The rotor is the main spinning element of the rotor spinning machine. Yarn quality, yarn character, operating performance, productivity, etc., all depend chiefly on the rotor. The most important parameters of the rotor that exert influence are (see Fig. 74):
- the inclination of the rotor wall (a);
- the coefficient of friction between the fibers and the surface conditions of the rotor wall (b);
- the design and the positioning of the rotor groove (c);
- rotor groove diameter (d) and rotor speed.
On considering this broad range of influences, and taking account also of the weight of the influence exerted, it is readily apparent that there can be no such thing as a universal rotor. Out of the multiplicity of rotors on offer, the spinner has to select the one best suited to the raw material, yarn product, and spinning conditions. Rotors are replaceable elements in all rotor spinning machines.
The rotor, see Fig. 75, consists of rotor shaft (a) with wear protection in some cases, rotor cup (b) with rotor groove (C) and rotor wall (d). The wall inclination is necessary so that fibers emerging from the feed tube and passing to the wall can slide downward. Depending upon the material and area of use, the angle of the rotor wall to the vertical ranges between 12° and 50°. This angle is dependent upon the make but will in all cases be smaller, the higher the rotation speed for which the rotor is designed. At the internal periphery in the lower region of the rotor cup, there is usually a groove that varies in width. This groove serves to collect fibers.
Rotors are made of steel and are in general surface-treated or coated to give them a longer useful life. The following means, which are customary and proven in mill practice, are available for protecting rotors against wear:
- diamond/nickel coating;
- boron treatment; or
- a combination of both processes.
The diamond coating usually consists of diamond dust embedded in a nickel layer and is the same as that used for protecting the opening rollers against wear. Boronized rotors and boronized rotors with an additional layer of diamond coating have twice the lifetime of a diamond-coated rotor. However, the surface structure of the rotor wall changes depending on the type of treatment (boron or diamond coating), and thus also its influence – which should not be underestimated – on both yarn quality and spinning stability and the tendency for deposits to form in the rotor groove. The best possible compromise between long service life of the rotor, good yarn values and stable spinning conditions is achieved with the combined boron/diamond coating. The rotor is a part subject to wear and must therefore be replaced periodically. Wear mainly affects the groove.
The configuration of the rotor groove determines whether the yarn is bulky or compact, hairy or lean, and whether the yarn quality is excellent or only adequate and the spinning stability low or high. The groove also affects the extent to which dust and dirt tend to accumulate in the rotor. Depending upon the raw material used, the desired yarn characteristics and yarn values, different groove designs are used in practice.
Wide grooves produce a soft, bulky yarn with rather low strength, while narrow grooves produce a compact, strong yarn with low hairiness. Wide grooves are therefore used in the production of yarns for knitted fabrics, homespun-type fabrics and coarse articles; narrow grooves are used for yarns required for the production of stronger fabrics with a smooth appearance. A fairly narrow groove is in most widespread use in classical short staple mills. The tendency to form moiré effects is also greater with the narrower groove, because fairly large dirt particles can jam in the groove.
A speed range in which the rotors in question produce optimum results, in terms of technology as well as spinning stability and energy consumption, is assigned to each rotor diameter. The speed ranges overlap between rotor diameters, with the energy consumption of the smaller rotor diameter being more favorable at the same rotor speed.
Fig. 76 shows the speed range and the highest possible rotor speed for the individual rotor diameters.
The smaller the rotor diameter, however, the higher the number of system-related wrapper fibers. The view generally held earlier and also valid at that time, that yarn twist must inevitably be increased when reducing the rotor diameter, is now no longer valid to the same extent. Optimized spinning elements, especially rotors and draw-off nozzles, as well as improvements in fiber guidance and spinning geometry mean that soft-twist knitting yarns can also be spun using small rotors (30 - 33 mm diameter). However, in these spinning operations spinning tension must not be too high, i.e. rotor speeds must be well below their maximum range.
The rotor diameter should in any event be large enough to permit fiber formation in the groove without technological disadvantages. A certain amount of space is needed for the fiber mass, i.e., larger rotor diameters have to be used for coarser yarns and vice versa. A relationship – albeit not very close – also exists between fiber length and rotor diameter. As a rule of thumb, rotor diameter should not exceed 1.2 times staple length, otherwise fiber integration in the rotor groove is disturbed. In mill operations staple lengths of 38 or 40 mm are also spun successfully (albeit only in certain cases) on rotors in the 30 - 32 mm range.
Fig. 77 describes the configuration and the properties of the different rotor and groove shapes. In principle:
- Narrow groove angles and small groove radii (T and K rotors) are suitable for all raw materials and are used to manufacture smooth weaving yarns with good regularity and high yarn tenacity.
- Narrow groove angles with large groove radii (G rotors) are also suitable for all raw materials and are preferably used for bulky knitting yarns.
- Rotors with wide groove angles (U and DS rotors) are suitable for bulky knitting and denim yarns in cotton and its blends with man-made fibers. The different groove shapes and groove radii are chosen according to the type of denim yarn (weft or warp yarn, rope or beam dyeing, etc.).
- The TC rotor is outstandingly suitable for manufacturing high-quality denim yarns and at the same time is characterized by excellent running properties. Compared to the T rotor, groove angle and groove radius are larger, but the groove shape has been retained. Especially shifting-resistant yarns are produced when processing man-made fibers and viscose with the TC rotor.
- The GM rotor can be used very flexibly in the fine count cotton yarn sector, for both weaving and knitting. Compared to the G rotor, groove angle and groove radius are larger, but the groove shape has been retained