Process and production engineers often find the simplest solution to an actuation challenge is the best, with fewer moving parts and reduced complexity usually equating to more reliability with less wear and downtime.
However, even the simplest ideas sometimes have in the past sometimes faced difficulty in achieving market acceptance when they run up against established practice. Like people in many other fields, process engineers and production managers often find it easier to stick with the known.
“Even in those instances where the simpler alternative delivers higher reliability and efficiency at less cost, there has always been an effort involved in changing established ideas and practices. But that could be changing as current economic conditions compel industry to think again to find the best and most cost-competitive methods of production,” says James Maslin, who is National Sales and Marketing Manager of Air Springs Supply Pty Ltd, which is national distributor for some of the very simplest pneumatic actuators and isolators available, Firestone air springs, or air bags.
Known as Airstroke™ actuators or Airmount™ Isolators, these flexible-wall, bellows-type air cylinders are, in essence, tough, fabric reinforced rubber balloons of different shapes engineered to perform different tasks. They can be small enough to fit in the palm of a hand, or more than a metre across and capable of producing 40,000 kg of force. They are typically used for high-force, low-stroke applications and for rapid cycle equipment.
“For many applications, including machinery actuation and metalworking, a major advantage of the Airstroke air spring actuators is that they don't use the guides and seals found in traditional pneumatic cylinders. This difference is the key to many of their benefits in rigorous production environments, ranging from metal stamping and metal fabrication through to mineral processing and conveying applications. Their hygienic performance – they’re easy to wash down and aren’t affected by water or grime – means they are also suited to food, beverage and primary production machinery and high speed labelling, sealing and packaging tasks.
To acquaint engineers and production managers with the possibilities of Airstroke and Airmount actuators and isolators, Air Springs has developed an extensive website (www.airsprings.com.au) that includes global applications of these technologies, as well as complementary products, such as Pickers and Grippers for manufacturing and packaging plus solid Marsh Mellow ® isolators.
Capabilities and differences
Air springs have capabilities and limitations that set them apart from traditional metal-wall cylinders driven by the pumps and compressors in nearly every production plant. The very different appearances of the cylinders is illustrated at left.
It is important to remember, however, that, regardless of their appearance, air springs are indeed cylinders - and that their performance has a critical bearing on the overall efficiency of pneumatic systems powered by pumps and compressors.
What is a cylinder?
In geometry, it's the rotation of a parallelogram around one of its sides. In fluid power, it's a device that extends axially when pressurised, causing motion or exerting a force.
Traditionally, a fluid power cylinder has had the general geometric shape described above and illustrated at left. In function, however, it's not necessary for an axial force/motion generator to be specifically cylindrical in shape.
Most production and process engineers involved in manufacturing, materials handling and motion control are familiar with the traditional cylinder design. It contains a piston sliding within a housing of circular cross-section connected to the work by a rod passing through one end of the device.
This design necessitates several guides and seals, which align and seal the sliding surfaces. These allow a pressurised, contained column of fluid to apply force to the piston.
An air spring uses none of these components to contain and channel its column of fluid. This difference is the key to its functionality.
An air spring contains its column of air in a fabric-reinforced rubber envelope, or bellows. The ends are sealed by bead plates, which are crimped around the bead of the bellows. These plates contain the attachment hardware for the part, normally a blind tapped hole called a blind nut. An air fitting, generally in one bead plate, allows fluid (air) to be introduced into the chamber. The fabric in the side wall of the bellows restricts radial expansion, so pressure is built up, causing axial extension.
Air springs are available in a variety of styles, sporting differing components that control the shape and path of axial extension, but their basic design is the same. Each style is, in essence, a heavy-duty balloon.
In order to select the appropriate air spring, users need to know the force necessary, the linkage motion and any special environmental concerns. A broad range of air springs is available to Australian industry. Airstroke actuators from Air Springs Supply, for example, give 40-40,000kg of pushing or lifting power. Offering power strokes of up to 350mm, Airstrokes are powered by simple, basic compressor equipment found in nearly every factory.
Australian manufacturing and industrial plants use them as ram cylinders, die cushions, counterbalances, clamps, lifters, valve operators, flexible connectors, shock absorbers and isolators. Because of their power and durability, they have even been used to split an enormous coal dragline for maintenance, Australian and international uses have included:
- Web and cable tensioning and roll changing on presses and production machinery
- Conveyor stops and gravity gates; conveyor line actuation for direction changes and pallet handling
- Belt takeup and roller friction brake on conveyor equipment employed in process and packaging applications; skate wheel right angle gravity transfer section on a conveyor
- Scissor lifts, gate valves and die strippers
- Isolation of computers, cameras and vibration-sensitive quality control technology,
- High speed metal stamping, clamping, forming, gluing and laminating
- Isolation of vibrating screens, generators, motors and compressor equipment
- Actuation of mineral processing and primary processing machinery, particularly where resistance to grimy environments is important
- High frequency stress testing of metal products
- Metal press counterbalances, paper sizing presses and forming presses
Benefits and Limitations
- Air springs can be used instead of more expensive hydraulic systems when applying large forces. Sizes are available from fewer than 80mm to more than nearly 1000mm (3in to 38in) in diameter. The larger sizes allow force up to 40,000 kg each using only 7 bar (100psi) air pressure. The capital cost of an air spring is normally less than half that of a pneumatic cylinder with equivalent capabilities
- Compact installation. The way a flexible-wall air spring operates is as follows: it is compressed to its minimum height then extends when pressure is applied. In most cases, the minimum height is considerably less than the available stroke. As a result, air springs can be put in a very compact space and extended to more than twice their starting height. This is a tremendous benefit in floor-mounted lifting devices.
- Side load flexibility. Because an air spring has a flexible, compliant bellows wall, instead of seals or guides, the bellows follows the path of least resistance. This means users don't have to worry about side loads caused by misalignment.
- Ease of attachment. Since the bellows bends, bead plates don't have to remain parallel, this significantly simplifies attachment, especially when linkage is at an angle. As long as the bellows' side walls aren't over-extended or over-compressed, users can stroke through an angle without clevises.
- Constant force. The lack of seals also means lack of friction. In many cases, a constant force needs to be applied to a moving object. With traditional cylinders, the sliding seals can stick, providing a jerky motion that can damage equipment. Air Springs outlast cylinders in most high-speed applications. They don't require lubrication and thus have a lower system cost.
- Curtailed air and production losses. Air springs contain no moving parts to break, wear, leak compressed air, or to cause costly disruptions of production.
- Suitability for aggressive environments. Since there are no seals sliding against exposed surfaces, an air spring can often survive abrasive and corrosive environments that require special consideration when a conventional cylinder is used.
Although air springs are beneficial for scores of applications where they are not currently considered, there are some limitations to consider when specifying them. They are as follows:
- Air springs are single-acting: some outside force must be used to retract them to their minimum, or starting, height. Frequently, this outside force is gravity - or it can be another air spring, or coil spring.
- The available stroke of an air spring is limited by the length of the side wall. This length is determined by stability concerns (Length-to-diameter considerations). These design needs can frequently be met by specifying either single, double or triple-convoluted air springs. However, air spring users should not stretch the wall in extension or pinch it in compression. This goes for linear as well as angular movements. Therefore, air springs are normally used for high-force, low-stroke applications.
- One critical component of an air spring is rubber. This must have good elongation, flexibility and abrasion resistance - factors which limit the choice of elastomers available. These determine the temperature and chemical compatibility of the product. Temperature ranges from -20 deg c to more than 95 deg c (-65 to 220F) are available in most designs. Normally, users should keep air springs away from petroleum-based fluids and chemicals that attack rubber (Although it is worth noting that these same air springs are used as suspension components in the latest semi-trailers, tourist coaches and in smooth riding express trains.
And a bonus
Not only are air springs Airstrokes ideal actuators for many applications, but the structurally identical Airmounts are also a highly efficient isolator. An air spring not only lifts, but also isolates an object. For instance, it can lift and support a vibrating load such as a shaker or vibrator without concern for wear on components. Isolation efficiencies frequently exceed 99 per cent.
As Australian states move toward more stringent workplace and environmental guidelines, this isolation capability is becoming a major factor in machinery design
Certified to ISO 9001: 2008 Quality Management Systems, Air Springs Supply represents world class products including:
- Firestone air bags, from the world’s top selling supplier of air springs for heavy vehicles
- Airstroke ™ Airmount™ actuators and isolators from Firestone for industrial applications
- Marsh Mellow(R) rubber mounts for industrial applications
- Ride-Rite™ and Coil-Rite™ airbag helper kits for working vehicles and 4wds
- Pickers and grippers for manufacturing, packaging and food and beverage applications
- Pronal Elastomer and environmental protection products, including pipeline systems
- Contitech air springs for heavy automotive including bus, prime movers, trailers and rail applications.
Free technical information
Would you like to learn more about the technology discussed in this exclusive report? Readers can receive more information by contacting Air Springs Supply Pty Ltd.