Nonlinear frictions are inherent characteristics of the rotary actuators used for velocity servo mechanisms and may often degrade the overall system behavior. The design of a sensorless velocity observer for such actuators poses a challenging task if stiction and Coulomb friction dominate the actuation. The necessary parameters, including the nonlinear friction coefficient for a fractional horsepower brushed DC motor, are determined through hardware experiments prior to the present simulation study. The robustness of Thau's observer against the contact resistance uncertainty originating from the motor brush is investigated under a wide operating range of the motor input through simulation results. Finally, a trade-off between stability and performance robustness is studied concerning feasible desired eigenvalues of a critically damped observer.