Hydraulic rotary actuators are compact units which generate very high torque in small spaces using hydraulic pressure. Since they can be controlled precisely and easily despite their high power, HKS rotary actuators have been a success in almost every area where limited rotary movements with high torque are required. This design is based on a system with a multiple helical gear. The linear movement of the piston is converted into a rotational movement via the multiple helical gears. The rotational movement will be greater if the linear movement of the piston is longer. FORMERLY ALSO CALLED SWIVEL MOTOR OR ROTARY MOTOR