Rotary actuators are compact and efficient, and produce high instantaneous torque in either direction. This makes them widely used in precision control systems and in heavy-duty machine tool, mobile, marine and aerospace applications.
Rotary motor actuators are coupled directly to a rotating load and provide good 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.
Because they are fully enclosed, they withstand harsh conditions and are protected from dust, dirt, and moisture.
The most common types of rotary actuators are vane, rack-and-pinion, or helical designs.
Vane actuators are a good fit for applications requiring extremely high endurance, high rotational speeds, and a need for constant control of movement without the need to maintain the load in a particular position. Their compact size makes vane actuators good for use in applications requiring positioning, harmonic motion, and high speed oscillating motion and are used in tool machines, robotics and for handling containers.
In a single-vane actuator, the vane is rigidly attached to a central shaft in a cylindrical housing. The housing is divided into two chambers by a second vane or abutment shoe fixed to its internal diameter and extending to the output shaft. Hydraulic fluid flows to these chambers through connection ports closed to the fixed shoe. Pressure difference between the chambers acts on the moving vane area, producing torque directly on the output shaft. Flow in and out of these chambers rotates the vane and shaft. Geometry usually limits the rotary movement of a single-vane actuator to 280°.
Double-vane actuators produce twice the torque and less than half the rotation of single-vane actuators. Two vanes and barriers provide a balance that counteracts the tendencies of unbalanced loads.
Mechanical efficiencies range from 80 to 95% and vane actuators transmit torques to nearly 700,000 lb-in.
Rack-and-pinion actuators are ideal for the highest demands on torque. They allow extensive swiveling where there is restricted building room in an axial direction. The rack-and-pinion actuator requires considerably larger dimensions compared with the rotary vane motor to transfer two torques of equal size.
Rack-and-pinion actuators are particularly useful for heavy-duty applications because they tolerate heavy side and end loads and can accommodate large bearings. Because of their constant torque output characteristics and resistance to drift, they are often used for precision control.
Rack-and-pinion actuators have low shock resistance, zero internal leakage, and also have enclosed moving parts.
Helical actuators convert the linear rotation of a piston to rotational movement by multiple helical gears. The rotational movement is greater if the linear movement of the piston is longer. Applications for helical actuators have found use for boom positioning, head rotation, and wheel steering in agriculture harvesters, for example, and in lifting positioning, and steering applications in the construction, energy, marine, material handling, military, mining industries, and valve operations.
The same torque is produced with both clockwise and counterclockwise rotation. Average efficiency is typically 70% for helical actuators. Because the angle of rotation is determined by actuator length, in theory, any rotation is possible. Many actuators have 90°, 180°, and 360° rotations as standard.
Other types of hydraulic rotary actuators include enclosed piston-crank, scotch-yoke, and bladder designs. Enclosed piston-crank designs have adjustable stroke for variable shaft rotation up to 110°. A rod connected to a crank arm drives the rotating shaft. In scotch-yoke actuators, two pistons are connected by a common rod. At the beginning and end of the stroke, torque output is twice the value produced at the stroke’s midpoint. Applications that require a high breaking torque to move the load find this type of actuator appropriate. In bladder designs a pair of rubber bladders are alternately pressurized and exhausted to produce the driving force. When pressurized, the bladder pushes against a cup-shaped lever arm that rotates the output shaft. Zero internal leakage makes this actuator highly accurate and resistant to contamination.
Selecting a hydraulic rotary actuator
- Duty (heavy, standard)
- Rotation (180º, 360º, 200º/220º)
- Maximum output torque
- Maximum holding torque
- Acceptable backlash
- Maximum bearing loads (moment, thrust, radial)
- Hydraulic fluid
- Hydraulic fluid operating temperatures
- Mounting (flange, foot)