tapered transversely to the longitudinal axis of the interior spline seats 24 to define the interior splines 25 the latter of which correspond to the asymmetrical exterior spline slots 20 respectively in a preferred embodiment of the invention the angle 34 a 34 defined by the angular driven face 19c and a flat edge of an interior spline seat 24 is in the range of from about 80 to about 90 degrees and preferably about 85 degrees accordingly the drive segments 17 will nest stack and interlock and yet articulate transversely with respect to each other in driving relationship to shape the drive string 10 as illustrated in fig1 with the asymmetrical exterior splines 19 of one segment registering with the interior spline seats 24 of an adjacent segment this interlocking registration is not rigid but permits lateral or transverse movement of the drive segments 17 in the interlocking and nested configuration such that the drive segments 17 can easily bend and articulate to conform to the bend illustrated in fig1 and yet maintain an interlocking driving relationship of high integrity due to the matched drive faces 19a and driven faces 19c which articulation also serves to dampen the eccentric.