1. Field of the Invention
The present invention relates to drawoff tubes for open-end spinning machines.
2. The Problem
Conventionally in open-end spinning machines the thread being spun is clamped against rotation about its axis by the nip of drawoff rollers. The thread portion between the spinning rotor and the drawoff rollers is twisted by the thread being bent at the infeed end of the drawoff tube so that the bent portion bears against the tube mouth and the thread is turned so that it is rolled around the circumference of the tube mouth. A portion of the thread extending between the tube mouth and the binding point of the fiber collection surface of the spinning rotor acts like a crank which effects the circumferential travel of the thread around the drawoff tube mouth. In prior apparatus, the rotor speed has been very high in order to provide enough torsional force on the thread portion in the drawoff tube to permit transmission of torsional force past the bearing point of the thread on the tube mouth to the free thread end for binding the fibers on the fiber-collection surface onto the free thread end. Such transmission is necessary in order to prevent the thread end from being pulled away from the fiber-collection surface, thereby creating a thread break. However, such high speed creates an undesirably hard or tight twist in the thread.
3. Prior Art
As disclosed in DT-PS 1.560.305, the prior art teaches that the bearing force of the thread against the mouth of the drawoff tube can be intermittently relieved by the use of complicated vibrational drive mechanism. Another alternative for improving the torsional transfer to the thread end is disclosed in DT-OS 1.560.302 and U.S. Pat. No. 3,805,505, which suggest that the torsional force applied by the spinning rotor can be more effectively transmitted to the thread-binding point on the fiber-collection surface by roughening the mouth of the drawoff tube with notches or ribs.
Suitable vibration mechanism is very expensive, and the roughened tube mouths abrade and roughen the surface of the thread.