by means of a transmission mechanism such as a set of pinion gears not shown to ensure an automatic balancing of the gripping arm 24 by the counterweight 26 whether on off load or carrying a load and whether or not an external force is applied the back gear motor 42 controlling the orientation of the counterweight 26 around the axis bb is sensitive to a signal delivered by means 44 able to detect a slanting or tilting of the entire machine around an axis ring 30 these detection means 44 may be embodied in various ways in particular these means may be sensitive to a slanting of the support 22 with respect to vertical so as to continuously bring back the axis aa of the support to vertical by acting on the back gear motor 42 controlling the pivoting of the counterweight 26 around the axis bb thus the detection means 44 may include in one of the branches of the lower inverted 34 u 34 shaped section of the support 22 a pendulum type device comprising a suspended weight used to determine what is strictly vertical and a potentiometer sensitive to the relative angular position of the suspended weight with respect to the axis aa this potentiometer produces a signal representative of the slanting of.