Single-acting cylinders transfer the hydraulic force in one direction only. The cylinder may be pulling or pushing. For the cylinder to return to its original position, a force in the opposite direction is necessary. The opposite effect can be achieved by using a built-in spring system. The counterforce is often the load that the cylinder moves.
These cylinders are most often designed as a flask sealed with two chambers in the cylinder, or as plungers with one chamber and without the piston seal. In a cylinder with one chamber the pressure force acts directly on the rod end.
These cylinders are often fitted with only one oil connection. In flask sealed cylinders, the non-power transmitting chamber can be equipped with air filters or a connection with the return to tank. That way, excess oil van be led away from the cylinder.