TM 1-1500-204-23-2cylinder housing is stationary and the piston moves is used for the following explanation of basic servo operation and ofservo sloppy finks and bypass valves. Such a servo is shown in figure 4-138.(2)Operation. To exemplify servo operation, let us assume that the servo pilot valve has beenmoved a given distance to the right from the off position and is to be held there. If the given distance is 0.250 Inch, thefollowing will result:•Fluid under pressure will flow through the now unblocked selector valve cylinder port and will enter theleft chamber of the servo cylinder housing.•The piston will be forced to the right and will carry the selector valve housing with it since that housing iswelded to the piston rod.•When the piston has moved to the right exactly 0.250 inch, the selector valve housing will have movedto the right over the now stationary pilot valve. This will result in returning the selector valve to the offposition. Since the flow of fluid is now stopped, the piston will travel no farther.j.IrreversibleValves. Irreversible valves assist hydraulic servos in a manner similar to that in whichratchet valves assist actuating cylinders. Both ratchet valves and irreversible valves provide defense against movementof the unit by an externally applied force. Irreversible valves are used with hydraulic servos in a helicopter flight controlsystem to block the travel of feedback forced from their point of organ in the rotor head and blades to the control stickfigure 4-139 illustrates basic operation of the Irreversible valve. The check valve in this mechanism allows fluid from thepump to flow in the normal direction as shown by the arrows. However, feedback force tends to move the servo piston inthe opposite direction to the pump produced pressure. This tends to force fluid backward through the irreversible valveThe check valve blocks this rearward flow of fluid and keeps the servo piston from yielding to feedback force. The reliefvalve serves as a safety device to limit the pressure produced by feedback-induced movement of the servo piston. Itopens to allow fluid to bypass to the return line if the feedback-produced pressure exceeds a preset safe limit.Figure 4-136. Ratchet Valve (Static)4-137
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