44 of the slide 37 and due to it being impossible for the slide to continue its downward movement it is the assembly of the chassis which is displaced vertically upwards in the direction of the arrow 84 of fig1 thus bringing about the raising of the means for rotationally driving the chassis namely the caterpillar tracks 32 with their drive wheels 33 and their return wheels 34 at the end of this movement and as shown by fig1 and 13 the means for driving in translation are raised from the ground by a value t which is between 5 and 10 mm fig1 shows that during this elevation movement of the chassis the plate 42 comes into contact with the nut 66 of the rod 63 and thus exerts a traction movement on this rod in the direction of the arrow 84 via the intermediate bearing lever 59 this movement is transmitted in the opposite direction that is to say in the direction of the arrow 83 and with multiplication to the finger 54 the result thereof is that the centerer 9 of this finger comes into contact with the precentering recess 77 which due to its arrangement coinciding with the theoretical vertical is off centered with respect to the actual vertical axis of the slide and consequently with that of the chassis.