Rotary actuators perform fractional (45°, 90°, 180°, etc.) turn or a defined limited number of full (360 deg) turns. Many rotary actuators are used to operate valves in the refineries, chemical plants, water works, etc. The remotely controlled reliable operation of these devices is crucial for the automation in process control.
Electric rotary actuators are much more common than hydraulic or pneumatic ones. Valves are scattered usually over a large area of the processing facility. Separate motors and pumps for each actuator would not be economically feasible. A central pump station with long hydraulic lines is also not acceptable in most cases. Hydraulic rotary actuators are used mostly in applications requiring a high torque tilting, turning movement and where already a hydraulic system is already available. The majority of hydraulic rotary actuators convert the linear motion of a hydraulic cylinder into rotary by using a rack and pinion arrangement. Vane type actuators are also in use for special applications.
Electric rotary actuators basically consist of an electric motor and a gearbox used to lower the motor speed and boost the torque. This does not mean, however, that a gear motor is automatically a rotary actuator. A rotary actuator still needs certain additional devices to control the fractional turn and/or generate a feedback signal. Depending upon the application requirements and space restrictions a wide variety of motors can be used. For simple applications a simple squirrel cage induction motor (or brake-motor) with a set of appropriate limit or proximity switches is sufficient. For high positioning accuracy and intermediate positions DC-, AC-servo, or stepper motors are used. In some low torque applications an electric motor alone can satisfy the requirements, but in most applications a suitable gearbox is needed. A variety of suitable commercially available standard gearboxes can be used. Rotary actuators can be created by integrating standard (motors, gearboxes, rotary switches etc. ) or from specially designed components. The low torque units frequently feature an integrated, specialized design, the higher torque units are mostly compiled of standard components restrictions a wide variety of motors can be used. Typical electric rotary actuator consisting of an electric motor, helical shaft mount multiple stage gear-box and a feedback/limit switch combination mounted to the opposite end of the output shaft in an appropriate housing. The application requirements and restrictions govern the actuator type and size selection. Listed below are the most important parameters for the selection along with the most commonly used units:
• required torque
• required speed
• positioning accuracy
• prime mover type (electric motor/supply Voltage)
• control requirements
• Feedback signal
• Ambient conditions
• Duty cycle (op. cycles per hour)
• Expected lifetime