Stretching is to achieve by the drafting machine roller surface line speed increases. Therefore, theory of draw ratio can by the drafting machine roller surface line speed ratio: first draw ratio = /; the secondary stretching ratio = /; total drawing ratio = = /; type, respectively, indicating first, second, third drawing machine roller surface line speed (M / min). Under normal circumstances, the total draw ratio of the machine is 6 ~ 3, the first level of the stretching ratio is about 90% ~ 80% of the total draw ratio, the 20% - level stretching factor is only 10% ~ second.

According to the requirement of hot stretching, between the first drafting machine and the second drawing machine set water bath drawing groove, and between the second and the third drawing machine with steam heater.

Tension heat setting machine is intended to eliminate the internal stress in tow after tension, reduce the heat shrinkage rate. Tension heat setting machine and the roller surface line speed, if than the third drawing machine roller surface line speed and low in fiber will have a retraction, retraction than = / = (-) / type in: ---- heat strain stereotypes machine and the roller surface line speed.

According to the theory of the design of the draw ratio, while in the actual production, due to the existence of the phenomenon of slipping, the actual stretching ratio is slightly lower than the theoretical value. Slip coefficient of seven roller traction machine is about 3%.

Draw ratio should be to trace the regulation, so in the transmission system of the drafting unit, often without tooth chain CVT or gear type gearbox. If the joint production of a variety of fiber is not much, but also can be used to change the change of the gear to change the tension.