Abstract: A bi-directional linear actuator in which the threads of the drive member and its follower are not exposed and where all force bearing components are concentrically connected to a load is disclosed. The actuator includes an actuator assembly having an output cylinder, an input shaft, transmission rollers therebetween, a prime mover, and a housing assembly. The output cylinder of the actuator assembly is moved between retracted and extended positions by the prime mover. Rotation of the prime mover produces a corresponding rotation of the input shaft. The input shaft carries a plurality of transmission rollers about its circumference. The output cylinder includes an internally threaded bore. By inhibiting the rotation of the output cylinder, the engagement of the threads of the output cylinder with the transmission rollers provides a corresponding linear movement in the output cylinder along its longitudinal axis.