By Dave Lane
Ormond Beach, Fla.
|Pneumatic rotary actuators, like this rack-and-pinion model, are used in machine automation and material-handling applications, and for operating valves and brakes. |
Pneumatic rotary actuators produce oscillating motion by using compressed air to turn an output shaft through a fixed arc. Like other air-powered devices, they offer benefits to machine builders such as ruggedness, compact size, and cool running operation. They are simple and efficient, and can generate high instantaneous torque in either direction of rotation.
They are often used to pivot a joint when a conventional cylinder proves impractical due to space, weight, or motion limitations. Rotary actuators can mount right at the equipment joint without taking up the long stroke lengths required of cylinders. Also, they are not limited to a 90° pivot arc, as is typical of cylinders. Actuator shafts can rotate 180° or more, and make intermediate stops, depending on the design.
Applications abound, including mixing, clamping, indexing, and lifting; transferring, turning, and positioning containers in material handling equipment; and operating valves and brakes.
Today, pneumatic rotary actuators are generally used in two major areas: machine automation and control of processes and systems. Here’s a brief look at the functional requirements and actuator characteristics for each.
The components that make up an automated system must be selected based on performance requirements, and that certainly holds for rotary actuators that provide mechanical motion. The most obvious requirement is that actuator speed must match system operating rates. Cycle time includes not only actual part motion, but the time required for preparatory functions for the next operation – such as pressurizing and exhausting the device.
|Rack-and-pinion actuators are available in many different versions, but torque output is constant and equal in both directions. |
Depending on the forces and mechanical impacts an actuator imparts to the adjacent structure or products being handled, deceleration rates and cushioning within the actuator may be concerns. Both are usually adjustable.
Automated systems often produce, handle, or assemble small parts, so the size and shape of a machine’s operating elements can have practical influences on equipment layout and operation. For that reason, actuator size and inertia often must be kept as small as possible.
Also, automated systems must be cost competitive and the ability to quickly create or modify systems is an increasingly important cost consideration. Actuators which can be configured in a basic installation – but also can be plumbed to different connecting points or operated with various shaft-motion patterns – are valuable components for machine builders.
For instance, most rotary-actuator shafts travel between two positions over a fixed angle. Designers can sometimes make easy revisions to system operating schemes by providing other motion patterns: say three, four, or five different positions around arcs that are determined by valve operation. Rack-and-pinion actuators often provide this capability.
Actuators for control
Most industrial-process valves require 90° of motion from the fully open to fully closed position. Therefore, as a matter of practice, most actuators in this group rotate valve elements 90°. They usually act at moderate rates, partly to ensure a tapering off of flow in the pipe or conduit to avoid abrupt flow stoppage and resultant pressure spikes.
|Vane actuators are relatively simple with few moving parts. Single-vane models rotate about 280°, double-vane versions are limited to about 100° of movement. |
Ordinarily, valve-operating actuators are configured so they do not seat opposing elements under load – flow forces handle the job. Thus, impact is not a concern when the actuator completes its motion.
|This simple actuator provides about 100° of rotation and sinusoidal torque output. |
Process-control facilities often have relatively extensive systems set up to maintain operations in a controlled pattern over an extended period of time. While they often do not require configuration or functional changes, they do demand stable, durable components.
Actuators must provide the required torque within a compact package and usually do so for years. In many cases, the system is outdoors, so components must be built to withstand this environment and remain efficient over the long haul. Because simple, two-position operation is generally standard, spring-return valves can reduce complexity and lower costs.
Designers specifying pneumatic rotary actuators to suit an application should consider factors such as operating pressure and temperature, the actuator’s rotation angle and maximum torque output, and axial and radial load capacities.
|Scotch-yoke actuators generate high breakaway torque. |
Shaft diameter and length – or for rotating tables, table diameter, height, and mounting-bolt pattern – are additional concerns. Accessories to consider include cushions or shock absorbers for high-impact conditions and switches or encoders where position feedback is required.
Different types of actuators use different means to generate rotary motion, and most fit specific types of applications. Here’s a brief overview.
Rack-and-pinion actuators use fluid pressure to drive a piston connected to a gear rack, which rotates a pinion. Standard units typically rotate 90°, 180°, or 360°, but units with much greater shaft rotations are available. Torque is constant and equal in both directions.
Units with two parallel piston-rack units balance loading on the bearings and double torque output, as both racks engage one centrally located pinion.
|Piston-and-chain actuators and produce several complete rotations of the output shaft. |
Adaptability has made the rack-and-pinion actuator popular in several forms for automation. Probably the most widely used is the tie-rod version. It has been commercially available longer and can be built in more configurations than other types. And it can house large bearings to handle difficult shaft loads in heavy-duty applications.
Extruded-body actuators are a form of rack-and-pinion actuator that offer several application advantages. In general, they cost less, are more compact, and present a cleaner appearance. But the extruded body can limit the degree of rotation in off-the-shelf units.
Turntable actuators are low-profile devices configured to provide torque around a vertical axis. Pneumatic elements are grouped into a small space, and the actuator must have accurate bearings to handle offset loads – as the diameter of the rotating table can far exceed that of the actuator itself.
Multi-position actuators are well suited for sorting and selective-assembly operations. Multiple-position, rack-and-pinion actuators are available that rotate the output shaft to several positions by switching the pressure porting, with system logic controlling the valves. Output positions can be in any sequence, letting the actuator stop at or pass any intermediate position.
Three-position actuators are the most widely used. A dual-rack, tie-rod unit is equipped with internal stop tubes on one side of the cylinder body. Pressurized pistons in this side can be driven to the stop tubes to provide an intermediate shaft position. Removing the pressure on this side lets pressure on either piston on the other side drive the shaft to an end position.
For four or five positions, a dual-rack, tie-rod actuator is fitted with stop tubes in cylinders in the same pattern as the three-position unit. In addition, two auxiliary cylinders and pistons are mounted outboard. They have push rods extending from their inner bases that pass through bearing bores in the inner end caps. Pressure on the outboard cylinders drives the push rods inward to provide the additional stops. Two auxiliaries provide four position stops, four provide five position stops.
Indexing actuators operate in fixed steps in a single direction. In conveyor or other repetitive operations, they provide a simple, precise means to generate a single rotation step.
Multi-motion actuators provide more than just rotary motion. For example, a rack-and-pinion actuator can be combined with a linear actuator via a coupling with a splined connection to the linear cylinder. It provides simultaneous and separately controllable rotary and linear motion.
Multipurpose rotary actuators
Some pneumatic rotary actuators are suited for both automation and control tasks. For instance, compact and simply plumbed vane actuators are useful and effective in both closely integrated automation systems, and in valve-actuation process applications.
Single-vane actuators have a cylindrical chamber in which a vane connected to a drive shaft rotates through an arc. Two ports are separated by a stationary barrier. Differential pressure applied across the vane rotates the drive shaft until the vane meets the barrier. Reversing pressure at the inlet and outlet ports changes the rotation direction. For rotations less than about 280°, a single vane actuator is attractive for its compact size.
Dual-vane actuators have two diametrically opposed vanes and barriers. This construction provides twice the torque of the single vane unit but is limited to about 100° of rotation.
Vane actuators have few parts and less-critical fits than many other types of rotary actuators, and this can make them easier to service. However, sealing the internal vanes can be a challenge and bypass leakage can be a problem. Position-holding capabilities may be limited without external controls.
Enclosed piston-crank actuators consist of a single cylinder with the rod connected to a crank arm that drives the rotating shaft. The devices are typically pressure-actuated in both directions and equipped with moveable stops for adjustment of stroke.
The simple mechanism provides about 100° of rotation. This actuator is compact, and built-in bearings overcome side thrust forces. Torque generation follows a sinusoidal pattern. Maximum torque is at midstroke, so select these actuators to drive a load based on the minimum output torque.
Scotch-yoke actuators have two pistons connected rigidly by a common rod. A central drive pin on the rod engages a double yoke keyed to the output shaft, which turns through arcs to a maximum 90°.
Torque output at the beginning (breakaway torque) and end of stroke is twice that at the midpoint (running torque). This characteristic is often desirable because many applications require high breaking torques to move and accelerate a load. Single and double-acting models are available.
Piston-and-chain actuators use a circular drive chain that travels over a sprocket on the actuator shaft and is held in tension by an idler sprocket. A large piston attached to the chain runs in a cylinder; a smaller piston and cylinder on the opposite side of the chain loop seals the system.
Ports admit pressure from each end of the enclosed system. Rotation reverses by reversing porting. It develops shaft torque by the difference in area of the pistons driven by the applied pressure.
A piston-chain actuator can provide several complete rotations. The design is limited by the strength of the chain and sprocket, but torque is constant throughout the stroke.
Piston-mounted rack-and-pinion actuator. In this design, the piston and rack are one functional part, with the rack an extension machined on one face of the piston. The piston can be double acting to drive the shaft in either direction. With two piston-rack combinations at opposite ends of the cylinder, dual torque results along with balanced loads on bearings. Extended rotations well beyond 360° are possible.
For more information, contact the author at Rotomation Inc, 11 Sunshine Blvd., Ormond Beach, FL 32174, (386) 676-6377, or visit rotomation.com.
Use flexible shaft couplings to eliminate side loading due to shaft misalignment. And when side loading is unavoidable, support the output shaft with auxiliary bearings, if the unit is not built to handle such loads. (Some rack-and-pinion designs, for instance, are available with integral bearings that can support significant overhung loads.)
To bleed air, mount the actuator so supply ports are on top. Or supply a suitable air-bleeding device for the system. Larger actuators often have built-in bleed valves.
Finally, do not install rotary actuators where contaminants in air lines are likely to collect – for example, at the system’s low point.