The nonlinear hydraulic actuator which includes a hydraulic cylinder and a riding cut-off valve was considered. The valve supplies the oil to the actuator’s hydraulic cylinder. The riding cut-off valve is an overlapping valve. Therefore the time constant in that type of actuator is variable and depends of the valve position. Equations for the value of the time constant at a steady state in an overlapping range depending on the size of the overlapping and a radial backlash were obtained. We have performed calculation of an equivalent time constant for that type of actuator with harmonic disturbances of different frequency and amplitude applied at the input. We have considered a method of evaluating the time constant for the actuator within overlapping range and its influence on the stability of the control system for a steam turbine with this type of actuator and friction forces in the system.