The top rollers are not positively driven. They can be either one-piece rollers (spinning preparation machines) or twin rollers ( roving frames, ring spinning machines) (see Fig. 5). Ball bearings are used almost exclusively in the roller mountings. The thick coating forming the roller surface is made of synthetic rubber. An important characteristic of this coating is its hardness. Soft coats surround the fiber strand to a greater extent than harder ones and thus guide the fibers better. On the other hand, they wear out more quickly. A soft coating is therefore used where good guidance is necessary, i.e. where few fibers have to be moved with high draft levels (e.g. at the front rollers of the ring spinning machine). Where this is not required, harder coatings are mostly used. Hardness is specified in terms of degrees Shore. The following ranges are defined:
- soft: 65° - 75° Shore
- medium: 75° - 80° Shore
- hard: above 80° Shore
Normally the diameters of top rollers are between 25 and 40 mm.
Maintenance of the top rollers
Since the coatings wear out during spinning, they must be reground from time to time. This is done periodically in accordance with a predetermined schedule, using grinding discs or rollers that remove material from the coating in very, very small layers.
If the thickness of the coating has been reduced to a defined minimum, i.e. if it has become too thin to provide adequate elasticity under pressure, then the old coat must be removed and a replacement must be glued onto the core.
Top roller pressure
To clamp the fibers, the top rollers must be forced at high pressure toward the bottom rollers. This pressure (loading) can be generated by means of:
- spring weighting (the most usual form);
- pneumatic weighting (Rieter);
- hydraulic systems (hardly used);
- magnetic weighting (was used by the former Saco Lowell company).
Nowadays only spring weighting and pneumatic weighting are used. The first is very uncomplicated, robust and easy to handle when dealing with machine faults; the second is in some cases more regular, and allows easy and quick changes of roller weighting exactly according to requirements as well as partial unloading during longer machine stoppages. It also avoids the need to adjust the weighting to a new roller diameter after roller grinding, which is sometimes necessary for spring weighted systems.