Rotary Actuators

Nov. 15, 2002
Rotary actuators produce oscillating power by rotating an output shaft through a fixed arc.

Rotary actuators produce oscillating power by rotating an output shaft through a fixed arc. They are compact, simple, and efficient. They produce high instantaneous torque in either direction and require only a small space and simple mountings.

Rack-and-pinion actuators use fluid pressure to drive a piston connected to a gear rack, which rotates a pinion. Standard units are available with rotation of 90, 180, or 360°. They can be obtained with two parallel piston-rack units to double output torque. Outputs to 35 million lb-in. are available.

Vane actuators consist of a shaft mounted in a cylindrical housing, with one or more vanes attached to the shaft. Applying fluid pressure to the vanes produces shaft rotation. An internal barrier between housing OD and shaft divides interior volume into two chambers. For this reason, single-vane actuators are normally limited to about 280° of rotation, and double-vane actuators to about 100°. Torque is directly proportional to vane area and effective fluid pressure. Vane actuators have torque outputs as high as 500,000 lb-in.

Helix actuators have helical grooves in the piston rod that convert linear to rotary motion. Helical actuators are available with standard rotations varying from 100 to 370° with outputs to 15,000 lb-in.

A relatively new actuator gaining wide acceptance uses double helical gearing. This design features two moving parts: the piston sleeve, which reciprocates and rotates; and the output shaft, which only rotates. As the piston sleeve reciprocates in helical actuators, the outer spline engages the ring gear and causes sleeve rotation. At the same time, the inner spline engages another set of helical teeth on the output shaft. This causes relative shaft rotation in addition to that of the piston sleeve.

Planetary actuators increase helix angle and reduce actuator length by replacing sliding action with rolling action. Planetary rollers on the piston between the helical shaft and housing grooves provide an arrangement similar to the gears in a planetary speed reducer. As in double helical actuators, planetary actuators have two basic moving elements, the piston assembly and shaft assembly.

Piston movement causes rollers to follow helical grooves in the housing, forcing piston rotation. Simultaneously, the rollers follow helical grooves in the shaft, forcing shaft rotation. Design is such that 90° piston movement results in 180° of shaft rotation. Large-diameter bearings and mounting flanges on this unit can carry large moment, thrust, and radial loads.

Linear cylinders consist of a simple cylinder with a pin-ended rod connected to a crank arm that drives the rotating shaft. These devices are typically pressure actuated in both directions and are equipped with adjustable stops for accurate adjustment of stroke. Stroke is ordinarily adjustable from 85 to 100°.

Fail-safe variations on the basic cylinder are used where a power failure or fluid loss could suspend the controlled object in a dangerous position. Fail-safe actuators are spring-loaded to ensure the return of the shaft to a safe position -- they are available with torque outputs to over 5,000 lb-in.

Scotch yoke actuators provide torque from a linear cylinder mechanism. They can be either single or double acting, producing torque as high as 45 million lb-in., driving through comparatively short arcs -- about 90° maximum. Output torque is not constant, but increases as the piston moves away from its center position.

Sprocket actuators provide long rotations. Up to five complete turns (1,800°) and torques to 23,500 lb-in. are available from sprocket actuators. In these devices, two pistons, a chain, and a sprocket convert fluid pressure into torque. The large piston acts as the driver, pulling the chain. The smaller piston seals against fluid leakage past the return side of the endless chain.

Bladder actuators route fluid into rubber bladders that push against a cup-shaped lever arm to provide rotary motion up to 100°. As long as bladders remain intact, there can be no leakage across the lever arm, so the actuators provide excellent angular accuracy. Bladders can be compounded for compatibility with almost any fluid medium, and are insensitive to abrasive particles in the fluid.

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