in the frame member 19 it locates the roller 1 accurately relative to the feed duct 8 and the roller 2 in accordance with settings applied previously or during manufacture a leaf spring 20 fixed to the frame member 19 by a screw 21 applies spring bias to the plate 14 so as to tend to maintain it in its position in the frame member 19 the spring is designed to apply only sufficient force to counteract the turning moment generated by pressure from the belt 17 in use under normal spinning conditions the plate 14 remains in position in the frame member 19 and hence the settings between the rollers 1 and 2 and the feed duct 8 are maintained however on an end break or any other fault occuring whereby an excessive amount of fibres enters the confined space defining the yarn formation area the pressure developed by the excess fibres tends to lift the roller 1 away from the feed duct by pivotting movement about the pivot 18 thus avoiding excessive force on the rollers and feed duct and possible resultant damage the axis of the pivot 18 lies in a plane parallel to one containing the axes of the rollers 1 and 2 and hence movement of the roller 1 is perpendicular to that plane it will be noted that the roller 2 tends to move any excess material away from the feed duct whereas the roller 1 tends to move it into the.