  We show by numerical simulations that a non rotationally symmetric body,
whose orientation is fixed and whose center of mass can only slide along a
rectilinear guide, under the effect of inelastic collisions with a surrounding
gas of particles, displays directed motion. We present a theory which explains
how the lack of time reversal induced by the inelasticity of collisions can be
exploited to generate a steady average drift. In the limit of an heavy ratchet,
we derive an effective Langevin equation whose parameters depend on the
microscopic properties of the system and obtain a fairly good quantitative
agreement between the theoretical predictions and simulations concerning
mobility, diffusivity and average velocity.
