Hydraulic actuators deliver the greatest thrust and are usually mechatronic, but are bulky and rarely used today — the Three Gorges ship-valves use hydraulics.
Electric actuators are weaker in explosion safety, their motors are not fast, and the motor is easily damaged when travel is blocked or the stem jams. Despite continual improvement and expanding use, they remain less widespread than pneumatic actuators.
In pneumatic actuators actuating and regulating mechanisms form one unit, of diaphragm or piston type. Pistons have long stroke for high-thrust duties; diaphragms have small stroke and drive the stem directly. With simple structure, large thrust, smooth reliable action and intrinsic safety, pneumatics dominate safety-critical processes such as chemicals and refining.
Hydraulic mechanisms use pressurised oil as the power source. Compared with purely mechanical and electric drives they offer:
Drawbacks: oil viscosity varies strongly with temperature — unsuited to very low or high temperatures; hydraulic components need precision machining and fitting, raising cost.
Pneumatic mechanisms use compressed air to transmit power and control signals. Advantages over hydraulics:
Drawbacks: air is compressible, so speed stability is a little worse (gas-hydraulic combinations give better results); low working pressure (0.3–1.0 MPa typical) limits output force; noisy — silencers needed at high-speed exhaust.
