Employing fluid power to achieve mechanical motion can be implemented via hydraulics using uncompressible liquids or via pneumatics using compressible gasses; typically, air. The latter provides several distinct advantages, making it a popular choice for many mechanical applications. Indeed, this isn’t a recent development, as using pressurized air for industrial applications and automated machinery has a history extending back well over 100 years.
In most facilities, the most basic utilities are electricity, natural gas, and water. Many users find compressed air is a convenient fourth utility, usually produced by using electricity. This article examines pneumatic advantages in industrial applications.
Automated equipment may employ a myriad of motion tasks such as clamping, gripping, positioning, lifting, pressing, shifting, sorting, and stacking. Most of these are two-position actions with repeatable end stops, but multi-position actions are often also required. Even more advanced are adaptive uses — possibly including closed-loop control for more precise positioning — such as tensioning, pressing, labeling, embossing, crimping, and cutting.
Pneumatics are well suited for moving and holding parts and tooling for industrial machinery and processes. A classic example is called “pick-and-place” in which pneumatic systems are used to achieve horizontal and vertical travel of a gripper moving parts through a manufacturing process.
Power Transmission Options
Linear motion is a common action required by automated equipment and machinery. Pneumatic and hydraulic cylinders easily transmit power into linear motion using cylinders that extend and retract. Electric-powered systems generally need to use belts, pulleys, chains, sprockets, or clutches to translate rotational motion into linear force, unless more expensive and specialized linear motor technology is used for very light loads. Figure 1 indicates the general advantages for each method of linear power transmission.
These competing technologies can be evaluated for each application. Some machines may operate using pneumatics entirely, but it is also common for larger machines to use two or three of these power transmission methods.
Function, Simplicity, and Economy
Fluid power systems, which include both pneumatics and hydraulics, typically produce more power with a smaller footprint than electric systems, and this often makes them the technology of choice for machining and other applications where significant clamping or positioning forces are required.
For fluid power, the associated control components often involve only a small control valve, regulator, and flow controls to control a cylinder’s direction, force, and speed. Electrically actuated systems may require an electronic controller, multiple I/O points, communication cables, and possibly encoder feedback, along with more complex automation system integration.
Hydraulic systems are the most expensive of the three options to purchase, operate, and maintain. These and other concerns typically limit their use to applications where their very high peak power output is required, since they deliver the highest power density of the three options.
Pneumatic equipment generally has lower up-front design requirements than other options and is overall the least expensive. Not only are the installation materials and components relatively economical, but once a pneumatic system is in operation, the maintenance such as replacing seals or even a whole cylinder is often much cheaper than servicing, let alone replacing, an electric actuator. Troubleshooting can also be easier for pneumatic systems compared to electric.
One historic downside of pneumatic systems has been the noise of the air and clattering hardware; however, this has become less of a concern due to improved designs and accessories, allowing the hardware to run quieter and with less impact.
Since the source compressed air is typically created from electrically driven compressors, there are conversion losses. Where electric actuator options can be implemented, they can claim superior energy efficiency. It can be argued that pneumatic systems cost more than electrical systems from an operating standpoint; however, when all functional, design, installation, and operation factors are taken into consideration, the scale is often tipped in favor of pneumatics over hydraulic or electrical options.
Pneumatic Hardware Basics
Most industrial facilities already have an air supply system consisting of one or more compressors and storage tanks. These systems must produce, clean, and dry the compressed air before it is distributed throughout the facility.
Once compressed air is delivered to an automated machine, there are several types of common pneumatic components (Figure 2):
Air preparation system (shutoff/lock-out, combination filter/regulator, soft start valve)
Tubing, hoses, and distribution manifolds
Control valves and manifolds (manual, air pilot, solenoid-operated)
Air cylinders and actuators
Cylinder position sensors
Discrete pressure switches
Specialty components and accessories
A basic pneumatic system consisting of air preparation, tubing, a control valve, and a cylinder is shown in Figure 3.
The air prep system should include a manual and lockable shutoff valve for isolating the equipment during servicing. A filter, water trap, and pressure regulator ensure the air is clean, dry, and at a suitable pressure. The air prep system may also include a lubricator, but it’s usually not necessary unless pneumatic rotary tools are in use.
An electrically operated soft-start valve serves two purposes. The first is to slowly pressurize the downstream system when energized so devices don’t slam into position. The second is to quickly dump air pressure downstream during an emergency stop, guard open, or similar safety event so actuated equipment stops as quickly as possible.
Tubing, hoses, distribution manifolds, and push-to-connect fittings are the most popular ways of distributing air on a machine. Hoses are flexible to allow for motion, and push-to-connect fittings make it quick to install and service systems. Distribution manifolds are a compact way to provide many air connections.
Compressed air is delivered to control valves or valve manifolds. Each valve can be manual, air-piloted, or solenoid-operated, with each used to turn the air supply off and on. In turn, these control valves feed control air to a variety of downstream equipment such as pneumatic cylinders, actuators, and grippers for power transmission.
Pneumatic cylinder position sensors and pressure switches are commonly used to provide feedback to the automation system regarding the physical location of the actuators and the status of supply pressure. There are also a wide variety of specialty components to perform additional fine-tuning of pneumatic operation such as flow controls, quick exhaust valves, hand valves, check valves, inline pressure regulators, and hard or cushioned end stops. Also, gauges and indicators can be added to provide additional information and diagnostics for operators and maintenance personnel.
Pneumatic Design Basics
The best design practice is to start at the actuators and mechanically determine the required force, stroke length, and speed. This will drive the compressed air requirements, from which the designer can work backwards to size the upstream supply elements.
Another part of the design is picking the proper automation elements. Different types of control valves are available based on whether the equipment should fail in a certain position or in last position upon an electrical failure. Designers should also consider what type of position switches will be needed to achieve positive control. A cost-effective balance must be determined between providing all possible options for consistency, as opposed to just providing the bare minimum at each location.
Electric motion systems can offer highly accurate and programmable motion, and hydraulic systems are sometimes needed to achieve very high levels of force. But for many industrial equipment applications, pneumatic systems are the best choice for achieving mechanical motion with the best combination of functionality, simplicity, and economy.
This article was written by Pat Phillips, PE, Product Manager, Fluid Power and Mechanical Product Division, for AutomationDirect, Cumming, GA. For more information, visit here.