head each threaded roller 3 and 3 39 is provided with needle bearing 14 mounted on fixed axle 15 which in turn is supported in the fork of the assigned roller head arm in this manner the roller 3 is freely rotatable about the axle 15 the unbalancing weight 12 which extends radially is provided on its outer surface with a cam 16 which cooperates with a spring loaded ball 17 arranged together with its biasing spring 18 in a bore of the assigned fork the ball 17 is located at the upper dead center point of the roller 3 and prevents the latter from remaining in this undesired upper position as indicated in fig4 resilient disks for example of rubber or cup springs of metal are arranged between the inner wall of the fork and the faces of the roller so as to allow for a small axial displacement of the threaded roller during the operation of the tangential roller head in the embodiment according to fig5 and 6 the unbalancing weight of each weight is created by the provision of axial bores 20 distributed along half of its periphery so that the remaining half has a larger mass and consequently by the effect of the force of gravity the heavier part always assumes the stable starting position at the lower dead center point also in this modification the threaded roller 3 is also mounted on a needle bearing 14 supported on a fixed axle 15 the advantage of this embodiment in comparison with the preceding one is the symmetrical arrangement of the roller 3 relative to the bearings of the axle 15 by contrast in the.