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Air springs – an actuator alternative

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article image The tiny plastic moulded Airomatic.

AS process automation gathers pace in Australia - and as new conveyor designs permit higher processing speeds - additional stresses and strains are being generated on vital system components such as actuators.

Because of the higher stroke speeds required, often coupled with greater tolerance to side and uneven loads for reliability, process and plant engineers have to look outside their traditional terms of reference to fresh approaches.

Air Springs Supply national sales manager Simon Agar said most process and production engineers would agree the simplest solution to a problem was often the best.

“Fewer moving parts and reduced complexity usually equates to less wear and more reliability."

The actuation alternative offer by Air Springs involves some of the very simplest pneumatic actuators available, Firestone Airstroke actuators, which are tough, flexible wall bellows engineered into different shapes to perform different tasks. They contain no internal moving parts to break, bend, cause internal scoring or require lubrication.

"Another of their advantages for conveyor applications is 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," Mr Agar said.

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. Because Airstroke actuators have no sliding seal, there is no breakaway friction as with conventional cylinders.

Airstrokes also possess the unique capability of stroking through an arc without a clevis. Angular motion of up to 30 degrees is possible, along with the design advantage of generally less complex linkages.

"We don't for a moment claim that Airstrokes are the ideal solution for every materials handling application, but we do know that they can offer considerable performance and cost benefits for many high-speed operations and those requiring reliable performance in aggressive environments," says Mr Agar.

Air springs can be used instead of more expensive hydraulic systems, for example, when applying large forces, or slower and bulkier conventional metal sleeve cylinders.

Airstroke sizes are available from about 31mm diameter to 940mm. These offer compact starting heights of 30.48mm and 139.7mm respectively (with the tiny new plastic moulded Airomatic polyactuator - a different design - having a starting height of just 16mm.

The smaller sizes provide a wide range of forces required for conveyor and processing operations, while the largest sizes allow force up to 40,000kg each using only 7 bar (100psi) air pressure.

Yet the capital cost of an air spring is normally less than half that of a hydraulic or pneumatic cylinder with equivalent capabilities, says Mr Agar, adding that Airstrokes may also be considered in installations requiring:

* 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.

* 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.

* 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. 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.

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