The sheet feeder can be mounted on a typewriter or on a printer in a system for processing data. The sheets (12) which are accommodated in a feed magazine (28) are passed to the platen roller (210) by friction stripper rollers (250). After the printing operation, the sheets are passed on to a collection table (150) positioned in front of the feed magazine. The stripper rollers are permanently connected to the platen roller by way of a mechanical transmission (212, 218, 226, 245, 248) and rotate in the same direction as the platen roller. The stripper rollers (250) are mounted on an oscillating arm (252) to be selectively moved towards or away from the first sheet to be fed, by means of a first cam (254). Before a fresh sheet is fed, a second cam, (242), also coupled to the platen roller, has to be brought to a reference position by rotating the platen roller enough to expel the current sheet. The platen roller is then rotated in the reverse direction, whereby the second cam actuates a lever (262), thereby to close a clutch (260). During subsequent rotation of the platen roller in the forward direction, the clutch rotates the first cam (254) through one revolution for engaging the stripper rollers (250) with the top sheet (12), thereby effecting stripping and feed of a sheet. At the end of the rotary movement through one revolution of the cam, the stripper rollers are moved away from the sheets and the clutch is opened, making the cams inoperative independently of subsequent rotary movements of the platen roller in one direction or the other.