Servo system bolted on design

A method of using a hydrostatic servo assembly unit to control a displacement of a hydrostatic unit, an open or closed circuit hydrostatic transmission, a steering device, a flap mechanism, or any other bi-directional moveable device/mechanism, the hydrostatic servo assembly unit comprising a servo housing in which at least one servo piston is arranged to move linearly relative to a servo cylinder formed in the servo housing, wherein the servo assembly unit comprises a movable output element protruding outside of the servo housing which can be mechanically coupled to a displacement element of a variable displacement hydrostatic unit, wherein the at least one servo piston comprises two piston heads each sealing a cylindrically shaped pressure chamber arranged coaxially along a longitudinal bore axis and formed at either side of the servo housing, wherein each pressure chamber is formed by an end cap, such that outwardly facing front faces of the two piston heads are configured to be pressurized by servo pressure in the respective pressure chamber in order to move the at least one servo piston, wherein each pressure chamber is hydraulically connected to a pressure source such that the servo pressure can be alternately provided to the pressure chambers, wherein at least one servo spring is arranged between the two piston heads by means of two spring seats, the two spring seats including radial outer portions and radial inner portions with reference to the longitudinal bore axis, where the radial outer portions abut against the end caps and the radial inner portions are movable by the two piston heads in order to compress the at least one servo spring when one of the pressure chambers is pressurized with the servo pressure, wherein a servo piston rod of the at least one servo piston is connected operatively to a first end of an eccentric mechanism such that a second end of the eccentric mechanism rotates when the at least one servo piston is moving, and wherein a portion of the eccentric mechanism is located inside of the servo housing and the second end of the eccentric mechanism acts as the movable output element and protrudes outside the servo housing.