An essential element of a functioning spinning unit is automatic rotor cleaning capability. This is one of the major advantages of the rotor spinning system compared with other spinning processes, which are unable to clean the raw material fed in again at the spinning position itself.
While the large majority of these extraneous particles are eliminated by trash removal in the opening roller housing (refer to section Trash removal), light trash particles and dust in particular can reach the rotor in the air required for fiber transport and be deposited together with the fibers in the fiber collecting groove of the rotor. These deposits can either interfere with twist integration in the rotor groove to such an extent that thread breaks occur, or the deposits continue to accumulate in the rotor groove without provoking thread breaks, but continuously changing the groove geometry. This in turn results in a creeping change in yarn quality. Deposits in the rotor groove which are not distributed uniformly over the rotor circumference, but occur at certain points, result in periodic yarn defects known as moiré effect.
In order to limit the negative impact of these deposits, the rotor groove must be cleaned at certain intervals. This can involve automatically interrupting the spinning process after a preset period of time, whereupon the spinning robot approaches the spinning position and cleans the rotor. However, this preventive cleaning means that each cleaning process entails an interruption of the spinning process and this results in principle in an additional join in the yarn due to the subsequent piecing process. Furthermore, this also causes a deterioration in machine efficiency. Preventive cleaning of the rotor is therefore performed only in exceptional cases, especially when spinning linen and severely soiled raw materials.
In mill operations and the overwhelming majority of applications rotor cleaning is performed automatically at each piecing operation at the spinning position, i.e. at each end down, each quality stop and each package change. Since a clean rotor groove is the precondition for both successful spinning start-up and high piecing quality, on modern systems the rotor groove is cleaned by means of a rotating cleaning head. The cleaning head cleans the rotor groove with 2 scrapers, while 3 air jets clean the rotor slip wall and the groove. The cleanliness of the rotor groove and the rotor that is required for trouble-free spinning operations is adequately assured by the frequency of the piecing process and the resulting cleaning intervals.
Although it sounds paradoxical, the absence of ends down during spinning, which many mill operators may wish for, is not always worthwhile. If a package were to reach its full size without any thread breaks, the risk that deposits would form in the rotor groove when using contaminated raw materials and thus cause creeping changes in yarn quality would naturally be very high. The resulting costs in downstream processing would then be considerably higher than the minimal loss of efficiency due to remedying a certain number of ends down. For one thing, piecings produced automatically do not differ substantially from ordinary yarn, either in cross-section or in tenacity, nor is the efficiency of the machine significantly affected by a limited number of ends down.
Essentially, two systems are used to clean the rotors: pneumatic cleaning by means of compressed air and mechanical cleaning by means of scrapers. Both systems are also used in combination (see Fig. 20).
During rotor cleaning the surface of the draw-off nozzles and the draw-off tube are also cleaned. Further modules clean the nozzle surface and the draw-off tube attached to it, either mechanically with a brush, or pneumatically with an air-jet.