A rotary actuator includes an actuator body having a first end and a second end defining an interior of the actuator body therebetween, an axis X of the body being between the first and second ends. Each end is provided with a port arranged to receive hydraulic fluid. The actuator also includes: mounting parts extending from the ends into the interior of the actuator body a screw mounted to the mounting parts within the actuator body between the first and second ends, the screw mounted to the mounting parts via splines to allow linear axial translation of the screw relative to the mounting parts. The screw having one or two walls extending across an inner diameter d of the screw.