<p>Serious transient vibrations often occur in a vertical-axis washing machine during the starting stage of the spin drying process. For solving this problem, a new type of rotational damper installed between the tub and the suspenders is designed, which can enhance lateral damping of the suspension system along radial directions. Damping force on the tub provided by the damper is derived, based on which vibration model of the washing machine is constructed. Bifurcation analyses are carried out to investigate influences of the rotational damper on stabilities of the suspension structure, and interactions between damping coefficient of the damper and several other design parameters are discussed. Variations of the unstable regions of the washer associated with the liquid balancer are studied, and the effect of the rotational damper on eliminating the unstable region induced by insufficient axial damping of the suspenders is discovered. The effectiveness of the damper for reducing transient vibrations of the tub is validated through experiments, and impacts of the damper on steady-state vibrations of the tub and the cabinet are presented. Conclusions can be drawn that the damper is not only effective for reducing transient vibrations, but also beneficial to stabilities of the washing machine.</p>