Abstract: A linear actuator includes a sleeve, a spindle rotatable in both directions, a threaded threaded nut driving a piston rod, and a motor capable of driving the spindle through a transmission, a self-braking unit being arranged in the transmission to stop the actuator whenever the power is been shut off. All axis force will be blocked outside the sleeve, and thus transmission can transmit torque smoothly. Transmission is designed as a coupling that makes good flexibility to assembly. Parts are circular design to make.