Some machine actions require rotary motion for only a portion of a turn. Using a hydraulic motor to perform a partial-turn function is expensive and it is difficult to accurately stop a motor at a specified degree of rotation. A clevis-mounted cylinder, attached to an arm and keyed to a shaft, produces rotary action, but is limited to 90° or less. At 90° rotation, a cylinder/lever arrangement has half torque or less when it starts and nears the end of stroke. To obtain partial- or multiple-turn rotary action and/or accurate stopping of rotary output, use one of the rotary actuators shown in this chapter. Figure 19-1 pictures the symbols for air- and hydraulic-operated rotary actuators.
Figure 19-2 provides simplified cutaway views of vane-type rotary actuators. The figure depicts both single- and double-vane-types. The vanes attach to an output shaft and have seals around their periphery. When fluid pressure on a given vane area pushes it through the body cavity, the output shaft turns with a given torque. The maximum rotation of vane rotary actuators is limited to approximately 280° in a single-vane model and approximately 100° in the double-vane configuration.
A double-vane rotary actuator sends fluid to the push side of the opposite vane through drilled passages in the shaft, as shown by dashed lines and arrows. Pressurized fluid at the CW port turns the output shaft clockwise. Pressurized fluid at the CCW port turns the output shaft counterclockwise.
Most vane-type rotary actuators operate at lower pressure and torque limits of 2500 to 5000 in. lb. Some manufacturers do make units that operate at up to 3000 psi, with torque in excess of 700,000 in. lb.
Vane-type rotary actuators have no effective way of internally cushioning or limiting the degree of rotation. An external method must be used to limit rotation or cushion the load. Some manufacturers offer a valve and stroke-limiting package that makes rotation degrees adjustable and gives variable deceleration and cushioning. Check manufacturers’ catalogs for more information on these packages.
Figure 19-3 illustrates one design of a rack-and-pinion type rotary actuator. This cutaway view shows a double-rack design that has fluid in the area where the pinion runs. This configuration requires a high-pressure shaft seal but assures that the rack and pinion is well lubricated. With fluid piped to the CW port, the output shaft turns clockwise. With fluid piped to the CCW port, the output shaft turns counterclockwise. This design works best in pneumatic or low-pressure hydraulic applications. The torque range usually does not exceed 2500 to 3500 in. lb.
The cutaway view in Figure 19-4 shows another style rack-and-pinion type rotary actuator. This design has opposing pistons with a rack gear as the piston rod. Fluid only enters the blind side of the piston so the pinion shaft never sees pressure. When fluid enters one of the piston cavities, that piston moves, pushing the rack gear to drive the pinion, and producing rotary output. With fluid piped to the CW port, the output shaft turns clockwise. With fluid piped to the CCW port, the output shaft turns counterclockwise.
The rack-and-pinion design rotary actuators shown in Figure 19-4 are available with a second rack gear and pistons mounted on the opposite side of the pinion. This double-piston setup produces twice the torque in both directions of rotation.
Optional stroke limiters select a precise stopping point at any degree of rotation less than maximum. Also available are cushions that decelerate rotation speed near the end of the stroke. Cushions are adjustable and not affected by the stroke limiter option in the same rotary actuator. This type rotary actuator is available with an optional hollow output shaft.
Rack-and-pinion rotary actuators operate equally well on pneumatic or hydraulic pressure (up to 3000 psi). They generate torque up to 200,000 in. lb for air service, and up to 15,000,000 in. lb and higher for hydraulic service. Output shafts turn any number of degrees up to five rotations according to piston and rack gear size.
Figure 19-5 shows a simplified cutaway view of a spiral-shaft rotary actuator. (There are several variations of spiral-type rotary actuators, but all function similar to this diagram.) The spiral-shaft rotary actuator has a keyed, non-rotating piston with a hollow rod. The hollow rod has a set of internal spiral grooves that mesh with the spiral shaft. The spiral-grooved shaft only has rotational movement and extends through the housing as an output shaft. With fluid piped to the CW port, the output shaft turns clockwise. With fluid piped to the CCW port, the output shaft turns counterclockwise.
One available option is a stroke limiter that allows a precise stopping at any degree less than maximum. Also available are cushions to decelerate rotation speed near the end of stroke. Some manufacturers make this type rotary actuator with an integral cylinder that adds linear movement to the output shaft.
The spiral-shaft rotary actuators in Figure 19-5 operate equally well on air or hydraulic power. They operate at pressures up to 3000 psi and produce torque up to 20,000 in. lb for air service, and up to 5,000,000 in. lb for hydraulic service. Output shafts normally rotate 360° with more turns available on special order.
Figure 19-6 shows a simplified cutaway view of a chain-and-sprocket rotary actuator. It consists of a large-diameter power piston with a roller chain attached to both sides. The roller chains go around a sprocket at both ends and attach to both sides of a smaller isolation piston. When pressurized fluid enters a port, it pushes against both pistons with equal force. Because the power piston has more area, it moves away from incoming fluid. The smaller isolation piston regenerates into the incoming pump flow. (To find the effective working area, subtract the area of the isolation piston from the area of the power piston.) With fluid piped to the CW port, the output shaft turns clockwise. With fluid piped to the CCW port, the output shaft turns counterclockwise.