each comprise a respective tubular body 52 fitted to shaft 49 and having a cylindrical outer surface 53 the axis 54 of which forms a given angle 55 with axis 51 each jaw 47 also comprises a ball bearing 56 having an inner ring fitted to surface 53 of tubular body 52 and a bar 57 in turn comprising a central hub 58 fitted to an outer ring of bearing 56 and two opposite arms 59 extending radially outwards from hub 58 the two arms 59 are fitted respectively on the free ends with a known sealing device 60 and a tappet roller 61 fitted in rotary manner to respective arm 59 and engaging a track 62 which is formed on relative drum 39 and is parallel to axis 51 of shaft 49 and crosswise to path p when shaft 49 oscillates about axis 51 bar 57 would therefore also oscillate about axis 51 if it were not forced by track 62 and by angle 55 formed between axes 51 and 54 to oscillate about an axis 63 extending perpendicular to the fig3 plane at the intersection of axes 51 and 54 transverse jaw 48 comprises a crank 64 fitted to shaft 49 between tubular bodies 52 of the two lateral jaws 47 and connected to a rocker arm 65 fitted to a shaft 66 the opposite ends of which are mounted for rotation through the two drums 39 to enable shaft 66 to rotate with respect to the two drums 39 and by virtue of said cam device 50 about an axis.