An actuator for an electric parking brake system has an outer housing. A planetary gear train and an electric motor are disposed in the outer housing. At least one gear connects the motor to the planetary gear train. The planetary gear train comprises a multi-stage planetary gear mechanism. A gear housing accommodates the planetary gear mechanism. The planetary gear train further comprises a centering pin extending through the gear mechanism, such that the planetary gear mechanism rotates about the centering pin. Each stage has a sun gear, a ring gear, a plurality of planet gears meshed with the sun gear and ring gear, and a planet carrier rotatably supporting the planet gears.