dynamometric measuring unit represented by 38 comprises a hollow shaft 40 which can be either an extension of the principal shaft 20 or in a preferred embodiment a subassembly intended to be mounted on the principal shaft 20 by a threaded part not shown and arranged adjacent to the output of the motor 18 the hollow shaft 40 comprises at its lower end a threaded male part 42 intended to interact with a corresponding female threaded part at the upper end of the string of pipe not shown the dynamometric measuring unit 38 comprises a series of sensors solidly mounted on its outer surface and forming a measuring unit in the example illustrated the unit comprises two traction gauges 44 and 46 a pair of torsion gauges 48 50 a temperature gauge 52 a pair of longitudinal accelerometers 54 56 and three transverse accelerometers 58 60 62 moreover the unit is equipped with a pressure sensor 64 the data generated by the sensors are processed by an electronic conditioning circuit 66 which permits a digital multiplexing of the signals the electronic circuit is linked to a pair of annular tracks 68 70 arranged around the shaft 40 a brush holder assembly 72 mounted so as to rotate with respect to the shaft 40 is intended to pick up the signals from the annular tracks 68 70 this assembly 72 is linked to a connector 74 arranged on a flange 76 which is also mounted so as to rotate freely on the shaft 40 these elements.