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Airmounts in a state of perfect tension

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article image Detail of the Longhorn vibrating screen installation

A PROBLEM sometimes encountered with conventional industrial screening equipment is its inability to provide the many speed variations required for handling a diversity of very fine materials.

Simple metal springs may be too rigid for a multitude of tasks, while sophisticated hydraulics may be too complicated for the standards of flexibility and versatility required for very fine materials.

An option in such cases is Air Spring’s Airmount isolators, which are, in essence, immensely strong rubber and fabric-reinforced balloons, or bellows.

Because Airmounts use air as the isolation media rather than a solid material, they provide less of a pathway for transmitting high frequency vibration.

In addition to reducing structurally transmitted vibration and noise transmission, they enhance fine control. Because they do not exhibit the chatter that conventional coil springs do, they are better suited for handling changing loads or overload conditions than coil springs.

Any given Airmount (and they range in load carrying capacity from 40-40,000 kg) can easily handle a substantial change in load by simply adjusting the air pressure, whereas coil springs are designed for a very narrow operating range.

Also, because of their nearly constant natural frequency, Airmounts react in a much less violent manner than coil springs during start-up and shut-down conditions, as the input frequency changes.

"The versatility of Airmounts for screening applications was borne out recently during a study tour in the US where I came across one of the most versatile screening applications I have encountered," Air Springs Supply managing director Andrew Cameron said.

The application involved Longhorn Crusher Manufacturing, Texas. Unlike traditional gravel crushers, Longhorn crushes very fine material like kaolin powders, salts, fertilisers, chemicals and glass.

The crushing process includes a rigorous sifting through various size screens to ensure the material's uniformity, so the final product meets the precise specifications for its intended use.

"A good example is fertiliser," says Howard Fager, owner of Longhorn Crusher. "Fertiliser has to be ground to a specific size so it will broadcast evenly. If it is too small, it'll fly into the atmosphere, and if it's too large it won't broadcast properly."

Another example is salt, which can be ground finely enough to melt quickly as a food flavouring or in larger granules to effectively melt ice on roads.

Longhorn grinds glass beads for reflective road signs, turns kaolin clays into powder for use in paper production, and prepares fine granular for roofing material applications.

A key step in the grinding process is passing the ground matter through vibrating screens that sift out debris and large particles.

A variety of screens and cloths can be stretched across two vibrating bars for sifting. The screener is installed at an angle, so that gravity and vibration work together for optimal sifting.

Longhorn uses four Firestone Airmount isolators on each vibrating deck powered by a motor.

The Firestone Airmounts are fixed directly to a cross-member frame of two-inch box tubes, which are connected to the side plates of the screener box.

The Airmounts lift the vibrating rail off the cross members and provide perfect tension for the cloth.

This unique setup permits the operators to adjust the pressure of the air springs (from 0-210kPa, 0-30psi) based on the size of the object being sifted.

Each high-speed vibrating screen has only six movable parts: a motor, four Firestone Airmounts and the vibrating rail.

Mr Cameron says one of the most frequent applications of Airmounts in Australia is in simple isolation of vibrating screens, which are widely employed in industries ranging from food and primary product processing through to mineral processing.

Using Airmounts in capacities from a few hundred kilograms to tens of thousands, severe occupational health and safety issues have been overcome through isolation efficiencies of 90-99 per cent.

In addition to overcoming problems of vibration transmission, Airmounts inflated to different pressures (and occasionally filled with different mediums, such as water or solutions instead of air) can be used to change the nature of the vibratory effect being produced by the vibrator motor.

Because friction is reduced, and there are no internal moving parts to break or wear, they are very suitable for high-repetition tasks involving either isolation or actuation.

The flexible air springs' ability to arc without a clevis - to bend and to tolerate uneven and fluctuating loads - also means they are suitable for materials handling tasks such as tensioning webs, belt take-up on conveyors, or powering scissors lifts.

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