Nowadays, the rotors on all rotor spinning machines are driven using the friction drive principle, i.e. by a tangential belt in contact with the rotor shafts on each side of the machine. Other systems, such as driving the rotors by individual motors, have not become established in mill operations. We distinguish between two different rotor bearing systems:
- Direct rotor bearing (Fig. 21), in which tangentially driven rotor shaft (a) is encased in ball bearing housing (b). The ball bearing rotates at the same speed (rpm) as the rotor shaft driven by the tangential belt. This bearing principle limits rotor speeds to approx. 110 000 rpm. Although direct bearings would be ideal, individual motors have also been unable to establish themselves for this rotor drive, on cost grounds.
- Indirect rotor bearing, in which the rotor shaft, also driven tangentially, runs on two pairs of supporting discs arranged side by side (see Fig. 22). With the supportdisc bearing the rotor speed is reduced at a ratio of 1:8 to 1:10 relative to the bearing of the supporting discs, depending on the diameter of the discs, so that these bearings run at speeds of only 16 000 to a maximum of 20 000 rpm (depending on the diameter of the supporting discs), even at rotor speeds of 160 000 rpm. For one thing, this bearing system permits much higher rotor speeds than direct bearings, and at the same time the service life of indirect bearing systems is significantly higher than that of directly driven bearing systems. High-performance rotor spinning machines operating at speeds of up to 160 000 rpm are therefore operated only with indirect rotor bearing.
As already stated, with both bearing systems the rotors are driven by a tangential belt on each side of the machine, the speed of which can be adjusted either by stepped speed pulleys or steplessly by means of an inverter drive. Tangential belt (a) is engaged with the rotor shafts via pressure rollers (b) to drive the rotors (see Fig. 23). If a spinning position is stopped and the rotor cover opened, the tangential belt is disengaged at this spinning position by raising the pressure roller and the rotor shaft is brought to a standstill by a brake positioned between the supporting discs. Since the rotor is held in position only by the light pressure of the tangential belt on the support-disc coatings, it can be removed very easily without the use of tools for replacement or examination and re-fitting.
While the tangential arrangement of the rotors is important for the rotor drive, the axial positioning of the rotor is the prerequisite for fiber feed to the rotor and thread take-off from the rotor to occur under absolutely identical conditions at each spinning position. Whereas both the tangential and the axial position of the rotor are defined by the fixed ball bearing housing in the case of direct rotor drive, the rotor on support-disc bearings also has to be fixed in position in the axial direction. The rotor is fixed in position axially by slightly crossing the pair of supporting discs, so that the rotor is pressed backward with some force (toward the spinning beam). Various bearing systems are available for absorbing this backward axial pressure:
- Steel ball or hybrid bearings: the axial thrust of the rotor is absorbed by a steel ball rotating in an oil bath. The front of the rotor shaft and the steel ball are subject to severe wear due to mechanical friction, despite oil lubrication. In more modern bearing systems the front of the rotor shaft is therefore ceramic-coated. This axial bearing system has been used by almost all machinery manufacturers in recent decades. However, the fundamental drawbacks of this system – high spare parts consumption, a high level of cleaning and maintenance effort and severe soiling due to sticky deposits in the axial bearing zone – have encouraged the development of modern bearing systems which are now used at least on high-performance rotor spinning machines.
- Magnetic bearings (see Fig. 24 + Fig. 25). The end of the rotor shaft is fixed in position without contact in a magnetic field created by annular magnets. Accurate radial positioning of the rotor shaft is the precondition for the functioning of this system, which as far as is known to date has no speed limitations.
- EC bearings (Fig. 26 + Fig. 27). The end of the rotor shaft runs (in contrast to the oil bearing) on a steel ball embedded in grease. The housing is sealed, grease cannot escape, and the bearing is largely maintenance-free.
- AERObearings (Fig. 28 + Fig. 29). In this bearing system an air cushion provides axial support for the rotor. This air cushion is provided by a compressed air supply of 6 bar to each spinning position. This system requires neither oil nor grease, sticky deposits are avoided, and in the immediate vicinity of the air cushion the permanent current of air ensures continuous cleaning (selfcleaning effect). Other advantages of this system are low maintenance effort and spare parts consumption. The accurate, level surface of the end of the rotor shaft is the precondition for trouble-free operation.