News Releases

July 2010

Air actuators' missing link can drive metal stamping technology to new efficiencies

One of the weakest links in high speed metal stamping and fabrication machinery can be the seals in traditional cylinders used to actuate the equipment involved.

This is especially critical in aggressive atmospheres laden with dust and waste, where such cylinders can clog up and wear prematurely over the millions of cycles for which they must operate.

One simple way to avoid this potential problem is to employ cylinders that totally lack such seals, says pneumatic actuation and isolation specialist Simon Agar. Mr Agar - who is General Manager of Air Springs Supply Pty Ltd - has more than 20 years experience with Firestone air spring actuators known as Airstrokes®.

These highly engineered rubber and fabric air bags are flexible-wall, bellows-type air cylinders which are ideally suited to engineering of assemblies for high-repetition tasks, for which they are inflated and deflated rapidly to achieve their purpose.

"Traditional cylinder designs contain 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. 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.

"Each style is, in essence, a heavy-duty balloon. 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."

In order to select the appropriate air spring, you need to know the force necessary, the required stroke 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 and international uses have included:

• High speed metal stamping
• Forming presses
• Die strippers
• Quench and pickling tank actuation
• Conveyor line actuation (mining and manufacturing)
• Conveyor stops and gravity gates
• Pallet handling equipment
• Web tensioning
• Vibrating screens and compressor equipment
• High frequency stress testing of materials
• Automotive metal press counterbalances
• Belt takeup and roller friction brake on conveyor equipment

Australian industrial plants also use them as ram cylinders, die cushions, counterbalances, clamps, lifters, valve operators, flexible connectors, shock absorbers and isolators.

Mr Agar says Airstrokes have capabilities and limitations that set them apart from traditional metal-wall cylinders driven by the pumps and compressors in nearly every production plant. It is important to remember, however, that, regardless of their appearance, they are indeed cylinders - and that their performance has a critical bearing on the overall efficiency of pneumatic systems.

Benefits and Limitations

1. Benefits. Key reasons for using air springs include:

• Cost benefits. 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 and conveyors, for example.
• 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.
• Durability. 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, including heavy industry and mining. 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.

2. Limitations

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. 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 trains).

And a bonus

"We certainly don't pretend they are the ideal actuator for all applications," says Simon Agar, " but we do contend that they are the ultimate actuator for many applications for which they are not always considered, including metal stamping and forming applications.

"And not only are air springs ideal actuators for many materials handling applications, but also they are a highly efficient isolator. An air spring not only lifts, but can also be used to isolate an object highly effectively (in which application they are known as Airmounts®)."

Airmounts can lift and support a vibrating load such as a shaker or vibrator without concern for wear on components. They can stop a motor passing noise, harshness and vibration (NVH) into the surrounding plant - or isolate a computer from surrounding NVH. Isolation efficiencies frequently exceed 99 per cent.

As Australian states move toward more stringent workplace and environmental guidelines - and as computerized automation equipment is more commonly used - this isolation capability is becoming a major factor in machinery design.

"Engineers shouldn't overlook air springs because they are simple," says Simon Agar. "Many of the best engineering innovations are the simplest. Sometimes, however, their very simplicity can work against initial market acceptance.

"Engineers being sceptical types, they question an idea that seems too good to be true. This is certainly true in Australia of the air spring - and many projects may be the poorer for it. Surely it is worth the time just to see if this might be the case?"

Free technical information

Would you like to learn more about the technology discussed here? Readers can receive more information by contacting AirSprings Supply Pty Ltd, PO Box 215 West Ryde, 2114, ph (02) 9807 4077, fax (02) 9807 6979.

For more information about Air Springs Supply's national distribution and technical support network, please contact Air Springs Supply Pty Ltd, 10 Angas St, Meadowbank, Sydney 2114, ph (02) 9807 4077, fax (02) 9807 6979, sales@airsprings.com.au