Between take-off roller (1) and transverse sliver condenser (3), some manufacturers include two smooth steel rollers, arranged one above the other (Fig. 117). They can run without loading, in which case they serve simply as guide rollers, or they can be loaded with a pressure of about 15 N/cm and are thus converted into crushing rollers. Where cotton with medium to high dirt content is being processed, additional cleaning can be carried out here by squashing the foreign particles (the fragments fall away immediately after the rollers or in the subsequent machines).
In some models, the rollers are ground with a barrel shape. With this arrangement their central sections cannot escape the pressure – the pressing effect is the same over the full width. Clean fiber material should not be crushed. Owing to the absence of dirt particles, the full roller pressure would be exerted on the fibers, resulting in fiber damage.
This would show up directly in the breaking strength of the yarn. Sticky cotton (honeydew) should also be carded without crushing, as should cotton with a high proportion of seed particles, because of the danger of lap formation at the rollers (again sticky effect).
With the high cleaning efficiency in high performance cards this arrangement is out-dated.