hydraulic actuator[hī′drȯ·lik ′ak·chə‚wād·ər]
a device for shifting the control members of a hydraulic actuating mechanism, with simultaneous power amplification of the controlling action. Commonly used types of hydraulic actuators are those employing a throttling control or a jet control. The first type of design (throttling) is used more frequently and includes designs with and without feedback and designs featuring combinations of control types. Such designs are simple and are reliable in operation, and they do not change the basic characteristics of the hydraulic mechanisms with which they are used. A hydraulic actuator consists of two basic mechanisms: a control device (variable throttles, such as nozzles with slide gates or paired slide valves having an initial axial gap) and an actuating device (for instance, the piston of an actuating mechanism or a controlling slide valve).
In a hydraulic actuator the working fluid from a pressure line enters the control system through the stationary (constant) throttles and is carried to the variable throttles and to the actuator chambers. The electrical input signal is passed through an electromechanical converter and controls the position of the gate slide. The displacement of this slide changes the cross-sectional ratio of the actuating openings through which the pressure fluid passes (that is, the ratio of the clearances between the nozzles and the gate slide). Simultaneously, the pressures in the actuator chambers are changed, thus causing a displacement of the slide valves.
The power amplification coefficient of a hydraulic actuator often exceeds 100,000. Hydraulic actuators with a load feedback or with a speed feedback not only cause a power amplification of the controlling action but also substantially improve the static and dynamic characteristics and the efficiency of hydraulic control systems and allow less stringent requirements for precision and quality of manufacture of the main subassemblies of the hydraulic actuators. When compared with other types of power amplifiers or boosters (such as electromechanical amplifiers), modern hydraulic actuators offer the advantage of a low metal weight per unit of power, often not exceeding 50 g per kilowatt of output power.
V. A. KHOKHLOV
A cylinder or fluid motor that converts hydraulic power into useful mechanical work. The mechanical motion produced may be linear, rotary, or oscillatory. Operation exhibits high force capability, high power per unit weight and volume, good mechanical stiffness, and high dynamic response. These features lead to wide use in precision control systems and in heavy-duty machine tool, mobile, marine, and aerospace applications. See Control systems
Cylinder actuators provide a fixed length of straight-line motion. They usually consist of a tight-fitting piston moving in a closed 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-way directional valve functions to connect one cylinder port to the hydraulic supply and the other to the return, depending on the desired direction of the power stroke.
Limited-rotation actuators are used for lifting, lowering, opening, closing, indexing, and transferring movements by producing limited reciprocating rotary force and motion. Rotary actuators are compact and efficient, and produce high instantaneous torque in either direction. Figure 2 shows a piston-rack type of rotary actuator. Hydraulic fluid is applied to either the two end chambers or the central chamber to cause the two pistons to retract or extend simultaneously so that the racks rotate the pinion 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 of mechanical and electrical power transmissions.
Motor actuators are generally reversible and are of the gear or vane type.