Abstract: A pneumatic actuator having a double-acting piston separating two pressure chambers, spring means to urge the piston in one direction; and valve means adapted to admit compressed air to one of the chambers and thus to load the spring means, then to transfer some of said air into the other of the chambers at a selected phase-position of said piston, and then to open the one chamber to atmosphere so that the combined forces of the spring means and of the compressed air acting on the piston in said other chamber complete the power stroke of the actuator.