Process control in the ring frame section is important since it affects the properties of the final yarns directly. Long-term variation should not be normally introduced by the ring frame, assuming that all ring frames have the same draft, roller grip is adequate and no slippage occurs. It should be ensured that all ring frames of the same make and running in the same count have the same change pinion and back roller wheels. As far as possible, a pinion change shall be avoided in ring spinning.
Between-Bobbin Variation
A high correlation exists between count variation of between-bobbins and the total count variation. A positive correlation of lesser extent is noticed between ‘within-bobbin’ variation and total count variation.
Frame to frame differences in count due to varying pinions or back roller wheels are found to be present in some mills.
Count also differs significantly between days which can be attributed to changes in relative humidity, incorrect pinion changes and also shifts in the average weight of lap.
Differences in hank between machines in the preparatory department and channelizing of material are the other sources of count variation.
Adequate attention is not paid by some of the mills to the proper feed of the creel bobbins. Sometimes, the creels are fed diagonally and beneath the roving bars, as a result of which undue stretch occurs.
Damaged skewers, accumulation of lint in the creel and trumpet, mis-shaped trumpet, improper back zone draft or weighting of top roller can also cause roving stretch. The presence of creel draft variation can be checked by marking each rove end close to the bobbin.
An advanced mark indicates slack roving and coarse count and a receding mark points to excessive stretch and fine count.
Excessive variation in tension between bobbins due to differences in spindle speeds, spindle-out of centre etc., can also result in between-bobbin variation. Slippage of top rollers due to poor lubrication or insufficient weighting is also one of the causes for count variation.
Strength Variation in Ring Frame
Strength variation could be largely dependent on count variation. It is observed that about 50% of the variation in strength as well as about 50% of the variation in strength CV mainly due to count variation.
The contribution of the CV of count to the CV of strength would be about 1.5 times the CV of count.
Variation in humidity is one of the major causes for the relatively higher strength variation.
Some of the causes for the relatively high strength variation are excessive yarn unevenness, difference in twist, mechanical imperfections in drafting like eccentric rollers, worn aprons, inadequate top roller pressure in draw frames and ring frames etc., and thick and thin places arising from the spindle vibration.
Setting between the rollers
In order to avoid the creation of drafting waves and to reduce short term irregularity (U %) of yarn, proper roller settings must be adopted. A back zone setting of 55 mm for fibers with length up to 30mm and a back zone setting of 60 mm for fibers with length greater than 30 mm will help to produce yarns with minimum irregularity.
Top roller Pressure & Shore Hardness
Insufficient loading of top rollers leads to erratic movement of the fibers due to fiber slip between the drafting rollers. This, in turn, will lead to high level of short term unevenness of yarn.
Studies show that a top roller pressure of 18 kg (for cotton spinning) improves yarn evenness by 1 U% and reduces thick and thin places by 15 to 40% in different counts as compared to the 9 kg pressure.
Use of soft cots (shore hardness of 70 o to 75 o) generally improves yarn quality by reducing slip between the cot and the bottom fluted roller.
Soft cots with a top roller pressure of 18 kg counts below 50s and 15kg, in counts finer than 50s will result in improved yarn quality.
Draft distribution
The total draft and break draft employed in spinning influence the amount of irregularity added in spinning and depend on the quality of roving and condition of the ring frame.
Break draft in ring frame is mainly to break the mild twist in the roving. Higher the break draft, greater will be the fiber breakage at the back zone. If the TM in the roving is higher, then comparatively higher break drafts could be employed. While using higher break draft the back roller setting should be wider to obtain good results.
