My answer to What is the principle of a hydraulic actuator?
Answer by Arûn Râinâ:
A cylinder or fluid motor that converts hydraulic power into useful mechanical work. The mechanical motion produced maybe linear, rotary, or oscillatory. Operation exhibits high force capability, high power per unit weight and volume, goodmechanical stiffness, and high dynamic response. These features lead to wide use in precision control systems and inheavy-duty machine tool, mobile, marine, and aerospace applications. See
Cylinder actuators provide a fixed length of straight-line motion. They usually consist of a tight-fitting piston moving in aclosed cylinder. The piston is attached to a rod that extends from one end of the cylinder to provide the mechanical output.The double-acting cylinder (Fig. 1) has a port at each end of the cylinder to admit or return hydraulic fluid. A four-waydirectional valve functions to connect one cylinder port to the hydraulic supply and the other to the return, depending on thedesired direction of the power stroke.
Limited-rotation actuators are used for lifting, lowering, opening, closing, indexing, and transferring movements by producinglimited reciprocating rotary force and motion. Rotary actuators are compact and efficient, and produce high instantaneoustorque in either direction. Figure 2 shows a piston-rack type of rotary actuator. Hydraulic fluid is applied to either the two endchambers or the central chamber to cause the two pistons to retract or extend simultaneously so that the racks rotate thepinion gear.
Rotary motor actuators are coupled directly to a rotating load and provide excellent control for acceleration, operating speed,deceleration, smooth reversals, and positioning. They allow flexibility in design and eliminate much of the bulk and weight ofmechanical and electrical power transmissions.
Motor actuators are generally reversible and are of the gear or vane type.