Rieter

Preparation for doffing

Index

For process-related reasons a cop takes up only 30 - 100 g of yarn. Nevertheless, it takes 1 - 30 hours to fill it. The limited capacity of the cop compels yarn manufacturers to add a further process subsequently, i.e. rewinding. Another drawback of the small package is the need to doff the full cop at relatively short intervals and replace it by an empty tube - a rather complicated process. Several preparatory operations have to be performed so that this change can be made rationally and without causing higher rates of ends down (Fig. 51) .

If the empty tubes have been prepared for the change and if the ring rail has reached its uppermost position (II), the ring rail and the  balloon checking rings are lowered (III) in order to reach the cop more easily. At the same time the yarn guide eyelets are tilted upward (IV), since only then can the cop be removed over the spindle. The ring rail is moved to a lower position (Fig. 52) than that at which the new winding operation then begins (a). The lower position is referred to as the underwinding position (b) and the starting position as the piecing position (a). The underwinding position has a specific function – it produces a thread reserve. This is because yarn continues to be delivered while the ring rail is being lowered and several turns are wound around the finished cop as a so-called reserve winding (Fig. 53). This should consist of no more than 3 - 4 turns, perhaps only 1 1/2 to 2 turns in the case of high-tenacity yarns.

When the ring rail reaches the underwinding position (2), delivery is still not interrupted, so that several yarn turns attach themselves here as a yarn ring. During manual doffing this thread reserve is still on the tube, during automatic doffing it is on the spindle. The reserve is necessary so that the yarn continues to be held on the spindle when the cop is doffed. Otherwise a yarn break would occur. Various systems are currently known that actively clamp the yarn on the spindle in order to keep the reserve thread as short as possible and thus virtually eliminate the disturbing yarn residues occurring later when the reserve is removed. On modern machines all these preparatory processes for doffing occur automatically.

Fig. 51 – Preparation for doffing

Fig. 52 – Underwinding position (b) and piecing position (a) of the ring rail

Fig. 53 – Reserve winding (1) and underwinding (2)