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206     Cha pte r  F i v e




                                             “90° bend radius”



                       Rod or pipe



                       “Radius of curvature”
                             or
                         “Bend radius”
                                           “Degrees per rod”
          FIGURE 5.14  Drill rod or product pipe curvature terminology. (Source:
          Outside Plant Consulting Services.)




          may be able to withstand a single bend cycle corresponding to a rela-
          tively sharp radius of curvature, but the rotation of the rod during the
          boring operation results in flexural cycles, which may eventually
          cause cumulative fatigue failure. The diameter of the drill rod is an
          important parameter affecting its stiffness, steering capability, and
          the allowable bend radii.
             There are several terms that may be used to describe the bending
          capability of a drill rod, including the “radius of curvature,” (bend
          radius) “90° bend radius,” and “angular change per rod,” as illustrated
          in Fig. 5.14. The significance of the radius of curvature dimension, and
          the 90º bend radius terms are often a source of confusion in the Mini-
          HDD industry. Whereas engineers are familiar with the radius of cur-
          vature (or bend radius) dimension, construction personnel often use
          the 90º bend radius dimension, equal to the distance along a 90° portion
          (quadrant) of the perimeter of the circular path, since the latter term
          may be more readily identified or measured during a boring operation,
          for a bend in a horizontal plane. Similarly, the maximum angular
          change per rod is a practical measure of the rod behavior. It is impor-
          tant that the construction personnel understand the significance of the
          terms in relation to drill rod specifications to avoid overstressing the
          rod or, conversely, underutilizing its capability.
             The following relationships are applicable:
          90º bend radius (ft) = 90 × rod length (ft)/angular change (deg/rod)
                                                               (5.2a)

          radius of curvature (ft) = 90º bend radius (ft)/1.57  (5.2b)
          radius of curvature (ft) = 57.3 × rod length (ft)/angular change (deg/rod)
                                                               (5.2c)
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