When rotary output is one or two turns or less, a hydraulic motor could be used but repeat stopping accuracy could be a problem. There are several designs of rotary actuators that give rotary output for limited numbers of revolutions (usually under one revolution).
Figure 15-30 shows one way to achieve rotary action when the motion is 90° or less. A clevis-mounted cylinder attached to a lever arm that is driving a shaft gives no less than half the cylinder force times the lever arm length. In the example shown, torque on the shaft would be approximately 4000 lb-in. when the cylinder starts and finishes and approximately 6600 lb-in. when the angle between the cylinder and lever arm is 90°. Often maximum torque is only required at the end of stroke so arrange cylinder mounting to give the greatest torque at that point. Remember, retract force is less than extend force which could cause the cylinder to stall when reversed.
The vane-type rotary actuator in Figure 15-31 is a common design for both pneumatic and hydraulic fluids. The vane has seals around the edges where it contacts the housing to control leakage. Fluid entering from the left, as shown, pushes the vane away and forces fluid out the opposite port as it turns the output shaft. A single-vane rotary actuator is usually limited to 270° rotation or less. A double-vane rotary actuator usually only turns 90° or less. Torque is equal to the vane area times input pressure times the radius from the center of the output shaft to halfway across the vane. The symbol shows a semi-circle to indicate rotary action that is not capable of being continuous in either direction.
Another common rotary actuator is the single-cylinder rack-and-pinion rotary actuator shown in Figure 15-32. Several companies make these units in single- and dual-cylinder models with torque outputs up to and above 300,000 lb-ft. They can be pneumatic- or hydraulic-actuated. This rotary actuator design can turn more than one revolution because turns are in direct relation to pinion gear size and rack gear length. It may also be supplied with cushions, and/or stroke limiters for smooth adjustable stops. Fluid entering ports at the cylinder ends forces the piston to move away and drive the rack gear against the pinion gear. The pinion gear continues to rotate until the opposite piston bottoms out. Reversing flow to the pistons reverses output shaft rotation. Torque is equal to piston area times input pressure times the radius of the pinion gear.
The helical gear rotary actuator shown in Figure 15-33 shows another design that can have more than one revolution of the output shaft. The number of rotations is in direct relation to gear teeth angle and the non-rotating piston stroke. Fluid entering and pushing against one side of the non-rotating piston forces it to move and impart the turning action to the output shaft through the helical gear and nut arrangement. This rotary actuator design is able to have deceleration built in and may be purchased as a double-shaft model.
The chain-and-sprocket rotary actuator in Figure 15-34 depicts another way of achieving limited rotary output with more than one turn capability. The number of turns is in direct relation to the sprocket size and the working piston stroke. It can be used with pneumatic or hydraulic fluids. Fluid entering the CCW port pushes against the working piston and the isolating piston at the same pressure. Because the working piston has more area it will move left while the isolating piston moves right. The effective thrust is pressure times the area of the working piston minus pressure times the area of the isolating piston. The result of these calculations times the radius of the sprocket determines torque.
There are some other rotary actuator designs, but in most cases, they are variations of the ones presented here. For circuits using rotary actuators see the authors upcoming e-book Fluid Power Circuits Explained.