Process and production engineers often find the simplest solution to an actuation challenge is the best. Fewer moving parts and reduced complexity typically equate to more reliability, less wear and tear and reduced downtime.
However, even the simplest and most obviously practical ideas sometimes face difficulty achieving 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 pneumatic isolation, actuation and suspension specialist Vinh Lam, of Air Springs Supply Pty Ltd, which is national distributor for some of the very simplest pneumatic actuators and isolators available globally, 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.
Complementing Airstrokes and Airmounts are tough, no-maintenance solid rubber-and-fabric Firestone Marsh Mellow® isolators, which offer further design flexibility.
"For many applications, including machinery actuation, automation and high repetition 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," says Vinh.
This difference is the key to many of their benefits in rigorous production environments, ranging from automated high-speed processing and despatch 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 Supply has developed an extensive website compendium, "Firestone Problem Solvers" that includes global applications of these technologies, as well as complementary products, such as 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 found in nearly every production plant. The quite different appearances of the cylinders is illustrated below.
Regardless of their appearance, air springs are indeed cylinders.
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 blind tapped holes called blind nuts. 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 required, the linkage motion, the available space, and any special environmental concerns.
Applications
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 in large numbers to lift 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
- Isolation of vibrating screens, generators, motors and compressor equipment
- High speed metal stamping, clamping, forming, gluing and laminating
- Actuation of mineral processing and primary processing machinery, particularly where resistance to grimy environments is important
- Metal press counterbalances, paper sizing presses and forming presses
Vinh says that in addition to being a maintenance-free alternative to conventional pneumatic cylinders, air springs can be used instead of more expensive hydraulic systems when applying large forces.
Sizes are available from fewer than 80 mm to more than nearly 1000 mm (3 in to 38 in) in diameter. The larger sizes allow force up to 40,000 kg each using only 7 bar (100 psi) air pressure. The capital cost of an air spring is normally less than half that of a pneumatic cylinder with equivalent capabilities.
They also offer compact installation, side-load flexibility, ease of attachment, curtailed air (energy) losses, constant force and durability. They don't require lubrication and thus have a lower system cost.
"In many cases where they may be overlooked, air springs and Marsh Mellows are the ideal substitute for OEM actuators and isolators that have outlived their usefulness," says Vinh Lam. Firestone air springs can also substitute directly for many other brands of air spring.
"Naturally, different types of springs have different performance characteristics suited to particular tasks. These should always be discussed with suppliers having wide experience in the particular area of application being considered," said Vinh.
