A servo is a combination of a selector valve and an actuating cylinder in a single unit. When the pilot
valve of a servo is opened by the operator, it is automatically closed by movement of the servo (or
actuating) unit as explained below. Hydraulic servos are used in aircraft when precise control is
necessary over the distance a component moves.
Typical Hydraulic Servo. Figure 2-9 shows a typical hydraulic servo. In operation, when the pilot
valve is displaced from center, pressure is directed to one chamber of the power piston. The other
chamber is open to return flow. As the power piston travels the pilot valve housing travels because the
two are attached. The pilot valve itself is being held stationary by the operator, and the ports again
become blocked by the lands of the pilot valve stopping the piston when it has moved the required
Figure 2-9. Hydraulic Servo Incorporating Sloppy Link and Bypass Valve.
Servo Sloppy Link. Notice the servo sloppy link in Figure 2-9. It is the connection point between
the control linkage, pilot valve, and servo piston rod. Its purpose is to permit the servo piston to be
moved either by fluid pressure or manually. The sloppy link provides a limited amount of slack between
connecting linkage and pilot valve. Because of the slack between the piston rod and the connecting
linkage, the pilot valve can be moved to an ON position by the connecting linkage without moving the