Bottom rollers are made of steel and are mounted in roller stands or in the frame by means of needle, roller or ball bearings. They are positively driven from the main gear transmission or by a separate drive. In order to improve their ability to carry the fibers along, they are formed with flutes of one of the following types (see Fig. 4):
- axial flutes (a),
- inclined flutes (spiral flutes) (b),
- knurled fluting (c).
Knurled fluting is used on rollers receiving aprons, to improve transfer of drive to the aprons. Other rollers have axial or, increasingly, spiral fluting. The latter gives quieter running and more even clamping of the fibers compared with axial fluting. Rolling of the top rollers on spiral flutes takes place in a more even manner and with less jerking. The diameter of the bottom rollers can lie in the range of 20 - 90 mm, but normally diameters between 25 and 50 mm are used. A drafting arrangement includes three to six such rollers. In long machines (e.g. ring spinning machines) the bottom rollers are made up by screwing together short lengths. Distances between the rollers of the drafting arrangement are usually adjustable and can then be adapted to the fiber lengths.