Single acting cylinders
In single acting hydraulic actuators, the pressure of hydraulic fluid forces the piston to move in one direction (in or out) which results in the actuator only being able to develop either a pushing or pulling force. A single acting actuator has only one access to the hydraulic fluid. The other end is fitted with a filter. The return travel of the piston is either forced by gravity (its own weight or external load), or by a spring.