0 92 0035 fig2 b also illustrates by the broken line a conical bore option option a for the nipple inner bore configuration which is preferred fig2 a shows the hollow rod in the union zone with an outside diameter devu and an internal diameter of the rod at the extreme surfaces of the first and second ends corresponding to the end of the thread difr it also shows the outside diameter of the hollow rod dev labeled as devu dev because there is no upset end acting as the union the ratio of the maximum external diameter devu either of a separate connector element or the upset type end of integral connector element union to the external diameter of the rod dev as illustrated at fig3 a 7a and 9 a is maintained within the following range hence for a maximum fixed diameter the mean polar momentum of the hollow rod and connector string is greater than that for a solid pump rod of equal cross section diameter transmitted rotation moment or torque is therefore greater in a hollow rod column than in a solid rod column this is also a determining factor in the resistance to the u201c backspin u201d phenomenon or counter rotation of the rod string additionally the ratio between the starting diameter of the torque shoulder on the connecting element dht and the internal diameter of the hollow rod at the thread free end difr is maintained as follows 0037 fig3 a gives further references at the assembly in which the ratio of the maximum diameter of the union devu to the diameter.