Rotary actuators are used to provide torque from hydraulic pressure.
They are widely used in industrial and marine environments for applications such as clamping, tensioning, opening and closing valves or equipment, tilting or indexing etc.
Rack and pinion actuators utilise hydraulic cylinders to drive a horizontal rack which then rotates a gear wheel, allowing greater or less than 360 degrees of rotation, although only over a limited stroke
Vane style rotary actuators use a simple, single vane with higher pressure on one side than the other to rotate it.
Helical gear style rotary actuators use hydraulic pressure to force two helical gears against each other, therefore forcing them to rotate.
There are three main types of rotary actuator, rack and pinion, vane or helical gear.
Actuators may have on or several cylinders to apply the torque.
Our images and descriptions only focus on rack and pinion style actuators although the same issues apply to other types as well.
All rotary actuators contain some form of cylinder, therefore the same design and performance issues will apply as with a standard cylinder. When the cylinder is not round, as in the vane actuator, potential sealing problems become a lot worse.
Some level of backlash is likely when the valve switches from one direction to the other. This backlash is likely to get worse as the drive gears wear. There can also be a significant reduction in overall efficiency from using the mechanical gear drive.
We've included a comprehensive specification guide from Parker here .
Complications can arise because when the hydraulic fluid may be sealed against atmosphere, as in a rod seal, rather than more fluid, as in a bore seal. This increases the risk of contamination and environmental seal damage. Carefully check the area where the rack and pinion are located. If this is enclosed but has a positive area change, then there will be air transfer with the atmosphere outside which runs the risk of drawing in new contaminants that may not be able to escape. If the actuator is used to operate a valve containing another fluid then the risk of cross-contamination must be considered and the inclusion of tale-tale holes to see any leakage should be considered.