On a variable-pitch axial forced-draft fan the blade-pitch mechanism is hydraulic — yet a plain electric actuator sits right on the fan. This puzzles every newcomer; 27 forum replies explain the role of that actuator precisely: it does not provide the force; it gives the orders.
The most widely endorsed reply: the electric actuator repositions the hydraulic control valve, switching the oil fill-and-drain passages; the hydraulic system then fills or drains the different chambers of the cylinder, the cylinder barrel slides axially, and the blade angle changes. In one line: the electric actuator commands, the oil pushes.
The servo unit comprises a control valve, oil cylinder, adjusting piston and guide shaft, mounted behind the impeller and rotating with it; the valve housing does not rotate. The external electric actuator moves a push-pull rod that slides the (non-rotating) valve sleeve axially, admitting pressure oil into the rear or front chamber of the cylinder; the differential pressure moves the piston in the same direction and by the same stroke as the sleeve (slightly lagging), and the force is transmitted through the guide shaft to the adjustment disc — opening or closing the blades.
The chain is: DCS command → actuator push-pull rod → sleeve displacement → oil routing switched → cylinder piston moves → blade angle changes. Oil supplies the force; the actuator decides direction and position.
Never operate the electric actuator with the oil station shut down — the linkage breaks easily. The hydraulic system is the power source; forcing the actuator with the oil station off can damage the push-pull rod and servo unit.
Hydraulics move big blades smoothly with little control force; the electric actuator brings remote control, 4~20mA positioning, diagnostics and bus integration. "Electronics supply the brains, hydraulics supply the muscle" — the same philosophy as turbine governing, and of every large fan in the plant.
