Abstract: A gas bearing roll shell assembly comprises an outer rotatable roll shell carried on an externally pressurized gas bearing between the roll shell and a stationary axle. The roll shell has end rings, one of which has turbine buckets recessed into its bore. The axle carries a valve body having part-chordal passages aligned with the turbine buckets and a rotatable valve for selectively driving the roll shell in one direction or the other. In a modified form, selection is limited to driving in a single direction by a pin and slot arrangement. The assembly is especially useful as an overrun roller in textile machinery involving the high speed transport of threads, yarns or the like.