A conventional false twist texturing machine is provided with a plurality of false twist texturing units, and false twisting is carried out by passing the yarn along as it is false twisted by the false twist texturing units. A false twist texturing unit in such a conventional false twist texturing machine looks like the example shown FIG. 4. This conventional false twist texturing machine is comprised of a plurality of false twist texturing units 1 arranged in a row perpendicular to the plane of the page. Each false twist texturing unit 1 is provided with a first feed roller 4 which draws the yarn Y (filament yarn) from out a supply package 3 held in a creel stand 2. In the conventional false twist texturing machine, the first feed roller 4 is arranged such that it receives driving force from a line shaft shared by all false twist texturing units 1, and all the false twist texturing units are driven at a common rotary speed.
A yarn Y, which passes in sequence from the first feed roller 4 to a first heater 5 and to a cooling plate 6 is imparted with a false twist by a false twist device 7 (a nip twister). The yarn Y imparted with the false twist then passes sequentially through a yarn tension detection unit 15, a second feed roller 8, a second heater 9 and a third feed roller 10. It is then wound into a package 12 by a winding device 11, completing the false twist texturing process.
The false twist device, that is, nip twister 7, as shown in the enlarged view of FIG. 5, is provided with a pair of crossed belts 13, 14 which nips the yarn Y, propelling and twisting the yarn Y. The twist created by the nip twister 7 is transmitted as far as the first feed rollers 4 and is heat-set by first heater 5. In other words, the nip twister 7 causes the upstream side to become the twisted side and the downstream side to become the untwisted side.
Thus, the belt-type nip twister 7 is arranged such that it nips the yarn Y using two endless belts that are crossed in contact with each other. By rotationally driving each belt, the yarn Y can be passed along and twisted. Both the nip pressure (contact pressure) of the belts and the rotational speed (the belt surface speed) affect the yarn tension T2 of the downstream (untwisted) side. The tension T2 of the untwisted yarn Y, in turn, affects the quality and characteristics of the yarn Y after it is untwisted. Additionally, changes in the running time of the belt also alter the tension T2 of the untwisted yarn Y, even if the nip pressure and rotational speed remain constant.
Normally, by adjusting the nip pressure and maintaining the constant tension T2 of the untwisted yarn Y2, the quality and characteristics of the yarn Y are kept constant. Specifically, as shown in FIG. 4, the yarn tension detection unit 15 for detecting the tension T2 of the untwisted yarn Y is provided on the downstream side from the belt type nip twister 7 to constantly monitor the tension T2 of the untwisted yarn Y. To control the tension T2 of the untwisted yarn Y, a target tension is pre-set and the detected tension T2 of the untwisted yarn Y is compared against the target value. If the detected value differs from the target value, the nip pressure of belts 13 and 14 are adjusted based on the disparity. When the nip pressure of the belts 13, 14 is adjusted, the tension T2 of the untwisted yarn on the downstream side is altered. Thus, it is possible to have the detected tension value approximate the target value.
There is variation, however, in the yarn quality of the packages 12 produced among the different false twist texturing units 1, as well as within the packages 12 of the same unit 1, in the conventional false twist texturing machine. The causes of this variation are mainly the mechanical or yarn supply-related problems. The mechanical causes are yarn slippage at the feed rollers 4 and positional differences between the upper and lower levels of the supply packages 3 in the creel stand 2. The supply-related cause is that there is variation in the degree of elongation of the individual yarns supplied to the false twist texturing units 1. This variation occurs when different supply packages 3 are used, or as the yarn is taken off the same supply packages 3.
When there is variation in the condition of the supply side yarn due to such mechanical or supply-related problems, the tension T1 of the yarn between the first feed roller 4 and the false twist device 7 (tension of the twisted side) changes, and thus variation of the tension T1 between the false twist texturing units 1 arises even if the yarn is drawn at a constant draw rate (the ratio of the surface speed of the second feed roller 8 to that of the first feed roller 4) at the false twist texturing units 1. As a result, the yarn quality varies between each false twist texturing unit 1, and further, varies within each wound package 12. This occurs even with conventional control systems that keep the tension of the untwisted yarn T2 constant.