  A trapping mechanism is observed and proposed as the origin of the anomalous
behavior recently discovered in transport properties of overdamped ratchets
subject to external oscillatory drive in the presence of quenched noise. In
particular, this mechanism is shown to appear whenever the quenched disorder
strength is greater than a threshold value. The minimum disorder strength
required for the existence of traps is determined by studying the trap
structure in a disorder configuration space. An approximation to the trapping
probability density function in a disordered region of finite length included
in an otherwise perfect ratchet lattice is obtained. The mean velocity of the
particles and the diffusion coefficient are found to have a non-monotonic
dependence on the quenched noise strength due to the presence of the traps.
