  In a recent paper by Casado-Pascual [Phys. Rev. E \textbf{97}, 032219
(2018)], directed motion of a sphere immersed in a viscous fluid and subjected
to zero-average biharmonic forces is studied. The author explains the
dependence on the relative amplitude of the two harmonic components of the
average terminal velocity from the perspective of a general formalism. In this
Comment, this explanation is shown to be in general incorrect, while the theory
of ratchet universality together with the vibrational mechanics approach
provide a satisfactory explanation of major aspects of the observed phenomena.
