Process and device for producing a yarn having alternate twists of opposite directions

A process for the production of a yarn which possesses along its length alternate zones of S and Z twist, the process involving the steps of positively delivering a single yarn via a delivery device, imparting a false twist to the yarn by means of a false twister acting continuously on the yarn and located at a position spaced from the delivery device, intermittently varying the distance over which the twist extends back upstream of the false twister, such intermittent variation being achieved by means of a blocking device in the form of a positive delivery device acting intermittently on the yarn at an upstream point close to the false twister, the duration of action of the blocking device approximately corresponding to the time taken for a point on the yarn to travel from the delivery device to the false twister, and winding up the yarn so that the wound up yarn possesses, along its length, alternate zones of S and Z twist.