plate 18 against the flange of a sleeve 19 dowelled to the shaft 14 the shaft 14 is rotatably mounted by way of bearings 20 in a sleeve 21 provided with an external collar for fixing to the frame 13 to the upper gear wheel 9 is also fixed a gear wheel or sprocket 22 which will be described hereinafter the two chains 8 are connected together at their vertically aligned points of articulation by pivots 23 fig2 which are spaced along the chains on each of the pivots 23 is disposed the sleeve 24a with which a support member 24 is rigid forming two vertically aligned seats 24b in which the tubular stem 25 is inserted the stem 25 is slidable vertically in the two seats 24b and is prevented from rotating by an appendix 26a of a clamp 26 locked approximately at the centre of the stem the appendix being guided in an elongated slot 24c the stem is forced upwards by a spring 27 interposed between the lower seat 24b and clamp 26 below this seat an element 28 made of elastic material and shaped in the form of a sucker is fixed to the tubular stem 25 above the upper seat.