Rieter

Influencing factors

Index

Fig. 49 – Effect of roller hardness on the friction field

Fig. 50 – Effect of roller diameter on the friction field

Both the spinner and the machine designer can exert strong influence on the  friction field, via:

  • pressure of the top rollers;
  • hardness of the top roller coverings;
  • roller diameter; mass of the fiber strand;
  • density of the strand;
  • cross-section of the strand;
  • width of the strand;
  • twist in the strand.

The individual parameters produce the following effects: High roller pressure causes strong compression and a correspondingly long friction field, but only up to an optimum pressure. Since, in modern drafting arrangements, pressures have already reached the optimum, no further improvement in fiber guidance can be expected from pressure increases. Very hard top rollers, e.g. steel rollers (Fig. 49, a), give very high pressure in the center of the nip line. However, since the outer layers can evade the pressure, there is a steep decline in the pressure curve from the center towards each edge. It is therefore clear that the friction field cannot be very long in directions away from the nip line. An improvement is obtained with a covering (Fig. 49, b) of medium hardness, and the optimum for loose but compact fiber material is a soft covering (Fig. 49, c), since it completely surrounds the fiber body. Similar results are obtained with rollers of different diameter (Fig. 50). Rollers of larger diameter, which spread the total pressing force over a greater area, give a lower pressure peak but a larger pressure width. The increased friction penetrates more deeply into the drafting zone.

The mass of the fiber body exerts its effect mainly through the number of fibers. A very low mass is identical with a lack of contact surface and hence a lack of friction. The friction field is short.

High density, i.e. strong compression, facilitates wide spreading of pressure and friction and thus gives a long friction field.

The cross-section of the body of fibers is of decisive importance. A thin strand, which readily moves apart, can take up neither pressure nor friction and therefore does not give a well-defined friction field.

This is a problem in so far as the fibers spread out during each drafting operation; the body of fibers thus becomes gradually broader. Attempts are made to oppose this by compressing the fiber strand within condensers in the drafting arrangement.

However, this is not optimally effective, since undesired delaying forces are produced by friction at the stationary condensing elements, and the resulting broad fiber ribbon is not really rounded but only folded on itself. Only a round cross-section gives the optimum result. Better still is a strand having protective twist, which holds the fiber mass together in a round and compact form (i.e. roving).

If influence is to be exerted on the friction field by adjustment of individual parameters, then it should be borne in mind that strong interactions are found throughout the whole drafting process.