Continuous variation of the operating conditions arises during winding of a cop. This variation is especially large with regard to changes in the winding diameter, i.e. when wraps have to be formed on the bare tube (small diameter), and then on the full cop circumferences (large diameter). This occurs not only at the start of cop winding (formation of the base); such changes arise at very short intervals in each ring rail stroke as demonstrated by the example illustrated in Fig. 92.
It has already been mentioned that tensile force FF must be assumed tangential to the cop circumference because it arises from the winding point. Frictional force FH undergoes only small variations; it can be assumed to be the same in both cases.
The components FT of the yarn tension are then also equal. However, owing to the difference in the angle a the tensile forces FF are different. The same dependence of the tensile force FF on the angle a can be seen from the formulas given above.
The result is that the tensile force exerted on the yarn is much higher during winding on the bare tube than during winding on the full cop diameter because of the difference in the angle of attack of the yarn on the traveler. When the ring rail is at the upper end of its stroke, in spinning onto the tube, yarn tension is substantially higher than when the ring rail is at its lowest position. This can be observed easily in the balloon on any ring spinning machine. If the yarn tension is measured over time, then the picture in Fig. 93 is obtained.
The tube and ring diameters must have a minimum ratio, between approximately 1:2 and 1:2.2, in order to ensure that the yarn tension oscillations do not become too great.