  Molecule- and solid-state gears build the elementary constituents of
nanoscale mechanical machineries. Recent experimental advances in fabrication
technologies in the field have strongly contributed to better delineate the
roadmap towards the ultimate goal of engineering molecular-scale mechanical
devices. To complement experimental studies, computer simulations play an
invaluable role, since they allow to address, with atomistic resolution,
various fundamental issues such as the transmission of angular momentum in
nanoscale gear trains and the mechanisms of energy dissipation at such length
scales. We review in this chapter our work addressing the latter problem. Our
computational approach is based on classical atom-istic Molecular Dynamics
simulations. Two basic problems are discussed: (i) the dominant energy
dissipation channels of a rotating solid-state nanogear adsorbed on a surface,
and (ii) the transmission of rotational motion and frictional processes in a
heterogeneous gear pair consisting of a graphene nanodisk and a molecular-scale
gear.
