstruct either of the annular fluid flow path  105  or the inner rod fluid flow path  107  of the drill string  102 . Specifically, because the groove  123  surrounds the inner rod fluid flow path  107  of the inner drill rod  116 , the cross apertures  135  position the pins in such a way where they never obstruct fluid flow. 
     The cross apertures  135  can have a variety of different shapes. In some examples, the cross apertures  135  have a width A (e.g., a diameter) at least equal to the width G of the groove  123  of the inner drill rod  116 . 
       FIGS. 18 a  and 18 b    depict an inner rod coupling  618 . The inner rod coupling  618  is substantially similar to the inner rod coupling  118  discussed above. The inner rod coupling  618  includes flow sleeve  637  that is configured to allow fluid flow therethrough so to allow generally axial fluid flow within the annular fluid flow passage  103 . Like the flow sleeve  137  described above, the flow sleeve  637  includes a plurality of flow sleeve fluid passages  647  that are positioned around the periphery of the inner rod coupling  618 . In some examples, the flow sleeve fluid passages  647  are sized and shaped to allow adequate flow therethrough. In some examples, the flow sleeve fluid passages  647  can be slots. 
       FIGS. 19-21  show perspective views of the flow collar  119 . The flow collar  119  includes a downhole end  173  and an uphole end  183 . 
     The flow collar  119  includes a first interior portion  185  that has a first interior diameter and a second interior portion  187  that has a second interior diameter. In some examples, the first interior portion  185  has a smaller interior diameter than the second interior portion  187 . Further, in some examples, the first interior portion  185  is configured to be press fit onto the downhole end  109  of the inner drill rod  116 . 
     Similar to the flow sleeve fluid passages  147  discussed above, the flow collar  119  includes a plurality of peripheral fluid passages  127 . The peripheral fluid passages  127  allow fluid flow from the uphole end  183  to the downhole end  173 . Specifically, when installed on the inner drill rod  116 , fluid flows around the outside of the flow collar  119 , through the peripheral passages  127 , and between the second interior portion  187  and the inner drill rod  116 . 
     The flow collar  119  further includes an outer rod interfacing surface  191 , similar to the outer rod interfacing surface  163  of the inner rod coupling  118 . The outer rod interfacing surface  191  is configured to periodically contact the outer drill rod  114  of the drill rod assembly  106  of which the flow collar  119  is a part. The outer rod interfacing surface  191  aids, along with the outer rod interfacing surface  163  of the inner rod coupling  118 , in retaining the inner drill rod  116  within the outer drill rod  114 . In some examples, the outer rod interfacing surface  191  is a continuous annular surface that extends around the entire perimeter of the flow collar  119 . Once the outer rod interfacing surface  191  interfaces with the outer drill rod  114 , the inner drill rod  116  cannot move further toward the uphole end  111  of the drill rod assembly  106 . Thus, the flow collar  119  also reduces the amount of axial force that can be introduced to the inner rod coupling  118 . 
       FIG. 22  shows a longitudinal cross-section of the drill head  110 . The drill head  110  is connectable to the outer drill rods  114  and inner drill rods  116  of the drill string  102 . The drill head  110  includes a downhole end  136  and an uphole end  138 . Further, the drill head  110  includes a replaceable drill bit  140 , a drill bit shaft  142 , an end casing  144 , a plurality of drill bit shaft bearings  146 , a drive coupling  148 , a drive shaft  150 , a main casing  152 , and an optional sonde  154  positioned within the main casing  152 . In some examples, the drill head  110  can include an outer rod adapter  255  to connect the drill head  110  to the outer drill rods  114  of the drill string  102  and the inner rod coupling  118  to connect the drill head  110  to the inner drill rod  116 . 
     The inner drill rods  116  of the drill string  102  are collectively used to drive the rotation of the drill bit  140  via the drive shaft  150 , the drive coupling  148 , and the drill bit shaft  142 . The outer drill rods  114  of the drill string  102  are collectively used to rotate and/or control the rotational orientation of the main casing  152 , which is connected to the end casing  144 . 
     The replaceable drill bit  140  can have a variety of different configurations and, in some examples, can be a tri-cone bit. The replaceable drill bit  140  is mounted to a downhole end  141  of drill bit shaft  142  at the downhole end  136  of the drill head  110 . 
     The drill bit shaft  142  is rotatably mounted within the end casing  144  via the drill bit shaft bearings  146  making the drill bit shaft  142  rotatable relative to the end casing  144  along a drill bit shaft axis  156 . The drill bit shaft axis  156  is parallel to an end casing axis  158 . The drill bit shaft  142  includes drive features  160  at an uphole end  143  that are configured to mate with the drive coupling  148  to facilitate torque transfer between the drive coupling  148  and the drill bit shaft  142 . The drill bit shaft  142  also includes an inner fluid flow cavity  145  that allows drill fluid flow to transfer from the drill string  102  to the drill bit  140 . 
     The drive coupling  148  is positioned between the drill bit shaft  142  and the drive shaft  150  within a recess  157  of the end casing  144  to facilitate the transfer of torque between the drill bit shaft  142  and the drive shaft  150 . Specifically, the drive coupling  148  receives the drill bit shaft  142  at a downhole end  162  and the drive shaft  150  at an uphole end  164 . The drive coupling  148  includes a coupling fluid flow passage  161  to allow fluid flow from the uphole end  164  to the downhole end  162  and then on to the inner fluid flow cavity  145  of the drill bit shaft  142 . 
     The drive shaft  150  includes a downhole end  166  and an uphole end  165 . The uphole end  165  is configured to attach to the inner drill rods  116  of the drill string  102 . In some examples, the inner rod coupling  118  can be secured to the uphole end  165 . The downhole end  166  includes drive features  168  that are torque transmitting and radial load bearing. The downhole end  166  of the drive shaft  150  is configured to mate with the uphole end  164  of the drive coupling  148 . The drive shaft  150  is rotatable about a drive shaft axis  167  and is positioned within the main casing  152 . In the depicted example, the drive shaft axis  167  is parallel with a main casing axis  169 . The drive shaft axis  167  is not aligned and is not parallel with the end casing axis  158  and the drill bit shaft axis  156 . In some examples, the drive shaft axis  167  and the drill bit shaft axis  156  are angled at an angle θ with respect to one another between about 1 degree and 5 degrees. In some examples, the drive shaft axis  167  and the drill bit shaft axis  156  are angled at an angle θ equal to about 2 degrees from one another. In some examples, the misalignment can be adjustable to alter the steering characteristics of the drill head  110 . 
     The drive shaft  150  has an outer diameter OD that is smaller than an inner diameter ID of the main casing  152 . A drive shaft fluid flow passage  170  is disposed between the inner diameter ID of the main casing  152  and the outer diameter OD of the drive shaft  150 . In some examples, the drive shaft fluid flow passage  170  is an annular fluid flow passage between the drive shaft  150  and the main casing  152 . The drive shaft fluid flow passage  170  is in communication with the fluid flow path  103  of the drill string  102  at the uphole end  138  of the drill head  110 . Further, due to the location of the drive coupling  148  and the drive shaft  150 , the drive coupling  148  and drive shaft  150  are surrounded by fluid flow from the drive shaft fluid flow passage  170 . This allows drilling fluid to be in communication with the drive features  168  of the drive shaft  150  and the uphole end  164  of the drive coupling  148 . 
       FIG. 23  shows an outer assembly  174  of the drill head  110  that includes the end casing  144  connected to the main casing  152 . Further, as shown, the outer rod adapter  255  is connected to the main casing  152 . In some examples, a sonde  154  (i.e., probe or beacon) can be positioned within the main casing  152 . The misalignment of the end casing axis  158  and the main casing axis  169  is fixed so as to allow the outer assembly  174  to interact with the bore hole to allow steering of the drill string  102  along a generally horizontal path. 
       FIG. 24  shows an inner assembly  172  of the drill head  110  that includes the drive shaft  150 , the drive coupling  148 , and the drill bit shaft  142 . The inner assembly  172  is configured to drive the rotation of the drill bit  140  via the inner drill rod  116  of the drill string  102 . As shown, the drill bit shaft  142  and the drive shaft  150  are both straight members that are axially misaligned at the drive coupling  148 . In some examples, the misalignment of the drive shaft  150  with the drive coupling  148  is adjustable. 
       FIG. 25  shows an exploded longitudinal cross-section of the inner assembly  172 . As shown, the drill bit shaft  142  includes a projection  175  at the uphole end  143 , and the drive coupling  148  includes a recess  176  at the downhole end  162 . The drive features  160  of the drill bit shaft  142  are configured to mate with drive features  178  of the drive coupling  148  located within the recess  176 . Further, the drive coupling  148  also includes a second recess  177  at the uphole end  164  that includes drive features  180  within the recess  177  that are sized and shaped to mate with the drive features  168  of a projection  179  the drive shaft  150 . In some examples, the drive coupling  148  can include one or more projections and mate with recesses on either, or both, the drill bit shaft  142  and the drive shaft  150 . 
     A perspective view of the drill bit shaft  142  is shown in  FIG. 26 . A side view of the drill bit shaft  142  is shown in  FIG. 27 . At the downhole end  141 , the drill bit shaft includes an interface  181  that is sized and shaped to mate with the drill bit  140 . In some examples, the interface  181  is a threaded interface. The drill bit shaft  142  is rotatable about the drill bit shaft axis  156 . The drill bit shaft  142  also includes a bearing portion  182  that is configured to interface and rotate about the drill bit shaft bearings  146 . 
       FIG. 28  shows a transverse cross-section of the drill bit shaft along line  28 - 28  of  FIG. 27 . As shown, the drive features  160  are a series of faces  184  each with a generally planar construction. In some examples, the projection  175  of the drill bit shaft  142  can have a generally polygonal cross-section. In the depicted embodiment, the drive features  160  of the projection  175  form a generally hexagonal profile. In some examples, the projection  175  can also include transitional surfaces  186  between the drive features  160  to allow for slight misalignment between the projection  175  of the drill bit shaft  142  and the recess  176  of the drive coupling  148 . 
       FIG. 29  shows a perspective view of the drive coupling  148 .  FIG. 30  shows a side view of the drive coupling  148 , and  FIG. 31  shows a cross-sectional view of the drive coupling  148  along line  31 - 31  in  FIG. 30 .  FIG. 32  shows an end view of the drive coupling  148 . 
     In the depicted example, the coupling fluid flow passage  161  includes a plurality of radial fluid flow passages  188  and an axial fluid flow passage  190 . The radial fluid flow passages  188  allow fluid communication between an exterior  189  of the drive coupling  148  and the recesses  176 ,  177 . As shown in  FIG. 33 , the radial fluid flow passages  188  are positioned around the drive coupling  148  and are in communication with an axial fluid flow passage  190 . In some examples, the drive coupling  148  can include a single radial fluid flow passage  188 . 
       FIG. 32  shows the downhole end  162  of the drive coupling  148 , and  FIG. 34  shows the uphole end  164  of the drive coupling  148 . The drive features  178 ,  180  of each of the recesses  176 ,  177  are torque transmitting and radial load bearing. In some examples, the drive features  178 ,  180  include a plurality of faces  192 ,  193  that form a polygonal cross-section. In some examples, the faces  192 ,  193  form a hexagonal profile. The faces  192 ,  193  can form any cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming (e.g., lobes, flat faces, curved faces, etc.). In some examples, the faces  192 ,  193  are at least partially heat treated. 
     As shown in the longitudinal cross-section of  FIG. 33 , the recesses  176 ,  177  are connected to one another by the axial fluid flow passage  190 . In some examples, the axial fluid flow passage  190  can be as wide as the recesses  176 ,  177 . In other examples, the axial fluid flow passage  190  is disposed between two end faces  194 ,  195  of each recess  176 ,  177 . In the depicted example, the end wall  195  of the uphole recess  177  has a non-planar construction. In some examples, the end wall  195  has a shape that matches a corresponding shape of an end face  196  of the downhole end  166  of the drive shaft  150 . In some examples, the end wall  195  can have a concave shape. In some examples, the drive coupling  148  includes a longitudinal axis  197  that is generally aligned with the drill bit shaft axis  156  when the drill head  110  is assembled. 
       FIG. 35  shows a perspective view of the drive shaft  150 . In some examples, the drive shaft  150  can be a solid, straight shaft without a bend. 
       FIG. 36  shows a zoomed-in perspective view of the downhole end  166  of the drive shaft  150 . The drive features  168  of the downhole end  166  of the drive shaft  150  are torque transmitting and radial load bearing. In some examples, the drive features  168  of the downhole end  166  include a plurality of faces  198 . In the depicted example, the projection  179  of the drive shaft  150  is configured to be received within the recess  177  of the drive coupling  148 . Accordingly, once received within the drive coupling  148 , the drive shaft  150  can transmit torque through the drive coupling  148  and bear radial loads while the drive shaft axis  167  remains misaligned with the drive coupling axis  197 . 
     In some examples, a portion of the downhole end  166  of the drive shaft  150  (e.g., the projection  179 ) has an outer profile that is generally spherical. In some examples, a portion of the downhole end  166  has an outer profile that is generally an ellipsoid. In other examples, a portion of the downhole end  166  has an outer profile that is generally a prolate spheroid. In other examples still, a portion of the downhole end  166  has an outer profile that is a prolate spheroid with the plurality of faces  198  having a rounded shape. The faces  198  together form a profile that has a generally hexagonal transverse cross-section (shown in  FIG. 40 ). In other examples still, a portion of the downhole end  166  is a crowned spline. 
       FIG. 37  shows a side view of the drive shaft  150 .  FIG. 38  shows a transverse cross-section of the drive shaft  150  along line  38 - 38  of  FIG. 37 . As shown, the faces  198  form a generally polygonal cross-section. In some examples, the cross-sectional profile can be generally hexagonal. In some examples, the drive features  168  of the drive shaft  150  include transitional faces  201  positioned between circumferentially consecutive faces  198 . In some examples, the transitional faces  201  reduce binding between the projection  179  and the drive features  178  of the recess  177  of the drive coupling  148 . In some examples, the faces  198  are immediately adjacent the transitional faces  201 . In some examples, the faces  198  are at least partially heat treated. In other examples, only about half of each face  198  is heat treated. 
       FIG. 39  shows a transverse cross-section of the drive shaft  150  along line  39 - 39  of  FIG. 37 . The drive shaft  150  includes radial fluid ports  202  and an axial fluid port  204 . The axial fluid port  204  is configured to be in fluid communication with the inner rod fluid flow path  107  of the inner drill rod  116  of the drill string  102 . The axial fluid port  204  is configured to transmit fluid to the radial fluid ports  202  and into the drive shaft fluid flow passage  170 . 
       FIG. 40  shows a transverse cross-section of the drive shaft  150  along line  40 - 40  of  FIG. 37 . The drive shaft  150  includes a plurality of torque-carrying uphole end faces  206  that form a generally polygonal cross-sectional profile. In some examples, the uphole end faces  206  have a generally hexagonal profile. The uphole end faces  206  can form any cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming (e.g., lobes, flat faces, curved faces, etc.). In some examples, the uphole end faces  206  are configured to mate with the inner rod coupling  118  so as to receive torque from the inner rod coupling  118 . 
       FIG. 41  shows a transverse cross-section of the drive shaft  150  along line  41 - 41  of  FIG. 37 . The drive shaft  150  includes a non-torque-carrying surface  208  that is configured to be captured within the inner rod coupling  118 . However, in the depicted example, the non-torque-carrying surface does not receive torque from the inner rod coupling  118 . 
       FIG. 42  shows a longitudinal cross-section of the drive shaft  150  along line  42 - 42  of  FIG. 37 .  FIG. 43  shows a zoomed-in side view of the uphole end  165  of the drive shaft  150 . The uphole end  165  of the drive shaft  150  includes a groove  210  that is configured to receive at least one pin (not shown) to retain the inner rod coupling  118 . The groove  210  is positioned between the torque-carrying uphole end faces  206  and the non-torque-carrying surface  208 . In some examples, the groove  210 , torque-carrying uphole end faces  206 , and the non-torque-carrying surface  208  are substantially similar to the torque-carrying section  121 , groove  123 , and non-torque-carrying section  125  of the uphole end  111  of the inner drill rod  116 . 
       FIG. 44  shows a zoomed-in side view of the downhole end  166  of the drive shaft  150 . As shown, each face  198  has a rounded shape that has a radius of curvature that extends in an axial direction along the drive shaft  150 . In some examples, a midpoint  199  of each face  198  is a greater distance away from the drive shaft axis  167  than end points  200  of each face  198 . 
       FIG. 45  shows a zoomed-in schematic cross-sectional view of the drive shaft  150  positioned within the drive coupling  148 . As described above, the drive shaft axis  167  is misaligned with the drive coupling axis  197 . Specifically, the drive coupling axis  197  is aligned with the drill bit shaft axis  156 . 
       FIG. 46  shows a cross-sectional view along line  46 - 46  of  FIG. 45 . In some examples, the transitional faces  201  do not make contact with the drive features  178  of the recess  177  and, thereby, allow fluid flow around the projection  179  while the projection  179  is mated with the drive features  178  of the drive coupling  148 . 
     Therefore, when the drive coupling  148  and drive shaft  150  are positioned within the drill head  110 , fluid flow is permitted from the drive shaft fluid flow passage  170  into the drive coupling  148  at both the recess  177  and the radial fluid flow passages  188 . Such fluid flow allows for a lubricated connection between the drive shaft  150  and the drive coupling  148  at the recess  177 . Fluid flow is further permitted along the axial fluid flow passage  190  in the drive coupling and then finally into the inner fluid flow cavity  145  of the drill bit shaft  142 . 
       FIG. 47  show a drill head  211  with an uphole end  209  and a downhole end  207 , according to another embodiment of the present disclosure. The drill head  211  includes a drive shaft  250  that includes a recess  252  at a downhole end  254 . The recess  252  is configured to mate with a projection  256  attached to a drill bit shaft  242  having a casing axis  258 . The recess  252  is configured to transfer torque from the drive shaft  250  to the drill bit shaft  242 . In some examples, the projection  256  is substantially similar to the projection  179  of the drive shaft  150 , described above. Further, the recess  252  of the drive shaft  250  is substantially similar to the recess  177  of the drive coupling  148 , described above. 
       FIG. 48  shows the drill bit shaft  142  coupled to the drive shaft  150  via a drive coupling  748 . As shown, the drive coupling  748  is substantially similar to the drive coupling  148  described above. The coupling  748  includes a pair of recesses  776 ,  777  that are configured to mate with the drill bit shaft  142  and the drive shaft  150 , respectively. Each recess  776 ,  777  includes drive features  778 ,  780  that are torque transmitting and radial load bearing. As shown, the drive features  780  of the recess  777  that receives the drive shaft  150  can have a cross sectional profile that generally matches the cross sectional profile of the projection  179  of the drive shaft  150 . In some examples, the drive features  780  are rounded, or curved as the drive features  780  extend in a longitudinal direction generally towards an uphole end  764  or a downhill end  762  of the drive coupling  748 . In some examples, the drive features  780  form a polygonal lateral cross-sectional profile, like the drive features  180  described above. In some examples, the drive features  780  have a generally hexagonal lateral cross-sectional profile. In some examples, the drive features  780  can form any lateral cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming. In some examples, the drive features  780  are at least partially heat-treated. 
     It is considered within the scope of the present disclosure that any drive shaft and drive coupling disclosed herein can have generally rounded longitudinal cross-sectional profiles. Like in the example shown in  FIG. 48 , both the drive features  168  of the draft shaft  150  and the drive features  780  of the drive coupling  748  can include rounded longitudinal cross-sectional profiles. Like in the example shown in  FIG. 45 , the drive features  168  of the draft shaft  150  have rounded longitudinal cross-sectional profiles while the drive features  180  of the drive coupling  148  have straight/flat longitudinal cross-sectional profiles. In other examples, the drive features  168  of the draft shaft  150  have straight/flat longitudinal cross-sectional profiles and the drive features  180 ,  780  of the drive coupling  148 ,  748  have rounded longitudinal cross-sectional profiles. 
     In some examples, the drive coupling  748  and/or the drive shaft  150  can be assembled with one another to prevent decoupling from one another during a drilling operation. In some examples, the assembly to prevent decoupling can include press-fitting the drive coupling  748  and drive shaft  150  together. In some examples, the assembly to prevent decoupling can include heating at least one of the drive coupling  748  and drive shaft  150  prior to coupling. In some examples, the assembly to prevent decoupling can include providing a seam on the drive coupling  748  (or the drive shaft  250  as shown in the embodiment shown in  FIG. 47 ) to allow the drive coupling  748  to be separated into multiple pieces. The multiple pieces can then be secured around the drive shaft  150  by, for example, a fastener such as an adhesive, a bolt(s), a screw(s), a weld, or other type fastener. 
       FIG. 49  shows a flow collar  819  adjacent a drive coupling  848  and within the drill head  110 , according to one example of the present disclosure. 
     The flow collar  819  is substantially similar to the flow collar  119 . The flow collar  119  is shown positioned around drive shaft  150 , adjacent the drive coupling  848 . In some examples, the main casing  152  defines a recess  203  in communication with the recess  157  of the end casing  144  when the end casing  144  and the main casing  152  are attached to one another. In some examples, the flow collar  819  is positioned within the recess  203  of the main casing  152 , around the drive shaft  150 . The flow collar  819  aids in preventing axial movement of the drive coupling  848  within the recess  157  of the end casing  144 , yet also permits fluid flow from around the drive shaft  150  to around the drive coupling  848 . 
     The flow collar  819  includes a plurality of peripheral fluid passages  827 . The peripheral fluid passages  827  allow fluid flow from the annular fluid flow path  105  around the drive shaft  150  to an annular fluid flow passage  849  defined between the flow collar  819  and the recess  203  and also between the recess  157  and the drive coupling  848 . Therefore, fluid is not only allowed around the projection  179  within the drive coupling  848  (i.e., coupling lubrication), but fluid flow is also facilitated by the flow collar  819  to flow around the drive coupling  848  within the recess  157 . In some examples, the flow collar  819  is positioned within the recess  157 . In some examples, the flow collar  819  is positioned to move freely within the recess  203 . In other examples, the flow collar  819  is press fit into at least one of the recesses  157 ,  203 . 
     The drive coupling  848  is substantially similar to the drive couplings  148 ,  748  disclosed herein. Accordingly, the drive coupling  848  has a pair of recesses  876 ,  877  at downhole and uphole ends  862 ,  864  that are configured to mate with the drill bit shaft  142  and drive shaft  150 , respectively. In the depicted example, the drive coupling  848  includes a coupling fluid flow passage  861  that includes at least one radial fluid flow passage  888  and an axial fluid flow passage  890 , the radial fluid flow passage  888  extending between an exterior surface  889  and the axial fluid flow passage  890 . 
     The exterior surface  889  of the drive coupling  848  includes portions that have different outer dimensions (e.g., outer diameters) to allow fluid flow around the drive coupling  848  within the recess  157  of the end casing  144 . Specifically, fluid flow is permitted around the exterior surface  889  of the uphole end  864  of the drive coupling  848 . Fluid can travel in and out of the radial fluid flow passage  888  so as to lubricate the recesses  876 ,  877 . Therefore, portions  891  of the exterior surface  889  are dimensioned smaller than the recess  157  of the end casing  144  to allow fluid flow therebetween. However, alignment of the drive coupling  848  within the recess  157  is desired to reduce premature wear. In order to stabilize the drive coupling  848  within the recess  157 , the drive coupling  848  includes balancing features  850  disposed on exterior surface  889  that are configured to aid in stabilizing the drive coupling  848  within the recess  157  of the end casing  144 . However, sufficient space must be maintained between the recess  157  and the drive coupling  848 , because, during a drilling operation, the drive shaft  150  transfers rotation to the bit shaft  142  through the drive coupling  848 , thereby rotating the drive coupling  848 . Because of this, at least at points during the drilling operation, the drive coupling  848  rotates with the drive shaft  150  within, and relative to, the recess  157  in the end casing  144 . 
     The balancing features  850  are dimensioned more closely to the dimension of the recess  157 , and larger than the portions  891 , to permit rotational movement between the drive coupling  848  and the recess  157  but limit substantial relative movement transverse to the end casing axis  158  between the drive coupling  848  and the recess  157 . In some examples, this aids in reducing movement (e.g., wobbling) of drive coupling  848  generally perpendicular to the end casing axis  158 . Such movement can be brought on by bending forces exerted on the drive coupling  858  by the drive shaft  150 , specifically the projection  179  exerting forces within the recess  877 . The bending forces can originate uphole in the inner drill rod  116  of the drill string  102 . Relative movement of the drive coupling  848  within the recess  157  can cause the projection  179  in the recess  877  of the drive coupling to loosen (i.e., “walk”) within the recess  877  of the drive coupling  848 . Such walking can distribute bending forces from the drive shaft  150  differently, thereby causing wear at the drive coupling  848 , the recess  157 , and/or the drill bit shaft  142 . By reducing relative movement of the drive coupling  848  in the recess  157 , the loosening of the connection between the projection  179  of the drive shaft  150  and the recess  877  of the drive coupling  848  is reduced, thereby limiting premature wear. 
     In some examples, the balancing features  850  include uphole balancing features  852  at the uphole end  864  and downhole balancing features  853  at the downhole end  862  of the drive coupling  848 . However, because stabilizing and fluid flow is desired, especially around the uphole end  864 , the uphole balancing features  852  include fluid flow passages  851  to allow fluid flow between uphole end  864  and the recess  157  of the end casing  144 . 
     As shown in  FIG. 49 , the projection  179  of the drive shaft  150  is shown to be positioned within the recess  877  of the drive coupling  848  so that a force inducing portion  860  is aligned with a connection of the end casing  144  and the main casing  152 , traverse to the end casing axis  152 . Such alignment is depicted as plane F. 
       FIG. 50  shows a perspective view of the drive coupling  848 .  FIG. 51  shows a side view of the drive coupling  848 .  FIG. 52  shows a longitudinal cross-section of the drive coupling  848  along line  52 - 52  in  FIG. 51 .  FIG. 53  shows an uphole end view of the drive coupling  848 . As shown, the balancing features  850  are generally disposed on the exterior surface  889  at the downhole end  864  and uphole end  862 . As shown in  FIGS. 49-53 , uphole balancing features  852  include the fluid flow passages  851 . The uphole balancing features  852 , as shown in  FIGS. 49-52 , are generally rectangular projections. However, it is considered within the scope of the present disclosure that the uphole balancing features can be configured in a variety of different ways to achieve stabilization and allow fluid flow therethrough. In other examples, the uphole balancing features  852  can be secured to the exterior surface  889  of the drive coupling  848  by, for example, a fastener (e.g., bolt, adhesive, weld, etc.). 
       FIGS. 54-57  depict a drive coupling  948  with uphole balance features  952  that are partiality spherical in nature.  FIGS. 58-61  depict a drive coupling  1048  with uphole balancing features  1052  in the form of a sleeve  1053  with a plurality of fluid flow passages  1051  disposed therein. Alternatively, as shown in  FIG. 62 , a recess  1157  of an end casing  1144 , which are substantially similar to the recess  157  of the end casing  144  described above, can include a sleeve  1153  disposed therein (i.e., press fit, fastened, or integrally formed with) to act as a balancing feature for a drive coupling positioned within the recess  1157 . In some examples, the sleeve  1153  is substantially similar to the sleeve  1053 . Accordingly, a drive coupling, such as the drive coupling  148  described above, can be positioned within the recess  1157 . 
       FIG. 63  shows a perspective view of the gearbox  124  with a sub saver  300  installed on a front end. The gearbox  124  is configured to drive the drill rod assemblies  106 , specifically the outer drill rods  114  and inner drill rods  116 . In some examples, the sub saver  300  can first be installed onto the inner and outer drive shafts of the gearbox  124 , and then a drill rod assembly  106  can be attached to, and driven by, the sub saver  300  and gearbox  124  assembly. The sub saver  300  is attached at a rear end  302  to a front side  502  of the gearbox  124  and further configured to attach to the outer and inner drill rods  114 ,  116  at a front end  304 . 
       FIGS. 64 and 65  show perspective views of the sub saver  300 . The sub saver  300  includes an inner rod member  306  contained within an outer rod member  308 . The outer rod member  308  is configured to drive the outer drill rod  114  of the drill rod assembly  106 , and the inner rod member  306  is configured to drive the inner drill rod  116  of the drill rod assembly  106 . 
       FIG. 66  shows a longitudinal cross-section of the sub saver  300 . The sub saver  300  includes an inner assembly  301  that is configured to be positioned within, and rotated separately about a longitudinal axis  303  of the sub saver  300  from, the outer rod member  308 . The inner assembly  301  includes the inner rod member  306 , a sub saver coupling  310 , an inner rod adapter  312 , and a sub saver spring  314 . 
     The inner rod adapter  312  is positioned within the sub saver coupling  310  together with the inner rod member  306 . In some examples, both the inner rod adapter  312  and the inner rod member  306  are retained within the coupling using pins  316  positioned in respective grooves  318 ,  320 . Such a pin and groove arrangement is substantially similar to the pin and groove arrangement of the inner rod coupling  118 , inner drill rod  116 , and drive shaft  150  described above. In some examples, the groove  320  of the inner rod member  306  has a width G 2  that is greater than the width of the pins  316 . In some examples, an elongated groove having a width greater than the width of the pins  316  can be defined by the inner rod adapter  312 , instead of the inner rod member  306 . In other examples still, an elongated groove having a width greater than the width of the pins  316  can be defined by cross apertures  332  of the sub saver coupling  310 . 
     In operation, the inner rod adapter  312  and sub saver coupling  310  are slidably attached to the inner rod member  308  so as to be configured to move axially along the longitudinal axis  303  separate from the inner rod member  306 . During such axial movement, the inner rod adapter  312  and sub saver coupling  310  act upon the sub saver spring  314  that is captured between the inner rod member  306  and the sub saver coupling  310 . The sub saver spring  314  biases the sub saver coupling  310  and inner rod adapter  312  to a first position. The first position is a position of the inner rod adapter  312  in which there is no force exerted by the inner rod adapter  312  on the sub saver spring  314  by an inner drill rod  116 . Accordingly, the inner rod adapter  312  can be positioned in any position between the first position and a position where the spring  314  is completely compressed. 
     As noted above, the inner and outer drill rods  116 ,  114  have differing lengths and each drill rod assembly  106  is configured to allow movement of the inner drill rod  116  within the outer drill rod  114 , such movement being limited by the flow collar  119  and the inner rod coupling  118 / 618 . However, this movement results in different relative positioning of the uphole ends  111  of the inner and outer drill rods  116 ,  114  of the most-uphole drill rod assembly  106 . For example, in some situations, the outer rod interfacing surface  163  of inner rod coupling  118 / 618  is spaced away from the uphole shoulder  117   a  of the outer drill rod  114 , and in other examples, the outer rod interfacing surface  163  of inner rod coupling  118 / 618  is contacting the uphole shoulder  117   a  of the outer drill rod  114 . Therefore, to accommodate this relative positioning, the sub saver  300  includes the sub saver spring  314  that allows the sub saver  300  to attach to both the inner and outer drill rods  116 ,  114  of the drill rod assembly  106  regardless of their relative positioning. Further, this relative movement aids in preventing damage to drill rod assembly  106 , specifically the inner drill rod  116  and the inner rod coupling  118 / 618 . 
     Similar to each drill rod assembly  106 , in some examples, the sub saver  300  includes an inner flow path  307  and an annular flow path  305 . The inner flow path  307  is disposed along the axis  303  of the sub saver  300  within the inner assembly  301 . The annular flow path  305  is configured to be disposed between the inner assembly  301  and the outer rod member  308 . In some examples, the sub saver  300  can just include an annular flow path  305  and no inner flow path  307 . 
       FIG. 67  shows a perspective view of the inner assembly  301  of the sub saver  300 , and  FIG. 68  shows an exploded view of the sub saver  300 . 
     The inner rod member  306  is configured to be attached to an inner drill rod drive shaft assembly  510  of the gearbox  124 . The inner rod member  306  includes an axial fluid flow passage  322 , a radial fluid flow passage  324 , a torque-carrying portion  326 , the groove  320 , and a non-carrying torque portion  328 . 
     The axial fluid flow passage  322  is configured to allow fluid flow along the axis  303  of the sub saver  300 . Further, the axial fluid flow passage  322  can receive fluid from the gearbox  124  and transfer fluid out of the radial fluid passage  324  to the annular fluid flow passage  305  of the sub saver  300 . 
     The inner rod member  306  can include torque transferring features (i.e., the torque-carrying portion  326  and groove  320 ), in addition to the non-torque-carrying portion  328 , that are substantially similar to the features of the inner rod coupling  118 . Specifically, the inner rod member  306  can have a polygonal cross-section at the torque-carrying section  326  that is configured to mate with, and be coupled with, the sub saver coupling  310 . The torque-carrying section  326  can be of any cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming (e.g., lobes, flat faces, curved faces, etc.). As mentioned above, in some examples, the groove  320  of the inner rod member  306  can have a width G 2  that is greater than a width of the pin(s)  316 . This allows the sub saver coupling  310  to move axially with respect to the inner rod member  306 . The movement of the sub saver coupling  310  with respect to the inner rod member  306  is limited by radial walls  319  of the groove  320 . Depending on the axial movement desired, the groove  320  can have a range of widths G 2 . During movement, the pins  316  slide within the groove  320  while a portion of an inner bore  330  of the sub saver coupling  310  slides freely over the torque-carrying section  326 . This allows for a non-binding telescopic connection that can account for relative positioning of the inner and out rods  116 ,  114  and, due to the configuration of the inner bore  330  of the sub saver coupling  310  and torque-carrying section  326 , simultaneously transfer torque. 
     The sub saver coupling  310  includes the inner bore  330  that is configured to mate with the torque-carrying section  326  of the inner rod member  306  and with the inner rod adapter  312 . The sub saver coupling  310  includes a plurality of cross apertures  332 , similar to the apertures  135  of the inner rod coupling  118 , that are configured to receive the pins  316 . Each cross aperture  332  is sized and configured to retain each pin  316  so as to retain the inner rod adapter  312  and inner rod member  306  within the inner bore  330  of the sub saver coupling  310 . 
     The inner rod adapter  312  is configured to interface with an inner rod coupling  118  located on an uphole end  111  of a drill rod assembly  106 . Accordingly, the inner rod adapter  312  can have a polygonal cross-section at a first section  334  that mates with the inner bore  133  of the inner rod coupling  118 . Further, the inner rod adapter  312  can include a second section  336  that includes a torque-carrying portion  338 , the groove  318 , and a non-torque-carrying portion  340  that are substantially similar to the features of the inner rod coupling  118 . The second section  336  is configured to be retained within the sub saver coupling  310  by at least one pin  316  that captures the groove  318  of the inner rod adapter  312 . The inner rod adapter  312  can also include an inner flow path  342  so as to provide fluid flow to the drill string  102 . Further, in some examples, the inner rod adapter  312  can be replaced separately from the entire inner assembly  301 . 
     The sub saver spring  314  is configured to interface with the sub saver coupling  310  and be positioned around a portion of the inner rod member  306 . Specifically, the sub saver spring  314  is configured to surround a portion of the torque-carrying portion  326  of the inner rod member  306  and be captured between a sub saver coupling face  311  and an inner rod member face  313 . 
       FIG. 69  shows a side view of the inner assembly  301  of the sub saver  300 . 
       FIG. 70  shows a cross-section of the inner rod adapter  312  taken along line  70 - 70  in  FIG. 69 . In the depicted example, the first section  334  of the inner rod adapter  312  has a hexagonal cross-section. However, in other examples, the first section  334  can have a variety of different cross-section shapes. 
     As noted above, the inner rod adapter  312  is configured to mate with the inner bore  133  of the inner rod coupling  118 . Specifically, the first section  334  is configured to slidably mate with the inner bore  133  of the inner rod coupling  118 . Because this connection is made by mechanically moving the sub saver  300  into engagement with the inner rod coupling  118  of the drill rod assembly  106 , it is advantageous for the first section  334  of the inner rod adapter  312  to be properly mated within the inner bore  133  of the inner rod coupling  118  to prevent potential damage to the inner rod coupling  118  and inner rod adapter  312 . To promote this alignment, the first section  334  of the inner rod adapter  312  includes a plurality of faces  335  that are arranged in a polygonal pattern that match the shape of the inner bore  133 . In some examples, the faces  335  are flat. In other examples, the faces  335  are rounded. Due to the configuration of the faces  335 , the faces  335  facilitate torque transfer while minimizing the chance of misalignment within the inner rod coupling  118  by allowing for a sliding connection with the inner bore  133  of the inner rod coupling  118 . The faces  355  result in a simplified construction that is resistant to damage. For example, even if the faces  335  are partially deformed (i.e., by accident, by wear, etc.) proper alignment with the inner bore  133  of the inner rod coupling  118  can still be possible. This is not the case with a more complicated cross-sectional profile where damage to such a profile can result in the inability to mate with a drill rod assembly or result in a jammed connection between the inner rod coupling and the sub saver that can cause damage to the drill rod assembly and/or a sub saver. 
     Further aiding in aligning the inner rod adapter  312  with the inner bore  133  of the inner rod coupling  118 , the inner rod adapter  312  is configured to be spring loaded by way of the sub saver spring  314 . Therefore, during engagement, even if the inner rod adapter  312  is misaligned with the inner bore  133  of the inner rod coupling  118 , the sub saver spring  314  and the non-binding telescopic movement between the sub saver coupling  310  and the torque-carrying portion  326  of the inner rod member  306  prevents the inner rod adapter  312  from forcibly engaging with the inner rod coupling  118 , which could potentially lead to damage of the inner rod coupling  118  and the inner rod adapter  312  of the sub saver  300 . Therefore, in some examples, the sub saver spring  314  allows the inner rod adapter  118  to self-align and slidably engage with inner rod adapter  312 . 
     In some examples, at least portions of the faces  335  of the inner rod adapter  312  are heat treated to discourage wear and accidental damage. Further, in other examples still, the inner rod adapter can include a sliding feature (not shown) to promote a telescopic connection. Such a sliding feature can include a coating, treatment, or other material that promotes a low friction connection disposed on the faces  335  of the inner rod adapter  312 . 
       FIG. 71  shows a cross-section of the inner rod adapter  312  and the sub saver coupling  310  taken along line  71 - 71  in  FIG. 69 . The torque-carrying portion  338  is shown to be mated with the inner bore  330  of the sub saver coupling  310 . Such mating allows torque to be transferred from the sub saver coupling  310  to the inner rod adapter  312 . The torque-carrying portion  338  can form any cross-sectional profile that is configured to transfer torque while minimizing friction and the potential for jamming (e.g., lobes, flat faces, curved faces, etc.). 
       FIG. 72  shows a cross-section of the inner rod adapter  312  and the sub saver coupling  310  taken along line  72 - 72  in  FIG. 69 . As shown, the non-torque-carrying portion  340  does not engage the inner bore  330  of the sub saver coupling  310 . 
       FIG. 73  shows a cross-section of the inner rod member  306  and the sub saver coupling  310  taken along line  73 - 73  in  FIG. 69 . Similar to the non-torque-carrying portion  340  of the inner rod adapter  312 , the non-torque-carrying portion  328  of the inner rod member  306  does not engage with the inner bore  330  of the sub saver coupling  310 . 
       FIG. 74  shows a cross-section of the inner rod member  306  and the sub saver coupling  310  taken along line  74 - 74  in  FIG. 69 . Similar to the torque-carrying portion  338  of the inner rod adapter  312 , the torque-carrying portion  326  is shown to be mated with the inner bore  330  of the sub saver coupling  310 . Such mating allows torque to be transferred from the inner rod member  306  to the sub saver coupling  310 . In the depicted example, the torque-carrying portion  326  of the inner rod member  306  has a polygonal cross section. In other examples, the torque-carrying portion  326  of the inner rod member  306  has a hexagonal cross-section. However, in other examples still, the torque-carrying portion  326  can have a variety of different cross-section shapes. 
     Like the inner rod adapter  312 , the inner rod member  306 , specifically the torque-carrying portion  326 , has a configuration to facilitate the telescopic connection between the sub saver coupling  310  and the torque carrying portion  326  of the inner rod member  306 . Such movement occurs when the inner rod adapter  312  and the sub saver coupling  310  axially move with respect to the inner rod member  306 . While the pins  316  of the sub saver coupling  310  are configured to be positioned within, and movable along, the groove  320 , the inner bore  330  of the sub saver coupling  310  slides over the torque-carrying portion  326 . Specifically, the torque carrying section  326  includes a plurality of faces  327  that are configured to slide smoothly within the inner bore  330  of the inner rod coupling  310 . In some examples, the faces  327  are flat. In other examples, the faces  327  are rounded. Due to the configuration of the faces  327 , jamming or binding between the inner bore  330  and the torque-carrying portion  326  is minimized. By not binding or jamming, it ensures that the inner rod adapter  312  and sub saver coupling  310  can freely move with respect to the inner rod member  306  when needed. If the connection between the inner rod member  306  and the sub saver coupling  310  were configured in such a way to allow periodic jamming (e.g., a cross-section having a more complicated profile such as a spline), there is a chance that the connection with the inner rod adapter  312  and the inner coupling  118  of a drill rod assembly may be misaligned. Such misalignment could damage the inner rod coupling  118 , inner rod adapter  312 , and/or portions of the drill rod assembly  106 . However, by configuring the inner rod adapter  312  and the inner rod member  306  with torque-carrying portions  338 ,  326  that are resistant to jamming or binding, the chance of misalignment and subsequent damage to the components is reduced. 
     In some examples, at least portions of the faces  327  of inner rod member  306  are heat treated to discourage wear and accidental damage. Further, in other examples still, the inner bore  330  of the sub saver coupling  310  and/or the torque carrying section  326  can include a sliding feature (not shown) to promote a telescopic connection. Such a sliding feature can include a coating, treatment, or other material that promotes a low friction connection disposed on or between the sub saver coupling  310  and/or the torque carrying section  326 . 
       FIG. 75  shows a longitudinal cross section of a sub saver  400  according to one embodiment of the present disclosure.  FIG. 76  shows an exploded view of the sub saver  400 . 
     The sub saver  400  operates in a substantially similar way to the sub saver  300  in that the sub saver  400  is configured to accommodate a range of relative positions between the outer and inner drill rods  114 ,  116  of the drill rod assembly  106  using a sub saver spring  401 . The sub saver  400  is attached at a rear end  402  to the front side  502  of the gearbox  124  and configured to attach to inner and outer drill rods  116 ,  114  at a front end  404  of the sub saver  400 . The sub saver  400  includes an inner rod member  406 , an outer rod member  408 , a sub saver coupling  410 , and an inner rod adapter  412 , all of which are substantially similar the components described above with respect to the sub saver  300 . 
     However, in the sub saver  400 , the sub saver spring  401  is positioned between and within the inner rod adapter  412  and the inner rod member  406 . Such positioning allows for the spring-loaded relative movement of the inner rod adapter  412  with respect to the inner rod member  406  so that the inner rod adapter is biased to a first position. The first position is a position of the inner rod adapter  412  in which there is no force exerted by the inner rod adapter  412  on the sub saver spring  401  by an inner drill rod  116 . When a force is received by the inner rod adapter, the inner rod adapter  414  can compress the spring  401  as needed to accommodate the relative positioning of the outer and inner rods  114 ,  116  of the drill rod assembly  106 . Accordingly, the inner rod adapter  412  can be positioned in any position between the first position and a position where the spring  401  is completely compressed. 
     The inner rod adapter  412  is slidably mated within the sub saver coupling  410  while the inner rod member  406  is fixedly mounted to the inner rod coupling  410 . To accommodate differing relative positioning of the outer and inner rods  114 ,  116 , the inner rod adapter  412  can slide within a recess  414  defined within the sub saver coupling  410 . The inner rod adapter  412  can be retained within the recess  414  using a variety of different methods. In one example, the inner rod adapter  412  can be retained within the recess  414  using a retainer ring  416 . In other examples, the inner rod adapter  412  can be retained within the recess  414  using a single pin, or a plurality of pins (not shown). 
       FIG. 77  is a perspective view of the gearbox  124 , and  FIG. 78  shows a side view of the gearbox  124 . As described above, the gearbox  124  is positioned on the rack  126  and configured to engage and rotate each drill rod assembly  106  about their respective longitudinal axis and further couple each drill rod assembly  106  with an immediately preceding downhole drill rod assembly  106 . 
     When driving drilling rod assemblies into the ground, the gearbox  124  is configured to travel toward the break out mechanism  128  while pushing the drill rod assemblies  106  into the ground. Simultaneously, the gearbox  124  is configured to selectively drive (i.e., rotate) both the outer and inner drill rods  114 ,  116  of the drill rod assembly  106 . 
     When pulling drill rod assemblies  106  from the ground, the gearbox  124  is configured to move on the rack  126  away from the break out mechanism  128  while simultaneous selectively rotating the outer and inner rods  114 ,  116  of the drill rod assemblies  106 . 
     The gearbox includes a front  502 , a rear  504 , a housing  505 , at least one outer drill rod drive motor  506 , an inner drill rod drive motor  508 , an inner drill rod drive shaft assembly  510  (i.e., an inner rod drive shaft) and an outer drill rod drive shaft assembly  512  (i.e., an outer rod drive shaft). Further, the gearbox  124  includes attachment features  511  that are configured to mount the gearbox  124  to the rack  126 . 
     The gearbox  124  is configured to drive (i.e., rotate) the drill rod assemblies  106  at the front end  502  of the gearbox  124 , and is also configured to receive drilling fluid via a fluid swivel  514  at the rear  504  of the gearbox  124 , which will be described in more detail below. 
     The outer and inner drill rod drive motors  506 ,  508  can be hydraulic motors that are configured to be operated using an on-board hydraulic system (not shown) of the drilling machine  104 . In some examples, the gearbox  124  utilizes two outer drill rod drive motors  506   a ,  506   b  and a single inner drill rod drive motor  508 . 
     The outer drill rod drive motors  506 , together, are configured to drive the rotation of the outer drill rod drive shaft assembly  512 , thereby driving the outer drill rod  114  of the drill rod assembly  106 , and thereby driving all coupled outer drill rods of the drill string  102 . 
     The inner drill rod drive motor  508  is configured to drive the rotation of the inner drill rod drive shaft assembly  510 , thereby driving the inner drill rod  116  of a drill rod assembly  106 , and thereby driving all of the coupled inner drill rods  116  of the drill string  102 . Further, in some examples, the inner drill rods  116  are connected to the drive shaft  150  of the drill head  110  and, therefore, the inner drill rod drive motor  508  is configured to drive the rotation of the drill bit shaft  142  and the drill bit  140 . 
     In some examples, the gearbox  124  is configured so that no relative axial movement between the inner drill rod drive shaft assembly  510  and the outer drill rod drive shaft assembly  510  is allowed. 
       FIG. 79  shows a front view of the gearbox  124 , and  FIG. 80  shows a cross-section of the gearbox  124  along line  80 - 80  of  FIG. 79 . 
     The outer drill rod drive motors  506  are configured to drive a pair of gears  516  and  518 . These components are configured to provide rotational drive torque to the outer drill rod drive shaft assembly  512 . Specifically, power is transferred from the motors  508 , to the gear  516 , to the gear  518 , to an outer drill rod head shaft  520 , and then to an outer drill rod drive chuck  522 . 
     The outer drill rod head shaft  520  is configured to be substantially contained and supported within the housing  505  of the gearbox  124 . Specifically, the outer drill rod head shaft  520  is configured to be in communication with a gearbox lubricating fluid (e.g., oil) contained within an internal cavity  521  of the housing  505 . Further, a pair of bearings  524  are configured to support the outer drill rod head shaft  520  within the housing  505 . 
     The outer drill rod drive chuck  522  is configured to be removably coupled to the outer drill rod head shaft  520  at the front end  502  of the gearbox  124 . The outer drill rod drive chuck  522  is further configured to couple to the end of an outer member of the drill string  102 . In some examples, the outer drill rod drive chuck  522  is coupled to the outer drill rod head shaft  520  by a plurality of fastener  523 . In some examples, the outer drill rod drive chuck  522  is configured to be further coupled directly to an outer drill rod  114  of a drill rod assembly  106 . In other examples still, the outer drill rod drive chuck  522  is configured to be threaded directly to an outer rod member  308 / 408  of the sub saver  300 / 400 . 
     The inner drill rod drive motor  508  is positioned at the rear  504  of the gearbox  124 . The inner drill rod drive motor  508  is configured to directly provide rotational drive torque to the inner drill rod drive shaft assembly  510 . Specifically, power is transferred from the inner drill rod drive motor  508  to an inner drill rod head shaft  526  and then to an inner member of the drill string  102 . In some examples, the inner drill rod head shaft  526  is configured to be coupled to an inner rod member  306 / 406  of the sub saver  300 / 400 . In other examples, the inner drill rod head shaft  526  can be directly coupled to an inner drill rod  116  of a drill rod assembly  106 . 
     In some examples, the inner drill rod head shaft  526  can be supported within the housing  505  by a pair of bearings  528 . Further, like the outer drill rod head shaft  520 , the inner drill rod head shaft  526  is configured to be in communication with a gearbox lubricating fluid (e.g., oil) contained within the internal cavity  521  of the housing  505 . 
     The inner drill rod drive motor  508  also includes an axial drilling fluid passage  529  that is generally axially aligned with the inner drill rod head shaft  526 . The axial drilling fluid passage  529  is defined by the motor  508  and configured to receive drilling fluid at a first end  530  from a drilling fluid source (not shown) via the fluid swivel  514 . The axial drilling fluid passage  529  then delivers the drilling fluid to the inner drill rod head shaft  526  at a second end  532  of the axial drilling fluid passage  529 . Specifically, the inner drill rod head shaft  526  receives the drilling fluid at a head shaft axial drilling fluid passage  534  that is isolated from the inner cavity  521  of the housing  505 . The inner drill rod head shaft  526  then delivers the drilling fluid to the inner drill rod of the drill string  102 . In some examples, drilling fluid is delivered from the inner drill rod head shaft  526  to the inner flow path  307  of the sub saver  300 . In some examples, the drilling fluid is delivered from the inner drill rod head shaft  526  to the axial fluid flow passage  322  of the inner rod member  306  of the sub saver  300 . 
     The fluid swivel  514  is configured to deliver drilling fluid to the axial drilling fluid passage  529  of the inner drill rod drive motor  508 . In some examples, the fluid swivel  514  can be connected to a drilling fluid pump (not shown) which is connected to a drilling fluid reservoir (not shown). In some examples, the fluid swivel  514  is configured to freely rotate about an axis  536  so as to accommodate the movement of the gearbox  124 . In some examples, the fluid swivel can be removably installed to the inner drill rod drive motor  508 . 
       FIG. 81  shows a zoomed-in view of the front  502  of the gearbox  124  of the longitudinal cross-section section in  FIG. 80 . The gearbox  124  further includes a drilling fluid seal  538 , an oil seal  540 , a weep cavity  542 , and at least one weep indicator  544 . 
     In order to prevent drilling fluid contained within the drill string  102  from entering back into the gearbox  124 , specifically the cavity  521 , the gearbox  124  includes the drilling fluid seal  538  that is positioned between the inner drill rod drive shaft assembly  510  and the outer drill rod drive shaft assembly  512 . Specifically, the drilling fluid seal  538  is positioned between the inner drill rod head shaft  526  and the outer drill rod drive chuck  522 . The fluid seal  538  can be a variety of different types of seals. In one example, the seal  538  is a ceramic seal. In some examples, the drilling fluid seal can be positioned between the inner drill rod drive shaft assembly  510  and the outer drill rod drive shaft assembly  512  where it can be easily accessed for maintenance. As shown, to access the seal  538 , an operator must only remove the outer drill rod drive chuck  522 . 
     Conversely, in order to prevent oil from entering into the drill string from the cavity  521  of the housing  505  of the gearbox  124 , the gearbox  124  includes the oil seal  540  positioned within the housing  505 , between the inner drill rod drive shaft assembly  510  and the outer drill rod drive shaft assembly  512 . Specifically, the oil seal  540  is positioned between the outer drill rod head shaft  520  and the inner drill rod head shaft  526 . Therefore, in some examples, the oil seal  540  is positioned closer the rear  504  of the gearbox  124 . Such positioning of the oil seal  540  allows the outer drill rod drive chuck  522  to be removed from the outer drill rod head shaft  520  without having to drain the oil from the cavity  521 . This arrangement eases maintenance. 
     The gearbox  124  further defines the weep cavity  542 . The weep cavity  542  is defined between the inner drill rod drive shaft assembly  510 , the outer drill rod drive shaft assembly  512 , the drilling fluid seal  538 , and the oil seal  540 . During normal proper operation, the weep cavity  542  contains no oil and no drilling fluid, thanks to the oil seal  540  and the drilling fluid seal  538 . However, if either the oil seal  540  or the drilling fluid seal  538  malfunctions, the weep cavity  542  is configured to receive any fluid that escapes either seal  540 ,  538 . 
     In some examples, the weep indicator  544  is configured to indicate when fluid is present within the weep cavity  542 . In some examples, the weep indicator  544  is a sensor disposed within the weep cavity  542 . In other examples still, the weep indicator  544  is a passage defined in the outer drill rod drive shaft assembly  512 . Further, in some examples, the weep cavity  542  can be vented to atmospheric pressure by way of the at least one weep indicator  544 . Because drilling fluid within the housing  505  of the gearbox  124  can damage components quickly and oil within the drill string  102  is not preferred, the weep cavity  542  and weep indicator  544  allow for an indication of such a malfunction so that the operator can cease operation before damage is done to the components of the drilling system  100 . 
       FIG. 82  shows a side view of the gearbox  124  with the outer drill rod drive chuck  522  removed. In the depicted example, once the outer drill rod drive chuck  522  is removed, the drilling fluid seal  538  remains positioned around the inner drill rod head shaft  526 . In some examples, the drilling fluid seal  538  separates into two halves, one that attaches to the inner drill rod head shaft  526  and one that attaches to the outer drill rod drive chuck  522 . 
       FIG. 83  shows a cross-section of the outer drill rod drive chuck  522  taken along line  83 - 83  in  FIG. 82 . In the depicted example, the outer drill rod drive chuck  522  includes a plurality of weep indicators  544 . As shown, the weep indicators  544  are radial weep passages positioned around a periphery of the outer drill rod drive chuck  522 . The weep passages  544  allow for any leaked fluid (e.g., oil or drilling fluid) that enters the weep cavity  542  to escape the weep cavity  542 , thereby providing a visual indication to the operator that a malfunction has occurred. In other examples, the weep indicators  544  can be disposed in the outer drill rod head shaft  520  in addition to, or in replacement of, the outer drill rod drive chuck  522 . 
     The process of driving the drill rod assemblies  106  into the ground requires control of the gearbox  124  to perform a number of steps. In one example, some of these steps are performed automatically by the controller  550  (shown in  FIG. 2 ), while in other examples, all of these steps are performed automatically by the controller  550 . 
     First, when the gearbox  124  has reached its most downhole position on the rack  126 , the break out mechanism  128  clamps the drill string  102 , and the gearbox  124  can uncouple to move back uphole along the rack  126 . The step of uncoupling requires the outer drill rod drive shaft assembly  512  to rotate in a reverse direction as it unthreads from the outer rod  114  of the drill string  102 , while at the same time the gearbox  124  has to move uphole on the rack  126  to separate from the drill string  102 . During this process, the inner drill rod drive shaft assembly  510  simultaneously slides out of engagement with the inner rod  116  of the drill string  102 . In one example of this step, the controller  550  automatically applies oscillating, relatively low torque to the inner drill rod drive shaft assembly  510 , specifically the inner rod head shaft  526 , whenever the break out mechanism  128  is clamped onto the drill string  106 , and the control signal (e.g. generated from the controller  550  via the controls  552  or automatically generated from the controller  550 ) for the outer drill rod drive shaft assembly  512  is operated to rotate in a reverse direction, or the control signal (e.g. generated from the controller  550  via the controls  552  or automatically generated from the controller  550 ) to move the gearbox  124  along the rack  126  is operated to move uphole. In one example, the oscillating torque is limited to a maximum of 150 ft-lbs. 
     Once the gearbox  124  has reached its most uphole position on the rack  126 , a singular drill rod assembly  106  is positioned (e.g., by a rod loader assembly mechanism, not shown) into alignment with the drill string  102  and the gearbox  124 . The gearbox  124  is then moved downhole and into engagement with the singular drill rod  106 , including coupling of the outer drill rod drive shaft assembly  512  and the outer rod  114  and simultaneous coupling of the inner drill rod drive shaft assembly  510  and the inner rod  116 . In one example of this step, the controller  550  automatically applies an oscillating, relatively low torque to the inner drill rod drive shaft assembly  510 , specifically the inner rod head shaft  526 , whenever the break out mechanism  128  is clamped onto the drill string  102 , and the control signal (e.g. generated from the controller  550  via the controls  552  or automatically generated from the controller  550 ) for the outer drill rod drive shaft assembly  512  is operated to rotate in a forward direction, or the control signal (e.g. generated from the controller  550  via the controls  552  or automatically generated from the controller  550 ) to move the gearbox  124  along the rack  126  is operated to move downhole. The controller  550  may also include closed loop control wherein the movement of the inner drill rod drive shaft assembly  510  is measured to ensure that the inner drill rod drive shaft assembly  510 , specifically the inner rod head shaft  526 , oscillates through a total angle range of 120 degrees, plus or minus 60 degrees, during this step. In one example, the oscillating torque is limited to a maximum of 150 ft-lbs. 
     Once the gearbox  124  is coupled to the singular rod  106 , the gearbox  124  continues to move downhole on the rack  126  pushing the singular rod  106  into engagement with the drill string  102 . Engaging the singular rod  106  with the drill string  102  requires the outer rods  116  to thread together while the inner rods  114  couple simultaneously. In one example of this step, the controller  550  automatically applies an oscillating, relatively low torque to the inner drill rod drive shaft assembly  510 , specifically the inner rod head shaft  526 , whenever the break out mechanism  128  is clamped onto the drill string  102 , and the control signal (e.g. generated from the controller  550  via the controls  552  or automatically generated from the controller  550 ) for outer drill rod drive shaft assembly  512  is operated to rotate in a forward direction, or the control signal (e.g. generated from the controller  550  via the controls  552  or automatically generated from the controller  550 ) to move the gearbox  124  along the rack  126  is operated to move downhole. The controller  550  may also include closed loop control wherein the movement of the inner drill rod drive shaft assembly  510 , specifically the inner rod head shaft  526 , is measured to insure that the inner rod head shaft  526  oscillates through a total angle of 120 degrees, plus or minus 60 degrees, during this step. In one example, the oscillating torque is limited to a maximum of 150 ft-lbs. 
     The various embodiments described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the following claims.