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                       FIGURE 20.97  Single rod double-acting piston actuator (Atos).
















                       FIGURE 20.98  Actuators simbols.


                       This involves actuating forces and feed speeds which are different in the two directions, with the same
                       feed pressure in the two thrust chambers.
                         Hydraulic actuators are able to support external overloads, as, if the load exceeds the available thrust
                       force, the rod stops or reverses, but generally does not suffer any damage. Cylinders may get damaged
                       however, or at least suffer a drop in performance, when they have to support loads which are not applied
                       along the axis of the rod, that is, with components in the radial direction, as reactions are generated on
                       the rod supports and piston bearings, which leads to fast wear of the same and reduces the tightness
                       with oil leakage as a result.
                         The main features of a linear actuator are its bore, its stroke, its maximum working pressure, the type
                       of working fluid, and the way its connections are fitted.
                         The symbols of the different types of actuators can be seen in Fig. 20.98.

                       Valves
                       Valves are the components in hydraulic circuits that carry out the task of regulating the hydraulic power
                       sent to the actuator. Their role is to turn the oil flow on or off or to divert it according to needs, thereby
                       permitting adjustment of the two fundamental physical magnitudes of fluid transmission: pressure and
                       flow rate. They are subdivided as follows on the basis of the operations they carry out:
                          • directional valves
                          • on-off valves
                          • pressure regulator valves
                          •flow-rate regulator valves
                       In servomechanism applications valves with the continuous positioning of the moving components in
                       them, said flow proportional valves or servovalves, and pressure proportional valves are used.

                       Directional Valves
                       Directional valves determine the passage and the  flow direction of the oil current by means of the
                       movement of appropriate moving parts contained in them, actuated from outside. Directional valves,


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