  We review a family of models recently introduced to describe Brownian motors
under the influence of Coulomb friction, or more general non-linear friction
laws. It is known that, if the heat bath is modeled as the usual Langevin
equation (linear viscosity plus white noise), additional non-linear friction
forces are not sufficient to break detailed balance, i.e. cannot produce a
motor effect. We discuss two possibile mechanisms to elude this problem. A
first possibility, exploited in several models inspired to recent experiments,
is to replace the heat bath's white noise by a ``collisional noise'', that is
the effect of random collisions with an external equilibrium gas of particles.
A second possibility is enlarging the phase space, e.g. by adding an external
potential which couples velocity to position, as in a Klein-Kramers equation.
In both cases, non-linear friction becomes sufficient to achieve a
non-equilibrium steady state and, in the presence of an even small spatial
asymmetry, a motor effect is produced.
